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BERNARD ET AL: X RADIATION FROM VAN de GRAAFF ACCELERATOR ION SOURCES
181
X-RADIATION FROM VAN DE GRAAFF ACCELERATOR I O N SOURCES*
+
D. L. Bernard, N. Vo Long
and J . L. Rinehart
The i n t e n s i t y of x - r a d i a t i o n was measured
as a function of p o s i t i o n along t h e ion source
axis of t h e model JN and model CN p o s i t i v e ion
Van de G r a a f f accelerators of t h e University of
V i r g i n i a . The measurements were made with two
x - r a y spectrometers b u i l t t o accommodate the
d i f f e r e n t physical arrangements o f t h e J N and
CN a c c e l e r a t o r s . Back streaming e l e c t r o n beam
p r o f i l e s are deduced from t h e i n t e n s i t y d i s t r i b u t i o n s . Suggestions f o r reducing t h e x-radiat i o n emanating from the a c c e l e r a t o r s a r e given.
Introduction
Although Van de Graaff a c c e l e r a t o r s have
been used as research t o o l s f o r over f i f t e e n
y e a r s , no work has been published on t h e locat i o n and i n t e n s i t y of r a d i a t i o n emanating from
t h e accelerator during i t s operation. From the
p o i n t of view o f r a d i o l o g i c a l health, a
knowledge of the source and i n t e n s i t y of x-radia t i o n could lead t o e f f e c t i v e use of shielding
as w e l l as modifications of design r e s u l t i n g i n
t h e reduction of r a d i a t i o n hazards f o r personnel.
The University of V i r g i n i a model J N and
model CN Van de Graaff a c c e l e r a t o r s , designed
and manufactured by High Voltage Engineering
Corporation, are capable of a c c e l e r a t i n g s i n g l y
charged p o s i t i v e ions up t o 1MeV and 5.5 MeV,
respectively. During t h e process of being accele r a t e d , the p o s i t i v e ions may c o l l i d e with residu a l gas or electrodes i n t h e a c c e l e r a t o r tube,
or both, thus l i b e r a t i n g e l e c t r o n s which a r e acc e l e r a t e d toward t h e ion source. These electrons upon s t r i k i n g t h e aluminum electrodes o f
t h e accelerator tube and components of t h e i o n
source produce bremsstrahlung.
Small o r i f i c e s
i n t h e path o f the back streaming electrons may
become point sources of r a d i a t i o n which can be
e a s i l y detected by an x-ray spectrometer with
appropriate s p a t i a l r e s o l u t i o n .
Two spectrometers with s p a t i a l r e s o l u t i o n
defined by l e a d collimators were b u i l t t o accommodate t h e d i f f e r e n t p h y s i c a l arrangements
of t h e model J N and model CN a c c e l e r a t o r s . The
sp t i a l resolution was measured by displacing a
Cog0 source perpendi u l a r t o t h e a x i s of t h e
spectrometer, the Cog' source being a t t h e same
d i s t a n c e from the spectrometer d e t e c t o r as from
t h e v e r t i c a l axis of t h e a c c e l e r a t o r tube during
t h e course o f the experiment. The spectrone t e r s were then used t o measure t h e location,
i n t e n s i t y , and energy of t h e x-radiation ext e r n a l t o t h e insulating tanks of t h e accelerat o r s while viewing along t h e a x i s of t h e accele r a t o r tube, the ion source and source gas
bottles.
Experimental Method
Description of the Apparatus.
The arrangement o f t h e apparatus used with
++, University
of Virginia
the J N a c c e l e r a t o r i s shown schematically i n
Figure 1. The x-ray spectrometer-detector w a s
pointed toward t h e axis of t h e accelerator tube.
The collimator i n f r o n t of t h e detector was made
of two Pb b r i c k s separated by a distance of 38 cm.
Both blacks were 10.2 cm wide, 13.3 cm high, 7 cm
t h i c k and had a 1.3 cm diameter hole through t h e i r
centers. The r e a r lead b r i c k was placed d i r e c t l y
i n f r o n t of a 23 cm long c y l i n d r i c a l Pb s h e l l
which served as a s h i e l d f o r t h e detector from
the s c a t t e r e d r a d i a t i o n . The w a l l thickness of
t h e Pb s h e l l was 2 . 5 cm. The detector included
a 5 . 1 cm diameter by 5.1 cm long Na1IT.E) scint i l l a t o r coupled t o an RCA 634a photomultiplier
tube enclosed by a Mu metal s h i e l d . The lead
collimator and t h e c y l i n d r i c a l lead s h e l l enclosing the d e t e c t o r r e s t e d on a mobile c a r t
whose height w a s such t h a t t h e detector a x i s w a s
at t h e same height a s t h e accelerator tube a x i s .
The d e t e c t o r could then be moved along t h e ext e r n a l s i d e o f t h e a c c e l e r a t o r tank and p a r a l l e l
t o t h e accelerator tube and ion source axes. The
forward edge of t h e d e t e c t o r collimator w a s 91.4
cm from t h e a c c e l e r a t o r tube axis. The spectrome t e r s p a t i a l r e s o l u t i o n was 4.3 cm a t t h i s
distance
A similar spectrometer w a s used for t h e
measurements on t h e CN accelerator as shown i n
Figure 2. I n t h i s arrangement, consideration w a s
given i n designing t h e spectrometer s h i e l d and
collimator t o t h e high l e v e l of background radia t i o n i n the v i c i n i t y of t h e accelerator under
normal operating conditions. The r e s u l t i n g spectrometer shielding and collimator contained more
lead than t h a t used with t h e J N accelerator.
The d e t e c t o r , d e t e c t o r s h i e l d , and collima t o r r e s t e d on a platform suspended along t h e
outside of t h e CN Van de Graaff i n s u l a t i n g tank
by means of a s e r v i c e crane. S t e e l r o l l e r s on
the a c c e l e r a t o r end of t h e spectrometer platform
constrained t h e spectrometer t o move along two
tracks fastened t o t h e i n s u l a t i n g tank. The
distance from t h e collimator forward edge t o t h e
axis of the a c c e l e r a t o r ion source was 119 cm.
The end apertures of t h e 40 cm long collimator
were .5 cm, r e s u l t i n g i n a s p a t i a l r e s o l u t i o n of
1.5 cm a t t h e ion source a x i s . A 7 cm t h i c k
lead b r i c k w a s used as a s h u t t e r f o r background
measurements during t h e runs. V e r t i c a l positioning of the spectrometer and a c t i v a t i o n of t h e
s h u t t e r was done remotely from t h e Van de Graaff
c o n t r o l room. The h o r i z o n t a l positioning w a s
done by dropping plumb l i n e s f o r e and a f t of t h e
spectrometer onto a l i n e perpendicular t o and
i n t e r s e c t i n g t h e a c c e l e r a t o r tube axis. A v e r t i c a l p o s i t i o n i n d i c a t o r whose output was read i n
t h e a c c e l e r a t o r c o n t r o l room was attached t o t h e
spectrometer platform. The v e r t i c a l p o s i t i o n of
the spectrometer along t h e i n s u l a t i n g tank w a s
reproducible t o within 2 mm. Marks on t h e ext e r n a l side of t h e i n s u l a t i n g tank corresponded
t o t h e height of vaxious p a r t s of the ion source
.
PAC 1967
182
IEEE TRANSACTIONS ON NUCLEAR SCIENCE,
measured from t h e top o f t h e a c c e l e r a t o r base
p l a t e . The i n t e n s i t y of x-radiation versus
p o s i t i o n along the i o n source was measured with
a model CN R.F. ion source with and without modi f i c a t i o n f o r nanosecond pulsing.
The operating conditions of t h e a c c e l e r a t o r
were selected i n such a way as t o duplicate o p
e r a t i n g conditions e x i s t i n g f o r a l a r g e number
of d.c. beam experiments being conducted with
t h e CN Van de Graaff accelerator of t h e Univers i t y of Virginia. These conditions are: (I) 200
p s i of i n s u l a t i n g gas; ( 2 ) a d j u s t i n g t h e ion
source parameters u n t i l a d.c. beam of 4 microamperes of protons i s extracted from t h e accele r a t o r ; and (3) varying the terminal p o t e n t i a l
f r o m 3 t o 5 million v o l t s i n s t e p s o f 1m i l l i o n
v o l t s . No measurements were taken with t h e modi f i e d ion source operating i n i t s pulsing mode.
S p a t i a l resolu$fon and energy c a l i b r a t i o n
measurements
Before x-radiation i n t e n s i t y measurements
were undertaken, t h e s p a t i a l r e s o l u t i o n of t h e
spectrometer was determined. The c r i t e r i o n f o r
s e l e c t i n g t h e s p a t i a l r e s o l u t i o n w a s motivated
by the speculation t h a t a narrow p e n c i l of back
streaming electrons could e n t e r t h e e x i t canal
of t h e ion source g l a s s b o t t l e and s t r i k e t h e
g l a s s b a f f l e , thus c r e a t i n g a source of bremsstrahlung i n a plane above t h e top of t h e accele r a t i n g region. Preliminary measurements on t h e
JN accelerator spectrometer indicated t h a t about
4 c m s p a t i a l r e s o l u t i o n a t t h e ion source axis
would be s u f f i c i e n t t o d e t e c t any such e f f e c t .
Preliminary measurements with t h e CN spectrome t e r , however, indicated s t r u c t u r e i n t h e radia t i o n y i e l d curve o r i g i n a t i n g from components
o t h e r than t h e g l a s s b a f f l e and which could be
resolved with 1.5 cm s p a t i a l r e s o l u t i o n . The
s p a t i a l resolution of t h e spectrometers could
be varied by a l t e r i n g t h e s i z e of t h e collimator
apertures.
The method used t o measure t h e s p a t i a l resol u t i o n of t h e CM spectrometer i s shown i n Figure
3 together with the r e s o l u t i o n function. The
same method was employed with t h e JN spectrome t e r . A Go6' p r a y source was positioned i n t h e
l i n e of s i g h t of the.spectrometer a t a distance
from t h e f r o n t face of t h e spectrometer collima t o r equal t o t h a t of t h e a c c e l e r a t i n g tube axis
i n the course of making the a c t u a l measurement.
I n t h e case o f t h e CN spectrometer, t h i s
distance w a s 119 cm; f o r t h e J N spectrometer it
was 91 cm. The v e r t i c a l height of t h e source
was read from a cm s c a l e . The s i g n a l s from t h e
N~I(TB) s c i n t i l l a t o r - d e t e c t o r were f e d t o a f a s t
s c a l e r a f t e r being properly shaped and amplified
The full-width-at-half maximum of t h e locus res u l t i n g from measuring r e l a t i v e y i e l d per u n i t
t i m e versus 7-ray source p o s i t i o n was defined
a s t h e s p a t i a l resolution. This was 1 . 5 cm f o r
t h e CN spectrometer and 4.3 cm f o r t h e J N spectrometer
A Na22 7-ray source was used f o r t h e energy
c a l i b r a t i o n of each spectrometer with the elect r o n i c configuration shown i n Fig. 3. I n t h e
JUNE 1967
case of the JN spectrometer, pulse height discrimination was made on pulses from t h e detectr
o r corresponding t o a b i a s equivalent t o a pulse
height from .03 MeV p r a y s . L i n e a r i t y of output
versus photon energy extended t o 0.9 MeV. I n
t h e case of t h e CN spectrometer, t h e pulse height
discriminator l e v e l s e t t i n g was f i x e d by t r i a l
and e r r o r while observing x-radiation from t h e
CN accelerator. The high counting r a t e measured
i n t h i s configuration r e s u l t e d i n appreciable
blocking time of t h e multichannel analyzer and
s c a l e r when no pulse height discrimination was
used. A t 5 m i l l i o n v o l t s on t h e a c c e l e r a t o r
terminal, t h e discriminator l e v e l , adjusted t o
make blocking time n e g l i g i b l e , corresponded t o
280 keV x-rays.
This w a s done a f t e r t h e amplif i e r dynamical range was adjusted such t h a t 3MeV
of x-radiation could be detected.
Results and Discussion
-The r e s u l t s of t h e measurement on the JN acc e l e r a t o r a r e presented i n Fig. 4. The y i e l d
of x-radiation i s p l o t t e d as a function of pos i t i o n along t h e i o n source. The smooth curve
w a s drawn t o connect t h e p o i n t s . The centroid
of t h e most i n t e n s e peak f a l l s between t h e
second and t h i r d electrodes of t h e accelerator
tube. The second peak of much less i n t e n s i t y
seems t o be c o r r e l a t e d i n p o s i t i o n with t h e ion
source gas r e s e r v o i r which c o n s i s t s of a gas
b o t t l e of hydrogen and one of deuterium. This
i n t e n s i t y d i s t r i b u t i o n w a s measured f o r a
deuteron beam current of 10 icroamperes with a
hydrogen gas pressure of l(rgcm of Hg i n the
accelerator tube. The terminal voltage w a s .75
m i l l i o n v o l t s . It was found t h a t t h e presence
of a p o s i t i v e ion beam (deuterons) was necessary
for t h e production of any s i g n i f i c a n t r a d i a t i o n .
This agrees w e l l with t h e view t h a t the ion beam
ionizes t h e r e s i d u a l gas i n t h e accelerator tube
and t h e l i b e r a t e d e l e c t r o n s s t r i k e the aluminum
electrodes of t h e a c c e l e r a t o r tube and components
of the ion source r e s u l t i n g i n t h e emission of
bremsstrahlung. A l i k e l y explanation f o r the
existence of t h e second peak i s t h a t it i s due
t o x-rays produced i n t h e low energy end of t h e
accelerator tube and s c a t t e r e d a t 90° by t h e gas
r e s e r v o i r by Compton s c a t t e r i n g processes. The
shoulder on t h e r i g h t hand s i d e of t h e main peak
could be a t t r i b u t e d t o x-rays produced by electrons s t r i k i n g t h e b a f f l e of t h e g l a s s b o t t l e
of the source. An estimate of t h e electron beam
p r o f i l e which can be made from t h e i n t e n s i t y dist r i b u t i o n and t h e dimensions of t h e o r i f i c e s of
t h e ion source' i s t h a t about 15 percent of the
electron beam i s contained within a diameter of
3.5 cm.
The measurement made on t h e d.c. beam R.F.
ion source of t h e CN Van de Graaff i s shown i n
Fig. 5 ( a ) . This i n t e n s i t y d i s t r i b u t i o n w a s
measured while a c c e l e r a t i n g a beam of 4 microamperes of,protons with a hydrogen gas pressure
of 4 x 10-o cm of Hg i n t h e accelerator tube.
The accelerator terminal voltage was 4 million
v o l t s . The i n t e n s i t y of x-radiation was
PAC 1967
BERNARD ET AL: X RADIATION FROM VAN de GRAAFF ACCELERATOR ION SOURCES
measured at every 1.5 cm position increment
a l o n g t h e upper p a r t of t h e accelerator tube and
t h e ion source. The centroid of t h e most intense
p e a k f a l l s a t a p o s i t i o n corresponding t o t h e
b a s e o f t h e ion source b o t t l e . A second peak i s
measured a t the upper shoulder of t h e focus electrode nozzle. Appreciable r a d i a t i o n i s emanating
f r o m a point j u s t above t h e base of t h e focus
n o z z l e . There i s also an indication of radiat i o n coming f r o m t h e f i r s t electrode of t h e acc e l e r a t o r tube. There w a s an appreciable amount
of r a d i a t i o n from t h e upper accelerator tube and
ion source region when t h e beam w a s turned off.
I n f a c t , w i t h 4 million v o l t s on t h e terminal
a d no beam, the i n t e n s i t y d i s t r i b u t i o n measured
c l o s e l y duplicated t h a t with a beam, t h e only
difference being t h a t t h e magnitude of t h e int e n s i t y was decreased by a f a c t o r of 2 with no
beam. This implies t h a t a l a r g e f r a c t i o n of t h e
e l e c t r o n s s t r i k i n g material i n s i d e t h e upper end
of t h e accelerator tube and t h e ion source axe
s e t f r e e by t h e p o t e n t i a l difference between t h e
a c c e l e r a t o r tube electrodes. Similar i n t e n s i t y
d i s t r i b u t i o n s were observed with 3 and 5 m i l l i o n
volts on the terminal. The peak heights a t 3
m i l l i o n v o l t s were a f a c t o r of 12 lower than
t h o s e a t 4 million v o l t s ; those at 5 million
volts were 1.5 times higher. No s i g n i f i c a n t
r a d i a t i o n was observed from t h e g l a s s b o t t l e .
It i s estimated from t h e i n t e n s i t y d i s t r i b u t i o n
measured w i t h the d.c. beam ion source t h a t 66
p e r c e n t of t h e electron beam has a diameter of
3 cm.
Fig. 5(b) shows t h e x-radiation intensit y d i s t r i b u t i o n measured with t h e model CN ion
source modified f o r nanosecond pulsing. The int e n s i t y w a s measured a t each 1.5 cm increment of
spectrometer position along t h e upper accelerat o r tube and ion source. This d i s t r i b u t i o n w a s
measured w i t h 4 million v o l t s on t h e terminal,
4 microamperes of proton beam and 4 x 10-6 cm of
Hg o f hydrogen pressure i n t h e accelerator tube.
The centroid of t h e most intense r a d i a t i o n f a l l s
on t h e upper shoulder of t h e focus electrode
nozzle and t h e nozzle aperture. The i n t e n s i t y
d i s t r i b u t i o n measured with t h i s ion source exh i b i t s more s t r u c t u r e than t h a t measured with
t h e d.c. beam R.F. ion source. This i s consiste n t with t h e f a c t t h a t components axe incorpor a t e d i n t o t h i s ion source which axe absent i n
t h e CN source described above. It i s i n t e r e s t i n g t o note t h a t an appreciable amount of elect r o n s s t r i k e the extraction electrode and g l a s s
b a f f l e of t h e source b o t t l e as evidenced by t h e
s i z a b l e bremsstrahlung peaks observed i n t h e
distribution
1 m i l l i c u r i e ~ 0 6 0source placed a t an equivalent
distance from t h e J N spectrometer. A second
source of r a d i a t i o n o f l e s s i n t e n s i t y was found
t o o r i g i n a t e i n the v i c i n i t y of t h e gas reserv o i r above the ion source. This could be due
t o bremsstrahlung being s c a t t e r e d a t goo by t h e
reservoir b o t t l e s . From these r e s u l t s , it i s
recommended t h a t the region around t h e focus
electrode of t h e accelerator tube be shielded
more e f f e c t i v e l y or t h a t t h e material of t h e
electrode be changed t o a smaller 2 material
( e .g. beryllium) t o reduce r a d i a t i v e loss of
t h e electrons.
The CN accelerator i o n sources exhibit more
s t r u c t u r e i n t h e i n t e n s i t y d i s t r i b u t i o n measurements. This s t r u c t u r e p e r s i s t s a t 3, 4, and 5
million v o l t s on the terminal, although the int e n s i t y of t h e x-radiation observed increases
rapidly with terminal voltage. The most intense
r a d i a t i o n observed with t h e d.c. beam R.F. ion
source came from the base of t h e ion source g l a s s
b o t t l e . Intense bremsstrahlung was also observed
as o r i g i n a t i n g a t the upper focus electrode
nozzle shoulder and a t t h e base of t h e nozzle.
Fabrication of these components with low Z
material would s i g n i f i c a n t l y reduce t h e i n t e n s i t y
of the r a d i a t i o n observed. The bremsstrahlung
i n t e n s i t y d i s t r i b u t i o n measured with t h e model
CN nanosecond pulsing k i t exhibited appreciably
more s t r u c t u r e than t h a t measured with t h e d.c.
beam ion source. The most intense r a d i a t i o n was
found t o o r i g i n a t e a t t h e upper focus electrode
nozzle shoulder and aperture, which warrants a p
p r o p r i a t e measures for shielding o r a l t e r i n g fabr i c a t i o n of these components t o reduce such int e n s i t i e s . s i g n i f i c a n t r a d i a t i o n appears t o
o r i g i n a t e a t t h e b a f f l e of t h e g l a s s b o t t l e .
A study of t h e e l e c t r o n beam p r o f i l e shows
t h a t t h e beam of back streaming electrons i n t h e
CN accelerator appears t o be smaller i n cross
section than t h a t i n t h e J N accelerator. No estimate i s made of t h e e l e c t r o n current s t r i k i n g
the ion source components because of t h e complexi t y of t h e s c a t t e r i n g which bremsstrahlung undergoes before reaching t h e spectrometer.
Acbowledgement
The authors wish t o acknowledge t h e advice
given by Dr. Lawrence Cranberg and M r . Denis
H i l l s during t h e course of t h i s experiment. P a r t
of t h i s work w a s subject f o r M.A. theses of two
of t h e authors (N.V.L. and 2. L.R.) under the
d i r e c t i o n of Dr. Lawrence Cranberg.
.
summary
The presence of a p o s i t i v e ion beam of t h e
JN a c c e l e r a t o r was necessary f o r the production
of any s i g n i f i c a n t radiation. Most of t h e bremsstrahlung produced i s .a r e s u l t of electrons
s t r i k i n g t h e focus electrode arc? t h e t h i r d elect r o d e of t h e accelerator tube. k m t h e operat i n g conditions discussed above; t h i s source of
r a d i a t i o n w a s about f i v e times as intense as a
I a3
References
1. R. D. Evans, The Atomic Nucleus (McGrawH i l l Book Co., Inc.,
2.
*
195357600.
The dimensions were obtained from
drawings of t h e 1 MeV Van de Graaff
accelerator furnished by High Voltage
Engineering Corp., Burlington, Mass.
Footnotes
Work p a r t i a l l y supported by t h e National
Science Foundation.
PAC 1967
IEEE TRANSACTIONS
184
ON NUCLEAR SCIENCE, JUNE 1967
Footnotes
Continued :
+ Present address, Saigon, South Viet Nam.
++ Present address, U. S. Army Combat Developments Command, Air Defense Agency,
Fort Bliss, Texas.
CORONA STABILIZER
Nz 8 COz AT 2OOp.l
-P
HIGH VOLT&bE TERMINAL
ACCELERATOR TUBE
I
Pb BRICKS
I /-No1
I
CRYSTAL
Mu METAL SHIELD
DETECTOR PLATFORM
CYLINDRICAL Pb SHELL
PHOTOMULTIPLIER 8 PREAMPLIFIER
Scale in cm.
0
0 ASSOCIATE0 ELECTRONK EWlPMENT
Fig. 1. A schematic diagram of the x-ray spectrometer used in measuring the x-radiation intensity distribution of the model J N Van de Graaff
positive ion accelerator.
B
I
0
24
-
FOCUS ELECTRODE
01
16
Fig. 2. A schematic diagram of the x-ray spectrometer used in measuring the x-radiation intensity distribution of the model CN Van de
Graaff positive ion accelerator.
GAS RESERVOR
E
I
I
I
COUNTS PER MINUTE
I
2
X
I
3
IO*'
Fig. 3. The graph represents the spatial resolution
function of the CN spectrometer. Below is a
diagram of the arrangement of the Y-ray
source and spectrometer used for measuring
the spatial resolution. This arrangement is
similar to that used with the J N spectrometer.
PAC 1967
BERNARD ET AL: X RADIATION FROM VAN de GRAAFF ACCELERATOR
-t 704
7
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185
ION SOURCES
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I IO
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Fig. 4. This graph shows the intensity of x-radiation measured as a function of relative position along the
JN ion source. The error bars represent the spatial resolution of 4.3 cm of the spectrometer. A
profile of the ion source is illustrated.
PAC 1967
186
IEEE TRANSACTIONS ON NUCLEAR SCIENCE, JUNE 1967
Fig. 5. The intensity of x-radiation measured as a
function of relative position along the CN ion
source (a) without and (b) with modification for
nano-second pulsing. The spatial resolution in
each case was 1.5 cm. A profile of the unmodified and modified ion source is illustrated.
The maximum count rate in (a) i s 17.4 x lo4
counts/min, in (b), 6.8 x 104 counts/min.
PAC 1967