Paper
Radiation Safety Management
Measurement of the Spatial Distribution the Combination of Activation Kazuyoshi
Masumoto,Akihiro
Vol.1,No.1(12-16)
of Neutrons in an Accelerator Room Detectors and an Imaging Plate
Toyoda,Kazuyoshi
Eda and Toyoyuki
by
Ishihara
RadiationScienceCenter,HighEnergyAcceleratorResearchOrganization ReceivedJune 6,2002;accepted September20,2002 In order tectors After out
to measure
and the
plate
of the
detectors.This
and
were
room.The
method
is suitable obtained
to make
of neutrons
tried.Activation
detectors
accelerator
correction.The
loss points
distribution
was
end of operation,the
plan
decay
the spatial
an imaging
plan
collected
was
and
overlaid
for the simultaneous image
an effective
data
were pasted
with
useful
for reducing
room,the
combined
set on monitoring
points
on the corresponding
an imaging
measurement
are very
design
in an accelerator
detectors
plate
monitoring
to develop
of the activities
for grasping
the
of many
the propagation
the activation
use of activation
in the accelerator
of the accelerator
points
activity
deroom.
on a lay-
of activation
detectors
without
of neutrons
from
components
and
any
the beamaccelerator
buildings. Key words:activation
detector,imaging
plate,neutron,accelerator,spatial
is useful to detect the temporal variation of the dose rate.The
1.Introduction The activation has been
of accelerators,peripheral
a serious
accelerator
problem
facilities primary
nents
acceleration
during
by the primary
limited.From arily
their
emitted
of them as the
is high,but
beam-loss
pumps
radioactivity
and the activated neutron
flux
activation ing.The
area
is very
is necessary
reactions,and materials
obtained
to propose
results
a manual
of the accelerator
the degree
and the
aid to design
of beam
structure
transport
components
to reduce
portional
counter
or a He-3
monitored
proportional
with
counter.This
Correspondence to K.Masumoto Radiation Science Center,High Energy Accelerator Oho,Tsukuba,305-0801,Japan
12
Research
in each
are time-consuming
in a limitation
of the number
acand of de-
imaging plate
of the radioactivity
and
ma-
a BF-3
induced
one by one.Therefore,the
the
pro-
method
Organization,
near the surface of various natural prod-
ucts or the slightly contaminated
surface.8) One of the authors
used activation detectors and the IP to monitor the bremsstrahlung and photoneutron
profiles radiated from a Pt-target bom-
barded by an electron
linear accelerator.9)
tors were polyethylene for the detection
are continuously
radioactivity
detector
(IP)has been developed as a new radiography technique.4-7) For example,the IP was applied to measure the distribution
terials. Neutrons
of neutrons.
by the activation
the
the activa-
and the surrounding
track
methods,however,are distribution
and calculation
results
with a film
and a fission
tectors treated in one experiment.Recently,an
of neutron
shield
this method,the
tedious,which
of the build-
an effective
the spatial
has to be measured
such
is not negligible,
wide.Therefore,measuring
detector
can also be monitored
method.In detector
to obtain
tivity measurement
of buildings.Although is low,it
Neutrons
is also monitored
using CR-39 plastic.These
some
circuits,tubes,
to estimate
of the accelerator
is
are second-
of the surrounding
by neutrons
inarea
dose of neutrons thermoluminescence
not suitable
compo-
the activated
badge,a detector
and disman-
points,neutrons
or the construction caused
cumulative
of
radioactivity
by nuclear
the activation
protection
accelerator
and transport.The beam
and shields
of maintenance
hits various
magnets,frames,cables,electrical
vacuum
tion
beam
in all directions
cause
devices
for the radiation
in cases
tling.1-3)The
duced
distribution.
film,nickel,gold
of bremsstrahlung
Activation and aluminum
and fast neutrons
detecfoils by us-
ing the 12C(y,n)11C,58Ni(y,n)57Ni,197Au(y,n)196Au and 27A1(n ,a)24Na reactions,respectively.The beam profile inTEL:0298-64-5491 FAX:0298-64-1993 e-mail:
[email protected]
Radiation
Safety
Management
Masumoto
side the irradiation
chamber
tion of the distance
was clearly obtained
from the Pt-target
after being contacted
as a func-
by analyzing
with activation
the IP
detectors. to
detect the low flux of neutrons in the accelerator room.Then, this method
of neutrons
in an accelerator
nique is performed
to measure
is set on a monitoring operation,(2)each monitoring
the spatial
activation
point in the accelerator detector
is collected
plan is directly
develop the radioactivity image of all detectors simultaneously
detector
room during on the
room after ir-
overlapped
on the IP to
of all monitoring points,(4)the is read out with an imaging
and(5)the
overlapped
and the plan are composed
latent analyzer
picture of a image data
on a computer
The
measurement
neutron
belongs
to
kyo,and
flux
the
is
erator
were
tips
used
of
for
der
different
and
reaction
the
the and
the
with
end
plan
of
IP
(BAS
to
Film
an with
software of
the
color was
image
of as
accelerator
October30,2002
the
from 2000 to 10000 sec for each activa-
tion detector.Each
foil was set on a Ge-detector
measurements. The PSL of gold foils was measured ing analyzer(BAS
image room
for data on
Film
Co.)was
the
the IP for a certain changed
exposure
was
In this work,Au
a computer
each
time was
the
the
of
on
thermal
neu-
based on the assumption
that the
its neutron capture cross-
observed
that the activity of 196Au
at the position
small,
of the maximum
neutron flux near the cyclotron.
uniformly
in sensitivity
on IP sensitivity
irradiated
The results
were
of the IP should be con-
PSL values of various with one another.After
by an X-rays,the
positions the IP was
PSL intensity per unit
measured at six random sampling points.
were in good agreement
detector
their %RSD was 4.1%.Therefore,the
overlaid
did not depend on the position
with one another,and detection
sensitivity
of the IP.
simulwas
of
PSL
the
plan
radioactivity on the IP.The
was
the activity
60 min.The
ranging
and PSL factor from PSL inten-
exposure obtained
of
sure time.The
contained
from 15 to 200 Bq of 198Au,were times were also changed
results
linearly increased in proportion
activation the
between
sity to the activity of 196Au,nine gold foils,which
activation by
Relationship
In order to obtain the conversion
intensity
each
and Na have been and C for fast neutrons.
reaction is dominant,and
firmed in advance,because
Photo
unit
and flux calculation
of its high sensitivity.
on the IP were compared
IP
Co.).The
display.
exposure
foils were chosen to monitor
area(PSL/mm2)were
map
superimposed
time.The
and discussion of Activation detector
The homogeneous
neutron
detectors
of
position
with the bio-imag-
gold foils were contacted with
from 2 to 60 min.
Uniformity
MeV moni-
strained
time,the
activity
in
the
25
(BAS-1000,Fuji
luminescence
automatically
-1000)after
induced by the 197Au(n,2n)196Aureaction was negligibly
of
detectors of
FUJIX
surface during
section is 1550 barn.It was confirmed
cm.
a two-dimensional by
counting
the
monitored
points
of
the
a tape
50
exposure analyzer
photo-stimulated
detector.The the
activities
appropriate
(MacBAS,provided
calculated
of
Photo
sample
used for y-ray measurements.The
un-
of
also on
intervals
III,Fuji
obtained
was
activation
the
of
of
hung
monitoring
a bio-imaging
Co.).A
detectors
protons
distribution
at
a small ro-
Co.)for
de-
walls
operated
number
line
floor
measure
by
and
was
were
the
activation
floor
as
operation,the on
the
maximum
vertical
Electric
multi-
time was changed
resonance
thickness)
induced
cyclotron
energies,such
the
pasted
taneously.After scanned
changing,was
and was barely
on
EG&G)and
EX,Mitsubishi
trons because
Accel-
in
as
198Au
beam-transport
to
plan.The the
set the
detectors,which
ceiling
After
To-
measured.
MeV.The
the
flux
of
were
47.The
with
collected on
was
activation
from
30
12 um
neutron
was
and
of
along the
of Energy
mm0and
when
particles
TRUMP,SEIKO
bot(Movemaster
gold foils were
spectrometer,which
(Model-7229P,Canberra),a
The flux was calculated
the
room
points
flux
analyzer
selected
gamma-ray
used for thermal neutrons and Al,Bi
SF-Cyclotron,which
Study,University
radioactivity
detectors
deuterons
toring
the
factor from PSL intensity
As activation detectors,In,Au,Co,Mn
detector
Branch,High
foil(11
monitoring
,ƒÁ)198Au
accelerator
at
Nuclear
Tanashi
gold
the
the
activation
Organization,Tokyo.
Activation
by
in
the
measured for
located
tectors,and 197Au(n
was
Center
Research Small
of
automatic
a Ge-detector
3.Results Selection and
and PSL intensity
display.
2.Experimental Setting
activity
of 198Au in Bq,nine
measured.An channel
tech-
and pasted
point of a plan of the accelerator
radiation,(3)the
between
In order to obtain the conversion
comprises
distribution
room in this work.This
as follows:(1)each
Relationship to the activity
It was not sure that the above method was applicable we applied
13
et al.
from 2 to
are shown in Fig.1.The to the activity
PSL
and the expo-
PSL of 198Auactivity could be measured
3 to 5% RSD for exposure
set
within
times of 10 to 20 min.Then,the
13
14
Measurement
of the spatial
Fig.1.Relationship different exposure
exposure
time
between times.
was
set
empirical
relation
(exposure
times,min),was
Spatial
The
shows
It was
was
found
the activity neutron
tron
the neutrons
beam
room.The
of 'Au,for
not
detectors
and an imaging
plate
Fig.2.Spatial distribution of the neutrons of 25-MeV proton acceleration.
observed
in the case
Fig.3.Spatial distribution of the neutrons of 30-MeV deuteron acceleration.
observed
in the case
colored
spots
order
corresponds
the deflector
Table1.Minimum
to the 102
of the cycloside
of the
that neutrons
were
of the cyclotron.The
the distance
to obtain
means
and blue means
was confirmed
that this method
the mea-
color
of the opposite
with
distrib-
show
detector.The
that
near
was 2uA.
uniformly
than
flux decreased
of activation
of this figure.
higher
produced
14
were
part
the deflector
neutron
by the combination
the proton
current
near
mainly
flux.In
room
beam
of 103/cm2/sec
window.It
neutron
cyclotron
flux
extraction
confirmed
work.An
neutron
much
It was
this
instance,orange
flux of the order
was
in the
to the upper
of the activation
/cm2/sec.The
throughout
and the average
transported
positions
min
the result of an experiment,where
that
room
obtained.
uted in the cyclotron suring
20
of neutrons
was 25 MeV
beam
in an accelerator
'Au,PSL=1.11•~(activity,Bq)•~
distribution
Fig.2 energy
of neutrons
Bq of 198Auand the PSL value under
on
for
distribution
from
was useful more
and
data
Maximum
the
simultaneously,
flux
the cyclotron
were positioned
on the floor and walls of
room as shown in Fig.3.The
of neutrons
deuteron
ergy was 30 MeV and the beam was transported side of this figure.The
neutron fluence
en-
to the left
in case of deuteron
beam was higher than that in case of proton
the cyclotron. to measure
about 50 samples
beam.Table
1
gives the minimum and maximum
values of the neutron flux
obtained
acceleration.The
in the case of deuteron
ence between the maximum
observedin
the
case
of 30-MeV
differ-
flux and the minimum
deuteron
irradiation
Radiation
flux was
.
Safety
Management
Masumoto
(a)Monitoring
15
et.II.
positions
(b)PSL values
Fig.4.Vertical change of neutrons obtained at five positions. (a)Measuring positions,(b)the PSL values observed.
about one order.
of which
were about
the specific
Vertical distribution
of neutrons in the cyclotron room
building quite
between the cyclotron and the switch magnet.as shown in Fig.4(a).Six activation detectors were hung on a string from the ceiling at intervals of 50 cm.The heights of monitors were 0.15,0.65,I .I5,1.65,2.15 and 2.65m from the floor and the
near the deflector
beam line was 1.15m from the floor.The observed result is shown in Fig.4(b).The thermal neutron flux gradually de-
been
creased with the distance from the cyclotron.On the other hand,no vertical dependence of the thermal neutron fluence
The
concentration
0.06
By/g,which
could be observed.This might be explained as meaning that the fast neutrons emitted from the beam loss point in the
value,0.059
cyclotron were gradually thermalized in their flights and reflected by the floor and the ceiling.Therefore.the thermal
tion
of the floor
with
a boron.
similar
and 3.The
in the concrete
maximum
activity
assumed.The
epithermal
was
and thermal
for 20 years
mal and epithermal
cross-section
By/a.In
order were
flux during
of "Co
agreed
with
op-
cyclotron
of Co is 10 ug/g.The quite
observed
conditions
neutron
and 100 days
per year.The
has ther-
are 37 and 74 barn. obtained
value
the observed
was
maximum
Bq/g.
Such
an information
is very
useful
to reduce
and wall by covering
the higher
an activaflux region
4.Conclusion The suring
activation neutrons
detector emitted
to grasp
method
by beam
the spatial
distribution
use of activation
the cyclotron room The radioactivity induced in the concrete wall was measured and compared with the result of a neutron-flux measurement.Concrete samples were obtained at various points
useful.because
the activity
using
the empirical
relation.A
on the floor,wall and ceiling,and measured with a Ge-detector for analyzing the residual radioactivity.The major radionuclides were "Co and 152Eu.the elemental abundances
could
be observed
at an exposure
be measured verted
is very effective
of neutron
detectors of many
simultaneously.The
to the activity
for mea-
loss in an accelerator
the combined
30.2002
0.059
of 'Co,following
was
in Fig.2
was
104 n/cm2/sec,respectively.The
operated
activity
observed
of 'Co,which
in concrete in
October
shows
of the cyclotron
of 'Co
of the cyclotron,was
the activity
eration
distribution
to the that of neutron-flux
to calculate
In order
Comparison with the induced radioactivity
of60Co
as an example.The
The vertical distribution of neutrons was measured in the same manner.The five measurement positions were selected
part of the neutron fluence might become almost uniform in the vertical direction.This result should be confirmed by comparing with a theoretical calculation of the neutron propagation.
10 and l utg/g,respectively.Fig.5
activity
value
of detector is directly
any decay
neutron
fluence,
and an IP was very
pieces
PSL
in By without
room.
can con-
correction
by
flux of 103 n/cm2/sec
time
of 10 min
within
a
few % of RSD. One
of the most
important
goals
of radiation
safety
con-
15
16
Measurement
of the spatial
distribution
of neutrons
in an accelerator
room by the combination
of activation
detectors
and an imaging
plate
Fig.5.Specific activity of 60Co in the concrete in the SF-cyclotron building.
trol is to reduce ponents
the residual
activity
and buildings.The
derstanding
of neutron
ing of neutrons.Such decommissioning
image
data contributes
propagation data
will
of the accelerator
to our un-
and to the effective be help to reduce
and decontaminating
com-
shield-
the cost of
an accelerator
facility.
ries No.188 (IAEA. Vienna. 1979). 3) Thomas. R. H. and Stevenson. G. R.: Radiological
safety as-
pects of the operation of proton accelerators. Technical Reports Series No.283 (IAEA, Vienna, 1988). 4) Abe. K.: Appl. Phys., 67. 677(1998). 5) Mori. C. and Matsumura. A.: Nucl. Instr. Methods. A353. 371 (1994). 6) Sonoda, M., Takano, M., Miyahara, J., Kato. H.: Radiology.
Acknowledgements The authors of NTT
are grateful
photonics
to Dr.M.Kato
laboratory
and Dr.K.Shikano
for offering
148, 833 (1983).
the BAS-1000.
7) Amcmiya. Y. and Miyahara. J.: Nature. 336. 89 (1988). 8) Mori. C.. Matsumura, A.. Suzuki. T.. Miyahara. H.. Aovama. T.. Nishizawa, K.: Nucl. Instr. Methods. A339. 278 (1994).
References 1) Patterson,
H. W. and Thomas,
ics, (Academic 2) Swanson,
W. P.: Radiological
tion of Electron
16
R. H.: Accelerator
Press, New York,
Linear
Health Phys-
1973), p.501.
Safety
Accelerators.
Aspects Technical
of the OperaReports
9) Masumoto,
K., Ohtsuki,
T., Miyamoto,
Y., Zaidi, J. H.,
Kajikawa, A., Haba. H. and Sakamoto. K.: J. Radioanal. Nucl. Chem.. 239, 495 (1999).
Se-
Radiation
Safety
NIanagement
