201 relative to the range of all charged particles produced by slow neutron capture in °Li or 1B. The recorder- source combinations were placed in aluminumholders under pressure supplied by a thin laver of sponge rubber to insure good surface contact. In some expo- sures the packages were placed in Cd boxes so that a correction for epithermal neutrons could be made. The unperturbed thermal neutron flux in the standard pile at the position where the packages were placed was assumed* to be 4100 + 200 neutrons/cm-" sec. The ratio of the Au activity for a given exposure time with and without the ®LiF was 0.42 + 0.02. The corresponding ratio for the 1°B was 0.50 = 0.01. The lack of flux depression in the case of the 4B is due to its small mass. Thus, the thermal neutron flux at the ®LiF surface was 1720 + 100 neutrons/cm? sec, and on the back side of the *LiF and !°B sources are absorbed without producing tracks. CONCLUSIONS The full energy of the alpha particles from SLitr.a«) induced by slow neutrons is 2.0 MeV. Most of the alpha particles from !°B(n,«) will have a maximum energy of 1.5 MeV. For thick !°B and °LiF sources most of the alpha particles entering the CAB surface will be below its threshold ~1.6 MeV. For the !°B sources the *Li recoil nuclei will also produce tracks. Thus, the 1°B is especially well suited for thermal neutron detection when used with the CAB plastic. Whereas the research reported here indicates that the CAB '°B or *L1 combinations offer great promise for slow neutron detection, further studics are needed the corresponding flux at the B was 2050 + 100 to put the results on a completely quantitative basis. The track recording rate for the °LiF was 9.65 = own calibrations. The threshold, detection efficiency, neutrons/em? sec. 0.23 tracks/em* sec in the neutron flux of 1720 neutrons/em* sec. The detection efficiency is thus (5.6 + 0.3) <X 10~% tracks per thermal neutron. The corre- sponding numbers for the B are (26.7 = 1.1) tracks/cm?-see in the thermal neutron flux of 2050, or (1.30 = 0.08) X 107? tracks/thermal neutron. The above calculated efficiencies are for thermal neutrons incident over a 2 x solid angle. For thermal neutrons incident isotropically, the above values should be divided by two, since all neutrons incident * Based on a measurement carried out by Harvey Casson in 1961 with gold foils. The flux is being redetermined with eold foils and natural uranium-mica combinations.“ Anyone planning to use the CAB should carry out his etc., depend upon the manufacturing process of the actual material used and may even vary from batch to batch from the same manufacturer. The authors are indebted to Harvey Casson for helpful discussions on the thermal neutron flux meas- urements, REFERENCES 1. Fleischer, R.L., Priee, P. B., and Walker, R. M. Ann. Per, Nuel. Set. 16, 1 (1965). 2. Ringo, G. R. Private communication. 3. Livingston, M.8., snd Bethe, H. A. Rev. Mod. Phys. 9, 245 (1987). 4, Gold, R., Arman, R. J., and Roberts, J. TE. Nuel. Sev. Eng. 34, 13 (1968). PERMANENT DAMAGE OF ‘LiF THERMOLUMINESCENT DOSIMETERS BY FAST: NEUTRONS Jacob’Kastner, Keith Eckerman,* B. G, Oltman, and Pete Tedeschi It has been postulated that lattice heating may be the mechanism for the effects of fast neutrons on the gamma-ray response of TLD-700. In the experiment to be described, LiF was exposed to neutron energies of 6-14 MeV. The observed decrease in thermoluminescent response was established to be due in part to permanent damage of the dosimeter and definitely not attributable to lattice heating. INTRODUCTION That there was an effect of fast neutron exposure on the 7LiF thermoluminescent response to gamma rays was reported earlier.) The decrease in thermoluminescent response observed seemed to be independent of the total neutron exposure and neutron energies * Civil Engineering Department, Environmental Division, Northwestern University, Evanston, Illinois. Health below 1 MeV. Furthermore, as no permanent damage was observed, it was postulated that neutrons of less than 1.0 MeV may be introducing lattice vibrations giving effects similar to the thermal effects of a TLD heater. In order to verify this hypothesis, it was decided to investigate this phenomenon at neutron energies of 6 to 14 MeV. The following is an account of this study and its results, EXPERIMENTAL PROCEDURE The gamma-ray responses of TLD 700 dosimeterst were determined with radium gamma rays using a + Extruded rods, 14 X 14 X 6 mm, Harshaw Chemical Co.. Cleveland, Ohio.