A. Abragam

Bio: A. Abragam is an academic researcher from Harvard University. The author has contributed to research in topics: Quadrupole & Zeeman energy. The author has an hindex of 4, co-authored 4 publications receiving 574 citations.

Influence of electric and magnetic fields on angular correlations

Abstract: The theory of the influence on angular correlations of perturbing interactions in the intermediate state is reformulated to allow the description of the effects of time-dependent as well as of static perturbations. For static interactions of the nuclear electric quadrupole moment with crystalline fields of axial symmetry in polycrystalline sources, attenuation factors are calculated for the coefficients of the various terms in the expansion of the correlation function in Legendre polynomials. No matter how strong the quadrupole interaction, some anisotropy must remain for polycrystalline sources but, for the same interaction in simple single crystals, the anisotropy can be either undisturbed or completely destroyed, depending on the orientation of the crystal. Fields of lower symmetry are shown also to leave, for polycrystalline sources, some anisotropy. Expressions for the influence of randomly fluctuating interactions, such as must exist in liquid sources, are calculated and these predict arbitrarily complete destruction of the correlation under certain conditions, but explain the more nearly unperturbed results usually found with such sources. For electronic shells having magnetic moments, the influences of electronic paramagnetic relaxation and of anisotropy of the hyperfine structure interaction are examined. An applied static magnetic field in the presence of static quadrupole interactions in polycrystalline sources is shown to have differing effects depending on the relative strengths of the two interactions. Application of a magnetic field directed toward a counter cannot reduce the disturbance of the intermediate state in liquid sources, except under special circumstances. The influences of an applied field in the presence of time dependent anisotropic hyperfine structure interactions are discussed. Finally, the feasibility of resonance experiments, for the precise determination of nuclear moments in the intermediate state, is explored.

Proton Relaxation Times in Paramagnetic Solutions. Effects of Electron Spin Relaxation

Abstract: The proton relaxation time in solutions of paramagnetic ions depends, among other factors, on the relaxation time of the electron spins, τs. It is shown that the latter, for ions of the iron group, is determined mostly by the distortion of the hydrated complex by collisions with other water molecules. The theory provides a quantitative explanation for the decrease in T2 in Mn++ (and other) solutions in very high magnetic fields. The experimentally observed field and temperature dependence of the proton relaxation times, T1 and T2, for ions of the iron group is compared with theory and the features which depend on τs are stressed.

A determination of fundamental Zeeman parameters for the OH radical

Abstract: Measurements of the gas-phase E.P.R. spectra of OH in the J = 9/2 and 11/2 levels of the X2II state at 26 GHz and 35 GHz respectively are reported. Confocal and semi-confocal optical resonators have been used in place of the more conventional microwave cavities for these experiments. The data are analysed, together with previous measurements by Radford on other rotational levels in the determination of six independent g factors: These parameters are interpreted in terms of the electronic structure of the OH radical. In agreement with previous workers, it is found that the major contaminant of the X2II state is the A2Σ+ state and that this pair of states is a good example of Van Vleck's pure precession hypothesis.

An investigation of the chemical stability of a monomer/polymer gel dosimeter

Abstract: The aim of this work is to investigate the temporal stability of a polyacrylamide gelatin hydrogel used for 3D monomer/polymer gel dosimetry techniques involving different methods of analysis. Long-term instabilities for a similar gel have recently been reported, but differ markedly from those described in this work. Two kinds of long-term instabilities are described. One affects the slope of the dose-R2 plot and is related to post-irradiation polymerization of the comonomer/polymer aggregates. It is observed that post-irradiation polymerization only lasts 12 hours after irradiation. The other instability affects the intercept of the dose-R2 plot, lasts for up to 30 days and is related to the gelation process of gelatin. Further studies were performed on gelatin gels of varying compositions to obtain a better understanding of the molecular mechanism that causes the instability due to gelation. The studies included observations of the spin-spin and spin-lattice relaxation rates in combination with diffusion measurements and optical measurements. It is shown that the heating history during the manufacture of the gel affects the absolute R2 value of the gel but not its variation. The findings presented in this study may help in producing more stable and reproducible monomer/polymer gel dosimeters.

Nuclear quadrupole interaction studies by perturbed angular correlations

Abstract: A comperhensive study was made of the applicability of gamma‐ray angular correlations to the determination of quadrupole interactions in metals and insulating solids. Dynamic effects were studied in solutions and gases. A total of fourteen gamma‐ray cascades were employed. Several nuclear spins were confirmed and the quadrupole moments of ten excited nuclear states were determined or estimated from the data. Quadrupole coupling constants were determined for excited states of the following nuclei in metallic host lattices of the same element: 44Sc, 99Ru, 111Cd, 117In, 187Re, 199Hg. Coupling constants were also measured for the following isotope (lattice) combinations: 99Ru(Zn, Cd, Sn, Sb), 100Rh(Zn, Ru, Cu5Zn8, Pd2Al, PdPb2), 111Cd(In, Hg, Tl, CdSb, Cd3Ag, Zn, Ga, In, Sn, Sb, Bi, AuIn, InBi, In2Bi), 115In(Cd), 117In(Cd, Sn), 131I(Te), 181Ta(HfB2, HfSi2), 204Pb(Cd, In, Sn, As, Sb, Bi, Hg, Tl, PdPb2). Systematic variations of e2qQ with host‐lattice structure were observed and host and solute properties were found to be separable to some extent for nontransition metals. The nuclei 111Cd, 115In, 117In, 199Hg, and 204Pb were used to determine a total of fifty quadrupole coupling constants in insulators, including twenty with nonzero asymmetry parameters, which give oscillatory but aperiodic correlation functions. It was strikingly (and exhaustively) demonstrated that good determinations of quadrupole coupling constants could be made following isomeric transitions (IT) (with no elemental transmutation) and beta decay (with elemental transmutation). However, in no case was it possible to derive a coupling constant from a gamma‐ray cascade preceded directly by electron‐capture (EC) decay, presumably because the sudden creation of a K‐hole, and the Auger and ``shake‐off'' events that follow, destroy the chemical integrity of the species under study. Relaxation times were determined for a number of liquid samples. Studies of dimethyl‐111mCd in various buffer gases showed that the spin memory was lost in one collision with heavy molecules, but that light molecules required several collisions.