Description
Several explanations have been proposed to explain the bump, i.e., the excess of reactor antineutrinos observed by all short-baseline experiments around 6 MeV [1]. It has been shown [2], that out of the hundreds of fission products whose $\beta^-$-decay originates the reactor antineutrino flux, only a fraction affects the high-energy portion of the spectrum where the bump has been observed. Of these, some have a long-lived excited state, or isomer, whose direct population through fission is expressed by the Isomeric Yield Ratio (IYR), the fraction of the fission yield that populates the isomer over the total yield of the isotope. When the isomers decay by $\beta^-$-decay, the IYR directly influences the antineutrino spectra, as it represents the weighting factor of two (or more) $\bar{\nu}$ spectra with as many $\beta^-$-endpoint energies.
Following a reevaluation of the available experimental data on IYRs [3], we performed a comprehensive sensitivity study that aimed at estimating to what extent IYRs of specific fission products impact the reactor antineutrino spectrum in the energy range where the bump is observed.
In the first phase of this work, we estimated the total that the newly evaluated IYRs have on reactor antineutrino spectra. Starting from the JEFF-3.3 fission product yields, whose IYRs are obtained from the Madland and England semiempirical model [4], we obtained a reference antineutrino spectrum with the summation method [5]. We then modified the JEFF-3.3 IYRs using the values recommended by Sears, et al., and obtained an updated antineutrino spectrum. We repeated this process for all 4 major actinides, and - calculating the ratio of the modified $\bar{\nu}$ spectrum and the reference JEFF-3.3, we obtained the curve in the figure. Including the updated IYR values results in an overall increase above 5 MeV, that reaches up to +30% at 8 MeV.
In a first-of-its-kind sensitivity study, we also looked at the impact that other IYRs would have if their value, never determined experimentally, was different from the one based on the Madland and England model. This resulted in a list of high-importance fission products (e.g., $^{134}$Sb, $^{97}$Y or $^{100}$Nb), whose IYR would impact the antineutrino spectrum by up to 5% in the bump energy range.
References
[1] Hayes, A. C., J. L. Friar, G. T. Garvey, Duligur Ibeling, Gerard Jungman, T. Kawano, and Robert W. Mills. "Possible origins and implications of the shoulder in reactor neutrino spectra." Physical Review D 92, no. 3 (2015): 033015.
[2] Sonzogni, A. A., T. D. Johnson, and E. A. McCutchan. "Nuclear structure insights into reactor antineutrino spectra." Physical Review C 91, no. 1 (2015): 011301.
[3] Sears, C. J., A. Mattera, E. A. McCutchan, A. A. Sonzogni, D. A. Brown, and D. Potemkin. "Compilation and Evaluation of Isomeric Fission Yield Ratios." Nuclear Data Sheets 173 (2021): 118-143.
[4] Madland, David G., and Talmadge R. England. Distribution of independent fission-product yields to isomeric states. No. LA-6595-MS; ENDF-241. Los Alamos Scientific Lab., NM (USA), 1976.
[5] Vogel, Po, Go Ko Schenter, Fo Mo Mann, and R. E. Schenter. "Reactor antineutrino spectra and their application to antineutrino-induced reactions. II." Physical Review C 24, no. 4 (1981): 1543.
