Production cross sections of ${}_{105}^{261}\mathrm{Ha}$ from the ${}^{250}\mathrm{Cf}{(}^{15}\mathrm{N},4n)$ and ${}^{243}\mathrm{Am}{(}^{22}\mathrm{Ne},4n)$ reactions

We have measured the production cross section of 1.8-s ${}^{261}\mathrm{Ha}$ from two different reactions. It was produced in the ${}^{250}\mathrm{Cf}{(}^{15}\mathrm{N},4n)$ reaction at 84 MeV and in the ${}^{243}\mathrm{Am}{(}^{22}\mathrm{Ne},4n)$ reaction at 116 MeV. Our rotating wheel system with a special parent-daughter stepping mode was used to detect α-α correlations between ${}^{261}\mathrm{Ha}$ and ${}^{257}\mathrm{Lr}.$ We measured 13 and 9 correlations in the two reactions, respectively. Assuming a 100% α branch, we have determined the production cross section of ${}^{261}\mathrm{Ha}$ to be $0.51\pm 0.20\mathrm{nb}$ in the ${}^{250}\mathrm{Cf}{(}^{15}\mathrm{N},4n)$ reaction at 84 MeV, and $0.25\pm 0.11\mathrm{nb}$ in the ${}^{243}\mathrm{Am}{(}^{22}\mathrm{Ne},4n)$ reaction at 116 MeV. Based on the number of fission events observed in the latter reaction, we have been able to set an upper limit of 18% for the spontaneous fission branch of ${}^{261}\mathrm{Ha}.$