Response to the comment by W.H. Schwarz et al. on “Joint determination of 40K decay constants and 40Ar∗/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology― by P.R. Renne et al. (2010)

We appreciate the opportunity to address the comment by Schwarz et al. (2011) concerning our calibration of the Ar/Ar chronometer (Renne et al., 2010). In particular, we thank Schwarz et al. for partially clarifying the sensitive dependence of liquid scintillation counting (LSC) data on the branching ratio (kb/ke) for determination of the K half-life. This dependence is as stated “. . . easily overseen [overlooked] . . .” because it is embodied in a correction for relative detection efficiency of c versus b radiation, and the equation(s) governing the efficiency dependence on branching ratio are not provided by either of the papers presenting LSC measurements (Grau Malonda and Grau Carles, 2002; Kossert and Günther, 2004). The physical basis for this correction and its application to LSC counting data are referred to an internal report by Grau Carles and Grau Malonda (1997) that we have been unable to obtain. Kossert and Günther (2004) attributed their choice of branching ratio to a reference given simply as “Helmer, R.G., 1998. Table de Radionucléides”, and did not discuss the merits of this choice which is well outside the range of most modern compilations as represented by the open symbols in their Fig. 1. It remains unclear how Kossert and Günther (2004; Table 5) obtained a half-life for K that is identical to that of Grau Malonda and Grau Carles (2002) despite having assumed different values for the branching ratio (8.21 and 8.35, respectively). It is also unclear what role the assumed (e.g., Beckinsale and Gale, 1969) direct electron