Measurement of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Th</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>229</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> Isomer Energy with a Magnetic Microcalorimeter
Tomáš Šikorský, Jeschua Geist, Daniel Hengstler, Sebastian Kempf, L. Gastaldo, C. Enss, C. Mokry, J. Runke, Ch. E. Düllmann, P. Wobrauschek, Kjeld Beeks, Veronika Rosecker, Johannes H. Sterba, Georgy A. Kazakov, Thorsten Schumm, A. Fleischmann
Abstract
We present a measurement of the low-energy (0-60 keV) γ-ray spectrum produced in the α decay of ^{233}U using a dedicated cryogenic magnetic microcalorimeter. The energy resolution of ∼10 eV, together with exceptional gain linearity, allows us to determine the energy of the low-lying isomeric state in ^{229}Th using four complementary evaluation schemes. The most precise scheme determines the ^{229}Th isomer energy to be 8.10(17) eV, corresponding to 153.1(32) nm, superseding in precision previous values based on γ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b_{29}=9.3(6)% and b_{42}<0.7%.