14—23.
SLOW NEUTRON S
335
The nucleus, thermal—neutron cross-sections do not vary in a
regular manner with atomic weight as in the case of fast neutrons
(section 14—12).
ln many cases they are only a few times larger
than the fast neutron values; i.e., of the order of1 to 10 >< 10”24
cm.2
But, in certain cases, exceedingly large cross—sections have '
been observed, particularly among the rare earths. Selected
values are given in table 14—2.
Fermi and his collaborators83 have shown that there are certain selective absorptions of slow neutrons. Thus, the exceedingly
large cross—sections observed in certain cases are due to the 7550—
nance
capture of neutrons of a given energy. Neutron groups of
this type are listed in table 14—2.
TABLE 14——2.——FOR NEUTRONS
El
Cross-section, >< 1024, cm.2
Main
RËÎÎÊÊÊÏŒ
Group
ement
“___—“___—
P
Name
Thermal
Resonance
Fast
rocess
Volts
Li6
900
1.8
a
B10
3000
1.6
a
Rh
125
6000
44”
"D”
80
14000
22”
3.0
“A"
Ag
//
"
'
.“B”
30
200
22
D—°
total 55
120
Cd
2600
N80000
C
-
0.16
“C"
In
85
54’
1-6
"D”
55
16'
2.6
total 140
I
9.4
150
4.6
140
Gd
30,000
__
Ir
280
600
C19 hf
1-9
HD”
Hg
440
5.8
C
C = capture, a = alpha emission.
(