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.     

Charge radii and shape transitions in short-lived Hg,Au and Pt isotopes

The hyperfine structure and the isotope shift of very neutron-deficient Au and Pt isotopes have been determined at the on-line isotope separator ISOLDE/CERN by resonance ionization mass spectroscopy combined with pulsed-laser induced desorption of the implanted radioactive sample. The changes of the mean-square charge radii were determined for the isotopes¹⁸⁴Au (T_1/2=53 s) and¹⁸³Au (T_1/2=42 s) as well as for 15 isotopes of platinum in the range between¹⁹⁸Pt (stable) and¹⁸³Pt (T_1/2=6.5 min). The strong deformation of¹⁸⁵Au (|β₂|≃0.25) persits down to¹⁸³Au. In¹⁸³Pt nearly the same value of |β₂| is reached but the deformation is build up rather smoothly in contrast to the neighbouring isotopes of gold and mercury. The magnetic moment of¹⁸³Pt was found to be μ₁=+0.51(3)μ _N .     

Relative intensity and internal conversion coefficient measurements in the decays of 51Cr, 137Cs, 192Ir,and 207Bi

An “internal conversion coefficient spectrometer” utilizing Ge(Li) and Si(Li) detectors has been used to make precision measurements of relative gamma-ray intensities for transitions following the decay of ¹⁹²Ir and ²⁰⁷Bi, relative internal conversion line intensities for some transitions from ¹⁹²Ir, and conversion coefficients for some transitions from the decays of ⁵¹Cr, ¹³⁷Cs, and ¹⁹²Ir. Limits and values are given for $\text{E2}\text{M1}$ mixing ratios and penetration effects for the 320.1-keV transition in ⁵¹V, the 484.5-keV transition in ¹⁹²Os, and the 604.4-keV transition in ¹⁹²Pt. The consequences of the results for nuclear structure are discussed.     

Half-life measurements and gamma spectroscopy in the decay184Re m →184W

The half-lives of the intrinsic states at 1,285.8 and 1,502.2 keV in¹⁸⁴W have been measured in the decay of¹⁸⁴Re ^m to be 8.33±0.18Μs and 2.35±0.10 ns, respectively. These half-lives determine absolute gamma-ray transition probabilities of eight transitions and yield upper limits for three transitions. The decay scheme of¹⁸⁴Re ^m →¹⁸⁴W is improved through measurements of delayed gamma-ray spectra with a Ge(Li) detector. Spin and Nilsson model interpretations are discussed.     

Measurements of multipolarities in 227Ra as tests of evidence for stable octupole deformation

Multipolarities of ~30 transitions in ²²⁷Ra have been established by measuring conversion electrons following the β⁻ decay of ²²⁷Fr. For this purpose a “mini-orange”-type electron spectrometer has been constructed. The ²²⁷Fr isotopes were produced by the ISOLDE on-line separator at the CERN Synchro-cyclotron. Internal conversion coefficients were obtained from singles spectra and also from simultaneous γe⁻ and γγ coincidence measurements. The new results support the placement of levels and transitions in the earlier level scheme but require changes in the previously assigned parities for four of the levels. Also, one E0 transition was identified. The results are consistent with previous interpretations for most of the levels that have been used to argue in favour of a small permanent octupole deformation for ²²⁷Ra.     

Nuclear structure of194Pt

The decay of¹⁹⁴Ir has been thoroughly studied resulting in the construction of a decay scheme consisting of 26 excited states and 69 transitions. Nine new levels and 29 new transitions were added to the previously known scheme. Of these, levels at 1,893.6 and 2,053.0 keV as well as 9γ-ray transitions are new to the¹⁹⁴Pt level scheme. Preciseγ-ray energy and intensity measurements as well as quantitative coincidence measurements were performed, and the internal conversion spectrum was investigated with a Si(Li) detector. Directional correlation measurements were performed for selected cascades with the major result being the unique assignment of 0⁺ to the new level at 1,893.6 keV. The structure was interpreted within the framework of the PPQ model as well as the effective-core picture including nuclear triaxiality.     

Concerning internal conversion coefficients in Hf180

The gamma-ray relative intensities from transitions in Hf¹⁸⁰ following the 5.5 hour Hf^180m decay have been measured using a bent-crystal gamma-ray diffraction monochromator and a least-squares fitting technique. From these measurements it was possible to deduce a value for the internal conversion coefficient for the 93.3-keV transition of α _T ⁹³ =4.91±0.23. From previous measurements of conversion electron intensities byEdwards andBoehm and our gamma-ray relative intensities, internal conversion coefficients for all transitions except the 57.5-keV transition were obtained. These coefficients agree well with the previous determinations byEdwards andBoehm, however, our measurements have improved precision, particularly in the case of 501.3-keV transition. The present measurements of α _K for the 215.3-, 332.5-and 443.8-keVE2 transitions are 11% lower than theoretical values while α_K for 93.3 keV E2 transition agrees closely with the theoretical value. These results are in close agreement with the previous measurements ofEdwards andBoehm. The present experimental α_K for the 501-keV transition agrees closely with the theoretical α_K for an E3 multipolarity.     

Nicole: New on-line orientation facility at ISOLDE/CERN

This paper presents off-line and on-line orientation results obtained with the new on line nuclear orientation device NICOLE in CERN. Magnetic moments of^187,185Pt ^g and¹⁸⁶Ir ^m , electric quadrupole moment ratios between Pt isotopes with mass number 185, 187 and 189, the spin of the 2 h isomer of¹⁸⁶Ir and a new decay scheme of¹⁸⁴Au involving a metastable state are established. The shape variation versus mass number of the Pt isotopes leads to sign change of the spectroscopic quadrupole moment between A=187 and 185.     

Nuclear level structure of99Tc

The gamma rays of⁹⁹Tc excited through negatron decay of⁹⁹Mo have been studied by internal conversion spectroscopy, using a high resolution double focusing beta-ray spectrometer. Conversion electrons were observed for all transitions reported before. In addition a new gamma ray of energy 989.37±1.02 keV could be identified and ascribed for the first time to the decay of⁹⁹Mo. Gamma-ray energies have been determined with high precision. Multipolarity data, obtained from the measurements of absolute or ratio of conversion coefficients of gamma rays, were utilized for assigning possible spins and parities to the levels of⁹⁹Tc. From our present data combined with other studies spins and parities; 7/2+, 1/2?, 5/2+, 3/2?, 3/2+ and 5/2? have been given to the 140, 142, 181, 514, 922 and 1131 keV levels respectively. The experimental level scheme is discussed in terms of theoretical predictions.     

Configuration assignments and decay of 126La high-spin bands

High spin states of ¹²⁶La have been populated using the reaction ¹¹⁶Sn+¹⁴N at 68 MeV. γ-rays and conversion electrons were detected with the GAREL array. Multipolarities of the lowest-lying in-band dipole transitions have been determined from the deduced internal conversion coefficients. Experimental B(M1)/B(E2) ratios have been derived for the bands and compared with calculated values using the D?nau-Frauendorf geometrical model. Configurations are proposed for the bands comparing them with cranked shell model calculations and on the basis of the measured B(M1)/B(E2) ratios. The β-decay of ¹²⁶La has also been revisited. The population of the ¹²⁶Ba levels gives a probable spin value of five for the decaying high-spin ¹²⁶La state with T _1/2≈ 64 s which may indicate a signature inversion in the πh_11/2νh_11/2 band. Received: 12 October 1999     

Decay studies of101m Rh and101g Rh

The level structure of¹⁰¹Ru has been investigated from the decays of^101m Rh and^101g Rh. For the gamma-ray measurements Ge(Li) detectors with effective volumes of 24 and 43 cm³ were used for singles and coincidence work, and several new transitions were observed. Conversion electron spectra from the^101m Rh decay were investigated with a 50 cm double-focussing beta spectrometer.K-conversion coefficients were determined by combining internal conversion and gamma-ray intensities. For the stronger transitions probable multipolarity assignments have been obtained from these coefficients and also fromK/L andL-subshell ratios. All transitions observed were fitted into a decay scheme, in which tentative spin-parity assignments of all states are given.     

Conversion-electron spectroscopy in the decay of superdeformed 135Nd

We have investigated the structure of the superdeformed band in ¹³⁵Nd using conversion-electron spectroscopy. The transitions within the superdeformed band show conversion coefficients which are consistent with an E2 multipolarity. From the conversion coefficients of transitions linking this band with known low-lying states its positive parity was established. Hence, a direct determination of the parity of a high-spin superdeformed band could be performed for the first time.     

Studies of the low-lying levels in192Pt and192Os

The energy level schemes of¹⁹²Os and¹⁹²Pt have been established on the basis ofγ- γ coincidence studies using a dual parameter data collection system. Ge(Li) detectors were employed to study the gamma spectra produced in the E.C. andβ ^? decays of¹⁹²Ir to¹⁹²Os and¹⁹²Pt, respectively. Thirteen new transitions and three new levels at 1,146.95, 1,237.35 and 1,913.76 keV are suggested. Relative intensities from singles measurements, branching ratios,α(K) and log ft values were calculated and multipolarities, spins and parities deduced. Comparisons are made with predictions of the Interacting Boson Model calculated on the basis of an O(6) to SU(3) transition.     

A Si(Li) detector system for conversion coefficient measurements and application to the decays of51Cr and87Y

An electron detector system with a 3 mm × 100 mm² Si(Li) detector with a cooled FET and vacuum air lock for changing sources was constructed. The absolute efficiency was measured as a function of energy for use in the measurement ofK conversion coefficients. With the Si(Li) and a Ge(Li) detector, theK conversion coefficient of the 320 keV transition in⁵¹V was measured to be 0.00156 ± 0.00008 where theK conversion coefficients of the 661 and 279 keV transitions in¹³⁷Ba and²⁰³Tl were used as standards. From absolute detection efficiency measurements, theK conversion coefficients of the 388 and 483 keV transitions in⁸⁷Sr were measured to be 0.16 ± 0.02 and 0.0024 ± 0.0003, respectively.     

Internal conversion studies of transitions in96Mo

The internal conversion spectrum from the decay of⁹⁶Tc has been investigated using a 50 cm radius double-focussing beta-ray spectrometer. Relevant features of the internal conversion spectrum from the decay of⁹⁶Nb have also been reexamined in the same way. Special interest has been focussed on investigating a proposed doublet level structure at about 2.44 MeV and higher lying levels in⁹⁶Mo. Internal conversion coefficients have been obtained for most transitions by combining the measured conversion electron intensities obtained in this work with recently reported gamma-ray intensities from Ge(Li) detector measurements. A level scheme has been compiled and deduced transition multipolarities have been used for a discussion of spins and parities for the levels.     

First evidence of the magnetic character of dipole transitions in the “oblate bands” of199Pb

Internal conversion coefficients have been determined from online measurement of electron- and y- ray emission related to the dipole transitions in the so-called oblate collective bands in ¹⁹⁹Pb.The results strongly support the M1 (or M1+E2) character of these transitions.     

Tentative level diagram of146Pm below 1.1 MeV

The level scheme of the doubly odd nucleus¹⁴⁶Pm was investigated by means of in-beam spectroscopy. The¹⁴⁶Nd(p, n) reaction in the 3.5–8 MeV proton energy interval on enriched targets has been used to populate the exited states of¹⁴⁶Pm. High resolutionγ-ray spectra using Ge(Li) and hyperpure Ge detectors and conversion electrons with a mini-orange spectrometer were measured. Withγ-exitation functions andnγ-coincidences usingnγ-discriminated NE231 neutron detectors transitions in¹⁴⁶Pm were identified,γγ-coincidences gave information aboutγ-cascades. With these data a level scheme of 38 levels below 1.1 MeV exitation energy has been deduced including 73 transitions. For 12 transitions experimental conversion coefficients have been evaluated, which agree with the expected values forM1/E2-transitions.     

Excited states in195Pt from the decay of 2.3h195Irg, 3.7h195Ir m and 183d195Au

The decay of two previously reported isomers in¹⁹⁵Ir has been investigated with Ge(Li) and Si(Li) detectors by observing β and γ rays as well as conversion electrons in single and coincidence measurements. To about 30 of the 85 observed transitions a multipolarity was assigned. The energies of the gamma rays from¹⁹⁵Au were also determined accurately. The decay scheme of¹⁹⁵Ir shows 13 new levels in¹⁹⁵Pt between 389 and 931 keV. Probable spin and parity assignments could be made to most levels. The results are compared with those of a recent Coulomb excitation study, reaction work and theoretical calculations.     

The level structure of 184Os from 184Ir decay and 185Re(p, 2nγ) in-beam spectroscopy

Levels of ¹⁸⁴Os populated in the decay of 3.1 h ¹⁸⁴Ir and in the ¹⁸⁵Re(p, 2nγ) reaction have been investigated. The measurements included γ-ray singles, β⁺ ray endpoint, conversion coefficient, β⁺-γ coincidence and detailed γ-γ coincidence determinations. The results have established an extensive ¹⁸⁴Os level scheme, which includes well developed ground state, γ-vibrational and K = 3 octupole bands and which accommodates all the intense transitions observed in both the radioactivity and in-beam γ-ray measurements. Deviations of the level energies in the K^π = 0⁺and K^π = 2⁺ bands and of the interband reduced transition probabilities from the predictions of the strong-coupling model are discussed in terms of the rotationvibration interaction, and the systematics of the octupole vibrational excitations in even-mass W and Os nuclei are reviewed. It is concluded that the ¹⁸⁴Ir ground state configuration has a spin of 5, and that it contains large admixtures of K = 0 or K = 1 character.     

Study of the radioactive decay of192Ir

Theβ ^? and electron capture decays of¹⁹²Ir to the levels of¹⁹²Pt and¹⁹²Os respectively have been investigated to verify the existence of some of weak gamma transitions reported earlier. Gamma-gamma directional correlation measurements have been done for 13-cascades. Multipole admixture of 201, 283, 485 keV gamma transitions in¹⁹²Os and 296, 308, 416, 604 keV gamma transitions in¹⁹²Pt have been deduced. A spin value of 4⁺ has been reaffirmed on the basis of almost zero intensity of 1201 keV gamma ray (4⁺→0⁺) and 416→(468)→316 keV cascade directional correlation measurements.