Ultracold neutrons (UCN) have been produced using the cold neutron (CN) beam FUNSPIN at SINQ on cryogenic oxygen (O-2), tetradeuteromethane ((CH4)-H-2), and deuterium (H-2(2)) targets. The target cell (40mm long, fiducial volume about 45 cm(3)) was operated between room temperature and 8K and UCN were produced from gaseous, liquid and solid targets. UCN rates have been measured as a convolution of UCN production and transport out of the target and to the detector. At least within the accessible temperature range of this experiment, deuterium outperforms the other materials. Copyright (C) EPLA, 2011

Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the Los Alamos National Security, LLC for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requests that the publisher identity this article as work performed under the auspices of the U.S. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness.

We report a measurement of the reflection of ultracold neutrons from flat, large-area plates of different Fermi potential materials with low surface roughness. The results were used to test two diffuse reflection models, the well-known Lambert model and the micro-roughness model which is based on wave scattering. The Lambert model fails to reproduce the diffuse reflection data. The surface roughness b and correlation length w , obtained by fitting the micro-roughness model to the data are in the range 1\( \le\) b \( \le\)3 nm and 10\( \le\) w \( \le\)120 nm, in qualitative agreement with independent measurements using atomic force microscopy.

A new type of ultra-cold neutron (UCN) source based on the spallation process is under construction at PSI. The essential elements are a pulsed proton beam with highest intensity (Ip≥2mA) and a low duty cycle (1%), a lead spallation target, a large D2O moderator and a solid deuterium (sD2) converter system. Spallation neutrons are thermalized in the D2O, further cooled and partially downscattered into the ultra-cold neutron range in the sD2. The expected UCN density is higher than 1000UCN/cm3 in typical experiments, an increase of almost two orders of magnitude over the best source currently available (at ILL). The new UCN source is expected to come into operation in 2009.

We have investigated the properties of the ultracold neutron converter materials deuterium D2, oxygen O2 and heavy methane CD4 in the temperature range between 8K and room temperature. The experimental program was performed at the FUNSPIN beamline of the Swiss Spallation Neutron Source (SINQ) at Paul Scherrer Institut (PSI). In this paper the measured cold neutron total cross-sections for D2, O2 and CD4 are presented.

At the Paul Scherrer Institute (PSI), a very intense source of ultracold neutrons (UCN) is being built. The UCN converter of solid deuterium must be contained in a vessel. Produced UCN leave that vessel through its top lid. To decide on the design of the vessel and the top lid, we have measured the transmission of neutrons with velocities between 3 and 20 m/s through different material foils. Contrary to expectations, we found that transmission through aluminium and aluminium alloys is equal or even higher compared to zirconium and reactor-grade zirconium alloys, respectively.

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.

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.

We report the characterization of diamond-like carbon (DLC) surfaces to be used for the storage of ultracold neutrons (UCN). The samples investigated were 100–300-nm-thick tetragonal amorphous carbon (ta-C) coatings produced by vacuum-arc technology on thin foils (0.1–0.2mm aluminum, stainless steel, PET). The diamond sp3 fraction was determined by X-ray photoelectron spectroscopy (XPS) to be in the range 45–65%. Secondary-ion mass spectroscopy (SIMS) and elastic recoil detection analysis (ERDA) yielded consistent results for the hydrogen contribution (about 1×1016cm−2 within the top 20nm), strongly concentrated within a surface layer of 1nm thickness. The boron contamination was found to be around 50at.ppm. The fractional hole area of the coatings is on a level of about 1×10−4. Temperature cycling of mechanically pre-stressed samples between 77 and 380K revealed no detrimental effect.

A measurement of the production of ultracold neutrons from velocity-selected cold neutrons on gaseous and solid deuterium targets is reported. The expected energy dependence for two-particle collisions with well defined neutron and Maxwell-Boltzmann distributed molecular velocities is found for the gas target. The solid target data agree in shape with the phonon density-of-states curve and provide strong evidence for the phonon model including multiphonon excitations.

The Fermi potential Vf of diamond-like carbon (DLC) coatings produced with laser-controlled vacuum arc deposition and that of diamond, Al, Si, Be, Cu, Fe and Ni was measured using two different methods, (i) transmission of slow neutrons through foils in a time-of-flight experiment and (ii) cold neutron reflectometry (CNR). For diamond-like carbon in transmission we obtain Vf=(249±14)neV. This is approximately the same as for beryllium and consistent with the theoretical expectations for the measured diamond (sp3) content of 45%. For an sp3-content of 67%, we find Vf=(271±13)neV from reflectometry, again in agreement with theory. These findings open new perspectives in using DLC as storage volume and neutron guide coatings for ultracold neutron sources.

A method for including medium-energy fission into neutronic calculations for spallation systems is described and discussed. The basis is a semi-empirical evaporation plus fission model able to treat a wide range of nuclear states in the mass region above about 100amu with nuclear excitation energies up to 1000MeV. In combination with suitable nuclear physics models describing the entrance channels and a suitable evaporation algorithm, this is able to give a full description of the de-excitation of the intermediate nuclear states formed in interactions with target nuclei from Ag to Cf by (i) medium energy (up to a few GeV) nucleons and pions, (ii) thermal and fast neutrons and (iii) light ions with energy up to several 100MeV/A.

We use a description of the work carried out to determine the radioactive inventory for a redundant beam-dump from the PSI accelerator complex, as an illustration of techniques for the classification and characterisation of accelerator waste and how some difficulties can be circumvented. The work has been carried out using a combination of calculation and sample analysis: The inventory calculation effectively involves a large scale Monte-Carlo transport calculation of a medium-sized spallation facility and for the sample analysis, standard radiochemical analysis techniques have had to be extended to include AMS measurements so as to allow measurement of some of the long half-life, waste disposal relevant, nuclides.

It has been shown recently that diamond-like carbon (DLC) with a sp3 fraction above 60% is a better wall coating material for ultracold neutron applications than beryllium. We report on results of Raman spectroscopic and XPS measurements obtained for diamond-like carbon coated neutron guides produced in a new facility, which is based on pulsed laser deposition at 193nm. For diamond-like carbon coatings on small stainless steel substrates we find sp3 fractions in the range from 60 to 70% and showing slightly increasing values with laser pulse energy and pulse repetition rate.

At PSI currently a source of ultra cold neutrons (UCN) is being built. This source is based on the spallation process using a high intensity proton beam (Ip ≥ 2 mA) with a low duty cycle (1 %) hitting a heavy metal spallation target. The neutrons will be thermalized in heavy water and down scattered into the UCN regime in a 30 dm 3 solid deuterium (sD2) converter. The UCN will be stored in a large UCN storage volume (2 m 3 ) from where they will be piped to the experiments. We expect a UCN density in the storage volume of 3000 per cm 3 . Start-up is planned for 2007.

The storage of ultracold neutrons (UCN) in a combined magnetic, gravitational, and material trap is described. Wall materials investigated were diamondlike carbon (DLC) coatings on solid and flexible foil substrates as well as beryllium coatings on solid substrates. The loss coefficient per wall collision, eta, and the depolarization probability beta were measured simultaneously as a function of temperature (from 70 to 400 K) and energy (from 30 to 80 neV). The results at 70 K are eta=(0.7 +/- 0.1)x10(-4),beta=(15.4 +/- 1.0)x10(-6) for DLC on polyethyleneterephtalate (PET) foil and eta=(1.7 +/- 0.1)x10(-4),beta=(0.7 +/- 0.3)x10(-6) for DLC on aluminum foil. At room temperature the loss coefficients are larger by a factor of about 2 whereas the depolarization probabilities are found to be independent of temperature. The corresponding values for Be at room temperature are eta similar to 5x10(-4),beta similar to 10x10(-6). The DLC results for beta and for the temperature-dependent part of the loss coefficient, eta(T), are interpreted in terms of incoherent scattering by hydrogen. The hydrogen admixture was measured independently by elastic recoil detection analysis to be about 1x10(16) atoms/cm(2). The data do not support the hypothesis of hydrogen being chemically bound within the top layers of the DLC. Using two different models with a thin waterlike film on top of the substrate we obtain consistency between the temperature-dependent loss contribution and the measured hydrogen contamination.

A high-fluence proton irradiation of neptunium was the last experiment in PSIs programme ATHENA related to accelerator-based transmutation. The principal aim of the programme has been to provide experimental data for the validation of theoretical models in nucleon–meson transport codes, with emphasis on the mass yield distribution of fission and spallation products. An improved mass spectrometry method has allowed the direct derivation of isobaric production cross sections with only minor corrections and an estimate of the fission cross section by integration in the fission hump. In a second sample position of the irradiation head, a repetition of the previous ATHENA experiment with thorium was possible, profiting from the improved mass spectrometry technique. The experimental results are better predicted by the code FUSSPOT than by HETC/RAL, both used at PSI.

To complete our study of ultracold neutron (UCN) storage-vessel coatings, we have measured the Fermi potential for neutrons on diamondlike carbon coatings produced by laser induced vacuum arc deposition. A sample with an sp3 content of 0.45, measured using, for the first time, neutron transmission had a Fermi potential of (249±14)neV. A second sample with an sp3 fraction of 0.67, measured using cold neutron reflectometry, gave (271±13)neV. These values complete the demonstration that there is a viable alternative to Be in UCN physics.