Measurement of scintillation efficiencies and pulse-shapes for nuclear recoils in NaI(Tl) and CaF2(Eu) at low energies for dark matter experiments

Measurements have been performed with a 2.55 MeV mono-energetic neutron beam of relative scintillation efficiency and pulse-shape for nuclear and electron recoils in NaI(T1) and CaF2(Eu). Scintillation efficiencies in NaI(T1) relative to 60 keV gamma events were found to be 27.5 +/- 1.8% for Na recoils (recoil energy E-rec > 4 keV) and 8.6 +/- 0.7% for I recoils (E-rec > 10 keV). Relative scintillation efficiencies in CaF2(Eu) for Ca and F recoils show some evidence for a fall with energy (17% to 8% for F) for 10 keV < E-rec < 100 keV. Pulse-shape analysis of NaI(Tl) data gives a mean photo-electron arrival time ((t) over bar(i)) of 263 +/- 15 ns for Na events (visible energy E-vis in the range 2-8 keV and 272 +/- 10 ns for I events (2 keV < E-vis < 5 keV). The [(t) over bar(i)] of 4 keV < E-vis < 54 keV electron events in NaI(TI) is found to rise with energy (320 ns to 440 ns). Similar analysis of CaF2(Eu) data shows [(t) over bar(i)] for 2 keV < E,, < 33 keV Ca, F and electron events to rise with energy (660 ns to 850 ns) with no evidence for pulse-shape differences. (C) 1998 Elsevier Science B.V. All rights reserved.