Cross-Section Measurements from 40 to 450 MeV for the Production of 14C from Silicon and Oxygen: Better Estimates for Cosmogenic Production Rates

Details Cross-Section Measurements from 40 to 450 MeV for the Production of 14C from Silicon and Oxygen: Better Estimates for Cosmogenic Production Rates Cross-Section Measurements from 40 to 450 MeV for the Production of 14C from Silicon and Oxygen: Better Estimates for Cosmogenic Production Rates

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..438s&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 438

Physics

Cosmogenic Nuclides, Solar Cosmic Ray Flux, Constancy, Solar Flare Particles, Spallation

Scientific paper

Cosmogenic nuclides in extraterrestrial materials allow studies to be made of the solar cosmic ray (SCR) flux over time periods in the past [1], the constancy of the galactic cosmic ray flux, and even of the sample's recent history. To interpret such measurements, especially for SCR-produced nuclides, it is essential that the cross sections for the reactions of all cosmic ray particles with each constituent of the sample be very well known. Approximately 98% of SCR particles are protons and their interactions are the major source of cosmogenic nuclides in the surface layers of extraterrestrial materials. Until the development of accelerator mass spectrometry (AMS), few of the needed cross sections were known well enough to be used with confidence. Now using thin target irradiations and the improved sensitivity of AMS, good cross-section measurements for the production of these cosmogenic radioisotopes can be made. Preliminary cross-section measurements for 16O(p,3p)14C and natSi(p,x)14C made at the Harvard Cyclotron Laboratory (HCL) using SiO2 and Si targets have been reported for the proton energy range 65-160 MeV [2]. They confirm earlier data for the 16O(p,3p)14C cross section and are the only measurements for the natSi(p,x)14C cross section. New measurements made at the cyclotron at the University of California at Davis for proton energies from 40 MeV to 67.5 MeV and at TRIUMF for proton energies from 200 to 450 MeV have extended the energy range over which these cross sections are well known, including the important region near the threshold of the excitation function. In all cases, targets of SiO2 and Si were irradiated in thin target conditions that kept the energy loss in a single target to <2 MeV. The total energy lost in the entire target stack for the Davis irradiations was <8 MeV; for the TRIUMF irradiations <1 MeV; and at HCL it ranged from <5 MeV at 160 MeV to <10 MeV at 65 MeV. Thus both the secondary neutron production within the target stack and loss of protons due to scattering were minimized. The proton fluence was determined using Faraday cups and the 27Al(p,3p3n)22Na reaction measured in aluminum monitor foils. All the samples were analyzed for 14C at Arizona using well known methods [3,4]. Details of these measurements and new values of the 14C production cross sections will be presented in the context of their importance to lunar sample and meteoritic studies. These measurements represent the first data available from Si, SiO2, Al, Mg, and C targets that have already been irradiated at some proton energies. These targets will be analyzed for 14C, 10Be, 26Al, 7Be, 22Na, and the noble gases, while additional relevant target materials will be irradiated in this ongoing systematic study. References: [1]) R. C. Reedy and K. Marti (1991) in The Sun in Time, 206, Univ. of Arizona. [2] J. M. Sisterson et al. (1992) LPSC XXIII, 1305. [3] A. J. T. Jull et al. (1989) GCA, 53, 2095. [4] T. W. Linick et al. (1986) Radiocarbon, 28, 522.

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