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International audience; Currently the most accurate measurements of neutron lifetime are being performed at the Petersburg Nuclear Physics Institute with ultracold neutrons (UCNs) stored in a gravitational trap. A modified setup with a large gravitational trap and cooling to 10–15 K is presented. The results of measurements of temperature dependence of UCN losses in collisions with walls, which were coated with a perfluorinated grease (Fomblin UT 18), at 300–77 K, are detailed. The probable heat inflow to the trap is estimated, and the feasibility of cooling to indicated temperatures in experiments with the modified setup is demonstrated.

A research program aimed at studying fundamental interactions by means of ultracold and polarized cold neutrons at the GEK-4-4′ channel of the PIK reactor is presented. The apparatus to be used includes a source of cold neutrons in the heavy-water reflector of the reactor, a source of ultracold neutrons based on superfluid helium and installed in a cold-neutron beam extracted from the GEK-4 channel, and a number of experimental facilities in neutron beams. An experiment devoted to searches for the neutron electric dipole moment and an experiment aimed at a measurement the neutron lifetime with the aid of a large gravitational trap are planned to be performed in a beam of ultracold neutrons. An experiment devoted to measuring neutron-decay asymmetries with the aid of a superconducting solenoid is planned in a beam of cold polarized neutrons from the GEK-4′ channel. The second ultracold-neutron source and an experiment aimed at measuring the neutron lifetime with the aid of a magnetic trap are planned in the neutron-guide system of the GEK-3 channel. In the realms of neutrino physics, an experiment intended for sterile-neutrino searches is designed. The state of affairs around the preparation of the experimental equipment for this program is discussed.

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.

A new measurement of the neutron lifetime, carried out with the aid of a large gravitational spectrometer made in Petersburg Institute of Nuclear Physics (PNPI) is presented: τn = 881.5 ± 0.7 ± 0.6 s. In our experiment the measurement of neutron lifetime is carried out using the method of storing ultracold neutrons in a material trap with gravitational barrier. Further improvement of the obtained result can be achieved at the helium temperatures. Here we present our neutron lifetime result, modified installation scheme and the first results of cryogenic tests are discussed.

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 Earth's 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., Comment: 14 pages, 22 figures

International audience; The lifetime of the neutron is one of the key physical quantities used to determine the weak interaction parameters and to test predictions of the theory of primary nucleosynthesis. The lifetime of the neutron has been measured in the reported experiment by the method of storing neutrons in a material trap with a gravitational valve. Fomblin grease UT-18 hydrogen-free fluorine polymer has been used as coating. The resistance of the coating to repeated cooling down to 80 K combined with heating up to 300 K has been studied. The probability of losses in the trap is as small as 1.5% of the neutron decay probability. The lifetime of the neutron τ$_{n}$ = (881.5 ± 0.7$_{stat}$ ± 0.6$_{syst}$)s obtained at the new step is in good agreement with a commonly accepted value of (880.2 ± 1.0) s presented by the Particle Data Group.

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 comparison of results of cross section measurements by the different methods can provide useful information on existence of long-range interaction. The total neutron cross sections of He, Ne, Ar, Kr, and Xe were measured using a method of ultracold neutrons. Measurements with ultra cold neutrons confirm the discrepancy between coherent cross-section of scattering for He measured by a neutron interferometer and scattering cross-section measured by the transmission method. The discrepancy makes up 5.3 standard deviations.

We have measured the free neutron lifetime τ n by storage of ultra-cold neutrons (UCN) in a Fomblin coated UCN trap of in situ variable size. The method was initially developed by W. Mampe et al. (1989) [10] with MAMBO I and improved by the addition of a prestorage volume yielding a well defined UCN spectrum for storage in the main trap. By extrapolation to infinite trap size using the time scaling method we obtain for the free neutron lifetime τ n = ( 880.7 ± 1.3 ± 1.2 ) s . Data from different UCN spectra, trap temperatures and storage times were used for the evaluation. The present result is compared with other experimental neutron lifetime data.

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. In our experiment we measure the storage time of UCN in the material trap coated with a hydrogen-free fluorine-containing polymer (Fomblin grease UT-18). The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. The achieved storage time is only 1.5% less than free neutron lifetime. Using additional surface, which can be plunged into the trap to change the collision frequency of UCN with walls, we calculate free neutron lifetime by extrapolation to zero collision frequency. The result of the measurements with this new experimental setup is τ n =881.5±0.7 stat ±0.6 syst s τn=881.5±0.7stat±0.6systs which is consistent with the conventional value of 880.2±1.0 880.2±1.0 presented in Particle Data Group. In conclusion, we present an analysis of currently-available data on measurements of neutron.

Present experimental and astrophysical limits do not exclude that the neutron (n) oscillation into mirror neutron ( $$n'$$ ), a sterile state exactly degenerate in mass with the neutron, can be a very fast process, in fact faster than the neutron decay itself, in which case it would have very interesting implications in cosmology and astrophysics. This process is sensitive to the magnetic field. Namely, if the mirror magnetic field $$\mathbf {B}'$$ exists at the Earth, $$n{-}n'$$ oscillation probability can be suppressed or resonantly amplified by the applied magnetic field $$\mathbf {B}$$ , depending on its strength and on the angle $$\beta $$ between $$\mathbf {B}$$ and $$\mathbf {B}'$$ . We present the results of ultra-cold neutron storage measurements aiming to check the anomalies observed in previous experiments which could be a signal for $$n{-}n'$$ oscillation in the presence of mirror magnetic field $$B'\sim 0.1$$ G. From the analysis of the experimental data new lower limits on $$n{-}n'$$ oscillation time as a function of $$B'$$ were obtained, assuming that the mirror magnetic field is constant in time: $$\tau _{nn'} > 17$$ s (95 % C.L.) for any $$B'$$ between 0.08 and 0.17 G, and $$\tau _{nn'}/\sqrt{\cos \beta } > 27$$ s (95 % C.L.) for any $$B'$$ in the interval ( $$0.06\div 0.25$$ ) G.

We describe the production, characterization and performance of magnetron-sputtered Be coatings on aluminum, copper and stainless steel substrates. The coating thickness is typically 300 nm. Small samples were characterized by means of Optical Microscopy (OM), Atomic Force Microscopy (AFM) and X-ray induced Photoelectron Spectroscopy (XPS) and Elastic Recoil Detection Analysis (ERDA). The coating quality and adhesion following thermal cycling and neutron irradiation were tested with respect to future applications as storage containers in Ultracold Neutron (UCN) sources. The fractional uncoated area was determined to be 10−4 to 10−5 by OM and XPS. These results were confirmed by foil transmission measurements with UCN in the energy range 180 to 230 neV. The storage time of a 250 l Be-coated Cu container was determined for UCN energies up to 60 neV at room temperature and around 90 K and the wall loss factor η extracted. We obtained η (90 K)=2.7×10−5 and η(300 K)=1.1×10−4 in good agreement with previously published results.

We are going to use a polarized cold neutron beam and an axial magnetic field in the shape of a bottle formed by the superconducting magnetic system. Such configuration of magnetic field allows to extract the decay electrons inside a well-defined solid angle with a high accuracy. The electrostatic cylinder with a potential of 25 kV defines the detected region of neutron decays. The protons, which come from this region will be accelerated and registered by a proton detector. The use of coincidences between the electron and proton signals allows in considerably suppressing the background. The final accuracy of A -asymmetry will be determined by the accuracy of the neutron beam polarization measurement, which is at the level of (1–2)×10 −3 as it was shown in previous studies.

We have studied the low-energy heating of ultracold neutrons during their storage. On fomblin, it is greatly suppressed for the low temperature liquid and at solidification. For non-magnetic solid materials, upper limits of 5×10 −9 (Be) and 3.6×10 −8 (Cu) per collision were found. Therefore, low-energy heating is not the reason for “anomalous losses” of (2–3)×10 −5 per collision on beryllium surfaces.

The studies of spectral dependences for neutrons passing through solid deuterium were carried out using a vertical beam of very cold neutrons with the wavelength λ∼40–150 A. The results show the dependence of the observed neutron scattering cross sections on the method of preparation of a solid deuterium sample and on the ortho-para composition of deuterium.

Chemical stimuli (glutamate, glycine, cAMP, and AMP) cause slow and pronounced changes in thermostability of the organisms ofDaphnia magna andEnchytraeus albidus, which can be explained by functioning of not adapted or partially adapted chemoreceptive cells. In experiments with action of purines on the thermostability, the presence inE. albidus of two different chemoreceptors, specific accordingly to cAMP and non-cyclic purines, is shown. A complex extremal stimulus—reaction function, with several optimal values of intensity of the chemical stimulus, at its action on theD. magna andE. albidus thermostability, shows that the elements (neurons or their systems) with the extremal input-output function take part in coding the chemosensory information by the nervous system of these animals.

The results of experiments and calculations for a solid-deuterium UCN source are presented. It was experimentally shown that the UCN yield is increased by a factor 3 at the solidification of liquid deuterium and a factor 10 at a temperature 10 K, with respect to the liquid phase. The gain factor for UCN yield with respect to hydrogen gas or deuterium gas at room temperature is equal to 1230.

Building on the experience that Mampe gained with his ultra-cold neutron (UCN) storage bottle MAMBO I (Phys Rev. Lett. 63 (1989) 593) the successor experiment MAMBO II has been designed and set up. The basic method of this experiment relies on the storage of UCN in a rectangular glass vessel of variable size with walls covered by Fomblin oil. A preliminary value for the lifetime of 881.0±3 s has been measured.

The temperature dependence of the loss factor for ultracold neutrons owing to heating at thermal energies on the surface of a beryllium sample is studied. The probability of heating ultracold neutrons is anomalously high throughout the entire measured temperature interval, but especially at low temperatures.

The subbarrier passage of ultracold neutrons through beryllium foils and coatings with a probability much higher than that of tunneling is observed. This effect may be responsible for the so-called anomalous loss of ultracold neutrons.

The search for CP violating forces between nucleons in the so-called axion window of force ranges lam between 2x10^-5 m and 0.02 m is interesting because only little experimental information is available there. Axion-like particles would induce a pseudo-magnetic field for neutrons close to bulk matter. A laboratory search investigates neutron spin precession close to a heavy mirror using ultracold neutrons in a magnetic resonance spectrometer. From the absence of a shift of the magnetic resonance we established new constraints on the coupling strength of axion-like particles in terms of the product gs x gp of scalar and pseudo-scalar dimensionless constants, as a function of the force range lam, gs x gp x lam^2, 10 pages, 3 figures

Mirror matter is considered as a candidate for dark matter. In connection with this an experimental search for neutron - mirror neutron (nn') transitions has been carried out using storage of ultracold neutrons in a trap with different magnetic fields. As a result, a new limit for the neutron - mirror neutron oscillation time has been obtained, tau_osc >= 448 s (90% C.L.), assuming that there is no mirror magnetic field larger than 100 nT. Besides a first attempt to obtain some restriction for mirror magnetic field has been done., 11 pages, more detailed analysis of data

Our experiment using gravitationally trapped ultracold neutrons (UCN) to measure the neutron lifetime is reviewed. Ultracold neutrons were trapped in a material bottle covered with perfluoropolyether. The neutron lifetime was deduced from comparison of UCN losses in the traps with different surface-to-volume ratios. The precise value of the neutron lifetime is of fundamental importance to particle physics and cosmology. In this experiment, the UCN storage time is brought closer to the neutron lifetime than in any experiments before:the probability of UCN losses from the trap was only 1% of that for neutron beta decay. The neutron lifetime obtained,878.5+/-0.7stat+/-0.3sys s, is the most accurate experimental measurement to date., Comment: 38 pages, 19 figures,changed content

The idea of a hidden sector of mirror partners of elementary particles has attracted considerable interest as a possible candidate for dark matter. Recently it was pointed out by Berezhiani and Bento that the present experimental data cannot exclude the possibility of a rapid oscillation of the neutron n to a mirror neutron n' with oscillation time much smaller than the neutron lifetime. A dedicated search for vacuum transitions n->n' has to be performed at weak magnetic field, where both states are degenerate. We report the result of our experiment, which compares rates of ultracold neutrons after storage at a weak magnetic field well below 20 nT and at a magnetic field strong enough to suppress the seeked transitions. We obtain a new limit for the oscillation time of n-n' transitions, tau_osc (90% C.L.) > 414 s. The corresponding limit for the mixing energy of the normal and mirror neutron states is delta_m (90% C.L.) < 1.5x10-18 eV., 16 pages, 8 figures, 2 tables

We present a new value for the neutron lifetime of 878.5 +- 0.7 stat. +- 0.3 syst. This result differs from the world average value (885.7 +- 0.8 s) by 6.5 standard deviations and by 5.6 standard deviations from the previous most precise result. However, this new value for the neutron lifetime together with a beta-asymmetry in neutron decay, Ao, of -0.1189(7) is in a good agreement with the Standard Model., 11 pages, 9 figures; extended content with some corrections

Experimental data shows anomalously large Ultra Cold Neutrons (UCN) reflection losses and that the process of UCN reflection is not completely coherent. UCN anomalous losses under reflection cannot be explained in the context of neutron optics calculations. UCN losses by means of incoherent scattering on material defects are considered and cross-section values calculated. The UCN capture cross-section on material defects is enhanced by a factor of 10^4 due to localization of UCN around defects. This phenomenon can explain anomalous losses of UCN., Comment: 13 pages, 4 figures

The first preliminary results of neutron lifetime measurements with a gravitational UCN trap are presented.