Total Neutron Cross Section of Chlorine and Carbon

The total neutron cross section of chlorine has been measured with the Materials Testing Reactor crystal spectrometer (0.03-70 ev) and fast chopper (16-15 000 ev). The chlorine cross section drops from 45 barns at 0.03 ev to a low of 1.3 barns at 5 kev. A sharp resonance is observed in ${\mathrm{Cl}}^{35}$ at 405±6 ev. A two-sample-area analysis of this resonance gives the limiting values; $\Gamma <0.6\mathrm{}$ ev and ${\sigma }_0>580\mathrm{}$ barns. For these limits, ${\Gamma }_n=0.14\mathrm{}$ ev. Another level was observed at 8.7±0.5 kev. A good fit to the cross-section data is given by a single-level Breit-Wigner formula for a negative-energy resonance in ${\mathrm{Cl}}^{35}$ with spin 2 and the following parameters; $E_0=-140\mathrm{}$ ev, ${{\Gamma }_n}^0=0.72\mathrm{}$ ev, ${\Gamma }_{\gamma }=0.48\mathrm{}$ ev, $R=4.74\mathrm{}\times {10}^{-13\mathrm{}}$ cm, and ${\sigma }_{\mathrm{pot}}=1.13\mathrm{}$ barns. From these parameters, one would obtain ${\sigma }_{\mathrm{incoh}}=7.8\mathrm{}\pm 0.7$ barns and ${\sigma }_{\mathrm{act}}\left(0.025\mathrm{} \mathrm{ev}\right)=34.5\mathrm{}\pm 3.4$ barns.