Indian Journal of Pure & Applied Physics
Vol. 48, November 2010, pp. 771-773
Estimation of radiation source term and concrete shield thickness
for low energy particle accelerators
D S Joshi* & P K Sarkar
Health Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India
*E-mail: [email protected]
Received 17 July 2010; accepted 13 August 2010
Electron beams with varying energy (0.5-15 MeV) have been extensively used for processing of various materials.
Main source of radiation is bremsstrahlung radiations (X-rays) generated when electron beam strikes high Z materials. The
emergent spectra have angular distribution and with increasing electron energy X-ray emission rate peaks in the forward
direction. Photo neutron generation in such electron accelerator is less significant as it generates after certain threshold
energy. The accelerated protons interact with matter and lead to the primary radiological hazards, which consist of prompt
radiation (neutrons and gamma). Another source is delayed radiation which is due to induced radioactivity in accelerator
component. Thus, in electron accelerator bremsstrahlung radiation and in proton accelerator, neutrons are the main sources
of radiation that decide the shield thickness. The emergent radiation has angular distribution and accordingly shield
thickness also varies. Radiation source terms are evaluated for 3, 7, 10 and 15 MeV electron beam and 10/20 MeV proton
beam accelerator. Required concrete shield thickness for housing these accelerators is also evaluated and presented in this
paper. Emergent radiation yield values are verified by Monte Carlo stimulations and are found to be in good agreements.
Keywords: Particle accelerators, Radiation source term, Shield thickness, Bremsstrahlung radiation, Neutron radiations
1 Introduction
The charged particle accelerators have played key
role in the fields of basic and applied sciences. The
electron generated photon beams are finding vast
applications in the fields of radiation therapy and Xray radiography. When X-rays fall upon material, they
get scattered, absorbed or transmitted. The scattered
or transmitted X-rays in conjunction with the imaging
techniques can be used in constructing the
radiographic images. X-rays for this purpose can be
generated when electron beam strike materials like
tantalum or tungsten. The commercial devices
available for this purpose range from 0.5 to 15 MeV.
Low energy proton beam accelerator is used as a
neutron source, radioactive beam facilities,
radioisotope production and research. Because of its
wide range of applications more low energy proton
accelerators are build at various places around the
world. The accelerated protons interact with matter
and lead to the primary radiological hazards, which
consist of prompt radiation (neutrons and gamma) it is
there when accelerator is in operation. Another source
is delayed radiation and it is due to induced
radioactivity that continues to emit radiation after the
accelerator is shut down. In the present paper,
radiation source term estimation for typical 3, 7, 10
and 15 MeV electron beam and 10/20 MeV proton
beam accelerators has been studied.
2 Methodology
2.1 Source term evaluation in electron beam accelerator
The main radiation source to be considered for
radiation protection in the present facility is the
bremsstrahlung X-rays produced when the electron
beam interacts with high Z materials like lead and
tungsten/tantalum. With increasing electron energy
the angular distribution in X-ray emission rate peaks
in the forward direction1-3.
The 0° and 90° dose rates for the electron beams of
various energies when fall on optimum thick high Z
target materials are evaluated and presented in
Table 1 (Ref. 1). The 0° dose rate decides shield
thickness for front wall while 90° dose rates decides
the shield thickness for sidewalls and roof (Fig. 1).
Dose rate Hx at distance d from target and
transmitted through shield thickness S can be
calculated as:
  S −T2  
− 1+ 

D
H x = 10 × 2 × 10   Te 
…(1)
d
where Hx is transmitted dose rate from shield (µGy/h),
D is dose rate from the source at 1m (Gy/h), d is
distance between X-ray source and reference point
(meter), S is Concrete shield thickness (meter), T1 is
first tenth value layer (meter) and Te is subsequent
tenth value layers, in equilibrium (meter).
6
INDIAN J PURE & APPL PHYS, VOL 48, NOVEMBER 2010
772
Required tenth value layers (TVL’s) of concrete
shield thickness are taken from Ref. 1. Table 1 also
gives the required concrete shield thickness (density2.35 g/cc) to reduce the dose rate to acceptable level
(1 µSv/h) as per AERB Safety Directive 2/91.
2.2 Photoneutrons
Photoneutrons generate when electron beam
interacts with the material of the structural
components. However generation of photo neutrons
occurs above certain threshold energy defined for
such material. The photoneutron threshold energy lies
Table 1— Evaluated radiation source term for electron beam
accelerator (beam loss at W target)
Electron Direction Dose rate Required concrete shield
beam energy angle at 1m Gy/h
thickness (cm)
(MeV)
per mA
TVL For <1 µSv/h at 5 m
3
7
10
15
0°
90°
0°
90°
0°
90°
0°
90°
8.4×102
2.3×102
1.05×104
5.3×102
2.7×104
9.0×102
6.0×104
1.2×103
23
20
35
30
40
35
43
38
180
150
310
250
370
270
410
300
Fig. 1 — Layout of 10MeV electron accelerator
in the range 6-13MeV, exception being hydrogen and
beryllium (2.2 and 1.67MeV, respectively). Electron
above 10 MeV may encounter structural component
like Pb, Fe/Cu (threshold and gamma energy are 6.75
and 10-11 MeV, respectively). Nuclear cross section
for (γ, n) reaction is low (<1mb). The yield of
neutrons (per incident particle) in a target material is
given by the expression4:
Y ( E0 ) =
ρN 0
A
∫
E0
k th
σn (k )
dLγ
( E0 , k )dk
dk
…(2)
where E0 is the incident electron kinetic energy in
MeV, N0 is Avogadro’s number, ρ is the material
density in gm/cc, A the atomic weight of the target
material, k the photon energy, σn(k) is the
photoneutron cross section as a function of photon
energy k and dLγ/dk photon differential track length.
For lead material (used as a collimator) neutron
yield will be 2×1010 neutrons/s/kW and is much less
as compare to photons.
3 Source Term Evaluation in Proton Accelerator
A DC proton beam will be generated by Electron
Cyclotron Resonance (ECR) plasma ion source at
about 40mA intensity and extraction voltage of
50keV. This beam will be accelerated to 10 and 20
MeV in four stages. Typical schematic view of
20MeV Low Energy High Intensity Proton
Accelerator (LEHIPA) is shown in Fig. 2. The spilled
protons from the acceleration channel up to energies
of <6 MeV interact with matter mostly by elastic
scattering, except for certain low mass matter like
lithium, beryllium and deuterium (uncommon
materials). The inelastic nuclear interaction by
protons in beam component would cause noticeable
secondary radiation (mostly neutrons) from DTL-1
(10 MeV) operation onwards.
Fig.2 — Schematic layout of 20 MeV proton linac
JOSHI & SARKAR: LOW ENERGY PARTICLE ACCELERATORS
773
Table 2 — Evaluated radiation source term for proton beam
accelerator (beam loss at Cu target)
Proton Neutron Neutron Dose rate Required concrete
beam
yield
source
mSv/h at shield thickness (cm)
Energy
n/p
strength
1m
TVL For <1 µSv/h
(MeV)
(near target)
at 5 m
n/s
10 MeV 0.0012
20 MeV
0.005
7.54×109
3.14×10
10
7.5×101
30
110
1.09×103
34
170
Fig. 4 — Neutron spectrum from 20MeV proton on Cu target
the area under the curve. For 10 Mev electron beam
on target W yield values obtained are 0°-5.32 and 90°0.4 photons/e and for 20 MeV proton beam on target
Cu value is 0.00356 neutrons/p. Yield values then
multiplied by beam current loss to get radiation
source term as particles/s. By considering source as
point source flux at 1 meter from the target surface is
evaluated and then converted in to dose using the
ICRP dose conversion coefficients6.
Fig. 3 — Photon spectrum from 10 MeV electron on W target
Neutron yield values per proton of energy
10/20 MeV incident on copper/iron targets5 is used
and source term n/sec is evaluated for typical 1 µA
beam loss. Neutron flux thus obtained is multiplied by
respective flux to dose conversion factors to get
neutron dose rates in mSv/hr at 1m. Evaluated neutron
dose rate at 1 meter per µA proton beam loss are
given in Table 2. The 90°/0° dose rate ratio is taken as
0.5 (Ref. 1).
4 Monte Carlo Calculations
Computer stimulation is done to get emergent
radiation spectra when high energy particle incident
on target material. Monte Carlo code EGS4 is used to
get photon spectra when 10 MeV electron beam is
incident on thick tungsten target. Emergent photons
(no. of photons/MeV/Sr) were scored in different
angular bins. Photon spectrum in forward direction
(0-5 degree) and sideward direction (87.5-92.5
degree) is shown in Fig. 3. FLUKA Monte Carlo
computer code is used to get neutron spectra when
20 MeV protons are incident on thick copper (Cu)
target; emergent neutrons (no. of neutrons/MeV/Sr)
from the target surface were scored using option user
yield. The resultant neutron spectrum is shown in
Fig. 4. The integral yield value per incident particle is
5 Conclusions
In electrons accelerator bremsstrahlung radiation
and in proton accelerator neutrons are the main
sources of radiation which decide the shield thickness.
The emergent radiation has angular distribution and
accordingly shield thickness also varies. Yield values
per incident particle and hence radiation source term
generated by using imperiacal relation closely
matches with the Monte Carlo results. Induced
activity in accelerator component will remain even
after accelerator made off and has to be assessed
before making any personal entry in to the accelerator
cell. Administrative control in the form of radiation
interlocks will help in controlling accidental
exposures.
References
1 NCRP Report No. 51, Radiation protection designed
guidelines for 0.1 to 100 MeV particle accelerator facilities,
1977, p 30, 95, 107, 115.
2 NCRP Report No. 144, Radiation protection for particle
accelerator facilities, 2003, p 45.
3 IAEA, Technical Report Series No. 188, Radiological safety
aspects of the operation of electron linear accelerators, 1979,
p 52.
4 Swanson W P, Health Phys, 37 (1979) 347.
5 IAEA, Technical Report Series No-283, Radiological safety
aspects of the operation of proton accelerators, 1988, p 210.
6 ICRP Publication 74 ( Pergamon press, Oxford), Ann ICRP, 26
(1997) 200.