nucteonics DATA SHEET no. 24 
Fission-Product Yields from 
By SEYMOUR KATCOFF 
Chemistry Department, Brookhaven National Laboratory, Upton, New York 
FISSION OF A HEAVY NUCLEUS, such as 
U2%®, by thermal neutrons results in two 
fission fragments that recede from each 
other with a total kinetic energy of 
about 170 Mev. In general, their 
masses are unequal—the most probable 
heavy mass is around 139, and the most 
probable light mass is around 95. 
However, products have been found in 
detectable amounts throughout the 
mass region 72-161. 
The percent probability per fission 
of forming a given nuclide, or a given 
chain, is defined as its fission yield. 
Since the fission fragments are formed 
with an excess of neutrons, they 
undergo a series of 8- decays, in a 
“chain,” until they attain stability 
(Table 2, p. 82). 
Fission yields can be determined by 
radiochemical or mass-spectrometric 
means. 
In the radiochemical method, the 
number of fissions is usually determined 
by direct fission counting of a small 
aliquot of the fissile material exposed 
simultaneously to the same neutron 
flux as the main sample. The latter is 
radiochemically analyzed for the radio- 
nuclides of interest. Their absolute dis- 
integration rates can be determined by 
4mr counting or by gas-phase counting. 
However, in most of the older work the 
beta counting was done with end-win- 
dow G-M counters, and correction to 
disintegration rates was accurate only 
to +10%. Frequently the number of 
fissions was not determined directly in 
each experiment; only the relative 
yields of nuclides were measured and 
Ba'‘° was used as the standard. 
The mass-spectrometric method has 
been applied to measuring fission yields 
of stable and long-lived isotopes of Kr, 
Fission Yields from . . . 
The fission-yield data of Table 2 (p. 82) are 
plotted versus mass number in the U**® curve 
(circles). The sum of the yields under each peak 
is very close to 100%, as expected for binary fission. 
This requirement of summation to 100% each, for 
the light and heavy groups of fission products, was 
used as an aid in normalizing some of the data in 
Table 1 (p. 80). Such a procedure was necessary 
because few of the fission yields have been meas- 
ured accurately on an absolute basis. 
Small ‘‘fine-structure”’ peaks are clearly shown 
in some of these yield-vs-mass curves. 
these peaks are at masses 100 and 134; they result 
from a preference for the formation of fragments 
with a closed shell of 82 neutrons. 
The U** curves illustrate the effect on mass dis- 
tribution of increasing the neutron energy above 
thermal. The greatest change is the increase in 
the probability of symmetric fission. 
neutron fission, the increase is about 100-fold; for a 
fission-neutron spectrum, the increase is four-fold. 
The other changes are: a small drop in the peak 
yields and a moderate increase in the most asym- 
metric modes of fission, i.e., a rise in the wings of 
the yield-vs-mass curve. 
The other two sets of curves show fission-product 
distributions from thermal-neutron fission of U?*3, 
and Pu?®*’ and from fission-neutron fission of Th?%? 
and U?%, 
236 
Rb, Sr, Zr, Mo, Ru, Xe, Cs, Ba, Ce, Nd 
and Sm. The absolute number of 
atoms of each nuclide can be deter- 
mined by the isotepe-dilution tech- 
nique (1). The number of fissions can 
be determined by measuring the change 
in the B!°/B!"! ratio in a BF; flux moni- 
tor irradiated simultaneously in the 
same neutron flux as the fission sample. 
Conversion to fissions is effected by use 
of the known ratio of B!°-absorption 
cross section to the fission cross section. 
Many of the yields from U*** thermal- 
neutron fission were measured in this 
way (2). In most of the other mass- 
spectrometric work to date, only the 
relative yields have been measured. 
In compiling these tables, first 
priority was given to mass-spectro- 
metric data since these are more 
accurate (+3%) than most of the 
radiochemical data (+10%). Where 
aa q 
TTT erry 
oA Lid TRS Ly 
S5)5255 2===7) 
a eeceens, 
~ ECO 
For U?*5, = = 2 SSSaSeeee5 
= a 5 aes aiS= 4 
Smeal cP 
d MCCOY 
SS SS SSS SS SSS 
A SEEEE= 
For 14-Mev- I] i 
ror 
eS SSS 
FEE see 
0.000! in 
70 80 90 100 Il0 120 130 140 150 
Mass Number 
