Extending the maximum age achievable in the luminescence dating of sediments using large quartz grains: A feasibility study N. Chauhan a, * , S. Anand b , T. Palani Selvam b , Y.S. Mayya b, c , A.K. Singhvi a a Physical Research laboratory, Planetary and Geosciences Division, Ahmedabad 380 009, India b Health, Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai 400 085, India c Homi Bhabha National Institute, Mumbai 400 085, India article info Article history: Received 20 October 2008 Received in revised form 16 January 2009 Accepted 17 June 2009 Keywords: Environmental dosimetry Luminescence Geochronology Quartz Monte-Carlo simulations abstract The feasibility of extending the upper dating limit in the luminescence chronometry of sediments using centimeter size quartz grains has been investigated. In general for such large sized quartz grains, the total dose is from radioelements external to the grain and this dose gets progressively attenuated towards the grain centre. Thus, for a centimeter size grain, a finite portion of the inner volume largely receives only the gamma dose. Such a reduced dose implies a delayed onset of saturation and hence offers the prospects of a higher age limit. Monte-Carlo simulations were used to compute beta dose depth distribution inside such grains when irradiated by beta particles from 40 K, 212 Bi ( 232 Th-series) and 234m Pa and 214 Bi ( 238 U-series). These computations suggest that quartz grains of up to 6–10 mm diameter would have an inner core of w2–6 mm that receives minimal beta dose. Given that gamma dose is only a third to a fourth of the total dose, this approach offers the prospect of a three to four fold increase in the age limit obtainable by luminescence methods. This contribution discusses the conceptual formalism, computational aspects and outlines some of the practical difficulties and remedial measures for routine applications. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The upper limit for the luminescence age is defined by a combination of the saturation dose (Chawla et al., 1998) and the stability of the signals. Quartz has a demonstrated stable lumi- nescence signal (w10 8 ka, Aitken, 1985) but the low saturation dose for OSL measurements implies an upper age limit of 100 ka. Feldspars have an order of magnitude higher saturation dose, however at times it suffers from athermal fading. This limits its applicability. Considerable efforts have been made to extract a more stable signal, via the choice of measurement wavelength, measurement temperature or measurement of decay with storage time, (Fattahi and Stokes, 2004; Lamothe and Auclair, 2000, Lamothe et al., 2003; Wallinga et al., 2000a,b; Wintle and Murray, 2006). The other possibility that enabled dating older events using quartz, is in low annual dose environments (Huntley and Prescott, 2001). Such low dose environments however are not routinely available. We therefore explored another and possibly simpler alternative of using large grains, where the interior receives only a fraction of the annual dose seen by a standard 100 mm grain, (Fig. 1(a); Aitken, 1985; Mejdahl, 1979). This implies a slower progression to saturation, and consequently of dating over an extended time range. Typically, in a sediment matrix with w1 ppm 232 Th, w1 ppm U and w1% K, the b dose contribution for a typical grain size of 100 mm is w70% and the remaining 30% is from gamma and cosmic rays. About w85% of the beta dose is contributed by 40 K, w13% by U-series and w2% by 232 Th-series (Adamiec and Aitken, 1998). In a typical sediment, dominant contributors to the beta dose are 40 K, 212 Bi ( 232 Th-series) and 234m Pa and 214 Bi ( 238 U-series). Table 1 lists their maximum beta energies (b max ), corresponding ranges under Continuously Slowing Down Approximation (CSDA), and the net contribution to beta dose for unit concentrations. At this stage of conceptual development, we worked with the above isotopes as the others make a minor contribution to the absorbed dose. The present study computes the depth dependence of beta dose rate from the above radioactive sources using Monte-Carlo simulations and analytical approach. It also identifies possible limiting factors including the extent of photobleaching of geological luminescence, in view of possible attenuation of light penetration in large grains. * Corresponding author. Tel.: þ91 7926314370. E-mail address: chauhan@prl.res.in (N. Chauhan). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.06.009 Radiation Measurements 44 (2009) 629–633