Ultra-cold neutron production with superfluid helium and spallation neutrons

Ultra-cold neutrons (UCN) production in superfluid helium with spallation neutrons is discussed. A source is described, where superfluid helium is located in a cold moderator of deuterium at 20 K surrounded by a thermal moderator of heavy water at 300 K. A lead target is installed in the thermal moderator for neutron production via a medium energy proton induced spallation reaction. A Monte Carlo simulation showed that a UCN density of the order of 10⁵ n/cm³ is achievable with an acceptable heat load for the helium cryostat.     

Spin exchange rate constant for collisions of metastable helium atoms with rubidium atoms

The spin exchange rate constant C _se in the He(2³S₁)-Rb(5²S_1/2) system has been measured for the first time in experiments on the optical orientation of metastable helium atoms in the presence of rubidium atoms. In the temperature interval T = 293–348 K, this value is C _se = (1.8 ± 0.4) × 10^?9 cm³ s^?1. The chemiionization rate constant, which has been simultaneously measured in collisions of these particles, is C _ci = (3.1 ± 0.6) × 10^?9 cm³ s^?1.     

Electrical properties of silver selenide thin films prepared by reactive evaporation

The electrical properties of silver selenide thin films prepared by reactive evaporation have been studied. Samples show a polymorphic phase transition at a temperature of 403 ± 2 K. Hall effect study shows that it has a mobility of 2000 cm²V^?1s^?1 and carrier concentration of 10¹⁸ cm^?3 at room temperature. The carriers are ofn-type. X-ray diffraction study indicates that the as-prepared films are polycrystalline in nature. The lattice parameters were found to bea= 4.353 Å,b= 6.929 Å andc = 7.805 Å.     

Direct nn-Scattering Measurement With the Pulsed Reactor YAGUAR

Although crucial for resolving the issue of charge symmetry in the nuclear force, direct measurement of nn-scattering by colliding free neutrons has never been performed. At present the Russian pulsed reactor YAGUAR is the best neutron source for performing such a measurement. It has a through channel where the neutron moderator is installed. The neutrons are counted by a neutron detector located 12 m from the reactor. In preliminary experiments an instantaneous value of 1.1 × 10¹⁸/cm²s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as International Science & Technology Center (ISTC) project No. 2286.     

On the Measurement of the Neutron Lifetime Using Ultracold Neutrons in a Vacuum Quadrupole Trap

We present a conceptual design for an experiment to measure the neutron lifetime (~886 s) with an accuracy of 10⁻⁴. The lifetime will be measured by observing the decay rate of a sample of ultracold neutrons (UCN) confined in vacuum in a magnetic trap. The UCN collaboration at Los Alamos National Laboratory has developed a prototype UCN source that is expected to produce a bottled UCN density of more than 100/cm³ [1]. The availability of such an intense source makes it possible to approach the measurement of the neutron lifetime in a new way. We argue below that it is possible to measure the neutron lifetime to 10⁻⁴ in a vacuum magnetic trap. The measurement involves no new technology beyond the expected UCN density. If even higher densities are available, the experiment can be made better and/or less expensive. We present the design and methodology for the measurement. The slow loss of neutrons that have stable orbits, but are not energetically trapped would produce a systematic uncertainty in the measurement. We discuss a new approach, chaotic cleaning, to the elimination of quasi-neutrons from the trap by breaking the rotational symmetry of the quadrupole trap. The neutron orbits take on a chaotic character and mode mixing causes the neutrons on the quasi-bound orbits to leave the trap.     

Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid ⁴He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an 833−63+74s trap lifetime, consistent with the currently accepted value of the free neutron lifetime.     

Stabilization of High Concentrations of Nitrogen Atoms in Impurity-Helium Solids

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ～8×10²⁰ cm^?3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 10¹⁹ cm^?3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.     

The effects of neutron irradiation damage on the superconducting properties of Nb3Sn

Specimens of Nb₃Sn have been irradiated by fast neutrons at 70°C to doses in the range 3 × 10²¹ neutrons m^?2 to 9 × 10²³ neutrons m^?2. Their critical temperatures were depressed linearly with dose, to less than 4.2 K after about 3 × 10²³ neutrons m^?2. The critical temperature recovered to their initial values in anneals of 2 hours at 900°C, and in 64 hours at 750°C.The critical current can be enhanced by low neutron doses, particularly at high fields, but is always depressed by higher doses.The observations are shown to be consistent with a qualitative model, and in the light of this the likely consequences of irradiation at operating temperature are considered.     

Charge carrier transport in gate-voltage-controlled heteroepitaxial indium arsenide layers

The charge carrier transport coefficients of InAs epilayers, grown on semi-insulating GaAs by chemical vapor phase heteroepitaxy, were investigated by means of gate-voltage-controlled electrical and galvanomagnetic measurements made on metal-oxide-semiconductor structures. The capacitance versus gate voltage dependence of such structures indicates that in the extrinsic temperature region the epilayer surfaces are accumulated for V_g = 0 and flat-band conditions apply for V_g ≈?31 V. It is shown that if the epilayer thickness is corrected for depletion then the epilayer Hall coefficients and conductivities are independent of V_g and have bulk-like values and that the electron mobility has its bulk-like value and is independent of V_g in depletion. In accumulation, the epilayer properties are considered in terms of a composite two-layer model: a bulk-like region of thickness d_b with an average flat-band electron density n_b = 2.5 × 10¹⁵ cm^?3 and mobility μ_b = 7.5 × 10⁴ cm² V^?1 s^?1 and a surface-like region of thickness d_s with a gate-voltage-dependent surface charge density n_sd_s and mobility μ_s where n_sd_s (+30 V) = 2.06 × 10¹² cm^?2 and μ_s(+30 V) = 1.47 × 10⁴ cm² V^?1 s^?1. The monotonic decrease in μ_s with V_g is attributed to scattering of the conduction electrons by localized surface charges which decrease the specularity of the epilayer surfaces.     

Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)

The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte-Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of Φ26×26 cm² with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D⁺ beam line in order to reduce the fast neutron and the γ ray components in the imaging beam. When the neutron yield is 3×10¹² n/s and the L/D is 50, the thermal neutron flux is 5×10⁵ n/cm²/s at the imaging plane, the Cd ratio is 1.63 and the n/γ ratio is 1.6×10¹⁰ n/cm²/Sv.     

Energy Release Channels During Destruction of Impurity-Helium Condensates

Injection of an impurity-helium gas jet passed through a radiofrequency discharge into a volume of superfluid helium leads to the growth of nanoclusters of impurity species which form impurity-helium condensates (IHCs). IHCs are porous materials with very low impurity density (～10²⁰ cm^?3). High average concentrations of stabilized free radicals can be achieved on the large total surface (～100 m²/cm³) of impurity nanoclusters. Warming of the IHCs leads to the destruction of the samples and formation of excited atoms and molecules as a consequence of the recombination of stabilized free radicals. We studied the influence of the nitrogen content in neon-helium and krypton-helium gas mixtures on the thermoluminescence spectra accompanying the destruction of the IHC samples, which were formed by using these gas mixtures. The energy release channels in the IHC samples were revealed from analysis of the thermoluminescence spectra.     

Cryogenically pumped laboratory metallurgical installation

The paper is concerned with the laboratory metallurgical installation designed for heat treatment and zone melting of refractory metals without a crucible. A 10kW electron-beam gun serves as a heater. The installation is evacuated from the atmospheric pressure to the ultimate vacuum using cryopumps only. Particular attention is given to hydrogen pumping. Performances of helium adsorption and condensation cryopumps with an efficiency up to 10⁴/s^?1 for hydrogen have been studied experimentally. The consumption of liquid helium was found to be 40–50cm³ h^?1 for adsorption pumps and 7cm³h^?1 for condensation pumps. It is shown that the application of the condensation pump with a surface cooled down to 2.3 K seems to be most efficient.     

Thermonuclear fusion neutrons under “Currentless plasmas” in Heliotron E

Thermonuclear fusion neutron emissions of up to 3 × 10⁹ neutrons/s have been observed in Heliotron E deuterium plasmas. “Currentless plasmas” were produced by electron cyclotron resonance (53 GHz, 150 kW) and further heated by hydrogen neutral beam injection (2.2 MW, H⁰→D⁺). It is found that all observed neutron emissions have thermonuclear origin due to the absence of hard X-ray background. Agreement between neutron and charge exchange ion temperature measurements (500 eV < T_i(0) × 900 eV) have been found at intermediate densities $\text{(1.5 ×}\text{n}{}_{\mathrm{<mtext>e</mtext>}}\text{× 3 × 10}{}^{13}\text{cm}{}^{\mathrm{?3}}\text{)}$. The neutrons reported in this paper are the first observations of pure thermonuclear fusion neutrons in a helical heliotron plasma confinement device.     

Decay of Turbulence Generated by Spin-Down to Rest in Superfluid 4He

We report on the extension of the experiments (P.M. Walmsley et al., Phys. Rev. Lett. 99:265302, 2007) on the decay of quasiclassical turbulence generated by an impulsive spin-down from angular velocity Ω to rest of superfluid ⁴He in a cubic container at temperatures 0.15 K–1.6 K. The density of quantized vortex lines L is measured by scattering negative ions. Following the spin-down, the maximal density of vortices is observed after time t～10Ω^?1. By observing the propagation of ions along the axis of the initial rotation, the transient dynamics of the turbulence spreading from the perimeter of the container into its central region is investigated. Nearly homogeneous turbulence develops after time t～100Ω^?1 and decays as L ∝ t ^?3/2. The effective kinematic viscosity in T=0 limit is ν=0.003κ, where κ=10^?3 cm²?s^?1 is the circulation quantum.     

Fish-Tail Effect and Its Evolution Under Neutron Irradiation in Li-Doped YBa2Cu3O7−x

The evolution of the critical current density of Li-doped YBa₂Cu₃O_7?x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 ≤ B ≤ 6 T) temperature (5 ≤ T ≤ 85 K) and neutron fluence (0 ≤ Φ ≤ 9.98 × 10¹⁷ cm^?2). At fluences lower than 10¹⁷ cm^?2, a second peak in j _s vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10¹⁷ cm^?2, the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B _infl. A dependence of B _infl on fluence is proposed.     

Effect of neutron radiation on the photoelectric parameters of <Emphasis Type="Italic">p-n</Emphasis>-InSe structures

The effect of irradiation with fast reactor neutrons at an effective energy of 1 MeV and a fluence within Φ = 1 × 10¹⁴?5 × 10¹⁵ n/cm² on the photoelectric parameters of p-n-InSe homojunctions obtained in direct optical contact between p- and n-type semiconductors has been studied. The exposure to fast neutrons leads to an increase in the rectification coefficient and the diode ideality factor of the current-voltage characteristics with increasing neutron dose. No significant changes have been observed in the photosensitivity spectra of p-n-InSe structures irradiated to various doses, which allows these structures to be recommended for the creation of radiation-resistant photodetectors.     

Lattice parameter study of silicon uniformly doped with boron and phosphorus

Powder and single-crystal X-ray techniques have been employed to obtain precise lattice parameters of silicon uniformly doped with boron or phosphorus. Good agreement is found between the two methods. Previous accurate determination of the CuKα₁, effective wavelength has yielded λ=1.540621±0.000006 Å. Particular care has been devoted to the chemical and electrical characterization of the alloys, whose maximum dopant concentrations were 8×10¹⁹ atoms cm^?3 for P and 4.4×10²⁰ atoms cm^?3 for B. A linear dependence of lattice parameter on concentration has been found for P in the whole examined range, while for B a deviation from the linear trend starts at about 2.25×10²⁰ atoms cm^?3. Tetrahedral radii are found to be 1.176 Å for pure Si, 1.07 Å and 0.91 Å respectively for dissolved substitutional P and B. Values of the linear lattice contraction coefficient, volume size factor, Vegard's law factor and elastic strain energy in both alloys are reported and discussed. The deviation from linear trend in borondoped alloys is analysed and it is shown that the phenomenon is insensitive to heattreatments and does not depend on the degree of ionization of boron atoms.     

Experimental modeling of fast neutron-beam impact on Pt using Ar+ beams

Using the methods of field ion microscopy, we studied radiation-induced defects on an atomically clean surface and within a subsurface volume of platinum initiated by the interaction of neutron (E > 0.1 MeV) and Ar⁺ beams (E = 30 keV). It is shown that interaction of fast neutrons (E > 0.1 MeV, F = 6.7 × 10²¹ m^?2, and 3.5 × 10²² m^?2) with platinum leads to the same radiation damage in the volume of Pt as that produced by beams of Ar⁺ ions (E = 30 keV, F = 10¹⁶ ion/cm²) and is observed at a depth of about 1.5–2 nm beneath the irradiated Pt surface. Thus, we have carried out modeling of neutron impact with matter when replacing the neutron beam by an ion beam that causes the same radiation damage in the bulk of the material.     

Transmission of slow neutrons (8–15 Å) through superfluid liquid helium

Transmission of very slow neutrons through superfluid liquid helium has been measured from 8 to 15 Å neutrons at temperatures between 0.45 K and 2.4 K. The results are compared with the single phonon emission and absorption theory.     

New ceramic material for thermal and dielectric isolation at low temperatures

A new perovskite material has been found, Pb $\text{(}\text{Mn}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{Ta}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)O}{}_3$ which has the smallest thermal diffusivity of any known, densified solid at low temperatures (9 × 10^?3 < k < 5 × 10^?2 cm² sec^?1 between 4–10 K). This results from a large specific heat (～ 0.1 J cm^?3 K^?1 at 10 K) and small thermal conductivity (～ 0.7 mW cm^?1 K^?1 at 10 K). The large specific heat is due to the combination of a small Debye temperature, 149 K, a low-lying Einstein mode, 4.52 cm^?1, and a large density, 9.68 gm cm^?3. The 4.52 cm^?1 mode is believed to be due to a small concentration of oxygen vacancies in the ceramic. The thermal conductivity is limited by phon scattering from localized excitations characterized by phonon mean free paths on the order of the lattice constant (4 Å). It is suggested that the local excitations may be associated with ferroelectric micro-regions. The dielectric constant and loss tangent at 4.2 K and 1 KHz are 351.7 and 0.0073, respectively, and show very small dispersion. The thermal expansion coefficient almost exactly matches that of Nb metal from 300 to 77 K (the lower limit of the measurement).