Measurement of the ${}^{65}\mathrm{Cu}(n,\gamma )$ cross section using the Detector for Advanced Neutron Capture Experiments at LANL

Improving our understanding of the origin of the elements in the observable universe as well as the nature of the environments responsible for their production has been of paramount importance to the nuclear physics community. More than half of the isotopes of these elements are created via neutron-capture processes, and thus accurate measurements of the salient underlying nuclear physics, such as neutron-capture cross sections, masses, and $\beta$-decay half-lives are crucial. Of particular importance to the synthesis of isotopes in the mass range of $A\approx 60$ to $A\approx 90$, via the weak $s$ process, are the neutron-capture cross sections of ${}^{63,65}\mathrm{Cu}$, where large discrepancies between measurements exist. Recent measurements have addressed these discrepancies for ${}^{63}\mathrm{Cu}$ [Weigand et al., Phys. Rev. C 95, 015808 (2017)], but questions still remain for ${}^{65}\mathrm{Cu}$. In this paper we report a new measurement of the ${}^{65}\mathrm{Cu}(n,\gamma )$ cross section performed using the Detector for Advanced Neutron Capture Experiments located at the Los Alamos Neutron Science Center of Los Alamos National Laboratory. The Maxwellian-averaged cross section (MACS) for ${}^{65}\mathrm{Cu}(n,\gamma )$ at $kT=30$ keV deduced from this work is ($37.0\pm 0.3_{\mathrm{stat}.}\pm 3.3_{\mathrm{sys}.}$) mb. The impact on weak $s$-process nucleosynthesis of new MACS values, calculated over the range of $kT=5$ to 100 keV, for ${}^{65}\mathrm{Cu}$, combined with the updated MACS for ${}^{63}\mathrm{Cu}$ [Weigand et al., Phys. Rev. C 95, 015808 (2017)] and ${}^{63}\mathrm{Ni}$ [Weigand et al., Phys. Rev. C 92, 045810 (2015)], were investigated. Results of this investigation show an increase of predicted nucleosynthesis yields of elements of Zn to Zr by as much as 20% .

Figure 1

Path of the $s$ process in the vicinity of ${}^{65}\mathrm{Cu}$. Isotopes shown in black are observationally stable, isotopes shown in pink are unstable, and ${}^{65}\mathrm{Cu}$ is highlighted in light blue. Blue arrows indicate neutron capture while red arrows indicate ${\beta }^{-}$ decay.

Figure 2

Beam-monitor yields, $Y_{BM}$, divided by the relevant beam-monitor reaction cross sections, ${\sigma }_{BM}$, for the ${}^{65}\mathrm{Cu}$ data. Below 3 keV all data are from the ${}^6\mathrm{Li}$ monitor, between 3 and 20 keV the data are the average of the ${}^6\mathrm{Li}$ and ${}^{235}\mathrm{U}$ monitors, and above 20 keV the data are exclusively from the ${}^{235}\mathrm{U}$ monitor.

Figure 3

(a) $E_{Sum}$ as a function of $E_n$ for the ${}^{197}\mathrm{Au}(n,\gamma )$ data. (b) Extracted yield for each $E_n$ bin in (a) shown as black squares while a fit of the ENDF/B-VIII.0 ${}^{197}\mathrm{Au}(n,\gamma )$ cross section, corrected for the response of the target-moderator assembly, to the data is shown in red.

Figure 4

Example fit of a single $E_n$ bin along the $x$ axis of Fig. 3 projected onto the $y$ axis. The raw data from the projection, shown in black, is a combination of foreground neutron capture, shown in dotted blue, and scattered-neutron background, shown in dashed red.

Figure 5

Counts as a function of $E_{Sum}$ and $E_n$ for the ${}^{65}\mathrm{Cu}(n,\gamma )$ data.

Figure 6

Counts as a function of $E_{Sum}$ and $E_n$ for the ${}^{65}\mathrm{Cu}(n,\gamma )$ data following the corrections for event pileup, dead time, and crystal pileup. The contribution from ${}^{63}\mathrm{Cu}(n,\gamma )$ from the ${}^{63}\mathrm{Cu}$ contaminant has also been subtracted.

Figure 7

Demonstration of background characterization for a single $E_n$ bin of the ${}^{65}\mathrm{Cu}$ data. The ${}^{65}\mathrm{Cu}$ data are shown in black, and the scattered-neutron background, obtained from the ${}^{208}\mathrm{Pb}$ data and scaled using Eq. (1), is shown in dashed red.

Figure 8

Energy differential ${}^{65}\mathrm{Cu}(n,\gamma )$ cross section shown in black squares. The ENDF/B-VIII.0 ${}^{65}\mathrm{Cu}(n,\gamma )$ cross section, corrected for the response of the target-moderator assembly, is shown in red.

Figure 9

The final composition of the models at the presupernova stage for 15, 20, and 25 $M_{\odot }$ massive star models from [18, 19] computed using the kepler code [20, 21, 22]. Unstable isotopes to have been allowed to decay for ${10}^{16}$ s. The left panels show the composition relative to the solar abundance distribution while the right panels show the relative difference when the three updated $(n,\gamma )$ cross sections are used.