Physics & Astronomy ETDs

Publication Date

Fall 12-21-2017


Metallic Magnetic Calorimeters (MMCs) are low temperature particle detectors which can be used for a wide range of applications including high-resolution γ -ray spectroscopy. High energy resolution in γ -ray spectroscopy is desired for Non Destructive Assay (NDA) of nuclear material and improved measurements of nuclear data. Our group has been developing γ -ray MMCs that should ultimately provide energy resolution more than an order of magnitude better than the benchmark detector technology, high purity germanium detectors (HPGe). Two components of these MMCs have been developed as a part of this dissertation: an on-chip passive superconducting persistent current switch and an improved microfabrication technique for electroforming cantilevered particle absorbers thick enough to have adequate stopping power for gamma rays. MMCs can require large on-chip magnetizing currents of order ∼100 mA to achieve optimal performance. To minimize noise injected from room-temperature current supplies, it is useful to trap these currents in on-chip persistent superconducting loops. These loops have so far used electrically heated persistent current switches. However, wire count can be reduced and on-chip design flexibility increased by using a passive superconducting persistent current switch with a superconducting critical temperature (T c ) intermediate between T c of the Nb loop and the operating temperature of the MMC. In addition, it is desirable for the T c of the switch to be above the regenerv ation temperature on the single-shot adiabatic demagnetization refrigerators (ADRs) that are widely used in the field. We have developed a new passive persistent current switch based on niobium-tantalum (NbTa) alloy shunts. With this approach we have demonstrated trapping of on-chip persistent currents up to 150 mA with no evidence of flux creep over 20 h, and persistence of 100 mA trapped current through several regeneration cycles of our ADR with a regeneration temperature of 2 K. MMC absorbers for higher-energy photons can require substantial thickness to achieve adequate stopping power. We developed a new absorber fabrication process using dry-film photoresist to electroform cantilevered, thick absorbers from gold. The new method is fully compatible with the microfabrication process for Superconducting QUantum Interference Devices (SQUIDs), and requires much less time, effort, and rework than other methods that have been used for this purpose. With this method we have so far electroformed 30 ➭m thick absorbers attached to MMC devices and demonstrated fabrication of 100 ➭m thick absorbers on test structures by layering two dry-film resists. Both of these improvements have been successfully applied to our MMC designs. With the completed prototype devices our collaborators at Lawrence Livermore National Lab have recently demonstrated 38 eV energy resolution for 60 keV γ -rays from 241 Am test source, while a reasonable estimate for HPGe resolution would be 310 eV. To our knowledge this is the best MMC energy resolution performance that has been achieved to date. This demonstrates feasibility of our detectors for high-resolution γ -ray spectroscopy.

Degree Name


Level of Degree


Department Name

Physics & Astronomy

First Committee Member (Chair)

Prof Stephen T. Boyd

Second Committee Member

Prof Dinesh Loomba

Third Committee Member

Prof Paul R. Schwoebel

Fourth Committee Member

Prof Adam A. Hecht



Document Type