LEGEND Overview | Experiment Nuclear and Astroparticle Physics

*Figure 1: LEGEND collaborators at the May 2023 collaboration meeting in Rome, Italy*

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) collaboration builds on the successful aspects of the Majorana Demonstrator and GERDA collaborations in its search for neutrinoless double-beta decay (0νββ) in the 76Ge isotope. LEGEND is implementing a phased approach in its goal to reach a discovery sensitivity of 1028 years. The first phase, LEGEND-200, is currently taking data at the Laboratori Nazionali del Gran Sasso (LNGS) in the heart of Italy. It just began its search 0νββ taking its first data in late 2022 with 142 kg of high purity 76Ge detectors deployed in its liquid Argon (LAr) cryostat. The second phase, LEGEND-1000, aims to deploy 1000 kg of 76Ge in a similar LAr cryostat to reach the discovery goal of the collaboration.

Despite being the most abundant particle in the universe that has mass, there is much to be discovered about the neutrino including its physical nature and its exact mass. Neutrinos could be Dirac in nature, requiring the neutrino and anti-neutrino to be separate particles, or they could be the first particle to be Majorana in nature where the neutrino is its own antiparticle. Double beta decay (2νββ) provides a good mechanism to probe the mass scale and nature of the neutrino. If the neutrino is Majorana in nature then the potential for no neutrinos to be emitted in double beta decay is possible resulting in neutrinoless double beta decay (0νββ). If 0νββ is discovered it would immediately present the nature of the neutrino and the rate of the decay would provide insight into the mass scale of neutrinos. If the neutrino is Majorana in nature then 0νββ should be able to be observed in any isotope that decays by 2νββ.

Figure 2: (Left) Typical 2νββ is demonstrated on the left where two neutrinos and two electrons are emitted from the nucleus. Some neutrino exchange process is demonstrated in the center figure resulting in no neutrinos being emitted in the 0νββ process on the right. (Right) With no neutrinos present to carry away the energy from the double beta decay, all of the energy is carried by the two electrons which are immediately detected. This results in a monoenergetic peak for 0νββ compared to the spectrum of energies in 2νββ.

One of the isotopes that undergoes 2νββ is 76Ge. Ge detectors are conveniently well-understood technology. These detectors can be created with extremely high purity and can also be enriched in the 76Ge isotope on the order of 90%. These detectors can achieve incredible energy resolution while being operated within a reasonable temperature range of 70K to 100K. Developments in Ge detector technology have led to point contacts for signal readout which allows for amazing pulse shape discrimination and larger detector masses, both of which are vital to the success of LEGEND. All of these benefits combine to make Ge detectors a powerful tool in the search for 0νββ.

LEGEND-200 has currently deployed 142 kg of Ge detectors into its LAr cryostat with plans to install more in the near future. The experiment hopes to obtain a total of 1000 kg*yr of data over the next 5 years. The ENAP group supports LEGEND-200 in nearly every aspect of the experiment from hardware and electronics development to data acquisition and analysis.

Several test stands are constructed in the ENAP lab spaces. The Full Chain Test (FCT) is a large liquid Argon cryostat is designed to recreate the entire L-200 electronics chain using identical parts and detectors that are installed at LNGS. With the ability to easily identify potential problems within the electronics chain, this test proved to be very valuable by saving the collaboration significant time and effort during the installation and commissioning of the L-200 array. The FCT is now being repurposed to study interesting characteristics of a subset of detectors in L-200 by recreating identical situations in the ENAP laboratory. A second test stand that aims to characterize detector responses to alpha and beta radiation on the sensitive passivated surfaces is currently active in the ENAP labs as well.

Figure 3: The LEGEND-200 experimental setup. (Left) a cross-sectional view into the water tank and LAr tank of LEGEND-200. The cleanroom, where the electronics operate and detector strings are assembled, is above these tanks on the fourth floor. (Right) A cross-sectional view of the detector array when fully assembled. Two barrels of wavelength-shifting fibers (green) comprise the LAr veto system surrounding the twelve strings of Ge detectors (grey).

ENAP has carved out large roles in data acquisition and analysis within the LEGEND collaboration. L-200 utilizes the Object-orientated Real-time Control and Acquisition (ORCA) software for all of its data acquisition and monitoring. ORCA was created and is continuously being developed and maintained by the ENAP group. Data analysis is a specialty for a large portion of ENAP members. Ge detectors’ excellent signal quality provides the ability to endlessly improve the data analysis routines for LEGEND.

Figure 4: Current proposed experimental setup for LEGEND-1000. There are four arrays of Ge detectors similar to LEGEND-200 that are deployed in copper re-entrant tubes containing radioactively pure under-ground LAr. Similar to LEGEND-200 a tank of LAr will surround the Ge array and this will be contained within a water tank for a muon veto system.

LEGEND-1000 (L-100) is currently development stage. The ENAP group plans to continue their strong roles in hardware and electronics testing, DAQ development, detector development, and analysis as L-1000 moves forward. The FCT will be rebuilt into a larger test stand to perform similar electronics and detector tests for L-1000. Characterizing L-1000 detectors and developing new detector geometries is another area where significant contributions will be made. ORCA continues to be the baseline data acquisition software for the L-1000 experiment and will require significant updates and maintenance efforts in the coming years. In order for L-1000 to reach its large exposure and stringent background goals significant developments in data analysis will be needed and contributed to by the ENAP group at UNC.

August 11, 2023, by Brady

Visit the LEGEND official website – https://legend-exp.org/