195 ENVIRONMENTAL NEUTRON MEASUREMENTS WITH SOLID STATE TRACK RECORDERS. J. H. Roberts,* R.A. Parker,* Fo J. Congel, Jacob Kastner, and B. G, Oltinan The suitability of using *B and ‘LiF in contact with cellulose ucetute butyrate (CAB) for the measurement of slow neutron densities has been investigated. The upper energy threshold of clear Diacel cellulose nitrate (CN) and of the CAB have also been measured. The CAB in good contact with thick *B or °LiF sources offers good promise for the detection of slow neutrons. For the actual CAB samples used, the CAB-"B combination had anefficiency of 1.3 X 10° tracks per thermal neutron incident in a 2m solid angle. The corresponding number for the CAB-“Li combination is 5.6 * 107 tracks per thermal neutron. INTRODUCTION The measurement of low level slow neutron den- sities with highly efficient and stable detection systems with low background is important for neutron dosimetry in a variety of environmental conditions. The suitability of cellulose nitrate (clear Diacel CN) and cellulose acetate butyrate (CAB) for registration of alpha and ‘Li particles from 1°B(n,a) and of alpha particles from °Li(n,«) has been studied. These studies have included a measurement of the upper en- ergy threshold of these materials for alpha track reg- istration. The motivation for the present studies is an application to the measurement of slow neutron levels over land and water due to cosmic radiation. These stud- les are now being undertaken. ETCHING PROCEDURES A 6.25 N NaOH solution was used to etch both types of plastic. This is nearly that suggested by Fleischer et al for CN.“ For the clear Diacel CN a room temperature etch of 2 hr gave excellent track Diacel CN were hard to distinguish from surface im- perfections. Thus the CAB was found to be much more promising for this application, THRESHOLD STUDIES Measurements of the upper energy threshold for alpha particles were carried out with a thin *44Am source, Two methods, one involving keeping the source-recorder distance fixed and varying the air pressure (Diacel CN), and the other, mvolving varying the source-recorder distance in air at constant pressure (CAB) were used. For the Diacel CN the source-recorder distance was 5.65 em. The angle of incidence of the alpha particles on the plastic was ~45°. The souree-recorder combina- tion was placed inside a chamberso that the air pres- sure could be regulated as desired. The main group of alpha particles from 744Am have an energy of 5.48 MeV. The source used had an activity of 2 x 10° disintegrations/min. Strips of CN were exposed for 90 min at 26° C and pressures of 30.5, 33.0, 35.6, 38.1, 40.6, 43.2, and 45.7 em of Hg. These correspond to in- cident alpha particle energies at the CN surface of 3.0, 2.8, 2.5, 2.2, 1.9, 1.5, and 1.1 MeV respectively.¢ Good tracks were obtained up to an alpha energy of 1.9 MeV. and no tracks were visible at all above 2.5 MeV. For the CAB, similar measurements indicated that the diameter of the etch pits decreased with increasing particle energy and were no longer visible for alpha particles above ~1.6 MeV. It is planned to refine these measurements on both the Diacel CN and the CAB. quality. However, by increasing the temperature to 40° C the etching time could be reduced to 19 min without anv significant increase in etch damage. Fol- SLOW NEUTRON DETECTION EFFICIENCY OF CELLULOSE ACETATE BUTYRATE IN CONTACT WITH THICK 18 that good track quality in the CAB was obtained by etching for 2.5 hr at 62° C. slow neutrons in the standard sigma pile at the lowing a suggestion of G. R. Ringo) it was found Typical alpha tracks in the CAB are shownin Fig- ures 159 and 160. The etched damage in the CAB resulting from alpha particle bombardment was more like circular pits than tracks. These etch pits are distinctive and readily distinguished from surface damage produced by the etchant in the unirradiated sur- face. When the surface was irradiated with alpha particles from a thick source, very short tracks in the * Northwestern University. AND ®LIF SOURCES Source-recorder combinations were irradiated with Argonne National Laboratory. Calibrations were made bv sandwiching a disk of CAB between a gold foil and a *LiF or ?°B source. These sources had a smooth flat surface so that good contact was possible. The °Lif crystals were square, 2 cm on a side and 2 mm thick. The !°9B was in the form of circular disks, 8 mmin diameter and 2 mm thick. Both sources were “black” to slow neutrons, and were infinitely thick + Based on the range-energy curve given by M. 8S. Livingston and H. A. Bethe.”