312
14.
ARTÏFIClAL TRANSMUTATION
All of the protons
have sufcient energy to penetrate a small
thickness of the absorber.
The number of protons penetrating
the absorber remains constant as more and more absorbing mate—
rial is introduced until a critical thickness of absorber is reached;
then, the number of protons drops rapidly to a lower, constant
value. Thus, there are a number ofprotons in the beam which are
all stopped together, or nearly so. This means that there is a group
of protons
with a denite range (71). The remaining protons have
greater energy
and are able to penetrate a greater thickness of the
absorber.
Then, a second group,
whose maximum range is 7"2, is
stopped, etc. A plot of the slope of the curve in gure 14—441 will
show three peaks from which the mean ranges, R1, R2 and R3 of
r,
s
,,
È
i
r
b.
3
°.
1
°
_
.
.
%
%
'
20
30
40
50
60
70
24—
32
40
46
56
64
Ah?
EOU/VÂLEI‘VT
/N
CM
AIR
EOU/VALENÏ
/N
CM
(8)
(bl
Flo.
14—4.
An
integral—distribution—in—range curve.
Protons from aluminum
bombarded by alpha particles.
the three proton groups can be determined as in gure 12—4. This
differential-distribution—in—range curve will also show in what
manner the energies of the protons in a given group are distributed
about the average value.
'
.
If the integral-range curve has a continuous slope downward,
rather than the step-like form of gure 14—4a, the protons
have a
Wide range of velocities instead of a group structure.
In a number of transmutations, protons are emitted in groups,
each with a denite energy. For example, with the reaction
13Al27 + 21‘I(ä4 _) 15P31 ——> 148130 + 1H1,
Pose4, in 1929, obtained a curve like that in gure 14—461. Laten
Chadwick and Constable5 improved the experiment, obtained the
curve of gure 14—45, and thus showed that at least eight groups