Angular distributions,relative orbital angular momenta,and spins of fragments originating from the binary fission of polarized nuclei

The angular distributions of fragments originating from the binary fission of odd and odd-odd nuclei capable of undergoing spontaneous fission that are polarized by a strong magnetic field at ultralow temperatures and from the low-energy photofission of even-even nuclei that is induced by dipole and quadrupole photons are investigated. It is shown that the deviations of these angular distributions from those that are obtained on the basis of the A. Bohr formula make it possible to estimate the maximum relative orbital angular momentum of fission fragments, this estimate providing important information about the relative orientation of the fragment spins. The angular distributions of fragments originating from subthreshold fission are analyzed for the case of the ²³⁸U nucleus. A comparison of the resulting angular distributions with their experimental counterparts leads to the conclusion that the maximum relative orbital angular momentum of binary-fission fragments exceeds 20, the fragment spins having predominantly a parallel orientation. The possibility is considered for performing an experiment aimed at measuring the angular distributions of fragments of the spontaneous fission of polarized nuclei in order to determine both the spins of such nuclei and the maximum values of the relative orbital angular momenta of fission fragments.     

Perturbed angular distribution of recoil implanted19F-nuclei in a cobalt and copper environment

The magnetic hyperfine field acting on¹⁹F nuclei in a cubic cobalt lattice at a temperature of 450 °C has been investigated. Time dependent spin rotation has been observed following the excitation and recoil implantation with a pulsed α-particle beam using the reaction¹⁹F(α,α′)¹⁹F*. The hyperfine field isH _hf (450 °C)=31.4 ± 0.5 kG. The measured amplitude of the spin rotation is only 18% of the amplitude expected for the case where all¹⁹F* nuclei are assumed to be subject to the same hyperfine field without any further perturbation. This was established by means of other implantation experiments with a copper backing using first the pulsed beam technique followed by a γ-ray angular distribution measurement in coincidence with backscattered α-particles. In copper a fast perturbation was found, which reduces the anisotropy to about 80% within a short time interval immediately after the excitation.     

On the paramagnetic resonance and the longitudinal relaxation of ferromagnets in the critical region above Tc

Uniform dynamical susceptibility of cubic ferromagnets above T_c in magnetic field is investigated. Resonance frequences and damping constants are determined in the limiting cases of low and strong magnetic field. It is shown that in the exchange temperature region when 4πχ ? 1 owing to spin diffusion the uniform susceptibility is depended non-trivially on the frequency and the magnetic field. From this dependence the temperature behaviour of spin diffusion coefficient can be determined.     

Angular distributions of α particles emitted by oriented nuclei and their relation to nuclear deformation

Low-temperature spin orientation of radioactive nuclei is a nuclear spectroscopic method that allows us to obtain experimental data on nuclei and extranuclear fields. We present the results from measuring the angular anisotropy of α radiation emitted by transuranium nuclei of ^253,254Es, ²⁵⁵Fm, ^241,243Am, along with that of γ radiation from ²⁵⁰Bk nuclei oriented in an iron matrix at temperatures of 10–300 mK. The data allow us to establish the relation between the mechanism of α decay and nuclear deformation and to compare them to the theoretical data. We also measure the energy of magnetic hyperfine splitting for the investigated nuclei, and find the magnetic hyperfine field value for Es in Fe to be |B _hf | = 396(32) T. The nuclear magnetic moment for ²⁵⁴Es was determined, and its value was |μ(²⁵⁴Es)| = 4.35(41)μ _N .     

P-odd,T-even symmetries for products of the spontaneous fissioning of oriented nuclei

P-Odd, T-even asymmetries in angular distributions of products from the binary and ternary spontaneous fissioning of nuclei oriented in strong magnetic fields at ultralow temperature are described for the first time by means of the quantum theory of fission. Using the spin density matrix of a fissioning nucleus and considering the notable octupole deformations of the nucleus that appear in the vicinity of its scission point, we show that coefficients of the asymmetries in question are governed by the orientation parameters of a fissioning nucleus with Q = 1 and Q = 3. The coefficients obtained in this work are compared to the analogous coefficients in the fissioning of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.     

NMR studies of single crystals of the topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> at low temperatures

A powder sample and single-crystal plates of the topological insulator Bi₂Te₃ have been investigated using the ¹²⁵Te NMR method at room temperature and at low temperatures in the range from 12.5 to 16.5 K. The NMR spectra of the single-crystal plates have been studied in the orientation where the crystallographic axis c is directed parallel or perpendicular to the magnetic field. The spectra have been obtained by means of recording spin-echo signals and plotting their envelopes. It has been shown that the NMR spectra for the bismuth telluride powder and plates with the orientation c ⊥ B consist of two lines, which are presumably attributed to tellurium nuclei in two crystallographic positions in the bulk of the sample. The position and shape of the lines are determined by the chemical shift and the Knight shift. For the orientation of the plates c || B, the spectrum contains an additional component in the high-frequency region, which cannot appear due to the angular dependence of the line shifts caused by tellurium nuclei in the bulk of the topological insulator. At a low temperature, the additional line dominates in the spectrum.     

Control of the nuclear spin of the <Superscript>129</Superscript>Xe and <Superscript>131</Superscript>Xe isotopes in the spin-exchange interaction with <Superscript>87</Superscript>Rb atoms

Nuclear spin dynamics of the ¹²⁹Xe and ¹³¹Xe isotopes in an external magnetic field B ₀ is considered. Nuclear spin is pumped by the laser through ⁸⁷Rb, which transfers the electron spin to the ¹²⁹Xe and ¹³¹Xe nuclei in the spin-exchange interaction. The nuclear spin dynamics is controlled with a transverse magnetic field that causes nuclear magnetic resonance in both ¹²⁹Xe and ¹³¹Xe isotopes. Numerical calculations are performed to find conditions at which the transverse component of the nuclear spin in the established motion is of maximum and the slope angle relative to the vector of the constant magnetic field B ₀ is 45°. This regime is taken to be optimal for simulation of practical applications. It is also found that the pump of the nuclear spin of xenon is strongly attenuated when the rubidium polarization vector is turned to the plane perpendicular to the external magnetic field vector B ₀.     

Intermediate state perturbation in low temperature nuclear orientation

This paper deals with the perturbation of low temperature nuclear orientation in an intermediate state by an interaction not axially symmetric around the orientation axis. The directional distribution of the emitted radiation is factorized into initial orientation tensors and perturbation factors known from perturbed angular correlation theory. Low temperature nuclear orientation coefficients have been calculated numerically for¹¹¹Cd in a polycrystalline sample with axially symmetric electric field gradient. Nuclear orientation experiment on¹¹¹In in indium host confirms theoretical perturbation predictions and demonstrates the sensitivity of the method on different parameters. Moreover this orientation allows the determination, in magnitude and sign of¹¹¹In (ground state) nuclear magnetic moment: = +5.48(10)_n.     

Features of low-frequency spin dynamics in manganite LaMnO3 according to 139La NMR data

The spin-lattice and spin-spin relaxation times of ¹³⁹La are measured in manganite LaMnO₃. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei.     

Field and temperature dependence of electronic states populations and spin relaxation in Mössbauer absorption spectra of LiNbO3:57Fe(III)

Single crystal LiNbO₃:⁵⁷Fe(III) Mössbauer absorption spectra at 3 Tesla and 120...300K can be fitted using a dynamical spin Hamiltonian according to spin-lattice relaxation. At high fields (0.9...7 Tesla) and 4.2K the spectra can be fitted using an electronuclear spin Hamiltonian. The area of the subspectra due to each electronic state is not in accordance with a Boltzmann distribution at the bath temperature. From the population we calculate spin temperatures which depend linearly on the external magnetic field.     

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of ²²⁶Th nuclei formed via a complete fusion of ¹⁸O and ²⁰⁸Pb nuclei at laboratory energies of ¹⁸O projectile ions in the range E _lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E _c.m. ? E _B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.     

Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields

Using the trion as an optical probe, we uncover novel electron spin dynamics in CdSe/ZnSe Stranski-Krastanov quantum dots. The longitudinal spin lifetime obeys an inverse power law associated with recharging processes in the dot ensemble. No hint at spin-orbit mediated spin relaxation is found. At very weak magnetic fields (< 50 mT), electron spin dynamics related to the hyperfine interaction with the lattice nuclei is uncovered. A strong Knight field gives rise to nuclear ordering and formation of dynamical polarization on a 100-micros time scale under continuous electron spin pumping. The associated spin transients are temperature robust and can be observed up to 100 K.     

<Superscript>209</Superscript>Bi NMR spectrum of BiFeO<Subscript>3</Subscript> in the presence of spatial modulation of hyperfine fields

A complicated spin-echo spectrum was observed for ²⁰⁹Bi nuclei in a ferroelectric antiferromagnet BiFeO₃ in zero external field. This spectrum is the first example of nuclear quadrupole resonance of a system of diamagnetic atoms in a hyperfine magnetic field produced by the spatially modulated Fe³⁺ spin system and varying in orientation and magnitude. An attempt is undertaken to theoretically simulate the spectrum.     

Magnetic properties of Pm in PrNi

A magnetic property of Pm as an impurity in PrNi single crystal was investigated by means of the low-temperature nuclear orientation of^143,144Pm. The angular distribution of -ray anisotropies revealed that the direction of hyperfine field experienced by the nuclei lie in the (a, c) plane andmade an angle of 60° (5) or 240° (5) with respect to the crystallinec-axis. From the temperature dependence of the anisotropies the strength of the hyperfine field of Pm in PrNi was deduced to be 185(22)T. The hyperfine field of Co at the Ni site of PrNi was found to be less than 4T.     

Angular momentum dependence of 200Pb fission studied by comparison of 19F on 181Ta with 16O on 184W

The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with ¹⁹F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10⁻¹⁷s. The total fission cross section and angular distribution of fission fragments were also measured for ¹⁹F bombardment of ¹⁸¹Ta in the energy range 84.3–114.7 MeV and for ¹⁶O bombardment of ¹⁸⁴W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The ¹⁹F + ¹⁸¹Ta measurements, when compared with the present ¹⁶O + ¹⁸⁴W cross section and angular distribution measurements and earlier lifetime measurements for ¹⁶O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.     

POLAREX Study of polarized exotic nuclei at millikelvin temperatures

POLAREX (POLARization of EXotic nuclei) is a new facility for the study of nuclear magnetic moments and decay modes of exotic nuclei using the established On-Line Nuclear Orientation (OLNO) method. A radioactive beam of interest is implanted into a ferromagnetic host foil held at a temperature of order 10mK in a ³He - ⁴He dilution refrigerator. The foil is magnetized by an applied magnetic field and the nuclear spins become polarized through the internal hyperfine field. The angular distribution of decay products from the polarized sample is measured. Accurate values of nuclear moment are obtained by NMR. The new facility will have access to neutron-rich nuclides produced at the ALTO facility (Linear Accelerator at Orsay Tandem) by fission induced by electrons from the linear electron accelerator. Basic concepts and initial tests are outlined.     

Polarization of quadrupolar nuclei

A method of obtaining high polarization and pure spin states of impurity nuclei with a moderately strong quadrupole interaction in solid diamagnetic hosts whose nuclei have spin 1/2, a large g factor (like ¹H and ¹⁹F), and a high degree of polarization is proposed. The method employs cross-relaxation transitions of the impurity nuclei with the host spins (with adiabatic variation of the external magnetic field) and simple radio-frequency pulses that invert the host nuclei or give rise to two-spin resonance of the host and impurity nuclei. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 539–543 (10 October 1998)     

Magnetoresistance and magnetic phase transitions in a Pr11B6 antiferromagnet

In isotopically pure praseodymium hexaboride (Pr¹¹B₆) single-crystal samples, the transverse magnetoresistance Δρ/ρ has been measured in a temperature range of 2–20 K in magnetic fields up to 80 kOe. The field and angular dependences Δρ/ρ(H, ?, T ₀) reveal a new magnetic phase in the AFM state of Pr¹¹B₆ which is observable only for the external magnetic field orientation in a narrow angular range near H ‖ 〈110〉. The data remove the previous contradictions in the Pr11B6 magnetic phase diagram representation and can be explained under the assumption that the spin-polarized regions (ferrons) are involved in the formation of the complex magnetic structure in the Pr¹¹B₆ AFM state in the 5d band in the vicinity of the rare-earth ions.     

Investigation of the dynamical behaviour of the FH(CN−) centre in KCl with temperature dependent endor spectroscopy

Abstract

The temperature dependence of the electron nuclear double resonance (ENDOR) spectra of F_H(CN^?) centres in KCl was investigated in the temperature range between 10–220 K. At the lowest temperature of 10 K only one CN^? orientation with respect to the F centre electron is present, in which the nitrogen is thought to be nearer to the F-electron than the carbon. With the very small thermal activation energy of 2.9 meV the opposite orientation is occupied. The superhyperfine interactions of those first shell K nuclei nearest to CN^? and of the ¹³C interaction of the CN^? molecule are strongly temperature dependent between 10 and 60 K, following an exponential law with a thermal activation energy of 4.2 meV. It is assumed that a soft local mode involving those two nearest K nuclei and the CN^? is causing the strong temperature dependence. The shf interactions of ¹⁴N nuclei have not been seen, probably because of the dynamical effects.