1. NEEP 541 – Neutron Damage Fall 2002 Jake Blanchard

2. Outline Neutron Damage Definitions Modeling Inelastic Collisions Empirical Data

3. Introduction Neutron damage results from the production of PKAs by neutron-target collisions Neutrons, because they aren’t charged, behave differently from other particles we’ve discussed The mean free path of fast neutrons in most solids is on the order of 15 cm

4. Definitions

5. Thermal Neutrons produce damage through (n,  ) reactions, eg. 27 Al+n -> 28 Al+Q For this reaction, Q=7.73 MeV, T=1.1 keV Thermal Neutrons

6. Fast Neutrons At low neutron energy (<1 MeV), angular distributions of elastically scattered neutrons is isotropic As E increases, scattering is more forward Above a few MeV, scattering becomes inelastic (nucleus is excited, and later emits a photon

8. Inelastic Collisions Nucleus recoils in excited state Kinetic energy not conserved Excitation energy is Q Threshold energy exists Neutron is absorbed, then emitted (fairly isotropic)

9. Inelastic Collisions

14. Example N=0.0805 /cubic angstrom  el =3 barns Flux=10 15 n/cm 2 /s Displacement rate=8.5 10 16 disp/cm 3 /s R d /N=33 dpa/year

15. Plot of  d vs E for elastic and inelastic

16. Typical Fusion Dose Rates MaterialDose Rate (dpa/y) 316 SS6.8 V8.1 Nb5.1 Mo5.7 Al11.9 Ti11.1 14 MeV neutrons, 1 MW/m 2 wall loading

17. Sample Dose Rates (dpa/s) Magnetic fusion=3 10 -7 Inertial fusion=3 Fission= 10 -6 Ion beams= 10 -4 -10 -2 Electron beams= 10 -3

18. Spectra Fission Lethargy=E 

19. Accelerator Sources D-T interactions Spallation uses proton beams (hundreds of MeV) aimed at large targets

21. Recoil Spectra

22. Damage Cross Sections