A liquid hydrogen source of ultra-cold neutrons

A liquid hydrogen source of ultra-cold neutrons (UCN) developed for an experimental search for the electric dipole moment of the neutron is described. The results of an investigation of the yield of UCN from gaseous, liquid, and solid hydrogen as a function of temperature are presented. The UCN counting rate obtained at the output of the 6 × 7 cm² neutron guide tube is 5 × 10⁴ n/s. This counting rate corresponds to a flux of neutrons whose velocity along the axis of the neutron guide tube is below 7 m/s. Preliminary measurements of the UCN yield from liquid and solid deuterium have been carried out.     

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.     

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.     

Superthermal source of ultracold neutrons for fundamental physics experiments

Ultracold neutrons (UCNs) play an important role for precise measurements of the properties of the neutron and its interactions. During the past 25 years, a neutron turbine coupled to a liquid deuterium cold neutron source at a high-flux reactor has defined the state of the art for UCN production, despite a long history of efforts towards a new generation of UCN sources. This Letter reports a world-best UCN density available for users, achieved with a new source based on conversion of cold neutrons in superfluid helium. A conversion volume of 5 liters provides at least 274,000 UCN in a single accumulation run. Cyclically repeated operation of the source has been demonstrated, as well.     

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.     

New type of low temperature source of Ultra-cold neutrons and production of continous beams of UCN

We discuss a new type of source for Ultra-cold neutrons (UCN) in which the UCN are produced in a thin film on the walls of a cryogenic container. The UCN build up to a significant density inside the container, and the build-up time can be adjusted without effecting the UCN density. Applications to the production of intense, continous beams of UCN for scattering experiments are emphasized. The new source is well suited for installation inside the moderator of an intense neutron source.     

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.     

Solid deuterium source of ultracold neutrons based on a pulsed spallation source

A new type of source of ultracold neutrons (UCNs) is proposed. The source operates on the basis of a pulsed spallation source. Solid deuterium makes it possible to obtain UCN density 10⁴ neutrons/cm³ as a result of high gain at low temperatures and the possibility of withstanding high pulsed heat loads as a result of the high specific heat of solid deuterium. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 765–770 (25 December 1997)     

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.     

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.     

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...     

Spallation ultracold neutron source of superfluid helium below 1 K

For the production of high-density ultracold neutrons (UCNs), we placed 0.8 K superfluid helium in a cold neutron moderator. We resolved previous heat-load problems in the spallation neutron source that were particularly serious below 1 K. With a proton-beam power of 400 MeV×1 μA, a UCN production rate of 4 UCN cm(-3) s(-1) at the maximum UCN energy of E(c)=210 neV and a storage lifetime of 81 s were obtained. A cryogenic test showed that the production rate can be increased by a factor of 10 with the same storage lifetime by increasing the proton-beam power as well as (3)He pumping speed.     

Multiple scattering theory for slow neutrons (from thermal to ultracold)

The general theory of neutron scattering is presented, valid for the whole domain of slow neutrons from thermal to ultracold. Particular attention is given to multiple scattering which is the dominant process for ultracold neutrons (UCN). For thermal and cold neutrons, when the multiple scattering in the target can be neglected, the cross-section is reduced to the known value. A new expression for inelastic scattering cross-section for UCN is proposed. Dynamical processes in the target are taken into account and their influence on inelastic scattering of UCN is analyzed. Received 21 July 1999     

Search for low-energy upscattering of ultracold neutrons from a beryllium surface

We present results of a search for anomalous low-energy upscattering of ultracold neutrons from a beryllium surface. This upscattering is considered one for the possible reasons for UCN “disappearance” from very cold beryllium bottles, as observed in experiments. The indium foil activation method was used to measure a very low intensity flux of upscattered UCN. The (15–300) m/s velocity range of upscattered UCN is ruled out by these measurements at a confidence level of 90%. Zh. éksp. Teor. Fiz. 115, 141–148 (January 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

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.     

Experimental study of quasi-elastic scattering of ultracold neutrons

Ultracold neutrons (UCN) are lost from traps if they are quasi-elastically scattered from the wall with an energy gain sufficient to exceed the Fermi potential for the wall. Possible mechanisms of a quasi-elastic energy transfer are, for instance, scattering from hydrogen diffusing in an impurity surface layer or on surface waves at a liquid wall. Using two different experimental methods at the UCN source of the Institut Laue-Langevin we have investigated both the energy-gain and the energy-loss side of quasi-elastic UCN scattering on Fomblin grease coated walls. For Fomblin oil and similar new types of oil we report up-scattering data as a function of temperature and energy transfer. These low-temperature oils may be used in an improved measurement of the neutron lifetime, which requires extremely low wall reflection losses. Received 13 March 2002 Published online 31 July 2002     

New methodical developments for GRANIT

New methodical developments for the GRANIT spectrometer address further improvements of the critical parameters of this experimental installation, as well as its applications to new fields of research. Keeping in mind an extremely small fraction of ultra cold neutrons (UCN) that could be bound in gravitational quantum states, we look for methods to increase statistics due to: developing UCN sources with maximum phase-space density, counting simultaneously a large fraction of neutrons using position-sensitive detectors, and decreasing detector backgrounds. Also we explore an eventual application of the GRANIT spectrometer beyond the scope of its initial goals, for instance, for reflectometry with UCN.     

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.     

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.     

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.