8 Chapter 1. Properties and Sources of Radiation
decay by proton emission is the decay of Indium-109, which decays with a half life
(^5) of only 103μs.
109
53 I→
108
52 Te+p
This extremely small half life is actually typical of all of the known proton emitters
with the exception of a few that have half lives in the range of a few hundred
milli-seconds. Such short half lives severely limit the usefulness of such materials as
proton sources.
Just like proton emitters, it is also possible to produce isotopes that decay by
neutron emission. They suffer from the same extremely short half lives typical of
proton emitters, rendering them useless as neutron sources in normal laboratory
environments. Exception to this are a few isotopes, which decay by spontaneous
fission and in the process also emit neutrons. The most common example of this is
californium-252, which with a half live of 2.65 years is commonly used in laboratories
as a source of neutrons.
It is now worthwhile to write general equations for different types of radioactive
decays. For a nucleusXwithpprotons,nneutrons andeelectrons, which transforms
into another nucleusY, the general decay equations can be written as follows.
Alpha decay: np+pX → pn−+ 2 p−^4 Y + α
Electron decay: np+pX → np+1+pY + e + ̄ν
Electron capture: np+pX+e → np−+ 1 pY + γ + ν
Positron decay: np+pX → pn−+ 1 pY + e+ + ν
Gamma decay: np+pX∗ → np+pX + γ
Proton emission: np+pX → pn−+ 1 p−^1 Y + p
Here we have deliberately avoided using the termproton decayto describe the decay
by proton emission. The reason is that proton decay is explicitly used for the
decay of the proton itself, a process that is expected to occur in nature, albeit
with an extremely low probability. Dedicated detectors have been built around the
world to observe such a phenomenon. The reader is encouraged to verify that in all
these reactions the total electrical charge is always conserved. This conservation of
electrical charge is one of the fundamental laws of nature.
The general equation for spontaneous fission process can not be written as there
are a number of modes in which a nucleus may fission. That is, there is generally a
whole spectrum of nuclides in which a decaying nucleus may split. Also the number
of neutrons emitted is variable and depends on the particular mode of decay.
It should be remembered that during radioactive decays the emitted particles
originate from the nuclei. For example during the process of electron decay, a neutron
inside the nucleus decays into a proton, an electron and an anti-neutrino. The proton
stays inside the nucleus while electrons and anti-neutrino are emitted out. Similarly,
during gamma decay, the photon is emitted from the nucleus and is not the result
(^5) Half life is the time taken by half of the sample atoms to decay. The term will be formally defined later
in the Chapter.