Determination of absolute Np L x-ray emission intensities from Am-241 decay using a metallic magnetic calorimeter

Photon emission intensities of radionuclides, i.e. the number of emitted photons per decay, are the single most important decay data when the photon spectrometry technique is employed in ionizing radiation metrology. However, their precise measurement is problematic because they are usually determined by photon spectrometry with spectrometers having a detection efficiency calibrated with x-ray and gamma-ray emission intensities from other radionuclides. Therefore, these intensities are ultimately interdependent and correlated to some extent. A novel method was applied to determine the photon intensities of alpha-emitting radionuclides by measuring the ratio between the rate of photons in the full energy peaks and the rate of detected alpha-particles with the same detection set-up. Thus, there is no need to calibrate the detection efficiency and to standardize calibration sources by primary methods. The main condition to reach low uncertainties is to have a spectrometer with an intrinsic detection efficiency close to unity for the measured photons and alpha-particles. This condition was fulfilled by a metallic magnetic calorimeter (MMC) with an intrinsic efficiency of around 99% between 5 keV and 25 keV. In addition, the MMC provides an ultra-high energy resolution of 28 eV (full width at half maximum), facilitating the processing of the spectrum. The method was applied to determine L x-ray emission intensities from the decay of Am-241(alpha) -> Np-237 emitted between 11.9 keV and 22.4 keV. A total L x-ray emission intensity of 37.90 (12) per 100 disintegrations was obtained; the value agrees well with previous measurements and has a lower uncertainty. The uncertainties of the L x-ray groups L alpha, L eta beta and L gamma were improved by a factor of two. Moreover, due to the high-energy resolution of 28 eV, a detailed set of 33 L x-ray emission intensities are provided.