Measurement of Anisotropic Fast-Neutron Spectra with Nuclear Emulsion Techniques

Abstract

Improvements in a technique for deducing the spectrum of fast neutrons from the measurement of proton recoil tracks in nuclear emulsions are described. The source of neutrons may be anisotropic. Tests were made by placing Ilford L4 emulsion plates in an assembly of depleted uranium (0.21% ²³⁵U) in which neutrons are produced by photofission. The photofission was induced by bremsstrahlung produced when a pulsed 30-Me V electron beam from a LINAC hit a target of depleted uranium within the assembly. Special development with amidol developer was used to reduce chemical fog and gamma-ray background in the emulsions, permitting a measurement of proton tracks down to about two microns. The neutron spectrum from ≈ 0.3 to 2.5 Me V is deduced from the measurements. Computer programs were developed to make the calculations. Results up to ≈ 0.9 Me V are compared with the neutron spectrum measured by time-of-flight techniques. Satisfactory agreement is obtained.