International Journal of Modern Physics A

The current situation in double beta decay experiments, the characteristics of modern detectors and the possibility of increasing the sensitivity to neutrino mass in future experiments are discussed. The issue of the production and use of enriched isotopes in double beta decay experiments is discussed in addition.

TeO₂ bolometers have been used for many years to search for neutrinoless double beta decay in ${}^{130}$Te. CUORE, a tonne-scale TeO₂ detector array, recently published the most sensitive limit on the half-life, $T_{1/2}^{0\nu }>1.5\times 10^{25}$ yr, which corresponds to an upper bound of 140–400 meV on the effective Majorana mass of the neutrino. While it makes CUORE a world-leading experiment looking for neutrinoless double beta decay, it is not the only study that CUORE will contribute to in the field of nuclear and particle physics. As already done over the years with many small-scale experiments, CUORE will investigate both rare decays (such as the two-neutrino double beta decay of ${}^{130}$Te and the hypothesized electron capture in ${}^{123}$Te), and rare processes (e.g. dark matter and axion interactions). This paper describes some of the achievements of past experiments that used TeO₂ bolometers, and perspectives for CUORE.

Bolometers are cryogenic calorimeters which feature excellent energy resolution, low energy threshold, high detection efficiency, flexibility in choice of materials, particle identification capability if operated as hybrid devices. After 30 years of rapid progresses, they represent nowadays a leading technology in several fields: particle and nuclear physics, X-ray astrophysics, cosmology. However, further and substantial developments are required to increase the sensitivity to the levels envisioned by future researches. A review of the challenges to be addressed and potentialities of bolometers in the search for rare nuclear decays is given, with particular emphasis to the neutrinoless double beta decay physics case.

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay ($0\nu \beta \beta$) of ${}^{76}$Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of $10^{-2}\mathrm{\text{cts}}/(\mathrm{\text{keV}}\cdot \mathrm{\text{kg}}\cdot \mathrm{\text{yr}})$ was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the $0\nu \beta \beta$ decay of ${}^{76}$Ge $T_{1/2}^{0\nu }>2.1\times 10^{25}$ yr (90% CL), while the corresponding median sensitivity was $2.4\times 10^{25}$ yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to $10^{-3}\mathrm{\text{cts}}/(\mathrm{\text{keV}}\cdot \mathrm{\text{kg}}\cdot \mathrm{\text{yr}})$. After analyzing 23.2 kg⋅yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the ${}^{76}$Ge $0\nu \beta \beta$ decay was obtained: $T_{1/2}^{0\nu }>8.0\times 10^{25}$ yr (90% CL), reaching a sensitivity of $5.8\times 10^{25}$ yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg$\cdot$yr. If no signal is found by then the GERDA sensitivity will have reached $1.4\times 10^{26}$ yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of $0\nu \beta \beta$ decay of ${}^{76}$Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of ${}^{76}$Ge. After ten years of data taking, it will reach a sensitivity beyond 10${}^{28}$ yr, and hence fully cover the inverted hierarchy region.

The DAMA project has obtained many competitive or new results in the search for various rare nuclear processes. Most of them have been obtained with the help of many different high purity crystal scintillators which have been measured in the low-background DAMA set-ups located in the Gran Sasso underground laboratory of INFN In this paper, the main results will be summarized.

The direct detection of galactic dark matter particles requires ultra-low background conditions. NaI(Tl) crystals are applied in the search for these dark matter particles through their interactions in the detector by measuring the scintillation signal produced. The production of long-lived isotopes in materials due to the exposure to cosmic rays on Earth’s surface can be an hazard for these ultra-low background demanding experiments, typically performed underground. Therefore, production rates of cosmogenic isotopes in all the materials present in the experimental set-up, as well as the corresponding cosmic rays exposure history, must be both well-known in order to assess the relevance of this effect in the achievable sensitivity of a given experiment.

Here, analysis of the cosmogenic studies developed from the ANAIS experiment NaI(Tl) detectors are presented. Installed inside a convenient shielding at the Canfranc Underground Laboratory just after finishing surface exposure to cosmic rays and thanks to the prompt data taking developed, identification and quantification of isotopes with half-lives of the order of tens of days were allowed, and thanks to the long-term operation of the detectors long-lived isotopes have been also identified and quantified. Main results for the activation yields of iodine and tellurium isotopes, ${}^{22}$Na, ${}^{113}$Sn, ${}^{109}$Cd, and tritium are presented in this work, together with the estimate of the production rates for their activation by cosmic nucleons while on Earth’s surface based on a selection of excitation functions over the entire energy range of cosmic nucleons.

Low counting experiments (search for double $\beta$ decay and dark matter particles, measurements of neutrino fluxes from different sources, search for hypothetical nuclear and subnuclear processes, low background $\alpha$, $\beta$, $\gamma$ spectrometry) require extremely low background of a detector. Scintillators are widely used to search for rare events both as conventional scintillation detectors and as cryogenic scintillating bolometers. Radioactive contamination of a scintillation material plays a key role to reach low level of background. Origin and nature of radioactive contamination of scintillators, experimental methods and results are reviewed. A programme to develop radiopure crystal scintillators for low counting experiments is discussed briefly.

The neutrinoless $\beta \beta$ decay of atomic nuclei continues to attract fervent interest due to its potential to confirm the possible Majorana nature of the neutrino, and thus the nonconservation of the lepton number. At the same time, the direct dark matter experiments are looking for weakly interacting massive particles (WIMPs) through their scattering on nuclei. The neutrino-oscillation experiments on reactor antineutrinos base their analyses on speculations of $\beta$-spectrum shapes of nuclear decays, thus leading to the notorious “reactor antineutrino anomaly.” In all these experimental efforts, one encounters the problem of $\beta$-spectrum shapes of forbidden $\beta$ decays, either as unwanted backgrounds or unknown components in the analyses of data. In this work, the problem of spectrum shapes is discussed and illustrated with a set of selected examples. The relation of the $\beta$-spectrum shapes to the problem of the effective value of the weak axial-vector coupling strength $g_{\mathrm{\text{A}}}$ and the enhancement of the axial-charge matrix element is also pointed out.

Borexino was designed to measure solar neutrinos in the MeV or sub-MeV energy range. The unprecedented radiopurity of the detector has allowed the detection of geo-neutrinos and the determination of competitive limits on the rate of rare or forbidden processes. In this paper, we review the basic principle of neutrinos and antineutrinos detection in Borexino and we describe the results of the geo-neutrinos measurements and their implications. Then we summarize the search for Borexino events correlated with gamma ray bursts and for axion induced signals, and the limits achieved on Pauli forbidden transitions and on the electron charge conservation.

Stellar evolution and nucleosynthesis are interconnected by a wide network of nuclear reactions: the study of such connection is usually known as nuclear astrophysics. The main task of this discipline is the determination of nuclear cross-section and hence of the reaction rate in different scenarios, i.e. from the synthesis of a few very light isotopes just after the Big Bang to the heavy element production in the violent explosive end of massive stars. The experimental determination of reaction cross-section at the astrophysical relevant energies is extremely difficult, sometime impossible, due to the Coulomb repulsion between the interacting nuclei which turns out in cross-section values down to the fbar level. To overcome these obstacles, several experimental approaches have been developed and the adopted techniques can be roughly divided into two categories, i.e. direct and indirect methods. In this review paper, the general problem of nuclear astrophysics is introduced and discussed from the point of view of experimental approach. We focus on direct methods and in particular on the features of low-background experiments performed at underground laboratory facilities. The present knowledge of reactions involved in the Big Bang and stellar hydrogen-burning scenarios is discussed as well as the ongoing projects aiming to investigate mainly the helium- and carbon-burning phases. Worldwide, a new generation of experiment in the MeV range is in the design phase or at the very first steps and decisive progresses are expected to come in the next years.

The decay properties of long-lived excited states (isomers) can have a significant impact on the destruction channels of isotopes under stellar conditions. In sufficiently hot environments, the population of isomers can be altered via thermal excitation or de-excitation. If the corresponding lifetimes are of the same order of magnitude as the typical time scales of the environment, the isomers have to be treated explicitly. We present a general approach to the treatment of isomers in stellar nucleosynthesis codes and discuss a few illustrative examples. The corresponding code is available online at http://exp-astro.de/isomers/.