Download High power electron accelerators for radiation processing and its

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
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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
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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.
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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