Physics & Astronomy ETDs

Publication Date

Spring 5-16-2018

Abstract

A Metallic Magnetic Calorimeter (MMC) is a cryogenic calorimetric particle detector that employs a metallic paramagnetic alloy as the temperature sensor material. MMCs are used in many different applications, but this work will focus on their uses in high energy resolution gamma-ray spectroscopy. This technology is of great interest to the field of Nuclear Forensics and Nuclear Safeguards as a non-destructive assay for isotopic analysis of nuclear samples. The energy resolution of MMCs is an order of magnitude higher than the benchmark High Purity Germanium (HPGe) detectors that are currently used in the field and MMCs are also poised to outperform the current leading microcalorimeter, the Transition Edge Sensor (TES). This dissertation will cover the work in development of paramagnetic alloys of Ag and Er as the sensor material, and testing of two generations of devices.

The heart of the MMC is the paramagnetic sensor material. The workhorse paramagnet for MMCs is currently an alloy of Au and Er. Although Au:Er is a high performing alloy, it starts to falter at the temperatures below 100 mK which are desirable to maximize the performance of the MMC. Au has a nuclear electric quadrupole moment, which at low temperatures, has its energy levels split by the radial electric fields created by the Er ions. This effect causes the specific heat of the alloy to increase as temperature is lowered, which diminishes device performance. A promising alternate paramagnet is an alloy of Ag and Er. Ag, with both naturally occuring isotopes having a nuclear spin of I=1/2, does not have a nuclear electric quadrupole moment. A technical challenge to working with Ag is that it has such a high affinity for oxygen that the usual method of creating Au:Er alloys may not be sufficient for Ag:Er. Much greater care has to be taken in removing oxygen from the alloy, as during creation the oxygen could adversely alter the Er dopant, thus degrading performance. To combat this, a vacuum induction furnace was developed to achieve the best possible control over synthesis process parameters. Description of the new furnace and test results from a successful synthesis of a Ag:Er alloy are discussed.

The other half of the MMC is a high-performing Superconducting Quantum Interfence Device (SQUID) magnetometer. In previously reported devices, it was standard to have the sensing coil, magnetizing circuit, and paramagnet be on a separate chips from the SQUID magnetometer. The approach taken at UNM has been to integrate the SQUID magnetometer and paramagnetic sensor onto a single chip. Having an integrated device increases the performance of the MMC, at the cost of a more difficult fabrication process. Two exploratory wafers of magnetometers have been fabricated and tested for use as MMCs. The first wafer is a set of exploratory two-pixel devices, varying almost every aspect of the device to search for optimal device parameters. The second wafer consists of 14-pixel MMC arrays. A process of electroplating gold absorbers to the devices that now contain sensitive SQUIDs has been developed, using a two-mold system to define the legs and body of the absorbers. An initial Fe-55 spectrum from one of the new arrays is shown as a proof of concept measurement.

Degree Name

Physics

Level of Degree

Doctoral

Department Name

Physics & Astronomy

First Committee Member (Chair)

StephenT Boyd

Second Committee Member

Robin Cantor

Third Committee Member

Francisco Elohim Becerra-Chavez

Fourth Committee Member

Adam A Hecht

Language

English

Keywords

MMC, SQUID, gamma-ray, spectroscopy, superconducting, microcalorimeter

Document Type

Dissertation

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