High-accuracy measurement of Compton scattering in germanium for dark matter searches

Compton scattering with bound electrons contributes to a significant atomic effect in low-momentum transfer, yielding background structures in direct light dark matter searches as well as low-energy rare event experiments. We report the measurement of Compton scattering in low-momentum transfer by implementing a 10-g germanium detector bombarded by a ^137Cs source with a radioactivity of 8.7 mCi and a scatter photon captured by a cylindrical NaI(Tl) detector. A fully relativistic impulse approximation combined with multi-configuration Dirac–Fock wavefunctions was evaluated, and the scattering function of Geant4 software was replaced by our calculation results. Our measurements show that the Livermore model with the modified scattering function in Geant4 is in good agreement with the experimental data. It is also revealed that atomic many-body effects significantly influence Compton scattering for low-momentum transfer (sub-keV energy transfer).