The need for cross section measurements for neutron- induced reactions If no cross section measurement exists, alternative strategies are: The cross section for the corresponding proton-induced reaction is used. Theoretical models are used to estimate the needed cross sections. The cross section is inferred from analysis of the results from irradiating thick target stacks with protons. None of these strategies is as good as an actual measurement!

To remedy this situation We are measuring cross sections for neutron induced reactions: LANSCE to make an energy integrated (average) cross section measurement using ‘white’ neutron beams 0 – 750 MeV. In the first year we will measure cross sections for the production of: 10 Be, 14 C, Ne, 26 Al from O and Si

The aim of the experiment in or 3 irradiations. These times are calculated assuming 1.8 microsec spacing and ~4-5 nA protons on the W target. 50 x 50 mm SiO 2 and/or 50 mm diameter Si targets. 10 days using 3 mm thick targets: 1 day using 1 mm targets; SiO 2 (n,x) 10 Be SiO 2 (n,x) 26 Al or Si(n,x) 26 Al Si or SiO 2 (n,x) 20,21,22 Ne SiO 2 (n,x) 14 C 1/2 day using 1 mm thick targets: SiO 2 (n,x) 3 He and Si(n,x) 3 He. Total target irradiation time requested = 15 days In addition, we need ~4 days with a long micropulse spacing to characterize the low energy flux.

Experimental Procedure at LANSCE Neutron beams cover the whole target stack. Total stack thickness is designed to attenuate <10% of the beam at all neutron energies. Irradiation times are designed to produce the optimum number of product atoms for determination using AMS or MS by appropriate collaborators. Short-lived radionuclides are measured using non-destructive gamma-ray spectroscopy. AMS and MS determinations will be made later.

Experiments at LANSCE Target in target holder LANSCE: 4FP15R 2002 The energy spectrum ranges from 0.1 – 750 MeV. The neutron fluence is monitored directly using an uranium fission chamber.

CSiO 2 MgAl 7 Be 7.1     Na 9.1    Na 6.9    2.7 TiFeNiCu 46 Sc 49.8     V 10.0    Cr 40.4    Mn 9.6    Mn 69.7    Co 29.4   Co   Co   Co 6.5   1.7 Au 194 Au  Au  Au 15.1  1.8 Average cross sections measured at LANSCE include:

nat Cu(p,x) 60 Co from S. J. Mills, G. F. Steyn and F. M. Nortier, Appl. Rad. Isot. 43, 1019, 1992 MC-ALICE calculations courtesy of Mark Chadwick.

The excitation function was constructed from the measured values and the adopted values of W. S. Gilbert et al (1968) of 10 mb for En>60 MeV and ‘tweaked’ to get reasonable agreement with the average value measured at LANSCE..