ATOMIC ENERGY COMMISSION
OF THE DELAYED NEUTRONS"
Arthur H. Snell
V. A. Nedzel
H. W. Ibser
M. B. Sampson
Metallurgical Laboratory, University of Chicago
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"* A '0
OF THE DELAYED NEUTRONS
The Decay Curve and the Intensity of the Delayed Neutrons
Arthur H. S
Wilkinson*** and M. B. Sampson***
University of Chicago.
The delayed neutrons resulting from slow neutron fi
ssion of uranium
have been found to decay as a combination of exponentials with half
23 and 56
seconds and respective initial relative intensi
ties when activated to saturation of 0.4
, 0.5, 1.1,
1.0 and 0.14.
equilibrium conditions the delayed neutrons are 1.0 +
as the instantaneous fi
Now at Clinton Laboratories
, Oak Ridge,
Now at Los Alamos
P. O. Box 1661
+ Now at Univer
++Now at Physic
*** Now at Physi
sity of Wisconsin,
s Department, Co
Lcs Department, In
rnell University, Ithaca
idiana University, Blool
Molte than a year before the first nuclear chain reaction was initiated
on December 2
, the importance of the delayed neutrons in controll-
ing the reaction was foreseen.
In a letter to S. K. Allison, E,
ed out the necessity of knowing how many of the fission neutrons were
and suggested how this could be measured experimentally.
work was undertaken, using the University of Chicago cyclotron, and it
culminated in a Metallurgical Laboratory report by Snell, Nedzel and Ib-
ser, the results of which have since been quoted in the Smyth report. As
a part of the intensity measurement,
delayed neutrons was required. Whi
adequate at the time,
a study of the decay curve for the
le this was accomplished in a man
subsequent work in the same laboratory showed t
the analysis of the short periods had been incomplete.
It is the purpose
this paper to state the revised analysis of the delayed neutron decay cur
and to describe the intensity measurement more fully than was done by
The Decay of the Delayed Neutrons
The earlier work on this
and Wang2(decay period 12.5 .
seconds and 45 seconds)
subject was that of Roberts,
, Dunning and Slack3 (10
)f Gibbs and Thomson4 (no strong delayed
neutron periods.between 0.001 and 0.1 sec),
Lauritsen5 (12.3 and 0.1
and of Bostrm
- 0.3 second activities).
Snell, Nedzel and Ibser
used 106 lb of U308 arranged in a hollow shell which was surrounded wit
paraffin and placed about a foot from the beryllium target of the cyclotrc
. A boron trifluoride proportional counter was placed inside the
shell, and a thickness of 2 inches of paraffin filled the space between thl
counter and the U308.
P A of 7.5 Mev deut
The maximum cyclotron beam on the target was
:erons. Readings were taken by photographing siF
multaneously the mechanical recorder actuated by the scaler,
interpolation lights, a stopwatch which read to 1/100 second, and an elec
activities with half
. The clearest result was the appearance of two well-defined
-lives of 24 sec and 57 sec respectively.
saturation intensity, the 24 sec activity was about eight times as strong
as the 57 sec activity.
The earlier part of the decay curve could be ac-
counted for by activities with half-lives of 7 sec and 2.5 sec.
but the ana
was not definite enough to enable one to have confidence in these as
In later work upon the shorter periods, activations of about 1 second
were given, and the readings were t.ken without automatic recording. In
one man would call time intervals
while another would read the
nnrn)ic^ onA .n f1,4 ir nrnnlA w^annrA +hnw,
. .1-' *3 ... :
K K.K K. KKKKKKKKK
: 13' **
; !*:.:: '
liHla nvrnn~;na fk; e mP~hnR
- 4 -
could be made which could be averaged simply. The main result of this
work was the emergence of a fairly intense, well-defined activity with a
half-life of 4.5 seconds. The shorter activity seemed to have a half-life
of about 1.5 seconds, and the values of the half-lives of the previously-
discovered longer periods were modified to 23 seconds and 56* seconds
In a third phase of the experimentation, rapidly circulating uranyl
nitrate solution was used.
The stream pas
a two-stage turbine pump driven above its rated
through an irradiation cell near the target of the
past two BF3 counters in series and into a sump
surrounded with paraffin and massively shielded
om a reservoir through
speed by a 5 H. P. motor,
cyclotron, and thence
tank. The counters were
against direct cyclotrpn
neutrons. The flow time between the two counters could be v
altering the volume of tubing between them, or by by-passing
" solution around the second counter. The first counter was a
a first approximation the flow time to it from the irradiation
constant. With this apparatus, timing was reduced to measure
volume and rates of flow. Irradiation lasted 0.19 sec and the
on the decay
tion cell. At
counts as one
period but ga
curve was at
this and subs
ve for the hal
a half-life of
showed that a shorter peri
a time 0.32 sec after the liquid left
equent chosen points one could take
e results confirmed the presence of
f-life the value 4.4 sec; they indicate
1.8 sec. The first two points on the
od was also present; it could not be
some of the
the 4,5 sec
ed also an
accurately, but had a half-life of about 0.4 sec and a moderate saturation
A summary of all of this work gives the following results for the de-
layed neutron activities for fission of uranium 235:
No activities of longer half-life were observed. In looking for therm,
activations up to 30 minutes duration were given and the decay of the de-
laved neutrons was followed for 15 minutes before their activity became
dependence of their distribution in the paraffin surrounding the counters
was not taken into account. The fact tha
ferent energies has been established.6,7
since re-surveyed the delayed neutron a
dence into account.
The Intensity Experiment
t the different activities have dif-
Hughes, Dabbs and Cahn6 have
activities, taking the energy depen-
The experimental arrangement is indicated in Fig. 2 and may be de-
scribed as follows:
column was built, 26
Near the target of the cyclotron a movable graphite
" x 30" in cross section, and 96" long. This column
was supported in an aperture built into one of the water tanks with which
the cyclotron was surrounded.
It rode on roller chains so that for use it
could be rolled toward the target, between the magnet coils of the cyclo-
tron, until one end was a few inches from the target.
Farther back it was
completely surrounded by water or other effective shielding- from fast
Five feet from the target end of this column a diaphragm of
U308 was placed; it filled the whole cross
section of the column,
thick, and consisted of a sheet iron box containing 291 Ibs of U308. Along
the axis of the column, one the side of the diaphragm away from the tar-
get, a large BF3 counter was placed.
.cadmium 0.020" thick.
It was completely shielded with
The principle of the :experiment is now apparent.
When the cyclotron
was on, the neutrons from the target which penetrated the 5 ft of graphite
to reach the U30g and the counter would be almost all of thermal energies.
They, therefore, would cause fissions in the U O0, but for the most part
would not penetrate the cadmium surrounding the counter. On the other
hand, many of the neutrons originating in the U30g8 would still have enough
H. D. Smyth, Atomic Energy for Military Purposes,
ton University Press,
R. B. Roberts, R. C.
Appendix III. Prince-
Meyer, and P.
Meyer, L. R.
Wang, Phys. Rev. 55
Hafstad and P
E. T. Booth, J. R.
D. F. Gibbs and G.
K. J. Bostrm, J.
Dunning and F
G. Slack, Phys.
Thomson, Nature 144,
Koch and T
, 876, 1939.
Lauritsen, Nature 144,
D. J. Hughes,
Dabbs and A. Cahn, CP-3094.
Willard and E.
O. Wollan, MDDC 16,
energy when reMachitg the counter nearby to ee capturte in the cadmim
cTh h be th his
adbe counted. The experiment would, therebrt be to cot the "ins
itantaneous" fission neutrons emitted from the U308g diaphragm with the
cyclotron on, then turn off the cyclotron and immediately count the de-
The delayed neutrons were so weak that only a few were registered
even after counting instantaneous neutrons at the maximum trustworthy
rate permitted by the apparatus. Since the shape of the decay curve was
known with moderate precision, it was expedient to measure the intensity
of the delayed neutrons by taking the integral number arriving during a
certain time interval after turning off the cyclotron, rather than to attempt
The start of counting was accomplished
* the scaler which closed when the radiofre
cyclotron opened. This ensured a uniform
the stop of bombardment and the start of
interval was estimated to be about a fiftie
provided which removed the relay from Uth
possible to count when the cyclotron was a
upon keeping the cyclotron beam steady, a
bardment the other operator went through
Turned off ti
through a relay in the input of
iency power contactor of the
short time interval between
hunting. The length of this time
of a second. A switch was
circuit, thereby making it
on. One operator concentrated
nd after three minutes of bornm-
the following procedure:
one minute with the cyclotron on and noted the read
relay into the circuit, and reset the scaler to zero
he cyclotron (thereby automatically starting counting
2.0 minutes and noted the number of delayed counts
The bombardment and counting were then repeated.
The beam wa
kept at about 16 / A, which gave a counting rate with the cyclotron on
15,000 to 20,000 counts per minute. A day's work yielded nineteen sat
factory runs, the main trouble being variability of the cyclotron beam.
The delayed neutron counts of these nineteen runs were corrected for
spontaneous fission neutron background (5.0 per min), normalized to a
counting rate with the cyclotron on of 10,000 per minute, and then ave
ed. The mean result was that if, during bombardment, neutrons were
arriving at a rate of 10,000 per minute, then in the interval 0.02 sec to
2.0 min after removing the source of primary neutrons, 22.4 : 3.2 deli
neutrons were counted, where the 3.2 was the probable error of the m<
of the nineteen observations. Now the result of the analysis of the dec
curves can be expressed as
so that at zero time I(0)
. The integral count of 22.4 from t
120 seconds gives from (1) the value C
Thus I(0) is 1.44
counts per second or 0.86 percent of the counting rate with the cyclotro
This result must be modified by two corrections.
The more impor-
tant of these is an upward correction in the delayed neutron intensity,
arising from a blank experiment (i.e.
, with the U308 removed) in which
was found 365 counts per min per P A were obtained under the same con-
ditions as gave 1180 counts per min per p A with the U302 present. The
resulting factor 1.40 is clearly a considerable overcorrechon because it
neglects the effect of the absorption of the cyclotron neutrons in the thick
layer of U30g8 The other correction arises from missed counts, because
of the high counting rates needed to give a reasonable number of delayed
counts. It is a downward correction in the relative intensity of the delayed
neutrons. The net effect of the two corrections may be estimated at an
increase of about 15 percent, leading to the result that in a body of uranium
emitting fission neutrons, at equilibrium
of the neutrons are delayed by more than 0.02 seconds and have half-lives
of 0.4 sec or more. The error indicated here is a guess at the maximum
uncertainty arising from the probable error of the counts and the vague-
ness in the corrections applied. No correction was attempted for a differ
ence in the spatial distribution of the slowed-down fission and delayed
neutrons arising from their possibly different initial energies,
lack of knowledge of the energy spectra. The longitidunal arrE
the counter would tend to reduce such a correction.
Experimental arrangement used in initial measurement of deyed
neutron decay by Snell, Nedzel and Ibser.
Diagram of the apparatus used to measure the intensity of the
delayed neutrons relative to that of the instantaneous fission ne
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