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: 10Be, 14C, Ne, 26Al from

O and Si

The aim of the experiment in 20052 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 SiO2 and/or 50 mm diameter Si targets.

10 days using 3 mm thick targets: 1 day using 1 mm targets;SiO2(n,x)10Be SiO2(n,x)26Al or Si(n,x)26Al

Si or SiO2(n,x)20,21,22Ne SiO2(n,x)14C

1/2 day using 1 mm thick targets: SiO2(n,x)3He and Si(n,x)3He.

Total target irradiation time requested = 15 daysIn 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.

C SiO2 Mg Al

7Be 7.1 0.8

6.7 0.8 1.4 0.2 1.5 0.3

22Na 9.1 1.0 20.4 2.3 7.8 1.024Na 6.9 0.8 27.1 3.1 23.6 2.7

Ti Fe Ni Cu46Sc 49.8

5.9 4.3 0.6 1.4 0.2 1.0 0.1

48V 10.0 1.1

5.8 0.7 14.9 1.7

51Cr 40.4 4.6 24.7 2.8 8.2 0.952Mn 9.6 1.1 7.7 0.9 1.7 0.254Mn 69.7 8.0 17.5 2.3 9.8 1.256Co 29.4 3.3 3.4 0.457Co 130.0 15.0 19.2 2.358Co 110.0

13.128.4 3.2

60Co 6.5 0.8 14.5 1.7

Au194Au 145.0 17.0196Au 271.0

31.0198Au 15.1 1.8

Average cross sections measured at LANSCE 1998-2003 include:

natCu(p,x)60Co from S. J. Mills, G. F. Steyn and F. M. Nortier, Appl. Rad. Isot. 43, 1019, 1992

MC-ALICE calculations courtesy of Mark Chadwick.

0.01

0.1

1

10

100

1 10 100 1000

natCu(n,x)60Co

MC-ALICEThis workLANSCE average (1.25-750 MeV)KI99natCu(p,x)60Co MI92Other measurements

Cro

ss s

ecti

on (

mb)

Incident neutron energy (MeV)

7-27-2004

0.01

0.1

1

10

100

10 100 1000

27Al(p,x)22Na and 27Al(n,x)22Na

27Al(p,x)22Naderived excitation functionImamura et al.27Al(n,x)22Na iTL

average 27Al(n,x)22Na LANSCE

Cro

ss s

ecti

on (

mb)

Neutron energy (MeV)

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..