High-sensitivity cryogenic light detectors for rare event searches

Research project


In the comprehension of fundamental laws of nature, astroparticle physics is now facing two important questions:
1) What are the properties of neutrinos? Are they analogous to other well-known elementary particles? Among the unknown neutrino properties is the absolute mass scale and, most important, whether it is a Majorana particle or not. In contrast to other massive elementary particles, neutrinos may be indistinguishable from antineutrinos, thus validating the theory postulated by the Sicilian physicist in 1937 and that is still awaiting for an experimental confirmation. The only practicable way to verify this hypothesis is to search for a particular nuclear process: neutrinoless double beta decay. Its observation would allow to measure the absolute scale of neutrino mass and would give a definitive proof of the fact that they are Majorana particles: an invaluable improvement in our comprehension of the nature and its origin. A non observation of this decay would put the current scientific theories under important revisions.
2) What is the so called “dark” matter made of? Astrophysical observations imply that the largest part of the mass of the Universe is composed by a form of matter other than atoms and known matter constituents. We still do not know what dark matter is made of because its rate of interaction with ordinary matter is really low, thus making the experimental detection extremely difficult. Given our ignorance about the nature of these particles, the diversification of experimental techniques is essential for this research field. The observation of dark matter by different experiments may lead to a well established discovery and it would open a wide research area for the comprehension of the laws of matter and the Universe.

Both neutrinoless double beta decay and dark matter interactions are rare processes: they could be detected using the same experimental technique or even the same experiment. In both cases, detectors featuring large mass, low background and good energy resolution are needed. A very promising technique consists in the use of bolometers, cryogenic calorimeters where the absorbed particle energy is converted into a measurable temperature rise.

The CUORE experiment (Cryogenic Underground Observatory for Rare Events) will search for neutrinoless double beta decay of Te-130. CUORE is under construction at the Gran Sasso underground laboratories (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN), in Italy, and aims at becoming the most sensible experiment among its competitors. The detector will be an array of 988 TeO2 crystals with size of 5x5x5 cm^3 each, maintained at 10 mK by a dilution refrigerator. A possible obstacle to the successful outcome of the experiment may be represented by the detector materials themselves: they contain radioactive contaminants emitting alpha particles, with different nature with respect to the beta particles emitted in the searched process. In other bolometric experiments, the use of scintillating crystals makes it possible to discriminate the nature of interacting particles based on the amount of light they emit while being absorbed. Unfortunately, tellurium dioxide bolometers do not scintillate at cryogenic temperatures.
It has been demonstrated recently that the identification of beta particles in TeO2 crystals could be performed on the base of a different phenomenon, the Cerenkov effect. In the energy range of interest for neutrinoless double beta decay, beta particles emit Cerenkov radiation while alpha particles do not. However for beta particles with energies corresponding to the double beta decay, the light signal amounts to 80 eV only. This value is too low if compared to the 100 eV noise of the light detectors used so far, that are made of germanium discs operated as bolometers. A complete rejection of the alpha background could be obtained instead with a light detector featuring a noise as low as 10-20 eV. If the Cerenkov light emitted by the CUORE bolometers could be monitored with such detectors, the sensitivity of the experiment would improve by a factor of 6.
In parallel to the construction of CUORE, the LUCIFER collaboration is investigating the possibility to build an experiment using crystals featuring both good bolometric and scintillating performances. Among the investigated materials is ZnSe, which would be used for the search of neutrinoless double beta decay of Se-82. At energies corresponding to the double beta decay, beta particles interacting in ZnSe produce about 20 keV in light, a signal that allows a complete discrimination of alpha and beta particles using the currently available light detectors. However the same crystals could be used for dark matter detection: in this case the energies of interest are in the few keV range, much lower than the 3 MeV to be detected in double beta decay. Therefore also the amount of emitted light is much lower, about 70 eV, and cannot be measured by the light detectors used so far. Instead, with light detectors having a resolution of 10-20 eV, LUCIFER would become sensitive not only to double beta decay, but also to dark matter interactions.

The goal of this research project is to build high sensitivity light detectors in view of applications in CUORE, LUCIFER and other experiments based on the bolometric technique. We decided to focus on the development of kinetic inductance detectors (KID), a new technology in this research field. KIDs are superconducting detectors working at temperatures well below the phase transition. This technique, first proposed in 2003 for astrophysical applications, is now used to produce detectors made of pixels with an area of few mm^2 and a resolution lower than 1 eV. The main advantages of these detectors consist in their reliability, in the fact that they are easy to produce and, most important, in the possibilty to read a large amount of channels with a single pair of excitation/readout lines.
We plan to extend the use of KIDs to particle physics experiments, developing light detectors with an area of several cm^2 and a resolution between 5 and 10 times better than that achieved with the germanium light detectors we used so far. Moreover this would be the first application in which KIDs will be operated at 10 mK, much lower than the 100 mK temperature that is normally used in astrophysical experiments.
The research project will be composed of two parts: in the first one we will design and produce two light detector prototypes and will characterize them in terms of noise and light collection efficiency; in the second part of the project we will use the same prototypes for two measurements at LNGS, where they will be faced to ZnSe and TeO2 bolometers with the purpose of verifying the discrimination power obtained.

This research will put together the expertise of scientists from INFN, Sapienza University of Roma and University of Genova. Besides researchers with experience in the CUORE and LUCIFER collaborations, researchers with a background in astrophysics and KIDs will be involved and hired in this project. We already have access to most of the resources and facilities that will be needed for the development of this research, and we will start a collaboration with the Italian National Research Center (CNR) for the design and production of the KIDs. The aim of this project is not only to extend the use of KIDs to particle physics, but also for the involved researchers to acquire a know-how that could be spent in other research fields. This latter aspect should not be underestimated, given the growing interest of the scientific community in this kind of technology for applications such as X-ray detection or the study of q-bits.
Effective start/end date1/1/12 → …




beta particles
dark matter
alpha particles
Cerenkov radiation
elementary particles
research projects