A. P. Serebrov

New measurements of the neutron electric dipole moment

We report a new measurement of the neutron electric dipole moment with the PNPI EDM spectrometer using the ultracold neutron source PF2 at the research reactor of the ILL. Its first results can be interpreted as a limit on the neutron electric dipole moment of |dn| < 5.5 × 10−26 e cm (90% confidence level).

On the possibility of performing an experiment in the search for a sterile neutrino

At present, the possible existence of a sterile neutrino having a significantly smaller cross section of interaction with a substance than, for example, electronic antineutrinos from a reactor is being widely discussed. It has been suggested that, due to reactor antineutrino transition into a sterile state, one can observe both the oscillation effect at short distances (5–15 m) from the reactor and a deficiency of the reactor antineutrino flux at large distances. We have investigated the possibility of performing experiments in search for reactor antineutrino oscillations into a sterile state on research reactors. A model experiment has been carried out on a 16-MW WWR-M reactor at the Petersburg Nuclear Physics Institute with a view to implementing a full-scale experiment using a 100-MW SM-3 reactor at the Research Institute of Atomic Reactors (RIAR). Background conditions of these experiments have been studied for both reactors. It is concluded that the implementation of a full-scale Neutrino-4 experiment on the SM-3 reactor at the RIAR is possible.

On the possibility of experimentally confirming the hypothesis of reactor antineutrino passage into a sterile state

The “Neutrino-4” experiment for the 100-MW SM-3 reactor has been developed with the aim of testing the reactor antineutrino anomaly at Petersburg Nuclear Physics Institute. The advantages of this reactor for studying the antineutrino anomaly are (i) a low background level and (ii) small dimensions (35 × 42 × 42 cm) of the active zone. Operation of a position-sensitive antineutrino detector comprising five working sections and moving so as to cover a region of distances within 6–13 m from the active zone has been simulated by the Monte-Carlo method. The range of experimental sensitivity with respect to the oscillation parameters Δm2 and sin22θ is determined, which will make it possible to confirm the hypothesis of antineutrino oscillations into a sterile state.

New installation for measuring a neutron lifetime with a big gravitational trap of ultra cold neutrons

At present the highest precision of neutron lifetime measurements has been achieved by the experiment made in Petersburg Nuclear Physics Institute (PNPI) with a gravitational trap of ultracold neutrons (UCN). A new installation with a big gravitational trap is an advanced development of methods and approaches applied in the previous experiment. We are planning to attain the measurement precision of 0.2 s which is four times better than the existing level of precision. A model of the experiemnt has been made for simulation by the Monte Carlo method. This model allows one to imply the concrete value of the neutron lifetime, then to reproduce an experiemental procedure and to see if there is any difference between the implied value and the measured one. As a result of modelling one has determined systematic uncertainty related to the procedure of calculating the efficient frequency of collisions of UCNs. It is equal to 0.1 s. We have also carried on modeling of different construction units of the installation.

The project of ultracold neutron sources at the PIK reactor with superfluid helium as a moderator

The project of ultracold neutron sources at the PIK reactor with superfluid helium as a moderator is presented. The rate of producing ultracold neutrons in superfluid helium is 100 cm−3 s−1 at neutron flux density Φ(λ = 9 Å) = 109 cm−2 s−1 Å−1. At a moderator temperature of 1 K within the experimental volume of 351, the density of ultracold neutrons may be equal to 1.3 × 103 cm−3, which is two orders of magnitude exceeds that the currently existing ultracold neutron sources.

Creation of neutrino laboratory for carrying out experiment on search for a sterile neutrino at the SM-3 reactor

To check the existence of a sterile neutrino, a neutrino laboratory aimed at searching reactor antineutrino oscillations is created at the SM-3 reactor. A prototype of a neutrino detector with a scintillator volume of 400 L is moved at distances 6–11 m from the core of the reactor. Background conditions are measured. It is shown that the cosmic rays background is the main problem in the experiment. The prospects of the search for reactor antineutrino oscillations at short distances are discussed.

Neutron lifetime measurements with a large gravitational trap for ultracold neutrons

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. There remains the unsolved problem of a 3.9{sigma} discrepancy between measurements of this lifetime using neutrons in beams and those with stored neutrons (UCN). In our experiment we measure the lifetime of neutrons trapped by Earths gravity in an open-topped vessel. Two configurations of the trap geometry are used to change the mean frequency of UCN collisions with the surfaces - this is achieved by plunging an additional surface into the trap without breaking the vacuum. The trap walls are coated with a hydrogen-less fluorine-containing polymer to reduce losses of UCN. The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. At 80 K, the probability of UCN loss due to collisions with the trap walls is just 1.5% of the probability of beta-decay. The free neutron lifetime is determined by extrapolation to an infinitely large trap with zero collision frequency. The result of these measurements is 881.5 +/- 0.7_stat +/- 0.6_syst s which is consistent with the conventional value of 880.2 +/- 1.0 s presented by the Particle Data Group. Future prospects for this experiment are in further cooling to 10 K which will lead to an improved accuracy of measurement. In conclusion we present an analysis of currently-available data on various measurements of the neutron lifetime.

Neutron lifetime measurement on setups with gravitational trap

Currently, the best accuracy of neutron lifetime measurements has been attained in the experiment with a gravitational trap for ultracold neutrons (UCNs), performed at the Petersburg Nuclear Physics Institute (PNPI); the measured lifetime was 878.5 ± 0.8 s. A new setup with a big gravitational trap has been designed to continue the methods and approaches used in the previous experiment. It is planned to reduce the measurement error to 0.2 s, i.e., improve the existing accuracy by a factor of 4. The spectrometer was designed at PNPI and installed on the PF2/MAM beam at the Institute Laue–Langevin. Test experiments have been performed.

New measurements of the neutron electric dipole moment with the Petersburg Nuclear Physics Institute double-chamber electric dipole moment spectrometer

This article presents results of measuring the neutron electric dipole moment (EDM) made by the Institut Laue-Langevin (ILL) reactor using the Petersburg Nuclear Physics Institute (PNPI) experimental installation. A double-chamber magnetic resonance spectrometer with prolonged holding of ultracold neutrons has been employed. The results determine the upper limit for EDM neutron quantity equal to |dn| < 5.5 × 10−26e cm at a 90% confidence level.

Experiment neutrino-4 on searching for a sterile neutrino with multisection detector model

A laboratory for searching for oscillations of reactor antineutrinos has been created based on the SM-3 reactor in order to approach the problem of the possible existence of a sterile neutrino. The multisection detector prototype with a liquid scintillator volume of 350 L was installed in mid-2015. This detector can move inside the passive shield in a range of 6–11 m from the active core of the reactor. The antineutrino flux was measured for the first time at these short distances from the active core of the reactor by the movable detector. The measurements with the multisection detector prototype demonstrated that it is possible to measure the antineutrino flux from the reactor in the complicated conditions of cosmic background on the Earth’s surface.