A closer look at IRSL SAR fading data and their implication for luminescence dating

Abstract. Feldspar, used for infrared stimulated luminescence (IRSL) dating, is known as a dosimeter which might not completely retain the environmental dose over time, therefore leading to age underestimation. The dose leakage is believed to be caused by non-thermal (anomalous) charge redistribution in the crystal and reflected in an IRSL signal which diminishes with time accordingly. After laboratory irradiation, this signal decline may be monitored by successive IRSL readouts following increasing delay times. Hence, tests of anomalous signal fading are integral steps of IRSL dating procedures applied to feldspar and feldspar-bearing polyminerals. In these measurements IR-stimulation is in most cases preceded by thermal pretreatment (preheating) of the sample. Per common practise, preheating is performed immediately after laboratory irradiation to avoid unwanted electron redistribution assumed to occur if preheating is performed immediately before the delayed IRSL-readout. Here we compile a series of single aliquot regeneration (SAR) measurements questioning this practice. As a result, the fading measurements may possibly reveal post-irradiation afterglow. The results also suggest that data curves resembling anomalous fading may be caused by insufficient control of the readout temperature waning with increasing delay time. The unwanted effects are observed best for IRSL at room-temperature and on luminescence readers with an out-of-date steering software, but they are relevant also for IRSL at elevated temperature and on modern readers, likely including novel post-IR IRSL (pIRIR) protocols. For temperatures as homogeneous as possible during IRSL readout of the (fading) dose, we recommend preheating immediately prior to (delayed) IRSL-readout in order to avoid measurement artefacts either resembling entirely anomalous fading of the IRSL-signal or increasing the true values. It should be noticed that multifold SAR protocol and measurement parameters, like e.g. the type of luminescence reader or the use of N2 flow, may further modify the course of the data values and therefore the amount of the measured signal loss in a particular time interval after laboratory irradiation. Furthermore, calculations of signal fading (g-value) should consider only IRSL-readout after a minimum delay time after laboratory irradiation to avoid including possible post-irradiation afterglow in g-value determination. The measurements compiled in the present study were performed on polymineral fine grains extracted from loess-borne samples from southern Germany and a limnic sample from Mexico. Therefore, the observations are assumed to be not only of local or regional interest but they appear to be of general relevance to SAR fading tests. However, with respect to the likely varying temperatures during IRSL readout of the fading dose administered in the laboratory, the observations are at least partly owed to the promptly measured test dose for normalizing the preceding (fading) dose. This is in contrast to classical multiple aliquot additive (MAA) measurements in which preheating may be replaced by long storage of a sample after laboratory irradiation and in which fading tests may be designed to also correct for possibly (slightly) changing IRSL readout temperatures at different delay times. Thus, the observations are at least partly SAR-immanent.