Measurement of the ${}^{100}\mathrm{Mo}(\alpha ,xn)$ cross section at weak $r$-process energies

The weak $r$ process in neutrino-driven winds following a core-collapse supernova is thought to contribute to the cosmic abundances of the first $r$-process peak elements between Se and Ag. Sensitivity studies have found that the early nucleosynthesis in the weak $r$ process is primarily driven by $(\alpha ,xn)$ reactions due to the high temperatures, and that current nuclear physics uncertainties in the $(\alpha ,xn)$ rates result in significant uncertainties of the calculated abundances. The weak $r$-process path proceeds several nuclei away from stability where $(\alpha ,xn)$ reaction cross sections have not yet been measured. In this paper we report the ${}^{100}\mathrm{Mo}(\alpha ,xn)$ cross section (between 8.9 and 13.2 MeV in the center of mass, corresponding to 3.5–6.8 GK) in inverse kinematics using the Multi-Sampling Ionization Chamber (MUSIC) detector at the Argonne Tandem Linac Accelerator System (ATLAS) facility. With this first measurement of the ${}^{100}\mathrm{Mo}(\alpha ,xn)$ cross section, we have demonstrated the ability of MUSIC to measure $(\alpha ,xn)$ cross sections for $A$ up to 100, therefore paving the way for further measurements with radioactive beams at ATLAS or the Facility for Rare Isotope Beams.

Figure 1

Comparison of the energy losses in the first two strips of MUSIC showing the different components of the beam, at an energy of $\approx 475$ MeV and a gas pressure of 459 Torr. ${}^{100}\mathrm{Mo}$ is the dominant component and can be easily separated from the contaminants.

Figure 2

Energy loss plot of events in the fifth anode strip of MUSIC showing the separation of the different exit channels. The $y$ axis is the gain-matched summed energy loss over anode strips 5–15 in MeV and the $x$ axis is the energy loss as measured by the Frisch grid of MUSIC. The approximate analysis gate used for the $\alpha$-induced events is shown as the dashed oval. The more intense feature is the unreacted ${}^{100}\mathrm{Mo}$ beam.

Figure 3

Energy loss profiles of individual events in MUSIC (“traces”). Traces in black dot-dashed lines are from the unreacted Mo beam. $(\alpha ,xn)$ events occurring in strip 5 are plotted as solid red lines, showing a characteristic and persistent jump in energy loss that characterizes a change in $Z$. Large-angle scattering events are shown as blue dashed lines where there is an increase in the energy loss from the scattered $\alpha$ particle over a small number of strips before the energy loss reverts to a beamlike profile. For small-angle scattering events, shown in solid green lines, the scattered $\alpha$ particle travels further along the detector, causing a persistent jump in energy loss that is still smaller than that of ($\alpha ,xn$) events.

Figure 4

The measured ($\alpha ,xn$) cross section in this paper at 408 Torr (red squares) and 459 Torr (blue squares), compared with the activation measurements of ($\alpha ,1n$) from Graf and Münzel [14] (open circles), Esterlund and Pate [13] (open squares), and Szegedi et al. [15] (open triangles). Also shown are the calculated Hauser-Feshbach $(\alpha ,1n)$ and total $\alpha$-induced cross sections using the ATOMKI-V2 potential, from [15]. Error bars shown here are the statistical errors, and the cross sections for the two lowest energies measured at 459 Torr are only upper limits.