Dipole strength of ${}^{181}\mathrm{Ta}$ for the evaluation of the ${}^{180}\mathrm{Ta}$ stellar neutron capture rate

The photoabsorption cross section of ${}^{181}\mathrm{Ta}$ up to the neutron-separation energy is deduced using bremsstrahlung produced with an electron beam of 9.6 MeV energy. The analysis of the measured $\gamma$-ray spectra includes the quasicontinuum of levels at high energy. Simulations of $\gamma$-ray cascades are performed to estimate intensities of inelastic transitions and branching ratios of the ground-state transitions. The resulting photoabsorption cross section shows enhanced dipole strength in the energy range from 5 to 8 MeV, which may be related to a pygmy dipole resonance. The results of the present experiment are compared with predictions of a quasiparticle-random-phase approximation in a deformed basis. A combination of the present experimental data and $(\gamma ,n)$ data is used as an input to the statistical code talys applied to calculate cross sections and reaction rates of photonuclear reactions that are important for the nucleosynthesis of ${}^{180}\mathrm{Ta}$.

Figure 1

Spectrum of photons scattered from ${}^{181}\mathrm{Ta}$, corrected for room background and detector response, and simulated spectrum of the atomic background, multiplied with efficiency and measuring time. The four most intense peaks belong to transitions in ${}^{11}\mathrm{B}$.

Figure 2

Absolute photon flux at the target deduced from intensities of five known transitions in ${}^{11}\mathrm{B}$ (circles) using the efficiency calculated with geant4 and the relative flux calculated as described in the text (solid line).

Figure 3

Mean branching ratios and uncertainty range of ground-state transitions as obtained from the simulations of $\gamma$-ray cascades in ${}^{181}\mathrm{Ta}$.

Figure 4

Photoabsorption cross sections obtained from the first (black squares), the eighth (blue triangles), and the ninth (red circles) iterations in the simulation of $\gamma$-ray cascades. The iteration was stopped after the ninth step.

Figure 5

Photoabsorption cross sections before (black circles) and after (red circles) correction for inelastic transitions and branching ratios in comparison with $\left(\gamma ,n\right)$ data from Ref. [58] (green squares) and Refs. [9, 10] (blue triangles).

Figure 6

Total photoabsorption cross section of ${}^{181}\mathrm{Ta}$ obtained by combining the present $(\gamma ,{\gamma }^{\prime })$ data and the $(\gamma ,n)$ data of Ref. [58]. The data were averaged over 0.5 MeV bins to reduce statistical fluctuations. The black dashed line represents a double-Lorentz distribution with the parameters given in the text. The jagged green and smooth blue solid lines are the results of the QRPA calculations discussed in Sec. 3, folded with Lorentz curves of smearing widths of 0.25 and 3.0 MeV, respectively.

Figure 7

${}^{181}\mathrm{Ta}(\gamma ,n)$ cross sections as functions of $\gamma$-ray energy calculated using the code talys with various models for the input strength function. The uncertainty of the EPACS shown as a blue (gray) band results from the use of various nuclear level-density models (see text).

Figure 8

${}^{180}\mathrm{Ta}(n,\gamma )$ cross sections as functions of neutron energy calculated using the code talys with various models for the input strength function. The uncertainty of the EPACS shown as a blue (gray) band results from the use of various nuclear level density models.

Figure 9

Same as Fig. 8, but for ${}^{180m}\mathrm{Ta}(n,\gamma )$ cross sections.

Figure 10

${}^{180m}\mathrm{Ta}(n,\gamma )$ Maxwellian-averaged cross sections calculated using the code talys with various models for the input strength function compared with KADoNiS (version 0.2) (black circles). The uncertainty of the EPACS shown as a blue (gray) band results from the use of various nuclear level density models.

Figure 11

${}^{181}\mathrm{Ta}(\gamma ,n)$ reaction rates as a function of temperature, calculated using the code talys with various models for the input strength function. The uncertainty of the EPACS shown as a blue band results from the use of various nuclear level density models.

Figure 12

Same as Fig. 11, but for ${}^{180}\mathrm{Ta}(n,\gamma )$ reaction rates.

Figure 13

${}^{180m}\mathrm{Ta}(n,\gamma )$ reaction rates as a function of temperature, calculated using the code talys with various models for the input strength function compared with KADoNiS (version 0.2) (black circles). The uncertainty of the EPACS shown as a blue(gray) band results from the use of various nuclear level density models.