Indian Journal of Pure & Applied Physics
Vol. 50, November 2012, pp. 802-804
Utilization of 10 MeV RF electron linear accelerator for
research and industrial applications
M Kumar*, N Chaudhury, D Bhattacharjee, V Yadav, S R Ghodke, R Baranwal, J Mondal, R R Tiwary,
S Chandan, A R Tillu, V Sharma, R B Chavan, D Jayaprakash, R L Mishra, Mahendra Kumar, R Patel,
B Nayak, K P Dixit, S Acharya, V T Nimje, K C Mittal, D P Chakravarthy & L M Gantayet
Accelerator & Pulse Power Division, Bhabha Atomic Research Centre, Mumbai
*E-mail: [email protected]
Received 23 August 2012; accepted 28 September 2012
A 10MeV RF electron linear accelerator (Linac) is regularly used at Electron Beam Centre (EBC) Kharghar, BARC to
demonstrate the radiation processing of different research and industrial applications. The use of 10 MeV electron beam for
environment friendly waste disposal of printed circuit board (PCB) has been demonstrated. The 99.9% pure Cu foil has been
extracted from the PCB by imparting 300 kGy of radiation dose. The power diode of Bharat Heavy Electricals Limited
(BHEL) has been irradiated by imparting a dose of 4 kGy to achieve reduction in reverse recovery time (Trr) from 15 µs to
6 µs. An irradiation experiment of polymer blend samples has been conducted by RTDD, BARC at EBC to increase the
tensile strength of blends of low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) from 6 MPa to 7.8 MPa by
imparting a dose of 100 kGy. The alanine-EPR dosimetry has been done to optimize process parameters for food irradiation.
The semolina (cream of wheat) packets are irradiated at the Dmin (minimum dose level) of 250 Gy and Dmax (maximum dose
level) of 1 kGy. The dose between the Dmin and Dmax has been delivered using 10 MeV electron beam, and packets are kept
to collect further data. Various research experiments of BARC and other universities have been carried out to study the
effects of irradiation on materials.
Keywords: Printed circuit board, Alanine-EPR, Radio-chromic film, Spectrophotometer, Linac
1 Introduction
A 10 MeV RF electron Linac is used for radiation
processing either in electron beam mode or in X-ray
mode. The conveyor system is used for product
handling during the irradiation processing. Along with
its various subsystems, Linac is housed in a twostoried building with 2.6 meters thick concrete
shielding walls. The Linac structure with electron
gun, RF cavity, gate valves, SIPs, and beam transport
line is at first floor as shown in Fig. 1, whereas scan
magnet, scan horn, titanium foil, SIP for scan horn
and conveyor systems for material handling is at
ground floor as shown in Fig. 2. The electron beam at
50 keV is generated in electron gun with LaB6 cathode
and is injected into the on-axis coupled cavity linac
which accelerates the electrons to energy of maximum
10 MeV. A 2856 MHz, 6 MW Klystron based RF
power source is used to establish the required electric
field of 18 MV/m inside the linac.
After acceleration, the magnetic sweep scanner
deflects the beam in the scan horn and taken out in the
atmosphere through a 1000 mm × 70 mm, 50 µm
thick titanium foil window for radiation processing
applications. The complete linac up to titanium foil is
maintained in the vacuum of 10-7 torr with the help of
rotary backed turbo molecular pumps and sputter-ion
pumps. A low conductivity water cooled tantalum
target is used to convert the electron beam into X-ray
mode irradiation. The other sub-systems of RF Linac
are ozone removal system to remove the ozone from
scan horn area which is generated by the interaction
of electron beam with air molecules before hitting the
product to be irradiated. The low conductivity water
(LCW) cooling system is used to remove the heat
generated in various sub-systems of RF Linac during
operation. The air cooling system is used to remove
the heat by imparting the air flow via fans. These are
used to cool electron gun load, titanium foil, electron
gun etc.
2 Characterization of the Radiation Dose at 3 kW
Electron Beam Power
Every user requires a certain dose to be delivered to
his samples or products to achieve desired changes in
the product or samples. To ensure that a certain
amount of radiation dose has been delivered to the
product, the dose must be characterized with
established standards1. Thus before product
KUMAR et al.: UTILIZATION OF 10 MeV RF ELECTRON LINEAR ACCELERATOR
irradiation, full characterization of the radiation dose
was done with the help of Radiological Physics &
Advisory Division (RPAD), BARC. The irradiation
experiment was carried out with radio-chromic films
placed at desired locations within the target product to
be irradiated. After irradiation the films were
analyzed at spectrophotometer system at a wavelength
of 552 nm, to observe the net change in optical
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density (or absorbance) of the radio chromic films.
There is a calibration curve for absorbed dose with
respect to change in absorbance. On this basis the
dose delivered to the product at point of interest has
been decided. The radiation dose characterization has
been done in electron beam mode at 10 MeV beam
energy and 3 kW average beam power. A radiation
dose of approximately 21 kGy is delivered to the
product in each pass below the titanium foil, when the
product is moving at a speed of 100 mm per minute at
a vertical distance of 460 mm from the exit window.
The experiment has been done to characterize the
dose variation with respect to conveyor speed. It has
been verified that if conveyor speed is increased 10
times (i.e. 1000 mm/min) the dose falls 10 times2
(i.e. 2 kGy in a single pass). This linear variation has
been utilized extensively to deliver low dose for food
irradiation of FTD, BARC and diode irradiation of
BHEL.
3 Irradiation Experiments
3.1 Environment friendly waste disposal of pcb delivering
300 kGy dose using 10 MeV RF linac
The PCBs without any components mounted had
been used for the electron beam (EB) irradiation
study. Fig. 3(a) shows the sample of PCB taken
before irradiation with components mounted on it.
Fig. 3(b) shows the sample of PCB taken before
irradiation with components removed. The PCBs were
taken in required dimension into the quartz tube with
Fig. 1 — RF Linac showing Electron Gun, RF Cavity, SIPs etc at
first floor
Fig. 2 — RF Linac showing scan horn, titanium foil cooling
system and conveyor system at ground floor
Fig. 3 — PCB sample before irradiation with components (a)
mounted on it and (b) removed
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INDIAN J PURE & APPL PHYS, VOL 50, NOVEMBER 2012
3% NaOH (sodium hydroxide) solution. The solution
was filled in the container just up to the top surface of
the PCBs placed for exposure. The container was air
tight closed with the rubber cork and properly sealed
with cellophane tape.
The depth of penetration of EB with respect to the
PCB samples was calculated with the depth of
penetration of EB in water as reference. The 3%
NaOH solution serves three purpose; maintain the
temperature between 30-45°C, to dissolve the gases
liberated during the exposure process and also to
enhance the degradation of the adhesive layer to ease
the separation of copper. The samples were then
exposed to EB radiation with conveyor speed of
200 mm/min and dose of 10 kGy per pass. After the
exposure, the samples were removed from the process
at different stages of delivered doses for further
analysis. It has been found that the copper foil and
copper circuit from the printed circuit board can be
separated without any loss in quality and quantity
during the process. Electron beam processing proves
that it is an environmental friendly way to secure
99.99% pure copper from the composite. Even-though
polymer adhesive start degradation at 150 kGy, the
complete removal of copper foil and copper circuit
occurs easily only at 300 kGy. The electron beam
processing of thermo set polymers showed a new way
to degrade them to particular extent and reuse them
which is not possible in any thermal or mechanical
processing.
3.2 Irradiation of BHEL power diodes to reduce trr from 15 µs
to 7 µs delivering 4 kGy dose
The e-beam dose of 4 kGy has been standardized
for the irradiation of power diodes of BHEL to reduce
the reverse recovery time (Trr) from 15 µs to 6 µs.
One hundred (100) power diodes have been
irradiated, the whole lot has passed the quality control
test done at BHEL lab and are being used in their
power circuits. The dose of 4 kGy has been delivered
by operating RF Linac at beam power of 500 W, and
conveyor speed of 1000 mm/min3.
3.3 Irradiation of food products by FTD, BARC
The alanine-EPR dosimetry has been done by using
10 MeV RF Linac to optimize process parameters for
food irradiation4. The commercially available
semolina (cream of wheat/rawa) packets are irradiated
between the Dmin (minimum dose level) of 250 Gy and
Dmax (maximum dose level) of 1 kGy. The dose
between the Dmin and Dmax has been delivered using
10 MeV electron beam. Such a lower dose has been
delivered by operating the RF Linac at average beam
power of 3 kW and increasing the conveyor speed up
to 5000 mm/min. The dosimetry has been redone by
FTD, BARC and this has reconfirmed the capability
of dose variation by varying the conveyor speed.
3.4 Irradiation of polymer blend samples by RTDD, BARC
Blends of low density polyethylene (LDPE) and
ethylene vinyl acetate (EVA) were irradiated to
different radiation doses using 10 MeV accelerators.
The mechanical properties of the blends increased
with the radiation dose due to the radiation induced
cross-linking. Un-irradiated (un-crosslinked) blend
showed tensile strength of 6 MPa which increased to
7.8 MPa at a dose of 100 kGy. However, the plateau
region in tensile strength versus radiation dose curve
was not reached in the dose range studied indicating
cross-linking density of the blends which can be
further enhanced at higher doses. Thermal properties
of samples however did not show any improvement
on EB irradiation in the dose range studied. Further
experiments are proposed to irradiate the samples to
higher doses to achieve higher crosslink density.
4 Conclusions
The dose profile is uniform with ±5% variation
which is allowed as per international standard. The
output beam energy is experimentally verified and
that is 10 MeV. The system has been used the various
research and commercial experiments done by BARC
and other universities, out of which few has been
mentioned in this paper.
Acknowledgement
We gratefully acknowledge all the users of 10 MeV
RF Linac, who proposes the innovative uses of RF
Linac, Dr A V R Reddy, the Chairman of Accelerator
Utilization Safety Committee, and the members of the
committee for their constant guidance in making an
experiment successful.
References
1 Dovbnya A N et al, IEEE, 3 (1998) 3810.
2 Chaudhary N et al. Monte-carlo simulation & dosimetric
measurement for a 10 MeV RF electron linear accelerator
InPAC-2011.
3 Iliescu E et al. IEEE, 2 (1997) 323.
4 Consultants meeting on food irradiation, IAEA, Vienna,
16-18 October (1995).