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Indian Journal of Pure & Applied Physics Vol. 50, November 2012, pp. 772-775 High power electron accelerators for radiation processing and its safety aspects K C Mittal Electron Beam Centre, Accelerator & Pulse Power Division, Bhabha Atomic Research Centre, Mumbai 400 085 E-mail: [email protected] Received 23 August 2012; accepted 28 September 2012 Electron accelerators from 500 keV to 10 MeV energy are employed for surface irradiation, food preservation, medical sterilization, cargo scanning and other industrial products. At EBC, a 10 MeV Linac has been indigenously developed and is operational for radiation processing. This is an on-axis coupled cavity standing wave type of RF linac operating with 50-70 keV electron gun having LaB6 cathode and klystron-based RF source at 2856 MHz. The linac is currently operating at 10 MeV, 130 mA (peak), 10 µs pulses at a prf of 415 Hz and is employed for various experiments. A similar 9 MeV linac is operational for cargo-scanning. A compact 6 MeV linac is under development. A 500 keV, 3 kW dc accelerator based on Cockroft Walton multiplier has been operational for industrial surface processing. A 3 MeV dc accelerator based on parallel coupled multiplier column is operational at 1 MeV, 4.6 kW level. A 700 keV, 5 kW dc machine for cross-linking and graft polymerization is under development. A 100 MeV, 100 kW electron Linac for producing intense neutron source has been proposed for development. Technological developments, operating experiences, utilization for industrial applications and safety aspects of all facilities are described. Keywords: Electron accelerators, Radiation processing, Radiation safety, Industrial applications 1 Introduction High power electron accelerators in the range of 500 keV to 10 MeV energy have found numerous industrial applications1, including surface irradiation, food preservation, medical sterilization, cross-linking of polymers, graft polymerization, etc. These applications require doses in the range of about 5-500 kGy. To address the growing need for electron accelerators, Accelerator & Pulse Power Division (APPD) of BARC has taken up the indigenous design and development of high power dc and RF electron accelerators and their deployment for industrial applications. The salient features of these accelerators, operational experiences, beam utilization and safety aspects have been studied in the present paper. 2 Accelerators Developed by APPD/BARC The main aim of this indigenous development is to build high throughput accelerators which are inherently robust, offering stable operation on a 24×7 basis and requiring minimal servicing. Main features of dc accelerators and RF linacs are described here. 2.1 DC Accelerators The challenges in the development of dc accelerators include complex design of multiplier columns, achieving dimensional tolerances of high voltage components, gun power supplies floating at high voltage dome and their control and handling of high pressure gas systems used for HV insulation. Successfully implemented projects include 500 keV and 3 MeV accelerators (Fig. 1). The 500 keV, 3 kW accelerator2, based on Cockroft-Walton multiplier is operational for industrial surface processing. This is located at BRIT complex, Vashi. At Electron Beam Centre (EBC), a 3 MeV dynamitron type accelerator3,4, based on parallel-coupled multiplier column has been tested up to 1 MeV, 4.6 kW. For cross-linking and graft polymerization, a 700 keV, 5 kW machine is under development. 2.2 RF Linacs Design and fabrication of the accelerator cavities are one of the most critical aspects of RF linacs. The dimensional tolerances (20-50 microns) of individual cavities and surface finish of (0.2 micron) during fabrication and maintaining these accuracies after brazing several cavities together, are a major challenge. Design of high power robust electron guns, handling of high power RF sources and design of beam scanning devices for required beam uniformity are other critical issues in RF linac systems. These challenges have been overcome successfully in the development of 10 MeV industrial RF linac5 and 9 MeV linac6 for cargo-scanning as shown in Fig. 2. MITTAL: HIGH POWER ELECTRON ACCELERATORS 773 Fig. 2 — View of 10 MeV and 9 MeV linacs Fig. 1 — View of 500 keV and 3 MeV DC Accelerators Located at EBC, the 10 MeV linac is a pulsed on-axis coupled cavity standing wave type of RF linac operating at 2856 MHz, with 10µs pulse width and 400 Hz rep rate. Injector is a 50-70 keV LaB6 based electron gun while RF power is obtained from 6 MW klystron. This is operational at 3 kW beam power and is in regular use for industrial and research applications. The 9 MeV linac for cargo-scanning at ECIL, Hyderabad, is a similar linac where an X-ray beam is produced for a dose of 24Gy/min/m with a beam diameter of ~2.5 mm. It is well known that neutrons are generated via photonuclear and photo fission reactions from Bremsstrahlung photons. Typically, 50 MeV electrons at 5-10 MW beam power can produce a neutron flux of ~1.65 × 1014 n/s with a uranium target7. Based on this principle, 30 MeV linac for neutron generation is being designed and developed. It is also proposed to build a 100 MeV, 100 kW linac for an intense neutron source. This is proposed to be used for material science as well as ADS studies. 3 Dosimetric Experiments Estimation of dose levels and dose rate distribution is a basic need for industrial accelerators used for radiation processing. Dosimetric experiments, described below, have been carried out in the 10 MeV, 9 MeV and 3 MeV accelerators. In the 10 MeV linac, radio-chromic films of 10 mm x 10mm were arranged at suitable locations along the scanning direction and the dose-rate profile is observed at different distances from Ti exit window in static mode as well as dynamic modes at different speeds. The average dose-rates at 12, 20 and 36 cm are found to be 65, 40 and 21 kGy / min, respectively. As shown in Fig. 3, dose rate is uniform within ±8% over entire scan length. In dynamic mode, dose rate8 is uniform within ±5%. Similar results have been obtained in 3 MeV accelerator, with dose rate uniformity of ±5%. Dose-depth distributions have been measured with perspex and aluminium, having full penetration depth of 42 mm and 19 mm, respectively. This corresponds to output beam energy1 of ~9.5-9.79 MeV. For 9 MeV Linac, dose rate of X-rays produced after striking 2 mm thick tantalum (Ta) target by 9 MeV electron beam has been measured by using air 774 INDIAN J PURE & APPL PHYS, VOL 50, NOVEMBER 2012 4.4 Semiconductor Irradiation Diodes and thyristors on irradiation at 4 kGy, showed a marked reduction in the reverse recovery time from 15µs to 6µs. Fig. 3 — Dose rate distribution over 1 m scan length in static mode at different distances from exit window ionization chamber at 1 m from X-ray target. The maximum X-ray dose rate observed is 20.63 Gy/min at 1m from X-ray source, which satisfies the requirement for cargo scanning. 4 Radiation Processing Experiments using 10 MeV RF Linac Several experiments for radiation processing of materials have been carried out with the 10 MeV RF Linac9. These include industrial as well as research applications. For all the applications, it has been seen that the throughput of the linac at 3 kW is suitable for large throughputs in an 8 h operation. Some of the applications are given below: 4.1 Irradiation of PE Gaskets To improve softening temperature of PE gaskets from 70° to 270°C, electron beam irradiation is used to deliver a dose of ~360 kGy. 4.2 Irradiation of Food Products Food products require dose in 0.25-1.00 kGy regime. Dummy food products have been irradiated and the absorbed dose is in the range 0.69-1.01 kGy measured by using standard dosimeters. 4.3 Medical Product Irradiation Experiments were carried out by using paddy husk (dummy medical product) for simulating the dose distribution required for sterilization of medical products. Minimum dose delivered was 30 kGy and dose uniformity ratio (DUR) is about 2, while the threshold dose is 25 kGy to achieve desired sterility level 1and allowed DUR is 1.5 to 3. 5 Radiological Safety Unlike nuclear reactors and gamma irradiators, which are permanent sources of radiation, electron accelerators have the unique feature that prompt radiation can be stopped by electrically switching off the source of electrons, i.e. electron gun. In addition, RF power can also be switched off, thereby cutting off the source of radiation. This makes accelerators inherently safe. During operation of these accelerators, radiation is produced by energetic electrons, which can be stopped by small metal attenuators. Another source of radiation in accelerators is bremmstrahlung X-ray radiation, which is generated when the electron beam is incident on product material, conveyor system and structural material. Photo-neutrons may also be produced; but in accelerators used for radiation processing, neutron production is negligible, since operation is limited to 10 MeV. Whatever be the source of radiation, it is required that these accelerators are housed in shielded enclosures, made of mild steel, lead or concrete, according to their location and utility. It is necessary that shielding should be sufficient enough to reduce the dose to be below regulatory dose limits of 0.1 mR/h for occupational workers in occupied areas. For 10 MeV linac, concrete wall of 2.6 m thickness has been used for shielding10. Similarly, 1.5 m thick concrete wall acts as shield for 3 MeV accelerator. Lead sheets form the shield for the 500 keV accelerator. In order to ensure radiological safety, at EBC, various zones are demarcated. Radiation monitors are used and radiation survey is conducted in all the zones around the accelerators, particularly at different human occupancy areas. Plastic scintilators, gamma area monitors and environmental radiation monitors with audio-visual alarm units have been installed. The radiation monitors are interlocked with accelerator system, such that if the radiation level exceeds the permissible limit, the accelerator automatically gets tripped. Neutron monitors have also been installed. Personal dosimeters (TLDs and DRDs) are used for radiation workers and these are monitored quarterly. The radiation level survey is carried out periodically in all the human occupancy areas. The calibration of all the radiation detectors/monitors is checked routinely by using standard source. MITTAL: HIGH POWER ELECTRON ACCELERATORS 6 Non-radiological Safety Non-radiological safety systems include those for environmental safety, HV and microwave protection systems and industrial hygiene. 775 It is ensured that proper gaskets are used and flanges are properly tightened. Matched terminations are used at all ports. 6.3 Industrial Hygiene 6.1 Environmental Safety Systems There are some environmental safety issues associated with electron accelerators. One such issue is the use of SF6 gas11 as an insulator in dc accelerators. Excessive SF6 in the environment can lead to oxygen deficiency posing a serious health hazard. It is necessary to ensure that leakage of the gas is prevented and its concentration in the atmosphere is monitored for safe operation. SF6 detectors are used and pressure of the gas is interlocked with accelerator operation as a safety measure. Another environmental issue is connected with ozone, which is produced when the accelerated electrons, irradiating the products, react with atmospheric oxygen. Ozone, being corrosive, is a health hazard and it is necessary to keep the ozone levels below the equilibrium concentration level < 0.1 ppm in areas of human occupancy. Suitable ventilation (with air blowers) ensures that sufficient number of air changes is provided to maintain safe levels of ozone. Interlocks are provided such that entry into the radiation cell area is permitted only after the ozone level of 0.1 ppm is reached. 6.2 HV and Microwave Protection Systems This is another important safety system provided in electron accelerators. High voltage is used in accelerators for generating the required electric field gradient. Proper insulation is provided for high voltage stand-off and quality of insulation is periodically checked and maintained. This ensures that breakdown is prevented. Adequate grounding is provided to take care of leakage currents. Electromagnetic Interference (EMI) arising due to high voltage systems are adequately bypassed, so that malfunction of electronic/electrical systems is prevented. For microwave leakage, it is necessary that the fields are below the permissible level of <5mW/cm2. Restricted entry; entry and exit doors interlocked with the high voltage supply, search & scram system and emergency shutdown system ensure the safe operation of electron accelerators used for radiation processing. 7 Conclusions The electron accelerator program has been successfully launched and the industrial accelerators are in operation reliably. Safe operation of electron accelerators is ensured with the proper interlocks and other practices described above. References 1 IAEA Trends in Radiation Sterilization of Health Care Products, 2008. 2 Nanu K, et al. Development and Operational Experience of 500 KeV, 10 kW DC Electron Accelerator at BRIT Vashi, ProcSEBTA 2005, Mumbai, September 28-30, 2005, pp 467-475. 3 Mittal K C, Design and Development of 3MeV, 30 kW DC Industrial Electron Accelerator at EBC Kharghar, Proc SEBTA 2005, Mumbai, September 28-30, 2005, pp 476-486. 4 Acharya S, et al., Beam Trials at 1 MeV of a DC Electron Accelerator, ProcInPAC-2011, February 2011, New Delhi. 5 Mittal K C, et al., Operating Experience of 10 MeV Industrial RF Linac, ProcInPAC-2011, February, 2011, New Delhi. 6 Mittal K C, et al., Performance of the 9 MeV RF Linac for cargo-scanning, ProcInPAC-2011, February, 2011, New Delhi. 7 Swanson W P, Calculation of neutron yield released by electron irradiation on selected materials, SLAC-2042. 8 Chaudhary N, Monte Carlo Estimation and Dosimetric Measurement for a 10 MeV RF Electron Linear Accelerator, ProcInPAC-2011, Feb, 2011, New Delhi. 9 Kumar Mukesh, 10 MeV RF Linac Electron Beam utilization for demonstration of radiation processing of various industrial and research applications, in this conference. 10 DixitKP, et al., Safety Aspects of 10 MeV RF Electron Linac, in this conference. 11 SF6 Gas Handling system for 3MeV, 30kW EB Accelerator at EBC, Kharghar, Navi Mumbai