14—22.
TRANSMUTATIONS BY FAST NEUTRONS
331
_
neutrons have been used to produce a large number of different
transmutations.
For examples, see reference 19.
In 1934, Fermi31 discovered the radio—activity of the residual
nuclei produced in a large number of elements bombarded by
neutrons.
This has proven of great value, since the saturation
activity (horizontal line of gure 14—10) is a measure of the number of residual nuclei produced each second and, hence, of the
yields and cross—sections of neutron induced trans'mutations. The
capture—cross—sections range from 10“26 to 10“24 cm.2 Furthermore, since the active nuclei continue to radiate positrons or
negative electrons for appreciable lengths of time, it is possible to
carry out extended chemical tests and obtain direct proof of the
nature of the residual nuclei.
Thus, it has been found that, if Z
is the atomic number of the initial nucleus, then that of the resid—
ual nucleus is either (1) Z — 2, (2) Z — 1 or (3) Z.
(1) Alpha particles can be produced by fast neutrons in heavy
and in light elements and also, by slow neutrons in certain of the
light elements.
For example, the reaction
7N14 + 0111 = 5311 + 2He4 + E
(14—37)
can
only be produced by fast neutrons. The reaction is endo—
thermic; i.e., E = ——0.3 MeV. The equation, read backwards,
gives the reaction for the production of neutrons by the bombard—
ment of boron with alpha particles.
(2) Protons are emitted in those cases where Z decreases by
one unit.
These reactions are usually endo—thermic and can only
be produced by fast neutrons (with one known exception, namely,
With nitrogen).
In the following example,
8016 + on1 = 7N16* + 1H1 + E,
(14—38)
very fast neutrons, above 12 MeV, are
(3) When Z does not change, we have (a) 1nelast1c scattering,
(É) simple capture or, (5) emission of two neutrons. In the rst
case, a neutron enters the nucleus and a neutron
leaves.
The
nucleus is sometimes left in an excited state With subsequent
emis—
sion of gamma rays.
This process
is very probable for light and
medium-weight nuclei. In case (5), Heyn* has expla1ned several
* Heyn, Physica, 4, 160 (1937)-
TRANSMUTATIONS BY FAST NEUTRONS
331
_
neutrons have been used to produce a large number of different
transmutations.
For examples, see reference 19.
In 1934, Fermi31 discovered the radio—activity of the residual
nuclei produced in a large number of elements bombarded by
neutrons.
This has proven of great value, since the saturation
activity (horizontal line of gure 14—10) is a measure of the number of residual nuclei produced each second and, hence, of the
yields and cross—sections of neutron induced trans'mutations. The
capture—cross—sections range from 10“26 to 10“24 cm.2 Furthermore, since the active nuclei continue to radiate positrons or
negative electrons for appreciable lengths of time, it is possible to
carry out extended chemical tests and obtain direct proof of the
nature of the residual nuclei.
Thus, it has been found that, if Z
is the atomic number of the initial nucleus, then that of the resid—
ual nucleus is either (1) Z — 2, (2) Z — 1 or (3) Z.
(1) Alpha particles can be produced by fast neutrons in heavy
and in light elements and also, by slow neutrons in certain of the
light elements.
For example, the reaction
7N14 + 0111 = 5311 + 2He4 + E
(14—37)
can
only be produced by fast neutrons. The reaction is endo—
thermic; i.e., E = ——0.3 MeV. The equation, read backwards,
gives the reaction for the production of neutrons by the bombard—
ment of boron with alpha particles.
(2) Protons are emitted in those cases where Z decreases by
one unit.
These reactions are usually endo—thermic and can only
be produced by fast neutrons (with one known exception, namely,
With nitrogen).
In the following example,
8016 + on1 = 7N16* + 1H1 + E,
(14—38)
very fast neutrons, above 12 MeV, are
(3) When Z does not change, we have (a) 1nelast1c scattering,
(É) simple capture or, (5) emission of two neutrons. In the rst
case, a neutron enters the nucleus and a neutron
leaves.
The
nucleus is sometimes left in an excited state With subsequent
emis—
sion of gamma rays.
This process
is very probable for light and
medium-weight nuclei. In case (5), Heyn* has expla1ned several
* Heyn, Physica, 4, 160 (1937)-