A focusing spectrometer using a variable curvature quartz crystal has been developed for measuring total cross sections versus energy of small samples of rare elements, separated isotopes, and radioactive isotopes in the energy region 0.03 ev to about 1.5 ev. Samples having macroscopic cross sections from 0.1 to 1.0 square millimeters have been used as absorbers. The main features of the new instrument are described, and performance curves are given. The instrument was first used to show that a sample of rare earth fission product material has a resonance at an energy corresponding to the known resonance of ${\mathrm{Sm}}^{149}$. Test runs were made on normal cobalt, whose total cross section was found to fit the formula, $\sigma \left(\mathrm{barns}\right)=5.0+6.1\mathrm{}{\left(E \mathrm{in}\mathrm{ev}\right)}^{\frac{1}{2}}$. A resonance at about 1 ev, attributed in the literature to zirconium, was found to belong to hafnium. Erbium was found to have a neutron resonance at about 0.5 ev. ${\mathrm{Ni}}^{58}$ was found to be the isotope primarily responsible for the anomalously high scattering cross section of normal nickel, confirming results from neutron diffraction studies made simultaneously by other observers. Results are given also for ${\mathrm{Ni}}^{60}$. Analysis of the holmium measurements give a scattering cross section that is strongly energy dependent. Paramagnetic scattering is suggested as a possible explanation of the variation.

• Received 11 April 1952

DOI:https://doi.org/10.1103/PhysRev.87.487