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UNITED STAtTES ATOMIC ENERGY COMMISSION
THE ABSOLUTE MEASUREMENT OF THERMAL NEUTRON DENSITY
H. L. Anderson
P. G. Koontz
J. H. Roberts
This document consists of 4 pages.
Date of Manuscript: August 7, 1942
Date Declassified: January 2, 1947
This document is for official use.
Its issuance does not constitute authority
for declassification of classified copies
Lthn colic t1.' CiaLtlcia uitACUltflL Alu LILAC1
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THE A13SOLI TE MEASUREMENT OF THERMAL NEUTRON DENSITY
By H. L. Anderson, P. G. Koontz and J. H. Roberts
A method for dclcrr.:-.jng the absolute value of the thermal neutron density is described. For
measurements in a be.am of thermal neutrons, a boron trifluoride proportional counter filled with a
known amount of BF3 gas is used. For measurements inside a medium in which thermal neutrons are
diffusing, MnO2 detectors are used. These detectors were standardized against this BF3 counter in a
slow neutror beam. For the MnO, detector which we used (1.05 grams on an area 5 x 6 cm2 enclosed
in scotch tapt and mn as;-rd by wrapping around a 0.011 cm wall dural counter), the number of neu-
trons per second per em~ traversing the detector is 0.76 x the saturation activity of the detector in
counts per mir.ute.
STANDARD BORON TREFLUORIDE COUNTER
The capture of slow neutrons by boron leads to the process
sB'" + ,n ,Li' + =He'
By using a proportional counter filled with boron trifluoride gas and suitable cadmium shielding, it is
possible to record all the disintegrations by thermal neutrons which take place in a known volume of the
gas in the counter. The cross section of boron for the capture of thermal neutrons has been determined
by Anderson and Fermi in Report C-74 by measuring the absorption by BF- gas of the thermal neutrons
emerging from a graphite column. The same gas was taken to fill the proportional counter as. was used
in the absorption measurements. In the present use of a BF, proportional counter to make an absolute
measurement of thermal neutron density, it is assumed that all the thermal neutron captures by boron
lead to disintegration which will be recorded by the counter.' If this is the case, then the number of
disintegration observed per second when the counter is placed in a beam of slow neutrons will be given
=nvMo B (v) (1)
where nv is the number of neutrons which pa;s through 1 cm2 per second, M is the number of moles of
boron in the active volume nf thpe rnl ,tcr ra i,- t.d .- r.... C cic f .r per -- l for
neutrons of velocity v. The neutron flux nv traversing the counter may be obtained by measuring y
provided M is known, since for slow neutrons U B- 1, v, the product nv0 B (v) is independent of the par-
ticular.velociry distribution of tue neutrons. For neutrons having the average velocity of the Maxwell
distribution v, the value of UB (v) is taken to be 411 cm2 per mole.
*Some doubt may be thrown on this point by the work of Maurer and Fisk, Zeit. f. Physick 112:463
(1939), who reported a group of low energy protons which might escape detection.
2 1 MDDC 879
The standard BF3 counter which was used consisted of a nickel tube turned down to a wall thick-
ness of 0.025 cm; its internal diameter was 3.62 cm and its length was 30 cm. The counter was filled
with BF1 gas so that there were 7.31 x 10-5 moles per cm of counter length. The counter was com-
pletely covered with Cd except for an opening of 10 cm along its length in the center of the counter.
The absorption of the counter %all was determined before assembling the counter, by slipping the glass
and nickel tubes over a smaller BFH counter placed near a graphite pile from which thermal neutrons
were emerging, and observing the decrease in the counting rate. In this way the absorption factor of
the counter wall was found tco be 1.13. The effective cross section of this counter per cm of length for
neutrons having the average velocity of the thermal neutron distribution is equal to 0.0266 cm .
The BF, counter w;.s connected to a high gain linear amplifier and the disintegrations were counted
with the use of a scale C 16 i E ,:roer. In Figure 1 is shown the variation of the counting rate as a
function of voltage of the co,,ntLi. Curve I shows the results with no Cd in front of the 10 cm window.
This curve does not exhibit a very satisfactory plateau, principally due to the disintegration taking
place near the ends of the counter Lnd due to fast neutron recoils, both of which give too small pulses
at the lower voltages. The lilference in the counting rate taken without and with Cd in front of the
window is plotted a Curve i. Tnese counts are due only to C neutrons* which enter through the Cd
window. The platau exhibited by this curve is evidence for the fact that all the disintegrations due to
C neutrons taking place in tle counter ire recorded.
Figure 1. Variation of c,,uinuE; l ite with voltage of BF, cuunier in a neutron beam.
Curve I -Measurements with no Cd. Curve II-No Cd-Cd difference.
C neutrons are those slow neutrons strongly absorbed by Cadmium. The energy distribution of the
C neutrons which emerge from paraffin has a more pronounced high energy tail than does the thermal
_.a n .
I Figure 2. Apparatus for determining the
-i 10 CMk -, counting rate of BF, counter in neutron"
S BF3 COUNTER| beam. In standardizing the MnO2 detec-
S32a CM tors, the BF, counter was removed and a
B4G SHIELD-_ .. detector placed 32.4 cm above the top of
\ CM the paraffin in a position where the aver-
> LEAD age neutron was the same as that meas-
,/ ured by the counter.
SOURCE / *
STANDARDIZATION OF MANti',NESE DIOXIDE DETECTORS
Such a ooron counter is quite useful for the measurement of the neutron flux in a beam. For
measurements of the neutror density msile -- medium in which neutrons are diffusing, the counter may
not be used J it is large, for its presence w.11 perturb the neutron distribution in its neighborhood. In
order not to perturb the neutron distribution, we have used thin layers of MnO, as detectors of thermal
neutrons inside water or graphite. The cross section of these manganese detectors was obtained by
comparing their activity with the counting rate of the BF3 counter in a slow neutron beam.
The detectors were prepared by pressing a thin uniform layer of about 1.05 gm of MnO, powder
in a steel die by means of a small hydraulic press. The area of the layer was 5 x 6 cm2. The layer
was covered on both sides with scotch tape. To facilitate w rapping around our counters the foils were
ruled with parallel impressions spaced 1/8 inch apart. The foils were fitted snugly around the counter
For the comparison of the MnO.. detector with the standard BF, counter, we used an arrangement
which is shown in Figure 2. A 1 gram Ra+Be source was surrounded by paraffin and suitable lead
protection was provided to reduce the number of > -rays. The center of the counter was 31.4 cm above
the top surface of the paraffin and to enter the counter the neutrons from the paraffin had to pass
through a circular opening i.1 a boron carbide shield. The diameter of the hole was 13 cm. This
opening could be covered with a Cd sheet and measurements were always taken as the difference no
Cd-Cd so as to make sure that only C neutrons emerging from the paraffin through the 13 cm opening
were being recorded. This paraffin geometry was used for reasons of intensity. It might be thought
that a beam of truly thermal neutrons. such as could be nhla inPe 11ina granhitp wnould ho nrofrPrnh1p
Insofar as the absorption of manganese and boron both, presumably, vary according to l/v, the com-
parison would give the same result in both cases.
It should be pointed out that indium detectors cannot be compared directly with a boron counter
using the paraffin geometry of Figure 2 due to the presence of the strong resonance line of indium at
1.35 ev. The presence of this resonance line causes a deviation in the I 'v absorption law for indium
below the Cd cutoff. For measurements inside a diffusing medium, the effect of the perturbation due
Chemical analysis of this material showed Mn content to be 58.0% due to the presence of oxides
of manganese other than MnO,.
to the resonance activation of indium is much less, since inside water or graphite the ratio of the
density of thermal to resonance neutrons is much greater than m a slow neutron beam. If .45 gm 'cap
of Cd is used for determining the contribution of the resonance neutrons to the activity observed with-
out Cd, then the difference n the :tivation no Cd- 1.07 Cd-In-Cd is a quite reliable indication of the
thermal neutron density. Indium "2'tectors may be standardized against manganese detectors pro-
vided the comparison is male inside the same medium in which it is desired to make absolute thermal
neutron density measurements with the indium detectors.
With the geometry Af Figure 2, the effective length of the counter is somewhat longer than 10 cm
due to the obliquity of the neutrcns which traverse the opening in the Cd. To take this obliquity into
account, we have taken as the effective length of the counter 10.46 cm. We observed a counting rate
no Cd-Cd, 885 counts per minute. The manganese detectors were placed at the position of the diam-
etral plane of the counter and were displaced slightly from the center of the opening in the cadmium
shield of the counter, so that the average neutron intensity falling on them would correspond most
closely to that falling on the opening in the cadmium. The activity of the MnO, detector was measured
on a thin wall dural counter (Kanga).* The saturation activity no Cd-Cd was found to be 69.8 counts
per minute per gram of MnO2. From these data we may calculate that the apparent cross section per
gram of the MnOa detectors is given by
r, ',v) = 69 x 0.266 x 10.46 = .0"2 cm., gm (2)
It is clear that thi: cross section refers to the particular MnOO detectors measured on the particular
counter we used. The absolute value of the neutron density inside a medium in which thermal neu-
trons are diffusing -nay ')e (eterminec b/ irradiatirj .N MnO2 detector without and with Cd, and meas-
uring its saturation activity Ath (Mn) = no Cd-Cd, in counts per minute per gram on the same counter
as was used in standardizing it. Thus,
Ath (Mn) neutrons
nv -60022 = 0.76 Ath Mn) sec xcm (3)
60 x .022 sec x cm
A correction should be applied for the lowering of the neutron density in the neighborhood of a
detector, due to the absorption by the detector. In the Mn02 detectors which were used this amounts
to about 2% in paraffin or water and is negligible in graphite.
*The counter Kanga, which was used, was a dural counter with a wall thickness of about 0.011
cm of 2 cm diameter and a sensitive length of bout 7 cm. Its construction will be described in a
forthcoming report. The efficiency of the counter for 3-rays of Ra E and UX, was found to be .181 and
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3 1262 08910 5661
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