232 The Poetry of Physics and The Physics of Poetry
always greater than the mass of the daughter nucleus plus the mass of the
electron (or positron) and the neutrino. The mass which is lost is
converted into the kinetic energy of the daughter nucleus, the electron
(or positron) and the neutrino according to Einstein’s formula E = mc^2.
A great deal has been learned about nuclei by studying the products of
their radioactive decays. Much more has been learned, however, by
studying the effects of bombarding the nucleus with various projectiles
such protons, neutrons, electrons, photons, alpha particles and other
nuclei. There are two possible outcomes of subjecting the nucleus to
collisions with other particles. Either the particles will scatter off the
nucleus without changing their character or else they will initiate nuclear
reactions in which the incoming projectile and the nucleus undergo
change.
The very first collision experiments were performed using radioactive
materials as a source of alpha particles, which bombarded the nuclei of
the target, usually a thin-foil of some metal such as gold. Rutherford’s
discovery of the nucleus described earlier was the first such experiment.
As experimental techniques became more sophisticated, nuclear
physicists designed machines that accelerated charge particles through
electric potentials before directing them at a target. The first machine
of this type was the Van De Graff electrostatic generator. The large
electrostatic potential through which the protons were accelerated was
generated using friction. The energy of the particle that one obtained in
this manner was limited by the amount of potential difference the
machine could be made to hold. The next innovation in accelerators
avoided the problem of electric breakdown by recycling the charged
particle through the same potential difference many times. This was
achieved by forcing it into a circular orbit through the use of magnets.
The more sophisticated accelerators developed since the first cyclotron
are still based on this principle.
The detection of the products of a scattering experiment posed
another challenge to the ingenuity of the nuclear physicist. Rutherford
detected the alpha particles scattered from the gold foil using a material,
which scintillates each time it is struck by a charged particle. This simple
technique still forms the basis for many of today’s particle detectors. The
signals from the scintillating material are now automatically read by a
photoelectric cell whose signals are fed directly into a computer, which
analyzes the data, almost the instant it is gathered. Another device for
observing nuclear reactions is the cloud chamber. In recent times the