Stellar $s$-process neutron capture cross sections of ${}^{69,71}\mathrm{Ga}$

The stable gallium isotopes, ${}^{69,71}\mathrm{Ga}$, are mostly produced by the weak slow ($s$) process in massive stars. We report here on measurements of astrophysically relevant neutron capture cross sections of the ${}^{69,71}\mathrm{Ga}(n,\gamma )$ reactions. The experiments were performed by the activation technique using a high-intensity ($3–5\times {10}^{10}$ n/s), quasi-Maxwellian neutron beam that closely mimics conditions of stellar $s$-process nucleosynthesis at $kT\approx$ 40 keV. The neutron field was produced by a mA proton beam at $E_p=1925$ keV (beam power of 2–3 kW) from the Soreq Applied Research Accelerator Facility (SARAF), bombarding the liquid-lithium target (LiLiT). A 473-mg sample of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ of natural isotopic composition was activated in the LiLiT neutron field and the activities of ${}^{70,72}\mathrm{Ga}$ were measured by decay counting via $\gamma$ spectrometry with a high-purity germanium detector. The Maxwellian-averaged cross sections at $kT=30\mathrm{keV}$ of ${}^{69}\mathrm{Ga}$ and ${}^{71}\mathrm{Ga}$ determined in this work are 136(8) and 105(7) mb, respectively, in good agreement with previous experimental values. Astrophysical implications of the measurements are discussed.

Figure 1

The path of the $s$ process between zinc and arsenic. The numerical values are the terrestrial isotopic abundances in % (half-life) for stable (unstable) nuclides. When ${}^{69}\mathrm{Ga}$ captures a neutron, the product ${}^{70}\mathrm{Ga}$ either decays to ${}^{70}\mathrm{Ge}$ (99.59%) or to ${}^{70}\mathrm{Zn}$ (0.41%) with a half-life of 21.14 min. Following the neutron capture reaction on ${}^{71}\mathrm{Ga}$, the product ${}^{72}\mathrm{Ga}$ decays to ${}^{72}\mathrm{Ge}$ with a half-life of 14.1 h.

Figure 2

Diagram of the liquid-lithium target (LiLiT) and activation target assembly. The 1–2 mA ($\approx 9\mathrm{mm}$ full width) proton beam (dashed red arrow) impinges directly on the windowless liquid-lithium film. The light blue shows the liquid-lithium circulating flow (see [10] for details). The activation samples are mounted at the center of a ring target holder made of Al and positioned in the outgoing neutron cone (green dashed lines) at a distance of $\approx 6\mathrm{mm}$ from the liquid-lithium film surface. The targets are in a vacuum chamber separated from the LiLiT chamber by a 0.5 mm stainless steel concave vacuum wall.

Figure 3

An autoradiographic scan of the 25-mm-diameter gold foil (Au no. 33). Red indicates the area with the highest neutron irradiation followed by yellow, green and blue. A $\approx 2.5\text{−}\mathrm{mm}$ offset was observed in the neutron irradiation and is attributed to the vertical steering of the proton beam. This offset was taken into account in the neutron irradiation simulations.

Figure 4

Time record of the fission chamber count rate (left axis) and corresponding intensity of the proton beam current (right axis) during the irradiation. The two low-intensity intervals and gaps at beginning and end of the irradiation correspond to the calibration of the fission count rate against beam current measured at low intensity with a Faraday cup (see text). All intensity variations are taken into account by the $1/f_b$ correction factor [Eq. (2)]. The total integrated current was 0.96 $\mathrm{mA}\times \mathrm{h}$.

Figure 5

Efficiency of the HPGe detector used at a distance of 20 cm (black dots with $1\sigma$ uncertainties) determined by standard calibrated radioactive sources: ${}^{22}\mathrm{Na}, {}^{60}\mathrm{Co}, {}^{88}\mathrm{Y}, {}^{133}\mathrm{Ba}, {}^{137}\mathrm{Cs}, {}^{241}\mathrm{Am}, {}^{152}\mathrm{Eu}$, and ${}^{155}\mathrm{Eu}$. The red curve is a fit to the data with the expression: $\epsilon =a\times E_{\gamma }^{-b}$ and the dotted lines are the uncertainty ($1\sigma$). The main source of the efficiency uncertainty is the radioactive source activity; these uncertainties are given by the source manufacturers. The small value of the reduced chi square suggests that these uncertainties are overestimated.

Figure 6

$\gamma$-ray spectrum of the activated ${}^{\mathrm{nat}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ no. 2 sample. The spectrum was accumulated for 3000 s, starting 2464 s after the end of the neutron activation. The sample was located 20 cm from the HPGe detector. The main Ga isotope full-energy peaks, with energies in keV, are labeled.

Figure 7

Decay curves for ${}^{70}\mathrm{Ga}$. The uncertainties of each data point in this plot are only the statistical uncertainties of the activity measurement. $N_{\gamma }=N_{\text{act}}{I}_{\gamma }$ [see Eq. (2)] is the photon intensity ($\gamma \mathrm{s}$ per second) of the $\gamma$ transitions at the 176.3- and 1039.5-keV lines of the ${}^{70}\mathrm{Ga}$ decay, determined by $\gamma$ spectrometry. Excellent agreement is observed with the adopted half-life of ${}^{70}\mathrm{Ga}$ from the literature [21.14(5) min. 17], and between the number of activated ${}^{70}\mathrm{Ga}$ nuclei (see Tables 3 and 4) derived independently from each transition, using the respective adopted $\gamma$ intensities [17]; see text.

Figure 8

Secondary $\gamma$-ray spectrum of the ${}^{\text{nat}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ no. 1 sample after irradiation with primary $\gamma$ rays. The spectrum was accumulated for 636 s, starting 1275 s after the end of the irradiation. The proton energy was below the neutron production threshold, resulting in the production of only $\gamma$ rays. The sample was located 2 cm from the HPGe detector. No Ga lines are observed.

Figure 9

The simulated neutron spectrum impinging on the Ga target (black). Also shown is a fit (between 0 and 90 keV) to a Maxwell-Boltzmann (MB) flux (red), where the best fit is for $kT=41.8$ keV.

Figure 10

Comparison of this work's results (red) with previously measured MACS at $kT=30$ keV (black) for ${}^{69}\mathrm{Ga}$ [4] (left) and ${}^{71}\mathrm{Ga}$ [6, 7] (right). The previously measured MACSs were renormalized as specified in the KADoNiS v1.0 website [36]. The TOF measurement is displayed as a circle and the activation measurements as squares.