$(\alpha ,\gamma )$ cross section measurements in the region of light $p$ nuclei

The ${}^{90}\mathrm{Zr}(\alpha ,\gamma ){}^{94}\mathrm{Mo},{}^{92}\mathrm{Zr}(\alpha ,\gamma ){}^{96}\mathrm{Mo}$, and ${}^{74}\mathrm{Ge}(\alpha ,\gamma ){}^{78}\mathrm{Se}$ reaction cross sections were measured for the first time in an effort to expand the existing experimental database for $(\alpha ,\gamma )$ reactions relevant for the production of $p$ nuclei in the universe. In particular, the ${}^{90}\mathrm{Zr}(\alpha ,\gamma ){}^{94}\mathrm{Mo}$ reaction was identified by a sensitivity study for its potential impact on the $\gamma$-process mass flow in the region of light $p$ nuclei. The measurements were performed for energies $E_{\alpha }=9.5–12.0$ MeV at the University of Notre Dame using the SuN detector and the $\gamma$-summing technique. The results are compared to theoretical calculations from the talys and non-smoker nuclear reaction codes, and it is shown that the data greatly reduce the uncertainty in the cross section for the measured energies. The talys parameters that provide the best description of the experimental data are reported.

Figure 1

REACLIB stellar reaction rates [12] as a function of temperature for the photodissociation of ${}^{94}\mathrm{Mo}$ through the $(\gamma ,n),(\gamma ,p)$, and $(\gamma ,\alpha )$ channels. The width of the curves corresponds to a factor of 10 uncertainty in the $(\gamma ,\alpha )$ reaction rate and a factor of 5 uncertainty in the $(\gamma ,n)$ and $(\gamma ,p)$ rates. Within the $\gamma$-process window of 1.8–3.3 GK, the ${}^{94}\mathrm{Mo}(\gamma ,\alpha ){}^{90}\mathrm{Zr}$ reaction could have a rate higher than that of the ${}^{94}\mathrm{Mo}(\gamma ,n){}^{93}\mathrm{Mo}$ reaction and could therefore be a branching point in the $\gamma$ process [11].

Figure 2

Total $\gamma$-summed spectrum for an $\alpha$ beam impinging onto the enriched ${}^{90}\mathrm{Zr}$ target at $E_{\mathrm{c}.\mathrm{m}.}=9.94$ MeV, with the ${}^{90}\mathrm{Zr}(\alpha ,\gamma ){}^{94}\mathrm{Mo}$ sum peak at 12 MeV. The peak around 9 MeV in the spectrum comes from neutrons released in the ${}^{90}\mathrm{Zr}(\alpha ,n){}^{93}\mathrm{Mo}$ reaction. Also plotted is the normalized room background.

Figure 3

Total $\gamma$-summed spectrum for an $\alpha$ beam impinging onto the enriched ${}^{92}\mathrm{Zr}$ target at $E_{\mathrm{c}.\mathrm{m}.}=11.4$ MeV, with the ${}^{92}\mathrm{Zr}(\alpha ,\gamma ){}^{96}\mathrm{Mo}$ sum peak at 14.2 MeV. The peak around 12 MeV originates from neutrons emitted in the ${}^{92}\mathrm{Zr}(\alpha ,n){}^{95}\mathrm{Mo}$ reaction. Also plotted are the normalized room background and the beam-induced background from the tantalum backing. The inset shows a zoomed-in view of the sum peak and the linear background used when integrating the sum peak.

Figure 4

Total $\gamma$-summed spectrum for an $\alpha$ beam impinging onto the enriched ${}^{74}\mathrm{Ge}$ target at $E_{\mathrm{c}.\mathrm{m}.}=8.5$ MeV, with the ${}^{74}\mathrm{Ge}(\alpha ,\gamma ){}^{78}\mathrm{Se}$ sum peak at 14.5 MeV. The neutron-induced peak around 10.5 MeV comes from neutrons emitted in the ${}^{74}\mathrm{Ge}(\alpha ,n){}^{77}\mathrm{Se}$ reaction. Also plotted is the normalized room background spectrum.

Figure 5

Experimental cross sections for the ${}^{90}\mathrm{Zr}(\alpha ,\gamma ){}^{94}\mathrm{Mo}$ reaction compared to talys and non-smoker calculations. The three talys curves correspond to the upper limit, lower limit, and best-fit calculations (see text for details).

Figure 6

Experimental cross sections for the ${}^{92}\mathrm{Zr}(\alpha ,\gamma ){}^{96}\mathrm{Mo}$ reaction compared to talys and non-smoker calculations. The three talys curves correspond to the upper limit, lower limit, and best-fit calculations (see text for details).

Figure 7

Experimental cross sections for the ${}^{74}\mathrm{Ge}(\alpha ,\gamma ){}^{78}\mathrm{Se}$ reaction compared to talys and non-smoker calculations. The three talys curves correspond to the upper limit, lower limit, and best-fit calculations (see text for details).