Pile Oscillator Research Articles

Pile Oscillator Measurements of Eta

Pile Oscillator Measurements of Eta

The design of a pile oscillator for the ford nuclear reactor

A pile oscillator is a device that periodically varies the neutron population in a nuclear reactor. The population change may occur as a localized variation in neutron density, or the density of neutrons throughout the reactor may be caused to vary in a systematic manner. Such a variation may be effected by a periodic oscillation of the multiplication constant, keff, of the reactor. The required changes in keff have been achieved in reactor oscillators by mechanical oscillations of either absorbers or reflectors located in or near the core.

The thermal neutron absorption cross-section of 240Pu

A measurement of the thermal neutron absorption cross-section of 240Pu has been made using the Oak Ridge Pile Oscillator. Enriched 240Pu samples and almost isotopically pure 239Pu samples were compared with gold standards. All samples were sufficiently thin so as to offer negligible self protection. The 240Pu was found to give a response per atom 2·94 times that of gold. Upon correcting for the non-(1/ν) character of 240Pu absorption, a value of285 ± 15 barns is computed for the 2200 m/sec absorption cross-section. The value of 314 ± 30 barns calculated from the resonance parameters of 240Pu is in agreement with this measurement.

Properties ofs-Wave andp-Wave Neutron Resonances in Niobium

The neutron resonances of niobium were studied with the Argonne fast chopper. The transmission of a series of four samples was measured over the range of energy from 10 to 10 000 ev. Resonance parameters were deduced for thirteen levels, those below 700 ev. An examination of these parameters, calculations with the optical model, and a comparison of the cross sections at low and high energy all indicate that some of the resonances, even those below 100 ev, are due to a $p$-wave process. By using this evidence, average parameters are deduced. The radiation width is found to be 0.22 ev. The strength function $\frac{{{\overline{\ensuremath{\Gamma}}}_{n}}^{0}}{\overline{D}}$ for $s$-wave neutrons is 0.10\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$. The value of the corresponding quantity for $p$-wave neutrons is 4.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$. The resonance absorption integral deduced from the parameters is found to be in agreement with the result of a "pile oscillator" measurement.

XVII. Pile modulation and statistical fluctuations in piles

Abstract A theory is given of the statistical fluctuation of the number of neutrons in a pile. An experimental verification of this theory is described, and the results of the theory are used to evaluate the accuracy of the measurement of neutron absorption cross sections with a pile oscillator.

THERMAL NEUTRON CAPTURE CROSS-SECTION OF Na23 AND Mn55

The thermal neutron capture cross sections of Na23 and Mn55 have been determined using the activation method. The values are 0.53 ± 0.03 and 12.7 ± 0.3 barns respectively with respect to σAul97 = 93 barns. These agree well with recent pile oscillator results. The half-life for Mn56 is found to be 2.576 ± 0.002 hr.

The Nuclear Reactor as a Research Implement

Several of the more significant physical experiments which have been performed in recent years with nuclear reactors as measuring instruments or as neutron sources are described. The examples are selected partly because of their importance and partly with diversification in mind. Short descriptions are given of the pile oscillator method of measuring neutron absorption cross sections, the measurement of neutron capture gamma-ray spectra, the study of neutron resonances by time-of-flight, neutron diffraction, neutron reflection, and the radioactive decay of the neutron.

Thermal Neutron Capture Cross Sections

The thermal neutron capture cross sections of more than one hundred isotopes, including all the isotopes of 24 elements, plus several others, have been measured with the Oak Ridge National Laboratory pile oscillator.

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 ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$ 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, $191.2b$; ${\mathrm{Ag}}^{107}$, $29.9b$; ${\mathrm{Ag}}^{109}$, $83.7b$.