MJD ?detector unit? assembly begins

Last Thursday, working nearly a mile underground in the cleanest space in South Dakota, physicist Ryan Martin assembled the first of 70-some ?detector units? for theMajorana, which will look for a rare, yet-to-be-detected phenomenon called neutrinoless double-beta decay, is under construction in the Davis Campus on the 4,850-foot level.

The assembly took only about 2? hours. Still, it was a milestone. Martin had practiced the task many times on the surface at Lawrence Berkeley National Laboratory?attaching a polished germanium-crystal detector the size of a hockey puck to a wafer-thin glass electronics board?but he had never assembled a ?detector unit? while wearing four pairs of protective gloves.

The gloves are to protect the experiment, not Martin. The Majorana is underground to shield it from cosmic radiation. The germanium detectors ultimately will be placed in cryostats made of ultra-pure copper that, in turn, will be inside a copper shield inside the lead igloo?all of which will protect them from radiation from the surrounding rock. The detectors also must be protected from radiation from any other source during construction?including people, equipment and even specks of dust. That?s why everyone working in the assembly room wears hooded Tyvek coveralls and two pairs of nitrile gloves.

The detector units themselves are being assembled inside a nitrogen-filled glove box inside the clean room. Once Martin has inserted his already double-gloved hands into the glove-box gloves, he dons a fourth pair of gloves that have been positioned inside the box.

Then the delicate work begins. Each germanium crystal must be precisely connected to ?the fused silica (glass) low-mass front end board? (LMFE) upon which is mounted the electronic circuits that will relay signals from the germanium detectors. The LMFE is mounted on a copper leaf spring, and a pin makes the connection with the germanium.

By noon today, Martin and his team had assembled four detector units. ?It?s taken a long time to get to this point,? he said. And a lot of work remains to be done. These first detectors are made from natural germanium. They?ll be used in prototype detectors, to test the experiment before installation of the ?enriched? germanium?that is, detectors with a much higher percentage of the isotope 76Ge.

Neutrinoless double-beta decay, in theory, will result from the natural double-beta decay of the 76Ge isotope in the detectors, which will be both the source of the decay and the detection medium. The first two enriched detectors have been completed by ORTEC, a contractor in Tennessee. Enriched germanium will begin arriving here early next year.