Answer to Question #2376 Submitted to "Ask the Experts"

Q

How does beta decay result in the nuclear emission of gamma photons? For example, ¹³³Xe decays by beta decay, but also emits 81 keV photons. Where do the 81 keV photons come from? Most beta decay seems to emit one or more gamma photons. Also, why doesn't ³²P emit gamma radiation when it beta decays?

A

The ¹³³Xe nucleus (atomic number 54) decays spontaneously by simultaneous emission of a beta particle and an antineutrino (a particle with no charge and nearly zero or zero mass). These two particles do not normally exist in the nucleus, but are created in the process of the transformation. Immediately following decay, the nucleus is left with one more unit of positive charge than it had before, since it emitted a (negative) electron. It has become the nucleus for a ¹³³Cs atom (atomic number 55). The decay either leaves the ¹³³Cs nucleus in its state of lowest energy, i.e., the ground state, or in an excited state with excess energy still to be shed. If the ¹³³Cs nucleus is initially in an excited state, then it quickly emits the excess energy as a gamma photon and thus goes to its ground state; if it is initially in its ground state, then there is no gamma ray or further action. With ¹³³Xe, an 81-keV gamma photon is emitted in 37% of the transformations, and there is essentially no gamma photon the rest of the time.

With some atoms, beta decay always leaves the residual nucleus directly in its ground state. Examples of such "pure" beta emitters, i.e., with no gamma-ray emissions, are ³H, ¹⁴C, and ³²P, which you asked about, as well as ⁹⁰Sr and ⁹⁰Y.

Gamma-ray emission, when it occurs, usually follows quickly after radioactive decay. A few species of excited nuclei, however, take a relatively long time to emit the gamma ray.

Gamma rays also accompany alpha-particle decay, for the same reasons described here. Alpha decay typically leaves a residual nucleus in one of several possible excited states.

James E. Turner, CHP, PhD