Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 57

any mass to the neutrinos but recent experiments have suggested that they indeed
have masses, albeit very small (a feweV). They do not carry any electrical charge
and therefore are extremely difficult to detect.

Basic Properties of Neutrinos

Rest mass : Extremely small (few eV)

Electrical charge = Zero

Mean life > 1033 years

Internal structure : Believed to have no internal structure.

Neutrinos are extremely abundant in nature and are produced in large numbers
in processes like nuclear fusion, stellar collapse, and black-hole collisions. The nu-
clear fusion reactions inside the core of the Sun produce electron-neutrinos in huge
numbers. Some of these electron-neutrinos change into the other two types on their
way to the earth and as a result we receive all three types. Their average flux on
earth’s surface is on the order of 10^6 cm−^2 s−^1.
The degree of difficulty in detecting neutrinos can be appreciated by the fact that
a neutrino can pass through a thousand earths without making a single interaction.
Still, scientists have exploited their abundance to design dedicated neutrino detectors
around the world.

H.3 SomeOtherParticles

All of the particles mentioned above have their counterparts in nature. For example
there is an anti-electron called positron having positive electrical charge and all the
other properties of an electron. When a positron and an electron come close, they
annihilate each other and give out energy in the form of photons. Similarly protons
have anti-protons and neutrinos have anti-neutrinos.
Apart from these so called anti-particles there is a host of other particles, some of
which exist in nature while others are produced in laboratories by colliding particles
at very high energies. For example gluons are the particles that bind the protons
and neutrons together inside the nucleus by thestrong nuclear force. The process of
radioactive decay, which we discussed earlier, is the result of theweak nuclear force
between particles mediated by W and Z particles. During high energy collisions a
large number of particles are created. Most of these particles have extremely short
mean lives, but with present day technologies, their properties can be studied in
detail. This has shed important light on the nature of elementary particles and
their underlying physics.