Superconducting ultra-high energy resolution gamma spectrometers

We are developing superconducting gamma-ray and fast-neutron spectrometers for nuclear safeguards and nuclear attribution applications that require higher energy resolution than conventional semiconductor or gas detectors can provide. This work was initially motivated by nuclear forensics needs, where a fast and precise isotope analysis is essential to quickly attribute unknown nuclear materials to their source. It is also becoming important in nuclear safeguards for high-precision analysis on Pu or other complicated mixed isotope samples, for example for Pu/U mixtures or samples with high Pu-242 concentrations. Our spectrometers are based on measuring the increase in temperature upon photon or particle absorption with a sensor operated at the transition between its superconducting and its normal state. Low temperature operation reduces thermal fluctuations and thus allows an energy resolution an order of magnitude higher than for conventional high-purity germanium detectors. The approach can be adapted for different types of radiation with the appropriate choice of absorber material. This presentation focuses on our development of gamma-ray spectrometers consisting of a bulk absorber attached to a superconducting thin film Mo/Cu multilayer sensor. These detectors are operated at a temperature of ∼0.1K the end of a cold finger in a two-stage adiabatic demagnetization refrigerator. They have an energy resolution between 50 and 90 eV FWHM for energies below 122 keV. This energy resolution is sufficient to separate the emission form different actinide isotopes of interest for nuclear safeguards and material management. In particular, a resolution of 100 eV allows separating the lines from the different Pu isotopes, including Pu-242, in the 100 keV region as needed for MGA analysis of Pu isotope ratios, and separating the U-Th lines in the 92 keV region as needed for MGA-U. This improvement in energy resolution will improve the precision of non-destructive isotope analysis by Gamma-spectroscopy by an order of magnitude compared to conventional analysis using high-purity Germanium spectrometers. This improvement is due both to a reduction of the limiting statistical errors due to line overlap, and a reduction of the systematic errors due to self-absorption and Compton background subtraction. For increased spectrometer sensitivity, we are currently developing gamma-detector arrays and the redout electronics to read out each pixel at rates above 100 counts/s. The next-generation of superconducting gamma spectrometers will be based on a 112-pixel sensor array, with an active area of several cm2 and total count rate capabilities above 10,000 counts/s. Scale up to larger array sizes is possible if desirable