radioactive decay (To天同)

The following paragraph is extracted from the web-page given below:

http://library.thinkquest.org/27954/dequ.htm

"Gamma rays are a type of electromagnetic wave. For complex nuclei there are many different possible ways in which the neutrons and protons can be arranged within the nucleus. Gamma rays can be emitted when a nucleus undergoes a transition from one such configuration to another. For example, this can occur when the shape of the nucleus undergoes a change, implying that gamma rays can also be emitted during an alpha or beta decay. Since gamma rays carry no mass or charge, they do not change the atomic number or the mass number of the daughter nucleus."

The "different configuations" of the nucleus referred to in the paragraph is just the "different energy states" (or qunatum states) of the nucleus. When the nucleus changes from a high to a low energy state (without any change of the number of protons and neutrons), the excessive energy is emitted as gamma rays.

The following web-page also gives a simple explanation pn the emission of gamma rays.

http://library.thinkquest.org/10429/low/nuclear/nuclear.htm

The relevant paragraph is excerpted below:

"Gamma Emission is when an excited nucleus gives off a ray in the gamma part of the spectrum. A gamma ray has no mass and no charge. This often occurs in radioactive elements because the other types of emission can result in an excited nucleus".