Completing the nuclear reaction puzzle of the nucleosynthesis of ${}^{92}\mathrm{Mo}$

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme astrophysical environments. A particularly challenging part of that question is the creation of the so-called $p$-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average $\gamma$ strength function of ${}^{92}\mathrm{Mo}$. State-of-the-art $p$-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the ${}^{91}\mathrm{Nb}(p,\gamma ){}^{92}\mathrm{Mo}$ reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of ${}^{92}\mathrm{Mo}$. Based on our results, we conclude that the ${}^{92}\mathrm{Mo}$ abundance anomaly is not due to the nuclear physics input to astrophysical model calculations.

Figure 1

Production and destruction mechanisms for ${}^{92}\mathrm{Mo}$ that are known to contribute $\gtrsim 1\%$ to the final abundance.

Figure 2

Primary $E_x-E_{\gamma }$ matrix, $P(E_{\gamma },E_x)$, with the limits for the extraction of the of NLD and $\gamma \mathrm{SF}$ shown as dashed lines.

Figure 3

First-generation spectra from selected initial energies $E_i$(crosses) compared to the product of the level density $\rho (E_i-E_{\gamma })$ and transmission coefficient vectors $T\left(E_{\gamma }\right)$. The spectra are normalized to unity.

Figure 4

Experimentally extracted upper and lower limits of $\rho \left(E\right)$ for ${}^{92}\mathrm{Mo}$. The bin width of $E_x\approx 0.1$ MeV.

Figure 5

Upper and lower limits for $f\left(E_{\gamma }\right)$ compared to $(\gamma ,{\gamma }^{\prime })$ data from the ELBE accelerator [51], $(\gamma ,n)$ data for ${}^{95,96}\mathrm{Mo}$ [34], and $(\gamma ,n)$ data for ${}^{92,94,96}\mathrm{Mo}$ [34, 46]. The renormalized ${}^{94}\mathrm{Mo}$ OCL data are also shown [34, 42]. The $\gamma \mathrm{SF}$ model used as input to talys is also shown.

Figure 6

Shell-model calculations for ${}^{92,94}\mathrm{Mo}$ shown with the ${}^{92,94}\mathrm{Mo}$ OCL data [34].

Figure 7

Upper panel: Comparison between the data-guided talys 1.6 calculations and theoretical predictions for the astrophysical reaction rates. Lower panel: The accumulation of ${}^{92}\mathrm{Mo}$ in different zones for the reaction rate from the JINA REACLIB $\pm$ factor of 3 compared to the same calculations using the present experimental results as input.