COSMIC-RAY PRODUCED NE-21 IN TERRESTRIAL QUARTZ - THE NEON INVENTORY OF SIERRA-NEVADA QUARTZ SEPARATES

The study of cosmic-ray-produced radioactive and stable nuclides on th e surface of the Earth can provide relevant geomorphological and glaci ological information. At present, the cosmic ray production rates of s table Ne-21 are not well known. This study attempts to remedy the situ ation by determining the production rate ratio of Ne-21 and Al-26, P21 /P-26, in quartz. Al-26 concentrations and P-26 rates have previously been investigated for quartz separates of Sierra Nevada rocks which we re brought to the surface by glacial scouring during the Tioga period at the end of the last ice age [1]. We used splits of the same samples for our studies and found that Ne in these rocks represents a mixture of several components: trapped Ne, nucleogenic Ne-21 and Ne-22 produc ed by (alpha,n) reactions in oxygen and fluorine, respectively, as wel l as cosmic-ray-produced Ne, which is the component of interest in thi s study. The trapped component was substantially lost in one sample (W 86-12) by crushing and by a density separation of the grain sizes 38-9 0 mum and 90-125 mum, permitting the resolution of the in situ produce d Ne-21 into cosmic-ray spallation and (alpha,n) produced components a nd the determination of a lower limit to P21/P-26. In a second sample (W86-8) one split contained small enough amounts of nucleogenic Ne-21 to permit the determination of a reasonable upper limit to P21/P-26. T he two ratio determinations are consistent within error limits and the value adopted, 0.65 +/- 0.11 (2sigma), agrees with ratios observed in extraterrestrial matter. Apparently, P21/P-26 is thus not very sensit ive to the neutron spectrum. However, the observed production rate rat io is substantially larger than theoretical estimates for Si targets, reflecting poorly known neutron excitation functions. The above P21/P- 26 value, coupled to the observed Al-26 production rate [1], correspon ds to a Ne-21 production rate of P21 = 21 atoms g-1 a-1 in quartz or t o P21 = 45 atoms (g Si)-1 a-1 (at sea level and high latitudes). This rate is based on an adopted exposure age of 11,000 yr for our quartz s amples.