On the Slow Neutron Absorption Resonances in Iodine

Abstract

The 12-channel slow neutron velocity spectrometer at Cornell University is described. Reported here is a measurement with this spectrometer of the neutron cross section of iodine over an energy range extending from 0.0026 to 1000 ev. Below 3 ev, the iodine cross section was found to obey the 1/v absorption law. The total cross section at thermal energy ($kT=0.025\mathrm{}$ ev) was found to be 10.3×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom. The scattering cross section, found by extrapolating to zero time of flight the straight line which best fitted the data on a cross-section vs. time-of-flight graph, is 3.6×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom. A single resonance was observed at 20.3 ev. A region containing at least three resonances was observed between 25 and 50 ev. An indication of a resonance or group of resonances was found near 85 ev. An attempt was made to match a symmetrical type Breit-Winger formula to the observed data at the 20.3 ev resonance. The constants chosen in this manner for the resonance are: $E_r=20.3\mathrm{}\pm 0.5$ ev, ${\sigma }_0=80\mathrm{}\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom with ${\sigma }_0>40\mathrm{}\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom, and $\Gamma =0.45\mathrm{}$ ev with $\Gamma <0.8\mathrm{}$ ev. Consideration of the activities for three thicknesses of the absorber for this resonance showed that the absorbers were thick and that ${\sigma }_0{\Gamma }^2=15\mathrm{}\times {10}^{-24\mathrm{}}$ ${\mathrm{ev}}^2$ ${\mathrm{cm}}^2$/atom, neglecting the Doppler effect. In the neutron energy region between 25 and 50 ev, the relation between the activity and the absorber thicknesses used showed that the absorbers were thick for the resonances in this region. From the curve of activity as a function of thickness, it was found that $\Sigma r^{}{\left({\sigma }_{0r}{{\Gamma }_r}^2\right)}^{\frac{1}{2}}=32.5\mathrm{}\times {10}^{-12\mathrm{}}$ ev-cm/${\mathrm{atom}}^{\mathrm{½}}$ for these resonances, neglecting the Doppler effect. An attempt at fitting two resonances of about equal strength to the data proved to be impossible, and it was concluded that there are three or more resonances in this region. Consideration of the maximum widths of these resonances, that account for the absorption, indicated that the widths were less than 1 or 2 ev. This conclusion is reached regardless of the relation of the level widths to the Doppler width. Therefore, it is believed that there are no resonances for neutron energies between 0 and 70 ev which are so wide as to be incompatible with the present theory of nuclei. Since iodine has only one stable isotope, the resonances observed are for one type nucleus. It would be valuable to know exactly the number of resonances for a wide range of neutron energies; however, the apparatus used in the present experiment does not have sufficient resolution to resolve resonances with a spacing as small as 7 ev at 40-ev energy.