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191.Venning AJ, Brindha S, Hill B, Baldock C. Preliminary study of a nomoxic PAG gel dosemeter with tetrabis (hydroxymethyl phosphonium chloride as an antioxidant. J Phip: Conf Ser 3 2004; 155–158.

192.Courbon F et al. Internal dosimetry using magnetic resonance imaging of polymer gel irradiated with iodine-131. Preliminary results. Proc 1st Int Workshop Radiation Therapy Gel Dosimetry (Lexington, KY). In: Schreiner L J Audet C, editors. Canadian Ottawa, Ontario, Canada: Organization of Medical Physicists, 1999.

193.Gambarini G et al. Three-dimensional determination of absorbed dose by spectrophotometric analysis of FerrousSulphate Agarose gel. Nucl Instrum and Meth 1999; A 422:643–648.

194.Gambarini G. Gel dosimetry in neutron capture therapy. Proc 2nd Int Conf Radiotherapy Gel Dosimetry (Brisbane, Australia). 2001. p 89–91.

195.Gambarini G. et al. Fricke-gel dosimetry in boron neutron capture therapy. Radiat Prot Dosim 2002;101:419–422.

196.Ba¨ck S A et al. Ferrous sulphate gel dosimetry and MRI for proton beam dose measurements. Phys Med Biol 1999; 44:1983–1996.

197.Gustavsson H, Karlsson A, Back S, Olsson LE. Dose response characteristics of a new normoxic polymer gel dosimeter. Ph.D. dissertation Department of Medical Radiation Physics, Lund University, Malmo University Hospital. 2004.

198.Gustavsson H et al. Linear energy transfer dependence of a normoxic polymer gel dosimeter investigated using proton beam absorbed dose measurements. Phys Med Biol 2004;49: 3847–3855.

199.Swallow AJ. Radiation Chemistry: an Introduction London: Longman Group limited; 1973.

200.Ramm U, et al. Three-dimensional BANGTMgel dosimetry in conformal carbon ion radiotherapy. Phys Med Biol 2000;45: N95–N102.

201.Vergote Ket al. Comparisons between monomer/polymer gel dosemetry and dose computations for an IMRT treatment of a thorox phantom. In: Baldock- C, De Deene Y editor DOSGEL 2001, Proc 2nd Int Conf Radiotherapy Gel Dosemetry. Queensland University of Technology, Brisbane, Queensland, Australia.

202.Hepworth SJ et al. Dose mapping of inhomogeneities positioned in radiosentive polymer gels. Nucl. Instrum. Methods Phys Res A 1999;422:756–760.

203.Gum F, et al. Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans, Phys Med Biol 2002;47; N67–N77.

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206.Borges JA, BenComo J, Ibbott GS. A 3 Dimensional Gel Dosimetry Lung Equivalent (WIP), AAPM annual meeting, 10–14 Aug 2003, San Diego(CA); 2003.

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See also PHANTOM MATERIALS IN RADIOLOGY; RADIATION DOSIMETRY FOR ONCOLOGY; RADIATION THERAPY, INTENSITY MODULATED; RADIATION THERAPY SIMULATOR; RADIOSURGERY, STEREOTACTIC.

RADIATION, EFFECTS OF. See IONIZING RADIATION,

BIOLOGICAL EFFECTS OF; NONIONIZING RADIATION, BIOLOGICAL EFFECTS OF.

RADIATION PROTECTION INSTRUMENTATION

GLENN P. GLASGOW

Loyola University of Chicago

Maywood, Illinois

RADIATION PROTECTION INSTRUMENTATION

Radioactive materials and equipment that generate radiation are prevalent in industry, military, education, science and medical facilities, and even in the home. Many scientific instruments perform dedicated radiation measurement tasks; the nuclear power industry employs possibly the greatest number of instruments of different designs and degrees of sophistication. This article describes similar instruments commonly used for radiation protection in medicine. Instruments used for radiation dosimetry for medical treatments (e.g., radiotherapy ionization chambers,) and those used for medical treatments (e.g., nuclear medicine well-ionization chambers) are excluded. Included are instruments used for the general tasks of detecting radiation, determining the types of radiation or species of radionuclides present, determining quantities of radionuclides, and measuring radiation levels around materials and equipment. The focus is how instruments detect radiation, not their electronic circuity, which is described only briefly in a few instances. Before choosing an instrument, the user must know about the availability and choice of instruments, types and sources of radiation, special terms that describe quantities of radiation, and measures of biological dose equivalency that individuals receives in the presence of radiation. A science discipline, Health Physics, and a scientific society, the Health Physics Society, are devoted to these topics (1). Since the1988 first edition of this article, major changes in medical radiation protection instrumentation include the development of the Internet for dissemination, by manufacturers and vendors, of information about instrument designs, operating parameters, and performances; improved performance and electronic circuitry using chips with complementary metal oxide semiconductors (CMOS) microprocessor technology of various types; miniaturization of computer components that reduce the weight and size of instruments; and new

definitions and terms used to describe radiation quantities and units [Note: In addition to the common prefixes of kilo- (k), mega- (M), giga- (G), milli- (m), micro- (m), nano-

(n), pico- (p), note the use of the somewhat less familiar femto- (f; 10 15), atto- (a; 10 18), zepto- (z; 10 21), and yocto-

(y; 10 24)] (2). Manufacturers market smaller, compact survey meters, personnel dosimeters, and specialized detectors and monitors with improved performance. We review common features of instruments, such as ionization chambers, gas-proportional counters, Geiger–Mu¨eller (GM) tubes, scintillation and solid-state detectors, other lesscommon detectors, and photographic films.

AVAILABILITY OF INSTRUMENTS AND INFORMATION

Table 1 lists some major companies and manufacturers of radiation protection instruments, their worldwide web Internet addresses, and their electronic mail addresses. Commercial product catalogues, usually now available on the Internet, contain a wealth of specific information on the theory and operation of instruments. This company list represents no endorsement by the author; these companies were selected because their worldwide web Internet sites provide details about common radiation protection instruments advertised for research, laboratory, environmental, security, medical, and health physics (safety and protection) applications. Basic instruments require only modest modifications for specific field applications. Table 2 contains a typical product list of medical radiation protection instruments. Instruments are regularly reviewed in Technology Monitor articles in Health Physics (3). One general interest group shares information regarding procedures, selection, testing, and standardization of instruments (4). Basic radiation detection principles and instrument designs are described in university level science textbooks in Health Physics curricula; comprehensive descriptions appear in Knoll (5), Shapiro (6), Turner (7), and Gollnick (8).

CHOICE OF INSTRUMENTS

A radiation field often consists of multiple types of radiation. Instruments usually must have the capability to detect particular types of radiation and produce relative or absolute measures of their magnitudes, while discriminating against other types of radiation. Often the radiation energies must be measured. Common medical uses, Table 3, include equipment radiation surveys, area monitoring, area and personnel contamination surveys, personnel dosimetry, finding misplaced radioactive materials (radioactive seeds or sources), surveying radioactive packages, air sampling, and emergency response tasks. Individuals choosing radiation protection instruments for measurements preferably should know about the radiation environment under investigation. Is it predominantly photon

Table 2. A List of Some Medical Radiation Protection

Products

(X ray or g ray) radiation, charged particle (proton, beta particle) radiation, neutron radiation, or mixtures thereof? Spectroscopy measurements can determine the types and energy distributions, but often measurements require simpler detection or measurement devices. The radiation environment may be characterized by the maximum energy of radiation, whether the radiation source is continuous, as with an X-ray unit, rapidly pulsed as with some

linear accelerators, or is random decay from a radioisotope. Is the measurement made in the primary direct beam, or in scattered radiation beam filtered by radiation barriers? Choice of instruments depends on why the radiation is being measured. It is desirable to know the approximate magnitude of radiation, expressed in some appropriate units, and the approximate energy of the radiation. It is then possible to estimate personnel equivalent dose rates

in the radiation field. Multiple instruments with different features may be required to properly characterize and measure a radiation environment.

TYPES OF RADIATION

Radiation is a general term used to describe the emission and propagation of energy through space or a material. Texts describing instruments describe types of radiation (5–8). Mohr and Taylor, on behalf of The Committee on Data for Science and Technology, updated, through 2002, the numerical (Note: In the interest of better science, we present exact values as they are not widely published nor readily available!) parameters of common radiation types

(9). Radiation may be classified as directly ionizing, indirectly ionizing, or nonionizing (e.g., microwaves, laser lights, ultrasound, not further discussed here). Particulate forms of radiation (protons, electrons) possess one unit of electrical charge (160.217653 zC is the unit of electrical charge of an electron) and directly ionize atoms and molecules, as do particulate radiations with multiple charges, such as alpha particles.

Indirecty ionizing forms of radiation (X rays, g rays, neutrons) lack electrical charge, but interact with matter and produce secondary charged particles (electrons, positrons) that ionize atoms. The magnitude of the energy possessed by the radiation, frequently expressed in millions of electron volts (MeV) and the mass of particulate radiation, expressed in atomic mass units (1 amu is defined as 1/12 of the mass of the carbon atom, and equals 0.00166053886 ykg) are important physical parameters that arise in describing the properties of radiation.

Protons have a mass of 1.007276446 amu, possess one unit of positive charge and are one of the core particles of the nucleus. Protons are heavy charged particles, that lose energy mostly by ionization and excitation of atoms as they exert electromagnetic forces on the orbital electrons surrounding the nucleus. Loosing only a small fraction of energy during each interaction, protons move through matter mostly in a straight-line path leaving in their wake ionized or excited atoms. Heavy charged particles require great energy to penetrate tissue. A 10 MeV proton has a range of 0.11 cm, while a 100 MeV proton has a range of7.5 cm. Protons with several tens of million electronvolts of energy are used at research facilities with particle accelerators as probes to study nuclear structure. Neutrons with a mass of 1.008664915 amu are slightly more massive than protons, but lack charge and interact in matter primarily by collisions with protons to which they impart a portion of their energy during the collision. Neutrons are generally classified as thermal if their energies are < 0.5 eV, intermediate if their energies are > 0.5 eV, but < 0.2 MeV, and fast if it is > 0.2 MeV, but < 20 MeV. Lacking charge, neutrons are generally more penetrating in tissue than protons of the same energy and interact with atoms by elastic and inelastic collisions. Numerous radioactive sources serve as neutron generators; an alpha particle source, such as 241Am, may be mixed with a light metal, such as beryllium to produce neutrons by a (a, n) reaction. Nuclear reactors are prolific neutron generators and

numerous research facilities have accelerators capable of producing neutrons. Alpha particles, usually with several million electronvolts of energy, consist of two protons and two neutrons, and appear when certain nuclides decay into more stable nuclides, such as the decay of 238U to 234Th or the decay of 226Ra and certain daughters. While large mass and double charge prevents even the most energetic alpha particles from penetrating much beyond the most superficial layer of external tissue, alpha particles are hazardous when ingested into the sensitive epithelium of the lungs.

Electrons have a mass of 0.00054857990945 amu, a small fraction of the mass of a proton, but carry an equal quantity of negative charge. Electrons with several tens of million electronvolts of energy can be generated with electron linear accelerators and many other pieces of equipment are capable of generating less energetic, but still hazardous electrons. Electrons interacting in a material can also produce a spectrum of bremsstrahlung X rays with the maximum X-ray energy identical to the maximum energy of the electrons. These X rays are far more penetrating that the electrons. Beta particles with several million electronvolts of energy arise from the nucleus during certain radioactive decay processes. Negative beta particles, negatrons, or ordinary electrons, possess a spectrum of energies below their maximum energy. Many nuclear transformations yield multiple beta particles; a few, for example, the decay of 32P to 32S, yield a single negative beta particle. Positrons have the same mass as electrons, but carrying a positive charge and arise in certain radionuclide transformations, such as the decay of 22Na to 22Ne. Beta particles and positrons with several million electronvolts of energy are more penetrating than alpha particles and even minute quantities of radioisotopes producing these particles, such as 32P, can potentially produce damaging skin burns if spilled on the skin and left unattended.

Gamma rays frequently arise when a daughter radioactive nuclide in an excited state, decays by beta particle decay, or by other modes of decay to form a more stable nuclide, such as the decay of 60Co to 60Ni, yielding 1.17 and 1.33 MeV g rays. These electromagnetic rays can possess several million electronvolts of energy and, lacking charge and mass, the more energetic g rays can penetrate deeply into tissue and other materials. Following their interaction in a medium, such as tissue, they generate ionizing secondary electrons that actually produce the damage to cells.

X rays are a form of electromagnetic radiation arising from changes in the arrangements in the orbital electrons surrounding the nucleus, yielding characteristic X rays of several tens of kiloelectronvolts (keV) of energy. Another form of X rays, bremsstrahlung, are produced when energetic electrons with energy of several million electronvolts strike high Z targets yielding X rays with very high energies. Hence, X rays can span a broad energy range, from a few fractions of million electronvolts to several tens of million electronvolts depending on how they are produced. Like g rays, the most energetic X rays have great potential to deeply penetrate matter. The term photon is used to describe X rays, g rays, or other form of electromagnetic energy without referring to the method of production or source of the radiation.

504 RADIATION PROTECTION INSTRUMENTATION

Nuclei of atoms, such as a deuterium, 2H, may be accelerated in highly energetic linear accelerators as a probe to study the properties of the nucleus of various elements. Because of the great mass and charge, heavy charged particles must possess several tens of million electronvolts of energy to penetrate tissue.

In addition to this limited list of types of radiation,

numerous radioactive isotopes of the elements, such as 60Co, 137Cs, 131I, and 125I, and many more are widely used

in medicine for a host of applications. Radioisotopes, through decay, can produce alpha particles, g rays, beta particles, positrons, and X rays and each radioisotope has a unique spectrum of radiation that allows it to be identified even in the presence of other radioisotopes. While many forms of radioactive materials are encapsulated solids, others are unsealed and as liquids or as gases, are more readily dispersed during accidental releases. Hence, radiation is a term used to refer to many different forms of particulate and nonparticulate radiations with energies from fractions of a million electronvolt to tens and hundreds of million electronvolts. Obviously, the means of detecting radiation must be specific for the types of radiations present in a specific locale.

SOURCES OF RADIATION

The most energetic X rays, g rays, and heavily charged particles are found almost exclusively in government or

Table 4. International Radiation Concepts and Unitsa

university sponsored scientific accelerator research facilities. Photons (X and g rays) with energies as high as several million electronvolts are the most prevalent forms of radiation as equipment yielding X and g ray are widely used in medical facilities. X-ray imaging in hospitals is probably the single most common medical use of radiation, with diagnostic use of radiopharmaceuticals next in importance. Beta particle sources and electron producing equipment are the next most prevalent sources of radiation. Neutron producing sources and equipment are frequently found in university research laboratories but are less common in medical facilities.

RADIATION QUANTITIES AND UNITS

Radiation protection definitions and terms often lack clear meanings, as noted by Storm and Watson (10). International commissions make recommendations, but national councils and regulatory bodies in different countries (or even within the same country) adopt or apply the recommendations differently (11). As radiation quantities and units, Table 4, are used on instrument displays, users must understand both historical and newly adopted radiation units. Gollnick offers a useful review (8). We limit this discussion to popular historical quantities and units and provided brief, albeit, limited descriptions of current quantities and units recommended by the International Commission on Radiological Protection, Systeme International

d’ Unites, and the International Commission on Radiological Units and Measurements (11–13).

Counts (events) or count rates (events per unit time) are denoted on instruments that detect the presence and relative magnitudes of radiation. Count rates of a few counts per minute (cpm) to millions of cpm are possible, depending on the radiation field intensity.

Exposure, denoted by the symbol, X, is the measure of the ability of X and g rays of energies 10 keV to < 3 MeV to ionize air and is the quotient of DQ/Dm, where DQ is the sum of all charges of one sign produced in air when all of the electrons liberated by photons in a mass Dm of air are completely stopped. Exposure is expressed in a special unit, the Roentgen (R), equal to 258 mC of charge per kilogram of air at standard temperature and pressure. As with count rates, exposure rates generally vary from5 mR h 1 associated with natural background radiation to nearly 1 MR h 1 in a linear accelerator X-ray beam. Because of its historical use in science and medicine, exposure remains popular even though its continued use is not recommended in the Systeme International d’ Unites (14).

Kinetic energy released per unit mass of material, denoted in lower case, by the acronym, kerma, is a measure for indirectly ionizing radiations (photon, neutron) interacting in a material, of the total kinetic energy of all charged particles released per unit mass of material. Kerma possesses a collision component from the kinetic energy imparted by inelastic collisions with electrons and a radiation component (usually much smaller) from interactions with nuclei. For X rays absorbed in air, collision kerma in air is the product of exposure and the average

energy, W, required to produce an ion pair per unit electrical charge. Kerma is measured in gray (Gy), which is one joule (J) of energy released per kilogram (kg) of the medium. Turner offers a complete description of kerma and its relationship to other quantities (7).

Absorbed dose, denoted by the symbol D, is a measure of the amount of the released energy that is absorbed in the medium per unit mass of the medium. Under conditions of charged-particle equilibrium, with negligible energy loss, absorbed dose equals kerma. One joule of energy absorbed per kilogram of the medium, the gray, is widely used in medicine as a measure of absorbed dose, as is the submultiple, the centigray (cGy), 1/100th of a gray. The older special unit (now abandoned) of absorbed dose, rad, an acronym for roentgen absorbed dose, represents 0.01 J of energy absorbed per kilogram of the medium (1 Gy ¼ 100 rad).

The absorbed dose in a medium may be determined from the exposure in air at the same point in the medium by multiplying the exposure by a conversion factor, fmed that converts the exposure in air to dose in the medium. The factor, fmed, is slightly < 1 for most biological materials, except bone, where values as high as 3 occur for the soft X rays in the diagnostic energy range.

Different types of radiation produce different degrees of biological damage when the same amount of energy per unit mass is deposited in the biological system. The radiation weighting factor, wR, which replaced an older similar concept, quality factor, Q (now abandoned) is a measure of this phenomena (11). Radiation weighting factors of unity, Table 4, are assigned to most electrons, X and g rays, while

factors as high as 20 are assigned to alpha particles and fast neutrons. Hence, the biologically equivalent effect in tissue of the absorption of 1 Gy of alpha particles is 20 times more severe than the absorption of 1 Gy of 1 MeV g rays.

Equivalent dose, usually denoted by the symbol HT, is the term used in radiation control programs to monitor and record the biological equivalency of exposure to amounts of radiations of different energies that an individual has received. The equivalent dose in sievert (Sv), the product of the absorbed dose, D, in gray and the radiation weighting factor, wR, is commonly used to express the biologically equivalency of absorbed doses of particular types and energies of radiation. Historically, this equivalency was expressed in the special unit, rem (now abandoned) an acronym for roentgen equivalent human. As the fmed factor and radiation weighting factors, wR, 1 for most photon energies commonly encountered in many situations, the units for kerma (Gy), absorbed dose (Gy), and equivalent dose (Sv) are nearly numerically equivalent and are often used interchangeably, as were the older abandoned special units, R, rad, and rem.Table 4 summarizes these relationships.

Committed equivalent dose, usually denoted by the symbol HT(50), in sieverts, is employed to consider exposure within the body from internally deposited radioactivity. It represents the total cumulative dose delivered over 50 years to an organ system by ingested radioactivity.

Effective dose, usually denoted by the symbol E, considers the consequences of partial body radiation exposure (11). Tissue weighting factors, wT, account for the reduced effects that occur when only a portion (or organ system) of the body is irradiated. Values for wT (Table 4) range from 0.01 for bone surfaces to 0.2 for the gonads. The effective dose, E, in sieverts, is the sum, over the body, of the products, wT HT for each partially irradiated portion of the body.

Committed effective dose, usually denoted by the symbol, E(50), in sieverts, is employed to consider exposure from internally deposited radioactivity. The committed effective dose, E(50), is the sum, over the body of the products, wT HT(50), for each partially irradiated portion of the body > 50 years.

For individuals experiencing both external and internal radiation exposure, methodologies exist (omitted here) to combine effective dose and committed effective dose to estimate cumulative risks from both types of exposures (8).

Activity, denoted by the symbol A, describes an amount of radioactivity, expressed in decays per second (dps). One becquerel (Bq) equals one decay per second. The curie (Ci), the original term used to describe an amount of activity, equals 37 Gdps. Trace amounts of radioactivity are generally expressed in the microcuries (mCi) quantities and laboratory cleanliness standards are often expressed in picocuries (pCi) or smaller amounts. One becquerel equals27 pCi. Activities of millions of curies are commonly found in power reactors while curie and millicurie (mCi) quantities of materials are commonly used in medical applications.

COMMON FEATURES OF INSTRUMENTS

Radiation protection instruments in a facility can be broadly categorized as those (e.g., area monitors, personnel

506 RADIATION PROTECTION INSTRUMENTATION

scanners) used in fixed locations and those (e.g., GM detectors, survey meters) moved for use in multiple locations. Fixed instruments usually are large, heavy, and use permanent electrically power. By design, they may have more features and offer more sensitive detection or measurement features than their portable counterparts. Portable instruments generally are small, lighter, and battery powered. Some feature tripod stands for temporary uses of long duration. Instruments may be categorized as those that detect or measure a quantity of radiation and those that grossly or specifically identify types of radiation or radionuclides. Instruments of all types usually feature multiple signal ranges because of the wide variations in the signal (counts, exposure, dose, etc.) monitored. The number of photons or particulate forms of radiation from a sizable radioactive source or large piece of equipment frequently varies inversely (or approximately so) as the square of the distance of the finite size detector from the radiation source and multiple signal ranges, usually in multiples of 3 or 10, are required to map the spatial variation of the radiation around the source or equipment that will vary from the highest signals measured very near the source to the smallest signal measured at distances far from the source of radiation. Current devices often feature auto-zeroing and auto-ranging of scales. Instrument scales indicate the magnitude or measure of the radiation detected. Often, more than one unit will appear on the scales, or users may electronically select from multiple choices of units. Some instruments feature a rate mode, usually per second, minute, or hour or integrate mode that sums the signals over some predetermined time periods. Instrument scales may only be correct for specific radiation conditions under which the instrument was calibrated. Instruments can yield incorrect readings when used under noncalibrated radiation conditions.

Efficiency of a device is a measure of the number of output parameters (counts, pulses, etc.) produced relative to the number of input parameters (g rays, particles, etc.) producing the output. Efficiencies (absolute, intrinsic, relative, etc.) have technical definitions beyond the scope of this article. Different applications require instruments with different degrees of efficiency, but, generally, high efficiency is desirable.

Signal accuracy is highly variable and depends on the type of radiation monitored and the design of the instrument. The more intense and hazardous the radiation field the greater the necessity for more accurate measurements. Conversely, measurements in very low level radiation fields need not be as accurate as the risk presented to personnel is roughly proportionately less. For example, ionization chambers used as survey meters will be calibrated to be accurate to 10% at the one-third and twothirds of full-scale deflection while GM counters may only be 50% accurate over their full-scale range. An instrument should provide precise reproducible readings. Instruments need to reproducibly repeat measurements at the same locations when used repeatedly in the same radiation fields. Portable instruments often feature rugged weatherproof designs with lightweight features, such as ergonomic antifatigue handles to facilitate outdoor use for long times. Individuals with various skill levels frequently use

instruments; some personnel use instruments infrequently. In both situations, instruments subsequently suffer some misuse and abuse. Hence, simplicity of use is a highly desirable feature. Instruments should be designed with a minimum of controls or knobs to be adjusted, On, Off, and Battery Check switch positions must be clearly indicated and any scale selection switches should be labeled in an unambiguous manner. Some instruments feature audible signals whose intensity is proportional to the magnitude of the radiation signals. Such a feature often is useful on the most sensitive scale, but may be undesirable on the higher ranges; hence, usually the audible signal is switch selectable and may be turned off when desired. Potentiometer controls necessary during calibration adjustments and voltage setting control should not be so accessible that they can be easily changed. Such controls often are recessed or located on a rear panel so that they can only be changed in a deliberate manner. While every instrument will not possess all of the features discussed here, this discussion has included those found most commonly.

Compact, lightweight designs are easily achieved using microprocessors, liquid-crystal displays, modern CMOS electronics, and phenol, or acrylonitrile–butadiene– styrene (ABS) plastic cases. Some devices feature automatically backlight displays in low ambient light conditions. Current instruments frequently are available with either digital scales or analog scales; some offer both displaying a digital reading and an analogue bar graph emulating analogue meter movement. While digital scales are usually easily read in a constant radiation environment, they are inappropriate in rapidly changing radiation fields as the signal changes rapidly and rapidly changing digital readouts are difficult to read and interpret. For these situations, a freeze mode indicates a peak reading.

The radiation detector may physically be in the instrument with the associated electronics necessary to process the signals, connected to the counting electronics by a cable, or feature a remote reading capability, allowing the observer to stay in an area where radiation exposure is minimal. A cable connection is common in applications where the observer is physically in the radiation field monitored. Cable connectors, with instrument displays of the probe connected, allow the use of multiple probes or detectors (GM tubes, neutron probes, proportional counters, scintillation probes) with different features with a single count rate meter. Instruments with built-in detectors are free of cable problems, such as the loss of charge by poor cable insulation. Many devices feature an RS-232 interface or a universal serial bus (USB) cable that can connect to a computer; data software packages allow data retrieval, time–date stamps, or use parameter selections, such as programmable flashing displays and audible alarms, or measurements for specific applications. Some instruments feature data logging, the sequential capturing of hundreds or thousands of data points under different measurement conditions. Data captured by the instrument can be downloaded, by cable connection or by via infrared (IR) communication, to a personal computer for processing with a numerical spreadsheet (3).

Generally, instruments feature a battery check (a known scale deflection on an analog device or a brief

audible tone or indicator lamp on a digital device) that allows the user to determine that the battery possesses enough charge to operate the instrument successfully. Voltage stability is important; many instruments feature two power supplies, one for the counting electronics and another for the constant voltage required across the detecting volume. Currently, multiple 9 V alkaline batteries are widely used, providing 100–500 h of operation. The voltage across the detecting volume is usually required to be the most stable. A small variation in this voltage can lead to large changes in the observed signal, depending on the design and mode of operation of the instrument. Voltage stability of 0.1% is usually required for the voltage across the detector in most radiation detection instruments.

A zero check allows the zero point on the scale, previously set to zero in a radiation free environment, to be checked in the presence of radiation. Some devices feature auto-zeroing scales. Some instruments have an attached

constancy source, a minute quantity of radioactive material, such as 0.06 mCi of 238U or 10 mCi of 137Cs, which,

when placed on or near the detector in a predetermined geometry, yields a predetermined signal on the most sensitive range. Proper instrument use requires all three items, battery function, zero point, and known response, to be checked prior to each use. A reduced signal or complete loss of signal from the radiation protection instruments is particularly dangerous because the user falsely concludes that little or no radiation is present.

Radiation detectors generally are designed either to monitor individual events (counts) or pulses or to integrate (sum) counts or pulses that occur in such a short time interval that they cannot be electrically separated. In pulse mode, individual events or signals are resolved in 1 ms, 1 ns, or even smaller time intervals. In integrate mode, the quantity measured is the average of many individual events in some very short time period. Some devices feature signal integration when the device is used in a rate mode.

Response time of an instrument measures how rapidly an instrument responds to the radiation detected. The response time is short (fractions of a second) on the higher multiple scales and becomes longer (several seconds) as the scale multiple decreases with the longest resolving times occurring on the most sensitive scale. The response time (T) is called the time constant, and is proportional to the product of the resistance (R) of the electronic counting circuitry and its capacitance (C). (Fig. 1). Many instruments feature a slow response switch that allows electronic averaging of a rapidly varying scale signals.

Energy independence is desired for most radiation survey instruments, such as ionization chambers and GM counters; the signal is independent of the energy of radiation detected, but is proportional to the magnitude (counts, exposure, etc.) of the radiation field being monitored. However, many instruments exhibited a marked energy dependency at lower X or g ray energies; the signal varies as the energy of the radiation varies even a constant magnitude radiation field (Fig. 2). Knowledge of the energy dependency of an instrument and of the approximate energy of the radiation field to be monitored is essential in properly using a radiation detector. Whether the signal from the

V

Figure 1. Simple schematic of a gas-filled ion chamber. A voltage (V) is maintained across the central wire anode (An) and chamber wall cathode (Ct). An incident (g-ray (G) produces ion pairs; they move to the anode and cathode producing a pulse in the circuit containing a resistor (R) and capacitor (C).

instrument is higher or lower than it should be relative to the signal observed at its calibration energy depends on many parameters. Calibration of radiation detectors is required annually for some regulatory agencies; instruments usually display a calibration sticker indicating the most recent calibration date, the calibration source, or sources if several were used to obtain energy response of the instrument, the scale readings obtained (often in mSv h 1, mR h 1, or other multiples, or cpm), and the accuracy and precision of those readings, expressed as a percentage of the scale reading, any necessary scale correction factors to be used specific scales, and instrument response to a reference source containing a minute quantity of radioactive material. By performing a battery check

Figure 2. Typical relative response versus incident photon energy (kV): (A) is the ionization chamber; (B1) is a Geiger counter with thin window shield closed. (B2) is a Geiger counter with thin window shield open.

508 RADIATION PROTECTION INSTRUMENTATION

or by observing that the battery condition is acceptable by the absence of a low battery indicator, a zero-scale reading check, and meter response check using a reference source, the user can determine that the instrument is operating correctly before use. Moreover, the average energy of radiation at a particular location in an area is highly dependent on the relative amounts of primary and scattered radiation present and frequently varies within an area. Energy dependency is, of course, advantageous when it is desirable to measure the energy spectrum in addition to determine the intensity of radiation.

Some instruments are environmentally sensitive; graphs or tables providing correction factors as a function of temperature, pressure, and humidity indicate the degree to which the signal is altered by environmental conditions. For instruments with the detector volume open to the air, corrections based on thermodynamic gas laws for the mass of air present in the detector are employed.

Excessive humidity can cause incorrect instrument readings. Humidity can cause leakage of current in cables, at electrical contacts, and at other locations in the electronic circuitry. Usually, an instrument will require a warmup time of 1–2 min or longer. Proper warm-up allows electronic circuitry to stabilize and yields more stable and reproducible signal readings.

Strong radio frequency (RF) fields associated with some equipment generating radiation can cause improper signals in some radiation measurement instruments. The susceptibility of the instrument to strong rf fields will usually be discussed in the user’s manual.

Many radiation protection instruments exhibit geotropism, orientation (gravitational) dependency, or angular dependency because radiation incident from the sides and rear of the instrument are attenuated more by metal casing surrounding the counting electronics than radiation incident on the sensitive detecting volume. The proper orientation of the instrument for measurement in a radiation field and the degree of angular response will be indicated in the users manual or on the calibration certificate.

As previously noted, some devices are designed to identify different types of radiation or to identify species of radionuclides. Many detectors will feature a thin detection window of only a few milligrams per square centimeters of thickness protected by a thicker filter, a sliding, or rotating shield, that allows the least energetic forms of radiation, such as soft X rays and alpha and beta particles to be detected through the thin window when the thicker filter is removed. Conversely, with the shield in place alpha and beta particles are discriminated against and only higher energy radiations are detected. Other windowless detectors are designed to detect the low energy radiation by flowing a radioactive gas through the detector.

Radionuclide identification requires spectroscopy, the identification of the characteristic radiation spectra of multiple radionuclides each in the presence of others. Resolution is the measure of the abilities of devices to distinguish a single energy in multiple energy radiation spectra. Different applications require instruments with different degrees of resolution. Spectroscopy formerly was limited to using heavy fixed laboratory-based NaI(Tl)

detectors or Ge(Li) detectors often with a multichannel analyzer to quantify and identify radionuclides in test samples. Currently, in-field spectroscopy can be performed with small handheld or portable NaI(Tl)-based detectors or with portable high purity germanium (HPGe) detectors that allow radionuclide identification of the most common radionuclides.

IONIZATION CHAMBERS

The ionization chamber (Fig. 1) consists of a cavity, frequently cylindrical, with a positively charged central electrode (anode) insulated from the chamber walls (cathode) at negative potential. The direct reading pocket dosimeter, with an external dosimeter charger (Fig. 3) is a simple ionization chamber. When fully charged, an internal quartz fiber, visible under a magnification lens, is deflected to a ‘‘zero’’ reading. As the dosimeter is irradiated, the charge is reduced proportionally to the amount of radiation received. Older pocket chambers are being replaced with personnel detectors or monitors with more electronic versatility. As a survey meter, an external power source (Fig. 1) provides the voltage potential; a resistor and capacitor in parallel (or equivalent electronic circuitry) are used to collect the charge produced when ionization occurs in the chamber. The ionization chamber electrode polarity may be reversed for special applications. Incident X or g rays interact in air or a tissue equivalent gas, producing positive and negative ions in the chamber. If the voltage is sufficiently high to prevent recombination, that is, the positive and negative ions rejoining before they reach the charged surfaces, the negative ions will be attracted to the central electrode and the positive ions will be collected on the chamber wall. The collected charge flows to the capacitor and one electronic pulse is detected in the counting circuitry. In open air chambers, the filling gas is air at ambient temperature and pressure and appropriate corrections to the charge collected may be required as previously discussed.

Historically, ionization chambers were designed to measure exposure (R); newer instruments may offer equivalent dose readings (Sv or their submultiples) (Fig. 4). The walls of the chamber must be sufficiently thick for electronic equilibrium to be established, that is, the number of electrons entering and leaving the cavity is the same and the chamber walls are sufficiently thick to stop any electrons arising from the interaction of the radiations with the gas or in the chamber walls. Moreover, the chamber walls are usually designed of air equivalent materials. Sealed ionization chambers may be filled with a tissue equivalent gas and usually are designed to measure collision air kerma. The chamber size must be small relative to the dimensions of the irradiating beam so the chamber is uniformly irradiated. Typical ionizing voltages required across the sensitive detecting volume are 150–300 V (Fig. 5), sufficiently high to present recombination of the positive and negative ions, but not high enough to cause additional ionizations that amplify the signal. Ion chamber currents are low, usually 1 pA or 1 fA. A 10 mSv h 1 g ray field yields1 pA; extraneous currents must be minimized in order to

measure such a small current. Electrical leakages can occur across lint, dust, or loose conductive materials between interior conducting surfaces. Cable connections can exhibit greater leakage current in high humidity. A guard ring design in some chambers minimizes leakage and polarization currents that arise after the collection

Figure 3. Personnel dosimeters. Low dose (bottom left; Dosimeter Corp., Model 862;) and high dose (bottom right; Dosimeter Corp., Model 866) g- and X-ray pocket dosimeters, with charger (top left; Jordan Nuclear Co, Model 750-5). An alarming personal digital dose meter (center; Technical Associates, Model PDA-2) and a miniature pocket digital dosimeter (right; Aloka Co, LTD., Model MYDOSE-mini).

potential is initially turned on. The insulator between the outer and inner electrodes is divided into two segments with the conductive guard ring in between; any leakage current through the insulator is collected and prevented from contributing to the true current. The currents from ionization chambers are normally measured using a potential drop across a high resistor or a rate of charge method. The small currents from the ion chamber are amplified by a vibrating reed electrometer. The amplified

Figure 4. A portable ionization chamber survey meter (Cardinal Health; Inovision, Model 451). The display shows the features during the initial check phase immediately after turning on the instrument.

Figure 5. Voltage dependence of a gas-filled cylindrical ionization chamber: (a) Voltage is insufficient to prevent ion recombination.

(b) Ionization chamber voltages are sufficient to prevent recombination. (c) Proportional counter voltages, the number of secondary ion pairs is proportional to the number of primary ion pairs.

(d) Limited proportionality region. (e) Geiger voltages produce maximum number of ion pairs from a single primary ion pair. (f)Continuous discharge region.

510 RADIATION PROTECTION INSTRUMENTATION

current passes through a precision resister and the voltage drop across the resistor is proportional to the current. If collected on a capacitor, the rate of charge collected on the capacitor is proportional to the current. This latter method is used for smaller current measurements while the former is used in ionization chambers designed for more rugged use.

Ionization chambers normally exhibit good energy independence (Fig. 2) over a large energy range, and this makes them useful for measurement of X or g rays with energies from 7–1000 keV and for measuring low (1 mSv h 1) to high (10 mSv h 1 or higher multiples) dose rates. Open-air ionization chambers are less useful for low dose rates of < 1 mSv h 1. Pressurized (up to 8 atm) ionization chambers allow accurate measurements < 1 mSv h 1. Properly modified ionization chambers, with sliding shields, may be used to monitor alpha, beta, and neutron radiation. For example, an ion chamber with boron on its interior chamber wall or containing boron gas may utilize the high cross-section of boron for neutrons and the subsequent 10B, 7Li reaction, and the chamber will detect neutrons using the subsequent alpha particles from this reaction.

GAS PROPORTIONAL COUNTERS

Gas proportional counters (Fig. 6) have similar design features as ionization chambers, but employ higher voltages between the central electrode and the chamber walls. The typical operating voltages of 300 up to 1000 V (Fig. 5) are sufficiently high that, following an ionizing event in the chamber, the positive and negative ions generate additional ionizations so that the number of ions from the initial ionizing events are multiplied 1 thousand to 1 million times. The resulting signal is proportional to the energy deposited by the initial number of ionizing events. Propor-

tional chambers can be used in either the pulse or integrate mode, but the pulse mode is used most commonly. They are capable of detecting individual ionizing events. Because of amplification, the current from proportional chambers is much higher than those from ionization chambers. As the signal from a proportional current is dependent on the operating voltage, a highly stable power supply is required. The choice of detector gas in thin-window proportional counters depends on the type of radiation to be detected. For counting alpha particles, helium or argon gas frequently is used. For counting beta particles, a high multiplication gas is required, such as methane (CH4) or a mixture of a polyatomic gas and a rare gas, such as argon. The gasses also help make the proportionally of the chamber more independent of operating voltage. Proportional counters are generally cylindrical in shape and the central electrode is a very fine wire of uniform diameter as any variation in the electrode’s diameter causes small variations in the resulting signal. Gas (often a mixture of 90% argon and 10% methane) flow proportional counters usually have a sample of the radioactive material flow through the chamber. Either 2p (1808) or 4p (3608) solid geometries are used, and these systems are very useful for counting low energy beta particles, alpha particles, or very low energy photons. Proportional counters are useful for spectroscopy (energy determination) measurements.

Proportional counters have the ability to discriminate between alpha and beta particles by discriminating between the magnitudes of the signals produced. Gas proportional counters may be used to measure fluence or absorbed dose.

When neutron spectra are poorly known, neutron rem meters are used to estimate the equivalent dose for fast neutrons. Older style neutron detectors consisted of a proportional counter either lined with boron or filled with boron trifluoride gas; the boron has a high capture crosssection thermal neutron detector. The subsequent charged

Figure 7. Portable neutron survey meter (Cardinal Health, Inovision, Model190N). (Courtesy Cardinal Health.)

particles (alpha particles) from this reaction are readily counted. Current neutron detectors use 3He as the fill gas and detect both the proton and tritium, 3H, from the subsequent reaction (Fig. 7). Fast neutrons can be moderated in several centimeters of high density polyethylene to thermalize the neutrons for detection by the methods described. Olsher et al. (15) described recent improvements in neutron rem meter instrumentation.

¨

GEIGER–MUELLER COUNTERS

If the voltage on an ion chamber is increased to 900– 1000 V (Fig. 5), the proportionately exhibited at lower voltages is lost. Each initial radiation interaction in the walls or gas of the detector results in complete ionization of the gas in the detector. Interactions in the detector are spatially dependent, but generally, the following sequence occurs. Electrons produced following the initial ionizing event lose energy as they drift toward the anode. They lack enough energy to produce secondary ionization until they approach the anode when secondary ionization begins to occur. This secondary ionization builds up rapidly producing an avalanche of electrical charge in the detector. These processes reduce the potential difference between the central electrode and the chamber walls and the avalanche terminates. Once the necessary ionizing potential is reestablished, the detector is ready again. One undesirable aspect of the movement of the positive ions to the cathode and their resulting collisions with the cathode causes additional electrons to be ejected from the cathode. These additional electrons are undesired and may be controlled by manufacturing a tube containing a quenching and a filling gas. Organic quenching gases, such as ethanol or ethyl formate, are depleted by this process. An inorganic filling gas, such as chlorine,

recombines to provide a continuous supply (8). The energy of the undesired electrons dissociates these organic molecules rather than starting new avalanches in the tube. The number of organic molecules available for quenching limits this method of quenching. An alternative method uses halogen gases, usually bromide or chlorine. The extra energy of these electrons is used to disassociate these halogen molecules. As opposed to the organic molecules, halogen reassociates so that the same atoms are available again to continue the process.

Geiger–Mu¨ eller tubes generally are used as pulse-type detectors of radiation. Their response is a function of the intensity of the radiation field. The movement of the positive ions to the cathode, previously described, requires from 100 to 200 ms and during this time interval the GM tube is unable to respond to other radiation interactions, have not recovered (reassociated) and are unable to resolve additional events (7). In very intense radiation fields, the relative long resolving times of GM tubes creates periods in which the tube is insensitive to radiation events; the GM tube may not respond to radiation, giving a false zero or low reading when an intense field is present. However, GM tubes are excellent as very sensitive detectors of X and g rays in low level radiation fields. Commercial manufacturers offer at least three different GM tubes designs (Figs. 8,9) for specific applications. Pancake probes, with covers only a few milligrams per centimeter squared thick, allow the detection of alpha particles > 3.5 MeV, beta particles > 35 keV, and g-rays > 6 keV, while thin end window probes detect alpha particles > 4 MeV, beta particles > 70 keV, and g rays > 6 keV (Fig. 8). Some pancaketype probes (Fig. 9) feature removable tin and copper filters3 mm thick that allow energy compensated exposure rates measurements. Energy-compensated GM probes feature a design that reduces response energy dependency so that it responds more like an ionization chamber (Fig. 5). Geiger–Mu¨ eller instruments are basically count rate meters, but may be calibrated in exposure rate for a specified energy of photons. Use of the meter in other energy spectrums different from the calibration spectrum invalidates the meter reading in mSv/h and mSv/h, but still the instrument allows the detection of radiation in the count rate mode.

SCINTILLATION DETECTORS

Luminescence is a physical process in which a substance, a scintillator, absorbs energy and then reemits the energy in the visible or near visible energy range. Prompt scintillators that deexcite in 10 ns following luminescence exhibit many useful properties as radiation detectors. For every photon or particle detected, a single pulse is normally counted and the size of the pulse generated is related to the energy deposited by the radiation interacting in the scintillator. Scintillators exhibit great sensitivity and yield high count rates. They can measure fluence, exposure, or absorbed dose if calibrated for the energy range of interest. Moreover, their exceptional sensitivity allows measurement of radiation rates at or near background levels, such as 1 nSv h 1. Solid inorganic scintillators includes sodium

512 RADIATION PROTECTION INSTRUMENTATION

Figure 8. Portable GM counter (top; Ludlum Measurements, Inc. Model 14C) with an open side-window probe (left; Ludlum Measurements, Inc. Model 44-38), an end thinwindow probe (center; Ludlum Measurements, Inc., Model 44-7), and a pancake probe (right; Ludlum Measurements, Inc., Model 44-9).

iodide crystals with trace amounts of thallium, NaI(Tl); cesium iodide with thallium, Csl(Tl); cesium fluoride, CsF; zinc sulfide with silver, ZnS(Ag); and bismuth germanium oxide, BiGeO, also known as BGO. The trace amounts of impurities in these inorganic salt crystals serves as luminescent process activators that promote the efficient conversion of the incident radiation energy into light. The scintillator crystal is connected to a photomultiplier by direct contact or through a light pipe. The crystal and photomultiplier must be encased in a light tight case to prevent light leaks. Typical crystals are cylindrical, 1 in. (2.54 cm) diameter by 1 in. (2.54 cm) thick, but larger sizes (Fig. 10) are available for more sensitive measurements. The resulting photomultiplier signal is amplified by the

associated counting electronics. Detectors with thin windows are available for the detection of low energy X rays and energetic beta particles. Inorganic solid crystals are relatively dense and reasonably efficient for detecting higher energy photons (Fig. 11). However, they are also hydroscopic and to protect them from absorbing moisture are encased in light reflecting cases that promote good efficiency. Organic crystal scintillators produce their light by a molecular process. Anthracene and transtiblene are the most widely used organic crystal scintillators. Incoming radiation excites electrons to higher energy levels of vibrational states; the electrons subsequentially decay with a release of energy. Organic liquid scintillators are formed by dissolving organic scintillators in liquid organic

514 RADIATION PROTECTION INSTRUMENTATION

detecting heavy particles. Plastic scintillators are available in many physical configurations. Neutrons can be defected by incorporating a neutron sensitive material, such as lithium, into the solvent and the subsequent plastic scintillator. Nobel gas scintillators consist of high purity concentration of helium or xenon, which have the property that, following radiation interaction in the gas, both visible and ultraviolet (UV) light is emitted. While these materials exhibit a very short deexcitation time of 1 ns, they yield little light and the conversion of light is reasonably inefficient; nevertheless, they do have numerous applications when a fast response time is required.

As with GM probes, scintillation detectors are available as rectangular pancake probes to detect beta particles > 100 keV and g rays > 25 keV, thin scintillators to detect g and X rays > 10 keV, flashlight-like probes to detect alpha particles > 350 keV and beta particles > 14 keV, and conventional thick-crystal cylindrical probes for g and X rays > 60 keV.

Photocathodes have many applications in devices that measure or detect radiation, such as image intensifiers, vidicon tubes, and other detectors. Usually, the electronics required to amplify the initial signal does not have as short a resolving time as the detector proper, but with modern solid-state electronics, the resolving times have been shortened to less than microseconds. Because of their extreme sensitivity, scintillator detectors are useful when detection and subsequent identification, by spectral analysis, of a type of radioactivity is required (Fig. 13). While the spectral peaks associated with scintillators are reasonably broad, their energy resolution is sufficient to allow rapid identification of minute quantities of various radioisotopes

Figure 13. A multiple component NaI(Tl) spectroscopy system. Detector shield (lower left; Ortec) with a NaI(Tl) detector (not shown); amplifier/bias supply(upper center; Ortec, Model Acemate); spectral analyzer (lower center; Ortec, Model 92X Spectrum Master), and computer display (left).

Figure 14. A spectrum measured with a 1 1 in. (2.54 cm) NaI(Tl) dectector: (a) 137Cs full-energy (0.66 MeV) peak; (b) Compton edge (0.48 MeV); (c) Compton distribution; (d) backscatter peak (0.18 MeV).

(Fig. 14). Formerly limited to laboratory analysis, NaI(Tl) spectroscopy is now available in portable handheld units (Fig. 15) that, using quadratic compression conversion (QCC), can identify 128 radionuclides in real-time (1 s intervals). Quadratic compression conversion creates spectral energy peaks whose widths vary proportionally to the NaI(Tl) crystal’s energy resolution. All energy peaks are displays with the same peak width that allows radionuclides’ distinct spectra to be more readily identified. The electronics associated with scintillators must be extremely stable, but often variations are introduced by environmental factors. High permeability magnetic shielding materials, for example, Mu-metal, often are used to shield against stray magnetic fields. Temperature and humidity variation can produce undesirable electronic noise. As previously noted, some crystals are hydroscopic and the moisture absorbed can reduce the efficiency of the process. Pulse discrimination techniques are often used to distinguish one type of radiation from another.

SOLID-STATE RADIATION DETECTORS

Thermoluminescence (TL) is the emission of visible light released by heating previously irradiated solid-state crystals. The light emitted from a thermoluminescent crystal is proportional to the amount of radiation to which the crystal has been exposed, and this proportionality holds over a large range (102–105) of exposures. At very high exposures, nonlinearity is exhibited and the amount of visible light released is no longer proportional to the amount of radiation detected. Thermoluminescent dosimetry (TLD) materials (Table 5) commonly used in medicine, include lithium fluoride(LiF),availableinthree forms(TLD-100, -600,-700,), and lithium borate manganese (Li2B4O7:Mn) (TLD-800).

Other TLDs used in environmental dosimetry applications, calcium fluoride manganese (CaF:Mn) (TLD-400), calcium fluoride dysprosium (CaF2:Dy) (TLD-200), calcium sulfate dysprosium (CaS04:Dy) (TLD-900), and aluminum oxide (Al2O3:C) (TLD-500), are not further described here. X rays, g rays, and neutrons are easily detected with different TLD materials; the detection of higher energy beta particles is possible, but quantification of the amount of beta radiation and calibration of the solid-state detectors for beta radiation

Table 5. Properties of Some Thermoluminescent Materials

Figure 15. A portable NaI(Tl) surveillance and measurement (spectroscopy) system. (Berkeley Nucleonics Corp., SAM Model 935).

is often more difficult than for X and g rays. The TL materials exhibit an enhanced response to lower energy (< 200 keV) X or g rays as compared to higher energy (1 MeV) X or g rays. For LiF, the over response is only a factor of 1.2, but for lithium borate manganese (Li2 B4 O7:Mn) (TLD-800), there is an under response of 0.9. By adding filters, the energy response of a given type of crystal can be made more uniform and this is commonly done with TLD detectors used as personnel radiation monitors (Table 5).

516 RADIATION PROTECTION INSTRUMENTATION

Figure 16. Glow cures for TLD-100 (natural LiF) rods exposed to 10 R. (a) Without proper preparation annealing, multiple natural peaks occur; (b) Using a 1 h annealing at 400 8C and a 2 h annealing at 100 8C a smoother curve is obtained.

Physically, TLD materials consist of loose powder contained or embedded in plastic holders, compressed crystals (chips), extruded rods, and chips on a card in a configuration that allows reproducible heating of the detector materials to selected temperatures. The TL materials exhibit some undesirable features as radiation detectors. At room temperature, fading or loss of signals occurs, from < 0.5% per month for LiF to 5% in 3 months for Li2 B4 O7:Mn. The degree of fading can be controlled, to some extent, by proper preparation (annealing) procedures.

The optical readers generally consist of a heating pan or device that allows the TL material to be uniformly heated in a controlled manner, at a specified temperature for a given period of time. The heating device and material holder are directly below a photomultiplier tube that usually has some filters to remove any IR radiations and transmits light in the blue-green region of the visible spectrum. The signal from the photomultiplier is amplified and used to prepare a glow curve (Fig. 16) a plot of the intensity of light versus the heating cycle of heating elements. Different TL materials exhibit different glow peaks. Numerous peaks occur in the curve and either the area under the curve or the height of the major peak is chosen to be proportional to the amount of radiation to which the material was exposed. Proper preannealing (extended heating at a controlled temperature) and postannealing of the material will remove some smaller undesirable peaks (Fig. 16) leaving the main peak that is used for measurement purposes.

Current TL readers offer automatic features for glow curve analysis and processing of large numbers of samples (chips, rods, or cards).

Lithium flouride is the most commonly used TL material for personnel dosimetry and consists of natural lithium. Lithium-6 is preferentially sensitive to thermal neutrons and 7Li is insensitive to thermal neutrons, but sensitive to

g-rays. Hence, by using paired 6Li and 7Li materials, it is possible to measure thermal neutrons in the presence of g rays. Use of natural LiF detectors in radiation environments that contain low levels of thermal neutrons will yield incorrect dose equivalents for personnel, as the thermal neutrons will cause an apparent over response of LiF calibrated only to detect and measure g rays (16).

Optically stimulated luminescence (OSL) is the release of light by a phosphor following its irradiation by a laser. Aluminum oxide (Al2O3) containing carbon impurities exhibit OSL releasing a blue light when excited by a green laser light. Some personnel radiation monitors employ OSL technology that offers some improvement over TLD-based personnel radiation dosimeters (17). The OSL dosimeter offers greater sensitivity, stability, and accuracy than TLD dosimeters. Aluminum oxide is highly linear from 1 mSv to 10 Sv; there is little signal fading > 1 year. It does exhibit an energy dependency below 100 keV. However, unlike TLD chips, the aluminum oxide element can be reread multiple times to confirm an initial reading, an advantage for personnel dosimeter applications. The Luxel badge (Fig. 17; Table 6) contains the Al2O3 phosphor element, with 20 mg cm 2 open filter (paper wrapper), 182 mg cm 2 copper filter, and 372 mg cm 2 tin filter in a heat-sealed, light-tight hexagonal plastic badge (18). The combination allows detection of beta particles > 150 keV with a 100 mSv threshold and X and g rays > 5 keV with a 10 mSv threshold.

Photoluminescence (PL) occurs when the irradiated crystal emits visible light when exposed to UV light instead of heat. Silver activated glass encapsulated PL detectors are available in numerous shapes, sizes, and radiation levels as low as 100 mSv are detectable, but the detectors are commonly used to detect higher exposures. Appropriate filters can be used to make the energy response more linear, but at exposures of 0.1 Sv or higher the response of these detectors is nonlinear. These materials exhibit some signal fading that depends on the composition of the glass. Heating the glass detectors post postirradiation for 30–60 min at 150 8C yields maximum luminescence. Reannealing requires 40 8C for 1 h, which restores the material to its preirradiation state. For low level exposure measurements, care is required to keep glass detectors free of dirt, dust, and other materials that would reduce the amount of light transmitted through the detector.

Semiconductor materials used for radiation detection, Si, Ge, CdTe, HgI2, and GaAs, have band gaps, the region between the valence and conducting band, of < 2.2 eV. Electrons migrate from the valance to the conduction band, leaving holes, that act like positively charged electrons, in the valence band. In a p-type semiconductor, the current is carried by the positively charged holes; in an n-type semiconductor, current is carried by the electrons. Usually, a potential difference is maintained across the solid-state semiconductor such that the depletion layer is devoid of electrons and holes. The interaction of X rays, g rays, alpha particles, or beta particles generates additional electrons in the depletion layer; these are then swept away by the potential difference across the material, yielding a small current whose magnitude is proportional to the intensity of the incident radiation Energy required to generate

electron–hole pairs range from 3 to 6.5 eV. Semiconductors exhibit excellent linear response over a large energy range and greater efficiency than gas detectors. (For a full description of semiconductor physics, see Refs. 7 and 8.)

Surface-barrier detectors essentially consist of an n-type and p-type layers that function as anode and cathode, with an intrinsic I layer (depletion region), without electrons, in between, in which radiation interactions occur. Depletion regions range from 0.1 to 1 mm. The composite is commonly called P-I-N structure. With a moderate reverse bias applied, radiation interactions create electron–hole pairs with each electron and hole leaving, or depleting, the intrinsic layer, and, with appropriate circuitry, creating a detection and counting system.

Silicon surface-barrier diode counters are used for charged-particle (alpha and beta particle) detectors. Alpha resolution ranges from 12 to 35 keV; beta resolution ranges from 6 to 30 keV. Passivated Implanted Planar Silicon (PIPS) detectors use implanted contacts more rugged

Figure 17. The Luxel Dosimeter: Holder (top left); front view (top center); rear view (top right); rear view with attached Neutron 144 detector (bottom left); detector element showing Cu filter (right), the Al filter (left), and plastic filter (circle, lower center). (Courtesy Landauer, Inc.)

than conventional surface-barrier contacts. They can be costumed-designed for specific applications.

Germanium (Ge) detectors are used for g- X-ray spectroscopy to identify not only the amount of radiation present, but also to identify the type of radioactivity present. These detectors have excellent energy resolution and are used to identify the individual X and g rays from a radioisotope, and with peak fitting programs have the ability to resolve closely lying peaks (Fig. 18). Lithium drifted geranium, Ge(Li), detectors have been replaced by high purity pure geranium (HPGe) detectors that only required liquidnitrogen cooling (Fig. 19) or electrically refrigerated cryostats during measurements or use; otherwise they are kept at room temperature. Lithium drifted silicon, Si(Li), detectors are still used. With microprocessors, these instruments are now used for on-site identification of samples that may contain several different radioisotopes, a process previously limited to the laboratory. They are important tools in research facilities where minute quantities of many

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Figure 18. A g-ray spectrum measured with a 35 cm Ge(Li) detector with good energy resolution.

different radioisotopes may potentially accumulate and radioisotope identification is required.

Cadmium telluridide (CdTe) detectors are popular as they are hygroscopic, do not require a photomultiplier, require only a 50 V bias, and operate at room temperatures. They exhibit high sensitivity, but their energy resolution is intermediate between that of NaI(Tl) and Ge detectors. They are available in a multitude of small sizes for special applications.

OTHER DETECTORS

Other physical changes that arise in materials as a result of irradiation include coloration and nuclear activation. Color changes occurs in some materials following their irradiation. Thin films with a cyanide emulsion (GAFCHROMIC EBT) will exhibit a deep orange color following their irradiation by g rays to a dose of 10–100 Gy (19). These materials exhibit excellent linearity > 1–800 cGy of radiation dose. While developed for high dose radiation dosimetry studies on linear accelerators, they can be used for high dose radiation protection studies.

Neutrons may be detected by numerous nuclear reactions or by the process of counting the number of recoil proton tracks produced in certain neutron sensitive mate-

rials, materials, such as boron (20). A polycarbonate material CR-39 (allyl diglycol carbonate) is insensitive to X rays, g rays, and beta particles. However, incident neutrons collide with the protons, which produced changed particle tracks; the tracks are enhanced by chemically etching the polycarbonate, so they will be more visible under a microscope. This technology is used as one component in a composite personnel dosimeter used to monitor radiation therapy personnel working around linear accelerators with X-ray energies > 10 MeV (Table 6) (18).

Superheated Drop Detectors (SDD) consists of a small container of gel holding superheated drops 0.1 mm diameter. Neutrons produce recoil protons that strike the drops, causing them to vaporize, generating an audible pop or sound that can be counted. The detector is insensitive to g-rays, and is independent of neutron energy to14 MeV. Sensitivity is 80 bubbles per 10 mSv, with a minimum threshold of 1 mSv; there is a linear relationship between the number of bubbles and the neutron dose. The SDD technology has been incorporated into equivalent dose neutron survey meters with replaceable SSD cartridges that must be changed after exposure to certain maximum doses. Alternately, bubbles in samples can be visually counted to determine neutron dose (8).

PHOTOGRAPHIC DETECTORS

Photographic emulsions that darken in proportion to the amount of radiation they receive represent one of the earliest methods of detecting radiation. Most films consist of a thin plastic sheet with 0.2 mm emulsions one or both sides; the emulsion, usually silver halide granules in a gelatin mixture is covered with a thin protective plastic coat. Modern films have emulsions specific for optimal detection of certain energy and intensity of radiation. Radiation incident on the emulsion changes the clear silver halide ions, forming a latent image. During processing additional silver is deposited, the darkness of the film is determined by where silver ions are deposited on the film and the amount of silver deposited. While most films are limited to dynamic ranges of 103, multiple film packets can be used in combination to extend the dynamic ranges to 105 or higher. Fast films are sensitive to the lowest levels of radiation, while slow films require greater exposure to darken the films. Usually, the film is used in a protective cover, which can be a cassette, commonly used for imaging and including metal screens to enhance the image. Rapid processing film is wrapped in a thin light tight paper wrapping that prevent light leaks and may be used without a cassette.

Personnel monitors frequently use film as the radiation detector (Table 6). The small film packet in its light wrapper is carried in a plastic holder, but the film wrapping is sufficiently thin to allow the transmission of the low energies X rays, g rays, or beta particles. Films usually exhibit an enhanced sensitivity of several factors of 10, to lowest energies of radiation, below 100–150 keV, relative to their response to g rays with energies of 150–1.5 MeV. By using filters of copper, aluminum, and lead of varying thickness in the plastic holder in front of the film, estimates

of the energies of the radiations darkening the film can be made. One distinct advantage of film dosimeters is the permanent record generated. A disadvantage is that radiation incidence at an angle to the filter appears to have passed through a thicker filter than actually available. Film badges or monitors are available in numerous configurations for the body, head, wrist, hand, and fingers. Normally film badges are exchanged monthly if radiation levels are low; individuals working in a higher level radiation environment may be monitored more frequently.

Numerous environmental factors can fog photographic film producing erroneous personnel exposure readings. While films are free of electromagnetic interference, excessive humidity can influence results, as can excessive heat. Images in films can fade with time so prompt collection and processing of personnel monitors is important in obtaining accurate results. Special metal filters, such as cadmium, may be used in a film holder to produce a neutron sensitive film by the (n, a) reaction in cadmium. Film dosimeter are widely used as personnel monitors and the overall accuracy of a film dosimetry system is usually at least 50% or better depending on the energy range of use. Lower energy radiation present in small amounts yields the greater uncertainty in the accuracy of monitor readings. With film dosimeters,

the method of film processing is very important, as inconsistent methods of developing film will lead to substantial errors in the final results. Generally, film processors have their developer chemistry optimized for a particular type of film. Monitoring the temperature of the developing chemistry is very important and frequent use of calibration films, films previously given known exposures to radiation, is required to maintain the integrity of a film dosimetry system. Proper care and maintenance and rigorously scheduling of chemical developer replenishers are necessary.

ACKNOWLEDGMENTS

Equipment photographs courtesy of Todd Senglaub, Loyola Radiation Control, and Alvin Hayashi, Loyola Medical Media. I wish to thank Josephine Davis, Loyola Dept. Radiation Oncology, for her patience and excellent assistance preparing this manuscript.

BIBLIOGRAPHY

1.Health Physics Society. (No Date). Home Page. [Online] Health Physics Society. Available at http://www.health-physics.com. [2004, Nov. 16]. 2004.