Double beta decay experiments at Canfranc Underground Laboratory

Abstract

The first activities of the Canfranc Underground Laboratory ("Laboratorio Subterráneo de Canfanc", LSC) started in the mid-eighties in a railway tunnel located under the Spanish Pyrenees; since then, it has become an international multidisciplinary facility equipped with different services for underground science. The research activity at LSC is about Astroparticle Physics, dark matter searches and neutrino Physics; but also activities in Nuclear Astrophysics, Geophysics, and Biology are carried out. The investigation of the neutrinoless double beta decay has been one of the main research lines of LSC since the beginning. Many unknowns remain in the characterization of the basic neutrino properties and the study of this rare decay process requiring Physics beyond the Standard Model of Particle Physics can shed light on the lepton number conservation, the nature of the neutrinos as Dirac or Majorana particles and the absolute scale and ordering of the masses of the three generations. Here, the double beta decay searches performed at LSC for different emitters and following very different experimental approaches will be reviewed: from the very first experiments in the laboratory including the successful IGEX ("International Germanium EXperiment") for 76 Ge, which released very stringent limits to the effective neutrino mass at the time, to the present NEXT experiment ("Neutrino Experiment with a Xenon Time-Projection Chamber") for 136 Xe and future project CROSS ("Cryogenic Rare-event Observatory with Surface Sensitivity") for 130 Te and 100 Mo, both implementing innovative detector technologies to discriminate backgrounds. For the neutrinoless double beta decay channel and at 90% C.L., IGEX derived a limit to the half-life of 76 Ge of T 0ν 1/2 > 1.57 × 10 25 y while the corresponding expected limits are T 0ν 1/2 > 1.0 × 10 26 y for 136 Xe from NEXT-100 (for an exposure of 500 kg•y) and T 0ν 1/2 > 2.8×10 25 y for 100 Mo from CROSS (for 5 y and 4.7 kg of isotope). Activities related to double beta decays searches carried out in other underground laboratories have also been developed at LSC and will be presented too, like the operation of the BiPo-3 detector for radiopurity measurements of thin sheets with very high sensitivity. For each one of these experiments, the concept, the experimental setups and relevant results will be discussed.

Cosmogenic Activation in Double Beta Decay Experiments

“…The study of the neutrinoless DBD of 76 Ge using enriched semi-coaxial HPGe detectors provided the strongest bounds to the effective neutrino mass in the nineties, from IGEX ("International Germanium EXperiment") operated in the Canfranc Underground Laboratory (LSC) in Spain [61,62] and the Heidelberg-Moscow (HM) experiment [63,64], at the Gran Sasso Underground Laboratory (LNGS) in Italy. HM presented 90% C.L.…”

“…CROSS ("Cryogenic Rare-event Observatory with Surface Sensitivity") foresees the installation of arrays of both enriched Li 2 MoO 4 and TeO 2 bolometers to investigate the DBD of 100 Mo and 130 Te [130]. It is in the commissioning phase at the Canfranc Underground Laboratory [62]. Pulse shape discrimination techniques based on Solid-State-Physics phenomena are in development to identify and reject not only alpha but also beta surface contamination.…”

“…The technology of high-pressure xenon gas (HPXe) TPCs with electroluminescent amplification is being developed to investigate the neutrinoless DBD of 136 Xe by NEXT ("Neutrino Experiment with a Xenon TPC") in the Canfranc Underground Laboratory [62]. Several prototypes (with mass of ∼1 kg) were designed and operated in a first step to demonstrate the advantages of the technology, including a distinctive signal topology and very good energy resolution [158,159].…”

Cosmogenic activation in double beta decay experiments

“…As an example, Fig. 1 shows a compilation of excitation functions for 60 Co, a typical cosmogenic product in different targets, generated in natural germanium and in copper by nucleons taken from different sources, as made in Ref. [41]; the availability of a large amount of experimental data in copper allows a more reliable validation of calculations.…”

Estimation of nuclear matrix elements of double-$β$ decay from shell model and quasiparticle random-phase approximation

“…The 0νββ decay offers one of the limited methods to determine this mass scale. Largescale experimental projects around twenty [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] are in progress for observing the 0νββ decay with the assumption of the Majorana nature of the neutrino. For other methods to use nuclei, see Refs.…”