University of Wisconsin-Madison Physical Sciences Lab

Learn More: CUORE

[Conceptual drawing of DCS]

Early in 2008, PSL mechanical engineer Ken Kriesel and CAD specialist Glen Gregerson began design of a detector calibration system (DCS) for the Cryogenic Underground Observatory for Rare Events (CUORE) with Associate Professor Karsten Heeger and others of the UW-Madison Physics Department.

CUORE is a rare event experiment to search for the yet unobserved neutrinoless double beta decay. CUORE employs 988 tellurium oxide crystals cooled to 0.01 degrees Kelvin for improved sensitivity. The tellurium is about one-third composed of an isotope that will decay, so it both provides the source of decay and makes up part of the detector. The purpose of the detector calibration system is to provide an absolute energy calibration for each one of the crystals. The goal is to introduce weakly radioactive calibration sources through bent guide tubes into the cryostat and cool them to < 4K to not alter the working point of the bolometers. The guide tubes are a few millimeters wide and extend up to 4 meters in overall length. The strings are Kevlar or Vectran thread just 0.35mm (0.014?) in diameter, on which 1.8mm diameter copper tubes have been crimped after insertion of small activated iron wires. Teflon heat shrink is applied over the copper to improve sliding of the sources in the guide tubes to limit heat generation in the lower temperature portions of the cryostat.

Each string is attached to its own drive assembly. Each drive is equipped with a small stepper motor, encoder, load cell, home switch, kill switch, and stop arm. The design houses three drives and one proximity sensor in one chamber. Individual drives, source cooling clamp assemblies, guide tubes and brackets have been prototyped. A structure called 4ktest (see above image) has been designed, built assembled, and shipped to the Physics Department for assembly and testing. It is for test at 4 degrees Kelvin in the cryostat (before all the cryostat components have been built) of a portion of the DCS.

Work on CUORE at PSL has halted, while testing of component prototypes continues at Physics. Eventually there will be 12 drives, 4 chambers, 12 guide tube assemblies, and 4 source coolers, enough for the full complement of 12 calibration strings on CUORE. Work at PSL is estimated to resume in 2014. It will likely include redesign found necessary after the prototype tests, construction of the remaining chambers, and providing information for integration with the cryostat being designed in Italy.

It has been a challenge to pack a lot of functionality into a very compact vacuum chamber to fit within available space among other instruments atop the cryostat, while meeting the many unique requirements. Material choices are restricted by vacuum compatibility, cryogenic compatibility, and low background radiation requirements.

For more information on CUORE, see the CUORE web page.