Question

State the condition of instability for and describe what happens during a β+ decay

Answer 1

Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus, as shown in . Beta decay is a process that allows the atom to obtain the optimal ratio of protons and neutrons.

There are two types of beta decay. Beta minus (β) leads to an electron emission (e⁻); beta plus (β+) leads to a positron emission (e⁺). In electron emission an electron antineutrino is also emitted, while positron emission is accompanied by an electron neutrino. Beta decay is mediated by the weak force.

Emitted beta particles have a continuous kinetic energy spectrum, ranging from 0 to the maximal available energy (Q), that depends on the parent and daughter nuclear states that participate in the decay. The continuous energy spectra of beta particles occur because Q is shared between a beta particle and a neutrino. A typical Q is around 1 MeV, but it can range from a few keV to a several tens of MeV. Since the rest mass energy of the electron is 511 keV, the most energetic beta particles are ultrarelativistic, with speeds very close to the speed of light.

Since the proton and neutron are part of an atomic nucleus, beta decay processes result in transmutation of one chemical element into another. For example:

137Cs  137Ba + e⁻

11Na  10Ne + e⁺

Instability:

The periodic variation of the mass surface caused the pairing energy also causes a large number of even-even nuclei to be unstable with respect to two successive beta decays. This process is called double beta decay. The difficulty is that the probability of a double transition is extremely low. A gross estimate can be made by squaring the rate constant obtained above, and the number of decays from even large samples is at best one per day and at worst a few per year.