Angular Momentum Alignment Research Articles

Pseudospin symmetry and quantized alignment in nuclei.

Nine superdeformed bands in several nuclei of the mass-190 region all have transitions of the same or equivalent energies to within an average of about 1 keV--almost identical. Furthermore, it is found that transitions of the same energy do not always connect levels of the same spin, indicating noncollective angular momentum alignments that are all very nearly integers (0, 1\ensuremath{\Elzxh}, or 2\ensuremath{\Elzxh}). Possible explanations involving pseudospin are discussed.

Alignment processes and shape variations in 184Pt

High-spin states in the transitional nucleus 184Pt were populated via the reactions 154Sm( 34S, 4n) 184Pt and 172Yb( 16O, 4n) 184Pt. The yrast band was extended up to I = 28 ħ and six new side bands built on both neutron and proton quasiparticle configurations were observed. Shell correction-type calculations indicate variations of the nuclear shape in different bands, especially as a result of band crossings due to the process of angular momentum alignment. Comparison of the band characteristics are made between 184Pt and eight adjacent nuclei. The pattern of band crossings in these nine nuclei is considered from the viewpoint of blocking comparisons and of theoretical calculations. The competition between low-frequency vi 13 2 and πh 9 2 band crossings is discussed.

Competition between triaxial bands and highly deformed intruder bands around 180Os

Abstract Angular momentum alignment processes in nuclei around 180 Os are discussed within the deformed shell model using the Woods-Saxon potential and the monopole pairing interaction. The crossings between bands of different structures are analysed and predictions are made for the possible observation of highly deformed intruder bands. Predictions of Q o moments are presented for the highly deformed bands and tables of calculated deformations for 178–183 Os are given.

Direct inelastic scattering of N2 from Ag(111). IV. Scattering from high temperature surface

We have measured the rotational state distribution and the angular momentum alignment and orientation of N2 scattered from Ag(111) at 540 K. Using resonance enhanced multiphoton ionization (REMPI), we are able to probe the scattered flux as a function of the exit angle θexit. For a modestly glancing incident beam (θi =30°) and incident translational energy, Ei =0.3 eV, the angular momentum alignment (tumbling vs helicoptering) at both quasispecular detection (θexit=35°) and superspecular detection (θexit=50°) is only weakly dependent upon the surface temperature. However, the angular momentum orientation (clockwise vs counterclockwise rotation) is strongly affected by the surface temperature. Raising the surface temperature from Ts =90 K to Ts =540 K causes the orientation to decrease substantially. Stochastic trajectory calculations were carried out in conjunction with the experiments. They reveal that at low temperature there is an averaging over two important initial conditions: the two-dimensional impact parameter and the molecular orientation geometry. At high temperature there is also an averaging over the instantaneous positions and momenta of the surface atoms. Hence, a given two-dimensional impact parameter and molecular orientation geometry results in a greater range of final J states, angular momentum polarizations, and velocities (exit angles) at high temperature than at low temperature. The resulting ‘‘smearing’’ accounts for the changes in rotational state distribution and polarization as a function of exit angle observed at high temperature. The major effect of averaging over the positions of the surface atoms (thermal roughening) upon the orientation of the scattered N2 is to increase the exit angle averaging rather than to increase the in-plane forces.

Structure of superdeformed bands in the A ≈ 150 mass region

The structure of superdeformed rotational bands recently discovered around 152Dy is discussed within the deformed shell model based on an average Woods-Saxon potential with a monopole pairing force. A comparison with available experimental data is provided and detailed predictions for yet unobserved cases are given. Pronounced variations in the observed rotational pattern are attributed to the angular momentum alignment of the high- N intruder (quasi)particles.

Coulomb excitation of230Th with32S,84Kr and142Nd projectiles

The Coulomb excitation measurements for the230Th nucleus with32S,84Kr and142Nd projectiles are presented. The use of different projectiles allowed us to get information in the ground-state band and side bands. The energy spectrum of the ground-state band and of the lowest negative-parity band have been investigated up to the spin valueI=24+ andI=19−(21), respectively. Five side bands (K π=0+, 2 1 + , 2 2 + , 1−, 2−) were observed also. The branching ratios for a large number of transitions in the spin regionI≦10 for π=+1 andI≦9 for π=− 1 are analysed. The full set of experimental data contains information on the mixing of the adiabatic states and on the nuclear response to the electromagnetic field ofγ-radiation. It is shown that the experimental data may be explained taking into account the coupling of the ground-β- and twoγ-bands and also of theK π=0−, 1− and 2− negative-parity bands. An enhancement of the transitions from theγ-to theβ-band in respect to the transitions from theγ to the ground band and from theβ- to the ground band is reported. The mixing of the negative-parity bands is found to be typical for the alignment of the octupole-vibrational angular momentum. The strong spin dependence of the intrinsic matrix elements of the electric-dipole operator follows from the branching ratios of inter- and intra-band transitions from theK π=0− states.

Angular distributions of prompt?-rays in the light charged particle accompanied and binary fission of252Cf

Double-differential emission distributions W(B,Ex) in angle 8 and energy Ex of prompt x-rays released in binary and light charged particle (LCP) accompanied fission of 252-Cf have been measured. The x-ray anisotropies are rather similar for both fission modes but the shapes of the angular distributions differ considerably. The maxima of the angular distributions are shifted from zero degree in the binary mode to 8≈20.30 deg. in the LCPac. fission. This behaviour may be understood as an influence of the α-particle (predominantly emitted in equatorial direction) on the alignment of angular momenta produced in bending modes.

Shape variations, influence of pairing and alignment of angular momentum in superdeformed bands in the A ≃ 150 region

High-spin properties of superdeformed bands recently discovered around 152Dy are discussed within the Woods-Saxon cranking model with pairing. While 152Dy can be viewed as a “doubly magic” superdeformed system that displays a rigid rotation, a dramatic rise in the dynamical moment of inertia observed in 150Gd is attributed to the angular momentum alignment of the N=7 neutron intruder quasiparticles.

Optical determination of the J-dependence of angular momentum alignment in a heat conducting gas

We have directly observed the flow of angular momentum alignment in a heat conducting gas of rotating molecules (Kagan polarization). This alignment is measured as a function of the angular momentum, J , from the polarization of the laser-induced fluorescence of iodine gas. A difficulty to overcome is that the gas as a whole does not show alignment, so that the detection method must be able to distinguish between molecular fluxes parallel and antiparallel to the temperature gradient. This is achieved by making use of the relatively long lifetime of the excited I 2 molecule. The long standing question of the precise form of the angular momentum alignment with respect to the magnitude of J is now unambiguously answered for classical rotors: the Kagan polarization is proportional to J 2. Also the sign of the alignment has been determined.

Level structure of 83Zr.

The level structure of $^{83}\mathrm{Zr}$ was investigated via the fusion-evaporation reaction $^{54}\mathrm{Fe}$${(}^{32}$S,2pn${)}^{83}$Zr at beam energies between 105 and 120 MeV using in-beam \ensuremath{\gamma}-ray spectroscopy techniques. \ensuremath{\gamma} transitions from the decay of $^{83}\mathrm{Zr}$ were unambiguously identified by means of recoil-\ensuremath{\gamma}, neutron-\ensuremath{\gamma}, and x-ray-\ensuremath{\gamma} coincidences. The decay scheme was constructed on the basis of Compton suppressed \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, delayed \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, and angular distribution measurements. A total of 34 transitions were identified connecting 26 new energy levels and the previously known ground state. The lifetimes of two low-lying isomeric states were determined. A decoupled positive parity band built on the odd ${g}_{9/2}$ neutron configuration was observed up to spin (37/2\ensuremath{\Elzxh} with the first band crossing due to proton alignment showing a sharp backbend at \ensuremath{\Elzxh}\ensuremath{\omega}=0.5 Mev. A strongly coupled negative parity band based on a ${(5/2}^{\mathrm{\ensuremath{-}}}$ bandhead was observed up to spin (35/2\ensuremath{\Elzxh}. The angular momentum alignment is discussed and compared to neighboring nuclei. A theoretical analysis was performed using the Woods-Saxon cranking model. The results of the calculation are consistent with the experimental findings for $^{83}\mathrm{Zr}$ and its neighbors. Triaxial shapes are predicted for all observed bands in $^{83}\mathrm{Zr}$. At higher angular momenta a transition to well-deformed prolate bands involving the ${h}_{11/2}$ intruder orbitals is expected.

Optical Observation of Angular Momentum Alignment in a Heat-Conducting Gas

Optical Observation of Angular Momentum Alignment in a Heat-Conducting Gas

Alignment of CN from 248 nm photolysis of ICN: A new model of the à continuum dissociation dynamics

A KrF excimer laser (248 nm) is used to dissociate a low pressure (5–10 mTorr) sample of cyanogen iodide (ICN) and the resulting CN X 2Σ+ fragments are probed by laser-induced fluorescence (LIF) via various vibrational sequences of the B 2Σ+–X 2Σ+ transition. In addition to measuring relative vibrational and rotational populations in the CN X 2Σ+ photofragment, the alignment of rotational angular momentum in this fragment is determined from the variation in LIF intensity as the direction of linear polarization of the probe laser is rotated with respect to that of the photolysis laser. A unifying model is proposed for the à continuum photodissociation dynamics which is able to account for present and previous experimental measurements characterizing both the I and CN photofragments.

On octupole alignment in actinides

The alignment of the octupole angular momentum in the rotational states of the K π = 0 − band is analysed in the microscopic model. The model describes qualitatively the branching ratio for the E1-transitions from these states to the ground band states.

Experimental determination of scattering induced angular momentum alignment in state selected rotational transitions

Measurements of angular momentum alignment in molecular beam scattering are presented for scattering of Na2 by He, Ne, and Ar. A mechanism for the production of this alignment is discussed, based on mJ conservation for quantization along the momentum transfer vector. The experimental results differ in several aspects from the predictions of this simple mechanism. It is found that the differential cross sections are strongly dependent on mJ. Furthermore, it is found that the principal axis of the orientational distribution does not coincide with the momentum transfer vector. The discrepancy is attributed to a nonstrict mJ conservation in the collision. These experiments are therefore expected to provide a sensitive probe to the nonsudden aspects of molecular scattering.

Born approximation for collisional alignment

It is shown that the first order Born approximation is in qualitative agreement with many of the experimental observations on the angular momentum alignment of Na 2 molecules produced during collisions with He. A model for certain aspects of the second order Born approximation is introduced which is able to predict the observed deviation of the orientation of the alignment from being directed along the kinematic apse. As well, exact expressions for the alignment cross section are given in terms of reduced T-matrix elements.

Sub-Doppler laser-induced fluorescence measurements of the velocity distribution and rotational alignment of NO photofragments

The photodissociation of dimethylnitrosamine, (CH3)2N–NO, at 363.5 nm produces ro-vibrationally excited NO fragments. With two-photon laser-induced fluorescence the Doppler profiles of single rotational transitions were probed, using various polarization schemes for dissociation and probe lasers. These measurements provided a detailed product state distribution, the three-dimensional recoil velocity distribution of specific fragment states, and the alignment of their angular momentum. We present evidence of the presence of correlations between fragment recoil direction and alignment of fragment angular momentum.

Measurement of product rotational alignment in associative-ionization collisions between polarized Na(3p) atoms.

We have studied the effect of reactant Na(3p) polarization on the rotational angular momentum alignment of product ${\mathrm{Na}}_{2}$${\mathrm{}}^{+}$ ions arising from associative-ionization (AI) collisions. Our results show that sensitivity of the AI rate constant to initial atomic polarization persists even when all hyperfine states are populated with broadband (3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) pulsed laser excitation of Na${(\mathrm{}}^{2}$${P}_{3/2}$) and that the spatial distribution of product rotational angular momentum vectors is anisotropic. We discuss a qualitative description of the collision process consistent with our measurements which indicates that \ensuremath{\sigma}-orbital symmetry is preferred to \ensuremath{\pi}-orbital symmetry as the colliding partners approach.

Non-adiabatic effects and coriolis interactions in odd deformed nuclei

The alignment of the intrinsic angular momentum of a rotating nucleus into the rotational axis is studied in terms of a simple quasi-adiabatic approach. In the framework of this approach using the “quasi-particle + phonon” model with Harris non-adiabatic corrections it is possible to describe the rotational bands up to relatively high spins. In the case of235U it is demonstrated that this simple approach enables the qualitative description of the process of alignment of the intrinsic angular momentum with respect to the rotational axis.

High-spin states in172Hf

The reaction 128Te(48Ca,4n) has been used to extend the yrast sequence of 172Hf up to Ipi =30+. In addition, two negative-parity sidebands have been observed to Ipi =24- and 25-. The aligned angular momentum of these bands is seen to increase approximately linearly with rotational frequency at high frequencies.

Angular momentum alignment in the reaction154Sm+214 MeV32S

Angular momentum alignment in the reaction154Sm+214 MeV32S has been investigated using discrete-lineγ-ray techniques. The anisotropies of stretchedE2 transitions from the target-like fragments reach approximately three at the largest energy losses, implying rather strong alignment. The results are compared with the predictions of a simple model.