Highlights from GERDA: Probing the Majorana Neutrino Mass at 100 meV †
Abstract
:1. Introduction
2. The Setup
- Phase I was operated from 9 November 2011 to August 2013. Eight semi-coaxial germanium (Coax) detectors enriched up to ∼87% in Ge (Ge) from the former Heidelberg-Moscow (HdM) [9] and Igex (Igex) [10] experiments, and later 5 freshly produced Ge point-contact germanium detectors (BEGe), for a total mass of ∼18 kg of Ge, were operated bare in a 63 m liquid argon (LAr) bath. The LAr cryostat is in turn placed at the center of a 650 m water tank where a Cerenkov muon veto detector is installed. The experimental setup is fully described in [11].
- Phase II started on 20 December 2015 after 1.5 years of upgrade work: the Ge mass increased up to 35.6 kg, 20 kg in form of 20 BEGe detectors [12], 19 of which were properly functioning, the rest being the Phase I Ge-Coax detectors (15.6 kg) plus 3 Ge-Coax (7.6 kg). It was shown [13] that the BEGe detectors have improved pulse shape rejection features with respect to Coax: this is thanks to the highly non-uniform electric field, yielding to a large spread and gradient of the holes drift velocities in the detector volume, hence to longer and more stretched pulses, allowing better discrimination of multi-site (MSE) energy releases. The pulse shape discrimination of Compton scattered s is hence improved. In addition, to reject those s releasing energy in the Ge detectors after scattering in LAr, a volume of ∼50 cm diameter and ∼220 cm height was delimited by an Oxygen Free High Conductivity (OFHC) Cu foil lined with a reflector foil and equipped with 16 Photomultipliers (9 top, 7 bottom): the central 100 cm of the cylinder are equipped with 800 m Scintillating Fibers, replacing the Cu foil, read out by Silicon Photo Multipliers (SiPM).To further confine the LAr in intimate contact with the Ge detectors, hence minimize the rate of the extremely dangerous ∼3.5 MeV particles from K decaying at the detector surface1, a transparent mini-shroud made from the Borexino ultra-low radioactivity nylon [14], surrounds each detector string. K is a cosmogenic isotope contained in LAr at (100) Bq level. The mini-shrouds, the fibers, the reflector foils, the PMTS are coated with TPB2 a VUV wavelength shifter, shifting the 128 nm LAr scintillation light to match the PMTs and SiPM spectral sensitivity in the optical range. The Phase II array, the nylon mini-shrouds and the fiber curtain shroud are shown in Figure 2 and described in [15].
3. Data Taking
4. Data Treatment
5. Discussion and Results
6. Conclusions
Funding
Conflicts of Interest
References
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1 | K ( = 12.6 h; (–)) is the decay product of Ar ( = 32.9 y; ) |
2 | TPB: Tetraphenyl butadiene |
3 | both the energy calibration and energy resolution have some ± variations from one calibration to the other, usually ∼0.1–0.3 keV, but in some cases, larger. Hence the physics data are calibrated following the scale variations and an exposure-weighted FWHM is determined for each data set |
4 | time for the pulse from 10% to 90% of its amplitude |
Data Set | [kg · yr] | FWHM [keV] | BI 10 cts/(keV · kg · yr) | N | |
---|---|---|---|---|---|
Phase I-Golden | 17.9 | 4.3(1) | 0.57(3) | 11 ± 2 | 46 |
Phase I-Silver | 1.3 | 4.3(1) | 0.57(3) | 30 ± 10 | 10 |
Phase I-BEGe | 2.4 | 2.7(2) | 0.66(2) | 5 | 3 |
Phase I-Extra | 1.9 | 4.2(2) | 0.58(4) | 5 | 2 |
Phase II-COAX 1 | 5.0 | 3.6(1) | 0.52(4) | 3.5 | 4 |
Phase II-COAX 2 | 23.1 | 3.6(1) | 0.48(4) | 0.6 | 3 |
Phase II-BEGe | 30.8 | 3.0(1) | 0.60(2) | 0.6 | 5 |
Data Set | Enrich. | Active Vol | Peak | QC | LAr-Veto | PSD | Total |
---|---|---|---|---|---|---|---|
Phase I-Golden | 86.7 | 86.7 | 91.4 | >99 | - | 83(3) | 57(3) |
Phase I-Silver | 86.7 | 86.6 | 91.4 | >99 | - | 83(3) | 57(3) |
Phase I-BEGe | 87.8 | 91.0 | 90.0 | >99 | - | 92.1(1.9) | 66(2) |
Phase I-Extra | 86.7 | 87.2 | 91.7 | >99 | - | 83(3) | 58(4) |
Phase II-COAX 1 | 86.7 | 86.5 | 91.4 | 99.92 | 97.8(1) | 77(5) | 52(4) |
Phase II-COAX 2 | 86.6 | 86.4 | 91.4 | 99.92 | 97.7(1) | 71.2(4.3) | 48(4) |
Phase II-BEGe | 88.00 | 88.7 | 89.7 | 99.92 | 97.7(1) | 87.6(2.5) | 60(2) |
Isotope | Moles | FWHM | BI | Sensitivity | Limit | Limit Range | ||
---|---|---|---|---|---|---|---|---|
[kg · yr] | [keV] | cts/(keV · kg · yr) | [yr] | [yr] | [eV] | |||
Gerda [25] | Ge | 410 | 59.3 | 3.3 | 0.6 | 11.0 | 9.0 | 0.11–0.26 |
Majorana [26] | Ge | 340 | 26.0 | 2.5 | 5.0 | 4.8 | 1.9 | 0.24–0.52 |
Cuore [27] | Te | 1580 | 83.6 | 7.7 | 15.0 | 0.7 | 1.5 | 0.11–0.52 |
Cupid [28] | Se | 57 | 2.9 | 23.0 | 3.6 | 0.23 | 0.24 | 0.38–0.77 |
Exo-200 [29] | Xe | 550 | 56 | 71 | 1.7 | 3.7 | 1.8 | 0.15–0.40 |
KAMland-ZEN [30] | Xe | 2530 | 138 | 265 | 0.3 | 5.6 | 11.0 | 0.06–0.17 |
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Cattadori, C.M.; the GERDA Collaboration, o.b.o. Highlights from GERDA: Probing the Majorana Neutrino Mass at 100 meV. Universe 2019, 5, 55. https://doi.org/10.3390/universe5020055
Cattadori CM, the GERDA Collaboration obo. Highlights from GERDA: Probing the Majorana Neutrino Mass at 100 meV. Universe. 2019; 5(2):55. https://doi.org/10.3390/universe5020055
Chicago/Turabian StyleCattadori, Carla Maria, and on behalf of the GERDA Collaboration. 2019. "Highlights from GERDA: Probing the Majorana Neutrino Mass at 100 meV" Universe 5, no. 2: 55. https://doi.org/10.3390/universe5020055