Search

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.

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.

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 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.

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.

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