Radio-frequency absorption study of the parameters of high-T c materials irradiated with fast reactor neutrons

The magnetic properties of YBa₂Cu₃O_7−x ceramics of various densities and Bi₂Sr₂CaCu₂O₈ quasicrystals irradiated with neutrons at fluences of 10¹⁶–10¹⁹ cm⁻² are investigated by a radio-frequency method (in the frequency range f=100–150 MHz). The electromagnetic absorption is used to estimate the parameters of a Josephson medium: the lower critical field, the critical current density, the grain size and intergranular distances, and the penetration depth of the magnetic field in the sample. The hysteresis of electromagnetic absorption in the indicated superconductors is investigated; the hysteresis effect can be used to obtain data on flux pinning processes and flux creep dynamics. It is shown that an increase in the critical current density is observed as a result of the injection of radiation-induced pinning centers. Fiz. Tverd. Tela (St. Petersburg) 39, 28–34 (January 1997)     

The intense neutron generator of 14 MeV neutrons

An innovative intense neutron generator of 14 MeV neutrons for the irradiation of future reactor materials is presented. Negative pions are produced inside a 5–10 T magnetic field by an intense deuteron beam interacting with a carbon target. The pions and the muons from pion decay in flight are collected in the backward direction and stopped in a deuterium-tritium-hydrogen target of high density. Using an 18 MW deuteron beam at 1.5 GeV (12 mA=7.5 × 10¹⁶d/s), circa 10¹⁶gt^– /s can be generated, decaying to muons of which up to 10¹⁵ µ^–/s stop in the D/T/H mixture. Assuming X_c=100 fusions per muon, the µCF source produces 14 MeV neutrons with a source strength of up to 10¹⁷ n/s, i.e. a neutron power of 200 kW. The environment of the second target, the neutron source itself, can be made to resemble part of the Tokamak ring to be simulated for irradiation test samples.     

Anomalous absorption of thermal neutrons in copper in the presence of strong mechanical stresses

A study has been made of the interaction between an ultraweak flux of thermal neutrons (I _n=20 n/s·cm²) and a copper cathode during electrolysis in a 1M solution of KOH in H₂O. An effect involving the generation of appreciable elastic and plastic deformations, revealed as transverse bending of the copper plate and damage to its surface, was observed in the copper. It was shown that the average cross section for absorption of thermal neutrons in the copper is increased approximately twenty times compared with its equilibrium value. Fiz. Tverd. Tela (St. Petersburg) 40, 254–259 (February 1998)     

散裂中子源耦合慢化器的中子学研究

利用蒙特－卡罗方法研究了散裂中子源中耦合慢化器的中子学特性。给出了冷中子与热中子慢化器的中子能谱。甲烷慢化器提供了性能非常好的冷中子，在低功率的散裂中子源中得到了应用。计算了慢中子引出的角分布。由于较低的氢密度，液态氢的漫化能力低于水与液态甲烷，但是可以通过增加预慢化器来弥补这一问题。2cm厚的水预慢化器层可以大约减少热量在低温慢化器中的热量沉积33％而不破坏中子特性。     

Properties of straight and curved neutron guide tubes

The properties of straight and curved neutron guides for neutrons of the energy region 4·10^?7 eV≦E≦2·10^?6 eV (very cold neutrons) are investigated. Experimental data for the angular distribution of neutrons emerging from a straight neutron guide are presented and it is shown that they can be theoretically verified using Steyerl's approximation. In the second part we consider a strongly curved neutron guide. The effect of laminating such a guide is investigated experimentally and theoretically and the conditions for optimum transmission are discussed.     

<Emphasis Type="Italic">T</Emphasis>-odd angular correlations in the emission of prompt gamma rays and neutrons in nuclear fission induced by polarized neutrons

Study of the T-odd three-vector correlation in the emission of prompt neutrons from ²³⁵U fission by polarized cold neutrons has been continued at the facility MEPHISTO of the FRM II reactor (Technical University of Munich). The sought correlation was not found within experimental error of 2.3 × 10⁻⁵. The upper limit for the asymmetry coefficient has been set to |D _n | < 6 × 10⁻⁵ at 99% confidence level, whereas for ternary fission correlation coefficient D _α = (170±20) × 10⁻⁵. This limit casts doubt on a model that explains the three-vector correlation in ternary fission by the Coriolis mechanism. At the same time, five-vector correlation in the emission of prompt fission neutrons has been measured, which describes the rotation of the fissioning nucleus at the moment it breaks (ROT effect). At the angle 22.5° to the fission axis, the correlation coefficient was found to be (1.57 ± 0.20) × 10⁻⁴, while at the angle of 67.5° it is zero within the experimental uncertainty. The existence of ROT effect in the emission of prompt fission neutrons can be explained by the anisotropy of neutron emission in the rest frame of the fragment (fission fragments are aligned with respect to the axis of deformation of the fissioning nucleus), similar to the mechanism of ROT effect in the emission of prompt γ-rays.     

场发射冷阴极微栅孔阵列制备技术

实现了一种采用聚苯乙烯纳米球自组装技术和微机械制造技术加工的场发射阴极用亚微米栅极微孔阵列。设计了一套完整的工艺实验方案,首先采用微球自组装技术获得了亚微米级金属网孔掩膜,然后通过反应离子刻蚀技术获得了亚微米栅极孔阵列,从而实现了集成度高、分布均匀的周期性亚微米孔洞阵列的制备,微孔集成度达到108cm-2。实验研究了氧气刻蚀聚苯乙烯微球的规律。采用金属掩膜,四氟化碳干法刻蚀二氧化硅,获得了深度为500 nm的微孔。实验结果证明该工艺方案是一种获得大面积、均匀分布、集成度高的场发射冷阴极栅孔阵列的有效方法。     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Ladders in a magnetic field: a strong coupling approach

We show that non-frustrated and frustrated ladders in a magnetic field can be systematically mapped onto an XXZ Heisenberg model in a longitudinal magnetic field in the limit where the rung coupling is the dominant one. This mapping is valid in the critical region where the magnetization goes from zero to saturation. It allows one to relate the properties of the critical phase (H _c ¹, H _c ², the critical exponents) to the exchange integrals and provide quantitative estimates of the frustration needed to create a plateau at half the saturation value for different models of frustration. Received: 7 May 1998 / Revised and Accepted: 10 July 1998     

Abnormal features of neutron-nucleus low-energy interaction and the mechanism of inhibition of neutron-induced reactions

The features of the distant interaction of thermal and slow neutrons with nuclei are examined. From the Dirac equation analysis it was shown that this interaction is determined by the potential barrier located outside the nucleus. The height of the barrier is proportional to Z ²/A ^4/3 and reaches 110eV for even-even nuclei like U²³⁸ and 25-65eV for even-odd U²³⁵ , U²³³ , Pu²⁴¹ nuclei. This barrier is connected with the non-linear ponderomotive interaction of the neutron abnormal magnetic moment with the strong electric field of a nucleus. The barrier penetrability for thermal neutrons equals 0.9-0.98. For cold neutrons the penetrability decreases greatly and for ultracold ones it becomes very small. At unlimited decrease of the neutron energy, E \( \rightarrow\) 0 , the cross-section of any neutron-nucleus reaction \( \sigma_{{f(tot)}}^{}\) \( \rightarrow\) 0 . So such reactions become impossible. In this work the existence of separated neutron potential wells symmetrically located at r ₀ \( \approx\) (1.3-4.5)×10^-12 cm \( \approx\) (1.7-5)×R from even-odd U²³⁵ , U²³³ , Pu²⁴¹ nuclei is predicted. These wells with depths 0.1-5eV are the result of combined ponderomotive and pure magnetic interactions of the neutron abnormal magnetic moment with nucleus electric and magnetic fields. The presence of distant wells leads to the possibility of the existence of virtual or quasi-stationary neutron-nucleus molecules. Such wells can be virtual traps for thermal and cold neutrons. It was predicted that the neutron halo phenomenon may be connected with such traps.     

Lowering of the Populations of Excited Hydrogen and Helium Atoms in Plasmas Irradiated by a Laser Beam

The populations of excited hydrogen and helium atoms in non-L.T.E. plasmas have been calculated for two different physical situations, namely (i) no external radiation field present, and (ii) strong superposed radiation originating from a CO₂, Ruby or Argon Ion Laser. The radiation field intervenes in the collisional-radiative model via two new terms: Photoionization and stimulated recombination induced by the Laser radiation field. The solutions of the rate equations yield a lowering of the populations when a plasma is irradiated by a Laser beam. The lowering is different for cold recombining and hot diffusion-dominated plasmas. At high radiation densities a saturation effect occurs, since the photoionization rate becomes equal to the rate of stimulated recombination. Measurements of the populations of excited H and He atoms in a glow discharge irradiated by a CO₂ Laser beam of power density 10⁵ W/cm² are in broad agreement with the theoretical predictions.     

Interference of fission amplitudes of neutron resonances and <Emphasis Type="Italic">T</Emphasis>-odd asymmetry for various prescission third particles in the ternary fission of nuclei

Differential cross sections for reactions of the true ternary fission of nuclei that was induced by cold polarized neutrons were constructed with allowance of the effect that Coriolis interaction and the interference between fission amplitudes of neutron resonances excited in fissile nuclei upon incidentneutron capture by target nuclei exerted on angular distributions of prescission third particles (alpha particles, neutrons, or photons). It is shown that T -odd TRI- and ROT-type asymmetries for prescission alpha particles are associated with, respectively, the odd and even components of the Coriolis interaction-perturbed amplitude of angular distributions of particles belonging to the types indicated above. These asymmetries have angular distributions differing from each other and stemming from a nontrivial dependence of these components on the neutron-resonance spins J _s and their projections K _s onto the symmetry axis of the nucleus involved. It is shown that angular distributions of prescission photons and neutrons from reactions of the ternary fission of nuclei that is induced by cold polarized neutrons are determined by the effect of Coriolis forces exclusively. Therefore, the emerging T-odd asymmetries have a character of a ROT-type asymmetry and are universal for all target nuclei.     

Rotation of neutron spin in passing through a noncentrosymmetric single crystal

Rotation of the spin of cold neutrons passing through a noncentrosymmetric single crystal is observed. This effect is caused by the Schwinger interaction of the magnetic moment of a moving neutron with the crystalline electric field in a noncentrosymmetric single crystal and depends both on the direction of neutron trajectory in the crystal and on its energy. It is shown that the characteristic magnitude of the effect for α-quartz is ?(1–2)×10^?4 rad/cm over a wide wavelength range (from 2.8 to 5.5 Å) and is determined by the degree of beam monochromaticity [Δλ/λ=(2–5)×10^?2 in our experiment]. This magnitude corresponds to an electric field of ?(0.5–1)×10⁵ V/cm acting on a neutron. The measured value agrees well with the theoretical calculation.     

Reaction kinetics in polymer melts

We study the reaction kinetics of end-functionalized polymer chains dispersed in an unreactive polymer melt. Starting from an infinite hierarchy of coupled equations for many-chain correlation functions, a closed equation is derived for the 2nd order rate constant k after postulating simple physical bounds. Our results generalize previous 2-chain treatments (valid in dilute reactants limit) by Doi [#!doi:inter2!#], de Gennes [#!gennes:polreactionsiandii!#], and Friedman and O'Shaughnessy [#!ben:interdil_all_aip!#], to arbitrary initial reactive group density n₀ and local chemical reactivity Q. Simple mean field (MF) kinetics apply at short times, .For high Q, a transition occurs to diffusion-controlled (DC) kinetics with (where x_t is rms monomer displacement in time t) leading to a density decay . If n₀ exceeds the chain overlap threshold, this behavior is followed by a regime where during which k has the same power law dependence in time, , but possibly different numerical coefficient. For unentangled melts this gives while for entangled cases one or more of the successive regimes ,t ^-3/8 and t ^-3/4 may be realized depending on the magnitudes of Q and n₀. Kinetics at times longer than the longest polymer relaxation time are always MF. If a DC regime has developed before then the long time rate constant is where R is the coil radius. We propose measuring the above kinetics in a model experiment where radical end groups are generated by photolysis. Received: 2 June 1998 / Revised: 9 July 1998 / Accepted: 10 July 1998     

Hall effect plateaus and giant Shubnikov-de Haas oscillations in (BiSb)Telayered single crystals near the quantum limit

On bulk layered single crystals (Bi_0.25Sb_0.75)₂Te₃ with a hole concentration cm^-3 and a mobility cm²/Vs magnetoresistance and Hall effect investigations were performed in the temperature range T = 1.4 K ... 20 K in magnetic fields up to 18 T. For the magnetic field perpendicular to the layered structure giant Shubnikov-de Haas oscillations are measured; the positions of the maxima are triplets in the reciprocally scaled magnetic field. From the damping of the amplitudes with increasing temperature the cyclotron mass m _c = 0.12m ₀ is evaluated. Correlated with the SdH oscillations doublets of Hall effect plateaus (or kinks in low fields) are found. The weak well known Shubnikov-de Haas oscillations from the generally accepted multivallied highest valence band can be detected as a modulation on the giant oscillation. The high anisotropy of the SdH oscillations and their triplet structure in connection with the layered crystal structure lead us to suggest that the effects are caused by hole carrier pairing (mediated by the bipolaron mechanism) in quasi 2D sheets parallel to the crystal layer stacks. The measured Hall plateau resistances coincide with the quantum Hall effect values considering the number of layer stacks and the valley degeneracy of the 3D hole carrier reservoir. The ratio of spin splitting to Landau (cyclotron) splitting is observed to be . Received: 12 September 1997 / Revised: 8 January 1998 / Accepted: 22 January 1998     

Project of the ultracold and cold neutron source at the WWR-M reactor with superfluid helium as a moderator

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release (～4 × 10^?3 W/g) at a sufficiently high neutron flux (～3 × 10¹² neutrons/(cm² s)). This opportunity can be implemented in the reactor thermal column, which represents a 1-m-diameter channel adjacent to the reactor core. This diameter of the channel allows the arrangement of the core gamma shielding made of bismuth (15 cm thick), a graphite premoderator (300 dm³) at a temperature of 20 K, and a converter with superfluid helium (35 dm³) at a temperature of 1.2 K. Calculations show that the heat release in the source (20 W) can be removed by pumping helium vapor, and the density of ultracold neutrons in an experimental trap will be ～10⁴ neutrons/cm³, which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.     

Studies of parity and time reversal symmetries in neutron scattering from165Ho

We describe searches for parity and time reversal violations in the scattering of polarized neutrons from polarized and aligned¹⁶⁵Ho targets. We have completed a search with 7.1 and 11.0 MeV neutrons for P_oddT_odd terms in the elastic scattering forward amplitude of the form s. (I×K), wheres is the neutron spin,I is the target spin andk is the neutron momentum vector. The target was a single crystal of holmium, polarized horizontally along itsb axis by a 1 Tesla magnetic field. The neutrons were polarized vertically. Differences in the neutron transmission were measured for neutrons with spins parallel (antiparallel) toI×k. The P,T violating analyzing powers were found to be consistent with zero at the few 10⁻³ level: ρ_P,T(7.1 MeV)=−0.88 (±2.02) x 10⁻³, ρ_P,T(11.0 MeV)=−0.4 (±2.88) x 10⁻³. We have also attempted to find enhancements with MeV neutrons in P-violation due to the term s\k. We are preparing an aligned target cryostat for investigations of P_evenT_odd terms {bd(I\k)(I×k)\s} in neutron scattering. The target will be a single crystal cylinder of¹⁶⁵Ho cooled to 100 mK in a bath of liquid helium and rotated by a shaft from a room temperature stepping motor. The cylinder will be oriented vertically and the alignment (c) axis oriented horizontally. Warming or rotation of the sample allows one to separate effects that mimic the sought-after time reversal violating term.     

Field-electron emission from polyimide-ablated films

Polyimide-ablated film was deposited by using pulsed laser ablation of a polyimide target, and field-electron emission from the film was observed for the first time. The turn-on field of the polyimide-ablated film is 12 V/μm. The current density is 0.725 mA/cm², and the emission sites density is on the order of 10⁶/cm² at the applied field of 24 V/μm. The field-electron emission measurements indicate that this kind of film could be a new cold cathode material. It is suggested that the graphite-like clusters contained in the film play an important role in the field-electron emission. Received: 2 February 2000 / Accepted: 13 March 2000 / Published online: 9 August 2000     

The scission neutron spectrum of252CF (SF)

The scission neutron spectrum was obtained as a difference between the integral experimental fission neutron spectrum evaluated by Mannhart based on measurements in the laboratory reference system from several authors in the different neutron energy ranges, and the laboratory fission neutron spectrum stemming only from those neutrons which are evaporated from the fully accelerated fission fragments. The scission neutron spectrum was represented by a Weisskopf evaporation spectrump _s(? _l )=P ₀/(T _s)²·?_l·exp(??_l/T_s). A least squares fit gave for the fraction of scission neutrons with respect to all fission neutrons a value ofp ₀=(0.011±0.003) and for the pseudo scission neutron temperature a value ofT _s=(0.20±0.03) MeV. The low pseudo temperature is compatible with a cold nuclear matter of the prescission configuration. The low energy of the scission neutrons indicates that they come from a slowly moving source. The width of the distribution suggests that the source is small: radius of about 5 fm. This information conforms with the idea of satellite droplets which are formed when the neck snaps.     

Inverse beta decay of arbitrarily polarized neutrons in a magnetic field

We calculate the cross-section of the inverse beta decay process,v _e+n → p+e, in a background magnetic field which is much smaller than m _p ^2/e . Using exact solutions of the Dirac equation in a constant magnetic field, we find the cross-section for arbitrary polarization of the initial neutrons. The cross-section depends on the direction of the incident neutrino even when the initial neutron is assumed to be at rest and has no net polarization. Possible implications of the result are discussed.