Thermal neutron capture cross section for ${}^{56}\mathrm{Fe}(n,\gamma )$

The ${}^{56}\mathrm{Fe}(n,\gamma )$ thermal neutron capture cross section and the ${}^{57}\mathrm{Fe}$ level scheme populated by this reaction have been investigated in this work. Singles $\gamma$-ray spectra were measured with an isotopically enriched ${}^{56}\mathrm{Fe}$ target using the guided cold neutron beam at the Budapest Reactor, and $\gamma \gamma$-coincidence data were measured with a natural Fe target at the LWR-15 research reactor in Řež, Czech Republic. A detailed level scheme consisting of 448 $\gamma$ rays populating/depopulating 97 levels and the capture state in ${}^{57}\mathrm{Fe}$ has been constructed, and $\approx 99\%$ of the total transition intensity has been placed. The transition probability of the 352-keV $\gamma$ ray was determined to be $P_{\gamma }\left(352\right)=11.90\pm 0.07$ per 100 neutron captures. The ${}^{57}\mathrm{Fe}$ level scheme is substantially revised from earlier work and $\approx 33$ previously assigned levels could not be confirmed while a comparable number of new levels were added. The ${}^{57}\mathrm{Fe}\gamma$-ray cross sections were internally calibrated with respect to ${}^1\mathrm{H}$ and ${}^{32}\mathrm{S}\gamma$-ray cross section standards using iron(III) acetylacetonate $\left({\mathrm{C}}_{15}{\mathrm{H}}_{21}{\mathrm{FeO}}_6\right)$ and iron pyrite $\left({\mathrm{FeS}}_2\right)$ targets. The thermal neutron cross section for production of the 352-keV $\gamma$-ray cross section was determined to be ${\sigma }_{\gamma }\left(352\right)=0.2849\pm 0.015$ b. The total ${}^{56}\mathrm{Fe}(n,\gamma )$ thermal radiative neutron cross section is derived from the 352-keV $\gamma$-ray cross section and transition probability as ${\sigma }_0=2.394\pm 0.019$ b. A least-squares fit of the $\gamma$ rays to the level scheme gives the ${}^{57}\mathrm{Fe}$ neutron separation energy $S_n=7646.183\pm 0.018$ keV.

Figure 1

Plot of ${}^{57}\mathrm{Fe}$ transition probabilities, $P_{\gamma }(\%)$, ordered by their decreasing value, for all, primary, ground state (GS), and 14-keV level feeding $\gamma$ rays. The solid lines through each plot represent an exponential fit through the data below $N_{\mathrm{cut}}$, indicated by the vertical lines in each curve. Extrapolation of the fit to higher $\gamma$-ray numbers gives an estimate of the unobserved feeding. The parameters of each exponential fit are given in Table 4.

Figure 2

The $P_{\gamma }(\%)$ balance populating/depopulating levels in ${}^{57}\mathrm{Fe}$ following thermal neutron capture.

Figure 3

Plot of simulated ${}^{57}\mathrm{Fe}$ transition probabilities, $P_{\gamma }(\%)$, ordered by decreasing value for all, primary, and $\mathrm{GS}+14$ keV level feeding $\gamma$ rays and averaged over 40 realizations with model A. The dashed black lines through each plot are an exponential fit through the predictions.

Figure 4

Simulated vs experimental side-feeding transition probabilities, $P_{\gamma }(\%)$. The modeled side-feeding corresponds to the predicted feeding from levels above $E_{\mathrm{crit}}$ except for the capture state. Experimental side-feeding corresponds to the difference between the intensity deexciting levels below $E_{\mathrm{crit}}$, minus the capture state feeding, and the feeding intensity from levels below $E_{\mathrm{crit}}$. Calculations were done using models $A$ (blue markers) and $B$ (green markers) that are described in the text. The observed side-feeding is shown (red markers) for comparison.