New Experimentally Observable Gamma-ray Emissions from 241 Am Nuclear Decay Katrina E. Koehler, * Michael D. Yoho, Matthew H. Carpenter, Mark P. Croce, David J. Mercer, Chandler M. Smith, Aidan D. Tollefson, and Duc T. Vo Los Alamos National Laboratory Los Alamos, NM 87545 Michael A. Famiano Western Michigan University Kalamazoo, MI 49008 Daniel T. Becker, Johnathon D. Gard, J. A. B. Mates, Nathan J. Ortiz, † Joel N. Ullom, † and Abigail L. Wessels University of Colorado Boulder, CO 80309 Douglas A. Bennett, Daniel R. Schmidt, Daniel S. Swetz, and Leila R. Vale NIST Boulder Laboratories Boulder, CO 80305 (Dated: March 31, 2021) With the high resolution of microcalorimeter detectors, previously unresolvable gamma-ray lines are now clearly resolvable. A careful measurement of 241 Am decay with a large microcalorimeter ar- ray has yielded never before seen or predicted gamma lines at 207.72±0.02 keV and 208.21±0.01 keV. These have been made possible because of new microwave-multiplexing readout and improved anal- ysis algorithms for microcalorimeters. INTRODUCTION This letter presents two new gamma-ray emissions (spectral lines) for 241 Am and the possible energy level candidates from which these gamma rays might orig- inate. Plutonium and americium photon emissions around 208 keV are used for the non-destructive assay of plutonium mass by IAEA inspectors and material con- trol and accountability (MC&A) technicians working in Department of Energy laboratories [1]. A better under- standing of these emissions may lead to higher fidelity plutonium assays. These peaks were previously not resolvable due to the relatively low energy resolution of HPGe detec- tors compared to microcalorimeters. With a recently commissioned spectrometer, SOFIA (Spectrometer Op- timized for Facility Integrated Applications), measure- ments of Pu and Am samples showed two previously un- known gamma peaks at 207.72 and 208.21 keV. SOFIA is a 256-pixel superconducting transition-edge sensor mi- crocalorimeter array combined with high-bandwidth mi- crowave frequency-division multiplexed readout, provid- ing high efficiency and high count rate capability [2]. These capabilities combined with improved analysis al- gorithms for co-adding pixel data allowed these peaks to be clearly resolved. In this work, SOFIA used the SLEDGEHAMMER (Spectrometer to Leverage Extensive Development of Gamma-ray TESs for Huge Arrays using Microwave Multiplexed Enabled Readout) detector array, devel- oped by the University of Colorado as the first 207.25 207.50 207.75 208.00 208.25 208.50 208.75 Energy (keV) 10 2 10 3 10 4 10 5 10 6 10 7 Counts PIDIE6 μMux TDM HPGe FIG. 1: In the microcalorimeter spectrum (black and grey ) the 241 Am 208.0 keV line has two satellite peaks, which are not visible in the HPGe spectrum (dotted ). Recent advances [3, 4] in microcalorimeter instrumentation and data analysis algorithms make the distinctions even sharper between previous (grey ) and current (black ) microcalorimeter data. large microwave-multiplexed gamma microcalorimeter array [2]. SLEDGEHAMMER was operated in multi- ple cryostats before it was installed in SOFIA and is well characterized. Typical spectral resolution with SOFIA is 73 eV at 208.0 keV. For these spectra, 1.5 mm of Cd was used to attenuate the high intensity 59.9 keV 241 Am transition. Spectral features show up at 207.7 keV and 208.2 keV in all plutonium spectra, yet become arXiv:2103.15893v1 [nucl-ex] 29 Mar 2021