Gamma ray

Gamma rays (often denoted by the Greek letter gamma, γ) are an
energetic form of electromagnetic radiation produced by radioactivity or
other nuclear or subatomic processes such as electron-position annihilation.
Gamma rays are more penetrating than either alpha or beta radiation, but
less ionizing. They are a form of electromagnetic radiation. Gamma rays are
distinguished from X rays by their origin. Gamma rays are produced by
nuclear transitions while X-rays are produced by energy transitions due to
accelerating electrons. Because it is possible for some electron transitions
to be of higher energy than nuclear transition, there is an overlap between
low energy gamma rays and high energy X-rays.

 Nuclear processes
 Radioactive decay processes

    * Alpha decay
    * Beta decay
    * Gamma radiation
    * Neutron emission
    * Proton emission
    * Spontaneous fission

 ----------------------------
 Nucleosynthesis

    * Neutron Capture
         o The R-process
         o The S-process
    * Proton capture:
         o The P-process

Shielding for γ rays requires large amounts of mass. Shields that
reduce gamma ray intensity by 50% include 1cm (0.4 inches) of lead, 6cm (2.4
inches) of concrete or 9cm (3.6 inches) of packed dirt.

Gamma rays from nuclear fallout would probably cause the largest number of
casualties in the event of the use of nuclear weapons in a nuclear war. An
effective fallout shelter reduces human exposure at least 1000 times.

Gamma rays are less ionising than either alpha or beta rays. However,
reducing human danger requires thicker shielding. They produce damage
similar to that caused by X-rays such as burns, cancer, and genetic
mutations.

In terms of ionization, gamma radiation interacts with matter via three main
processes: the photoelectric effect, Compton scattering, and pair
production.

Photoelectric Effect: This describes the case in which a gamma photon
interacts with and transfers all of its energy to an orbital electron,
ejecting that electron from the atom. The kinetic energy of the resulting
photoelectron is equal to the energy of the incident gamma photon minus the
binding energy of the electron. The photoelectric effect is thought to be
the dominant energy transfer mechanism for x-ray and gamma ray photons with
energies below 50 keV (thousand electron volts), but it is much less
important at higher energies.

Compton Scattering: This is an interaction in which an incident gamma photon
loses enough energy to an orbital electron to cause its ejection, with the
remainder of the original photon's energy being emitted as a new, lower
energy gamma photon with an emission direction different from that of the
incident gamma photon. The probability of Compton scatter decreases with
increasing photon energy. Compton scattering is thought to be the principal
absorption mechanism for gamma rays in the intermediate energy range 100 keV
to 10 MeV (million electron volts), an energy spectrum which includes most
gamma radiation present in a nuclear explosion. Compton scattering is
relatively independent of the atomic number of the absorbing material.

Pair Production: By interaction in the vicinity of the coulomb force of the
nucleus, the energy of the incident photon is spontaneously converted into
the mass of an electron-positron pair. A positron is a positively charged
electron. Energy in excess of the equivalent rest mass of the two particles
(1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus.
The electron of the pair, frequently referred to as the secondary electron,
is densely ionizing. The positron has a very short lifetime. It combines
with 10-8 seconds with a free electron. The entire mass of these two
particles is then converted to two gamma photons of 0.51 MeV energy each.

Gamma rays are often produced alongside other forms of radiation such as
alpha or beta. When a nucleus emits an α or β particle, the
daughter nucleus is sometimes left in an excited state. It can then jump
down to a lower level by emitting a gamma ray in much the same way that an
atomic electron can jump to a lower level by emitting ultraviolet radiation.

Gamma rays, x-rays, visible light, and UV rays are all forms of
electromagnetic radiation. The only difference is the frequency and hence
the energy of the photons. Gamma rays are the most energetic. An example of
gamma ray production follows;

First cobalt-60 decays to excited nickel-60 by beta decay

   60Co --> 60Ni* + e- + ν?e

Then the Nickel-60 drops down to the ground state (see nuclear shell model)
by emitting a gamma ray.

   60Ni* --> 60Ni + γ

Uses:

The powerful nature of gamma-rays have made them useful in the sterilising
of medical equipment by killing bacteria. They are also used to kill
bacteria in foodstuffs to keep them fresher for longer.

In spite of their cancer-causing properties, gamma rays are also used to
treat some cancerous growths. Multiple concentrated beams of gamma rays are
directed on the growth in order to kill the cancerous cells. The beams are
aimed from different angles to focus the radiation on the growth while
minimising damage to the surrounding tissues.