$Q$ Spectroscopy With Superconducting Sensor Microcalorimeters

The total reaction energy
$(Q)$
of individual nuclear decays was measured using microcalorimeters with transition-edge-sensor (TES) thermometers. For alpha-decaying actinides (e.g., U-235, Pu-239, Np-237, Am-241),
$Q$
is in the 4–6 MeV range. Nearly all of this energy goes into the relatively light alpha particle, and approximately 100 keV is left over for the much heavier, recoiling daughter atom. Alpha-particle energy spectroscopy with TES-microcalorimeters has shown the ability to simultaneously resolve peaks that overlap in conventional alpha spectroscopy, with resolution now less than 1 keV full-width-at-half-maximum (FWHM) at 5.3 MeV. For total reaction energy spectroscopy, we use the same TES design as our alpha detectors, but embed a small radioactive sample (of about 1 Bq) directly inside an absorber designed to capture all the emitted particles (alpha, recoil nucleus, electrons, X-rays) with near 100% efficiency. We have measured
$Q$
-spectra of alpha-decaying isotopes with spectral resolution of 2–3 keV FWHM. For some actinide analytical problems, the
$Q$
-spectrum is simpler than the alpha-spectrum: fewer peaks, further apart, and easier to quantify. We will discuss sensor design, methods for embedding radionuclides, and spectral data.