γ-ray performance of a 1242 cm3 LaCl3:Ce scintillation spectrometer

We present the results of γ-ray measurements on a large 4″×6″ LaCl3:Ce crystal, characterized using radioactive sources over the range 14–3220 keV. The response of the crystal was found to be largely linear over the upper 90% of its dynamic range—the deviations at lower energies can be attributed to the shortcomings of the photomultiplier tube (PMT) and is in fact a generic problem when using high-gain PMTs with lanthanum halide scintillators. At 662 keV, a measured energy resolution of 4.1% FWHM was recorded at room temperature. In comparison to earlier generations of LaCl3 detectors, the internal background spectrum shows significantly less X-ray, β, γ-ray and α contamination. The integral background count rate in the energy region 20 keV–3 MeV was determined to be 1.8 cm−3 s−1, of which most (1.5 cm−3 s−1) can be attributed to the β continuum from 138La and only ∼0.04 cm−3 s−1 to the α complex from 227Ac and daughters. Using these data, we have determined the light output of α particles relative to the equivalent energy electrons (or alternately γ-rays) to be, α/γ=0.35±0.02.