The design of an ultra-low background thermosyphon for the Majorana Demonstrator

The Majorana Demonstrator (MJD) is an ultra-low background neutrinoless double-beta decay (0νββ) experiment that will deploy up to 40 kg of high purity germanium detectors (HPGe). The goal of this experiment is to demonstrate the feasibility of building a detector array with less than 1 event/ton year in a 4 keV region of interest around the 0νββ signal. HPGe diodes, when used as ionizing radiation detectors, need to be maintained at a temperature close to that of liquid nitrogen (77 K). This work describes the results of research and development toward a cryogenic system capable of meeting the ultra-low background requirements while providing the required cryogenic cooling capacity of 15–30 W. This paper shows the experimental results obtained using a two-phase horizontal thermosyphon using nitrogen as the working fluid. The cold tests show that the proposed thermosyphon has sufficient cooling power to handle the heat load of an MJD module. Results for the temperature gradient across the thermosyphon, cooling capacity, and design considerations demonstrate that the thermosyphon can effectively remove the calculated heat load of each module of the experiment.