MadSci Network: Physics
Query:

Re: Gamma Rays, Heat and Energy

Date: Wed Sep 1 14:30:49 1999
Posted By: Stephen Murray, Physicist
Area of science: Physics
ID: 935507549.Ph
Message:

Hi Andrew,

The short answer is yes, the material will heat up, by an amount that depends upon the total energy of gamma rays that you "shoot" at it.

In general, gamma rays can be absorbed by any material, but they are not as easily absorbed as are lower energy photons. If you look at the human body, for example, we absorb essentially all of the ultraviolet light that hits us (or why we get sunburns). The ultraviolet light can break molecular bonds, and ionize atoms. Ionization occurs when an electron in an atom or molecule absorbs the energy of a photon that is enough to completely strip the electron from the atom or molecule. The atom or molecule that is left behind is now missing an electron. The now free electron "pinballs" around, hitting nearby molecules, and depositing the energy that it gained from the photon as heat.

X-rays, which have higher energy than ultraviolet light are absorbed primarily by ionization, though they may also break up molecules. But, an x-ray photon is much less likely to be absorbed by an electron in a given atom or molecule than is an ultraviolet photon. As a result, many x-rays can pass right through us, which is why x-ray machines are able to work.

Gamma rays have even higher energy than x-rays, and are even less likely to be absorbed. When they are absorbed by an electron, that electron will be stripped from its molecule, just as in the case of ultraviolet light, and the electron will then collide with nearby molecules. Some of the energy may even go into breaking up nearby molecules, while the rest is spread around as heat.

High energy gamma rays may also be absorbed by the nuclei of atoms. It is very analogous to the electrons orbiting the nuclei, which may be in the ground state (closest to the nuclei) or in excited states (higher energy states, further away from the nuclei, which requires the absorption of energy). In a similar fashion, the nucleons (protons and neutrons) in the nuclei of atoms may be in either ground or excited states. The energies involved in exciting the nuclei are, however, much greater than those involved in moving electrons to excited states.

In a nuclear fission reaction, the energy that is produced is carried away by multiple channels. Some of it goes into kinetic energy (energy of motion) of the fission particles. These include the lighter nuclei that result from the breakup of the larger nucleus, and also the neutrons that are produced in the fission. Some energy also comes out in gamma rays and x-rays. These come about because the lighter nuclei are generally created in excited states, and emit gamma and x-ray photons when the nucleons return to their ground states.


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