Photomultiplier and filter combinations for the detection of relatively long wavelength (>600 nm) luminescence emissions from feldspar

Abstract

We describe our efforts to detect IR-stimulated (λ ~ 833nm) orange-red emission (λemission = 600-750 nm) luminescence from potassium feldspars for dating applications. The aim here is to provide basic information about the detection of orange-red IRSL, as an alternative to conventional IRSL approaches which typically exploits portions of the IR-stimulated UV-blue (λemission < 600 nm) luminescence. We describe modifications to a standard Risø reader, which make it possible to detect orange-red IRSL. Modification consist of various optical filter combinations and extended range, cooled photomultipliers. The use of a modified Electron Tubes D716A “green” bialkaline PMT (green tube) provides a greater quantum efficiency (0.1- ~5% QE) in the wavelength range 600-700 nm than the traditionally used 9635 bialkaline tube (1-negligible% QE @ 600-700 nm). An extended S20 9650 PMT (red tube) provides a greater quantum efficiency (~7-12% QE) in the wavelength range 600-700 nm than both the 9635 and D716A. A substantial disadvantage of using an extended S20 tube is its relatively high quantum efficiency (2.5%) at 830 nm. Filter combinations were examined by both a UV-visible spectrometer and direct measurement of filter/PMT transmission characteristics. Our data suggest that optimal PMT/filter combinations consist of using the green PMT + two FR 400S + BG 39 and either OG 590 or RG 665 for detection of either a broad orange-red (λ ~ 600-720 nm) or far red (λ ~ 670-720 nm), respectively. The green PMT and an Omega 625DF 50 filter is the optimal combination for orange-near red (600-660 nm) emission.