Comparison of Cross Sections for Neutron Reactions Using Different Neutron Energy Spectra

1. Irradiations at LANSCE May 2 – 19 2005 • Flight path 4FP15R was used, the same one used for NASA in 2001-3. • The accelerator was not running well. • Much of the time, it was running with a lower rep. rate of 40 Hz rather than 100 Hz. • Result: • Additional corrections required to account for the different rates of production during an irradiation. • Longer irradiation times were required to get the desired yields.

2. All NASA irradiations had 5 cms of polyethylene upstream. • In 2005, there were 15 cms of polyethylene upstream 5 cm 15 cm

3. 5 cm poly 15 cm poly • 2005: Comparison of measured spectrum with MCNPX calculation. • The increase in fluence at lowest energies is an artifact but we still need to understand what it is.

4. Ion chamber • Irradiation set-up at LANSCE in beam line 4FP15R • Looking upstream. • The transmission ion chamber is behind the lead shield

5. Targets irradiated: 5 x 5 cm targets of SiO2 and Si. • I made three separate irradiations to measure the cross sections for following reactions: • 1) O(n,x)14C and Si(n,x)14C; 100 Hz only. • 2) O(n,x)10Be and Si(n,x)10Be; Mixed 40 and 100 Hz. • 3) O(n,x)Ne and Si(n,x)Ne; Mixed 40 and 100 Hz. • Three runs were needed to keep the target stack thickness such that the neutron beam transmission was >90% at almost all but the lowest energies.

6. Monitor foils of Al, Ni, Cu and Au were included in the target stacks. • Cross sections for reactions producing relatively short-lived radionuclides were measured. • These can be used to: • 1) Check that the corrections made for the different run conditions were reasonable. • 2) Explore how the different neutron energy spectra affect the values measured for the energy integrated cross sections

7. Many energy integrated cross sections measured in 2005 were measured from 1998-2003 using the softer neutron energy spectrum. • Average cross sections for reactions producing the relatively short lived radionuclides are published in J. Sisterson and J. Ullmann, Nucl. Instr. Meth. B 234 (2005) 419. • These reactions have different thresholds, some are as low as 0.0 MeV • Comparing these values to those that we measured in 2005 may be valuable and give us insight into how the hardness of the neutron spectrum affects the average cross section.

8. In addition to these measurements of energy integrated cross sections, cross sections at unique neutron energies ranging from ~70 - ~150 MeV were measured at iThemab LABS, South Africa. • Cross sections for reactions in Cu producing relatively short lived radionuclides are published in J.M. Sisterson et al. Nucl. Instr. Meth. B 240 (2005) 617. • A paper reporting similar cross section measurements for reactions in Ti, Fe and Ni has been submitted (Nov. 2005) to Nucl. Instr. Meth. • For all the reported reactions, the cross section values at unique neutron energies are consistent with the average cross sections. • Thus, average cross sections are very useful as a guide to the values the cross section can assume as a function of energy.

9. Examples of some results: Do these graphs make sense? • Calculations were made where the shape of the energy spectra with 2 and 6 in of poly used were similar to those calculated using MCNPX: • The reaction has a low threshold energy and initially the cross section is high then falls to a smaller constant value, for example Ni(n,x)58Co. • Result: the cross section calculated for 6 in of poly tended to be lower than that for 2 in of poly.

10. Examples of some results: Do these graphs make sense? Calculations were made where the shape of the energy spectra with 2 and 6 in of poly used were similar to those calculated using MCNPX: (2) The reaction has a high threshold and the cross section is zero at low energies and then is like a step function, for example. Ni(n,x)52Mn. Result: the cross section calculated using 6 in of poly was higher than that calculated with 2 in of poly.

11. Future Plans • The budget for LANSCE was cut for fiscal 2006 • This means: • Reduced number of days available for experimenters. • The accelerator will run at 40 Hz only • As a result: • We must think carefully what we want to do next. • We must be efficient in our use of time. • Perhaps we should concentrate on measuring cross sections for which NO energy integrated cross sections have been measured.

12. Which targets should I irradiate next? Kuni’s ideal list is below. What other targets should be added to this list? Irradiated with 2 in poly 6 in poly Si  SiO2   Si3N4 x CaCO3  MgO Mg KNO3 K2CO3 Fe  FeS x Ni  Ba x