14.
ARTIFICIAL TRANSMUTATION
lithium were found29 to be very eective in ejecting neutrons from
various elements.
The cross—sections observed were of the order
of 10—27 cm.2
14—21.
Pair Production and Annü1ilation.—The
of
,
positrons and the number of negative electrons produced simul>
taneously by the bombardment of various substances with gamma
rays
has been found to be very nearly equal to each other (gure
14—95).
It is assumed that each gamma ray photon (hu) can
_
materialize in the form of one electron pair.30 The process has
_‘
only been observed to take place in the electro-magnetic eld of a
nucleus.
Since the self—energy of each electron is 77252 (equation
2—27) (N 0.5 Mev), the gamma ray must have an energy 277262
(N 1 Mev) or more in order to form one pair. Any additional
energy appears
in the form of kinetic energy of the two electrons
_
—
and the small recoil energy given to the nucleus. Thus, the
kinetic energy KE is
,
KE = [w — 2mc2,
,
(14—35)
where ]; is Planck’s constant, V is the frequency of the incident
_
gamma ray, 772 is the mass of an electron and 6 is the velocity of
light. The number of pairs produced increases as the square of the ,
atomic number. Hence, the use of lead in gure 14—95.
"
Thibaud and ]oliot have sent a known number of positrons‘,
toward heavy metal foils and counted the average number of
photons eje'cted.
They found two photons ‘for each posrtron,
emitted in opposite directions. It is assumed that a positron and
an extra—nuclear electron are annihilaz‘ed in the presence of the
nuclear forces.
Thus, “
.
_
.
i
"
:
=
2/w.
,
-
(14—36)
,,
Each
has been foundto have the proper energy; i.e.,'05
wave—length >\ = 0.024 angstroms.
prôdù“céioniiatioh in the bodywhose