The Influence of Dosimetry on Earlier Damage Equivalence Ratios

Since it was first noted that the experimental damage equivalence ratios for different neutron spectra obtained from change in transistor parameter behavior did not agree with the calculated ratios based on damage to silicon, an explanation has been sought for this apparent discrepancy. Investigations have included device processing techniques device geometry, and finally dosimetry. Careful dosimetry is of the utmost importance to these ratio measurements. Recently extensive studies of the White Sands Fast Burst Reactor (FBR), which is identical to the Sandia Pulsed Reactor (SPR-II), yielded information which suggested that a significant error was possible in the SPR-II dosimetry utilized in earlier studies. A test group at SAl726 (2N4251) transistors were used to make a comparison of the spectrum from the FBR and the SPR-II. The results of the device study showed a difference of 25-30 percent in fluence measured by Sandia dosimetry and by White Sands dosimetry for a given FBR exposure. This difference was for a 10 KeV energy threshold. These findings prompted an investigation of the Sandia dosimetry techniques used at SPR-II. This investigation showed, on the basis of dosimetry measurements excluding fission foils, that the 10 KeV/3 MeV ratio for SPR-II was 7.5 instead of 10 as presently used. The correction in dosimetry from SPR-II when applied to earlier damage equivalence ratios changes the 14 MeV-to-SPR-II damage ratio from 4.2 to approximately 2.9. This value (2.9) is in good agreement with other experimenters and calculations based on displacement damage in silicon.