Total absorption $\gamma$-ray spectroscopy of the $\beta$-delayed neutron emitters ${}^{87}\mathrm{Br}$, ${}^{88}\mathrm{Br}$, and ${}^{94}\mathrm{Rb}$

We investigate the decay of ${}^{87,88}\mathrm{Br}$ and ${}^{94}\mathrm{Rb}$ using total absorption $\gamma$-ray spectroscopy. These important fission products are $\beta$-delayed neutron emitters. Our data show considerable $\beta \gamma$ intensity, so far unobserved in high-resolution $\gamma$-ray spectroscopy, from states at high excitation energy. We also find significant differences with the $\beta$ intensity that can be deduced from existing measurements of the $\beta$ spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for ${}^{235}\mathrm{U}$ fission at cooling times in the range $1\text{–}100$ s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for $\gamma$ emission from neutron unbound states populated in the daughter nucleus. The $\gamma$ branching is compared to Hauser-Feshbach calculations, which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for ${}^{94}\mathrm{Rb}$, although much smaller, hints of the need to increase the radiative width ${\mathrm{\Gamma }}_{\gamma }$ by one order of magnitude. This increase in ${\mathrm{\Gamma }}_{\gamma }$ would lead to a similar increase in the calculated $(\mathrm{n},\gamma )$ cross section for this very neutron-rich nucleus with a potential impact on $r$ process abundance calculations.

Figure 1

Cross-sectional view of the detector geometry as implemented in the geant4 simulation code. ${\mathrm{BaF}}_2$ crystals in red. Si detector in blue. The beam enters from the left and is deposited on the tape (not shown in the figure) in front of the Si detector.

Figure 2

View of the beam-tube end-cap geometry as implemented in the geant4 simulation code. Visible elements are the detector holder (blue), holder screws and mounts (pink), silicon detector (red) with active area (green), aluminium roller (yellow), plastic roller (orange), and aluminium structural elements (white). For clarity the tape is not shown.

Figure 3

Relevant histograms for the analysis: parent decay (gray filled), daughter decay (pink), delayed neutron decay (dark blue), accidental contamination (light blue), summing-pileup contribution (green), and reconstructed spectrum (red). See text for details. The neutron separation energy $S_n$ and decay energy window $Q_{\beta }$ are also indicated.

Figure 4

$\beta$-intensity distributions: TAGS result (red line), high-resolution measurements (blue filled), from delayed neutron spectrum (gray filled). See text for details.

Figure 5

$\beta$-intensity distributions from TAGS. The thin black line is the adopted solution; the light-blue filled region indicates the spread of solutions due to the systematic effects investigated. See text for details.

Figure 6

Accumulated $\beta$-intensity distribution $I_{\beta \gamma }^{\mathrm{\Sigma }}$: TAGS result (red line) and high-resolution measurements (blue line).

Figure 7

Difference between average $\beta$ energies obtained by direct $\beta$ spectrum measurements (Tengblad et al. [38]) and from TAGS $\beta$-intensity distributions. TAGS results are from Ref. [12] (open circles) and from the present work (filled circles).

Figure 8

Comparison of $\beta$ spectra $S_{\beta }$. Tengblad et al. [38]: black circles; present TAGS result: dashed red line; present TAGS plus $\beta$ delayed neutron contribution: continuous red line; high-resolution measurements [1, 2, 3]: blue line.

Figure 9

Ratio of decay heat as a function of cooling time calculated for ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ when our TAGS data replace high-resolution data. Continuous line: photon component; dashed line: electron component. Red: ${}^{87}\mathrm{Br}$; green: ${}^{88}\mathrm{Br}$; blue: ${}^{94}\mathrm{Rb}$; black: all three isotopes.

Figure 10

Ratio of antineutrino spectra as a function of energy calculated for ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ when our TAGS data replace high-resolution data. Red: ${}^{87}\mathrm{Br}$; green: ${}^{88}\mathrm{Br}$; blue: ${}^{94}\mathrm{Rb}$; black: all three isotopes.

Figure 11

Ratio of antineutrino spectra as a function of energy calculated for ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ when our TAGS data replace the data of Tengblad et al. Red: ${}^{87}\mathrm{Br}$; green: ${}^{88}\mathrm{Br}$; blue: ${}^{94}\mathrm{Rb}$; black: all three isotopes.

Figure 12

Average $\gamma$ to total width from experiment (black line) and calculated for the three spin-parity groups populated in allowed decay (red, green, blue). The gray-shaded area around the experiment indicates the sensitivity to systematic effects. See text for details.