Detector, String, and Cryostat Assembly

Detector, string, and cryostat assembly is performed in the glovebox. The glovebox is divided into three sections, each engineered for the progressive stages of string building and assembly. Individual detectors are assembled in the detector glovebox. Multiple detectors are assembled into detector strings in the string assembly glovebox. Each string is loaded into a cryostat in the module glovebox; a total of seven strings can be loaded into a cryostat assembly.

Assembly of detectors and strings can only begin if the glovebox has been cleaned. Clean- ing involves wiping down surfaces with deionized water and ethanol. The glovebox is con- tinuously purged with LN boil-off. Particle counts are monitored at all times, and must return to normal levels after glovebox cleaning before work can commence. All parts and tools required for detector building are accounted for in the glovebox; any extra required parts are gathered from a nitrogen purged dry box and placed in the glovebox antechamber. Components in the antechamber must be purged with nitrogen gas for 24 hours before being moved into the work space.

A detector assembly consists of the germanium crystal, an EFCu high-voltage ring for detector biasing, a grounded point contact electrode for charge collection, and a low-mass front-end (LMFE) for signal readout. The assembly is held together with hollow EFCu hexagonal bolts and teflon nuts. All components are handled according to strict protocol outlined in the detector assembly procedure [110]. Building the detectors can be especially challenging because personnel working in the glovebox will often have four layers of gloves

(a) Detector Assembly (b) String Assembly

Figure 3.8: (a) A Majorana detector assembly. The germanium crystal is supported by an EFCu frame consisting of the HV ring (bottom), hexagonal bolts, and a detector mount. The LMFE is attached to the center of the detector mount (b) A string assembly consisting of a stack of four detectors being loaded into a module. Loading is performed in the module glovebox.

(2 from the cleanroom suit, the glovebox glove, and a clean glove over the glovebox glove) resulting in inhibited dexterity. The LMFE is particularly fragile due to its low mass required because of its proximity to the detector. Contact with the LMFE surface can damage the tiny wire bonds connecting the JFET to the pads on the silica board. Ultra-fine 50-gauge PICO-COAX cables [141] are epoxied to the board to connect the LMFE to a preamplifier or a diagnostic tool. After the LMFE is installed, the detector baseline is checked via the first stage preamplifier output to ensure no connections on the board were broken. An example of a detector assembly is shown in figure 3.8.

Once enough detector units are completed (four or five depending on the string size), they are transferred to the string building glovebox via an interlock chamber. The same cleanliness protocols as the detector glovebox apply when working in the string glovebox. Detectors are stacked and held together using three tie rods that fit through the hollow hex bolts. At the top of the string is an adapter plate that is used to attach the string

to a copper coldplate that’s kept in thermal contact with a mass held at LN temperature. Some additional hardware is used for cable management. Completed strings undergo initial testing and characterization outside of the glovebox in a custom-designed string test cryostat. After a string undergoes testing, it is then transferred to the module glovebox to prepare for cryostat loading. See figure 3.8.

The third chamber of the glovebox is the module glove box, used for loading strings into one of the Majorana custom built cryostats, see figure 3.11. The full module must be docked to this glove box to install strings into the cryostat. String loading is performed by raising the string using a lift to secure it in place while it’s bolted to a coldplate capable of holding seven strings. The coldplate is attached to a copper hoop that has been electron- beam welded to a crossarm tube2. Cabling from the string is run through ports in the coldplate and attached to an adapter on the opposite side. Additional cabling runs through the crossarm tube to the service body feedthroughs of the module vacuum system. The crossarm tube houses cabling and a thermosyphon tube for cooling of the coldplate [142], while simultaneously providing a vacuum path for cryostat pumping [110].

After all the strings are loaded and the cabling connections checked, preparations for sealing the cryostat are made. A thin copper IR shield is placed around the detectors to reduce IR shine that affects detector temperature, and thus leakage current, during operation. To seal the cryostat, the cryostat can is placed on the lift, raised up, and then carefully clamped to the hoop. A copper lid is placed on top of the hoop, completing the cryostat assembly.

Once initial roughing vacuum leak checks are complete, the cryostat is pumped down to ultra-high vacuum. This can take multiple days because of the large volume and the amount of material obstructing the crossarm tube, e.g. cabling, and IR shine-path shielding. 2_{E-beam welding was performed by Electron Beam Welding Associates in Indianapolis, IN. To minimize}

cosmic ray exposure, the parts were driven and detailed logs were kept of the time that the parts were above ground. A muon counter was taken along with the parts in an attempt to quantify cosmic ray exposure during the trip.

When the vacuum pressure is low enough, ∼ 10−5 Torr, the thermosyphon cooling system is pressurized to begin cooling the strings. Heat from the coldplate is carried away by the two-phase nitrogen working fluid in the thermosyphon tube. A dewar filled with LN, housing a condenser placed at a height above the coldplate, serves as a thermal reservoir for the two-phase fluid. A ballast tank is used for pressure equalization. Gravity stimulates the convective heat flow between the coldplate and dewar [142]. Once the detectors’ temperature is low enough they can be biased. Diagnostic data is taken while the module is attached to the glovebox to ensure the module is ready to be moved into the shield.