Red thermoluminescence (RTL) in volcanic quartz: development of a high sensitivity detection system and some preliminary findings

Abstract

As part of a general study exploring the suitability of the RTK of quartz for dating volcanic events, a modified Riso Reader appartus has been assembled and tested. Modification consisted of an alternative, cooled photomultiplier, and the incroporation of signal pass filters which have not previously been employed in RTL investigations. The use of an extended S20 9650 PMT provides a greater quantum efficiency in the wave range 600-800 µm than the traditionally used 9635 bialkaline tube. A substantially disadvantage of using such an extended tube relates to the relatively high thermal backgroung generated with the photocathodes. To overcome this limitation we have tested the PMT response via cooling down by c. -17°C. This reduced the background to an order of magnitude (-200cis). A cooled (-17°C) extended S20 Photomultiplier (PMT) demonstrated a signal increasing and background decreasing due to the PMT cooling up to 400°C. A range of filter combinations was examined by both empirical experiments of RTL on quartz extracted from a late Holocene, New Zealand dune sand sample (OXOD84713) and a uv-vis spectrophotometer. Of the filter combinations examined, a Hoya 2-63 and Schott BG-39 provided the best IR suppression and therefore signal to noise ratio. However, as the BG-39 limits a substantial component of the 600-620 µm RTL peak emission, it may be inadequate for relatively young or insensitive samples. In such cases a new IR suppression filter (Corion FR-400S) is recommended.