The objective of this proposal is to develop detectors with high efficiency and high resolution. Currently, the best high-purity germanium detectors (HPGe) used in nuclear spectroscopy provide a fractional energy resolution of only 0.5% in the 100 keV region important in nuclear spectroscopy. In many cases this resolution is inadequate to discriminate between threatening nuclear materials and more mundane substances which are part of the natural background. For example, the most common source of false alarms at US border crossings is the confusion between the naturally occurring emission of 226Ra (an element commonly found in clay-bearing materials and soils) at 186.211 keV and the185.715 keV emission of fissile 235U. Another spectroscopic challenge in nuclear materials analysis is separating the intense 104.2 keV line of 240Pu from the nearby 103.7 keV X-ray of Pu X. These and other line overlaps in the 100-200 keV region set the error budget for nondestructive analysis of mixed actinide samples. Determining the isotopic mix in a reactor fuel sample provides both a characteristic fingerprint and specific information about the intended purpose of the fuel (weapons production or electricity generation), the type of reactor, and the history of chemical processing, if any. Further, detectors with high efficiency and high resolution can provide new capabilities for detecting fissile material. Detectors with better resolution produce a better signal-to-noise ratio when measuring faint emission lines from trace quantities of radioactive material in the presence of background or overlapping lines. The new superconducting detectors proposed here offer the promise of 30 times better energy resolution than HPGe, and perhaps 1000 times greater cross-sectional sensitivity compared to cryogenic microcalorimeters.
The intellectual merit of the proposed activity derives from its potential to produce more than an order of magnitude improvement in the energy resolution of gamma ray detectors, and the scientific fallout this advance would have, not only for nuclear security, but also in astrophysics and nuclear medicine. The detectors proposed rely on an effect called kinetic inductance, in which the macroscopic inductance of the surface of a superconductor can be influenced by the creation of unpaired electrons (quasiparticles) through the absorption of ionizing radiation. The change in inductance is picked up by a high-Q microwave circuit, and the signal event is counted as with any another photon counter, but with the difference that the pulse heights can be measured with 30 eV energy resolution in the gamma absorption.
The broader impact of this effort will be felt in several areas. First in science the development of high-energy kinetic inductance detectors will influence the direction of gamma ray astronomy and perhaps even the search for dark matter. In technology, the impact will be felt in the development of non-contact nuclear assay methods of unprecedented sensitivity. Finally, in education, the participants in the research, the graduate student and postdoc, will be free to explore a wide open area as part of thesis and postdoctoral research. Finally the senior personnel will have something exciting to do, and something new to convey to the K-12 students in the science fair and outreach activities.
Project Outcome Report for CBET 1039309 This report covers the first year of a three-year ARI-grant sponsored jointly by NSF and DHS. The goal of this 3-yr effort is to develop a new type of gamma-ray detector with high energy resolution. The first-year period reported here was funded by the NSF under grant CBET 1039309. Support for years two and three is provided by DHS. Half of the first year was devoted to a technology study to select an optimum approach from several available, and the other half to the design and fabrication of the preferred implementation. This work is done in collaboration with the Quantum Calorimeter Group led by Joel Ullom at NIST, Boulder. Currently, the best high-purity germanium detectors (HPGe) used in nuclear spectroscopy provide a fractional energy resolution of only 0.5% in the 100 keV region. In many cases this resolution is inadequate to discriminate between threatening nuclear materials and more mundane substances which are part of the natural background. However, by using superconductors instead of semiconductors as detectors, the energy resolution can be improved by a factor of 10 to 0.05%, which provides adequate discrimination for nuclear security applications. Existing superconducting detectors measure the heat generated when ionizing radiation is absorbed, but these micro-calorimeter detectors require cooling to 100 mK, which restricts the technology to a lab environment. Our new detectors rely on a different detection mechanism to achieve an energy resolution similar to micro-calorimeters, but with much higher operating temperatures near 2 K, and thus are suitable for field applications. The new superconducting detectors rely on an effect called kinetic inductance, which changes in response to the absorption of ionizing radiation. This change in inductance affects the resonant frequency of a narrowband microwave circuit, which can be measured with microwave techniques. Many detectors can be coupled to a single read-out line in order to improve the sensitivity of the system to gamma rays. Our work this past year has been to design a superconducting microwave resonator that can be fabricated in small arrays with production techniques similar to those used for integrated circuits. With the help of a sophisticated computer-aided-design program for microwave circuits, a number of suitable designs were optimized and copied onto photo-mask layouts. These masks are used to control photo-etching of the microwave circuit on an inert substrate. A suitable gamma-ray absorber with dimensions 1 mm x 1 mm x 0.25 mm is then bonded to the top of the circuit to complete the detector. Circuit wafers will be produced in Nov. 2011 at the NIST fabrication facility, and tested starting in December. As mentioned earlier, this second-year effort is supported by DHS. Although the project goal is to produce radiation detectors for nuclear security requirements, the technology when proven will also have applications in gamma-ray astrophysics and the search for dark matter, the development of non-destructive nuclear assay methods, and eventually nuclear medicine.
