CANDLES

Double Beta Decay

"Double beta decay" is the process in which two neutrons in a nucleus are converted into two electrons and two protons. At the same time two anti-neutrinos are also emitted. This "two neutrino double beta decay", which emits the two anti-neutrinos, is allowed within the standard model of particle physics, although the decay rate is extremely small(half life = 10¹⁹~²¹).

On the other hand, there might be the other process of double beta decay where the emitted anti-neutrino is converted to the neutrino inside the nucleus and is absorbed. In this process, no anti-neutrinos are emitted. This "neutrino-less double beta decay" is allowed only if the neutrinos are Majorana particles.

"Neutrino-less double beta decay" has a very long half-life, which is longer than 10²⁴ years. In order to measure the decay, a detector system needs a large amount of double beta decay nuclei and low background condition.

CANDLES

In order to measure the rare decay, there are two key points for the detector system. The one is a large amount of double beta decay nuclei. The other is a low background condition.

To realize the low background condition, we selected ⁴⁸Ca among double beta decay isotopes. ⁴⁸Ca has an advantage of the highest Q-value. The Q-value corresponds to the sum energy of two electrons in the neutrino-less double beta decay. This large Q-value, which is larger than the energies of natural radiations, ensures the least backgrounds from them.

In this project, we study double beta decay of ⁴⁸Ca. The enrichment of ⁴⁸Ca is an only firm way to increase double beta decay nuclei without increasing backgrounds.

When we realize the ⁴⁸Ca enrichment and the low background measurement for double beta decay, sensitivity of the measurement will be much improved. Then, we will make a substantial contribution to the progress of astroparticle physics.

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