Total Absorption Spectroscopy with SuN

The Summing NaI (SuN) detector is a large volume scintillator designed to detect γ rays very efficiently. In the last couple of years, SuN has been used to study relatively heavy, neutron-rich nuclei using a technique called “Total Absorption Spectroscopy” or “TAS”. The TAS technique requires the use of a γ-ray calorimeter, such as SuN, in order to detect the emitted γ rays with an efficiency close to 100%, and extract the β-decay intensity (Iβ), feeding individual levels of the daughter nucleus. Iβ is an important quantity, as it links directly to the structure of the decaying nucleus, and is a sensitive tool for probing the nuclear shape.

Recently the SuN group has published the first TAS results on the proof-of-principle case of the β decay of gallium-76 into germanium-76. Even though germanium-76 is a stable isotope and this decay was originally measured 45 years ago, the SuN results showed that the previously measured decay was incomplete and significant intensity was incorrectly assigned to low energy levels (Figure). This phenomenon is known as the pandemonium effect. Our measurement showed once again the need for TAS measurements for the accurate extraction of β decay intensity.

In addition, germanium-76 is also a candidate for neutrinoless double β decay (0νββ), a process where the single β-decay is energetically not allowed, and the nucleus decays by a double β decay, without, however, the emission of the expected two neutrinos. This is an important process that has been proposed but has not yet been observed and several large-scale experiments are attempting to measure it for the first time. The recent SuN results were compared to available theoretical models in the 76Ge region and provided a new constraint for the theories used in 0νββ calculations.

Figure Caption: Total absorption spectrum with the SuN detector for the decay of 76Ga. The SuN experimental spectrum and best fit are shown in black and red. The literature Iβ values were used to create an equivalent spectrum (blue) which does not reproduce the experimental data. The inset shows a technical drawing of the SuN detector.

Work published in Physical Review C