Table Of Content..I
Z UNITED STATES ATOMIC ENERGY COMMISSION
AECD-3 158
FISSION SPECTRUM
BY Work done by
F. Bloch F. Bloch
H. Staub M. Hamermesh
D. B. Nicodemus
H. Staub
D. C. de Vault
August 18, 1943
Los Alamos Scientific Laboratory
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Technical Information Service, Oak Ridge, Tennessee
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ABSTRACT
Measurements of the spectrum of the fission neutrons of 25
are described, in which the energy of the neutrons is determined
from the ionization produced by individual hydrogen recoils. The
slow neutrons producing fission are obtained by slowing down the
fast neutrons from the Be-D reaction of the Stanford cyclotron.
In order to distinguish between fission neutrons and the remaining
fast cyclotron neutrons both the cyclotron current and the pulse
amplifier are modulated. A hollow neutron container, in which
slow neutrons have a lifetime of about 2 milliseconds, avoids the
use of large distances. This method results in much higher in-
tensities than the usual modulation arrangement. The results show
a continuous distribution of neutrons with a rather wide maximum
at about 0.8 MV falling off to half of its m&imum value at 2.0
MV. The total number of neutrons is determined by comparison
with the number of fission fragments. The result seems to indi-
cate that only about 30$ of the neutrons have energies below .8
MV. Various tests are described which were performed in order to
rule out modification of the spectrum by inelastic scattering.
This document is for official use.
Its issuance does not constitute authority
for declassification of classified copies
of the same or similar content and title
and by the same authors.
Reproduced direct from copy
as submitted to this office.
This report is based on LA-17, LA-lTA, WC-934.
.
AEC Oak Ridge, Tenn 12-5-51-33 5-W43 06 I
FISSION SPECTRUM
The neutrons emittesd in consequence of the fiesion of U25 haw
been : tudisd by various investigatorso Zinn and Seilard') have recorded
the proton and ci> reooils in an ionization chamber. Proton recoils from
fission neutrons have also been observed by Chadwick') in a photographic
8n-\ul!;~.Onan d by Bonnet+ and Bichrds" 2.a a Wilson cloud chamber. A rough
determtmtion of their enerGy has furthermore been obtained by Christy and
&nlny4) from the absorption ooav fficient in Bakero Tho results of'
C1i~ci;~+.;.afr: give a distribution wi-th tho InJlximum at 2 LIV and a half width of
abo~r:;1 ,3 I@?. Cnu absorption data yield an "sffective" energy of 2.2 W
and ,:x tracks ctbaerved in tie cloud chamber originally seemed to indicate
a c.3 ?iderablo number of nod%ro2s even above 4,s W0
The object of the present investigation was t.j redetermlns the
x
ener,: distribution of the fission neutrons of U25,, miking use of tho
c
,.
cycloxon a8 an abundant source of pirnary neutronso This h5-s %ha obirioue
ad~7ail;~goov er the method8 previously apglied to obtain good 8tQtistics
wj.61. L 1. short running times and to avoid the cumbersome geometrical
63c2.i. ~TIcesn ecessitated by low intensities. On the other hand. because
of great number of primary rieutrons from the cyclotron twrgei; with
I ..e
/
arm 108 in the same range aa those of tho secondary fission neukirons Lo
be o1?s3rved, G. rather intricate arrangement and method of r6COrding hAd
Co iis devised,
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The method of detection is essentially the same as that used by
Zinn and Szilmdl). The fission neutrons, emerging from a disk of tub-
alloy, produce proton recoils in a hydrogen-filled ionization chamber. The
ionization pulses of these recoils me amplified and those lying between -
an enera E and E+AE are selected and recorded. Assuming for the moment
k .
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that the range of the recoils is small compared to the linear dimensions - 0
of the chamber, so that its wall effects can be neglected, and letting 0
*
R(E) be the number of recoils in the chamber per unit energy and time, we
find the number N(E) of fission neutrons per unit energy and time emerging
from the disk to be
- E m
(11 N(E) =
dE
SDh (/I)
where S is the effective fraction of solid angle under which the recording
volume of the charnber is seen from the disk, D its effective depth, and
h the number of hydrogen atoms per unit volume, (p) is the scattering
cross section of the proton for a neutron of energy E. It is furthermore
assumed in (1) that, over the investigated range of energy, the neutron- Q
proton scattering is isotopic in a reference system moving with the
c
center of gravity of the two particles. This crom section is sufficiently
well known, both from experiment and theory51, and with all the other
quantities in (1)g iven, the problem reduces to the determination of the
recoil distribution R(E).
The source of primary neutrons was a Be target, bombarded with
a beam of 2.6 MV deuterons from the Stanford cyclotron. In order to
eliminate in R(E) all those recoils due to primaries, the neutrons from
.’
5 ) See paragraph 10 of the Handbook, L. A. 11, and the more detailed
discussion of an unhumbered Berkeley report by Bohm and Richman,
Dec. 21, 1942, catalogued in the L. A. library as UCRL-320.
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the target were slowed doh in paraffin and by the well known method of
delayed modulation of source and detector') only those effects due to
WrelY thermal neutrons were observed. Since thermal neutrons cannot
induce fission of the isotope u28, our investigation of the fission spectrum
ref ers to U25 alone.
Before entering into a more detailed description of OW method
it is necessary to discuss the essential determining factors. Since it
was to be foreseen that the spectrum would extend well above 2 MV, it was
important that the gas in the ionization chamber should have the relatively
high stopping power of &our 8 times that of' air. This requires a con-
siderable pressure of the gas which, with a feasible electrode spacing and
collection voltage, leads to collection times of the Tom in the neighbor-
hood of one millisecond. It is true that the collection time could be
considerably shortened by collecting unattached electrons; since,
however, the induced voltage pulse in the chamber is proportional to the
average distance from the collecting electrode to the region where the
electrons are liberated, recoils of the same energy would give pulses of
different size, depending upon their position relative to the next
positive electrodes. This would spoil the resolution to such an extent,
that electron collection must be deliberately avoided and care must be
taken that not only the positive but also the negative charges collected
are attached to heavy ions which have approximately the same collection
.
time
.I
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6) L. W. Alvarez, Phys. Rev. 54, 609, 1938.
C. P. Baker and R. F. Bacher, Phys. Rev. 2, 332, 1941.
i 1
With the oonparalively poor resolution which one get8 with B
oollection time of one millisecond it becomes necessary to work with the
oonsiderable time lag of' several milliseconds between the moment at which
%he aourae cm886 and that at which the detector starts to operateo It
was found that the primary f'ast neutrons together with the oapbre E. .
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x-rays give during the "on" period of the souroe an aooumulated pulse
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of the order of 100 W; although this pulse was largely neutralized by
a feed-baok oirouit, it skill rnras nreaesssry to waft about 6 millisecond8
after "turning:"off"t he sowoe before it oeased to interfere with %he
recording of the oomparatively few and small reooils from fission neutrons.
During thie time a neutron of 1/80 V energy would travel 15 merixfrs; to
wor~a t suoh distanoes f'rm the target would wuse an excessiwe loss of
intensity and we had $0 use a different arrangement to maintain good
intensity in spite of the neaessary time 1aG.
"hie armwment oonsists eseentially of a hollow container
z
dP;h walls havirg an albedo as close to unity as possible into whioh
Q
slow neut9-ons enter from the oydotron. Let 3 be the average mber
of times a neutron of velooity w o m be refleobd from the wall without a
being captured and let the linear dimensions of the oonfainer be L; then
it is obvioue that after a tin9 €3 o 53 &am their entering, the
V
density of slow neutrons inside, the oontaincsr ell saptill be of the orio
ginel order of xtgnitudeo 3 is oonnected with the mean number N of free
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paths which a neutron Immerses in the reflsuting substanue before
W
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For paraffin it is N 2 150, d g 6 SO that with Ls 100 om,
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TT Y 2.5~10 cm/sso one should expect a dehy time Q z 2 ~ 1 0 ~se'e ;
if instead graphite With f3g 1000 is used, ~e delay time oan for the
same dimensions be increased by a faotor 2.5.
A more quantitative theory of thu contaimr c9n be Given by
. -
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solving the diffusion equation
:
I
p
for the density of neutron8 with velooity v and mean f'ree pat?i 1.
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Assuming all radii of ourvature of the wall to be (large compared to
fil,
and its thioknesa large compared to one om easily 886 that
while no neutrons enter the oontainer the density inside will be uni-
form, and as a funotion of tine will dsorease aooording to
vt
(4) p = f o e -3L
where
wi%hV and A standing for the inside volume and wall are8 of the oon-
iner ree veo tivelye
For a uontainer with .swllrs of unifom material the quantity
in (4) fa given by (2). In our a88 the wall material aas aot mi-
fom but oonsfeted of oil in tin uans (this having the atme albedo a8
paraffin) lined on the inside with 12 om of graphite. TNs lining did
sot cause an exoeaeive u88 of graphite and yet helped oonsiderably to
incream the albedo. One obtains In this case of a lined wall, instead
1
-8-
where in our case N1 and prZ stand for tho numbex of f h s paths bofdre
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oapture in graphite and oil respeotively,& is tihe rnin ?me path in
I graphite and A the thickness of lining. Takirg &s 2 om, one obhin8
A = 12 cm, 3 = 9.5# i.8. inorease of the delay time 2 OVOT khat n:.";houf
v
lining by about a faotor I&*
For the discussion of the most suikble modulation sohone and
dimensions of the container it is best to note the ratio of slow Yreta-kmne
striking a body of' surfbe S per unit .time (averaged over Q full ~~~06uh.-
tion period) to those entering u e container per unit xhi?~ti w
soupce is in operation. It is given by
time of the container becones
