Low-energy modification of the $\gamma$ strength function of the odd-even nucleus ${}^{115}\mathrm{In}$

Photoactivation yield measurements on ${}^{115}\mathrm{In}$ have been performed at the ELSA facility with Bremsstrahlung photon beams over a range of endpoint energies between 4.5 and 18 MeV. The measured photoexcitation yields of the ${}^{115m}\mathrm{In}$ metastable state are compared with calculated yields using cross sections obtained with different models of the photon strength function. It is shown that additional photon strength with respect to the general Lorentzian model is needed at 8.1 MeV for the calculated yields to reproduce the data. The origin of this extra strength is unclear, because it is compatible with additional strength predicted in both $E1$ and $M1$ photon strength distributions by quasiparticle random-phase approximation calculations using the Gogny D1S force.

Figure 1

Experimental setup at ELSA.

Figure 2

Measured $\gamma$-ray spectrum of an activated natural In sample at $E_e=17$ MeV (15 h of counting). The 336.2-keV line corresponds to the deexcitation of the ${}^{115m}\mathrm{In}$ isomer.

Figure 3

Partial level schemes of ${}^{115}\mathrm{In}$ and ${}^{113}\mathrm{In}$ from Refs. [23] and [24], respectively.

Figure 4

Partial level scheme of ${}^{114}\mathrm{In}$ with the ${\beta }^{-}$ decay onto ${}^{114}\mathrm{Sn}$ from Ref. [25].

Figure 5

Partial level scheme of ${}^{112}\mathrm{In}$ with the ${\beta }^{+}$ decay onto ${}^{112}\mathrm{Cd}$ from Ref. [26].

Figure 6

Bremsstrahlung spectra impinging onto the In sample at the irradiation site obtained from Monte Carlo simulations (geant4) for incident electron energies between 4.5 and 18 MeV.

Figure 7

Measured and calculated yields of activated ${}^{196}\mathrm{Au}$ and ${}^{62}\mathrm{Cu}$ as a function of electron beam energies.

Figure 8

(a) Time decay spectrum of the 617-keV $\gamma$ ray of ${}^{112}\mathrm{Cd}$ fitted by the expected analytical expression of the decay taking the ${}^{112m}\mathrm{In}$ and ${}^{112gs}\mathrm{In}$ production yields as parameters. (b) Residuals of the fit.

Figure 9

Yields of $(\gamma ,n)$ reactions in the In, Au, and Cu targets per $\mu \mathrm{C}$ of electron beam.

Figure 10

Energy distributions of (a) photoneutrons produced by all $(\gamma ,n)$ reactions in the natural In sample. (b) Photoneutrons produced in the In sample and interacting by inelastic scattering in the sample ($d{N}_{\mathrm{int}}/dE$).

Figure 11

${}^{115}\mathrm{In}(n,n^{\prime }){}^{115m}\mathrm{In}$ excitation probability per photoneutron produced in the In or Au samples. Uncertainties are smaller than the data points.

Figure 12

Measured $(\gamma ,{\gamma }^{\prime })$ yields of ${}^{115m}\mathrm{In}$ produced in the sample per $\mu \mathrm{C}$ of beam.

Figure 13

Cross sections of the ${}^{115}\mathrm{In}(\gamma ,{\gamma }^{\prime }){}^{115m}\mathrm{In}$ reaction calculated with the talys code using the different available models of the PSF. See text for details.

Figure 14

Comparison between the measured photoexcitation yields of ${}^{115m}\mathrm{In}$ (solid black triangles) and simulated yields, calculated with the different PSF models available in the standard input parameters of talys. Color codes for calculated yields are identical to those in Fig. 13.

Figure 15

Comparison between the measured photoexcitation yields of ${}^{115m}\mathrm{In}$ and calculated yields obtained with the GLO $E1$ and the default SLO $M1$ PSFs and with either $E1$ (pygmy) or $M1$ additional photon strength. See text for details.

Figure 16

Photon strength functions used in this work to reproduce the data. See text for details.

Figure 17

$B\left(E1\right)$ reduced transition probability in ${}^{115}\mathrm{In}$ calculated in the QRPA approach with the Gogny D1S force.

Figure 18

$B\left(M1\right)$ reduced transition probability in ${}^{115}\mathrm{In}$ calculated within the QRPA approach using the Gogny D1S force.

Figure 19

Cross sections of the ${}^{115}\mathrm{In}(\gamma ,{\gamma }^{\prime }){}^{115m}\mathrm{In}$ reaction calculated using the different level density models available in the talys code. See text for details.

Figure 20

Comparison between the measured photoexcitation yields of ${}^{115m}\mathrm{In}$ and calculated yields obtained with the different nuclear level density models available in talys and the modified PSF introduced in this work. See text for details.

Figure 21

Total cross section of the ${}^{115}\mathrm{In}(\gamma ,n)$ reaction calculated using the GLO $E1$ strength function and the default SLO $M1$ strength function, and with either $E1$ (pygmy) or $M1$ additional strength. A vertical arrow points to the neutron separation energy.

Figure 22

Experimental and calculated cross sections of ${}^{115}\mathrm{In}(\gamma ,{\gamma }^{\prime }){}^{115m}\mathrm{In}$ obtained using the GLO model for the $E1$ PSF and either the default SLO $M1$ or the enhanced $M1$ photon strength (Goldemberg and Katz 1953: [14]; Burkhardt et al. 1955: [15]).

Figure 23

Effective integrated cross section as a function of the Bremsstrahlung spectrum endpoint energy $E_0$. See text for details (Carroll et al. 1991: [13]).