Members from two current-generation experiments — GERDA in the EU and Majorana Demonstrator (MJD) in the US — along with other experts in other low-background underground experiments joined forces to realize this new project. Nuclear Science Division, whose principal contribution will be low-background low-noise signal readout implementation, is a founding institutional member of LEGEND. In summer 2020, LEGEND successfully completed a test run of the prototype electronics for its first phase despite numerous obstacles effected by COVID-19.

For the first phase of LEGEND — LEGEND-200 — the plan is to operate up to 200 kg of ⁷⁶Ge-enriched germanium detectors using the existing GERDA infrastructure at Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The signal front-end readout scheme in LEGEND-200 follows MJD’s basic design. But it would need to be modified and tested for continuous operation in liquid argon. The 10-plus-meter signal path between the detectors and the backend processing electronics is also significantly longer than that of about 2 meters in MJD. Figure 1 shows the low-mass front-end developed for LEGEND-200, which maintains similar features as the MJD one.

Figure 2. The LEGEND-1000 ASIC test board. The ASIC, developed with LBNL LDRD funding, is the tiny speck mounted slightly off center on the left of this test circuit board.

In the prototype test at LNGS, NSD provided the front-end electronics and some other signal processing electronics in the backend. The tests were successfully, demonstrating that the performance has surpassed that in GERDA in energy resolution and signal-background discrimination power.

The GERDA infrastructure at LNGS is now being modified to allow the heavier detector payload in LEGEND-200. The goal of this phase of the experiment is to reach a 0νββ half-life sensitivity of better than 10²⁷ years in ⁷⁶Ge.

The second phase of LEGEND — LEGEND-1000 — is planned to have one ton of ⁷⁶Ge-enriched germanium detectors, which will have a 0νββ half-life sensitivity exceeding 10²⁸ years and encompass the full so-called “inverted-ordering” region of the neutrino mass scheme. NSD is leading the effort to develop an application-specific integrated-circuit (ASIC) to further improve the signal readout that has been achieved in LEGEND-200 (Fig. 2).