Project

The ACCESS project aims to develop and operate cryogenic calorimeters to evaluate with high accuracy the spectral shape of forbidden β- decay. In particular, the collaboration is now focused on indium oxide and indium iodine crystals to measure 115In β-decay. Further isotopes will be studied with natural and doped crystals.

Why cryogenic calorimeters?

The idea originates from the analyses performed by CUPID-0 (82Se), CUPID-Mo (100Mo) and CUORE (130Te) on the two-neutrino double β-decay mode. Such measurements showed that a detailed reconstruction of the experimental data with Monte Carlo simulations is not only possible but also very sensitive to the spectral shape of continuous contributions. The main advantages of this technique are summarized on the following points.

Cryogenic calorimeters based on semiconductor sensors are sensitive in a wide energy range, from few keV up to 10 MeV. This feature allows us to measure the internal contaminations by studying the alpha decays from natural radioactivity while searching for low-energy signals.
Energy resolution lower than 1% in the whole sensitive region guarantees identification and reconstruction of the background sources, resolving close peaks and distintive decay structures.
These detectors combine a containment efficiency for electrons higher than 80% with an intrinsic radiopurity due to contaminants segregation during the crystal growth, thus providing a promising signal-to-background ratio.

Project Overview

The ACCESS project proposes to use different crystals to measure the spectral shape of different β-emitters.

NATURAL CRYSTALS

Just a couple of forbidden β-decay emitters are natural isotopes with a high isotopic abundance (i.a.); this is the case of 115In (i.a. = 95.72 %), 113Cd (i.a.= 12.23%), and 87Rb ( 27.835%). In particular, the first two isotopes can be embedded in crystals suitable to be used as an absorber in a cryogenic calorimeter, such as indium oxide (In2O3), indium iodine (InI), and cadmium tungstate (CdWO4). These are the first crystals and isotopes to which ACCESS will focus on.

An other interesting candidate to explore as cryogenic calorimeter for the ACCESS purposes could be the PbWO4, the material used for the electromagnetic calorimeter of the CMS experiment at LHC. Indeed, the residual amount of 210Pb from natural radioactivity in such crystal could allow one to study the low-energy β-decay of 210Pb, and the subsequent decay of 210Bi. This material is currently under investigation for RES NOVA, a proposed experiment to detect Supernova Neutrinos with PbWO4 based cryogenic calorimeters.

DOPED CRYSTALS

Unfortunately, natural crystals would allow to study only a limited number of isotopes belonging to the long list of interesting candidates. A different approach is therefore needed to develop a technique able to assess simultaneously a higher number of isotopes. What we propose is to have a carrier crystal, e.g. TeO2 or Li2MoO4, doped with a specific β-decaying isotope during the crystal growth. For each isotope, a twin pair of crystals, one doped and one natural, can be operated simultaneously.

While the counting rate of the doped crystal will be dominated by the embedded β-source, the natural one will monitor internal and environmental backgrounds. A combined Bayesian fit of Monte Carlo simulations to the two spectra will ensure a precise reconstruction of both signal and background spectral shapes. The goal of ACCESS is to test this approach at least with an isotope, reasonably 99Tc.

THERMAL SENSORS

The natural choice for the ACCESS thermal sensors is the semiconductor thermistors, i.e. Neutron Transmutation Doped germanium (Ge-NTD), developed and successfully operated by CUORE, CUPID-0 and CUPID-Mo collaborations.

In natural crystals, the signal counting rate is determined by the crystal dimensions, which can be optimized to increase the signal to background ratio, even if some technical aspects of crystal growth could set a limit preventing to reach the real optimal point. For this application, NTD readout is very suitable, because of the expected counting rate of O(150mHz) which keeps the pile up probability below 10%.

Doped crystals exhibit a signal counting rate dependent on the source concentration, which can be adjusted by balancing the high signal to background ratio and the low pileup probability. In this case, NTD sensor may not be the most effective choice, because the slow time response limits the maxim tolerable activity to O (1 Hz). The Transition Edge Sensors (TES) can overcome such limitations.

Figure adapted from

A. Giachero 2017 J. Phys.: Conf. Ser. 841 012027

Working Groups

WG1 - NUCLEAR THEORY

Main tasks:

Theoretical modeling of the forbidden beta decay
Results interpretation

WG2 - CRYSTALS

Main tasks:

Material selection for the crystal production
Crystal production and procurement

WG3 - MONTE CARLO SIMULATIONS

Main tasks:

Geant4 Monte Carlo simulations
Detector response modeling

WG4 - EXPERIMENTAL MEASUREMENTS

Main tasks:

Detector design, assembly, operation and optimization
Data taking and signal processing

WG5 - DATA ANALYSIS

Main tasks:

Background modeling
Bayesian fit to the experimental data of Monte Carlo simulations

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