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SSNuceatFormemn Fisston
Vol. 68 *
ossible.. It is not feasible,however, in thislim::? The valueof the fission yield of the nucleus in
ited framework to indicate the indirect contri-*: percentage for U™ is given in column six. . It is
butions to the extent that will occur in the PPR.’ defined as thepercentage of the fissions leading to .
In giving references, it seems advisable to follow: thenucleus in question by direct formation and
the pattern of a normal survey article, 4. ¢.,; to®' decay of precursors. Most of the values were de‘list all results that present a contribution to the ’ termined relative to the value of 6.1% for 12.8d
bodyof information,citing the names of those who- Ba’ based on direct fission counting (F108).
carried out this work and the Project report in: Older fission yield values have been normalized |
whichit is described most completely." In addi- > to this value for Ba’, The majority of the fission
tion to giving references to Project reports, some” ' yield determinations were made onnatural ura- '
references to the PPR papers are included. .....- nium irradiated with pile neutrons. ‘In a few
cases, designated by the symbol (t), irradiations
Description of Table I, Characteristics
In preparing the present survey an attempt was
were made in a column containing neutrons in
thermal equilibrium at room temperature; such
made to evaluate critically the results of various. irradiations gave essentially the same values. A
investigators published in the open literature. and‘ a fission yield is given only for the nucleus for which
in the Plutonium Project reports. '..The values. . . it has been determined; in most c-.s the fission
given in Table I (pp. 2416-36)arelisted in order of, Yield will not be appreciably greater for succeed-
preference. Column one gives the atomic number
and mass number of the nucleus... Parentheses
around the mass numberindicate the mass assignment is uncertain, and an asterisk (*) denotes
that the nucleus is in a metastable excited state,
decaying by isomeric transition. _
The half-life is listed in column two with the
following abbreviations: ‘'s’’ for seconds, ‘‘m” for
minutes, “‘h” for hours, “d’”’ for days, and “y”’ for
years. In some cases an upperor lowerlimit to the
half-life has been established for a nucleus whose
radiations have not been directly observed. Half-
life values were selected on the basis of initial
purity of the activity and the number of halflives over which the intensity was measured, tak-
ing into consideration the estimated : precision
of individual measurements.
ae
.
Column three gives the mode of decay and the
type of radiations emitted. The following symbols are used: §~ for negative electrons emitted
from the nucleus, y for gamma rays, e~ for internal-conversion electrons, 2 for neutrons, and
“LT.” for isomeric transition. In the cases in
which it has been established that the nucleus
emits little or no gamzna radiation, this fact is
stated explicitly in columneight.
Column four refers to the investigators who
first isolated and identified the nucleus as a fission
product. As it was not always possible to establish absolute priority, the references serve mainly
as a guide for tl. approximate date of discovery
of each fission product nucleus.
The “‘class’’ of a nucleus, given in column five,
refers to the degree of certainty in the atomic
number and inass assigninent. The clusses are tu
be interpreted as follows:
A = element certain, isotope certain;
B = element certain, isotope proba fe;
C # element certain, isotope uncertain;
D = insufficient evidence.
.
Parentheses around the class rating signify that
the corresponding nucleus has not been observed
in the fission of uranium, althoughit is very prob-
Bbly formed in the process.
ing members of the decay chain (G147).
Thefission yields of the delayed neutron emitters Kr, Xe(™, and the last four nuclei listed in
Table I have not been determined directly. The
values in column six represent therefore the percentage of the fission neutrons emitted by the nucleus in question, as indicated in the table. These
data can be trarisformed to the percentagefission
yield of the neutron-decay process by multiplica- .
tion by the value for y, the number of neutrons per
fission. The value of » is given as 1-3 in the
Smyth report (S150).
.
|
The energy values of both nuclear beta spectra
and conversion electronsarelisted in column seven.
Only the maximum ‘energy of a nuclear beta
spectrum is given, and in cases where only Kono-
pinski-Uhlenbeck extrapolated values (K8) are
reported they are designated ‘(K.U.)}.”” In each
case the value is followed by a description of the
method employed in its determination using the
following
abbreviations:
.
Spect.
Abs.Al FP.
Abs.Al
= magnetic spectrometer or spectrograph
studies,
= absorption of the particles in aluminum
{range evaluated according to the
method of Feather (F4,F5,F6)),
= absorption of the particles in aluminum
(range estimated
visually or from the
Abs.Al, coinc, = absorption in aluminum of f-y coincidences,
CLCh.
= cloud chamber studizs,
The selection of values for the maximum energy
of a beta spectrum or the energy of conversion
electrons is based primarily on the method used
in their determination. The various methods
employed are given preference as to their reliability in the order listed above, In all cases where
the range in aluminum is given for a nuclear beta
component, the corresponding maximum energy
has
recalculated using a revised range-energy
relationship (G145).
Gammaray energiesare listed in column eight,
Each value is followed by a description of the
method employed in its determination and the
following abbreviations are used: