Storage of ultracold neutrons in vessels whose walls are made from graphite,fluorine polymer oil,or heavy-water ice

The possibility of attaining the calculated probabilities of the losses of ultracold neutrons (UCN) stored in vessels whose walls are made from graphite, fluorine polymer oil, or heavy-water ice is tested experimentally. It is found that UCN hitting the walls of a graphite vessel undergo additional inelastic scattering not predicted by the theory. It is shown that this scattering may be due to the presence of surface hydrogen that provides a channel of UCN leakage slightly varying with temperature. For vessels whose walls are coated with fluorine polymer oil, additional inelastic UCN scattering is also observed and is found to be efficiently suppressed with decreasing temperature. The experimentally observed and calculated values of the probabilities of UCN losses are shown to be in good agreement for vessels whose walls are made from heavy-water ice.     

Interaction of neutrons with nanoparticles

Two hypotheses concerning the interaction of neutrons with nanoparticles and having applications in the physics of ultracold neutrons (UCN) are considered. In 1997, it was found that, upon reflection from the sample surface or spectrometer walls, UCN change their energy by about 10^?7 eV with a probability of 10^?7–10^?5 per collision. The nature of this phenomenon is not clear at present. Probably, it is due to the inelastic coherent scattering of UCN on nanoparticles or nanostructures weakly attached at the surface, in a state of Brownian thermal motion. An analysis of experimental data on the basis of this model allows one to estimate the mass of such nanoparticles and nanostructures at 10⁷ a.u. The proposed hypothesis indicates a method for studying the dynamics of nanoparticles and nanostructures and, accordingly, their interactions with the surface or with one another, this method being selective in their sizes. In all experiments with UCN, the trap-wall temperature was much higher than a temperature of about 1 mK, which corresponds to the UCN energy. Therefore, UCN increased their energy. The surface density of weakly attached nanoparticles was low. If, however, the nanoparticle temperature is lower than the neutron temperature and if the nanoparticle density is high, the problem of interaction of neutrons with nanoparticles is inverted. In this case, the neutrons of initial velocity below 10² m/s can cool down, under certain conditions, owing to their scattering on ultracold heavy-water, deuterium, and oxygen nanoparticles to their temperature of about 1 mK, with the result that the UCN density increases by many orders of magnitude.     

Neutron lifetime and density of states of fluoropolymers at low temperatures

Present status of the measurements of the neutron lifetime is shortly reviewed. We report the inelastic neutron scattering measurement of the density of vibrational states G(ω) of two fluoropolymers, which are promising coating materials for the storage of ultracold neutrons (UCNs) in closed volumes covered with polymer film. From determined G(ω), we calculate the expected UCN loss coefficients.     

Measurements of ultracold-neutron lifetimes in solid deuterium

We present the first measurements of the survival time of ultracold neutrons (UCNs) in solid deuterium (SD2). This critical parameter provides a fundamental limitation to the effectiveness of superthermal UCN sources that utilize solid ortho-deuterium as the source material. These measurements are performed utilizing a SD2 source coupled to a spallation source of neutrons, providing a demonstration of UCN production in this geometry and permitting systematic studies of the influence of thermal up-scatter and contamination with para-deuterium on the UCN survival time.     

Startup of the high-intensity ultracold neutron source at the Paul Scherrer Institute

Ultracold neutrons (UCN) can be stored in suitable bottles and observed for several hundreds of seconds. Therefore UCN can be used to study in detail the fundamental properties of the neutron. A new user facility providing ultracold neutrons for fundamental physics research has been constructed at the Paul Scherrer Institute, the PSI UCN source. Assembly of the facility finished in December 2010 with the first production of ultracold neutrons. Operation approval was received in June 2011. We give an overview of the source and the status at startup.     

Neutron velocity distribution from a superthermal solid 2H2 ultracold neutron source

We have determined for the first time the velocity distribution of neutrons from a solid ²H₂ ultracold neutron (UCN) source. The spectrum rises sharply above 4.5m/s and has a maximum around 7m/s after transport in an 8m long guide. The number of neutrons in the UCN velocity range (< 7m/s) may be increased by a factor of two by placing the experiment 1m above the UCN source level.     

UCN anomalous losses and the UCN capture cross section on material defects

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 10⁴ due to localization of UCN around defects. This phenomenon can explain anomalous losses of UCN.     

Superfluid-helium converter for accumulation and extraction of ultracold neutrons

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.     

Measurement of the total cross sections of ultracold neutrons with noble gases and search for long-range forces

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.     

First production of ultracold neutrons with a solid deuterium source at the pulsed reactor TRIGA Mainz⋆

The production rates of ultracold neutrons (UCN) with a solid deuterium converter have been measured at the pulsed reactor TRIGA Mainz. Exposed to a thermal neutron fluence of n·cm^-2·pulse^-1, the number of detected very cold and ultracold neutrons ranges up to 200 000 at 7mol of solid deuterium (sD₂) in combination with a pre-moderator (mesitylene). About 50% of the measured neutrons can be assigned to UCN with energies E of where V _F(sD ₂) = 105 neV and V _F(guide) = 190 neV are the Fermi potentials of the sD₂ converter and our stainless steel neutron guides, respectively. Thermal cycling of solid deuterium, which was frozen out from the gas phase, considerably improved the UCN yield, in particular at higher amounts of sD₂.     

Direct experimental verification of neutron acceleration by the material optical potential of solid 2H2

We have measured the acceleration of neutrons by the material optical potential of solid 2H2. Using a gravitational spectrometer, we find a minimal kinetic energy Ec = (99+/-7) neV of neutrons from a superthermal ultracold neutron (UCN) source with solid 2H2 as an UCN converter. The result is in excellent agreement with theoretical predictions, Ec = 106 neV.     

Bulk effects in the coherent inelastic scattering of ultracold neutrons

With the use of a theory developed earlier, bulk effects in ultracold neutron coherent inelastic scattering are considered both for solid and liquid target samples related to energy and momentum exchange with phonon and diffusion-like modes. For the neutron in a material trap, differential and integral probabilities for the energy transfer per bounce are presented in a simple analytic form which exhibits parameter dependence. As an example, the theoretical values for the ultracold-neutron loss rate from a storage bottle with Fomblin-coated walls and stainless-steel walls are evaluated. A possible contribution from incoherent inelastic scattering is discussed.     

Ultracold neutrons extracted from a superfluid-helium converter coated with fluorinated grease

The European Physical Journal C - We report experiments on the production of ultracold neutrons (UCN) in a converter of superfluid helium coated with fluorinated grease (fomblin). We employed our...     

Neutron Guide System for Ultracold and Cold Neutrons at the WWR-M Reactor

Technical Physics - The results of calculation of fluxes of ultracold (UCNs), very cold, and cold neutrons at the output of neutron guides of the UCN source with superfluid helium at the WWR-M...     

Interference in the ultracold neutron transmission through thin layers

At the time-of-flight spectrometer for ultracold neutrons (UCN) with energies below 10^?4 eV at the FRM, Munich, the perpendicular neutron transmission through carbon and gold films of thickness 450 to 800 Å was measured as a function of neutron velocity in the range 11≧v _z≧3.7 m/s. The transmission curves obtained show the expected interference patterns due to interference between the partial waves transmitted and reflected at the two surfaces. From these curves absolute values for the thickness and density could be obtained. From the UCN transmission through a 4.71 μm thick copper foil a value a_coh=7.5±0.15 F was derived for the nuclear scattering length of Cu.     

Measured total cross sections of slow neutrons scattered by solid deuterium and implications for ultracold neutron sources

The total scattering cross sections for slow neutrons with energies in the range 100 neV to 3 meV for solid ortho-2H2 at 18 and 5 K, frozen from the liquid, have been measured. The 18 K cross sections are found to be in excellent agreement with theoretical expectations and for ultracold neutrons dominated by thermal up scattering. At 5 K the total scattering cross sections are found to be dominated by the crystal defects originating in temperature induced stress but not deteriorated by temperature cycles between 5 and 10 K.     

Unconventional trapping of ultracold neutrons

In unconventional storage experiments we filled ultracold neutrons (UCN) into a Fomblin-grease coated trap and then immediately removed the UCN from the storage volume by an absorber, until their residual density in the trap was measured to be negligible. When subsequently the absorber was withdrawn a significant number of UCN of higher energies emerged from the trap. Their appearance cannot be attributed to heating or cooling of residual UCN. Further experiments were performed to investigate the origin of these UCN which we call `late UCN'. We noticed that application of a magnetic field gradient at the trap wall as well as a replacement of Fomblin grease on the surface by Fomblin oil gave rise to small but measurable alterations of storage behavior. These phenomena are consistent with the hypothesis of temporary adhesion of a few UCN to a rough wall.     

Program of Fundamental-Interaction Research for the Ultracold-Neutron Source at the the WWR-M Reactor

The use of ultracold neutrons opens unique possibilities for studying fundamental interactions in particles physics. Searches for the neutron electric dipole moment are aimed at testing models of CP violation. A precise measurement of the neutron lifetime is of paramount importance for cosmology and astrophysics. Considerable advances in these realms can be made with the aid of a new ultracold-neutron (UCN) supersource presently under construction at Petersburg Nuclear Physics Institute. With this source, it would be possible to obtain an UCN density approximately 100 times as high as that at currently the best UCN source at the high-flux reactor of the Institute Laue–Langevin (ILL, Grenoble, France). To date, the design and basic elements of the source have been prepared, tests of a full-scale source model have been performed, and the research program has been developed. It is planned to improve accuracy in measuring the neutron electric dipole moment by one order of magnitude to a level of 10^?27 to 10^?28e cm. This is of crucial importance for particle physics. The accuracy in measuring the neutron lifetime can also be improved by one order of magnitude. Finally, experiments that would seek neutron–antineutron oscillations by employing ultracold neutrons will become possible upon reaching an UCN density of 10³ to 10⁴ cm^?3. The current status of the source and the proposed research program are discussed.     

Critical magnetic surface scattering of neutrons at grazing angles

We derive and discuss the cross-section for elastic and inelastic magnetic scattering of neutrons under conditions of total reflection on the surface of antiferromagnets near the bulk critical temperature.This work was supported by the Bundesministerium für Forschung und Technologie     

The scattering of thermal neutrons by crystals

The present article describes some experiments with thermal neutrons to investigate a variety of properties of crystals. Elastic scattering of the neutrons, like Bragg scattering of X-rays, helps to determine the atomic structure of crystals, and is particularly useful for locating light atoms, such as hydrogen, where X-rays fail. It also gives information on the arrangement of electron spins and the density distribution of unpaired electrons in magnetic atoms. Measurement of the change in energy of the neutrons in inelastic scattering gives the frequencies of the normal modes of vibration of the crystal, which are related to the interatomic forces. For magnetic crystals, inelastic scattering also gives the frequencies of spin waves, which depend on the magnetic interactions between the atoms. The article concludes with an account of the experimental techniques of thermal neutron scattering.