modified reader of the research type described elsewhere.‘*) The glow curves were recorded on a strip chart, and the area under the glow curve peak was obtained with a mechanical integrator. Each dosime- ter was weighed after readout and the weight recorded to the nearest 0.1 mg. The responses of the dosimeters were expressed as area/mg/Rad (precalibration). The dosimeters were then annealed at 400° C for 1 hr and 85° C for 18 hr. Two sets of these dosimeters were used. One set, referred to as the predosed dosimeters, were exposed to radium gamma rays before the fast neutron irradiation, and a second set, referred to as postdosed dosimeters, were exposed to radium gammarays after the fast neutron irradiation. Both sets were exposed to 4R of radium gammarays. The dosimeters were packaged in cadmium ‘holders, which consisted of two square cadmium plates 6 mm square and 1 mm thick. Each holder contained 4 predosed dosimeters, 4 postdosed dosimeters, and 3 back- ground dosimeters. The exposure to fast neutrons was obtained by using the "Li (p,n) ®Be reaction from an 18-MeV tandem accelerator. At these energies there is the pos- sibility that the residual nucleus, 7Be, could be left in an excited state as well as the ground state.‘?) Conse- quently, the resulting neutrons would not necessarily be monoenergetic. However, the cross section for this event proved negligible. Exposures were made by placing the dosimeters at 20° with respect to the beam and 20cm from the target. A °38U fission counter was placed symmetri- cally with respect to the dosimeters and the beam. Exposures were made over several hours for a fluence of approximately 10° neutrons/cm? as determined by the fission counter. After exposure and readout (observed area per mg), all dosimeters were annealed at 400° C for 1 hr and 85° C for 18 hr. The responses to TABLE 68. Neutron energy radium gamma rays were redetermined (postcalibration area/mg/Rad). The gamma background exposure during neutron irradiation was determined from the background dosimeters. The expected response was calculated using the calibration values. The damage factor was computed as the ratio of the observed response to the expected response, Since the background dosimeters also suffered a de- crease in thermoluminescent response, the background gamma exposure had to be corrected. The initially determined factor was used to correct the background data and the results applied in an iterative manner until there was less than a 20% change in the background correction. RESULTS AND DISCUSSION Tables 68 and 69 give the results obtained for the predose and postdose dosimeters respectively. A comparison of the precalibration (Col. 5) and the postealibration areas (Col. 6) showed that the dosimeters suffered a decrease in gammasensitivity. The glow curves, however, exhibited no marked changes. Furthermore, this damage was established to be permanent, because there was no recovery of gamma sen- sitivity with further annealing in the manner described above. The ratio of the precalibration to postcalibration, i.e., the irreversible or permanent damage factor (Col. 7), failed to account fully for the observed decrease in thermoluminescence, This can be seen by comparing the total damage factor (Col. 4) with the irreversible damage factor. There is a residual temporary loss of response or a reversible damage. In speculating about the mechanism which would account for this reversible damage, one has to rule out lattice heating owing to the presence of this factor in both the predosed and postdosed dosimeters. It ap- Fast Neutron Errect on Preposep TLD-700 Total damage (1) (2) (3) Mev Cale. area/ mg Obs. area/ mg 6.2 7.2 8.2 9.2 10.1 11.1 12.1 14.1 224.8 170.9 312.6 263.3 346.1 328.2 152.2 391.2 126.9 47.8 216.7 148.0 136.2 123.4 91.5 196.8 Irreversible damage (4) [(3)/(2)] (5) (6) Reversible (7) [(6)/(S)] (8) ((4)/(7)] 0.83 0.78 0.84 0.76 0.77 0.70 0.96 0.70 0.68 0.40 0.78 0.71 0.52 0.58 0.63 0.73 Precalib. area/ |Postcalib. area /, mg * Rad mg * Rad 0.570 0.30 0.65 0.54 0.38 0.40 0.60 0.49 34.0 14.4 26.1 19.8 16.3 16.6 8.6 16.0 28.1 11.5 21.1 15.4 13.3 12.4 8.2 10.6