Low Temperature Nuclear Orientation Research Articles

Hyperfine interaction studies in the David Shirley group, 1960–1975. II. Perturbed angular correlations and Mössbauer effect

In addition to the low-temperature nuclear orientation studies discussed in Paper I, David Shirley's research group at Berkeley was renowned for other types of studies of the interaction of probe nuclei with their electromagnetic environment, particularly perturbed angular correlations and the Mössbauer effect. The present paper discusses these other fields of research into hyperfine interactions and gives examples of the contributions of the Shirley group toward elucidating these interactions.

Hyperfine interaction studies in the David Shirley group, 1960–1975. I. Low-temperature nuclear orientation

Under the leadership of David Shirley, the hyperfine interactions group at Berkeley became one of the world’s leading laboratories for its diversity of studies of the interaction between probe nuclei and their environment. One branch of those studies, low-temperature nuclear orientation, concerned the radioactive decays of nuclei whose spins were oriented in a variety of electromagnetic environments at temperatures in the mK range. In the years 1960–1975, this group did pioneering research that produced more than 50 papers in the field, and at least 16 graduate students completed their dissertation research. The present paper gives a brief introduction to the field of low-temperature nuclear orientation, summarizes the main accomplishments of the Shirley group, and discusses examples of the variety of results obtained in the group’s research. Paper II deals with other hyperfine interaction studies by the Shirley group.

The on-line low temperature nuclear orientation facility NICOLE

We review major experiments and results obtained by the on-line low temperature nuclear orientation method at the NICOLE facility at ISOLDE, CERN since the year 2000 and highlight their general physical impact. This versatile facility, providing a large degree of controlled nuclear polarization, was used for a long-standing study of magnetic moments at shell closures in the region Z = 28, N = 28–50 but also for dedicated studies in the deformed region around A ∼ 180. Another physics program was conducted to test symmetry in the weak sector and constrain weak coupling beyond V–A. Those two programs were supported by careful measurements of the involved solid state physics parameters to attain the full sensitivity of the technique and provide interesting interdisciplinary results. Future plans for this facility include the challenging idea of measuring the beta–gamma–neutron angular distributions from polarized beta delayed neutron emitters, further test of fundamental symmetries and obtaining nuclear structure data used in medical applications. The facility will also continue to contribute to both the nuclear structure and fundamental symmetry test programs.

Evolution with applied field of the magnetic structure of TbNiAl4

Neutron powder diffraction data of TbNiAl4 has been re-examined using a representational analysis, allowing a new model of the magnetic structure to be deduced. The basis vectors obtained describe an ‘elliptical helix’ type structure in which the moments rotate in the ab-plane as one moves along the c-axis. This new model has been used to simulate the expected result of a Low Temperature Nuclear Orientation (LTNO) experiment involving 299 keV gamma-ray emission from the 160Dy daughter of aligned 160Tb nuclei. Results of the simulation along the a-axis appear to partly match currently existing experimental data, with good agreement in the magnitude of lost anisotropy.

A new spin-oriented nuclei facility: POLAREX

Using the On-Line Nuclear Orientation method, POLAREX (POLARization of EXotic nuclei) is a new facility allowing to study the anisotropic decay of spin-oriented nuclei. Based on the combination of on-line implantation of radioactive nuclei with Low Temperature Nuclear Orientation technique and Nuclear Magnetic Resonance, POLAREX allows to measure nuclear electromagnetic moments and ground-state spins, in the aim to get information about the wave function composition of the nuclear state. Polarized nuclei can also be used to study fundamental interactions involving nuclear -decay asymmetries. The POLAREX infrastructure will be installed at Accelerateur Lineaire aupres du Tandem d’Orsay in order to study neutron-rich nuclei, some of which have not been studied yet. Will be presented here, all the possibilities of this new facility and a non exhaustive scientific program.

Applications of low temperature nuclear orientation

The techniques of low temperature nuclear orientation (LTNO) and nuclear magnetic resonance on oriented nuclei (NMRON) are versatile tools used across a range of applications. Such applications include nuclear moment and hyperfine field measurements and testing of nuclear decay models, as well as condensed matter studies, particularly applied to magnetism. Following the tradition of such presentations, the techniques LTNO and NMRON, are briefly outlined along with the principal applications with a focus on some recent works.

Low temperature nuclear orientation studies of the magnetic structures of RNiAl4 in applied magnetic fields

Low temperature nuclear orientation (LTNO) is applied to thermal neutron irradiated single crystal (160Tb)TbNiAl4. The magnetic field dependence of the gamma ray anisotropy is recorded and a temporary reduction is seen only during the transit of the first TbNiAl4 metamagnetic transition at ∼7 T, indicating the changed magnetic structure of the higher field phase still has essentially a axis moment alignment. A similar behaviour is seen for (147Nd)NdNiAl4.

First measurement of the nuclear magnetic moment of 254Es

Angular distributions of α particles and γ rays emitted by 253,254Es, 255Fm, and 250Bk nuclei were studied using the low-temperature nuclear orientation method. Information on the hyperfine interactions of these actinide impurities in an iron host lattice is derived from experimental data and the value of magnetic moment of 254Es nucleus is determined.

βasymmetry parameter in the decay ofIn114

The $\ensuremath{\beta}$-asymmetry parameter $\mathit{\~A}$ for the pure Gamow-Teller decay of $^{114}\mathrm{In}$ is reported. The low-temperature nuclear orientation method was combined with a GEANT4-based simulation code allowing us, for the first time, to address in detail the effects of scattering and the magnetic field. The result, $\mathit{\~A}=\ensuremath{-}0.994\ifmmode\pm\else\textpm\fi{}0.{010}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.{010}_{\mathrm{syst}}$, constitutes the most accurate value for the asymmetry parameter of a nuclear $\ensuremath{\beta}$ transition to date. The value is in agreement with the Standard Model prediction of $\mathit{\~A}=\ensuremath{-}1$ and provides new limits on tensor-type charged weak currents.

A GEANT4 Monte-Carlo simulation code for precision [formula omitted] spectroscopy

The measurement of the β asymmetry parameter in nuclear β decay is a potentially very sensitive tool to search for non V – A components in the charge-changing weak interaction. To reach the required precision (percent level) all effects that modify the emission pattern of the β radiation, i.e. the geometry of the setup, the effect of the magnetic field on the trajectories of β particles as well as (back)scattering in the source, on the sample holder and on the detector, have to be correctly taken into account in the analysis of the data. A thorough study of these effects and a new method based on detailed GEANT4 Monte-Carlo simulations that was developed for this purpose is presented here. The code was developed for β asymmetry measurements by means of the Low Temperature Nuclear Orientation (LTNO) method, but can in principle be generalized to other experimental setups using other polarization techniques.

On the absence of appreciable half-life changes in alpha emitters cooled in metals to 1 Kelvin and below

The recent suggestion that dramatic changes may occur in the lifetime of alpha and beta decay when the activity, in a pure metal host, is cooled to a few Kelvin, is examined in the light of published low temperature nuclear orientation (LTNO) experiments on such sources cooled to as low as 25 mK, with emphasis here on alpha decay. In LTNO observations are made of the anisotropy of radioactive emissions with respect to an axis of orientation. Correction of data for decay of activities in metallic samples held at temperatures at and below 1 Kelvin for periods of several days has been a routine element of LTNO experiments for many years. No evidence for any change of half-life on cooling, with an upper level of order 1%, has been found, in striking contrast to the predicted changes, for alpha decay, of several orders of magnitude. The proposal that such dramatic changes might alleviate problems of disposal of long-lived radioactive waste is shown to be unrealistic.

Low temperature nuclear orientation observations of severe neutron activation damage in ( 160 Tb)TbNiAl 4

An unusually large attenuation (approximately 500%) in the low temperature nuclear orientation directional gamma anisotropy from unannealed thermal neutron irradiated single crystal (160Tb)TbNiAl4 has been observed in the region of saturation of the second order nuclear orientation coefficient, B2. It is suggested that the form of the remanent neutron activation recoil damage in unannealed (160Tb)TbNiAl4 is fundamentally electric quadrupolar in origin, as opposed to magnetic dipolar.

Magnetic Nanostructures: Radioactive Probes and Recent Developments

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Nuclear spin-lattice relaxation ofCu62at low temperatures in iron

The nuclear spin-lattice relaxation of $^{62}\mathrm{Cu}$ in iron has been studied with the low temperature nuclear orientation method. At temperatures ranging from 6.5 mK to about 100 mK and a magnetic field of 0.1 $T$ the relaxation constant for $^{62}\mathrm{Cu}$ in Fe was found to be ${C}_{\mathrm{K}}[$$^{62}\mathrm{Cu}$$]=4.34(25)\mathrm{s}\mathrm{K}$.

A new Brute force low-temperature nuclear orientation set-up to search for physics beyond the standard electroweak model

A new Brute Force Low-Temperature Nuclear Orientation set-up has been installed. The aim of this new apparatus is to search for a tensor component in the weak interaction. The set-up consists mainly of a 3He– 4He dilution refrigerator equipped with a 17 T superconducting magnet to orient nuclei and a Si PIN photodiode operating at a temperature of about 10 K for the detection of β -particles. The lowest temperature that has been achieved with this system until now is 4.9 mK in a magnetic field of 0.1 T and 8.6 mK in 14 T. The set-up is now fully operational and the first test measurements of the 60Co β -asymmetry have been performed.

A study of the induced magnetism in the Au spacer layer of Co/Au/CoO exchange-bias trilayers and related systems

We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at a magnetic/non-magnetic interface, applying low temperature nuclear orientation (LTNO) to Co/Au(x)/CoO trilayer systems. This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au γ-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurements on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.

A study of perpendicular Ag hyperfine fields induced at the interface inFe∕Agmultilayer systems

In a series of experiments the hyperfine fields of Ag at the interface with Fe were measured in $\mathrm{Fe}∕\mathrm{Ag}$ multilayers using the low temperature nuclear orientation technique. The thickness of the Fe layers was chosen so that the in-plane shape anisotropy would dominate over the perpendicular surface anisotropy. However, for multilayers with a well ordered multilayer structure as measured by x-ray diffraction, the Ag magnetic hyperfine fields were found to be nearly perpendicular to the layer plane. The out-of-plane magnetic structure of the induced Ag magnetic moments has been also supported by vibrating sample magnetometry measurements on the same samples. In addition, the magnitude of the induced Ag hyperfine fields are significantly reduced by roughness. Calculations using WIEN97 confirm that the Ag hyperfine fields are strongly affected by the number of nearest Fe neighbors and their distance.

Study the magnetic structure of FeCl 2·4H 2O crystals by LTNO technique and molecular field theory

We have grown FeCl 2·4H 2O crystals with monoclinic structure and have studied their magnetic structure using low-temperature nuclear orientation technique and molecular field theory. The results show that at zero applied fields the angle between the electronic spins of each sublattices with the easy axis of magnetization is about 38°. Using molecular field theory to fit the experimental data, we have obtained the exchange field, B e = 1.15 T and anisotropy field, B a = 2 T .

NMR/ON of 60Co Experiments on the Neutron Activated Fe-Co Alloy

The irradiation by thermal neutrons is a commonly used procedure for the preparation of radioactive samples for low-temperature nuclear orientation (LTNO) experiments. Such an irradiation is a potential source of irradiation damage. The ferromagnetic alloy Fe+0.5 Co was selected for this kind of radiation damage study by LTNO and the nuclear magnetic resonance of oriented nuclei (NMR/ON), respectively. It was found by LTNO that in an as-irradiated sample, smaller amount of 60Co nuclei is subject to regular hyperfine field Bhf=29.01 T in comparison with the annealed sample. The same LTNO single resonance line on the frequency f=165.6(2) MHz was observed with both as-irradiated and annealed samples, respectively. The resonant destruction of γ-ray anisotropy is significantly smaller in as-irradiated sample and higher rf field is to be used to obtain a sizable NMR/ON effect. The experimental data indicates a great spread of hyperfine field values in non-regular positions of 60Co nuclei without contribution to the NMR/ON.

Low-Temperature Nuclear Orientation of 144Pm in Metamagnetic (RE)NiAl4 Single Crystals

Magnetic properties of Pm ions in NdNiAl4 were investigated by low-temperature nuclear orientation of 144Pm. The observed γ-ray anisotropy as a function of external fields revealed a change in the magnetic structure of Pm ions at the metamagnetic phase transition of NdNiAl4. The extracted hyperfine field was 50(5) T which is substantially smaller than those previously reported.