332
14.
ARTIFICIAL TRANSMUTATION
reactions produced by fast neutrons as due to the emission of two
neutrons.
The residual nucleus is usually positron active and is
isotopic with the original nucleus, although reduced by one unit in
mass.
The half-lives of the radio—active substances are the same as
when produced in gamma—neutron reactions, in all cases where it
has been possible to make a comparison. Many cases of n-2n
reactions have been observed by Pool, Cork and Thornton.19
14—23.
Slow Neutrons.—We shall consider 5Z0w neutrons as
those whose energy ranges from zero up
to a few thousand volts.
Those whose energy is approximately 0.028 electron volts are
known as thermal neutrons.
In 1934, Fermi 32 found that the saturation activity produced
in a number of substances such as silver, cadmium, rhodium, etc.,
was greatly increased when the neutron source (radon + berylhum) was surrounded by several centimeters of a substance which
was rich in hydrogen, such as parain or water. He assumed that
.
the neutrons were slowed down in the hydrogen—rich material and,
_
hence, were easily captured. We shall return to the proof of this
a little later.
'
'
Slow neutrons can cause transmutati0ns in which alpha par—
;
ticles are emitted.
Thus,
3Li6 + on1 ——> 2He4 + 1H3
(14—39)
'
51310 + 0111 -—> 3Li7 + 2H64.
(14—40)
For thermal neutrons, the cross—sections are very large, 900 X
10—24 and 3000 X 10—24 cm.2, respectively, and hence, the reactions are very probable. The heavy particles (He4 and H3)
*
which are ejected are excellent ionizers of a gas. Hence, lithium
or bdron (about 1 mm. thick) is used as a lining in shallow chambers
…
or electrosc0pes for the detection of…neutrons.'
_
'
Protons are emitted when slow neutrons bombard nitrogen.
'
Thus,
-
‘
.
_
,
*
(
Ï
7N14+0n1—+604+1H1.
(14—41)v -
'
The simple capture of slow neutrons has also been observed.
'
example,
’
Ü
_
'
‘
Ï
-
“
_
' _
'
1H15+ 0111 ——> IH? + 7.
'
'
(14—42)
"
'
14.
ARTIFICIAL TRANSMUTATION
reactions produced by fast neutrons as due to the emission of two
neutrons.
The residual nucleus is usually positron active and is
isotopic with the original nucleus, although reduced by one unit in
mass.
The half-lives of the radio—active substances are the same as
when produced in gamma—neutron reactions, in all cases where it
has been possible to make a comparison. Many cases of n-2n
reactions have been observed by Pool, Cork and Thornton.19
14—23.
Slow Neutrons.—We shall consider 5Z0w neutrons as
those whose energy ranges from zero up
to a few thousand volts.
Those whose energy is approximately 0.028 electron volts are
known as thermal neutrons.
In 1934, Fermi 32 found that the saturation activity produced
in a number of substances such as silver, cadmium, rhodium, etc.,
was greatly increased when the neutron source (radon + berylhum) was surrounded by several centimeters of a substance which
was rich in hydrogen, such as parain or water. He assumed that
.
the neutrons were slowed down in the hydrogen—rich material and,
_
hence, were easily captured. We shall return to the proof of this
a little later.
'
'
Slow neutrons can cause transmutati0ns in which alpha par—
;
ticles are emitted.
Thus,
3Li6 + on1 ——> 2He4 + 1H3
(14—39)
'
51310 + 0111 -—> 3Li7 + 2H64.
(14—40)
For thermal neutrons, the cross—sections are very large, 900 X
10—24 and 3000 X 10—24 cm.2, respectively, and hence, the reactions are very probable. The heavy particles (He4 and H3)
*
which are ejected are excellent ionizers of a gas. Hence, lithium
or bdron (about 1 mm. thick) is used as a lining in shallow chambers
…
or electrosc0pes for the detection of…neutrons.'
_
'
Protons are emitted when slow neutrons bombard nitrogen.
'
Thus,
-
‘
.
_
,
*
(
Ï
7N14+0n1—+604+1H1.
(14—41)v -
'
The simple capture of slow neutrons has also been observed.
'
example,
’
Ü
_
'
‘
Ï
-
“
_
' _
'
1H15+ 0111 ——> IH? + 7.
'
'
(14—42)
"
'