An Apparatus for Low Energy Investigation of Intraatomic Bremsstrahlung Radiation*

Electromagnetic radiation produced from the photoejection of an electron from the inner shell of a single atom, termed Intraatomic Bremsstrahlung (IAB), is far less understood than other forms of final state bremsstrahlung, which accompany processes such as beta decay and K capture. The current best QED-based theory models IAB according to the low energy theorem, which describes the cross section of the phenomena and predicts an infrared divergence at low scattered-photon energy. Contemporary investigation of IAB was performed most recently by Jacobson et al. Phys. Rev. A 104, 042809 (2021), finding an over-prediction at the 5-sigma level of IAB radiation in the 3-7 keV range with a 46 keV incident photon. Our work is focused on the measurement of IAB radiation in the visible light spectrum three orders of magnitude lower in energy. Electron photoejection is caused in thin samples excited at 60 keV by a 40 mCi Am 241 source. We collect with a Fresnel lens the resulting radiation with single-photon precision using an ultra-low dark count photomultiplier tube. Our samples consist of mylar films between 0.9μm and 250μm thickness, which are expected to produce both direct IAB signal and secondary radiation, principally from ordinary bremsstrahlung due to the photoelectron. The preliminary data suggests a primary signal roughly linear with thickness as expected, but an unexpectedly low power for secondary signals in thick samples. This success is despite the possibility of x-ray scattering from our collection lens.