Light yield of cryogenic CsI down to 5.9 keV*

We measured the light yield of an undoped cryogenic CsI scintillator at four energies including the first measurement at 5.9 keV that is close to the detector threshold. A light yield of 33.4 ± 1.7 photoelectrons per keV electron-equivalent (PE/keV_ee) was achieved with an undoped CsI crystal coupled with a photomultiplier tube (PMT) at 77 Kelvin using the 5.9 keV X-ray from Fe-55 source and a light yield of 40.0 ± 2.0 PE/keV_ee was observed from the 59.5 keV γ-ray from an Am-241 source. Analysis procedures and reconstruction methods will be presented.