Wikipedia: Gamma rays

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Gamma rays (γ) are an energetic form of electromagnetic radiation usually produced by radioactivity or other nuclear reactions. Gamma rays are more penetrating than either alpha or beta rays.

Shielding for γ rays requires large amounts of mass. Shields that reduce gammay 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 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.

Gamma rays are almost always 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 exited 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 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.

/Talk