Measurement of Neutron Absorption Cross Sections with a Pile Oscillator

The following paper deals with the measurement of thermal neutron cross sections by a technique referred to as the pile oscillator. In this method, a neutron absorber is moved back and forth in a field of thermal neutrons such as that existing in a chain reactor. In the vicinity of the absorber there is a depression in the neutron flux and the motion of this depression past a nearby ion chamber produces an oscillating signal whose amplitude is proportional to the total neutron absorption cross section of the sample. In the instrument considered here, the oscillating signal generated in the ion chamber is amplified, rectified, and fed to an integrating circuit. The absorption cross section of an unknown sample is determined by comparing the charge accumulated in the integrating circuit with that produced by a standard gold absorber. The instrument will detect a total absorption cross sestion of about 10^-3 cm^2 and hence it is suitable for measuring cross sections of separated isotopes which are available only in small quantities. One of the major difficulties in making this instrument useful for absorption measurements has been the elimination of the effects of scattering by the sample. It was found that the signals caused by absorption and scattering are slightly out of phase with each other, and by properly phasing the rectifier, absorption cross sections which are no greater than 1 percent of the corresponding scattering cross sections can be measured. A number of thermal cross section measurements have been made with this instrument, most of which will be published separately; as illustrations, the following thermal cross sections are given: In, 191. 2b; Ag^107, 29. 9b; Ag^109, 83. 7b.