The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

X-ray Emission from Caesium Sputter Ion Sourceof 1. 7MV Tandetron Accelerator
Nobuyuki SUGIURA, Toshiso KOSAKO and Takeshi IIMOTO
Research Center for Nuclear Science and Technology, The University of Tokyo
2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
Abstract
The dose rate around I. 7 MV tandetron accelerator wasmeasured. High dose rates of X-ray were detected from aceasium sputter ion source. The dose rate was the order of10 fl Sv/hr. The spectrum of this X-ray has a peak at 25.8
keV. This X-ray showed directivity. This X-ray isconsidered as the bremsstrahlung, which is generated by thecollision of electron to extraction electrode.
This situation was simulated and analysed by EGS4
code. X-ray emission from extraction electrode was foundin all directions and the energy spectrum . After passing thewall of ion source, I mm thickness of SUS304, the energy
spectrum showed a single peak, which range was from 20
to 25keV. It is thought that the generation mechanism of X
rays could be confirmed analytically.

1 Introduction
In recent years, the installation of small accelerator isgrowing for the research field of ion beam science. The 1.7
MV tandetron was installed at Research Center for Nuclear
Science and Technology of the University of Tokyo. We
have reported the dose rates around the accelerator and theX-ray .leakage from the ceasium sputter ion source forproviding fundamental information on radiation protection
and control [1].
We accumulated further information on the X-ray
around the ion source and investigated the mechanism of Xray generation by using EGS4 simulation.

Fig. I Dose distribution around ceasium sputter ion source.
(ion species: C2+)

2 Radiation Dose around 1.7 MV Tandetron

2.1 Measurement method ofradiation dose
The ionizing chamber type survey meter (AE-133L,Applied Engineering Inc.) was used for the dose ratemeasurement. The spectrum measurement around the ionsource was done by Nai(Tl) scintillation detector with Be
window (Ohyo Koken Kogyo Co.).
The measured ion species were H\ He2+, C2+, N2+, se+,Cu2+ and Au2+. Terminal voltage was around 1.7 MV.

M .... (I!4VI

Fig.2 Spectrum of X-ray around ceasium sputter ion source.
(ion species: Au2+)

2.2 Dose distribution and spectrum ofX-ray
High dose rates were detected around the ceasium
sputter ion source. The dose rates were 35, 15 and 2.7 fl
Sv!hr when ion species were Au2+, Cu2+ and N2+,
respectively. The spatial dose distribution of X-ray aroundthe ion source in case of accelerating ~+ is shown in Fig. 1.
This X-ray emission had a directivity.
The spectrum of X-ray around the ion source in case ofaccelerating Au2+ is shown in Fig.2. The peak energy of Xray was 25.8 keV. The peak energy did not change by thedeference of ion species.
This low energy X-ray could be easily shielded by I mm
thick iron plate. The dose rate decreased from 35 fl Sv!hrto 0.8 fl Svlhr in case of Au2+ and from 15 fl Svlhr to 0.45
fl Svlhr in case of Cu2+.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

extraction electrode)3) detector: surface crossing detector-energy bin: 2.5 keY to 30 keY in every 2.5 keY-position: -75° to 75° , every 15°4)histrory: 10,000,000
3.3 Calculation results ofEGS4

Figure 4 shows the deference of directional distributionof Bremsstrahlung by electron beam direction. When theelectron collides with the target vertically (0 ° ), thedirectional distribution seemed almost uniform. On theother hand, when the electron comes slantways (-45 o ),large amount of Bremsstrahlung was detected in theopposite side.
Fig.3 Cross section of ceasium sputter ion source.
The potential at the ion source is 0 Y (earth),and extraction electrode is -30 kY [2].

1.0E+OO ~----'----------------,
9.0E -01 ~·····················- ·········· ········· ············· ......,,c.;.~~·-···················· ··The mechanism of the X-ray emission from the ion
source was speculated as follows:
I) Free electrons are generated with other anion species in
the ceasium atmosphere, especially from the surface ofion source.
2) The free electrons were accelerated by the potential
difference between ion source and extraction electrode.
3) The electrons were collided with the extraction electrodeand the Bremsstrahlung was generated.

8.0£-01 (-.... .......~-···-~---~.)L....•~\·····················...,7.0£-01 !

................................................ ,/ ...............•\.

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o;~ 3.0E-01

I :..........

+...............................----------!

····· .. ----··-·----\---------·--···-......

2.oE-o1 f··········r=~=·-- -······--··· \, ·········· ====:;:

I
11.0£-01
o.OE+OO w:.._.__..__--'---~--'---'----'--'--'---~4-.J
75' so· 45' 30' 1s' o· -15' -30' -45' -so· -75'
detector direction

3 Analysis of X-ray Emission by EGS4

Fig.4

Deferenceof directional
distribution
Bremsstrahlung by electron beam directionof

3.1 EGS4 Simulation CodeThe EGS4 code simulates the electromagnetic cascade
and the transport of electrons, positrons, and photons byMonte Cairo techniques. The following physical processes
are taking into account: I) bremsstrahlung production, 2)
positron annihilation in flight, 3) Moller and Bhabha
scattering, pair production, Compton scattering, coherent
scattering, photoelectric effect, and continuous energy lossof electrons and positrons with Moliere multiple scattering[3]. The general purpose user's version of the EGS4 code
system was developed to make an input files easily by usinga CG input [4].
In the present study, this general purpose user's versionof the EGS4 was used for the simulation of the generationof Bremsstrahlung from the ion source.

Figure 5 shows the EGS4 simulation results of energy
spectrum of Bremsstrahlung generated from extraction
electrode. As is shown in Fig.5, the more diagonally theelectrons enter, the larger the fraction of low energy X rays
is.

.~S.OE-01~-···L==l·················· ·················· ·· ......................... rj·5 6.0£-01 f--+--+--------------·- ·······

Ie4.oE-o1

3.2 Calculation conditions for EGS4

EGS4 simulation was done in order to analyse themechanism of X-ray emission around the ion source. The
calculation condition for EGS4 is as follows:!)source: electron, beam, 30keY (Beam direction wasvaried from 0° to 75° in every 15° )2)target: SUS303 (This was represent for material of an

0.0£+00 l_~-'----'--...l.:......:::C::c::4==a"=~~.....,.__05
1015302025
energy (keV)

Fig. 5-421-__J35

Energy spectrum of Bremsstrahlung generatedfrom extraction electrode by EGS4.

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

based on the speculation described in the end of section 2.2,
the obtained energy spectrum was the almost same as themeasured spectrum (Fig.2). Therefore, it is thought that thespeculated generation mechanism of X rays could be
confirmed analytically.

1.0E+OO
9.0E-01r--~-

S.OE-01

0.~7.0E-01.&:::~:~
..

1--..

j 4.0E-01

4 Conclusion""3.0E-01
2.0E-01
1.0E-01
O.OE+OO
051015202530

35energy (keV)

Fig. 6The spectrum after attenuation of 1 mm thickness
SUS304.

Table. I Linear attenuation coefficient used for estimationof effect of SUS304.
linear attenuationenergy (ke V)
coefficient (cm- 1)
5-7.5
8.69E+02
l.99E+03
7.5- 10
10- 12.5l.llE+03
12.5- 15
6.66E+02
3.82E+02
15- 17.5
2.60E+02
17.5-20
l.81E+02
20-22.5
l.47E+02
22.5-25
25-27.5
l.l3E+02
27.5-30
7.93E+Ol
Table 1 shows the linear attenuation coefficients forevaluation ofthe effect ofSUS304. As shown in Fig. 6, theenergy spectrum showed a single peak which range wasfrom 20 to 25keV after passing the wall of ion source, 1 mm
As a result of EGS4 simulation
thickness of SUS304.

The dose rate around 1.7 MV tandetron accelerator atRCNST, the University of Tokyo was measured. The dose
rates around a ceasium sputter ion source was relatively
high (the order of 10 1J. Sv/hr).
This X-ray was considered as the bremsstrahlung, whichis generated by the collision of electron to extraction
electrode. Based on this speculation, EGS4 situation wasdone. As a result, it is thought that the generation
mechanism of X rays could be confirmed analytically.

References[I] Nobuyuki SUGIURA et al., "X-ray leakage around 1.7
MV Tandetron Accelerator", Proc. lOth Symp.
Accelerator Scinece and Technology, JAERI-Conf 95021 416-418, (1995).
[2] High Voltage Engineering Europe B.V., Instruction
Manuals, (1993)[3] W. R. Nelson et al., "The EGS4 Code System", SLACReport 265, (1985).
[4] T. Momose et al., "Improvement in the EGS4 code
system - general purpose electron-photon Monte Cairotransport code system", 48-73, Proc. EGS4 User's
Meeting inJapan, KEK Proc. 91-14 (1991).
[5] J. Ishikawa, Ion source engineering, Ionex Co. (1986)[6] H. Kumagai., Accelerator, Kyoritu. Pub. (1988).
[7] Roy Middleton., "A Negative-Ion Cookbook", Univ. ofPennsylvania (1990)[8] R. Middleton, "A Versatile High Intensity Negative IonSource".

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