${}^{63}\mathrm{Cu}(n,\gamma )$ cross section measured via 25 keV activation and time of flight

In the nuclear mass range $A\approx 60$ to 90 of the solar abundance distribution the weak $s$-process component is the dominant contributor. In this scenario, which is related to massive stars, the overall neutron exposure is not sufficient for the $s$ process to reach mass flow equilibrium. Hence, abundances and isotopic ratios are very sensitive to the neutron capture cross sections of single isotopes, and nucleosynthesis models need accurate experimental data. In this work we report on a new measurement of the ${}^{63}\mathrm{Cu}(n,\gamma )$ cross section for which the existing experimental data show large discrepancies. The ${}^{63}\mathrm{Cu}(n,\gamma )$ cross section at $k_{B}T=25$ keV was determined via activation with a quasistellar neutron spectrum at the Joint Research Centre (JRC) in Geel, and the energy dependence was determined with the time-of-flight technique and the calorimetric $4\pi {\mathrm{BaF}}_2$ detector array DANCE at the Los Alamos National Laboratory. We provide new cross section data for the whole astrophysically relevant energy range.

Figure 1

The $s$-process path in the iron-zinc-region. Following neutron capture on ${}^{63}\mathrm{Cu}$, the product ${}^{64}\mathrm{Cu}$ decays according to [6] with a half-life of 12.7 hours either to ${}^{64}\mathrm{Ni}$ or to ${}^{64}\mathrm{Zn}$.

Figure 2

A pino simulation of the 25 keV neutron spectrum used for the activation (red) compared with the corresponding Maxwell-Boltzmann distribution (blue); see text for details.

Figure 3

The time dependence of the neutron flux for the second activation (blue) and the corresponding growth curve (red).

Figure 4

Result of the $\gamma$ counting after the first activation run. The $\gamma$ line at the left at 1345.77 keV originates from the decay of ${}^{63}\mathrm{Cu}$; the line at 1460 keV is background from ${}^{40}\mathrm{K}$.

Figure 5

A sketch of the DANCE array. Almost half of the crystals are blended out to give an insight. Colors distinguish between different crystal shapes; the blue disk indicates the neutron absorber.

Figure 6

The number of neutrons per eV plotted against the neutron energy. The neutron energy distribution at DANCE is continuously monitored with a ${}^6\mathrm{Li}$ detector. The origins of the absorption features at 500 eV and above 20 keV are neutron captures on beam line components and belong to Mn and Al, respectively.

Figure 7

A two-dimensional map of all detected capture events with $E_n$ on the $y$ axis and $E_{\mathrm{tot}}$ on the $x$ axis. Neutron capture resonances appear as horizontal stripes. For the most prominent resonances pile-up effects are visible to the right, but can be neglected, since only relatively low count numbers are reached. The $Q$ values of ${}^{63}\mathrm{Cu}$ (7.92 MeV) and the two most prominent barium isotopes, ${}^{138}\mathrm{Ba}$ (4.72 MeV) and ${}^{135}\mathrm{Ba}$ (9.11 MeV), are indicated.

Figure 8

(a) Total $\gamma$-energy spectrum measured with ${}^{63}\mathrm{Cu}$ (black) and ${}^{208}\mathrm{Pb}$ (red) for $E_n=588\pm 50$ eV. The energy range from $E_{\mathrm{tot}}=8.3$ to 9.2 MeV was used to normalize the background data for subtraction. The capture events between $E_{\mathrm{tot}}=5.5$ and 8.3 MeV were considered for the cross section. (b) Total numbers of events as a function of the neutron energy (black) and the normalized background spectra for subtraction, acquired with an empty target holder (blue) and with the lead sample (red). The binning is logarithmic with 50 bins per decade.

Figure 9

(a) The measured neutron capture cross section of ${}^{63}\mathrm{Cu}$ as a function of neutron energy in comparison to the evaluated data from TENDL-2015. The binning is logarithmic with 50 bins per decade in both cases. (b) The MACS data are shown as a red line and the uncertainties are depicted as a grey area. For comparison, data from [20] and JEFF 3.2 are plotted as well.

Figure 10

Comparison of our 25 keV cross section with data from older publications. The MACS from Pandey et al. (1977) [8] were calculated by Bao et al. (2000) [20].

Figure 11

Influence of a 1 mm copper backing on the neutron spectrum at the sample position (red) showing a pronounced self-shielding effect. For this calculation cross section data from TENDL-2015 was used, which is plotted in blue.