Speaker
Description
Neutrinoless double beta decay (0$\nu$2$\beta$) would provide experimental evidence for the Majorana nature of neutrinos. Its half-lives can be interpreted in terms of the effective neutrino mass through the phase-space factors and nuclear matrix elements (NMEs). However, theoretical uncertainties in the NMEs lead to an uncertainty of roughly one order of magnitude in the extracted effective mass. Precise measurements of two-neutrino double beta decay (2$\nu$2$\beta$) half-lives for multiple nuclei are therefore crucial to constrain these uncertainties.
Gadolinium-160 ($^{160}$Gd) is a candidate nuclide for double beta decay with a relatively high natural abundance of 21.9$\%$. However, its low Q-value of 1.73 MeV makes the observation of even 2$\nu$2$\beta$ decay highly challenging. Previous experiment using a 2-inch Gd$_2$SiO$_5$ (GSO) scintillator were unable to detect the 2$\nu$2$\beta$ decay due to significant background contributions from intrinsic radioactive impurities, such as uranium and thorium decay chains. As a result, a lower limit of 1.9 $\times$ 10$^{19}$ years was set on the 2$\nu$2$\beta$ decay half-life, while one of the theoretical calculations predicts a half-life of approximately 7.4 $\times$ 10$^{20}$ years.
The PIKACHU experiment is designed to overcome the limitations of the previous search by employing large Ce-doped Gd$_3$Ga$_3$Al$_2$O$_{12}$ (GAGG) single crystals to explore the 2$\nu$2$\beta$ decay in $^{160}$Gd. GAGG has several advantages over GSO. First, its light yield is approximately 60,000 photons/MeV, which is about six times higher than that of GSO, resulting in improved energy resolution. Second, $\alpha$ and $\beta$ ($\gamma$) events can be clearly distinguished using pulse shape discrimination techniques. Finally, the crystal growth technology for large GAGG single crystals is well established, enabling the preparation of large crystals with a higher $^{160}$Gd content and allowing for relatively straightforward detector scale-up in the future phases.
The experiment is conducted in two phases. Phase 1 aims to update the current lower limit on the 2$\nu$2$\beta$ decay half-life, while Phase 2 targets a sensitivity approximately one order of magnitude better than the previous experiments, with the ultimate goal of observing the decay.
Data acquisition for Phase 1 started on December 14, 2024, at the Kamioka underground laboratory, and more than one year of data have been accumulated to date. In Phase 1, two GAGG crystals containing a total of 355 g of $^{160}$Gd are employed. Their background levels are approximately an order of magnitude lower than those of conventional GAGG.
In this presentation, we introduce the concept of the PIKACHU experiment, the development of high-purity GAGG crystals, the experimental setup for Phase 1, and report the current constraint on the lower limits of the double beta decay half-lives.