Experiment – ORNL

The predecessor & Baseline Design

The Experiment

The predecessor experiments:

⁷⁶Ge is one isotope known to undergo two neutrino double-beta decay, making it a candidate to search for the yet-to-be-observed neutrinoless double-beta decay. The MAJORANA DEMONSTRATOR and GERDA are two Ge-based neutrinoless double-beta decay experiments that have established excellent energy resolution (2.5 keV FWHM) [Phys Rev C 100, 025501 (2019)] and background index of less than 1 x 10^-3 counts/(keV kg yr) [Science 365, 1445–1448 (2019)] at the energy region to set competitive half-life limits; the MAJORANA DEMONSTRATOR experiment has set a limit of greater than 2.7 x 10²⁵ yr from an exposure of 26 kg-yr while the GERDA experiment has set a limit of greater than 9 x 10²⁵ yr from an exposure of 82.4 kg-yr. GERDA recently announced a new result setting a leading limit of 1.8 x 10²⁶ yr from an exposure of 103.7 kg-yr.

The preliminary phase of the LEGEND program is LEGEND-200: 200 kg of Ge detectors (88% in ⁷⁶Ge) operated inside a cryogenic active shield using low-background components and electronics inside the existing GERDA infrastructure at LNGS in Italy. The experiment will probe neutrinoless double-beta decay with a sensitivity of T_1/2 > 10²⁷ yr (90% confidence level), corresponding to a range of the effective neutrino mass of about < 30-70 meV, within 5 years live time. Existing enriched detectors from the MAJORANA DEMONSTRATOR and GERDA experiments, as well as the procurement of new detectors of the larger inverted-coaxial, point contact (ICPC) detector style, are secured to meet a total detector mass of 200 kg. Construction is underway and data collection is expected to begin in 2021.

The baseline design:

The LEGEND-1000 baseline technical design is centered around the demonstrated low-background and excellent energy performance of p-type, point-contact high-purity Ge (HPGe) semiconductor detectors, enriched to over 88% in 76Ge The larger mass inverted-coaxial, point contact (ICPC) detectors will be the standard in use for LEGEND-1000. Approximately 400 individual ICPCs with an average mass of 2-3 kg will be instrumented for a total detector active mass of 1000 kg.

The background rejection power of ICPC-style detectors begins with the superior energy resolution, demonstrated to be as low as 0.12% FWHM at the region of interest, which is necessary to separate two-neutrino from neutrinoless double-beta decays. Beyond the tight energy cut, additional cuts based on pulse-shape analysis parameters offer a discrimination of backgrounds from the neutrinoless signal of interest. A pulse shape analysis of the charge-collection signature from a bulk double-beta energy deposition can be distinguished from a multiple-interacting gamma ray and surface events. The highly granular nature of the Ge detector array allows discrimination of background interactions that span multiple detectors. Finally, background interactions external to the Ge detectors are signaled by an active liquid Ar background-suppression shield.

The Ge detectors are split among four, 250-kg modules to allow commissioning of the array in stages and independent operation. In each module, the detector strings are immersed within the LAr active shield, sourced from radiopure underground Ar, to provide direct mitigation of background. Background sources that deposit energy in the LAr create scintillation light that is readout by a curtain of wavelength-shifting polystyrene fibers. The TPB (tetraphenyl butadiene) coated fibers are coupled to silicon photomultiplier photodetectors. Each of the four underground LAr modules are surrounded by natural LAr, with additional light collection, and supported within a vacuum-insulated cryostat, itself inside a water tank providing infrastructure and additional shielding.