Nuclear Physics
Electromagnetic Radiation

How are alpha beta and gamma radiation created?

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2011-05-31 21:42:28

Forms of all three can be created by nuclear reactions. Alpha

radiation is actually particles (a helium nucleus), beta

radiation consists of either electrons or positrons, and

gamma rays are the only one of the three that is actually a

form of radiation energy (photon).


Alpha decay involves the release of a helium nuclei (alpha

particle) from the nucleus of the parent nuclide. This reduces the

atomic number by two and reduces the atomic mass number by


Some physicists actually consider it an extremely tilted form

of spontaneous fission, with 24He2+ (alpha) being one daughter, and

AN-2AMU-4Something being the other daughter.


Beta- decay starts with the weak interaction that causes a down

quark in a neutron to change into an up quark, releasing a W-

boson. The neutron becomes a proton, increasing the atomic number

by one, and keeping the atomic mass number the same. The W- boson

then decays into an electron and an electron antineutrino.

Beta+ decay involves energy, which is used to change an up quark

in a proton into a down quark, changing the proton into a neutron,

decreasing the atomic number by one, and keeping the atomic mass

number the same. It also emits a positron, and an electron

neutrino. Sometimes, K capture is involved in order to achieve the

extra energy. K capture is when an inner (K) shell electron is

absorbed into the nucleus, transferring its energy into the

nucleus, followed by reshuffling of the electron cloud. K

capture can also occur without beta+ decay.


All of the above events, as well as things like fission, fusion,

neutron absorbtion, and a few others, can leave the nucleus in an

excited state. When it comes back down to ground state, a photon is

emitted with an energy that corresponds to the energy level

transition of the step that just occurred. This is a gamma ray.

Delayed Gamma

Usually, if there is going to be a gamma event following, say, a

beta- event, it occurs quickly, typically within 1 x 10-12 seconds.

Some nuclides, however, have a meta stable form where the gamma

event is delayed, sometimes for a very long time. In the case of

Technetium-99m, for instance, the gamma event has a half-life of 6

hours. This is very useful in the medical field, where

Technetium-99m can be tagged to certain biologically sensitive

materials, injected, and then scanned, such as for a heart scan,

impacting the body with only the gamma and not the beta.


Not asked, but answered for completness and due to its

similarity to gamma...

The electron cloud can also become excited, for various reasons,

such as K capture, or when an alpha particle flys by and steals an

electron. When it does so, it also "wants" to come back to ground

state. As each electron comes down in energy, it emits a photon,

just like the nucleus does. When this happens, it is called an

x-ray. Usually, gamma and x-rays have different energies, but there

is some overlap, and it is possible that they can be

indistinguishable from each other. For more information on the

difference between gamma radiation and x-rays, please see the

related question below.

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