COSMOGENIC CL-36 PRODUCTION IN CALCITE BY MUONS

At depths below a few metres, Cl-36 production in calcite is initiated almost entirely by cosmic ray muons. The principal reactions are (1) direct negative muon capture by Ca; Ca-40(mu(-),alpha)Cl-36, and (2) c apture by Cl-35 of secondary neutrons produced in muon capture and muo n-induced photodisintegration reactions. We have determined rates for Cl-36 and neutron production due to muon capture in calcite from a 20 m (5360 g cm(-2)) depth profile in limestone. The Cl-36 yield from muo n capture by Ca in pure calcite is 0.012 +/- 0.002 atom per stopped ne gative muon. The surface production rate of Cl-36 by muon capture on C a in calcite is, therefore, 2.1 +/- 0.4 atom g(-1)a(-1) at sea level a nd high latitude, approximately 11% of the production rate by Ca spall ation. If it is assumed that 34% of the negative muons are captured by the Ca atom in calcite, the alpha-yield from Ca-40 following muon cap ture is 0.043 +/- 0.008, somewhat lower than the result of a recent mu on irradiation experiment (0.062 +/- 0.020), but well within the extre mes of existing theoretical predictions (0.0033-0.15). The average neu tron yield following muon capture in pure calcite is 0.44 +/- 0.15 sec ondary neutrons per stopped negative muon, in good agreement with exis ting theoretical predictions. Cosmogenic isotope production by muons m ust be taken into account when dating young geomorphic surfaces, espec ially those created by excavation of only a few metres of overlying ro ck. Attention to isotope production by muons is also crucial to determ ining surface erosion rates accurately. Due to the deep penetration of muons compared to cosmic ray hadrons, the accumulation of muon-produc ed Cl-36 is less sensitive to erosion than that of spallogenic Cl-36. Although production by muons at the surface is only a small fraction o f production by spallation, the fraction of muon-produced Cl-36 in rap idly eroding limestone surfaces can approach 50%. In such cases, erosi on rates estimated using conventional models which attribute productio n solely to spallation will be in error by up to 40%. The difference i n sensitivity to erosion of spallogenic and muon-produced Cl-36 sugges ts methods for dating deeply eroded surfaces, checking the assumption of steady-state when calculating erosion rates, and unravelling multi- stage exposure and erosion histories. Copyright (C) 1998 Elsevier Scie nce Ltd.