Measurement of the Neutron Lifetime with Ultracold Neutrons Stored in a Magneto-Gravitational Trap

We report a measurement of the neutron lifetime using ultracold neutrons stored in a magneto-gravitational trap made of permanent magnets. Neutrons surviving in the trap after fixed storage times have been counted and the trap losses have continuously been monitored during storage by detecting neutrons leaking from the trap. The value of the neutron lifetime resulting from this measurement is τ_n = (878.3 ± 1.6_stat ± 1.0_syst) A unique feature of this experiment is the monitoring of leaking neutrons providing a robust control of the main systematic loss.     

Detailed analysis and Monte Carlo simulation of the neutron lifetime experiment

We performed a detailed analysis and the Monte Carlo simulation of the neutron lifetime experiment (Phys. Lett. B 483, 15 (2000)) because of the strong disagreement by 5.6 standard deviations between the results of this experiment and our experiment [1]. We found a few effects which were not taken into account in the experiment [3]. The possible correction is -5.5 s with uncertainty of 2.4 s which comes from initial data knowledge. We assume that after taking into account this correction the result of work [3] for neutron lifetime 885.4 ± 0.9_stat ± 0.4_syst s could be corrected to 879.9 ± 0.9_stat ± 2.4_syst s.     

Analysis and correction of the measurement of the neutron lifetime

Corrections have been introduced into the result τ_β = 885.4 ± 0.9_stat ± 0.4_syst s of our measurements of the neutron lifetime. The corrected value is τ_β = 881.6 ± 0.8_stat ± 1.9_syst s.     

Neutron lifetime and the background structure in a neutron magnetic trap

A new cycle of processing the 1985–1986 experimental data concerning the storage of neutrons in a magnetic gravitational trap was carried out. It was shown that the count rates determined for the background are multiples of the neutron decay constant. This is explained by the fact that the background in the experiment on the storage of ultracold neutrons was caused by the electron count from background neutrons penetrating through trap walls. Numerous measurements for a small and varying number of background neutrons in the trap make it possible to extract and use data for determining the mean neutron lifetime. This lifetime turns out to be τtn=900.01±0.15 s in a nonuniform magnetic field.     

超薄Fe(4?)膜磁特性极化中子反射研究

Ultrathin Fe film 200  V/4  Fe/900  V/MgO（100） has been prepared by molecular beam epitaxy （MBE）. The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity mea 关键词： 超薄Fe膜 磁特性 极化中子反射 分子束外延     

Hydrogen diffusion in β-phase titanium iron hydride

NMR pulse techniques have been used to measure proton relaxation times in high purity β-TiFeH_1.03 between 94K and 413K. Recent crystal structure parameters from neutron diffraction studies of orthorhombic β-TiFeD_x have been explicitly included in the analysis of the T₂ relaxation times to obtain more accurate hydrogen diffusion parameters than were possible from previous NMR studies of β-TiFeH_x. The room temperature (300K) hydrogen diffusion constant in β-TiFeH_x is verified to be about 1.2 ± 0.8 × 10^?12cm²/s, which is 10^?4–10^?5times smaller than in many other metal hydrides such as PdH_x or LaNi₅H_x.     

用正电子湮没寿命谱研究凝聚态甲烷的温度关系

在30—150K温度范围内,测量了高纯凝聚态甲烷中的正电子寿命谱随温度的变化。在固态甲烷中,正电子素(简称Ps)在自由体积中形成,形成率为²7%。正-正电子素(其符号为o-Ps)的寿命随温度变化的特征可用o-Ps在热激活空位型缺陷中的捕获来说明,且由Ps捕获模型和实验测得的o-Ps寿命求得缺陷激活能E_a=0.10±0.02eV。在液态甲烷中,Ps形成率高达36%,且o-Ps寿命长达5—7ns,这表明液态甲烷中形成了Ps气泡态。我们用经验公式估算了这种气泡的尺寸及其微观表面张力。 关键词：     

The lifetime of the 2pπu → 1sσg transition of the F2+ center in KCl

The lifetime of the 2pπ_u → 1sσ_g transition for the F₂⁺ center in KCl has been measured (15.8 ± 0.7 ns at 8 K) and found to be constant over the temperature range (8–90 K) investigated. No contradiction is found with the Aegerter and Lüty quantum yield data. A forecast for the lifetime of the 2pσ_u → 1sσ_g transition is also made, within the framework of the H₂⁺ model for the F₂⁺ center in alkali halides.     

Decay of 48K, 49K and 50K

The ⁴⁸K, ⁴⁹K and ⁵⁰K nuclides have been produced in high energy fragmentation and analyzed by mass spectroscopy techniques. Their half-lives have been measured as 6 ± 1 s, 1.1 ± 0.3 s and and 0.7 ± 0.3 s, respectively. The γ-rays from their radioactive decay have been observed and the corresponding γ-intensities measured. The nuclide ⁵⁰K is shown to be a delayed neutron emitter. The antianalog states in the daughter Ca nuclei with a $\text{(1d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}{}^{\mathrm{?}}$ neutron configuration, preferentially populated in the β-decay, have been located. The corresponding $\text{1d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ neutron single-particle energy is found to remain approximately constant for these neutron-rich Ca isotopes.     

Problem of the neutron lifetime measurements

The current status of the measurements of the neutron lifetime has been analyzed. The problem of experimental data discrepancy has been analyzed and systematic errors of some experiments have been found out. The corrected table of measurements is given and a new world average value of neutron lifetime 879.9 ± 0.9 s is presented.     

Magnon dispersion in TbAl2

Slow neutron inelastic scattering has been used to determine the magnon dispersion relation in TbAl₂ at 4.6 K. Energies of magnons in symmetry points are in meV: Γ 1.7±0.1 and 6.0±0.1, X5.1±0.2 (degenerate); L3.0±0.1 and 7.4±0.1 for the acoustic and optical modes respectively. An analysis of the experimental curves in terms of interatomic exchange parameters shows long range order and oscillations in real space.     

Spin-dependent scattering of slow neutrons on polarized 17O

The method of pseudomagnetism has been applied to measure the spin-dependent scattering length b_N of slow neutrons on ¹⁷O nuclei. A powder sample of CaO, enriched with ¹⁷O, has been used. In this sample, dynamic nuclear polarizations of ± 10 % have been achieved at 2.5 T and 0.2 K using paramagnetic impurities created by fast neutron irradiation (F-centers). b_N = 0.12 ± 0.04 fm has been found. This value is close to an earlier shell model calculation.     

Photo-physical properties of anisole: temperature, pressure, and bath gas composition dependence of fluorescence spectra and lifetimes

Anisole is a promising candidate for use as fluorescent tracer for gas-phase imaging diagnostics. Its high-fluorescence quantum yield (FQY) and its large Stokes shift lead to improved signal intensity (up to 100 times stronger) compared with the often used toluene. Fluorescence spectra and effective fluorescence lifetimes of gaseous anisole were investigated after picosecond laser excitation at 266 nm as a function of temperature (296–977 K) and bath gas composition (varying amounts of N₂ and O₂) at total pressures in the range of 1–10 bar to provide spectroscopic data and FQY for applications, e.g., in in-cylinder measurements in internal combustion engines. Fluorescence spectra of anisole extend from roughly 270–360 nm with a peak close to 290 nm at 296 K. The spectra show a red-shift with increasing temperature (0.03 nm/K) and O₂ partial pressure (5 nm from N₂ to air). In the investigated temperature range and in pure N₂ at 1 bar total pressure the effective fluorescence lifetime drops with increasing temperature from 13.3 ± 0.5 to 0.05 ± 0.01 ns. Increasing the total pressure of N₂ leads to a small decrease of the lifetime at temperatures above 400 K (e.g., at 525 K from 4.2 ± 0.2 ns at 1 bar to 2.7 ± 0.2 ns at 10 bar). At constant temperature and in the presence of O₂ the lifetimes decrease significantly (e.g., at 296 K from 13.3 ± 0.5 ns in N₂ to 0.40 ± 0.02 ns in air), with this trend diminishing with increasing temperature (e.g., at 675 K from 1.02 ± 0.08 ns in N₂ to 0.25 ± 0.05 ns in air). A phenomenological model that predicts fluorescence lifetimes, i.e., relative quantum yields as a function of temperature, pressure, and O₂ concentration is presented. The photophysics of anisole is discussed in comparison with other aromatics.     

A new experimental study of the circular polarization of np capture γ-rays

An installation using a light-water neutron trap in the reactor core as a proton target is described. Results of the main and control measurements are presented which permit one to conclude that the parity-violating circular polarization of the γ-rays from the np → dγ reaction is P_γ = (1.8 ± 1.8) × 10^?7.     

Determination of static displacements around non-metal vacancies in NbN1−c and TiC1−c by channeling

The results of channeling experiments, which have been performed on B1-NbN_1?c-single crystals with 0.07 ? c ? 0.18 and on a B1-TiC_0.9-single crystal in order to get information on lattice distortions due to non-metal vacancies, are presented. Angular yield curves were measured at temperatures of 295, 78 and 5 K. Monte-Carlo calculations with a defect model of mean static displacements of all metal atoms were used to analyze the experimental data. Static displacements for the Nb-atoms of 0.14 ± 0.02 Å and for the Ti-atoms of 0.04 ± 0.01 Å have been determined. The results on the Nb- and the Ti-displacements were in good agreement with results obtained by neutron diffraction experiments on B1-NbN_1?c powder samples and by diffuse neutron scattering experiments on B1-TiC- crystals, respectively.     

Measurement of the Rb D2 transition linewidth at ultralow temperature

The Rb D2 linewidth was studied using atoms cooled to a temperature of 50 μK that were contained in a magneto-optical trap. The transmission of a probe laser through the atom cloud was monitored using a CCD detector. The frequency of the probe laser was scanned across the resonance using an acousto-optic modulator. The observed lineshape was very well fitted by a Lorentzian function. The full width half maximum linewidth was examined as a function of the optical depth and the probe laser intensity. The extrapolated value at zero optical depth 6.062 ± 0.017 MHz corresponds to a 5P_3/2 lifetime of 26.25 ± 0.07 ns. This result agrees with lifetimes found in experiments that measured the temporal decay of fluorescence or photoassociation spectroscopy and is somewhat below the result of a relativistic many body perturbation calculation.     

Influence of Ni-concentration on the magnetic structure in Tb(Cu,Ni)2 system

A powder sample of orthorhombic Tb(Cu_0.7Ni_0.3)₂ has been studied by neutron diffraction at T = 4.2 K and above the Curie temperature. It is found that this compound is ferromagnetically ordered at 4.2 K. The magnetic moment of Tb in this compound is (9.2 ± 0.2)μ_B, with components (2.2 ± 0.2, 0, 8.9 ± 0.1)μ_B. The influence of the Ni-concentration on the magnetic structure is discussed in the whole Tb(Cu, Ni)₂ system.     

The antiferromagnetic structure of TbB4

The magnetic structure of the rare earth tetraboride TbB₄ (crystallographic space group P4/mbm) has been determined by neutron diffraction on a polycrystalline sample. Below the experimentally determined Néel temperature of T_N = (43±1) K TbB₄ is ordered antiferromagnetically. The data refinement yielded a magnetic moment value of (7.7 ± 0.2) μ_B/Tb ion at 4.2 K which we interpret as Tb⁴⁺. The magnetic structure is antiferromagnetic collinear with the moments perpendicular to the tetragonal axis.     

The magnetic structures of HoTiO3 and ErTiO3

The magnetic structures of rare-earth titanium perovskites, ErTiO₃ and HoTiO₃, have been determined at 4.2 K by neutron diffraction. The Er³⁺ moment of (8.5 ± 0.5) μ_B lies along [001] and is colinear with the titanium moment of (-0.7 ± 0.3) μ_B. The Ho³⁺ moment of (8.1 ± 0.5) μ_B is inclined at an angle of 24° to the bc plane and 32° to the ab pla so as to produce an antiferromagnetic ordering of the x component and a ferromagnetic ordering of the y and the z components. The titanium moment of (-0.55 ± 0.3) μ_B lies in the bc plane but its precise direction has not been determined.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.