3.3.5. Process control
The delivery and validation of a specified dose to a medical product are key concerns of operators of e-beam irradiation facilities [3.22]. In an IMPELA based irradiator, four of the parameters that directly influence the absorbed dose distribution in the product are controllable in real time — the electron energy, average beam current, scanned area and the product exposure time [3.23].
Analogue control systems were commonly used in early accelerator constructions. Interlock systems must fulfil safety requirements in addition to control and operation functions. Protection of the accelerator is provided against mechanical and electrical failures by the electrical interlocks in every accelerator component and installation. The feedback between the beam
current level and the speed of the conveyor is usually in place to provide constant dose to the irradiated products. At present, computer or micro- processor driven control systems are the only preferable solutions for modern accelerators. The most favourable features of such systems are:
—Automatic checking of initial data to avoid incorrect data entry and eliminate operator errors;
— Automatic startup and shutdown procedures;
— Automatic monitoring and control of every critical parameter;
— Simpler and better process control;
— Automatic conditioning;
— Data logging and graphic display;
— Higher reliability and simpler service procedures;
— Automatic control allows the reduction of skill levels required of machine operators;
— Control system based on validated software;
— Integrity of the process controlled on a real time basis (error detection);
— Graphic based operator interface (step by step instruction);
— Controlled access to the system (password and security). A control system (Digital Process Controller) computer and I/O co- processor have been developed to control accelerator operation [3.24]. The digital systems can be adopted easily for different accelerator construction and parameters. The system not only controls the current electron beam parameters but also provides necessary interlock safety system control and
usually can be applied to control the technological equipment during the irradiation process [3.25].
Measurement of parameters (such as energy, beam current, pulse repetition and scan width), calculation and recording are typically included. A Programmable Logic Control processor is usually used to control accelerator equipment. A PC system is used to provide necessary communication with the accelerator operator. An LCD touch sensing panel to realize ‘one button control’ is sometimes used to simplify the accelerator operation.
3.4. IN-LINE SYSTEMS
Currently, most of the developed countries use electron accelerators for the sterilization of medical products, as they are the safest and ecologically pure compared to all other known methods. The report by Auslender et al. [3.26] describes in detail the automated in-line installation for sterilization of single use syringes operating in the city of Izhevsk, Russian Federation. The syringes are irradiated from two sides inside the packs containing 250 units each. The packs are automatically turned over on the inclined part of the conveyor under the influence of their own weight. The syringes are positioned vertically along the beam fall. The ratio of the maximum absorbed dose to the minimum is 1:4. The production rate of installation is no less than 100 000 syringes/h. The instal- lation is based on the linear pulse electron accelerator ILU-6. It is a single cavity machine with electron energy up to 2.5 MeV and average beam power up to 20 kW. The pulse nature of the current and the automatic control system allow the absorbed dose to vary over a large range. The electron energy, beam current, pulse repetition rate, beam position in the exit window and transpor- tation of the treated products are computer controlled [3.26].
In the most technologically developed countries, due to high transport costs and time losses incurred during transportation, manufacturers of medical products are interested in in-house or in-line accelerators. Considerable effort has been put into reducing the size of accelerators, for example, the KeVAC accelerator (Fig. 3.6) and the MeVAC accelerator (3–10 MeV, 3–5 kW) were developed and equipped with their integrated sterilization tunnels. The systems are called SterStar and SterBox, respectively [3.27].
At several pharmaceutical production sites in Europe, products are treated with three low energy (200 keV), 1–10 mA accelerators for surface decontamination of products prior to entering a sterile area. The products (tubs containing pre-sterilized syringes) pass through an e-beam curtain before entering a filling machine. The KeVAC accelerators are placed inside a lead
housing, forming a self-shielded sterilization unit, the SterStar. Their triangular configuration ensures that the entire surface of the product is exposed to 25 kGy radiation dose. The conveyor system guarantees output and proper exposure time, while the isolator interface provides differential pressure and clean air inside the sterilization tunnel [3.27, 3.28].
3.5. X RAY IRRADIATORS
The electron to X ray conversion effect was discovered more than 100 years ago. Since this discovery, X rays have been widely applied in medical and industrial diagnostic instruments due to their unique properties. The efficiency of electron to X ray conversion is relatively low and depends on the composition of the target material and the energy of the electron beam. High penetration abilities of X rays provide a unique opportunity to irradiate large objects.
The efficiency of conversion and spatial distribution of X rays are the main parameters of any target for application in radiation processing. The target construction should be optimized to improve its technical and economical features. Under optimal conditions, only 7.6% of the total e-beam power is converted into a forward X ray stream for an electron energy of 5 MeV. Up to 76% of e-beam power has to be removed by a cooling system, while the remaining portion is lost by electron scattering, backscattering, etc., and adsorbed in the shielding. For some radiation processing applications, X rays are economically competitive and offer more flexibility than gamma sources (easy control of radiation, safety and intensity of radiation). Recent development in high power and high energy accelerators provides an opportunity to produce and use X rays for industrial applications [3.29–3.36].
An irradiator with an X ray converter is shown in Fig. 3.7, and the config- uration of the electron scattering horns with respect to product tote boxes is shown in Fig. 3.8.
To optimize the irradiation conditions and calculate product throughput, several parameters should be taken into account, such as density and size of the product package, radiation utilization efficiency, dose required and dose uniformity. Two-sided, two times irradiation (four passes) may be applied to improve dose uniformity and increase X ray utilization. Calculations show that
dose distribution can be similar or even better than that achieved in the case of gamma irradiators with the same throughput. A more sophisticated Palletron system was proposed to improve depth–dose distribution [3.37]. In this design, collimators are inserted between the X ray source and the product to shape the X ray beam, and a non-constant scanning of the e-beam is applied to obtain a uniform dose distribution along the vertical axis. A pallet load rotates in front of the X ray beam with a dedicated rotation speed profile. The Palletron® system yields a maximum to minimum dose ratio that is smaller than 1.5 for all densities between 0.1 and 0.8 g/cm3, however, at the cost of reduced throughput.
The prospective user of an X ray irradiator should clearly define the type of product that would be treated by the facility. Similar to gamma irradiators, products on pallets and those in continuously moving containers require very different materials handling; the materials handling system and source exposure to the beam should be carefully designed to meet the specifications of the sterilization process. In addition, the choice of irradiation container is product or facility dependent. After the requirements for the irradiation container are defined, the presentation to the source (beam) must be considered. One of the examples is shown in Fig. 3.7. Unfortunately, most current applications concentrate on X ray target selection before considering
the materials handling system to optimize easy operation, throughput maximi- zation and dose uniformity. Dose uniformity and throughput are seldom optimized simultaneously; some compromise has to be found to ensure technical and economical effectiveness of the facility.
3.6. CRITERIA OF ACCELERATOR SELECTION
Although there are many different types of accelerators offering a wide range of performance ratings, only a few would be suitable for a particular application. Table 3.5 lists important criteria that should help in making the most suitable selection of the accelerator.
The basic specification for electron energy and beam power should be derived from the process requirements (absorbed dose distribution, product size, shape and density, and throughput rate) to ensure satisfactory results with minimum capital and operating costs. Table 3.6 describes accelerator and facility basic parameters, which should be correlated with particular process requirements.
Nominal average beam power (kW) Type and range of beam power adjustment Nominal average beam current (mA) Beam current stability (%) Beam current setting Beam pulse repetition frequency (range)