# What are continuous energy spectra?

##### 1 Answer

"Continuous *Energy spectra*" in nuclear chemistry typically refers to the fact that kinetic energy of electrons (or positrons) released in beta decays can take *any* value from a specific range of energies.

#### Explanation:

The sum of all energy released in a nuclear reaction can be calculated from mass defect, the difference in the mass of the products and the reactants, by the equation

The amount of mass lost in a particular process of beta is definite, meaning that the sum of kinetic energy of all product particles shall have discrete values.

It is possible to set up a system of equations to solve for the final kinetic energy of the electron:

- The sum of final kinetic energy of the nucleus and the electron equals to the energy released in the decay;
- Momentum conserves

This system will yield a finite number of solution (one or two) if the decay produces only two particles: the nucleus and an electron. Hence one might expect to detect electrons that travel at some particular velocity near a collection of nucleus undergoing beta-minus decay.

However, experimental results disagree with the prediction; instead of giving discrete points, plotting kinetic energy against the number of particles possessing that amount of energy will produce a distribution similar to that of a Maxwell-Boltzmann distribution curve.

Both energy and momentum still have to conserve; the release of the antineutrino in beta-plus decays (or neutrino for beta-minus decays) as a third product of beta decays accounts for the continuous energy spectra. For a general beta-minus decay:

The sum of kinetic energy of all three products- the daughter nuclei, the electron, and the antineutrino- is a definite value.

Hence

The mass of the nucleus is much larger than that of the electron and the antineutrino such that its share of kinetic energy is negligible; the kinetic energy of the antineutrino, however, can vary significantly from