International Journal of Modern Physics A

Deep underground laboratories are multidisciplinary research infrastructures. The main feature of these laboratories is the reduced cosmic ray muons flux. This characteristic allows searching for rare events such as proton decay, dark matter particles or neutrino interactions. However, biology in extreme environments and geophysics are also studied underground. A number of ancillary facilities are critical to properly operate low background experiments in these laboratories. In this work we review the main characteristics of deep underground laboratories and discuss a few of the low background facilities.

The Gran Sasso National Laboratories (LNGS) of the National Institute of Nuclear Physics (INFN) are equipped with an underground ultra-low background facility namely the SubTErranean Low Level Assay (STELLA), which is in first line dedicated to material screening for the fundamental physics experiments installed in the underground laboratories. The high level of performance of the ultra-low-level detector systems allows for detecting extremely low radioactivity levels in materials down to the $\mu \mathrm{\text{Bq}}\cdot {\mathrm{\text{kg}}}^{-1}$ level. The installations and experimental setups using ultra-low background techniques will be described shortly, and an example of material screening will be discussed.

Rare events search experiments, like those dedicated to the direct evidence of dark matter or neutrinoless double beta decay, are among the most exciting challenges of modern physics. The sensitivity of such experiments is driven by the background, which depends substantially on the radiopurity of the materials used for the experimental apparatus. Cutting edge measurement techniques are needed for a fast, sensitive and efficient screening of these materials and the certification of their production. Trace element measurements of high sensitivity and quick execution are mandatory also in other fields like tracing the geographical origin of food, temporal and geographical assignment of cultural heritage or monitoring environmental radioactivity. This work is an overview of the inorganic mass spectrometry facility available at Gran Sasso National Laboratory (LNGS) for radiopure material screening and is especially focused on its ICP-MS instrumentation. Analytical methods developed to achieve lowest detection limits in different types of matrix, like metals, polymers, crystals and composite materials, are also indicated. Detection limits of $10^{-18}{\mathrm{\text{gg}}}^{-1}$ for ${}^{226}\mathrm{\text{Ra}}$, $10^{-14}{\mathrm{\text{gg}}}^{-1}$ for U and Th and $10^{-12}{\mathrm{\text{gg}}}^{-1}$ for K are attained through dedicated operation conditions of the instrumentation. Details are given on the results obtained for different experiments ongoing or under construction at LNGS.

A challenge in many present cutting-edge particle physics experiments is the stringent requirements in terms of radioactive background. In peculiar, the prevention of Radon, a radioactive noble gas, which occurs from ambient air and it is also released by emanation from the omnipresent progenitor Radium. In this paper we review various high-sensitivity Radon detection techniques and approaches, applied in the experiments looking for rare nuclear processes happening at low energies. They allow to identify, quantitatively measure and finally suppress the numerous sources of Radon in the detectors’ components and plants.

There are many potentially active isotopes among the lanthanide elements which are possible to use for low-background experiments to search for double $\beta$ decay, dark matter, to investigate rare $\alpha$ and $\beta$ decays. These kind of experiments require very low level of radioactive contamination, but commercially available compounds of lanthanides are always contamined by uranium, thorium, radium, potassium, etc. A simple chemical method based on liquid–liquid extraction has been applied for the purification of ${\mathrm{\text{CeO}}}_2$, ${\mathrm{\text{Nd}}}_2{\mathrm{\text{O}}}_3$ and $\mathrm{\text{Gd˙2O˙3}}$ from radioactive traces. Detailed schemes of purification procedure are described. Measurements by using HPGe spectrometry demonstrate high efficiency in K, Ra, Th, U contaminations reduction on at least one order of magnitude.

Experiments looking for rare events like the direct detection of dark matter particles, neutrino interactions or the nuclear double beta decay are operated deep underground to suppress the effect of cosmic rays. But, the production of radioactive isotopes in materials due to previous exposure to cosmic rays is a hazard when ultra-low background conditions are required. In this context, the generation of long-lived products by cosmic nucleons has been studied for many detector media and for other materials commonly used. Here, the main results obtained on the quantification of activation yields on the Earth’s surface will be summarized, considering both measurements and calculations following different approaches. The isotope production cross-sections and the cosmic ray spectrum are the two main ingredients when calculating this cosmogenic activation; the different alternatives for implementing them will be discussed. Activation that can take place deep underground mainly due to cosmic muons will be briefly commented too. Presently, the experimental results for the cosmogenic production of radioisotopes are scarce and discrepancies between different calculations are important in many cases, but the increasing interest on this background source which is becoming more and more relevant can help to change this situation.

The application of very low background techniques is fundamental in deep underground experiments dedicated to the investigation of rare processes. In this paper, the case of NaI(Tl) detectors is discussed mainly considering their crucial role for direct Dark Matter (DM) investigation. In particular, the DAMA/LIBRA apparatus, as a result of a long-standing activity focused on the optimization of NaI(Tl) detectors, is underlined.

Crystal scintillators are very promising detectors to investigate double beta decay of atomic nuclei. Recent achievements in development and application of tungstate and molybdate crystal scintillators, as well as prospects for the next generation double beta decay experiments are discussed.

The ZnWO₄ crystal is a very promising scintillator to investigate Dark Matter candidates able to induce just nuclear recoils by exploiting the directionality technique. In particular, the anisotropy of the light output for heavy particles in this scintillator can be considered to point out the presence — in the diurnal counting rate — of a signal produced by such Dark Matter candidates. In this paper the status of art of the performances of recently realized ZnWO₄ crystal scintillator is summarized and possible future improvements are discussed. Finally, the directionality approach to pursue this kind of Dark Matter investigation with the anisotropic crystal scintillator is addressed and reachable sensitivities — under given assumptions — in future experiments are discussed.

Many neutrino physics experiments use organic liquid scintillators, which present a unique advantage: enormous masses (in the order of tens ktons) with very low radioactive background can be reached by assembling a detector with organic liquid scintillators. Thanks to the very fast decay rate, it is possible to localize the event in space and time and discriminate it from the background signals. Furthermore, organic liquid scintillators are very efficient in alfa/beta discrimination and offer the possibility to dissolve solvents in the chemical compounds to enhance the signal.

Water purification is an important technique in high-mass low radioactivity experiments in modern physics. Water is frequently used both as a shielding and as the sensitive part of a particle detector in underground arrangements, especially in the frame of Astroparticle Physics studies. In this paper, I will describe the main purification techniques and discuss some of its performances.

Bolometers are low temperature particle detectors with high energy resolution and detection efficiency. Some types of bolometric detectors are also able to perform an efficient particle identification. A wide variety of radiopure dielectric and diamagnetic materials makes the bolometric technique favorable for applications in astroparticle physics. In particular, thanks to their superior performance, bolometers play an important role in the worldwide efforts on searches for neutrinoless double-beta decay. Such experiments strongly require an extremely low level of the backgrounds that can easily mimic the process searched for. Here, we overview recent progress in the development of low background techniques for bolometric double-beta decay searches.