The illicit trafficking and production of special nuclear materials (SNM) is a paramount global concern. Currently, scintillator detectors are used to determine whether SNM are present in incoming vehicles or cargo at border crossings. These detectors are cost effective and fast, but have a relatively poor spectral energy resolution. As a result, they cannot discriminate dangerous materials such as highly-enriched uranium (HEU) from common and innocuous background radiation. Even high purity germanium detectors, the current state of the art for gamma-ray detection, often do not have good enough resolution to clearly separate HEU emissions from background. Cryogenic microcalorimeters are a new class of detectors that provide resolution capable of separating these lines from any interference. The overall goal of this project is to provide the knowledge necessary to make these microcalorimeters an efficient and field usable instrument that will help solve real-world nuclear security and forensics problems. Specifically, we will design and implement measurements on the thermal properties of these low-temperature detectors that will enable faster, more efficient, array-compatible sensors with extremely high spectral resolution. These experiments will be designed and implemented by a team that includes a postdoctoral scholar, graduate and undergraduate student researchers, who will also work collaboratively with scientists at the National Institute of Standards and Technology and Los Alamos National Lab. The broader impacts of the project encompass education and technical training of post-graduate, graduate, and undergraduate students, and strengthening partnerships between the University of Denver, NIST and LANL. In addition, much of our work focuses on development and implementation of novel measurement techniques to probe thermal properties of detectors and their micromachined constituent materials. Many of these new techniques will be useful for a broad range of future experiments, and their development provides opportunities for students to learn techniques relevant to high-tech industry such as silicon micromachining and circuit design and characterization.
The intellectual merit of this award was focused on understanding heat flow from bulk superconductors into micromachined low temperature thermal isolation structures. Such composite structures, called microcalorimeters, can form detectors of energetic photons (gamma rays) capable of making very accurate measurements of the photon energies. Because the sensing mechanism relies on temperature measurement, understanding and optimizing the heat flow is crucially important to improve these devices. Our specific goal was to improve the speed and efficiency of these detectors in order to make the next generation of these devices more easily usable in the field for nuclear security and other applications requiring ultra-high resolution gamma-ray spectroscopy. Over the course of the project we made several important advances toward this goal. Through many experiments with different superconductors, isolation structures and adhesives used to attach them, we collected a great deal of information about the fundamental physics and materials science related to heat flow in the microcalorimeters. A brief summary of our results is that use of large amounts of low temperatre expoy in the assembly of the detector can be the cause of poor thermal performance, similarly long-lived excitations in many superconductors can contribute, and that even replacing the epoxy with low melting point superconducting metals does not dramatically improve the heat flow from the absorber to the thermometer. However we did find that attaching the absorber via diffusion bonding, using no dedicated adhesive but only mild heat and pressure, leads to dramatic improvements in the thermal time constants. This is a signifcant result both for understanding heat flow in these structures and building better detectors. In addition to testing various superconducting absorbers, we tested wide band-gap HgCdTe, a semiconductor with good stopping power for gamma rays near 100 keV and acceptibly low heat capacity. Our results are among the first to confirm that this is a promising absorber material for low temperature gamma-ray microcalorimeters. The most important broader impacts of this work are related to improved gamma-ray spectrometers that can be constructed that will improve border security and monitoring of nuclear safeguards.
