Electromagnetic dipole strength up to the neutron separation energy from ${}^{196}$Pt($\gamma ,{\gamma }^{\prime }$) and ${}^{195}$Pt($n,\gamma$) reactions

The dipole strength in the nucleus ${}^{196}$Pt was investigated using two different experimental methods. The photon spectrum from the deexcitation of a state after cold neutron capture in ${}^{195}$Pt is influenced by the dipole strength and nuclear level density in ${}^{196}$Pt as is the $\gamma$-ray spectrum from photon scattering on ${}^{196}$Pt. In a combined analysis of data from the research reactor in Budapest and the bremsstrahlung facility at the ELBE accelerator in Dresden, the geant4 code was used to calculate detector response and efficiency. Also the influence of non-nuclear scattered photons was determined and allows us to take into account the continuum of unresolved states. The statistical code $\gamma$dex was used to estimate branching ratios and compare simulated and experimental spectra. Using information from both experiments it was possible to obtain a temperature parameter of 600 keV for the constant temperature level density model. For the dipole strength a small extra strength over the tail of the giant dipole resonance in the region below the neutron separation energy was found.

Figure 1

Fluence distribution of bremsstrahlung photons for electrons with a kinetic energy of 9.5 MeV. The black circles show the known transitions in ${}^{11}$B. The dashed and solid lines show the flux without and with hardener, respectively, with their uncertainty (gray bands).

Figure 2

Experimental spectrum after background subtraction (red/gray) and response corrected spectrum (black) in the ${}^{195}$Pt($n$,$\gamma$) experiment.

Figure 3

The plot shows, in comparison to the experimental spectrum, the dependence of the simulated $\gamma$-ray spectrum after neutron capture on the photon strength function while using the same assumed level density. The experimental spectrum (transparent black bars) was corrected for background and detector response. The simulated results are divided into primary (red/dark gray bars), secondary (gray bars), and higher-order $\gamma$ rays (orange/medium gray bars). Panel (a) shows the first input with a temperature of 600 keV and the standard TLO strength function. Panel (b) uses the same level density with the strength function deduced from photon scattering. The value of $M$ describes the average number of photons per deexcitation cascade.

Figure 4

Response-corrected ($\gamma ,{\gamma }^{\prime }$) spectrum (orange/light gray bars) in relation to the simulated atomic background (black line). The subtraction of this leads to the spectrum (blue/dark gray bars) that is used as input for the deconvolution of inelastic transitions with $\gamma$dex.

Figure 5

Simulated enhanced ground-state branching ratios for 1${}^{-}$ states calculated by $\gamma$dex. The average branching ratios were multiplied by the factor $S$ which takes account of the enhancement of states with large ground-state widths in photon scattering experiments.

Figure 6

Deducing the dipole strength function by iterative approximation. The final experimental values (black dots) are shown together with the input strength functions. The $E1$ (blue dashed line) and the $M1$ (black dotted line) used as inputs are shown in comparison to the red line which is a combined fit of Lorentzian functions to the ($\gamma$,$n$) cross section and the resulting ($\gamma ,{\gamma }^{\prime }$) data. Panel (a) shows the first iteration, and panel (b) shows the sixth iteration. After this calculation the input and output are self-consistent within the error bars.

Figure 7

Final photoabsorption cross section (orange circles) from the experiment at ELBE. The blue triangles show the data not corrected for branching and feeding and the gray squares show the elastic cross section. The black stars represent existing ($\gamma$,$n$) data [8]. The black line is the $E1$ TLO model and the gray line depicts a combined fit of our data and the ($\gamma$,$n$) data (see also Fig. 6).