Atomic Energy for Military Purposes (The Smyth Report)

The Official Report on the Development of the Atomic Bomb Under the Auspices of the United States Government

By Henry De Wolf Smyth


Since the proton and the neutron are the fundamental particles out of which all nuclei are built, it would seem natural to use the mass of one or the other of them as a unit of mass. The choice would probably be the proton, which is the nucleus of a hydrogen atom. There arc good reasons, historical and otherwise, why neither the proton nor the neutron was chosen. Instead, the mass unit used in atomic and nuclear physics is one sixteenth of the mass of the predominant oxygen isotope, 016, and is equal to 1.6603 x 10-24 gram. Expressed in terms of this unit, the mass of the proton is 1.00758 and the mass of the neutron is 1.00893. (Chemists usually use a very slightly different unit of mass.)

The unit of electric charge used in nuclear science is the positive charge of the proton. It is equal in magnitude but opposite in sign to the charge on the electron and is therefore often called the electronic charge. One electronic charge is 1.60 x 10-19 coulomb. It may be recalled that a current of one ampere flowing for one second conveys a charge of one coulomb; i.e., one electronic charge equals 1.60 x10-19 ampere second.

The energy unit used in nuclear physics is the electron volt, which is defined as equal to the kinetic energy which a particle carrying one electronic charge acquires in falling freely through a potential drop of one volt. It is often convenient to use the million-times greater unit: million electron volt (Mev).

The relationships among the electron volt and other common units of energy are in the following table:

Mev 1.07 x 10-3 mass units
  1.60 x 10-6 ergs
  3.83 x 10-14 g. cal.
  4.45 x 10-20 kw. hrs.
mass units 9.31 x 102 Mev
  1.49 x 10-3 ergs
  3.56 x 10-11 g. cal.
  4.15 x 10-17 kw. hrs.
ergs 6.71 x 102 mass units
  6.24 x 105 Mev
  2.39 x 10-8 g. cal.
  2.78 x 10-14 kw. hrs.
g. cal 2.81 x 1010 mass units
  2.62 x 1013 Mev
  4.18 x 107 ergs
  1.16 x 10-6 kw. hrs.
kw. hrs. 2.41 x 1016 mass units
  2.25 x 1019 Mev
  3.60 x 1013 ergs
  8.60 x 105 g. cal.
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