A NEUTRON DETECTOR HAVING UNIFORM SENSITIVITY
10 KEV TO 3 MEV
A. O. Hanson
J. L. McKibben
Los Alamos Scientific Laboratory
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A NEUTRON DETECTOR HAVING UNIFORM SENSITIVITY
FROM 10 KEY TO 3 MEV
By A. O. Hanson and J. L. McKibben
A neutron detector having approximately uniform sensitivity from a few kilovolts neutron
energy to a few million volts energy is described. The arrangement known as a long counter con-
sis of a paraffin cylinder about 10 inches outer diameter x 12 inches long surrounding a long boron
proportional counter. Sensitivity curves are given for two of the best arrangements. The response is
flat over the above range to about 10%.
A neutron detector which has a uniform efficiency for neutrons of widely different energies has
many advantages for certain types of measurements. A large water bath containing slow-neutron de-
ifors in some form fulfills this requirement and has been very useful in determining the number of
neutrons emitted by various neutron sources.' The examination of the number of slow neutrons as a
ffnction of the distance from the source in such a water bath gives additional information regarding
t ne~uergy of the neutrons.? But there are many experiments where the use of a large water bath is
ethlr awkward or gives erroneous results due to effect of the degraded neutrons reflected from the
into the experimental setup and yet a detector having a uniform sensitivity to neutrons is required.
S:Ta.rder to achieve a high efficiency in a detector of reasonable size an attempt was made to find
a suitablee arrangement of paraffin surrounding a boron detector. The analogy with the water bath ex-
pejiment suggested that a long boron counter embedded in a block of paraffin would have a counting
rate which would not depend much on the energy of the neutrons. The first detector constructed con-
sisted of a boron-lined ionization chamber 20 cm long surrounded by a cylinder of paraffin 20 cm long
andt em in diameter, this was used with its axis pointed toward the neutron source. Preliminary
tests on the sensitivity of this counter showed that the sensitivity was very nearly the same from neu-
tron energies of from 0.4 Mev to 2 Mev; these tests served to encourage the development of counters
along the same lines. This type of detector will be referred to as a long counter.
The theoretical treatment of the sensitivity of this type of counter is quite complicated and has
not been worked out. There are, however, certain qualitative arguments which may help in under-
standing the behavior of these counters and which may serve to suggest further improvements.
Let us examine first an arrangement in which a long thermal-neutron detector is imbedded in a
laige (semi-infinite) slab of paraffin and in which neutrons of various energies are incident upon this
slab in the direction parallel to the axis of the detector. The neutrons entering the paraffin will be
slowed down primarily by the hydrogen atoms to thermal energies and some of these neutrons will
be captured by the central thermal-neutron detector and will be recorded as counts in some manner.
If the neutrons have a very high energy, the mean free path of these neutrons is initially large and
therefore will be slowed down an appreciable distance from the front face of the slab. After a number
of collisions, the mean free oath will be reduced to such an extent that these neutrons have a veiv
small chance of escaping out of the front face of the slab.
is not the case, however, for neutrons
of the order of 100 kev or less since in this case the mean free path of the neutrons
does not change appreciably
as the neutrons approach thermal energies.? These neutrons would there-
fore have a much larger probability of escaping out of the surface of the semi-infinite slab as corn-
pared to the high-energy neutrons.
This effect is partially compensated for by the fact that the less
energetic neutrons would need to make fewer collisions before becoming thermalized, but the effect
would be such that the detection efficiency for high-energy neutrons would be several times greater
than that for very low-energy neutrons.
If the sensitivity for low-energy neutrons
is to be approximately the same
of high energy, some modification of this idealized arrangement must be made.
as that for neutrons
Therefore the suc-
of the first long counter must be ascribed to a fortunate choice of the dimensions of the paraffin
block such that the increased probability of escape of high-energy neutrons compensated in some de-
for the escape of low-energy neutrons from the front face.
The effect of the
size of the paraffin
cylinder has been investigated roughly and is indicated in a later section of this report. Furthermore,
the size of the central cavity made by the detector
gation of its effect was made. It will be shown, hou
is perhaps important although no systematic investi-
rever, that the introduction of additional holes in
the front face affected the low-energy sensitivity of one of the counters appreciably.
DESCRIPTION OF COUNTERS
Two counters were used quite extensively and will be described here in detail.
The first consisted
of a central BF3 proportional counter 1 inch in diameter, which had an effective length of 8 inches.
This tube was surrounded by paraffin cylinders 12 inches long and 6, 8, and 12 inches in diameter.
The counter with the 8 inch paraffin cylinder is shown in Figure 1.
slightly from the front face of the paraffin block but is protected fr
The proportional counter protrudes
rom direct thermal neutrons by means
of a cadmium shield.
The proportional counter
is supported by means of ceresin wax in the center of
an aluminum tube which also
as an electrical shield.
The body of the counter was a 1/32 inch
wall brass tube and was soldered to Kovar-glass seals.
The central electrode consisted of a 10-mil
The 1/4-inch intermediate electrode was used
as a guard ring and was connected to ground.
The counter was filled with enriched BF3(80 percent B'0) to a pressure of
25 cm Hg.
With -2700 volts
on the outer shell the proportional counter gives a gas amplification of about 10.
The signal was further
amplified by means of a model 100 linear amplifier with a R-C time constant of 5 microseconds and
the pulse was counted by means of a model 200 discriminator and scale-of-64 circuit5
Except in the
where impure BF3 were used, owing to leaks in the tube or filling system, the bias curves
(counting rate against minimum pulse height recorded) were such that a change in the bias voltage by
a factor of two in either direction would not change the counting rate by more than
rate was not affected by the radiation from an unshielded 500-mg radium source used at a distance
of 40 cm from the counter.
The other counter, which will be referred to
as the shielded long counter, is shown in Figure 2.
The principle modification being that an additional paraffin and boron shield
so as to make it
less sensitive to neutrons which have been scattered about the room.
The proportional counter is
similar except that it was 10.5 long, 1/2 inch in diameter, and was filled with BF3 to a pressure of
40 cm. For most of the measurements made with these counters it was convenient to use a matched
pair of the counters in the arrangement shown
In addition to increasing the sensitivity
of the overall system such an arrangement minimizes errors due to exact positioning of sources.
The data on the sensitivity of the counters to neutrons of various energies were obtained by 3
8 inch OD long counter.
1) By comparing the counting rates in the counters due to various radioactive neutron
sources whose total neutron yield had been compared by some other method such
as the water bath
Photo-neutron sources used were Sb-Be and Y-Be. Alpha neutron sources used were
Po-BF36 and Ra-Be.
The energies of these sources were taken to be 0.023, 0.16, 2.2, and
While the energy of the photo-neutron sources should be well defined the alpha neutron
give a spectrum and the values given represent only average energy.
The assignment of an
average energy to the neutrons from Ra-Be
is especially dubious since the energy spectrum of these
neutrons extends out to about 14 Mev.
The fraction of neutrons below 0.1 Mev, however,
to be less than 10%.
2) The degradation of the energy of the neutrons from a given source by surrounding the
source with spheres of graphite and heavy water.
A graphite sphere, 24 cm in diameter, was used which had the effect of reducing the average
energy of the neutrons by
a factor of about
The heavy-water sphere, 20 cm in diameter, served to reduce the average energy of the neutrons
by a factor of 4 or more. Since neither graphite nor heavy water absorbs neutrons appreciably, the
number of neutrons emerging from the sphere would be the same
as that emitted from the source. A
change in the counting rate with the sphere around the source
was therefore taken
as a measure of the
change in the sensitivity of the counter to the modified spectrum of neutrons.
The use of DIO was, of
course, limited by the fact that any source having sufficiently high energy gamma rays would give rise
to photo-neutrons from the deuterium. In the
of yttrium it was found that the number of photo-
neutrons from deuterium due to the high-energy ray (-2.8 Mev)
and hence could be accurately taken into account.
was only 3%
of that due to the Be
use of homogeneous neutrons of known energy from the Li (p,n) and D (d,n) reactions.
In these experiments the flux of neutrons into the counters were determined by counting the fissions
occurring in a standardized sample of uranium
known, and the flux measurement are consider
235. The energies of these neutrons are accurately
ed to be reliable so that these points should be quite
The summary of the data on the first counter with 6-
, 8- and 12-inch cylinders are shown in
Figure 4. Since very few data were obtained with the 6 inch cylinder, the curve
is sketched in only
to indicate the general trend of the sensitivity curve
as the size
of the paraffin cylinder is reduced.
It is seen that the 8 inch cylinder
the best approximation to a uniform sensitivity over the
region shown. Other tests with neutrons absorbable by cadmium indicated that the sensitivity of the
counter to thermal neutrons
was about 70 on the scale used in-Figure 4.
The sensitivity of this
counter would be somewhat effected by the arrangement in which it
is used. For most of the tests
described here the counters were used
as a matched pair
as the arrangement shown in Figure 3 so
as to reduce errors due to the location of the sources.
This pair of counters was used in the center
of a room approximately 15 by 20 feet at a height of about 50 inches above the floor. In spite of pre-
cautions to keep all other material
as far from the counters
as possible about 15% of the counting
rate in the counters was due to scattered neutrons when a Ra-Be source was placed at a distance of
1 meter from the front face of the counters.
The absolute sensitivity of this counter was such that it
would give about 1 count for every 10s neutrons emitted from a source placed at a distance of 1 meter
from the front face.
The shielded counter
is less sensitive to scattered neutrons by a factor of about 3 and hence
largely eliminates this objectionable feature of the 8 inch long counter.
The sensitivity of this counter
to high-energy neutrons is considerably increased owing to the large
of paraffin in the shield
and the sensitivity to low-energy neutrons is therefore relatively low.
The use of holes in the front
face, however, increases the sensitivity to low-energy neutrons to a sufficient extent so that the re-
spouse curve of the counter
as that of the previous counter.
The effect of these holes
Experimental arrangement with a pair of 8 inch OD long counters.
1 2 3 4 5
Sensitivity of 8 inch OD and 12 inch OD long counters.
and circled X's
represent points obtained with Li? (p,n) and D (d,n) neutron
Although the curves are not continued beyond 3 Mev the general trends of the curves
above this energy are indicated by the points obtained with the Ra Be source.
0 1 2 3 4 5
Sensitivity of shielded long counters.
work done to indicate the trend of the sensitivity curve in this range of energies. No doubt further
improvement in the response curve to both high-energy neutrons and to low-energy neutrons can be
accomplished by the methods described or perhaps by new methods.
The detectors described here have been found useful in many problems requiring a high and
approximately uniform sensitivity to neutrons of various energies. Some of the applications have
been the determination of the yield and angular distribution of neutrons from various (p,n) and (d,n)
as a function of energy, the preliminary measurement of neutron yield from various radio-
active sources, and the determination of the number of delayed neutrons accompanying fission.
1. Amaldi, E.,
L. R. Hafstad, and M. A. Tuve, Phys. Rev. 51:896,(1937).
O'Neil, R. D., Phys. Rev. 70:1, (1946
Bailey, C. L., W. E. Bennett, T. Bergstralh, R, G. Nuckolls,
H. T. Richards, and J. H. Williams,
Phys. Rev. 70:583,
Frisch, D. H., Phys. Rev. 70:589,
4. Designed by M. Sands.
5. Designed by W. Higginbotham.
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