Cosmogenic Cl-36 production rates in meteorites and the lunar surface

Activity vs. depth profiles of cosmic ray produced Cl-36 were measured in metal from two cores each in the St. Severin and Jilin chondrites and in lunar core 15008. Production of Cl-36 in these samples range from high-energy reactions with Fe and Ni to low-energy reactions with Ca and K and possibly neutron-capture reactions with Cl-36. The cross sections used in the Reedy-Arnold model for neutron-induced reactions were adjusted to get production rates that fit the measured Cl-36 activities in St. Severin metal and in the lunar soil of core 15008. The Cl-36 in metal from St. Severin has a fairly flat activity-vs-depth profile, unlike most other cosmogenic nuclides in bulk samples from St. Severin, which increase in concentration with depth. In metal from Jilin, a decrease in Cl-36 was observed near its center. The length of Jilin's most recent cosmic-ray exposure was approximately 0.5 My. Lunar core 15008 has an excess in Cl-36 of about 4 dpm/kg near its surface that was produced by solar-proton-induced reactions. The calculated production rates are consistent with these measured trends in 15008.