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

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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.

We report the use of deuterated polystyrene (dPS) and deuterated polyethylene (dPE) for the storage of ultracold neutrons (UCN) in an experiment to search for a finite neutron electric dipole moment (nEDM). The Fermi potential of thin film coatings on silicon was measured by cold neutron reflectometry as V(dPS)=(161±10)neV and V(dPE)=(214±10)neV. The UCN loss factor at room temperature of a dPS-coated polystyrene insulator is η=(3±1)×10−4.

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.

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.

In case a mirror world with a copy of our ordinary particle spectrum would exist, the neutron n and its degenerate partner, the mirror neutron n', could potentially mix and undergo nn' oscillations. The interaction of an ordinary magnetic field with the ordinary neutron would lift the degeneracy between the mirror partners, diminish the n' amplitude in the n wave function and, thus, suppress its observability. We report an experimental comparison of ultracold neutron storage in a trap with and without superimposed magnetic field. No influence of the magnetic field is found and, assuming negligible mirror magnetic fields, a limit on the oscillation time taunn' > 103 s (95% C.L.) is derived.

A new type of ultracold neutron source based on the spallation process is under construction at PSI. The essential elements of this source are a pulsed proton beam with a highest intensity of (Ip > 2mA) and a low duty cycle (~1%), a lead spallation target, a large D2O moderator and a solid D2 (sD2) converter system. Spallation neutrons are thermalized in the D2O, further cooled in the sD2 and, finally, some of them are down-scattered into the ultra-cold neutron range (Tkin < 250 neV). The expected UCN density in the new source is 3000 UCN/cm-3, almost two orders of magnitude higher than with the best source currently available (at ILL).

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.

We have produced hydrogen-free diamond-like carbon (DLC) films by vacuum arc deposition for use as wall coating material in ultracold neutron (UCN) applications. The sp(3) fraction, the main quality factor for DLC used in UCN applications, was varied from 0.4 to 0.9, the coating thickness between 10 nm and 120 nm. The samples were characterized by using X-ray Absorption Near-Edge Spectroscopy (XANES), X-ray induced Photoelectron Spectroscopy (XPS), Laser induced surface Acoustic Waves (LAwave), cold neutron reflectometry and Raman spectroscopy at visible excitation wavelength. We observe reasonable agreement between the different results for film thicknesses below 20 nm. For larger thickness, we find that the surface-sensitive methods XPS and XANES yield smaller sp(3) fractions (by up to 20%) than the bulk-sensitive LAwave, being consistent with the assumption of a lower-density surface layer on a nominal-density bulk layer. (c) 2006 Elsevier B.V. All rights reserved.

Ultracold neutrons (UCN) with energy up to the Fermi potential were stored for the first time in a volume coated with diamondlike carbon. As a function of the UCN energy and wall temperature of the storage volume, we measured the wall loss parameter eta and obtained eta=(3.1 +/- 0.9)center dot 10(-4) at T=290 K and eta=(1.8 +/- 0.2)center dot 10(-4) at T=115 K.