Calorimetric low temperature detectors for low-energetic heavy ions and their application in accelerator mass spectrometry

The energy-sensitive detection of heavy ions with calorimetric low temperature detectors was investigated in the energy range of E=0.1-1 MeV/amu, commonly used for accelerator mass spectrometry (AMS). The detectors used consist of sapphire absorbers and superconducting aluminum transition edge thermometers operated at T similar to 1.5 K. They were irradiated with various ion beams ((13)C, (197)Au, (238)U) provided by the VERA tandem accelerator in Vienna, Austria. The relative energy resolution obtained was Delta E/E= (5-9) X 10(-3), even for the heaviest ions such as (238)U. In addition, no evidence for a pulse height defect was observed. This performance allowed for the first time to apply a calorimetric low temperature detector in an AMS experiment. The aim was to precisely determine the isotope ratio of (236)U/(238)U for several samples of natural uranium, (236)U being known as a sensitive monitor for neutron fluxes. Replacing a conventionally used detection system at VERA by the calorimetric detector enabled to substantially reduce background from neighboring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of (236)U/(238)U=6.1 X 10(-12) could be obtained, representing the smallest (236)U/(238)U ratio measured at the time. In addition, we contributed to establishing an improved material standard of (236)U/(238)U, which can be used as a reference for future AMS measurements. (C) 2009 American Institute of Physics. [doi:10.1063/1.3213622]