Search

Using a208Pb beam of the UNILAC accelerator at GSI we produced the new isotopes203Au and205Au and identified them by measuring theirß-decay properties. Their half-lives were found to be 60(6)s and 31(2)s, respectively, and theQ-value of203Au was determined to beQß=2040(60)keV. Nuclear structure information of203,205Hg was derived from this measurement. Furthermore, production cross-sections of203,205Au are given and compared to those obtained in other experiments.

The neutron-rich isotope179Yb was produced by using the multinucleon transfer reactions 11.6 MeV/u136Xe+Re and 11.4 MeV/u197Au+W. The isotope identification was achieved by combining mass-separation and decay spectroscopy. A decay scheme was constructed including a new isomeric state in179Lu with a half-life of 3.1(9) ms.

Thea decay of mass-separated188Pb and180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asa-X-t anda-e-t coincidence events were collected. Fine structure in thea decay of188Pb was measured for the first time, feeding the 21+ and 02+ states in184Hg. Thea decay of180,182Hg feeds low-lying states in176-178Pt. 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.

Using on-line mass separation of evaporation residues from the reaction58Ni +92Mo?150Yb*, a proton line of 1,055±6 keV energy and 0.42±0.10 s half-life was observed at mass number 147. The origin of this activity is very likely the direct proton decay of147Tm. Beta-delayed protons registered at the same mass position show a pronounced peak structure in their energy distribution. A lower limit of their half-life was set to 1 s.

Fusion reactions between58Ni projectiles and58Ni target nuclei were used to produce114Cs activity, which was studied by on-line mass separation and decay spectroscopy. The obtained half-life of 0.57±0.02 s and the probability ratio of 44±3 between betadelayed proton and beta-delayed alpha emission represent data with improved precision. The absolute branching ratios for both particle decay modes were determined to be (7 ±2)×10-2 and (1.6±0.6)×10-3, respectively. These branching ratios and the shapes of particle spectra are discussed with reference to the statistical model calculations. Groundstate alpha decay of114Cs was observed with an alpha energy of 3,226±30 keV and a branching ratio of (1.8±0.6)×10-4. An upper limit of the branching ratio for a possible ground-state proton decay was set at 5×10-4. From?-ray singles and?-? coincidence measurements, excited states in114Xe are proposed.

The EC/ß+-decay of the odd-odd nucleus100Ag was studied by means of total absorption?-ray spectrometry. Most of the Gamow-Teller strength was found to be concentrated at an excitation energy of 5.6 MeV in100Pd, the FWHM of this resonance being 1.5 MeV. The measured strength distribution which is interpreted within the BCS approximation as being due to the dominant population of four-quasiparticle excitations, resembles the distribution predicted by an advanced shell-model calculation for the98Ag ?98Cd decay.

We report here on the measurement of isotope shift and hyperfine splitting of190, 191, 193, 197Pb for the 723 nm atomic optical transition. Detailed analysis of the optical data has been done by combining them with the available muonic and electronicx-ray isotope shift data. The magnetic dipole moments and the electric quadrupole moments of the odd isotopes have been extracted from the hyperfine coupling constants of the atomic states involved in the optical transition used.

With a summation-free ß+-endpoint spectrometer the ß+-decay energies of104Sn,147Tb,148, 149Dy,149Ho,150Er, and151Tm were remeasured with improved accuracy. Combined with known proton and alpha decay energies, the resulting QEC values lead to atomic masses of very neutron-deficient isotopes including nuclei beyond the proton drip-line such as109I and113Cs. Furthermore, the Gamow-Tellerß-strength of the even-even nuclei104Sn,148Dy, and150Er is reevaluated with reduced uncertainty.

Within the framework of a systematic study of 0+?1+?-transitions in the100Sn region, the decay of the neutrondeficient isotope102Cd was investigated. This isotope was produced in16O+92Mo and58Ni+50Cr heavy-ion reactions and in proton-induced spallation ofnatSn, and mass-separated samples were prepared by means of the GSI and the ISOLDE on-line mass separators, respectively. Observations of X-rays,?-rays, and conversion-electrons studies have led to an improved102Cd?102Ag decay scheme which includes six 0+?1+ Gamow-Teller transitions. The half-life of102Cd was redetermined with higher accuracy to be 345±8 s. Using a Si(Li)-BGO-Ge spectrometer, the?+ endpoint energy of the main decay component was measured, yielding aQEC value of 2587±8 keV. This very small uncertainty ofQEC, combined with the improved knowledge of half-life and decay scheme, allows an accurate determination of the observed Gamow-Teller decay strengthBS(GT)=1.513±0.055 for102Cd. This result is discussed in comparison with predictions from model calculations.

The hyperfine structure splittings (hfs) and isotope shifts (IS) in the atomic transitions 5s2 5p21S0 ? 5s2 5p6s1,3P1 have been measured for the radioactive isotopes108-111Sn and all stable ones. The tin isotopes were prepared as fast atomic beams for collinear laser spectroscopy at the GSI online mass separator following a fusion reaction. Nuclear spins, magnetic dipole, electric quadrupole moments and changes in mean square charge radii have been determined. In109Sn the spinI=5/2 was measured for the nuclear ground state (T1/2=18 min) in contradiction to the literature value. The mean square charge radii show a parabolic behaviour with a maximum at N=66. This is interpreted by collective effects, which are considerably stronger than accounted for by theB(E2)-values.

Using the40Ca(4.2 MeV/u) +60Ni reaction and on-line mass separation, the decay properties of4696Pd50 were reinvestigated. From a comparison of experimental and theoreticalβ+/(EC +β+) probability ratios, theQEC value was deduced to be 3,450 ± 150 keV. The strength was determined for four 0+→1+ Gamow-Teller beta transitions and found to be quenched as compared to predictions of the single-particle shell model. For96Pd and94Ru the sources of this quenching are discussed.

The new proton-rich nucleus104Sn was produced in a50Cr(58Ni, 2p2n)104Sn reaction. Its half-life was measured to be 21±1s. From a newly established partial decay scheme, aQEC value of 4,000-300+650 keV was estimated. The resulting mass excess value for104Sn, combined with decay energies for a and proton emission, leads to mass excess values for the nuclei108Te,109I,112Xe, and113Cs. These mass values are compared to predictions of mass formulae.

Very neutron-deficient isotopes of rhodium, palladium, silver, cadmium and indium in theN=50 region were studied at the GSI on-line mass separator following bombardment of58, 60, 62Ni and63, 65Cu targets with 4.0 MeV/u ions of40Ca. The known properties of the ß-delayed proton precursor95mPd were confirmed, but a search for direct proton decay of95mAg was not successful. Beta-delayed proton activities observed at mass-numbers 94, 96, 97 and 100 were assigned to the precursors94Rh(66±14 s),96Ag(5.1±0.4s),97Cd(3-2+4s) and100In. In addition,94Pd(9.0±0.5 s) was identified from ß-?-X studies. Individual decay schemes of95m96Pd and96, 98Ag are reported.

Decay experiments for 148Ho 6−and 150Tm 6−were carried out at the GSI On-line mass separator using 5 MeV/A 58Ni-beams on 94Mo and 96Ru targets. Gammaγ-and γX-coincidences established the 150Tm 6−decay scheme, and e−gave firm Iπassignments for 148Dy and 150Er levels fed in the decays. It was found that the previously unknown $6_1^ -$state in 150Er receives significant β-strength, which strongly suggests $\pi {h_{11/2}}\nu s_{1/2}^{ - 1}$character for the 150Tm 6−isomer. The results are discussed in terms of the expected GT strength function. Shell model calculations for the $\pi h_{11/2}^4$yrast states are presented.

Thea-branching ratios (ba) of192,190,188Pb are measured using mass-separated sources. Different experimental set-ups are used — one detector as well as two detector set-ups — thereby detecting thea particles from the parent and/or viaa decay formed daughter nuclei, theß-delayed gamma radiation from the parent and/or viaß decay formed daughter nuclei and the Tl KX rays from electron capture decay. Values forba of 6.2(6) 10-5 and 4.0(4) 10-3 were found for192,190Pb respectively. For188Pb, limits on theba values were obtained: 0.03a<0.10. A careful analysis of the previously reportedba values showed that the discrepancies in theba values were not due to inadequate correction procedures, as was suggested, but to experimental uncertainties in the efficiency determination of the different detection set-ups and to an unreliableß-decay scheme for188Pb. Theba obtained in this work show that the leada decay is not faster than the Hga decay and that there is no need to assume a disappearance of theZ=82 shell closure halfway betweenN=82 andN=126.

With the use of the58Ni+50Cr fusion-evaporation reaction and chemically selective on-line mass separation, the EC/ß+ decay of104Sn was studied by?-ray and conversionelectron spectroscopy. In the course of these studies, a new 15.7-s isomeric state of104In was observed. Reinvestigation of theß-decays of the two heavier even isotopes of tin, produced in the58Ni+58Ni reaction and mass separated without chemical discrimination, brought new information on the decay of106Sn and108Sn. The QEC values of104Sn,106Sn and108Sn were determined to be 4550-250+300keV, 3200±100 keV and < 2270 keV, respectively. In the data analysis, the emphasis is put on the identification of the 0+ ? 1+ Gamow-Tellerß transitions and determination of their strengths. For104-110Sn, the total Gamow-Teller strengths are measured to be 20% to 30% of that predicted by the single-particle shell model with the inclusion of pairing correlations and core polarization. Predictions of the decay properties of the as yet unobserved102Sn and100Sn are made.

In theN=83 nucleus151Er we observed the isomericE3 transition in conversion electron measurements following on-line mass separation and located its 0.6 s 27/2- isomer at 2.586 MeV. Gamma-ray data establish a 4.7(3)% Gamow-Tellerß-decay branch proceeding to high-spin states above 2 MeV excitation in151Ho. A weakß-decay branch of 0.7(3)% was also observed for the 0.5 s 27/2- isomer in the149Dy isotone, and it was established that a similar 27/2-E3-isomer does not occur in153Yb. First results on the151Tm and151Er ground-stateß-decays are briefly discussed.

IrradiatingnatW/Ta targets with186W, the decay of the new isotope180Yb was observed and its half-life determined to be 2.4(5) min. Based on the measuredß-,?- and X-ray data, ther-process production of180mTa and the role of a possible isomer180mLu in this context are discussed.

Collinear fast beam laser spectroscopy has been used to measure the hyperfine structure and isotope shift in the atomic 5s2 5p2P3/2-5s2 6s2S1/2 transition (?=451 nm) of107–111In. Secondary beams of neutron deficient indium isotopes were prepared at the GSI on-line mass separator following fusion evaporation reactions. Magnetic dipole moments and electric quadrupole moments have been determined. The isotope shifts are discussed in terms of the change of the mean square nuclear charge radii and compared with the droplet model predictions and the deformation values calculated from the quadrupole moments.

Theß-decay branch of180Lu to180mHf is measured by an integral method tracing the amount of180mHf daughter activity, to be 0.46 (15)%. Samples of180Lu are prepared by on-line mass separation of136Xe-on-tungsten transfer reaction products. The amount of isobaric180mHf in the samples is strongly suppressed by the employed separator ion source; its directly produced fraction is derived from comparison with177mHf activity. The properties of lutetium and hafnium release from the ion source are separately determined. With this measurementr-process nucleosynthesis is estimated to account for at most 22% of the solar180mTa abundance.

The hyperfine structure and isotope shift of108gIn and108mIn have been investigated by means of collinear fast-beam laser spectroscopy in the resonance line at?=451 nm. The indium isotopes were prepared at the GSI on-line mass separator following a fusion evaporation reaction. For the first time, a FEBIAD ion source with bunched release of indium was used. Magnetic dipole moments, electric quadrupole moments and isotope shifts were determined. The present results lead us to assign the spinI=2 to the108In (T1/2=40 min) state. Spins and moments are discussed in the framework of thejj-coupling model.

The earlier preliminary assignment of a 1,055±6 keV proton line to direct proton decay of147Tm is supported by cross bombardment measurements and by a negative result from a positron-proton coincidence experiment. The half-life was remeasured to be 0.56 ±0.04s. For two types of thermal ion sources, overall efficiencies were estimated for on-line mass separation of known short-lived isotopes of promethium, europium, terbium, and holmium. Direct proton decay was searched for among very neutron-deficient isotopes of these elements, and of iodine and caesium. No evidence for direct proton decay was found. Based on estimated overall efficiencies, on calculated cross-sections, and on predictions from the gross-theory ofβ decay, half-life limits for direct proton decay were deduced.

Sources of187Pb were produced by the142Nd(48Ti, 3n)187Pb reaction followed by online mass separation of the products. Measurements of?-? anda-? coincidences and half-lives were performed. Twoa-emitting states in187Pb were found with half-lives of 15.2±0.3 s and 18.3±0.3 s, respectively. Thea-? coincidences yielded excitation energy and possible spin values for a few levels in183Hg. Gamma-transitions in187Tl were identified and a level scheme for187Tl is proposed on the basis of the coincidence relationships and the level systematics of the heavier odd-thallium isotopes.

Decay experiments for148Ho 6−and150Tm 6−were carried out at the GSI On-line mass separator using 5 MeV/A58Ni-beams on94Mo and96Ru targets. Gammaγ-andγX-coincidences established the150Tm 6−decay scheme, and e−gave firm Iπassignments for148Dy and150Er levels fed in the decays. It was found that the previously unknown 61−state in150Er receives significantβ-strength, which strongly suggestsπh11/2νs12/−1character for the150Tm 6−isomer. The results are discussed in terms of the expected GT strength function. Shell model calculations for theπh112/4yrast states are presented.

Using58Ni+50Cr and58Ni+54Fe reactions and on-line mass separation, the new isotopes103Sn and105Sn with half-lives of 7±3 s and 31±6 s, respectively, were identified via their beta-delayed proton decays. The relative yields and the possibility of reaching101Sn are discussed.

The QEC values of the ground-state and the isomer were measured to the 5750(40) keV and 5846(50) keV, respectively, corresponding to a148Tb ground-state mass-excess of ⦓70527(30) keV. The impact on mass calculations near146Gd is discussed.

The nucleus 151Er was investigated through γ-ray and conversion electron measurements from its 27/2−isomer decay produced in a HI reaction at the GSI on-line mass separator. Inaddition tothe known intense yrast cascade,a complex 8% decay branch to above-yrast states was identified. Among these, the 11/2+member of the νf7/2× 3−septet was unambiguously characterised. Its energy implies that the quadrupole moment of the 3−core has changed sign with respect to the lighter even-Z N=83 isotones.

Adedicated γ-ray measurement with mass-separated 147Dy activity gave the direct β-feeding intensities to the 147Tb πs1=2and πh11=2single proton states in decay of the 147Dyνs-11=2andνh-111=2activities.With these results the absolute transition rates for the three Gamow-Teller decay modes of 147Dy can be deduced.

Using an 11.4MeV/u136Xe beam of the UNILAC accelerator of GSI and anatW target, A=184 isobars were produced and separated by using an on-line mass separator. A new isomeric state was found in184Hf. This state decays with a half-life of 48(10) s and emits three ?-rays of 73, 482, and 555 keV as well as lower-lying cascade transitions. The hindrance of the 73 keV-E1 transition with ?K=8 is compared with those of other even-even hafnium isotopes. The half-life of184Lu was estimated to be between 15 and 25s.