Subatomic Physics

88 The Subatomic Zoo
Figure 5.6: Level diagrams
of nuclei and particles. The
notation is explained in the
text.
Subatomic particles are quantum systems, and
nearly all of the ones that are not elementary particles
possess excited states. Schematically the
level diagrams appear as shown in Fig. 5.6. Even
though the basic aspects are similar for nuclei and
particles, units and notation differ. In the case
of nuclei, the mass of the ground state is quoted
not for the nucleus alone but for the neutral atom,
including all electrons. The international unit for
the atomic mass is one twelfth of the atomic mass
of 12 C. This unit is called the atomic mass unit
and is abbreviated u. In terms of grams and MeV,
it is
1 u ≈ 1.66054 × 10 −24 g (mass)
≈ 931.494 MeV/c 2 . (5.23)
The masses of nuclear ground states are given in u.
The excited nuclear states are not characterized by
their masses but by their excitation energies (MeV
above ground state). In the case of particles, rest
energies are given, and they are quoted in MeV or
GeV. This procedure is arbitrary but makes sense
because in the nuclear case excitation energies are
small compared to the rest energy of the ground
state, whereas in the particle case excitation energies
and ground-state energies are comparable.
After these preliminary remarks we turn to mass spectroscopy, the determination
of nuclear masses. The first mass spectrometer was built in 1910 by J. J. Thomson,
advanced by F. W. Aston. The components of Aston’s mass spectrometer are
shown in Fig. 5.7. Atoms are ionized in an ion source. The ions are accelerated
by a voltage of 20–50 kV. The beam is collimated by slits and passes through an
electric and a magnetic field. These fields are so chosen that ions of different velocity
but with the same charge-to-mass ratio are focused on the photographic plate. The
positions of the various ions on the photographic plate permit a determination
of the relative masses with accuracy. However, the most accurate determination of
nuclear masses have been performed with ion traps (see Section 6.5 for a description
of Penning traps) where instead of measuring the deflection of charged particles in a
field one determines the frequency of oscillations in a field. In recent years there has
been great progress in using these techniques to accurately determine the masses of