Response of NaI, anthracene and plastic scintillators to electrons and the problems of detecting low energy electrons with scintillation counters

Abstract

The electron response of NaI(Tl) has been measured with magnetic-spectrometer-energy-selected electrons incident on cleaved crystal surfaces, in the energy range 5–1000 keV. The light output/keV (≡ response) shows a maximum in the neighborhood of 15–20 keV which is some 20% larger than that at 1 MeV; below 15 keV the response falls sharply reaching 80% of the peak value at ∼ 5 keV. The electron response of anthracene and a plastic scintillator (Pilot B) were measured under the same experimental conditions for comparison. Both anthracene and Pilot B show nonproportional response in the low energy region. At 500 keV the ratio of the responses NaI(Tl)/anthracene/Pilot B are 1.6/1.0/0.38; at 20 keV the ratios are 2.5/1.0/0.41. Decay curve analysis yields for some of the long lived states (excited by incident electrons) in NaI(Tl), 200–300 μsec, 7–9 msec, 40–80 msec, 4 sec, 1 min and 12 min; for anthracene 36 μsec, 120 μsec and 570 μsec. Relatively intense long lived states and more backscattering make NaI(Tl) not generally useful as a low energy electron detector. Uncovered anthracene mounted directly on a photomultiplier and the combination cooled to prevent sublimation of the phosphor in the high vacuum of the beta spectrometer provides a counter which has efficiencies measurable from the pulse height distributions down to ∼6 keV. At backgrounds of 300–500 cpm it has usable efficiencies down to 1 keV.