Photoneutron reaction cross section measurements on ${}^{94}\mathrm{Mo}$ and ${}^{90}\mathrm{Zr}$ relevant to the $p$-process nucleosynthesis

The photodisintegration cross sections for the ${}^{94}\mathrm{Mo}(\gamma ,n)$ and ${}^{90}\mathrm{Zr}(\gamma ,n)$ reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity $\gamma$-ray Source $\left(\mathrm{HI}\gamma \mathrm{S}\right)$ facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the $p$-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole $\gamma$-ray strength function. The resulting ${}^{94}\mathrm{Mo}(\gamma ,n)$ and ${}^{90}\mathrm{Zr}(\gamma ,n)$ photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the $p$-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.

Figure 1

Schematic drawing (not to scale) of the experimental setup in the $\mathrm{HI}\gamma \mathrm{S}$ Upstream Target Room (UTR). See the text for details of the detectors sketched in the figure.

Figure 2

$\gamma$-ray spectrum recorded by the 123% HPGe detector in anticoincidence with the NaI(Tl) annulus at a $\gamma$-ray beam energy of 9 MeV. The red curve represents a Gaussian fit from where the centroid of the full-energy peak of the $\gamma$-ray beam was determined. The peak at 8.5 MeV corresponds to the first-escape peak.

Figure 3

Axial and side cutaway views of the INVS detector showing the ${}^3\mathrm{He}$ tubes arranged in two concentric rings around a central cavity. See the text for information on dimensions.

Figure 4

icarus fitting plot for the excitation function of the ${}^{94}\mathrm{Mo}(\gamma ,n)$ reaction. The horizontal error bars represent the measured $\gamma$-ray beam energy widths.

Figure 5

icarus excitation function for ${}^{94}\mathrm{Mo}(\gamma ,n)$ of this work compared with the previous measurements [18, 43].

Figure 6

Excitation function for ${}^{90}\mathrm{Zr}(\gamma ,n)$ of this work compared with the previous measurements [44, 45].

Figure 7

Comparison between the present $(\gamma ,n)$ photoneutron reaction cross sections as a function of the $\gamma$-ray beam energy and the previous data [44, 45] for ${}^{90}\mathrm{Zr}$. Also included are the predictions obtained with the D1M+QRPA $E1$ and $M1$ strengths (solid line) and with the GLO model (dotted line).

Figure 8

Comparison between the present $(\gamma ,n)$ photoneutron reaction cross sections as a function of the $\gamma$-ray beam energy and the previous data [18, 43] for ${}^{94}\mathrm{Mo}$. Also included are the predictions obtained with the D1M+QRPA $E1$ and $M1$ strengths (solid line) and with the GLO model (dotted line).

Figure 9

Comparison between experimental [18, 43, 44, 45, 49, 50, 51] and theoretical $\gamma$-ray strength function as a function of the $\gamma$-ray energy for (a) ${}^{90}\mathrm{Zr}$ and (b) ${}^{94}\mathrm{Mo}$. The predictions correspond to the D1M+QRPA $E1$ (red dashed line), $M1$ (red dotted line), and $E1+M1$ (red solid line) strengths and the GLO $E1$ (blue dashed line), $M1$ (blue dotted line), and $E1+M1$ (blue solid line) strengths.

Figure 10

(a) ${}^{90}\mathrm{Zr}(\gamma ,n){}^{89}\mathrm{Zr}$ (circles) and ${}^{90}\mathrm{Zr}(\gamma ,p){}^{89}\mathrm{Y}$ (squares) stellar reaction rates, as a function of the temperature, obtained with the D1M+QRPA (red solid lines) or the GLO (blue dotted lines) $\gamma \mathrm{SF}$ shown in Fig. 9. (b) Same for ${}^{94}\mathrm{Mo}(\gamma ,n){}^{93}\mathrm{Mo}$ and ${}^{94}\mathrm{Mo}(\gamma ,\alpha ){}^{90}\mathrm{Zr}$, respectively.