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92
Radiation Detectors
When thermoluminescent materials are exposed to electromagnetic radiation such as
gamma rays, X-rays or UV rays, to charged particles such as beta rays, to uncharged particles
such as neutrons, or to other forms of radiation, electrons within the material are excited
from low energy levels to the electron traps at higher energy levels as shown in Figure 8a.
The electrons may stay at these higher energy levels for a long period of time. If the
material is heated, the added energy releases the trapped electrons, causing them to fall back
to the valence band (figure 8b) thereby emitting visible light- the thermoluminescence.
When a thermoluminescent material is heated to high temperature (e.g. 300°C) the
intensity of the luminescence increases at first (when more electrons in the traps are released)
and then decreases (when the number of trapped electrons decreases). This gives rise to a
peak in the luminescence, at a certain temperature. If there are several types of traps, several
peaks are observed at different temperatures as shown in figure 9. The heights of the peaks
(or the integrated area under the glow curve) are found to depend on the quantity of radiation
exposure. In a simple case, the dependence may be linear, which allows the dose to be
obtained from a measurement of the glow curve, after proper calibration. This is the principle
of thermoluminescence dosimetry (TLD).
Figure 9: Schematic representation of a glow curve
One of the applications of TLD is to monitor radiation exposures of personnel working
in medicine, industry and nuclear power plants. For getting reliable and reproducible results
the TLD chips are heated in a controllable and reproducible fashion during measurement
and later for annealing. Different materials have been used for TL dosimetry with CaSO4:Dy
and LiF:Cu being most common for personnel monitoring.
Radiation Detectors
93
Neutron detectors
Neutron detection is required at nuclear power stations and at other facilities where either
neutrons are produced or used. After the established role of cyclotrons in nuclear medicine
facilities for the production of positron emitters, measurement of neutrons (dose rate) has
become an important part for radiation protection. No single detector technology is ideal for
all applications but each detector type offers some unique features. Electronic counting
detectors such as BF3 counter, LiI scintillation detectors etc. can be successfully employed
for neutron measurements. A number of dose rate meters (known as rem meters) are
commercially available these days that provide direct reading of dose equivalent in Sv.
These systems use an electronic thermal neutron detector inside a moderator sphere. An
alternative to the rem meter is to employ a Bonner spectrometer, which is sensitive to entire
energy range of neutrons but complex to use, as it requires extensive data analysis.
Radiation detectors as survey meters
Survey meters are either ion chambers or Geiger counters. Basic principle of ion chambers
has already been described before. The ion type survey meters (commonly known as Cutie
Pie) have a wide range of scale for measurement whereas GM tubes are normally available
with low range of scale. Some of the modern GM tubes are also available these days with
wide range of scale. The response of Cutie Pie depends on the nature and energy of radiation
whereas the GM survey meter does not depend on these parameters, as its response is
independent of energy.
The pulse size in GM counter is independent of primary ionization hence it is not
possible to discriminate between different radiation types (alpha, beta, or gamma) in a GM
counter. To achieve high sensitivity an adequate voltage (above the threshold in the range
900-1200V) has to be applied between the electrodes.
The design of GM counters is modified for detecting alpha and beta radiation. The
tubes are either end window or a side window type. End window tubes are made of thin
mica to maximize penetration by alpha and beta radiation. The counter efficiency is very
high (up to 100 %) for alpha or beta but for gamma their efficiency drops to 1 to 2 percent.
Quality control of survey meters
Calibration
The suppliers at their own workshop/laboratory initially calibrate the survey meters with a
known and standard source of radiation (60Co,
is measured at a given distance for their accuracy and performance. The distance is then
doubled/tripled and the survey meter reading is recorded. The measurements at these various
distances should follow inverse square law. The expected value of dose rate in mSv/hr or
mR/hr at a given distance can be calculated with the help of the specific gamma ray constant
137
Cs). The exposure from the standard source
94
1000 1000
100 100
1 2 12
2( )
R R R
Radiation Detectors
() for the source radionuclide used. The measured value may be accepted if falls
within 20 % (preferably within 10%) of the correct (expected) value. A calibration factor is
accordingly determined for the instrument for routine use while measuring the dose rate.
The instrument should be calibrated at least once a year or after any repair. Periodic checks
with known activity available in the facility should be performed quarterly. The checks
should be performed in all of the ranges available with correction for their zero reading
if any.
The specific gamma ray constant (exposure rate constant) for the standard source is
known. For example it is 3.32 R/h/mCi at 1 cm for
3.32
exposure of = 3.32 × 1000 R/h at 1 cm =
137
Cs. Thus 1 Ci of
mR/h at 1 meter
137
Cs will give an
= 332 mR/h at 1 meter
The survey meter should show this value at 1 meter from a 1 Ci standard source of
137
Cs.
It is extremely important to measure the distance between the survey meter (center of the
chamber/tube) and the source accurately. The true/expected value may be calculated on the
day of measurement using decay correction for the radionuclide source. If the measured
value is within 10% of the expected value then the measurement may be repeated for a
distance of 2 meter and 3 meter or even 5 meter and compared with the expected values at
these distances. If the measured value varies proportionately at different distances then a
calibration factor (CF) is calculated for the survey meter. For example if the survey meter
gives an exposure of 300 mR/hr instead of 332 mR/h at one meter then a CF (= 332/300 =
1.1) of 1.1 should be used for all measurements. The value of CF should be same for
measurements at 2 m and 3 m distance. This should be noted that all the survey meters
available in the facility should be calibrated in a national reference laboratory accredited by
the national competent authority.
Dead time
This is the time interval immediately following an ionizing event. The counter is insensitive
during this period and does not respond to another ionizing event. This also known as
resolving time for GM tubes and is as high as 200 sec. Prepare two small point sources
and measure the count rate from each of them and then by putting them together side by
side. One may prepare the source by putting a drop of
99m
Tc on a filter paper; allow it to dry
and then cut in to two pieces. Put them very close to the survey meter and measure count
rate (R12), then remove one of them measure (R1) and then (R2). Early (1) suggested the use
of following formula to calculate the resolving time.
Resolving time () =
R R
1 2
Radiation Detectors
t o o
R R R
95
= Resolving time (seconds)
R1= count rate from one source
R2= count rate from second source
R12= count rate from both the sources
The true count rate can be estimated by the following formula (1):
R
o
1
R
o
Where Rt= True count rate
Ro= Observed count rate
= resolving time
Contamination monitors
Contamination monitors are either G.M. counters or scintillation detectors. They measure
the radioactivity in terms of count per unit time either in counts/sec or counts/min. The QC
test for a contamination monitor is performed with the help of a very low activity (kBq, <
1Ci) standard source preferably
from the source under a given geometry and the results are properly recorded for future
reference. Periodic checks are very helpful in ensuring their performance and accuracy. The
contamination monitors are useful for monitoring of working area, instruments and radioactive
waste. They should be accurate within 20 % (preferably within 10%) with proper measurement
conditions.
137
Cs. The count rate is measured at different distances
The user of survey instruments should have adequate knowledge about the preventive
maintenance of the instruments. When the instruments are not in use the batteries should be
removed and kept separately. In high humidity area, the instruments should be kept inside a
room having dehumidifier. In order to ensure good performance of survey instruments or
contamination monitors it is advisable that they are checked periodically with standard
sources available in the facility and calibrated periodically in a standard laboratory. In India
facility of calibration of radiation measuring instrument is available at Radiation Standard
Section, BARC, Mumbai. The instrument should be in good working condition for calibration.
The battery condition, possibility of radioactive contamination, proper operation of the
switches, and the zero adjustment should be properly checked before sending the instrument
to the standard laboratory for calibration.
96
Radiation Detectors
References and suggested reading:
1. Early PJ. Radiation detection, In Principles and Practice of Nuclear Medicine, ed. Paul J Early and D.
Bruce Sodee, Mosby publication (second edition) page 158, 1995.
2. Chandra R. Introductory Physics of nuclear medicine, 4th edition, Lea and Febiger, Philadelphia,
1992.
3. Cherry SR.,Sorenson JA and Phelps ME, Physics in nuclear medicine, 3rd edition, Philadelphia, PA:
WB Saunders, 2003.
4. Knoll GF, Radiation Detection and Measurement, 3rd Ed, John Wiley and Sons, New York 2000.
Radionuclide Dose Calibrator
G.S. Pant
Radionuclide dose calibrator (activity meter) is an instrument for measuring radionuclides
and radiopharmaceuticals in nuclear medicine. These instruments are basically well type
ionization chambers which are sealed after filling the air under pressure. The air is filled
under pressure in the chamber and sealed. It has two co-axial cylindrical electrodes
maintained at a given potential difference. The ionization produced inside the sealed chamber
by radiation emitted from a given radionuclide, placed in the well, is collected by the
electrodes as ionization current. This ion current is converted into a voltage signal that is
processed and displayed in digital form either in Becquerels (Bq) or Curies (Ci) or both. The
layout diagram of dose calibrator is shown in figure 1.
Figure 1: Lay out diagram of a dose calibrator
The ionization current hence the voltage signal produced by the chamber is directly
related to the amount of activity present in the well but will be different for different
radionuclides due to the difference in type, energy and branching fraction for the emitted
radiation. There are various sources of error while measuring activity in a dose calibrator.
They may be related to sample position or volume, non-linearity over a wide range of
activity (very small to very high), accuracy and precision etc. that need proper consideration
and testing.
9 7
98
Radionuclide Dose Calibrator
To get rid of the geometrical variation along the well, plastic holders for vials and
syringes are made available by the manufacturers so that the activity is measured always at
the same and appropriate location. This particular area of the ion chamber at a given depth
normally shows stability in measurement. The well is surrounded by a lead lining to provide
shielding for the operator and also reduces the effect of radiation coming from outside. To
eliminate the background radiation completely, its reading has to be subtracted from the
measured value. In modern dose calibrators there is provision for auto zero corrections.
When the lead lining is not provided by the supplier or additional shielding is added by the
users on their own then the instrument needs to be recalibrated and all the performance
parameters should be evaluated due to change in back scatter value.
It is necessary to maintain the dose calibrators always at their optimum performance for
which routine quality control tests are essential.
Acceptance tests
One of the most important aspects at the time of acceptance is the physical inspection of
the instrument and its accessories. All the parts/accessories, which are expected to be available
with the system, should be checked for their physical condition. Essential accessories needed
with the dose calibrator are :
• Standard radioactive sources (
137
Cs, 57Co,
• Vial holder and syringe hanger,
• Moly breakthrough/assay Canister.
133
Ba or 60Co),
The power and environmental conditions that are required for the instrument should be
made available in the facility.
Accuracy and precision
After completing the routine test for background measurement or zeroing the background
with auto zero facility; a standard radioactive source should be measured in the calibrator.
The mean of the measured values (A) and expected or true value of the activity are used to
evaluate the accuracy as follows:
)(exp
Accuracy
exp
valueectedorCorrect
valueectedCorrectvalueobservedMean
Example:
A given standard source of 57Co was measured in a dose calibrator and the mean of 15
measurements was observed to be 99.593 MBq. The correct or expected value at the time of
measurement was 100.64 MBq. Calculate the accuracy and precision
GSPant\Newbook\Final-2007\6-chp\98
100
Radionuclide Dose Calibrator
64.100
Accuracy
)64.100593.99(
100
= -1.04%
99
An accuracy within + 5% is good, within + 10% may be acceptable (for diagnostic
procedures) beyound with it needs corrective measures.
Precision
The precision, which determines the reproducibility/reliability of the measuring
instrument, may be measured in terms of standard deviation or coefficient of variation
(COV).
The mean value in the avove example = 99.593
Standard deviation (sd) for 15 measurements = 0.0458
Coefficient of Variation (COV) = 0.046%
With such low value of (sd) and (COV), it is clear that the instrument age very precise.
Lesser the value of COV better is the precision.
The instrument seems to be quite precise as the value of both standard deviation and
coefficient of variation are quite low. One can check the precision with other radioactive
sources available in the facility. For counting instruments Chi square test is the choice of
method for checking precision.
Measurement of standard sources in different isotope modes
All the available standard sources may also be measured in other isotope modes and the
ratios with its actual mode may be recorded. These ratios are constant and should not
change with time. The values may be recorded at the time of acceptance (reference values)
and then semi annually or after any repair of the instrument.
Test of stability
The standard source (normally
plotted against the time elapsed. Measured values on a given day of the week or month may
be used for plotting the graph. The expected value (continuous line in figure 2) and the
values within +5% and -5% (dotted lines) are also plotted along with the measured values
(dots). This allows checking the closeness of the measured values from the expected ones. If
the measured values are within 5%, the instrument is treated to be stable.
137
Cs) can be routinely measured and the values can be
Test of Linearity of Activity
The dose calibrator should be linear over a wide range of activity measurements. It is
therefore, advisable to measure activity from kBq to GBq (Ci-Ci) range. The linearity from
GSPant\Newbook\Final-2007\6-chp\99
100
Radionuclide Dose Calibrator
very low activity to very high activity is tested as there are situations when very small and
high activity samples are required to be measured in the laboratory. There are three methods
to check the linearity of activity.
Figure 2: Graph showing the stable performance of a dose calibrator. The dots represent the
measured values, regular line-expected value and dotted lines show the tolerance limit of ± 5%.
(a) Decay method: A sample with high activity source is placed into the well of the
calibrator with the help of a suitable holder and measured at a regular time interval till the
activity comes down to kBq (Ci) level. The measurement may take 2-3 days depending
upon the amount of activity at the beginning and half-life of the radionuclide used. The
activity time graph may be plotted and checked for linearity. Figure 3 shows the linearity
with 67Ga source at low level of activity.
Figure 3: Linearity of activity at low doses (67Ga source)
GSPant\Newbook\Final-2007\6-chp\100
Radionuclide Dose Calibrator
The demerits of this method are:
(a) It is time consuming
(b) There is an unavoidable gap between the measurements at the end of the day and
beginning of the next working day. To check the linearity in the range of gap region,
one should take the desired activity for that region of the graph and check the
linearity in the same manner. This method though time consuming, minimizes
radiation exposure to the staff involved in measurement.
101
(b) Graded volume method : With the help of a micro auto pipette a small volume of a
radionuclide (
99m
Tc or
131
I), corresponding to low activity, can be measured. The volume is
then increased in steps and the measured activity is recorded. The graph between volume(s)
and corresponding measured activity is plotted as shown in figure 4. This method is quite
fast but radiation exposure to the staff involved in the measurement is unavoidable.
Figure 4: Linearity of activity response in a dose calibrator with graded volume method.
(c) CALICHECK method: Some manufacturers provide a kit known as CALICHECK
KIT for checking the linearity of the calibrator. The kit consists of about 7 cylindrical coaxial metallic tubes (absorbers) of varying diameters. The first tube is black with a lead
shielding at the bottom end but there is no lead lining inside the cylindrical surface. The
other tubes are open at both the end but have lead lining (of different thickness) inside the
cylindrical surface. The diameters of all these tubes are made in such a way that each can fit
over the black tube. To differentiate from each other the tubes are given colors. The black
and the purple tube have the smallest and the largest diameters respectively. The red, orange,
yellow, green and blue tubes are in increasing order of diameter.
GSPant\Newbook\Final-2007\6-chp\101
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