Components of antineutrino emission in nuclear reactor

New ?_ee scattering experiments aimed at sensitive searches for the ν_e magnetic moment and projects to explore small mixing angle neutrino oscillations at reactors require a better understanding of the reactor antineutrino spectrum. Six components which contribute to the total ?_e spectrum generated in a nuclear reactor are considered. They are beta decays of the fission fragments of ²³⁵U, ²³⁹Pu, ²³⁸U, and ²⁴¹Pu and decays of beta emitters produced as a result of neutron capture in ²³⁸U and in accumulated fission fragments which perturb the spectrum. For antineutrino energies of less than 3.5 MeV and for each of the four fissile isotopes, the time evolution of ?_e spectra is given during fuel irradiation and after the irradiation is stopped. The relevant uncertainties are estimated. Small corrections to the ILL spectra are considered.     

Present and future experiments in nonequilibrium reactor antineutrino energy spectrum

Considerable efforts that have been undertaken in recent years in low-energy antineutrino experiments require further systematic investigations in line of reactor antineutrino spectroscopy as a metrological basis of these experiments. We consider some effects associated with the nonequilibrium of reactor radiation and residual emission from spent reactor fuel in contemporary experiments. The text was submitted by the authors in English.     

The anisotropy of inverse beta decay and antineutrino detection

The anisotropy of the positrons emitted in the reaction has to be taken into account for extracting an antineutrino signal in Superkamiokande. For the Sun, this effect allows a sensitivity to transition probability at the 3% level already with the statistics collected in the first hundred days. For a supernova in the Galaxy, the effect is crucial for extracting the correct ratio of events.     

Improved measurement of the reactor antineutrino flux and spectrum at Daya Bay

A new measurement of the reactor antineutrino flux and energy spectrum by the Daya Bay reactor neutrino experiment is reported. The antineutrinos were generated by six 2.9 GWth nuclear reactors and detected by eight antineutrino detectors deployed in two near(560 m and 600 m flux-weighted baselines) and one far(1640 m flux-weighted baseline) underground experimental halls. With 621 days of data, more than 1.2 million inverse beta decay(IBD) candidates were detected. The IBD yield in the eight detectors was measured, and the ratio of measured to predicted flux was found to be 0.946±0.020(0.992±0.021) for the Huber+Mueller(ILL+Vogel) model. A 2.9σ deviation was found in the measured IBD positron energy spectrum compared to the predictions. In particular, an excess of events in the region of 4–6 MeV was found in the measured spectrum, with a local significance of 4.4σ. A reactor antineutrino spectrum weighted by the IBD cross section is extracted for model-independent predictions.     

Precision measurements in nuclear beta decay

Precision measurements in nuclear beta decay provide sensitive means to determine the fundamental coupling of charged fermions to weak bosons and to test discrete symmetries in the weak interaction. The main motivation of such measurements is to find deviations from Standard Model predictions as possible indications of new physics. I focus here on two topics related to precision measurements in beta decay, namely: (i) the determination of the V _ud element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix from nuclear mirror transitions and (ii) selected measurements of time reversal violating correlations in nuclear and neutron decays. These topics complement those presented in other contributions to this conference.     

Inverse beta decay of arbitrarily polarized neutrons in a magnetic field

We calculate the cross-section of the inverse beta decay process,v _e+n → p+e, in a background magnetic field which is much smaller than m _p ^2/e . Using exact solutions of the Dirac equation in a constant magnetic field, we find the cross-section for arbitrary polarization of the initial neutrons. The cross-section depends on the direction of the incident neutrino even when the initial neutron is assumed to be at rest and has no net polarization. Possible implications of the result are discussed.     

Measuring of fissile isotope partial antineutrino spectra in direct experiment at nuclear reactor

The direct measuring method is considered to get nuclear reactor antineutrino spectrum. We suppose to isolate partial spectra of the fissile isotopes by using the method of antineutrino spectrum extraction from the inverse beta-decay reaction positron spectrum applied at Rovno experiment. This admits to increase the accuracy of partial antineutrino spectra forming the total nuclear reactor spectrum. It is important for the analysis of the reactor core fuel composition and could be applied for non-proliferation purposes.     

Coulomb corrections to nuclear beta decay

A calculation of the Coulomb correction to the nuclear beta decay is presented, in the context of the elementary-particle treatment of Kim and Primakoff, making use of the low-energy approximants to nuclear form factors.     

First results from KamLAND: evidence for reactor antineutrino disappearance

KamLAND has measured the flux of nu;(e)'s from distant nuclear reactors. We find fewer nu;(e) events than expected from standard assumptions about nu;(e) propagation at the 99.95% C.L. In a 162 ton.yr exposure the ratio of the observed inverse beta-decay events to the expected number without nu;(e) disappearance is 0.611+/-0.085(stat)+/-0.041(syst) for nu;(e) energies >3.4 MeV. In the context of two-flavor neutrino oscillations with CPT invariance, all solutions to the solar neutrino problem except for the "large mixing angle" region are excluded.     

Collective description of nuclear double beta decay transitions

A consistent treatment of the intrinsic and collective coordinates relevant for the calculation of matrix elements describing nuclear double beta decay transitions is introduced. The method, which was originally developed for the case of nuclear rotations, is adapted to include isospin and number of particles degrees of freedom. To illustrate its main features we apply the formalism to the case of Fermi transitions in a simplified model. From the corresponding results we conclude that the uncertainties found in many existing double beta decay calculations might be largely due to the mixing of physical and spurious effects in the treatment of isospin dependent interactions.     

Effect of synchrotron radiation on nuclear beta decay

The stimulation of the endothermic beta decay of stable nuclei by the field of synchrotron radiation has been analyzed theoretically in the framework of the photobeta decay mechanism. In contrast to works devoted to the effect of laser fields on beta decay, the action of the field directly on a nucleus rather than on a beta electron is considered (a sufficiently intense flux of hard photons whose energies exceed 60 keV allows this action). The rates of such a beta decay are calculated for a number of “parent nucleus-daughter nucleus” pairs for the relativistic case including Coulomb effects. For the most intense available sources of synchrotron radiation, the rate of stimulated beta decay for most nuclei under investigation appears to be characteristic of third-forbidden β^? transitions. The effect of synchrotron radiation on highly forbidden natural nuclear β^? decays is also analyzed. In particular, irradiation increases the rate of the β^? decay of the ₃₇ ⁸⁷ Rb and ₄₉ ¹¹⁵ In nuclei by 2% and by almost two orders of magnitude, respectively.     

Neutrino and antineutrino losses in nuclear processes

Energy, momentum and angular momentum losses in β-decay processes are ascribed to neutrinos and antineutrinos. These penetrating neutral particles, with peculiar symmetry properties, are responsible for sizeable irrecoverable energy losses in nuclear reactors and thermonuclear processes.     

First result from the MTV experiment at TRIUMF searching T-violation in nuclear beta decay

The MTV experiment (Mott Polarimetry for T-Violation Experiment) is running from 2009 at TRIUMF, which aims to search a large non-standard T-Violation in polarized nuclear beta decay. Existence of transverse polarization of electrons emitted from polarized ⁸Li nuclei, which are produced at TRIUMF-ISAC and stopped inside an aluminum stopper, is investigated. We utilize a Mott polarimeter consists of a planer drift chamber, measuring backward scattering left-right asymmetry from a thin lead analyzer foil. In this paper, preliminary results from the first physics run performed in 2010 are described.     

On the isospin breaking corrections to nuclear beta decay

We estimate the effect of the up and down quark mass difference to the neutron beta decay vector form factor at zero momentum transfer, by using an expansion in chiral SU(3) X SU(3) symmetry breaking to leading order. We find a deviation from a deviation from unity of the order of 3.5X10⁻⁵.