Thresholds Observed in the ${\mathrm{C}}^{12}(\alpha ,n){\mathrm{O}}^{15}$, ${\mathrm{Si}}^{28}(\alpha ,n){\mathrm{S}}^{31}$, ${\mathrm{S}}^{32}(\alpha ,n){\mathrm{Ar}}^{35}$, ${\mathrm{S}}^{34}(\alpha ,n){\mathrm{Ar}}^{37}$, and ${\mathrm{Cl}}^{35}\mathrm{}(p,n)\mathrm{}{\mathrm{Ar}}^{35}$ Reactions

The following thresholds have been measured in the ($\alpha ,n$) reaction: ${\mathrm{C}}^{12}$ 11.341±0.015; ${\mathrm{Si}}^{28}$ 9.30±0.05, 10.55±0.05, 11.83±0.05, 12.78±0.05; ${\mathrm{S}}^{32}$ 9.846±0.020, 10.69±0.05, 11.97±0.08; ${\mathrm{S}}^{34}$ 5.17±0.10, 5.96±0.05, 6.90±0.05, 8.04±0.05, 9.09±0.05, 10.09±0.05, 10.31±0.05 MeV. These results are in agreement with previously published results except for the ${\mathrm{S}}^{32}(\alpha ,n){\mathrm{Ar}}^{35}$ ground threshold which is 140 keV higher than expected. To confirm the value of the mass of ${\mathrm{Ar}}^{35}$, the ${\mathrm{Cl}}^{35}\mathrm{}(p,n)\mathrm{}{\mathrm{Ar}}^{35}$ threshold energy was measured and is 6.942±0.020 MeV. Since this result agrees with the accepted value of the ${\mathrm{Ar}}^{35}$ mass, the 140-keV discrepancy was not resolved. In the ${\mathrm{C}}^{12}(\alpha ,n){\mathrm{O}}^{15}$ excitation curve a prominent resonance of half-width 220±60 keV was found corresponding to a level in ${\mathrm{O}}^{16}$ of excitation 18.10±0.06 MeV. The cross section rises to about 22 mb at the peak of this resonance. Half-life determinations yielded the values: ${\mathrm{O}}^{15}$ 122.6±1.0, ${\mathrm{S}}^{31}$ 2.56±0.10, and ${\mathrm{Ar}}^{35}$ 1.76±0.03 sec.