Determination of the magnetic dipole moment of114Sb Via on-line nuclear orientation

The magnetic dipole moment of¹¹⁴Sb has been measured using on-line nuclear orientation (OLNO) at the UNISOR Nuclear Orientation Facility (UNISOR/NOF). The value was determined to be 1.72 (8) nuclear magnetons. The observed anisotropy of the 1299 keV transition was fitted as a function of temperature making allowance for incomplete thermalization of the¹¹⁴Sb nuclei prior to decay. The relaxation constant, C_k, is discussed, as is the ground state structure of¹¹⁴Sb.     

The measurement of beta asymmetries at UNISOR/NOF using external plastic scintillator detectors

The performance of two external plastic scintillator detectors has been tested in an off-line mode using a source of¹⁰⁶RuFe at UNISOR/NOF. The measurement of beta asymmetries at various temperatures has resulted in the reproduction of the previously-determined magnetic dipole moment for the 1⁺ ground state of¹⁰⁶Rh and in addition the conclusion that the sign of this moment is positive.     

On-line nuclear orientation study of184Au

The¹⁸⁴Au→¹⁸⁴Pt decay, studied on-line with the UNISOR facility at HHIRF, is discussed. Gamma-ray and conversion-electron spectroscopy of¹⁸⁴Pt as well as on-line nuclear orientation measurements of¹⁸⁴Au were done. A new low-lying level scheme of¹⁸⁴Pt is proposed. Two coexisting bands with different deformations and their respective γ-vibrational bands are established. Internal conversion coefficients for interband transitions between states with the same spin are extracted from the spectroscopy measurements. The relative E0 contents of the transitions are determined by combining internal conversion coefficients with E2/M1 mixing ratios deduced from gamma-ray anisotropies measured from oriented nuclei.     

Simple and inexpensive3He insert for an ac susceptometer

The design of a simple and inexpensive³He insert for the existing ac susceptometer is described. The system uses a home built mutual inductance bridge for the ac susceptibility measurements from 0·4 K to 300 K. Simple and inexpensive design with the top loading facility are the main features of this set up. The insert can also be used as a continuously³He operating refrigerator down to 0·5 K.     

Measurements of neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets bombarded by 0.5 and 1.0 GeV protons

Measurements have been carried out on neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets and the surrounding paraffin moderators bombarded by 0.533 and 1.0 GeV protons. CR-39 detectors were deployed on the surfaces of targets and moderators to record the neutrons produced in the targets. The measurements show that:

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First experiments and future prospects at the Daresbury On-Line Isotope Separator

The Daresbury On-Line Isotope Separator (DOLIS) has recently been commissioned, in conjunction with the Nuclear Structure Facility (NSF) 20 MV tandem accelerator, and a³He-⁴He dilution refrigerator. First experiments are concentrating on a study of the decay of the neutron deficient iodine isotopes, extracted from a FEBIAD ion source and implanted at 60 keV into an iron host at 15 mK. As well as measuring iodine magnetic moments, the role of proton excitations across the Z=50 shell gap is being investigated in the even-even tellurium daughter nuclei. An on-line laser facility is also being developed, and first measurements have been made for unstable samarium isotopes     

Electron spin resonance studies of 199HgH, 201HgH, 199HgD and 201HgD isolated in neon and argon matrices at 4K: an electronic structure investigation

Various isotopomers of the mercury hydride radical (HgH) have been generated in a microwave discharge and trapped in neon and argon matrices at 4 K for electron spin resonance (ESR) investigations. Both the dipolar (A_dip) and isotropic (A_iso) components of the nuclear hyperfine interactions have been directly measured for ¹⁹⁹Hg, ²⁰¹Hg, H and D. Electronic structure information for HgH in its X²Σ ground state obtained from the hyperfine data is compared with theoretical results from several different computational methods. The hyperfine interactions in HgH are unusually large with A_iso(¹⁹⁹Hg) = 6859(3), A_dip(¹⁹⁹Hg) = 446(3), A_iso(H) = 730(2) and A_dip(H) = 0(2) MHz. A standard analysis of the hyperfine interactions demonstrates the need for a more in-depth theoretical treatment of HgH that should include relativistic effects. An interesting shift in spin density is observed when deuterium replaces hydrogen in HgH. The decreased spin density on deuterium, which was demonstrated in earlier studies, can now be more fully investigated since these new measurements confirm an associated increase in spin density on mercury     

Determination of spin,magnetic moment and isotopic shift of neutron rich205Hg by optical pumping

Neutron rich²⁰⁵Hg (T _1/2=5.2 min) was produced and on-line mass separated at the ISOLDE facility at CERN. The polarization achieved by optical pumping via the atomic line (6s ²¹ S ₀?6s6p ³ P ₁,λ=2 537Å) was monitored by theβ decay asymmetry. Hyperfine structure and isotopic shift of the²⁰⁵Hg absorption line was determined by Zeeman scanning. In addition a magnetic resonance was performed on the polarized²⁰⁵Hg nuclei in the atomic ground state. The results are: \(I(^{205} Hg) = \tfrac{1}{2}\) (confirmed);μ _I (²⁰⁵Hg)=0.5915 (1)μ _N (uncorrected for diamagnetism); isotopic shiftδv^204/205=v(²⁰⁵Hg)-v(204Hg)=?1.8 (1) GHz.μ _I and IS are discussed briefly in the frame of current literature.     

Isotope shift measurements on the neutron-deficient thallium isotopes

Isotope shifts of the neutron-deficient thallium isotopes were measured on-line at the UNISOR mass separator. Spectroscopy was carried out using collinear fast beam laser spectroscopy in neutral thallium. The changes of the mean square charge radii were derived from the measurements. The changes in charge radii of the I=7 isotopes, including the newly measured¹⁸⁸Tl, are compared to the results in mercury and lead.     

Nuclear orientation as a tool for studying the structure of very unstable nuclei

With the availability of modern isotope separator on-line systems it has become possible to make broad and systematic studies of low-energy low-spin nuclear structure. A vital ingredient in such a program is unique spin-parity assignments to all low-lying levels. A most desirable complement to spin-parity information is detailed spectroscopic information. Obtaining such information far from stability is difficult because of low activity production. Nuclear orientation provides a means for obtaining spin assignments usingsingles measurements. This is less demanding on source intensities than - angular correlation coincidence measurements. Further, nuclear orientation can provide information on magnetic moments and on multipole mixing ratios. A number of structural problems are discussed: the need for unique spin assignments in systematics schemes; the need to distinguish between E2+E0 and M1 transitions; the importance of measuring E2-M1 mixing ratios; and the value of magnetic moment information. Particular emphasis is placed on the desirability of obtaining such information in the neutron-deficient Pt, Au, Hg, Tl, Pb and Bi isotopes, based upon the experimental program at the UNISOR facility.Work supported in part by U.S.Dept. of Energy, Contract No. DE-AS05-80ER10599.     

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.     

Electric hyperfine alignment of Au nuclei in KAu(CN)2 observed by the Mössbauer effect

A large electric hyperfine alignment of ¹⁹⁷Au nuclei has been observed by Mössbauer absorption measurements of the 77.3 keV γ-transition of ¹⁹⁷Au in diamagnetic KAu(CN)₂. the measurements were extended down to 36 mK by employing a ³He/⁴He dilution refrigerator. The nuclear spin-lattice relaxation time was estimated from the time dependence of the asymmetry in Mössbauer spectra. It is shown that the spectra of oriented specimen can be used for determination of the recoilless factor of the absorber.     

Integrated target-ion source unit for on-line production of radioactive short-lived isotopes

A new version of integrated target-ion source unit (ionising target) has been developed for the on-line production of radioactive single-charged ions. The target is able to withstand temperatures up to 2500 ^°C and acts also as an ion source of the surface and laser ionisation. Off-line and on-line experiments with the ionising target, housing tantalum foils as a target material, have been carried out at the IRIS (Investigation of Radioactive Isotopes on Synchrocyclotron) facility. The off-line surface ionisation efficiency measured for stable atoms of Li, Rb and Cs was correspondingly 6% , 40% and 55% at the target temperature of 2000 ^°C and 3-10% for atoms of rare-earth elements Sm, Eu, Tm and Yb at a temperature of 2200 ^°C. The off-line measured values of the ionisation efficiency for stable Gd and Eu atoms by the laser beam ionisation inside the target were 1% and 7%, respectively. The radioactive beam intensities of neutron-deficient rare-earth nuclides from Eu to Lu produced by the integrated target-ion source unit have been measured at a temperature of 2500 ^°C. The results of the integrated target-ion source unit use for on-line laser resonance ionisation spectroscopy study of neutron-deficient Gd isotopes have been also presented.     

Dynamically polarized target for the g 2 p and G E p experiments at Jefferson Lab

Recently, two experiments were concluded in Hall A at Jefferson Lab which utilized a newly assembled, solid, polarized hydrogen target. The primary components of the target are a new, high cooling power ⁴He evaporation refrigerator, and a re-purposed, superconducting split-coil magnet. It has been used to polarize protons in irradiated NH₃ at a temperature of 1 K and at fields of 2.5 and 5.0 tesla. Maximum polarizations of 55% and 95% were obtained at those fields, respectively. To satisfy the requirements of both experiments, the magnet had to be routinely rotated between angles of 0°, 6°, and 90° with respect to the incident electron beam.     

Holifield Radioactive Ion Beam Facility — status and plans for the near future

The Holifield Radioactive Ion Beam Facility is described. Utilizing the isotope separator on-line technique, this new user facility is coming on-line at the end of 1996. A new experimental endstation centered around the Recoil Mass Spectrometer is also described. Recent data taken on ⁷⁴Rb and ⁷⁵Rb is also presented.     

Alpha-decay study of188Pb and180,182Hg

Theα decay of mass-separated¹⁸⁸Pb and^180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asα-X-t andα-e-t coincidence events were collected. Fine structure in theα decay of¹⁸⁸Pb was measured for the first time, feeding the 2₁ ⁺ and 0₂ ⁺ states in¹⁸⁴Hg. Theα decay of^180,182Hg feeds low-lying states in^176?178Pt. One of these states could be identified as a 0⁺ state. The deexcitation pattern could be established. As a by-product an alpha-decay scheme of¹⁸⁰Au towards levels in¹⁷⁶Ir is given.     

EquilibriumK-shell excitation of highly ionized neon

Reorientation effect and multiple Coulomb excitation measurements have been performed for^{198, 200, 202, 1204}Hg. By measuring particle spectra after Coulomb excitation of an enriched¹⁹⁸Hg target with⁴He and¹⁶O beams, a positive quadrupole moment of Q₂ = 0.66±0.18 eb (for negative interference) or Q₂=0.80±0.18 eb (for positive interference) andB(E2,0→2)=0.991±0.006 e²b² were obtained. All otherB(E2) values and quadrupole moments were determined relative to these numbers from preciseγ-ray yields obtained from Coulomb excitation of a frozen^natHg target with⁴He and¹⁶O projectiles. Oblate shapes of all Hg isotopes were found. The data have been interpreted in terms of triaxial rotation and anharmonic vibrations.     

Time-resolved and time-integral on-line nuclear orientation measurements of neutron-deficient Hg−Au−Pt−Ir nuclei

The methods of time-resolved and time-integral on-line nuclear orientation have been applied to study short lived nuclei with the NICOLE facility (Nuclear Implantation into Cold On-Line Equipment) at ISOLDE-3 in CERN using beams of^182–186Hg. The half-lives in these decay chains are of the order of seconds and therefore comparable to the spin-lattice relaxation times of the nuclei in iron. As the relaxation rate depends strongly on the g-factor, g-factors of nuclei in the decay chains can be deduced from the observation of the time evolution of γ-ray anisotropy. Using this technique the existence of an isomer in¹⁸⁴Au has been found and the g-factors of¹⁸⁴Au,^184mAu and¹⁸²Au have been determined. Accurate half-lives have been extracted from the data. Time-integral nuclear orientation has been observed for short lived as well as longer lived isotopes of the Hg decay chains. From these measurements, after proper correction for incomplete relaxation, the magnetic moments of^183mPt,¹⁸³Ir and¹⁸²Ir have been derived. The applicability of the time-resolved nuclear orientation technique for nuclei far from stability and its possible limitations is discussed.