同位旋对用裂变截面探测鞍点前摩擦的影响

耦合了轻粒子发射的随机Langevin模型被用于计算三个Bi裂变系统因核耗散效应引起的相对于标准统计模型预计的裂变截面值的变化,σ_fdrop作为激发能的函数。发现209Bi的σ_fdrop要大于202Bi和195Bi的σ_fdrop,这表明高同位旋系统209Bi的裂变截面对摩擦强度有更大的敏感性。该结果建议,当实验上使用裂变激发函数来更精确地提取鞍点前摩擦强度时,应产生具有高同位旋的复合核。进而,通过Langevin计算拟合p+206Pb和p+209Bi反应的裂变激发函数数据,提取的鞍点前摩擦强度为（3~5）×1021 s-1。Langevin models that are coupled with particle emission are applied to calculate the drop of fission cross section caused by friction over its standard statistical-model value, σ_fdrop, as a function of excitation energy for three Bi nuclei with different isospins. We find that σ_fdrop become larger for 209Bi than for 202Bi and 195Bi, indicating a greater sensitivity of fission cross section to friction for high-isospin 209Bi. These results suggest that, to more accurately determine the presaddle dissipation strength by measuring fission excitation functions, it is best to populate those compound systems with a high isospin. Furthermore, we analyze the data of fission excitation functions measured in p + 209Bi and p + 208Pb reactions, which yield high-isospin 210Po and 209Bi systems, and extract a presaddle friction strength of (3-5)×1021 s-1 by comparing Langevin simulations with these experimental data.     

Nucleon-nucleon final-state interactions in the reactions He(p, d)2p, He(p, t)2p and He(p, He)pn

The energy spectra of deuterons, tritons and ³He particles from the reactions ³He(p, d)2p, ⁴He(p, t)2p and ⁴He(p, ³He)pn have been measured at angles between 6° and 60° lab. The ³He(p, d)2p reaction was studied at both 30.5 and 49.5 MeV incident proton energies, while the other two reactions were studied at 49.5 MeV only. The energy spectra are compared with calculations based on the Watson-Migdal model of final-state interactions.     

Adsorption of He on He crystal surfaces

We have compared the surface properties of crystals respectively grown from normal ⁴He containing 130 ppb of ³He and from ultrapure ⁴He (0.4 ppb). Below 0.4 K, ³He impurities are found to decrease both the surface tension and the step energy. Our results are consistent with the existence of two-dimensional bound states for ³He atoms at the solid-liquid interface. Quantitative agreement with the data is found with a binding energy _s ≈ 4.3 K and a ³He density saturating around 0.4 monolayer. The presence of steps is found to increase the binding energy by about 10 mK.     

He particle emission in fast neutron induced reactions

Cross sections for some (n, ³He + dp + n2p) and (n, + 2n2p + n³He + pt + dd + dnp) reactions induced by fast neutrons produced via breakup of 53 MeV deuterons on a Be target (E_n = 4–50 MeV; I_max at 22.5 MeV; FWHM = 15.8 MeV) were measured for isotopes of the elements I, La, Nd, Tb, Ho, Yb, W and Au by the activation technique using high-resolution γ-ray spectroscopy, wherever necessary chemical separation, and in several cases enriched isotopes as targets. Furthermore, ³He/ emission cross-section ratios were measured for Co, Ag, In, Cs, La, Ta, Au and Bi using a quadrupole mass spectrometer. The cross sections decrease as a function of increasing (N−Z)/A of the target nucleus. For target elements with Z 44, the ³He/ emission cross-section ratio increases with increasing Z; for elements with Z > 44, however, the trend is reversed. Hauser-Feshbach calculations suggest that the ³He/ emission cross-section ratios are not explainable by the statistical model.     

Measurements of interaction cross sections and radii of He isotopes

Secondary beams of ³He, ⁴He, ⁶He, and ⁸He were produced through the projectile fragmentation of an 800 MeV/nucleon ¹¹B primary beam. Interaction cross sections (σ_I) of all He isotopes of 790 MeV/nucleon on Be, C, and Al targets were measured by a transmission-type experiment. The interaction nuclear radii of He isotopes R_I(He) = (σ_I/π)^1/2 − R _I(T) where R_I(T) is the radius of the target nucleus, have been deduced to be R_I(³He) = 1.59 ± 0.06 fm, R_I(⁴He) = 1.40 ± 0.05 fm, R_I(⁶He) = 2.21 ± 0.06 fm, and R_I(⁸He) = 2.52 ± 0.06 fm.     

Pionic K X-rays in liquid He

Energies and lorentzian widths of pionic K X-ray transitions have been measured in liquid ³He with improved accuracy. The strong interaction on the π³He 1s level is found to be attractive and to produce an increase in the K transition energies of 34 ± 4 eV; the measured lorentzian width is 36 ±7 eV. Measured values are also presented for K X-ray energies in liquid π⁴He and μ⁴He, the lorentzian width of the 1s level in π⁴He, and relative intensities of K X-ray transitions in π³He, π⁴He and μ⁴He. The measurements are compared with those of others and with recent theoretical calculations.     

Studies on the track formation mechanism of the heavy ions in CR-39

The bulk etch rate for two types of CR-39 detector was measured as a function of temperature and the activation energies of bulk etching was determined. Experimental values of track etch rate were derived directly from the function of the succesive measured track length vrs. etching time for ²⁰⁹Bi, ¹²⁹Xe and ²⁰Ne ions.

The maximum etchable length of 13 MeV/u ²⁰⁹Bi and 13.04 MeV/u ¹²⁹Xe ions have been measured at and below these energies. A comparison of the measured and calculated track length data is presented.     

Electron screening effect in the reactions He(d, p)He and d(He, p)He

The cross section of the reactions ³He(d, p)⁴He and d(³He, p)⁴He has been measured at the center-of-mass energies E=5 to 60 keV and 10 to 40 keV, respectively. The experiments were performed to determine the magnitude of the electron screening effect leading to the respective electron-screening potential energy U_e=219±7 and 109±9 eV, which are both significantly higher than the respective values from atomic physics models, U_e=120 and 65 eV.     

Nonwetting of He and dilute He–He mixtures on Cs

For pure ⁴He on Cs a nonwet phase exists below ≈2 K but for dilute ³He–⁴He mixtures the wetting is reentrant. Measurements of the contact angle with dilute mixtures of liquid helium on Cs can be explained in detail in terms of ³He states together with ripplons at the Cs–He mixture interface, but it is impossible to account for the lower wetting temperatures or contact angles with only ³He states or ripplons. We discuss the influence of surface roughness of the Cs on the contribution of interface excitations to the free energy and suggest that the variety of contact angles found with different samples of Cs is due mainly to their degree of roughness.     

Double-Differential Cross Section of 5He Emission

The probability of ⁵He particle emission has been affirmed theoretically [J.S. Zhang, Science in China G47 (2004) 137]. In order to describe the ⁵He emission, the theoretical formula of the double-differential cross section of emitted ⁵He is to be established. Based on the pick-up mechanism, used for calculating the formula of d, t, ³He, α emissions, the theoretical formula of double-differential cross section of ⁵He is obtained, which is expressed in the form of Legendre coefficients. In the case of low incident energies, the configuration [J.S. Zhang, Science in China G47 (2004) 137; J.S. Zhang, Commun. Theor. Phys. (Beijing, China) 39 (2003) 83] is the dominant part in the reaction processes. The calculated result indicates that the forward peaked angular distribution of the composite particle emission is weaker than that of the emitted single nucleon due to pick-up nucleon from the Fermi sea. As an example, the reactions of n+¹⁴N have been calculated, and the Legendre coefficients of d, t, ³He, α, ⁵He emissions are obtained respectively. The results show that the forward tendency is decided by the average momentum per nucleon in the emitted composite particles. The larger the average momentum is, the stronger the forward tendency is.     

He superfluidity in the presence of aerogel

Aerogels introduce disorder into the p-wave-paired superfluid ³He and suppress T_c. Quantifiable (by small angle X-ray scattering) differences in the long-range structure of two identical density aerogels are primarily responsible for their different transition temperatures. We also demonstrate that alteration of the short-range correlations by the addition of ⁴He does not strongly affect T_c. Acoustic measurements of the fast and slow modes of ³He in aerogel are described. These can be used to explore the superfluid component. We also outline future prospects.     

The quasi-free breakup of He

A DWBA formalism is presented for the quasi-free breakup of ³He into elastic, inelastic and particle-transfer channels. The model assumes that only a subset of the projectile interacts strongly with the target while the remaining fragment acts as a spectator. The full DWBA is reduced to a rather transparent quasi-free reaction mechanism. Distortion of the spectator wave function due to the Coulomb field of the target is included. Calculations of angular correlations and projected energy spectra resulting from processes involving the binary breakup of ³He on ²⁸Si at E(³He) = 52 MeV are, apart from a normalization factor, in good agreement with the data.     

Remnant magmatic activity in the Coastal Range of East Taiwan after arc–continent collision: fission-track data and He/He ratio evidence

The magma activity of the North Luzon Arc is considered to have ceased, due to collision with the Asian continental margin, since late Miocene. New fission track dates of zircons taken from dike swarms in the central Coastal Range of East Taiwan show very young ages of 0.5 Ma. Furthermore, the high ³He/⁴He ratios (2.5 R_A; R_A is the air ratio) of hot spring gases near the same area indicate that more than 30% of a mantle-derived source component is necessary to account for the helium composition. These results suggest the existence of later remnant magmatic activity (more than 4 Ma) in the central Coastal Range. Therefore, arc magmatism may survive longer than the previously expected after collision.     

Are the solar-neutrino experiments suggestive of the existence of a resonance in the He + He system?

The possibility of the existence of a resonance in the ³He + ³He system, which may be important for explaining the solar-neutrino experiments, is discussed.     

极化3He系统核磁共振FID信号的激光探测

极化率测量装置是极化³He系统的重要组成部分之一.本文介绍了一套在极化³He系统上搭建的激光探测自由感应衰减核磁共振（FID NMR）信号的实验装置,该装置有望为³He极化率的高精度实时测量提供新的途径.激光探测法的原理是法拉第旋光效应,该方法使用一束线偏振光探测极化³He磁矩绕主磁场进动的FID信号,实验结果表明相较于拾波线圈探测方法,激光探测方法的信噪比提高了106%,对³He的极化率测量更精确.该探测方法有望替代传统的拾波线圈,广泛地应用于³He极化系统的极化率测量或精密测量实验中.     

Electrons on superfluid He

Low-frequency transport properties of the Wigner solid on normal and superfluid ³He have been studied. It is known that the Wigner solid deforms the free surface of liquid He to form a dimple lattice. The low-frequency resistivity is successfully interpreted by the ballistic and specular scattering of ³He quasiparticles from the dimple lattice. The resistivity decreases abruptly below the superfluid transition temperature. For superfluid ³He-A, the l-texture, presumably perpendicularly oriented to the free surface, has an essential influence on the resistivity.     

光泵抽运3He极化程度监控系统的设计与实现

极化³He的一项重要应用是中子的极化.中国绵阳研究堆（CMRR）已建立国内首个自旋交换光学泵浦（SEOP）极化³He中子极化系统.为了监测³He的极化率随时间的相对变化情况,本文首先设计了基于核磁共振（NMR）技术的³He相对极化率测量系统,通过Matlab控制程序实现了对³He相对极化率的定时检测.然后对拾波线圈的构形和信噪比（SNR）进行了优化.结果表明当绕线长度一样时,Brooks构形的线圈有利于提高SNR;当线圈的平均半径为（a₀+d）/√2（a₀为³He气室的半径,d为拾波线圈与气室之间的距离）时,其SNR最高.最后对该系统的本底噪声进行了测量,发现其主要来源于环境噪声（0.27 μV/√Hz）和数据采集（DAQ）卡的噪声（0.40 μV/√Hz）,系统的总噪声功率谱密度约为√0.16+0.073G² μV/√Hz（G为放大器的增益倍数）.     

Effect of He pre-plating on third sound in superfluid He

We recently reported observation of third sound resonances in superfluid films of pure ³He on a disk-shaped polished copper substrate. From the observed spectra of resonant modes, the third sound speed, the average superfluid density, and information on dissipation were obtained. The film thicknesses studied were larger than or comparable to the superfluid coherence length, which is near 100 nm. We describe preliminary results from efforts aimed at using third sound in thinner films on a substrate which has been coated with a few atomic layers of ⁴He.     

Neutron fluences and dose equivalents measured with passive detectors on LDEF

Neutron fluences were measured on LDEF in the low energy (< 1 MeV) and high energy (> 1 MeV) ranges. The low energy detectors used the ⁶Li(n,)T reaction with Gd foil absorbers to separate thermal (< 0.2 eV) and resonance (0.2 eV−1 MeV) neutron response. High energy detectors contained sets of fission foils (¹⁸¹Ta, ²⁰⁹Bi, ²³²Th, ²³⁸U) with different neutron energy thresholds. The measured neutron fluences together with predicted spectral shapes were used to estimate neutron dose equivalents. The detectors were located in the A0015 and P0006 experiments at the west and Earth sides of LDEF under shielding varying from 1 to 19 g/cm².

Dose equivalent rates varied from 0.8 to 3.3 μSv/d for the low energy neutrons and from 160 to 390 μSv/d for the high energy neutrons. This compares with TLD measured absorbed dose rates in the range of 1000–3000 μGy/d near these locations and demonstrates that high energy neutrons contribute a significant fraction of the total dose equivalent in LEO.

Comparisons between measurements and calculations were made for high energy neutrons based on fission fragment tracks generated by fission foils at different shielding depths. A simple 1-D slab geometry was used in the calculations. Agreement between measurements and calculations depended on both shielding depth and threshold energy of the fission foils. Differences increased as both shielding and thereshold energy increased. The modeled proton/neutron spectra appeared deficient at high energies. A 3-D model of the experiments is needed to help resolve the differences.     

A study of the Ti(He, t) and Ni(He, t) reactions to isobaric analogue states

The (³He, t) reaction populating 0⁺ and 2⁺ states in ^{58, 60}Cu and ^{46, 48}V which are isobaric analogue states (IAS) of the 0⁺ ground states and 2⁺ first excited states in ^{58, 60}Ni and ^{46, 48}Ti have been studied at an incident ³He energy of 24.6 MeV. Triton spectra were measured for the targets ^46,48Ti, ^natNi and ⁵⁸Ni and angular distributions for the 0⁺ and 2⁺ IAS of ^{46, 48}Ti and ^{58, 60}Ni determined. The data were obtained using a magnetic spectrometer and position-sensitive detectors. The results have been analysed using DWBA theory. The 0⁺ → 0⁺ transitions to analogue states are described quite well using a microscopic form factor derived from a nucleon-nucleon interaction. However, with a Gaussian form, the m.s. radius of this interaction is only limited to the region 0–9 fm². Comparisons with data at other incident energies indicate that the strength of the effective interaction is strongly energy dependent. The Coulomb energies and (³He, t) angular distributions of the states assigned as the 2⁺ analogues in ⁴⁸V and ^58,60Cu are not described well by the models investigated. The ⁴⁶V 2⁺ IAS angular distribution is reproduced by a microscopic calculation, however. The ratios of the 0⁺ → 2⁺ IAS to the 0⁺ → 0⁺ IAS transitions are used to deduce a quadrupole deformation for the valence neutrons. The difference in the quadrupole deformations of the matter and proton distributions, as determined by other means, is found to be correlated with those of the valence neutrons. Several transitions to non-analogue states are also investigated.