Physics and Engineering of Radiation Detection

1.3. Radioactivity and Radioactive Decay 5

calledfission fragementsand emits several neutrons. But this is not the only mode
of neutron decay. Another particle just mentioned was neutrino. Neutrino is an
extremely light and low interacting chargeless particle that was discovered in 1952.
It solved the mystery of the variable electron energy in beta decays: the electron,
being very light as compared to the other heavy decay product in beta decays, was
supposed to carry the same energy in each decay. However, it was observed that
the emitted electrons had a whole spectrum of energy with a cut off characteristic
to the decaying atom. It took several decades for scientists to discover that some of
the energy is actually taken away by a very light particle called neutrino. Now we
know that this particle in beta decays is actually anti-neutrino^3.
When nuclei emit subatomic particles, their configuration, state, and even iden-
tity may change. For example, when a nucleus emits an alpha particle, the new
nucleus has 2 protons and 2 neutrons less than the original one. Except forγ-decay,
in which the nucleus retains its identity, all other decays transform the nucleus into
a totally different one. This process is called radioactive decay.
There are a number of naturally occurring and man-made radioactive elements
that decay at different rates. Although the underlying mechanism of these decays
is fairly complicated, their gross outcome can be easily predicted by considering the
conservation of electrical charge. Before we write general decay equations, let us
first have a look at some examples.

Alpha decay:^22288 Ra →^21886 Rn + α

Beta decay:^13153 I →^13154 Xe + e + ̄ν

Gamma decay:^15286 Dy∗ →^15286 Dy + γ

Spontaneous fission:^256100 Fm →^14054 Xe +^11246 Pd +4n

Herenp+pXrepresents an elementXwithpprotons andnneutrons.X∗represents
an atom in an excited state.

The term beta decay as used in the above example is sometimes conventionally
used to represent only emission of electrons. However, there are actually three kinds
of beta decays: electron decay, electron capture, and positron decay. The first two
involve electrons while the third involves the emission of theanti-electronor positron,
which we will represent bye+in this book. A positron has all the properties of an
electron with the exception of electrical charge, which is positive in this case. In this
book the symbolewill be used to represent either the electron or the unit electrical
charge.
Electron capture occurs when a nucleus captures one of the electrons orbiting
around it and as a result goes into an excited state. It quickly returns to the ground
state by emitting a photon and a neutrino. The positron emission is very similar to
the electron decay with the exception that during this process instead of an electron
a positron is emitted. Let us have a look at examples of these two processes.

(^3) Anti-neutrino is the antiparticle of neutrino. The presence of neutrino inβ-decays was suggested by
Wolfgang Pauli. It was namedneutrinoby Enrico Fermi.