W. W. Havens

Nevis Synchrocyclotron Slow Neutron Velocity Spectrometer

A high-intensity, high-resolution neutron velocity selector system, employing the Nevis synchrocyclotron as a source of pulsed neutrons, is described. Detector counts are accumulated in a 2000-channel analyzer with 0.1-μsec channel width. Punch card data reduction techniques are discussed. Resonance spectra of PbI2 obtained using a 35-m flight path provide an example of the resolution and illustrate the use of the self-indication capture γ-ray detector scheme extensively employed. A planned flight path of 200 m is expected to provide a resolution of <1 nsec/m for neutron energies above 1000 ev.

Neutron Crystal Spectrometer with Range Extended to Subthermal Energies

A neutron crystal spectrometer is described which has the following features: (1) a useful range from 0.1 to 11.5 A, (2) exceptional reproducibility for high precision experiments, (3) complete automatic control for measuring sample transmission as a function of wavelength, (4) a temperature regulated sample changer with range from −196 to 300°C, and (5) direct recording of data on punch cards, and a FORTRAN program for the statistical analysis of the data on a digital computer. Three alternate techniques were investigated and developed for extending the range of the spectrometer to wavelengths longer than 1 A where the relative intensity of higher order Bragg reflections becomes large. In two of the methods, crystalline filters are used to attenuate shorter wavelength neutrons in the beam. In the third, a mechanical neutron velocity selector is used which transmits only the desired first order reflection. The residual higher order contamination in the spectrometer beam was measured to be less than 0.5% o...

SYNCHROCYCLOTRON 200-METER FLIGHT PATH NEUTRON VELOCITY SPECTROMETER

Our synchrocyclotron neutron time‐of‐flight spectrometer now uses a 200‐m flight path and two 2000‐channel analyzers. The many important modifications and improvements described in this paper have been made as a result of operating experience. The present operation uses ∼20 nsec bursts at an instantaneous intensity ∼5×1018 evaporation neutrons per second and a 60‐cps repetition rate. The resolution for energies above 1 keV is now limited by the time analyzers which have 100‐nsec detection channel widths, giving a resolution of 0.5 nsec/m in the keV region.

Recent Experimental Neutron Resonance Spectroscopy Results as a Test of Statistical Theories of Short and Long Range Order for Level Spacings

During 1968 and 1970, we obtained large amounts of high quality neutron resonance spectroscopy data using the Columbia University Nevis Synchrocyclotron. This report emphasizes our experimental results for even-even nuclei having 150 < A < 190. In the oast, attempts to make detailed comparison of experimental resonance energies for nuclei for such “best test” cases as Th232 or U238 with theory gave poor fits for those tests which assumed that a single s level population only was present, and were sensitive to the inclusion of a partial extra p level population. For 150 < A < 190, the s level strength function S0 is sufficiently greater than the p level strength function S1 that all p levels tend to be weaker than all but a very small fraction of the s levels, providing a better separation of the two populations. Our results for Erl66, Erl68, wl82, wl84, Sml52, and Ybl72 were of particularly good quality, and give good agreement with the following statistical tests. Except for Er168, they seem to have only s levels, and for Er168 the p levels can be cleanly separated out on the basis of their strength.

HIGH-RESOLUTION NEUTRON RESONANCE SPECTROSCOPY IN NATURAL FLUORINE, ALUMINUM, CHLORINE, AND POTASSIUM.

In order to understand the features of nuclear reactions, we need to be able to relate the cross sections to the internal properties of the nucleus. The simple Breit Wigner formule(1) has been intensively used with the necessary modifications for Doppler broadening in the analysis of resonances observed in neutron interactions with heavy nuclei. Another important theory is due to Kapur and Peierls(2) which employs two sets of eigenstates and eigenvalues. The boundary conditions required for their definition require the logarithmic derivatives of the interior wave function at each channel entrance to be equal to the logarithmic derivative of the radial outgoing or ingoing wave function, respectively, in that channel. These two sets of eigenstates are mutually, but not separately, orthogonal, and the necessary expansions must be made in terms of both of them. The resulting cross section expressions are very familiar in form to those of S-matrix theory, but the parameters are all energy dependent. The S-matrix formulation of nuclear reaction theory by Humblet and Rosenfeld(3) has the advantage of a physically clear definition of resonance energy and rather simple expressions for the cross sections.