12
1 Energy Release in Nuclear Reactions, Neutrons, Fission, and Characteristics …
Before invoking a reaction mechanism involving a (hypothetical) neutron, Chadwick devised a further test to investigate the remote possibility that 55-MeV gammarays might somehow be created in the α-Be collision. In addition to having the
“beryllium radiation” strike protons, he also directed it to strike a sample of nitrogen
gas. The mass of a nitrogen nucleus is about 14 mass units; at a conversion factor of
931.49 MeV per mass unit, the rest energy of a
14 N nucleus is about 13,040 MeV. If
such a nucleus is struck by a 54.4-MeV gamma-ray, (1.32) indicates that it should
acquire a kinetic energy of about 450 keV. From prior experience, Chadwick knew
that when an energetic particle travels through air it produces ions, with about 35 eV
required to produce a single ionization (hence yielding “one pair” of ions). A 450 keV
nitrogen nucleus should thus generate some 13,000 ion pairs. Upon performing this
experiment, however, he found that some 30,000 to 40,000 ion pairs would typically
be produced. These figures imply a kinetic energy of ~ 1.1–1.4 MeV for the recoiling
nitrogen nuclei, which in turn by (1.30) would require gamma-rays of energy up to
~ 90 MeV, a number completely inconsistent with the ~ 55 MeV indicated by the
proton experiment. Indeed, upon letting the supposed gamma-rays strike heavier and
heavier target nuclei, Chadwick found that “… if the recoil atoms are to be explained
by collision with a quantum [γ -ray], we must assume a larger and larger energy for
the quantum as the mass of the struck atom increases.” The absurdity of this situation
led him to write (Paper 2, p. 697) that “It is evident that we must either relinquish
the application of conservation of energy and momentum in these collisions or adopt
another hypothesis about the nature of the radiation.” To be historically correct, the
mass of beryllium atoms had not yet been accurately established in 1932, so Chadwick did not know the E γ = 14.6 MeV figure for certain. However, he was able to
sensibly estimate it as no more than about 14 MeV unless the beryllium nucleus lost
an unexpectedly great amount of mass in the reaction, so, as he remarked in his Paper
2 (p. 693), “… it is difficult to account for the production of a quantum of 50 MeV
from the interaction of a beryllium nucleus and an α-particle of kinetic energy of
5 MeV.”
The fundamental problem with the gamma-ray hypothesis is that for the amount
of energy Q liberated in the α-Be reaction, any resulting gamma-ray will possess
much less momentum than a classical particle of the same kinetic energy; the ratio
is p γ
p m =
Q
2E m , where E m is again the rest energy of the classical particle.
Only an extremely energetic gamma-ray can kick a proton to a kinetic energy of
several MeV.
Chadwick’s key insight was to realize that if the protons were in reality being
struck billiard-ball style by neutral material particles of mass equal or closely similar
to that of a proton, then the striking energy need only be on the order of the kinetic
energy that the protons acquire in the collision.
This is the point at which the neutron makes its debut. Chadwick hypothesized
that instead of the Joliot-Curie reaction of (1.21), the α-Be collision leads to the
production of carbon and a neutron via the reaction
4
2 He +
9
4 Be →
12
6 C +
1
0 n.
(1.33)
1 Energy Release in Nuclear Reactions, Neutrons, Fission, and Characteristics …
Before invoking a reaction mechanism involving a (hypothetical) neutron, Chadwick devised a further test to investigate the remote possibility that 55-MeV gammarays might somehow be created in the α-Be collision. In addition to having the
“beryllium radiation” strike protons, he also directed it to strike a sample of nitrogen
gas. The mass of a nitrogen nucleus is about 14 mass units; at a conversion factor of
931.49 MeV per mass unit, the rest energy of a
14 N nucleus is about 13,040 MeV. If
such a nucleus is struck by a 54.4-MeV gamma-ray, (1.32) indicates that it should
acquire a kinetic energy of about 450 keV. From prior experience, Chadwick knew
that when an energetic particle travels through air it produces ions, with about 35 eV
required to produce a single ionization (hence yielding “one pair” of ions). A 450 keV
nitrogen nucleus should thus generate some 13,000 ion pairs. Upon performing this
experiment, however, he found that some 30,000 to 40,000 ion pairs would typically
be produced. These figures imply a kinetic energy of ~ 1.1–1.4 MeV for the recoiling
nitrogen nuclei, which in turn by (1.30) would require gamma-rays of energy up to
~ 90 MeV, a number completely inconsistent with the ~ 55 MeV indicated by the
proton experiment. Indeed, upon letting the supposed gamma-rays strike heavier and
heavier target nuclei, Chadwick found that “… if the recoil atoms are to be explained
by collision with a quantum [γ -ray], we must assume a larger and larger energy for
the quantum as the mass of the struck atom increases.” The absurdity of this situation
led him to write (Paper 2, p. 697) that “It is evident that we must either relinquish
the application of conservation of energy and momentum in these collisions or adopt
another hypothesis about the nature of the radiation.” To be historically correct, the
mass of beryllium atoms had not yet been accurately established in 1932, so Chadwick did not know the E γ = 14.6 MeV figure for certain. However, he was able to
sensibly estimate it as no more than about 14 MeV unless the beryllium nucleus lost
an unexpectedly great amount of mass in the reaction, so, as he remarked in his Paper
2 (p. 693), “… it is difficult to account for the production of a quantum of 50 MeV
from the interaction of a beryllium nucleus and an α-particle of kinetic energy of
5 MeV.”
The fundamental problem with the gamma-ray hypothesis is that for the amount
of energy Q liberated in the α-Be reaction, any resulting gamma-ray will possess
much less momentum than a classical particle of the same kinetic energy; the ratio
is p γ
p m =
Q
2E m , where E m is again the rest energy of the classical particle.
Only an extremely energetic gamma-ray can kick a proton to a kinetic energy of
several MeV.
Chadwick’s key insight was to realize that if the protons were in reality being
struck billiard-ball style by neutral material particles of mass equal or closely similar
to that of a proton, then the striking energy need only be on the order of the kinetic
energy that the protons acquire in the collision.
This is the point at which the neutron makes its debut. Chadwick hypothesized
that instead of the Joliot-Curie reaction of (1.21), the α-Be collision leads to the
production of carbon and a neutron via the reaction
4
2 He +
9
4 Be →
12
6 C +
1
0 n.
(1.33)