${}^{197}\mathrm{Au}$($n,\gamma$) cross section in the resonance region

The ($n,\gamma$) cross section of ${}^{197}\mathrm{Au}$ has been measured at n_TOF in the resolved resonance region, up to 5 keV, with the aim of improving the accuracy in an energy range where it is not yet considered standard. The measurements were performed with two different experimental setup and detection techniques, the total energy method based on ${\mathrm{C}}_6$${\mathrm{D}}_6$ detectors, and the total absorption calorimetry based on a 4$\pi$ BaF${}_2$ array. By comparing the data collected with the two techniques, two accurate sets of neutron-capture yields have been obtained, which could be the basis for a new evaluation leading to an extended cross-section standard. Overall good agreement is found between the n_TOF results and evaluated cross sections, with some significant exceptions for small resonances. A few resonances not included in the existing databases have also been observed.

Figure 1

The spectra of the energy deposited in the TAC measured with the Au and C samples compared to the case without sample (empty) in the neutron energy range $1<E_n<5000$ eV. The adopted thresholds for the deposited energy are indicated by dashed vertical lines.

Figure 2

Experimental capture yields for the ${}^{197}\mathrm{Au}$($n,\gamma$) reaction extracted with and without the selection criteria for the deposited energy $E_{\mathrm{TAC}}$.

Figure 3

Capture yields extracted with coincidence time windows of 26 and 52 ns to illustrate the detector dead time at the example of the first Au resonance at 4.9 eV. In the present analysis a time window of 26 ns was adopted.

Figure 4

The $E_{\mathrm{TAC}}$ spectra of ${}^{197}\mathrm{Au}$ for parasitic and dedicated operation mode for capture events around the 4.9-eV resonance.

Figure 5

Experimental distribution of the energy deposited in the TAC for neutron capture events around the 4.9-eV resonance in Au and a Monte Carlo simulation of the pileup effect for capture events taken from the same distribution.

Figure 6

Capture yields from data obtained in parasitic and dedicated beam mode for resonances with the largest dead-time corrections.

Figure 7

(Left) Experimental capture yield measured with Au and without sample. (Right) Experimental yields measured without sample and with the ${}^{\mathrm{nat}}$C and ${}^{\mathrm{nat}}$Pb samples, all analyzed with the same conditions for energy deposition and multiplicity. Resonances in the ${}^{\mathrm{nat}}$Pb yield are due to a small Sb contamination.

Figure 8

The neutron sensitivity of the n_TOF TAC as a function of neutron energy for different conditions.

Figure 9

Au capture yield and neutron background determined from experimental data using the second method described in the text.

Figure 10

Probability for $\gamma$-ray emission as a function of the depth in the capture sample illustrated for different capture cross sections.

Figure 11

Simulated response functions before (top left) and after weighting (top right). By the weighting procedure the original energy dependence of the efficiency (bottom left) becomes proportional to the energy of the registered $\gamma$ rays (bottom right).

Figure 12

(Top) Four-degree polynomial WFs obtained for two different neutron absorption rates. (Bottom) Response function of the ${\mathrm{C}}_6$${\mathrm{D}}_6$ detector system for simulated $\gamma$-ray cascades from the 4.9-eV resonance in Au. The specific neutron absorption coefficient $\lambda =A/\rho N_A{\sigma }_{\gamma }$ is determined by the radiative capture cross section ${\sigma }_{\gamma }$ in cm${}^2$ and the sample density $\rho$ in (g/cm${}^3$). $A$ and $N_A$ denote atomic number and Avogadro number.

Figure 13

Energy dependence of the background components at n_TOF determined from the measurement of a lead sample. At lower neutron energies, the ambient background due to thermal neutron activation shows the expected exponential trend. At higher neutron energies the background is dominated by delayed in-beam $\gamma$ rays (see text for details).

Figure 14

Capture yield of Au with Al and W filters in the beam to determine the background level.

Figure 15

Examples for resonance fits of the$C_6$${\mathrm{D}}_6$ data (solid symbols with error bars). The blue curverepresents the evaluated data listed in ENDF/B-VIIdatabase [16] and the red curve showsthe present fits.

Figure 16

Examples for resonance fits of the TAC data (solid symbols with error bars). The blue curverepresents the evaluated data listed in ENDF/B-VII database [16] and the red curve showsthe present fits.

Figure 17

(Left) Ratio of capture kernels versus resonance energy. (Right) The statistical distributions of the kernel ratios agree within 1.5% at an average standard deviation of about 8%.

Figure 18

Comparison of the capture kernels obtained from the TAC data and ${\mathrm{C}}_6$${\mathrm{D}}_6$ data as a function of resonance area (a) and peak resonance count rate of TAC data (b).

Figure 19

Comparison of the capture kernels obtained from the TAC and ${\mathrm{C}}_6$${\mathrm{D}}_6$ data versus ${\Gamma }_n/{\Gamma }_{\gamma }$ ratio (a) and the resonance strength $\mathrm{g}{\Gamma }_n$ (b).

Figure 20

Capture yield reconstructed from the ENDF/B-VII parameters and the corresponding TAC data. (a) The resonances at 209 and 255 eV are not included in the evaluation. (b) The evaluation ends at 4.83 keV.

Figure 21

Capture yield calculated from theENDF/B-VII parameters (dotted line) and folded with the experimentalresolution (thick solid line) compared to the contributionfrom multiple scattering, which was calculated without theresonance at 2652.6 (dotted line). The structure indicatedby the arrow is probably due to multiple scattering.

Figure 22

(Left) Ratio of capture kernels from ENDF/B-VII and the present ${\mathrm{C}}_6$${\mathrm{D}}_6$ results vs. resonance energy. (Right) The corresponding distribution of the ratios.

Figure 23

(Left) Ratio of capture kernels from ENDF/B-VII and the present TAC results vs resonance energy. (Right) The corresponding distribution of the ratios.