Neutron Transmission Measurements Research Articles

Radiation Widths of Levels in Nuclei near Closed Shells

An experimental investigation has been made of the variations in the radiation widths of nuclear energy levels for isotopes in the regions of the neutron magic numbers, where fluctuations in level spacing and neutron binding energy are largest. Neutron transmission measurements were made using the Brookhaven fast chopper, and the Breit-Wigner level parameters were obtained for resonances in the following target isotopes: ${\mathrm{Sr}}^{87}$, ${\mathrm{Sb}}^{121}$, ${\mathrm{Sb}}^{123}$, ${\mathrm{Ba}}^{135}$, ${\mathrm{La}}^{139}$, ${\mathrm{Nd}}^{145}$, and ${\mathrm{Pt}}^{195}$. Other elements around closed neutron shells, namely, Rb, Zr, Nb, Ce, and Tl, were examined but no accurate measurements could be made of the radiation widths for these elements. Results show that the fluctuations in the measured radiation widths are small compared to the large fluctuations in neutron scattering widths, and that they are related to the level spacing and the effective level excitation energy. An effect arising from the closed shells at 82 and 126 neutrons is observed. Analysis of all the available good data indicates that the essential features of the theory of Blatt and Weisskopf are generally valid; that is, radiation widths are strongly dependent on the effective level excitation energy and weakly dependent on the level spacing. Experimental results are compared with predictions from semiempirical formulas.

Boron-Loaded Liquid Scintillation Neutron Detectors

The general problems involved in constructing boron-loaded liquid scintillation neutron detectors are considered. The characteristics of particular counters which have been successfully used in neutron transmission measurements by the time-of-flight method are then described. The design of these counters was guided by the results of a Monte Carlo study of neutron capture in a boron-poisoned medium. This calculation gives the probability of neutron capture as a function of neutron energy, counter thickness, and time. The calculated results are compared with experimentally determined efficiencies. The advantages and problems encountered in using the boron-loaded liquid scintillator with the Argonne fast neutron chopper are discussed.

Neutron Transmission Measurements and Resonance Parameters in Cd

Neutron transmission measurements for cadmium have been determined from 14.5 ev to 11 kev. These data were taken with 0.04 to 0.07 \ensuremath{\mu}sec/meter resolution using the Materials Testing Reactor fast chopper. Isotopic assignments have been made to those resonances which are due to ${\mathrm{Cd}}^{111}$ and ${\mathrm{Cd}}^{113}$ by using a sample enriched in ${\mathrm{Cd}}^{111}$ and another sample depleted in ${\mathrm{Cd}}^{113}$. Breit-Wigner parameters have been obtained up to 215 ev and give values for $\frac{{{\overline{\ensuremath{\Gamma}}}_{n}}^{0}}{D}$ of (0.33\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ for ${\mathrm{Cd}}^{111}$ and (0.43\ifmmode\pm\else\textpm\fi{}0.13)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ for ${\mathrm{Cd}}^{113}$.

Neutron Cross-Section Measurements of Antimony, Gallium, Cadmium, and Mercury

The results of neutron transmission measurements on antimony, gallium, cadmium, and mercury, and on their separated isotopes are presented. Several methods are described for finding resonance parameters from neutron transmission measurements. Level parameters are reported for six resonances of ${\mathrm{Sb}}^{121}$; four of ${\mathrm{Sb}}^{123}$; three for ${\mathrm{Ga}}^{69}$; four for ${\mathrm{Ga}}^{71}$; and one each for ${\mathrm{Cd}}^{110}$ and ${\mathrm{Cd}}^{111}$. Isotopic assignments are made for three resonances in mercury.

Slow Neutron Resonances of Manganese, Bismuth, and Selenium

The apparatus associated with the Argonne fast neutron chopper and techniques that have been developed for its use in neutron transmission measurements are described. Of particular importance is the application of a boron-loaded liquid scintillation counter for the efficient detection of slow neutrons. The nature of the data that are obtained is illustrated by the results given for bismuth, selenium, and manganese. Several techniques for finding neutron resonance parameters and their application to the above data are are discussed. Resonance widths are found for the first two levels in bismuth. For the levels in selenium the measurements give only ${\ensuremath{\sigma}}_{0}{\ensuremath{\Gamma}}^{2}$. A complete set of parameters is obtained for the first three manganese resonances. The abnormally large ratio of neutron width to level spacing for manganese produces an unusual shape for the cross-section curve in the neighborhood of the second resonance which is interpreted as being caused by interference between the second and third resonances. For this interpretation we obtain $J=2$ for ${E}_{0}=337$ ev, $J=3$ for ${E}_{0}=1080$ ev, and $J=3$ for 2360 ev.

(n,γ) Detector for Study of Low-Energy Neutron Resonances with a Velocity Selector

Development of a neutron capture gamma-ray detector for study of neutron resonances by the time-of-flight method is described. The G.E. 100-Mev betatron is used as a pulsed source of neutrons. Gamma rays are detected by a pair of large annular counters, each containing toluene-terphenyl scintillation phosphor and four 5819 photomultipliers. The counters have a 4 in. diam opening for the neutron beam, which passes first through one, then the sample radiator, then the other; all centered on a common axis. Each counter subtends about 40 percent of the total solid angle, and has a detection efficiency of about 5 percent for 1-Mev gamma radiation. The two detectors are connected in coincidence, with a resolving time of about 10−8 sec. Use of the instrument for measuring capture level densities is illustrated by a resonance curve for gold. The detector is also used for measurement of neutron transmission with thick samarium oxide as the radiator. This application is illustrated by transmission curves for manganese.