Multi-collector configuration considerations and substrate relative sensitivity factor effects for age-dating measurements of particles by large geometry secondary ion mass spectrometry

Chronometry (a.k.a age-dating, AD) of materials by bulk mass spectrometric methods is a wellestablished technique based on analysis protocols that have been used in geological fields and by the non-proliferation communities for many years. Recently, it has been demonstrated it is possible to broaden the applicability of AD measurements to single uranium-containing particles using large geometry secondary ion mass spectrometry (LG-SIMS) showing that it is feasible to measure the U – Th mother-daughter chronometry pair in micrometer-sized particles. We present results building from that work focusing on two topics. The first topic will discuss the use of the multicollector configuration — common in LG-SIMS instruments — to measure all isotopes needed for the AD measurement simultaneously. Our protocol developed for U-Th AD measurements uses the mono-collector configuration with the preponderance of the counting time of an analysis cycle being on the Th. The multi-collector configuration allows the simultaneous counting of both Th and U, with the added advantage of allowing the collection of Pa and Th isotopes, too. Simultaneously counting all isotopes should improve the overall measurement precision, as well as eliminate transient artifacts during the analysis that could result in inaccurate data. We will also discuss results which show that different substrates can have a large effect on the relative sensitivity factor (RSF) of Th:U, which could result in inaccurate age estimates if this effect is ignored. We have previously reported a Th:U RSF of ~0.673 for measurements done on carbon planchettes under our specific analysis conditions. Carbon planchettes are not the only common substrate used by LG-SIMS labs. Others include silicon wafers, indium pucks, and stainless-steel disks. We have recently determined that the Th:U RSF measured on Si wafers is ~0.79, i.e. 17% higher than the RSF on carbon. This difference, as well as the RSF difference for other substrates, must be properly accounted for or AD results will be inaccurate.