A thin film source of ultra-cold neutrons

Ultra-cold neutron (UCN) densities have been calculated for a thin film source using realistic phonon spectra of the film material (hydrogen and deuterium). The UCN's build up to a significant density inside a cryogenic container whose walls support the thin films. The UCN's density dependence on the neutron temperature,T _n, was also investigated. It was found that the density reaches a maximum value atT _n20K. A deuterium film at low temperature (5K) andT _n=30 K yields a UCN density 10^–9 ₀ where ₀ is the total flux.Work supported in part by NSF Grant # DMR-8024662     

Operation of a superthermal ultra-cold neutron source and the storage of ultra-cold neutrons in superfluid Helium4

We report the first successful operation of a superthermal UCN source and the observation of the storage of Ultra-Cold Neutrons in superfluid Helium⁴ at Temperatures in the range 0.5–1.15K. From the observations we obtain the total cross section for UCN scattering by superfluid Helium. The temperature dependence of the total cross section is found to be consistent with calculations which include the effects of phonon absorption and phonon and roton scattering.     

Precision experiments with cold and ultra-cold neutrons

We present selected results from the first round of the Priority Programme SPP 1491. This programme gets funding from the German DFG and the Austrian FWF funding agencies. The aim of this programme is to address basic open questions in particle and astrophysics using a specific tool: the neutron, which allows the search for new physics becoming manifest itself as small deviations from expectations.     

Axial peaking of ultra-cold neutrons in a guide tube

A simple focussing device in the form of a cone has been used to produce a localised flux increase of ultra-cold neutrons.     

Main effects of the Earth's rotation on the stationary states of ultra-cold neutrons

The relativistic corrections in the Hamiltonian for a particle in a uniformly rotating frame are discussed. They are shown to be negligible in the case of ultra-cold neutrons (UCN) in the Earth's gravity. The effect, on the energy levels of UCN, of the main term due to the Earth's rotation, i.e. the angular-momentum term, is calculated. The energy shift is found proportional to the energy level itself.     

Scattering of ultra-cold neutrons (UCN) by superfluid helium at temperatures around 1 K

We report the first observation of the upscattering of UCN stored in a closed vessel filled with ⁴He. The observations allow the direct determination of the production rate and hence the density of the UCN stored in the helium, which are found to be in agreement with theoretical expectations. the technique offers the possibility of studying the interactions between excitations in the liquid at regions of temperature and wavelength inaccessible to presently available methods.     

Upscattering detected as the main reason for anomalous loss of ultra-cold neutrons in neutron storage experiments

Neutron bottles have been surrounded with an outer detection system in an attempt to detect neutrons which possibly leave the neutron bottle through the walls. Results give evidence that neutrons, undergoing inelastic processes at the inner surface, leave the bottle with energies of 10 meV. About 75 % of all neutrons captured in the bottle are lost in this way. This result is in contradiction with the results of several previous works.     

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)     

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.     

A cold pulsed source of hydrogen and hydrogen clusters: development and extreme-ultraviolet studies

We describe the development and application of a differentially pumped pulsed cold H₂ and (H₂)₂ beam source. The nozzle was cooled with liquid nitrogen and, actuated by a piezoelectric valve with pressures up to 5 bar, produced gas pulses 250 μs long. Strong limitations by collisions with the fast hydrogen molecules returning from the walls of the source chamber could be overcome by surrounding the volume around nozzle and skimmer with cold surfaces. As a consequence, the beam intensities for both the monomer and the clusters could be greatly enhanced. The valve was operated without any realignment for more than one year. The beam source was characterized by a number of photo-ionization studies. As a part of these efforts the presumably first laser-based single-photon ionization spectra of H₂ and (H₂)₂ were obtained. We estimate that more than ten $\mathrm{H}_{3}^{+}$ ions could be detected per laser shot at a wavelength of 74.2 nm and a pressure of 5 bar. The auto-ionization lines of H₂ were recorded with a resolution of about 0.54 cm^?1. A line width of about 629 MHz was obtained in the B←X band system of hydrogen observed by resonance-enhanced two-photon ionization. Further potential improvements of the experiment are discussed.     

Polarization of 22 MeV neutrons elastically scattered from liquid tritium

The asymmetries from elastic scattering of 22.1 MeV incident neutrons was measured for 11 laboratory angles between 40° and 118.5°. The extrema are −60% at 85° (lab) and +98% at 110° (lab).     

On the total cross-section for thermal neutrons of hydrogen in niobium

Neutron transmission experiments have been performed to determine the total cross-section of hydrogen interstitially dissolved in niobium. The wavelength and temperature dependence of the total cross-section can be described by a model with the protons as harmonic oscillators. No changes in the total cross-section were found for the different phases of the interstitials in the niobium host lattice. An upper limit for the jump-time of a proton in diffusion _D is given compared to its mean time of localization _L on an interstitial site. _D <0.2 _L is found.     

The RTM-70 accelerator as a pulsed source of neutrons and photons

The possibility of using the RTM-70 race-track microtron for pulsed production of photons and neutrons is analyzed. Optimum parameters of such sources are obtained through a computer simulation.     

A novel source of atomic hydrogen for passivation of defects in silicon

Palladium membranes have been used for decades for the separation of hydrogen from other gasses. In this letter the use of thin palladium leaves to act as sources of atomic hydrogen for silicon samples is explored. It has been assumed in the past that although hydrogen diffuses through palladium in atomic form, the atoms recombine to form molecular hydrogen at the surface. In this letter it is shown that hydrogen supplied to one surface of a palladium leaf can result in atomic hydrogen being released from the opposite surface at low pressure. This is demonstrated through the use of a palladium leaf in a direct plasma system which allows for atomic hydrogen to be supplied to a sample while avoiding exposure to UV radiation from the plasma and high energy charged particles. This method is shown to provide significant atomic hydrogen to silicon samples and improve passivation of silicon surfaces. Method of Shielded Hydrogen Passivation: Schematic of direct plasma chamber with a shield inserted between the plasma and the silicon sample.     

Penning型负氢离子源诊断

 为了给小型医用回旋加速器提供负氢离子,研制了一台Penning型负氢离子源。采用发射光谱法对该负氢离子源进行了诊断,同时结合离子源功率变化对离子源工作状态进行了分析。实验测量了不同氢气流量、离子源弧流及磁场条件下,该离子源等离子体氢原子巴尔末系中前三条谱线的相对光强和离子源功率变化,分析了不同工作条件对离子源工作状态的影响。结果表明：在可调节范围内,该离子源的工作状态主要受氢气流量的影响,对离子源弧流及磁场的变化不敏感。     

Penning型负氢离子源诊断

为了给小型医用回旋加速器提供负氢离子,研制了一台Penning型负氢离子源。采用发射光谱法对该负氢离子源进行了诊断,同时结合离子源功率变化对离子源工作状态进行了分析。实验测量了不同氢气流量、离子源弧流及磁场条件下,该离子源等离子体氢原子巴尔末系中前三条谱线的相对光强和离子源功率变化,分析了不同工作条件对离子源工作状态的影响。结果表明：在可调节范围内,该离子源的工作状态主要受氢气流量的影响,对离子源弧流及磁场的变化不敏感。     

Colossal light-induced refractive-index modulation for neutrons in holographic polymer-dispersed liquid crystals

We report strong diffraction of cold neutrons from an only 30 micro m thick holographic polymer-dispersed liquid crystal (H-PDLC) transmission grating. The light-induced refractive-index modulation for neutrons is about 10(-6), i.e., nearly 2 orders of magnitude larger than in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs a promising candidate for fabricating neutron-optical devices.