The antiferromagnetic and spin-flop phases of MnCl‧4H₂O have been investigated by observing the nuclear orxentatxon of ⁵⁴Mn in that material. The sublattice magnetizations in the absence of an external field were found to lie in a direction between the a and c crystal axes at an angle of 11.5° ± 3.5° to the c axis. The field dependence of the spin configuration in the spin-flop state indicates that second order anisotropy is significant in this system. The molecular fields were determined by combining the results of this work with other measurements of the-critical fields (Rives and Benedict, 1975). The results are: the exchange field is 11.05 ± 0.21 KOe; the biaxial single ion anisotropy fields are 0.75 ± 0.22 KOe and 2.35 ± 0.23 KOe along the a* and b axes respectively; the second order anisotropy field is 1.45 ± 0.19 KOe and the anisotropic exchange field is 0.1 ± 0.3 KOe. The spin-flop transition region was found to be adequately described by a 'domain' structure in which regions of antiferromagnetic phase and regions of spin-flop phase co-exist in the crystal. Measurements were made of the temperature dependence of the spin-flop transition field and, contrary to the extrapolated results of Rives and Benedict (1975) , the spin-flop field was found to decrease with decreasing temperature from 0.3K to 0.15K. If there is a minimum in the transition field it must occur at lower temperatures. The cooling of the MnCl‧4H₂O crystal which was held in contact with a copper heat sink by Apiezon N grease was fitted to the relation Q = kA (T₁ⁿ-T₂ⁿ) where T₁ and T₂ are the temperatures of the crystal and copper heat sink respectively, and A is the contact area. For n = 4 the value obtained for the constant k is (8.2 ± 1.9) x 10³ ergK⁻⁴ sec⁻¹ cm⁻². Nuclear orientation experiments were also performed on the systems ¹⁰³Ru-Fe and ⁵⁹Fe-Fe. The gamma-ray anisotropies for these systems (at temperatures of 10 and 15 mK, respectively) were very small; however, it was possible to determine limits for the magnitudes of the nuclear magnetic moments of the active nuclei. The ¹⁰³Ru moment was found to be greater than 0.15uN and the ⁵⁹Fe moment was found to be less than 0.9uN. J.E. Rives and V. Benedict, Phys. Rev. B12, 1908 (1975). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

We have analyzed the NMR spectra of a series of 19 di-halobenzenes having Cs or C₂v symmetry dissolved in a special mixture of nematic liquid crystals, i.e., 55 wt% 1132/EBBA-d₂. This mixture has the unique feature that dideuterium dissolved in it experiences a zero average electric field gradient. Therefore one can assume that for this mixture the contribution to the molecular order due to the interaction between the average electric field gradient and the quadrupole moment of the molecule can be neglected. It has been suggested that the orientational order also depends on the size and shape of the solutes and a model has been used to calculate the order parameters. Another model relates the ordering to the molecular polarizability anisotropy. A description of the ordering of molecules having Cs symmetry requires 3 independent order parameters and thus provides a strong test for the different models of orientational order. Comparisons are made between our experimentally determined order parameters and the theoretical values obtained using the size and shape, and the polarizability models. Very good agreement is obtained for the size and shape model. However, it is found that any mechanism, involving a molecular property which is approximately bond additive, can not predict the differences found between the ortho- and meta-dihalobenzenes. Consequently, a poor agreement results from the polarizability model when a bond-additive scheme is used to calculate the molecular polarizability. Further investigation of the correlation between order parameters and solute molecules suggests that the molecular shape dominates the orientational behavior. / Science, Faculty of / Chemistry, Department of / Graduate

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Low Temperature Nuclear Orientation (LTNO) is an important technique in the study of nuclei far from stability. The theory of LTNO and its applications to the measurement of static nuclear moments and other quantities of spectroscopic interest are reviewed. Among the nuclei off the line of stability those in the A ~ 75 region are of considerable interest, exhibiting large quadrupole deformations, triaxiality and shape coexistence. LTNO measurements performed on neutron deficient bromine isotopes have yielded the static magnetic dipole moments of the nuclei <sup>72g,72m,74m,75,76,77<sup>Br, Spectroscopic information on ^72-77Se and also identified the ground state spin of ⁷³Br as l/2‾ Existing odd A and odd-odd particle-rotor computer codes have been extended in order to include a Variable Moment of Inertia (VMI) asymmetric rotational core. The formalism necessary for this modification is developed. The measured magnetic moments are interpreted within the framework of this particle - VMI rotor model. It is shown that the systematic reduction in the moments of the odd A nuclei ^75-81Br characterizes the transformation of the prolate ground state configuration from largely π[301]3/2 in ^79,81Br to almost pure π[312]3/2 in ^75,77Br. This trend is fully consistent with the increase in deformation towards the lower masses suggested by the known electric quadrupole moments. In contrast, the ground state spin of ⁷³Br can only be interpreted in terms of an oblate nuclear shape. This is the first evidence for the predicted prolate-oblate shape transition in the bromine nuclei. In addition the magnetic moment of ⁷⁴Br^m, by identifying a π[431]3/2ν[422]5/2 configuration, strongly suggests a positive parity assignment for the isomeric state. Finally, an experiment to search for possible T-violation effects in nuclear gamma decay is described. Using coincidence techniques, a measurement of the T-odd P-even quantity (I ‧ k x e)(I ‧ k)(I ‧ e) has lead to a limit on the T-violating phase angle between the E2 and Ml matrix elements associated with the 604 keV gamma transition in ¹⁹²Pt of sin η = ±11(12) x 10^-3.

This thesis describes Nuclear Orientation and radiofrequency magnetic susceptibility experiments on the enhanced nuclear antiferromagnet holmium vanadate (HoVO₄) from 1 K down to about 1 mK and in applied fields of up to 2.4 tesla. Adiabatic demagnetisation of single crystals of HoVO₄ allowed a mean nuclear spin temperature of about 1 mK to be reached. R.f. magnetic susceptibility measurements at approximately 2.4 mK and in an applied field of about 1.5 x 10^-2 T identified the spin 'flop' phase of the nuclear anti-ferromagnet HoVO₄. The Néel temperature T_N was determined to be 4.0 (2) mK which is in good agreement with the value of 4.8 mK calculated for the dipole-dipole interaction of the enhanced magnetic moments at the Ho site. Gamma-ray anisotropy measurements of ^166mHoVO₄ showed that in low applied fields (B ≮ 0.5 x 10^-2 T) and temperatures of about 1 mK the antiferromagnetic domains were uniformly distributed in the a-a' plane of the tetragonal zircon crystal structure. This was interpreted in terms of pinning of the nuclei due to crystalline imperfections. The spin 'flop' phase was identified in an applied field of 1.2 x 10^-2 T which is in good agreement with the susceptibility measurements. Demagnetisation experiments down to the relatively high residual fields of 4.2 x 10</sup>-2</sup> T and 7.4 x 10</sup>-2</sup> T and temperatures of about 2 mK showed that the effective field at the nucleus was much smaller than the applied field thus indicating that there was some antiferromagnetic ordering even at these relatively high fields. The magnetic dipole moment of ^166mHo was estimated to be 3.60 (6) μ_N from gamma-ray anisotropy measurements of some gamma transitions of known NO parameters. This value is in good agreement with the value of 3.69μ_N calculated for the Nilsson configuration [6337./2]_n [5237/2]_p for the extra-core neutron and proton respectively. Mixing ratio (δ) measurements for gamma-ray transitions between the (K^π = 2⁺) γ-vibrational and (K^π = 0⁺) ground state rotational bands of the daughter nucleus ¹⁶⁶Er showed that Ml transition probabilities are predominantly via ΔK = 1 band mixing which is in good agreement with systematics for deformed nuclei in this mass region. Nuclear refrigeration experiments on a powdered compressed HoVO₄ - copper pill allowed a lattice temperature of 3.9 (3) mK to be reached. Thermal contact measurements on a similar pill showed that the effective area of contact between the salt particles and copper was only about 30 cm² which was a small fraction of the total surface area available. Preliminary work on a HoVO₄ - gold pill enabled a ⁶⁰Co ̲C̲o thermometer to be cooled to 6.0 (l) mK.

On-line nuclear orientation at low temperature has become an important technique for the study of nuclei far from stability, through measurements of nuclear moments and other quantities of spectroscopic interest. The theory of low temperature nuclear orientation and its application to the study of nuclear structure are reviewed. Of particular importance to the on-line measurement, in which a wide range of short-lived nuclei are available for study, is the question of how fast these nuclei can be cooled to the lattice temperature, and thus oriented. To address this, the theory of nuclear spin-lattice relaxation, relevant to the online technique, is outlined. In particular, quantitative methods to deal with cases in which the spin-lattice relaxation time is comparable with the isotope half-life have been developed and applied. One of the major current interests in nuclear structure physics is to investigate how the neutron-proton interaction influences the structure of nuclei that are transitional, between well established regions of spherical and deformed nuclei. In such nuclei, intruder excitations, which signal the onset of deformation, are observed low in energy. Using the Daresbury on-line isotope separator, an extensive study of the decay of ¹¹⁸I to ¹¹⁸Te has been performed using nuclear orientation techniques, combined with γ - γ and conversion electron spectroscopy measurements. Interpretation of the results obtained for ¹¹⁸Te within the framework of IBM-2, gives strong evidence for the existence of such an ,em>intruder configuration in this nucleus. On-line experiments have also been performed in which a range of neutrondeficient Cs nuclei has been oriented for the first time. In these measurements the hyperfine field of CsFe has been determined as (+)40.8(7) T, and also the Korringa constant for the system ¹²¹Cs^mFe has been measured (using a new technique) to be C_k = 0.059(l6)sK. These results have been applied to the case of ¹¹⁸Cs^m, for which the magnetic moment has been measured to be 5.4(1.1)nm. This large value clearly indicates the presence of the [404]9/2 orbital, which provides further evidence for the existence of intruder states in this region.

Low Temperature Nuclear Orientation (LTNO) is an important technique in the study of nuclei far from stability. The theory of LTNO and its application to the measurement of static nuclear moments and other quantities of spectroscopic interest are reviewed. The off-line facility at Oxford was used to study the decay of ¹³³I→¹³³Xe and ¹³⁵I→¹³⁵Xe. ¹³³I having Z=53 and N=80 has three protons above the closed shell Z=50 and two neutrons holes in N=82 shell, while ¹³⁵I has fully closed neutron shell since it has N=82, and they are of considerable theoretical interest since a wide variety of the theoretical nuclear models may be used to describe the observed levels close to the stable double closed shell structure. Another aim is to search for the nuclear magnetic dipole moment of the ground state of ¹³⁵I. Nuclear orientation of ¹³³IFe and ¹³⁵IFe enabled the mixing ratios of several transitions in the decay scheme of ¹³³I and ¹³⁵I to be determined. From temperature dependence for ¹³⁵I, the nuclear magnetic moment of ¹³⁵I has been deduced. Also temperature dependence for ¹³³I, analysed using a simple model, gave value for the magnetic hyperfine field that differed from previous published values. The method of combining nuclear orientation with NMR has become a very popular technique in recent years for determining nuclear magnetic dipole moments very precisely. The purpose of the NMR/ON experiment was to measure the hyperfine field with greater precision and to get some idea of the proportion of nuclei subject to it. Light Eu and Sm nuclei have attracted attention as systems with the number of protons right below the Z=64 subshell gap and the number of neutrons approaching N=82 major shell closure. Odd-proton, odd-neutron and odd-odd nuclei near the A=140 region have been investigated in the framework of the particle-triaxial rotor model. Main attention has been paid to explanation of experimental magnetic dipole and electric quadrupole moments of ground and isomeric states. Model predictions for deformation parameters of ^136-142Sm even-even cores have been extracted.

Graduation date: 1992