Possible origin of shoulder in the reactor antineutrino spectrum

Abstract

The re-analysis of measured $^{238}U$ integrated electron spectrum reveals a shape mismatch with respect to the so called ab initio calculation and the ratio resembles well with the recently observed reactor antineutrino shoulder or bump around 5 MeV prompt energy when plotted in a reduced scale where the first two moments (area and average) of the spectrum are made identical (KNO type scaling). However, such bumps are not seen in case of other actinides like $^{235}U$ and $^{239,241}Pu$ isotopes. Since the magnitude of the bump in $^{238}U$ is comparable with the experimental observations, $^{238}U$ can not be the only source of anomaly unless similar contributions come from other uranium and plutonium isotopes as well. Therefore, it is strongly conjectured that the integrated electron spectra of uranium and plutonium isotopes particularly in the $6$ MeV to $8$ MeV energy range may have additional contributions coming from the harder part of the reactor neutron spectrum which may not be present in the thermal measurements.