SLAC-TN-65-11
H. DeStaebler
T. Jenkins
W. R. Nelson
January 1965
REDUCTION OF THE THERMALNEUTRONFLUX IN
CONCRETEUSING PARAFFIN MODERATORS
SUMMARY
The estimated
residual
due to the sodium activity
drogenous
shield
plus
radiation
level
in the concrete
a benign
thermal
in the neighborhood
walls
neutron
of a scraper
would be reduced
absorber
were placed
if
a hyaround
the scraper.'
We have performed several experiments to determine the efficiency
of
We present the results
in a form which should prove useful
such shields.
in estimating
The results
the concrete
shown in Fig.
activity
3 show that
cadmium reduce the thermal
The absolute
yield
about 0.2 neutrons/BeV
calculation2'21
for
neutron
various
10 inches of paraffin
flux
of neutrons
(1.25
and other
X
geometries
by a factor
and beam powers.
surrounded
by
of 10.
from 40 MeV and 990 MeV electrons
is
in reasonable agreement with
10' n/see-watt)
experiments.3'4
A.
When an electron
duce neutrons
giant
with
resonance
beam strikes
and they are isotropic
that
culations
do not hit
the neutrons
the roughly
dinary
Very roughly
in the lab?
high energy interactions
shields
the concrete,
for
are around an MeV,
giant
resonance reactions,
This kind of argument
beam are meaningful for calIf the electrons
energy electrons.
much higher
in the concrete
arises
and are eventually
in the concrete,
very
The
a 70 MeV electron
1% of sodium (a = 0.5 barns)
with
reactions.
the same amount of photon
less abundant.
the major activity
are moderated
concrete
of the shower pro-
energies
as into
are appreciably
measurements with
of thin
their
are most abundant;
so high energy neutrons
suggests
the photons
a target,
a wide spread in energy from photo-nuclear
neutrons
energy goes into
INTRODUCTION
present
low sodium content
because
captured
in the aggregate.
there
by
For or-
may be a significant
manganese activity.5
This residual
activity
may be reduced by adding boron to the concrete
the thermal neutrons preferentially
(a = 3840 barns) rather than the sodium.'
mix,
capture
so that
Another way to reduce the activity
ture
them before
here.
Paraffin,
can be effectively
The neutron
bare or placed
verted
is to thermalize
they get to the concrete,
lucite,
or water
on the BLo
the neutrons
and cap-
which is the method investigated
act as good moderators.
The thermal
neutrons
captured by a l/32 inch cover of cadmium (u = 2300 barns).
fluxes were measured by the activation
of indium foils,
either
inside
to neutron
paraffin
fluxes
moderators.
by calibrating
The observed activities
the system against
and by making corrections
of known strength,6
For the Mark IV experiments
the fluxes
for
irradiation
neutron
were consources
and decay times.
were normalized
to one watt
source size was small compared with
the distance
of beam
power.
The effective
and the results
write
were analyzed
in terms of a point
approximately
g
cp = 2
4m
B(x)e
-X/A
T
neutron
flux
Gl
neutron
source strength
proportional
r
distance
(n/cm2-set)
- approximately
to absorbed beam power (n/see)
from source to foil
-.
2
-
involved,
source for which one can
X
B(x)
A
thickness
effective
of shield between source and fail
build-up
factor
effective
removal mean free
B.
A solid
placed
by various
X 8" length,
of paraffin
directly
a specified
time,
known strength
flux
neutron
a saturation
of 13.9 n/cm2-see,
ground counting
The results
and for
was
slabs were
and bare indium
by l/4
The foils
inch.
were irradiated
and counted.
The general
by indium foils is described
rate
of 1 count/see
a foil-weight
for
of 1.4210 gms.
a neuThe back-
rate was lpc/min.
are plotted
in Fig.
concrete
2 where we notice
with
Figure
shield.
:,:
thickness.
3 shows the reduction
c.
To check the point
inches
depth,
and we also see the effect
in activity
of the
as a function
source model we use a Pu-Be neutron
of lucite
of photoneutrons
the expected build-up
of moderator
Pu-Be EXPERIMENT
and a cadmium cover.
peaks at about 4 MeV and falls
very similar
and
of our system using a source of
counting
and attenuation
by l-1/2
detection
The calibration6
yielded
assembly,
geometry.
removed to a remote spot,
in the literature.7-11
Concrete
which were separated
experimental
and methods of thermal
water-cooled,
and a cadmium cover,
below the target
between the slabs,
1 shows the general
theory
tron
diameter
thicknesses
positioned
were centered
Figure
for
l-1/2"
in the 70 MeV beam of the Mark IV accelerator.
horizontally
foils
MARK IV EXPERIMENT#l
copper target,
surrounded
path
off
A typical
at zero and 12 MeV.
from such elements
to the P-u-Be spectrum,
as Ca, S, Bi,
source,
surrounded
Pu-Be spectrumL2
The energy spectra
Au, 0, and C13-17 look
and we assume that
copper is not too
different.
The same concrete
outdoors
in order
again irradiated
Figure
slabs were used; however,
to reduce scattering
and counted
The geometrical
correction
from walls,
for various
4 shows the experimental
(the
inverse
etc.
r
and Fig.
square)
was performed
Indium foils
source-to-concrete
set-up,
-3-
the experiment
were
distances.
5 shows the results.
accounts
for
most of
the differences
in the fluxes.
The average intercept
which is comparable
0.88 x 10~ n/see,
with
x = 0
is
the known source strength
One would expect this
1.94 X lo7 n/see, but somewhat lower.
foils
only determine the thermal flux,
fast
at
of
because the
whereas the source emits mostly
neutrons.
D.
As another
MARK IV EXPERIMENT#2
check on the point
foils
were placed
inside
foils
have a response
source approximation,
the trench
that
is independent
Foils
These units
1 count/see
were placed
source strength
is roughly
Neutron
10' n/see-watt,
5 inches
fluxes
6a).
about 7%).
*
Dirk
MARK III
in diameter
or -0.2
was the lead Faraday
The measured neutron
yields
the same devices
moderated indium
as a function
an
lengths).
used in
see
an SEM (efficiency
of
of angle were
Neutron/BeV
(iron)
O0
0.10
3o”
5o”
0.16
0.22
PO0
0.22
135O
0.26
this
struck
foils;
Angle with respect
to Beam Direction
Neet and Roger Coombes performed
port
(24 radiation
was measured with
-4-
disThe
n/BeV.
long
angles with
(cadmium covered,
beam current
a
per 8.26 n/cm'-sec.
beam from the p-foot
and 17 inches
were measured at five
The electron
with
EXPERIMENT*
a 990 MeV electron
the second Mark IV experiment
Fig.
were calibrated
at saturation
These
energy within
under the Faraday cup and at varying
Figure 7 gives the results.
structure.
E.
At the Mark III
neutron
6.
directly
tances along the accelerator
iron block
of incoming
beam energy was 40 MeV, and the target
The electron
cup.
the result
indium
of the Mark IV as shown in Fig.
the range of about 40 keV to 14 MeV.18
Pu-Be source with
moderated
experiment,
There is a trend toward higher yields at back angles.
No correction
was made for attenuation
or scattering
of the neutrons in the iron block,
which may account
for
0
at 0 .
the low value
The other
about 20% of the average which is not a very
principal
uncertainties
in the relative
and in the contribution
from the target,
nearby dense objects.
For example,
Faraday cup, and the 90' point
also
supported
effects
feet
it
the iron
is worth noting
square with
above that,
walls
and a thin
roof.
of the bunker wall
It
yields.
with
10 (see Fig.
general
target
that
there
the effective
sensitivity
falls
area is about 60 or 70
above a neutron
absorber
can reduce the concrete
reduction
trons
practical
tion
for
it
however,
significantly,
of activity
out a shield
at most comparable with
that
much more spread out so that
probably
dominates
penetrate
as well
and
by a factor
to surround
of
the scraper
the activity
level
-5-
with
beams some high energy neuand are hardly
attenu-
reduce the nearby radiation
level
depends on the distribu-
as on the total
activity.
from the high energy particles
from the giant
the radiation
a moderator
along the machine
First,
betwo reasons.
the shield
may still
because the radiation
activity
directly
might have been detected.
activity
for
is impossible
the moderator
the concrete
contribution
energy of about
is that
high energy electron
along the tunnel
resonance
case of the scrapers
might not be so great
are produced which easily
ated at all;
level
Secondly,
giant
CONCIJJSIONS
Mark IV experiment
In the
which
room-scattering
These would not have been detected
cause of the complex geometry
the shield.
block
is no noticeable
of the first
3 ).
shielding
(7 feet).
F.
nelltron
The
was taken near the lead
10 - 15 MeV,;18J1g but the lower energy secondaries
thermal
difference.
are about the same as the calculated
from the high energy neutrons.
The result
are within
about 12 feet high (on the average), thin walls
The iron block was about 20 feet downstream
is somewhat surprising
because the detector
the 135' point
the Mark III
at beam height
The observed yields
values
values are probably in the distance
from the neutrons scattered by
In connection
that
thick
significant
was taken atop a big
block.
four
resonance neutrons,
from the giant
near the scraper.
Withis probab lY
and it is
resonance neutrons
Thus for high
energies
we expect that
the radiation
level
a moderator
from the concrete
In the two Mark IV experiments
able with
giant
the predictions
resonance
and absorber
cross
in the neighborhood
the neutron
of a calculation
sections
mation A of shower theory,
results
agreement with
tion
loss into
flux
depth into
levels
curves given
for various
the concrete.
in Fig.
geometries
length
formula
loss
from Approxi-
is neglected.
Our
the measurements of Barber
shower theory
2 allow
after
taking
and to take account
one to estimate
and beam conditions,
This should
due to the sodium activity
and the like,
are compar-
and
ioniza-
account.
The build-up
neutron
of the scraper.
source strengths
in which ionization
George,3 who found good agreement with
lower
2 based on measured integrated
and the track
20
are in reasonable
would significantly
as a function
enable one to calculate
in the concrete
walls
of the attenuation
the thermal
of
radiation
around beam scrapers
in the concrete
of the
decay photons.
ACKNOWLEDGMENTS
It
is a pleasure
and assistance
In addition,
staff
with
to thank Dr. R. C. McCall and G. Warren for
the experiments,
and T. Shipman for
we wish to thank the Lawrence Radiation
at Berkeley
for
letting
us use their
-6-
thermal
counting
Laboratory
neutron
cavity.
discussions
the foils.
Health
Physics
REFERENCES
1.
H. DeStaebler,
SLAC-m-64-23,
"Radiation
Levels
March 1964.
in the Vicinity
2.
H. DeStaebler,
"Simple
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3.
W. C. Barber and W. D. George, 'Neutron Yields from Targets Bombarded
by Electrons,"
Physical Review l.&
1551 (December 15, 1959).
4.
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Bremsstrahlung
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DESY 64/11.
5.
W. S. Gilbert,
"Shut-down y-Radiation
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6.
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7.
R. L. Koontz and A. A. Jarrett,
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90
Shielding
of the Beam Scrapers,"
SLAC-TN-63-38,
von 4.8 Gev
'A Method of Calibrating
1958.
M. A. Greenfield,
--et al., "Measuring Flux Absolutely
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--
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10.
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11.
D. H. Martin,
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W. H. Hess, "Neutrons
13 *
C. Milone, "Energy Spectrum from Photoneutrons
Physical Review Letters 3, 43 (July 1, 1959).
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1957.
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14.
F.W.K. Firk and E. R. Roe, "Energy Spectra of Photoneutrons for Calcium
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Direct Interactions
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E. Clenentel and C. Villi
(ea.), Gordon and B&(63);
pp. 807-9.
15.
V. Emma, --et al., "Energy Spectra and Angular Distribution
of Photoneutrons from Carbon,ll Physical Review 1_1_8,1297 (June 1, 1960).
16.
W. Bertozzi,
Measurement of Photoneutron
--et al., "Time-of-Flight
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Physical Review 110, 790 (1958).
17.
from the
F.W.K. Firk and K. H. Lokan, 'Energy Spectrum of Photoneutrons
016
(7,n)
OX5,"
Physical
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321
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15, 1962!) .
Reaction
L. D. Stephens and A. R. Smith, "Fast-Neukron Surveys Using Indium-Foil
~~~~-8418, August 1958.
Activation,H
18.
19.
R. L. Bramblett,
--et al., "A New Type of Neutron
Nucl. Inst. and Methods 2, 1 (1960).
20.
B. Rossi,
21.
K. G. Dedrick and H. H. Clark, ttPhotoneutron Yield
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Prentice-Hall,
Inc.,1952.
from Excitation
of
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MACHINE
TUNNEL
(-
8 fi
x 8ft
)
FARADAY
CUP-
ACCELERATOR
BEAM+
-
_--_
NEUTRON
NEUTRON
DETECTOR
DETECTOR
*-----CADM!UM
COVER
APARAFFIN
SPHERE
<
INDIUM
FOII
( - 32
’ “THICK
)
j G’ldia)
(’ 0.005”
TH ICK, I .5”dia
211-I-A
FIG-Go.
MARK IV EXPERIMENT
No.2.
( NOT TO SCALE )
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