Cloud-chamber energy measurement of photoneutron sources


Material Information

Cloud-chamber energy measurement of photoneutron sources
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
8 p. : ill. ; 27 cm.
Hughes, D. J
Eggler, C
Argonne National Laboratory
U.S. Atomic Energy Commission
Technical Information Division, oak Ridge Directed Operations
Place of Publication:
Oak Ridge, Tenn
Publication Date:


Subjects / Keywords:
Neutrons -- Measurement   ( lcsh )
Neutron sources   ( lcsh )
Cloud chamber   ( lcsh )
Ionization chambers   ( lcsh )
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )


Bibliography: p. 8.
"Date Declassified: July 9, 1947"
Statement of Responsibility:
by D.J. Hughes and C. Eggler.
General Note:
Manhattan District Declassified Code
General Note:
"Date of Manuscript: June 6, 1947"

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University of Florida
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Full Text


D. J. Hughes
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,"... D. JI, Hughes
S:.i- C, Eggler

Argonne National Laboratory

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This document consists of 8 pages.
Date of Manuscript: June 6, 1947
Date Declassified: July 9, 1947

This document is issued for official use.
Its issuance does not constitute authority
to declassify copies or versions of the
same or similar content and title
and by the same authorss.

Technical Information Division, Oak Ridge Directed Operations
Oak Ridge, Tennessee

MDDC 1082

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By D. J. Hughes and C. Eggler

SThe energy distributions of the neutrons from several photoneutro sources have been measured
0eans of the range distribution of recoil protons produced in a hydrogen-filled cloud chamber. It
l..wn that slowing down in the source itself produces a broad spread in energy instead of the al-
t monoenergetic spectrum to be expected from a single y energy. The values of the mean neutron
E, and the maximum energy, EM, for the sources measured are as fo'lows:

Source E (Kev) EM (Kev)

Na-Be 800 1020
Na-DO 220 320
Mn-Be 300 375
<150 < 150
SIn-Be 300 375
.. <150 <150

1 Sb-Be 35 68

As the EM values should be directly related to the energies of each of the y sources, they are com-
Spared to the y energies as reported by different observers for each source.

S- The advent of chain-reacting piles has made available intense gamma sources which can be used
Sto produce photoneutrons by the photodislntegration of the deuteron or of beryllium. Such photo-
L. neutron sources have proved to be extremely useful and convenient neutron sources in the region
3.: JO' Key to 1 Mev. If only a single gamma ray above the photodisintegration threshold is present iiid
VI.: .the amount of deuterium or beryllium is jery small, then the emitted neutrons show praclirally no
.spread in energy. Such a monoenergetic neutron source is, of course, much superior to the usual
S al (a) Be source with its great spread in neutron energy. Actually, in order to obtain sufficient in-
I tensityy, the amount of deuterium or beryllium which must be used is large enough so that significant
I:S, i .dration of the neutrons occurs by elastic scattering and a broad band of energies results.

S During the year 1944, Wattenberg' investigated the production and calibration of Ihotconeutrnn
f-ources, using various gamma emitters produced in the Argonne pile. The gamma sources were sur-
Irounded by amounts of deuterium or beryllium about one centimeter in thickness in order to obtain
."fiffcient intensity, and as a result the nearly monoenergetic neutrons emitted in the phutodisinte-
tkation were slowed down appreciably in the surrounding material. Wattenberg estimated the averageC
I MDDC 1082 1

MDDC 1082

energy of his sources by measuring the average scattering cross section of hydrogen for the neutraoni:'
of each source. The energy corresponding to the observed hydrogen cross section was then obtained '
from the curve of Bohm and Richman.2 The energies as determined from the hydrogen scattering ,
were always less than that expected from the gamma energy, thus showing that the moderator did:
have a definite effect in lowering the average neutron energy.
In order to aid in the understanding of the actual energy distribution of the neutrons emitted.b .
the sources, it was decided to measure the neutron energies by means of recoil protons in a hydrog- ,i
filled cloud chamber. In addition, it was hoped that investigation of the actual energy distribution
would show whether some of the sources consisted of several neutron groups instead of a single one,..
Some of the gamma sources had been reported as having more than one gamma energy above the
photodisintegration threshold, but it was impossible to ascertain from the average energy, as meas- ...
ured by the hydrogen scattering, whether multiple groups were present or not.

The main difficulty in the measurement of the neutron energies with the cloud chamber is due to
the fact that, while the sources are only moderate in neutron strength, they are intense in gamma
activity. Thus a source strong enough to give one recoil proton per expansion when placed about two .'..
feet from the chamber must be several curies in gamma strength. Such an intense gamma activity
near the chamber makes it rather difficult to obtain clear proton tracks. It was found, however, that
if the photoneutron source was surrounded by several inches of lead to reduce the gamma activity to .
some extent, and if the expansion ratio of the chamber was kept quite low (which emphasizes proton.
over beta tracks), it was possible to obtain quite sharp contrast between the protons and the intense
background of beta tracks. The presence of the lead does not change the neutron energy spectrum
appreciably, because energy losses for elastic scattering with lead are small and inelastic scattering :.
does not take place at photoneutron energies.
The apparatus is shown diagrammatically in Figure 1. It was placed on supports about 6 feet
above the floor, in a large room, to reduce the scattering of neutrons from walls and floor. The gam- *
ma source could be lowered by remote control into a large lead pot on the floor while the chamber
was being adjusted, film changed, etc; then it could be replaced in operating position and several hun- ..'":.I
dred pictures taken without the necessity of approaching the apparatus. The photoneatro source it-
self is of the same construction as those described by Wattenberg, that is, a tube 2 cm in diameter i
and 5 cm long containing the gamma source, located in a cylinder of beryllium or a deuterium-filled ":;:,
cylindrical can 3.8 cm in diameter and 5.1 cm long with a 2.2-cm diameter axial hole.
The cloud chamber, 30 cm in diameter, is very similar to that described by Jones and Hughes.P ...
For most of the sources measured it was filled with hydrogen gas and water vapor to a pressure of
82 cm of mercury. The stereoscopic pictures were taken with a standard mirror arrangement using
35-mm Eastman XX film and an f 3.5 lens. The light source was a xenon-filled capillary, flashed by
discharging a 50-gf condenser bank at 2000 volts. The xenon lamp, a General Electric "Flashtube >'i
FT 26," has proved to be an extremely convenient cloud-chamber light source. ,i
The lengths of the recoil protons and their direction of motion relative to the incident neutroaisl;,':"
were determined by a stereoscopic reprojection of the negatives. The proton ranges in standard :m;:..i.::a,
were obtained by comparison with the observed range in the chamber of the alphas from a plutoni di*g -,.i
source mounted in the chamber. In the comparison, a correction was made for the fact that the ast p .-
ping power of the chamber for alphas relative to protons is a function of the proton range. The prti"!
range in air was converted to energy, using the range energy curves given by Livingston and Bethea*'
and the neutron energy calculated from the proton energy and the angle, 0, between the neutron and i
proton direction. The energy spectrum of the neutrons from the source could then be plotted from:':
the observed numbers of recoil protons as a function of energy.1 A correction must be made, of c.i
for the effect of the change in the scattering cross section of hydrogen with neutron energy, but beu ; i~
cause of the nearly monochromatic nature of the sources such a! correction is small.
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MDDC 1082

SCloud Chamber

Pu Alpha

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Figure 1

Photo- Neutron Source


Lead Shield
.BBe or DO0 Cylinder
Sy Ry Source

Brass Tube

Lead. Coffin for
Source Storage

If no slowing down takes place in the deuterium or beryllium of the source itself, then the energy
spread in the emitted neutrons should be very small. The energy spread in this ideal case is caused
by the difference in direction of the neutron and the gamma ray and is given by

I, =E c 2(A-1) (E y -Q
931 A

Where I is the energy spread, Ey the gamma energy, A the mass of target nucleus, and Q the thresh-
old energy. The value of this energy spread is usually of the order of one per cent, which is much
S: less than the energy spread caused by slowing down in an actual source. Because of the low atomic
Weight of deuterium and beryllium, they are excellent moderators and distort the neutron energy spec-
Strum even when present in small amounts. The resulting energy spectrum is expected to have a max-
imum neutron energy corresponding to that given by the gamma energy but an average energy some-
:;. what lower. The actual energy spectra were measured for several photoneutron sources of interest,
:. ld the findings will be discussed for each source.

The spectrum obtained from 106 recoil protons which were within 30 of head-on collision is
shown by the diagram of-Figure 2. It is seen that the distribution, while showing no great spread in similar to a Ra Be source, is by no means monoenergetic. The spectrum is, of course, dis-
orted to some extent by errors of measurement. However, if only those recoils are chosen which
Aire" whin 20' of head-on, the spectrum does not narrow appreciably, the width at half maximum,
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4] MDDC 1082

which is 33% of the maximum energy in Figure 2, changing only to 31% in the 20 case. The true "'
width is probably slightly less than the latter value, say about 25%

20 -




Jr '4* A
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0.2 0.4 0.6 0.8 1.0 1.2

E, Mev*
Figure 2
The most probable neutron energy from Figure 2 is about 825 Kev, while the arithmetic mean
energy is 800 Kev. Wattenberg finds a mean energy for this source from the mean hydrogen scatter-
ing cross section of 830 Kev. It seems then that the spectrum of Figure 2 is a good representation al
the actual neutron distribution of a source, made according to Wattenberg's method. i
The maximum energy in the spectrum can of course be identified with those neutrons which ate i.
unmoderated and which should therefore correspond to the energy calculated from the Na gamma en-."
ergy. The Na gamma energy has recently been measured as 2.76 Mev by Siegbahn,5 in agreement
with an earlier measurement of Elliot, Deutsch, and Roberts." Other measurements have given vle.
for the energy ranging as high as 2.94 Mev. The maximum neutron energy to be expected, assume .:
Ey to be 2.76 Mev, is marked as EM in Figure 2. Considering the inevitable straggling of the exp" ..
mental points, the agreement between EM and the observed upper limit is quite satisfactory. A gai ,
ma ray has been reported of energy higher than the 2.76-Mev gamma, but no tracks were found In-t4
present spectrum (or in the Na-D2O spectrum) to indicate the presence of such a gamma ray. If it
exists, it does not contribute an appreciable number of neutrons in the photoneutron source.

The spectrum based on 75 recoils is shown in Figure 2. The theoretical maximum, based
13-223-p .4ad

MDDC- 1082 [ 5

gamma energy of 2.76 Mev and a D1O threshold of 2.18 Mev, is indicated, and it is seen that the max-
Simum neutron energy corresponds quite well with the theoretical value. The most probable value of
the distribution and the average are at about 220 Kev (70% of EM), which is the same value as the
S mean energy which Wattenberg finds for this source by the hydrogen cross section method. The width
Sof the energy distribution at half maximum for the Na-DO source is 20% of EM.

M ln-Be
S Gamma-ray measurements',a for Mn5s had shown two gamma rays of energy 1.81 and 2.13 Mev.
As both these energies are higher than the threshold in Be, one would expect two groups of photo-
neutrons. Wattenberg actually found from the scattering cross section of hydrogen that the photo-
'eutrons he observed were due to a gamma ray of energy 1.83 Mev. (He also found an extremely
i:weak group of photoneutrons in deuterium which would indicate a 2.7-Mev gamma). As his method
SOf estimating energies from the hydrogen scattering cross section gives only the mean energy of the
neutron group, it was impossible for him to say if there were any neutrons from the 2.1-Mev gamma.
4 The energy spectrum of the Mn-Be neutrons was measured in the cloud chamber to determine if
only one group of the neutrons was present, as seemed likely from Wattenberg's result, or if two
groups were present, as would be indicated by the gamma energies. The results are shown in the
lower curve of Figure 3. It was found that two groups of neutrons were definitely present (no effort
was made to study the small number of neutrons which would be caused by the 2.7-Mev gamma). The
'. energy of the most abundant group was too low to be measured accurately, as the recoil protons were
of such short range. The higher energy group has an average energy of about 300 Kev, and it is pos-
S sible to estimate the energy of the gamma ray causing the group. The maximum energy EM is chosen
as 375 Kev, by taking EM slightly less than the apparent maximum energy in analogy with the spectra
a:. of Figure 2. A value of 375 Key for EM then gives 2.05 Mev for the gamma-ray energy. This energy
C.. is somewhat less than the earlier, value 2.13 Mev but agrees extremely well with a recent determina-
... tion of 2.06 Mev for this gamma ray by Siegbahn." It is definite then that the 2.06-Mev gamma pro-
Sduces photoneutrons in addition to the 1.81-Mev gamma.
i The relative numbers of photoneutrons in the low and high energy groups are about 90% and 10%,
:: respectively, as determined by counting recoil protons and correcting for the change of hydrogen
S cross section with energy. Because of the low intensity of the high energy group, it was not indicated
*.. as a discrete group by Wattenberg but it probably increased his average energy slightly. Thus the
1.83-Mev gamma-ray energy that he inferred is probably high for this reason. Siegbahn'sg recent
Determination of the low energy gamma is 1.77 Mev.

Indium is of doubtful value as a gamma emitter for photoneutron sources because of its short
half-life (54 minutes). However, if only one group of neutrons were present, then the difficulty caused
i'. by the short half-life would not be insurmountable. Gamma-ray measurements had indicated energies
of 1.8 (spectrometer) and 2.3 Mev (cloud chamber) for those higher than tne Be threshold, so it was
S decided to measure the photoneutron spectrum to see if several groups were actually present. The
i spectrum obtained, shown in the upper part of Figure 3, contains two groups of neutrons, one of mean
energy about 300 Kev and a group of energy too low to be measured accurately (about 100 Kev). The
..higher energy neutron group comprises 59% of the total and its EM for Na indicates a 2.1-Mev gamma
:.,.4tProi indium. The low energy group cannot'be measured accurately, but it indicates a gamma of rough-
i~i" ly 1.8 Mev. It seems, therefore, that both energies are present in indium and in such intensities that
;" they give roughly comparable groups of photoneutrons. Because of the presence of the two neutron
.,;. groups, the value of In Be as a photoneutron source is much reduced.

f:.. : ":

.,. The highest energy gamma ray from Sb has been reported'0,' as having energies ranging from

6 ] MDDC 102


10 -..2.

41% 59% -

,1 .2 .3 .4 .5 Mev

Mn-Be '


-5 n e
90 % 10/ .3/

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.1 .2 .3 .4 .5 Mev **
Figure 3

1.70 to 1.82 Mev. This discrepancy in the gamma ray is large enough so that the resulting discrepanc:r.
in the photoneutron energy is quite serious. Shariff-Goldhaber and Klaiber'" measured the energy a(
the photoneutron from an Sb-Be source and found EM to be 115 Kev, which would indicate a gamma -
energy of about 1.75 Mev. Wattenberg's value for the average energy of the photoneutrons is 24 .Ke, .4
from which he obtains a gamma energy of 1.67 Mev. The spectrum of photoneutrons was studied with: ::'
the cloud chamber mainly to investigate this rather large discrepancy. Because the neutron energy
is so low, the cloud chamber was operated at the lowest possible pressure, to increase the range .f
the protons to a measurable value. The chamber could be operated with hydrogen at a pressure of
about 8 cm, using water as the vapor. Under such conditions the range is about 25 times the range: :..
air, and a 20-Key proton, which would have a range of only 0.3 mm in air, will have a range of almost
a centimeter in the chamber. .
The source strength of Sb-Be is very low, so it was possible to obtain only 20 recoil protaons.::
caused by nearly head-on collisions. The distribution obtained is plotted in Figure 4. In spite i.
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eXtremely small number of tracks, it appears that a single group of neutrons of average energy of
ji bout 35 Kev is present. This value is in rather good agreement with the mean energy of the photo-
neitrons of 25 + 15 Key measured by Wattenberg. Because the neutrons are of such low energy, it
Should be possible to use the photoneutron energy to determine the gamma energy rather accurately.
:' Unfortunately, the straggling at the upper end of the measured spectrum of Figure 4 is quite large,
iand EM cannot be determined very accurately. However, EM seems to be within the range shown,
that is, 68 1 11 Kev. The value of the gamma energy resulting from this EM is 1.707 .012 Mev. The
Serror in this determination of the gamma ray is actually somewhat greater than 12 Kev, because the
: error in the photoneutron threshold should be included, as well as errors in the determination of the
Stopping power of the chamber gas. Inclusion of these two errors would probably increase the error
-i: the gamma energy to about 20 Kev.

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0 30 40 50 60 70 80 Kev

Figure 4

The gamma energy determined from the present experiment agrees very well with the latest
H .direct gamma energy determination of Kruger and Ogle, which gave 1.70 1 0.02 Mev. It is somewhat
blger than the value 1.67 Mev which Wattenberg obtains from the mean energy of the photoneutron
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8] MDDC 1082

We wish to extend thanks to Dr. A. Wattenberg for his help in preparation of the sources use: .'j
and for valuable discussion, to T. Brill for the design of cloud-chamber control circuits, and to t.1
Goldstein and S. Joshemski for aid in taking data. .

1. Wattenberg, A., Phys. Rev. 71, 497 (1947).:
2. Bohm, D., and C. Richman, Phys. Rev. 71, 567 (1947). :
3. Jones, H., and D. Hughes, Rev. Sci. Inst. 11, 79 (1940).
4. Livingston, M. S., and H. A. Bethe, Rev. Modern Phys. 9, 245 (1937).
5. Siegbahn, K., Phys. Rev. i
6. Elliot, Deutsch, and Roberts, Phys. Rev. 67, 273 (1945).
7. Deutsch, M., and A. Roberts, Phys. Rev. 60, 362 (1941).
8. Elliot, L. G., and M. Deutsch, Phys. Rev. 63, 321 (1943).
9. Siegbahn, K., Ark. Mat. Astr. Fys. 33A (2), Paper 10 (1946).
10. Kruger, P. G., and W. E. Ogle, Phys. Rev. 67, 273 (1945). "
11. Mitchell, Langer, and McDaniel, Phys. Rev. 57, 1107 41940).
12. Schariff-Goldhaber and Klaiber. Phys. Rev. 61, 733A (1942).

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