The TUNL-ITEP ββ decay setup was used in the TUNL Low Background Counting Facility (LBCF) to investigate the ββ decays of ¹⁰⁰Mo and ¹⁵⁰Nd to excited states of ¹⁰⁰Ru and ¹⁵⁰Sm respectively. These studies were accomplished by using a coincidence technique involving two high-purity germanium (HPGe) detectors. Below, you can see a generic level diagram for a double-beta decay to an excited final state. This state decays by de-excitation to the ground state, emitting characteristic gamma rays. In the generic case shown, the decay goes through the 0⁺₁→2⁺₁→0⁺_g.s. states, emitting E_γ1 and E_γ2 in coincidence.

created by M.J. Hornish
In the TUNL-ITEP ββ decay setup, ββ decay source is sandwiched between the two HPGe detectors. These detectors are operated in coincidence, so that if E_γ1 is seen in one detector and E_γ2 is seen in the other, an event is recorded. Surrounding the HPGe detectors is an annulus of sodium-iodide (NaI) detectors and plastic shields, which are operated in anti-coincidence with the detectors. This helps to remove accidental events and background which come from outside sources, such as cosmic ray muons. Furthermore, a lead house is stacked up around the annulus and plastic shields, which provides some passive sheilding.
created by M.J. Hornish
Here are the detectors without the annulus. Here the detectors are inside the annulus, and the lead house is halfway stacked. Finally, the completed lead house.