The limit of stability of proton-rich thallium isotopes: a search for the decay of177Tl

The fusion reaction⁷⁴Se+¹⁰⁶Cd¹⁸⁰Pb^* at a compound nucleus excitation energy E_x 40 MeV, has been used in a search for the decay of the unknown nucleus¹⁷⁷Tl. Evaporation residues were velocity and mass analysed prior to implantation into a position sensitive silicon surface barrier detector. No evidence was discovered for the proton or alpha-decay of¹⁷⁷Tl for a cross-section > 10 _– ⁵⁺¹⁰ nb. This indicates either that¹⁷⁷Tl decays too rapidly (t_1/2 < 1=">s) by proton emission for the decay to be detected, or that¹⁷⁷Tl is produced with a cross-section less than the limit established in the present experiment. The following nuclear decay half-lives were measured with improved accuracy;¹⁷⁷Hg (t_1/2 = 130±5 ms),¹⁷⁸Hg (t_1/2=250±25 ms) and¹⁷⁷Au (t_1/2=1180±70 ms). The experiment also provided the first direct confirmation of the correct mass assignment of the alpha-decay, E=6.26 MeV, to the decay of¹⁷⁶Au. Two additional halflife measurements of proton-rich Osmium isotopes are also presented from a previous similar experiment;¹⁶⁵Os (t_1/2=73±8 ms) and¹⁶⁶Os (t_1/2 = 194±17 ms).     

Complete spectroscopy of51,52V via the50,51V(n, γ) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched⁵⁰V and^natV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in⁵¹V and⁵²V were determined to beB _n (⁵¹V)=11051.11(17) keV andB _n (⁵²V)=7311.22(26) keV. The thermal neutron capture cross-section in⁵⁰V was measured to be 21 _–2 ⁺⁴ b. From 724 lines attributed to⁵¹V, 330 transitions, comprising 90% of the-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2⁺ or 13/2⁺ capture states in⁵¹V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2I15/2) region was parametrized with the Fermi gas model. The spin distributions of⁵¹V were analyzed and a spin cut-off parameter =2.8 (3) was deduced. — A nearly complete level scheme of⁵²V up to 3.5 MeV excitation and similar results concerning the level density and the primary -ray spectrum were obtained in the⁵¹V (n, )⁵²V study.Supported by Deutsches BMFT under contract 06GÖ141     

The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

New assignments of quantum numbers have been obtained for more than 50 levels in²⁸Si between 8953 and 15915 keV excitation energy. They are based on the measurement of -ray angular distributions or anisotropies on 29 resonances of the²⁷Al(p, ) reaction and on the²⁴Mg(a, ) resonance atE =3355 keV. A total of 15 high-spin states withI=5–7 has been obtained and the spectrum ofT=1 states has been identified up to 15915 keV excitation energy and a maximum spinI=6.     

Measurements of magnetic moments of Tc isotopes and the hyperfine field of Tc in Fe and Ni

We report measurements of nuclear magnetic resonance on oriented nuclei (NMR-ON) on⁹⁴Tc (I=7⁺,T _1/2=4.9 h),⁹⁵Tc (I =9/2⁺,T _1/2=20 h) and⁹⁶Tc (I =7⁺,T _1/2=4.3 d) in Fe and on⁹⁵Tc and⁹⁶Tc in Ni. In order to elucidate the discrepancies on the hyperfine field of TcFe in the literature, the resonances were measured with high precision as a function of the external magnetic field B_ext up to 2.0 T. In addition, TDPAC measurements were performed on⁹⁹TcFe at T=300 K and 13 K. TheT=0 K hyperfine fields of Tc in Fe and Ni were determined to be –314(3) kG and –51.8(5) kG, respectively. The g-factors of⁹³Tc,⁹⁴Tc,⁹⁵Tc,⁹⁶Tc and⁹⁹Tc(E=181 keV;I =5/2⁺) are redetermined as g(⁹³Tc)=1.405(14), g(⁹⁴Tc)=0.731(7),g(⁹⁵Tc)=1.321(13), g(⁹⁶Tc)=0.727(7), g(⁹⁹Tc)=1.390(17).We wish to thank E. Smolic and G. Seewald for experimental help. This work was funded by the German Federal Minister for Research and Technology (BMFT) under the contract number 06TM353/TP 4, by the Deutsche Forschungsgemeinschaft (DFG) under the contract numbers Ha 1282/3-1,2, and, partly, by the Kernforschungszentrum, Karlsruhe.     

Experimental investigation of vibrational excitations in230Th

Nuclear levels in²³⁰Th have been investigated in the decay of²³⁰Pa, the²³⁰Th(d, pn) reaction and the²³²Th(p, t) reaction. TheK =0 ₁ ⁺ , 2 ₁ ⁺ , and 2 ₂ ⁺ bands with band heads at 635, 781, and 1010 keV were observed up to the 8⁺, 9⁺, and 7⁺ levels, respectively. A second excited 0⁺ level was identified at 1297 keV which might be interpreted as the usual shape-oscillation. The branching ratios of theE2 transitions from the 0 ₁ ⁺ , 2 ₁ ⁺ , and 2 ₂ ⁺ bands are explained in the framework of the rotational model by taking into account the coupling of these bands with the ground-state band, and the coupling between the 0 ₁ ⁺ and 2 ₁ ⁺ band. A strong enhancement ofE2 transitions from the 2 ₁ ⁺ to the 0 ₁ ⁺ band reported earlier is not confirmed. For the octupole vibrations withK =0^–, 1^–, and 2^– theE1 branching ratios are analyzed in terms of the Coriolis coupling of these bands. An almost complete experimental set ofE1 transition moments from these negative-parity bands to the 0 _g ⁺ , 0 ₁ ⁺ , and 2 ₁ ⁺ bands was obtained. It is suggested that octupole correlations might be important in explaining theseE1 moments.We appreciate the help of Mr. Assmus with the proton irradiations and the financial support of the Kernforschungszentrum Karlsruhe GmbH. This work was supported by the Deutsche Forschungsgemeinschaft (grants Bo 1109/1-1, Gr 894/2-1, Gu 179/3-1 and He 1316/2-3) and by the Volkswagen Stiftung.     

The decay of 31 sec98Zr, 2.9 sec98Nb and 51 min98Nb

The- and-radiations of⁹⁸Zr and^{98m, g} Nb have been investigated employing scintillation and semiconductor spectrometers and coincidence techniques. Sources of⁹⁸Nb were produced by the⁹⁸Mo(n, p)⁹⁸Nb reaction, sources of⁹⁸Zr by fission of²³⁵U with thermal neutrons applying chemical separations. For⁹⁸Zr, a half-life of 30.7±0.4 sec and aQ -value of 2.3±0.2 MeV were obtained, for the⁹⁸Nb isomers, half-lives of 2.86±0.06 sec and 51.3±0.4 min, andQ -values of 4.8±0.2 MeV and 4.5±0.2 MeV, respectively. No-rays were observed in the decay of⁹⁸Zr. The decay of 2.9 sec⁹⁸Nb was found to involve 11-ray transitions. In the decay of 51 min⁹⁸Nb, 54-transitions were detected. Spin and parity of 1⁺ and 4^– were deduced for the isomeric states of⁹⁸Nb.     

Detection on NMR via the anisotropy of X-rays (X-NMR-ON)

Nuclear magnetic resonance on oriented nuclei has been detected for the first time via the destruction of the anisotropy of characteristic L_x-rays. The new method can be applied to isomeric states which decay only via highly converted transitions, for which the standard NMR-ON technique — detection of NMR via the anisotropy of -rays — is not applicable. The X-NMR-ON technique has been used to measure the magnetic hyperfine splitting of^193mpt (I=1322⁺; E=149.8 keV; T_1/2= 4.3 d) to be ¦ g _NB_HF/h¦=111.3 (3) MHz. with the known hyperfine field of –1280(27) kG the magnetic moment of^193mpt is deduced to be ¦¦=0.7417(14) _N. This magnetic moment differs strongly from the known magnetic moments of the 13/2⁺ isomeric states in Hg and Pb and^195mPt.     

Lifetime measurements and particle-rotor calculations for 42 105 Mo63

The half-lives of seven levels in¹⁰⁵Mo have been determined with high precision at the fission-product separator Josef. The-- triple coincidence method with plastic and BaF₂ scintillators was used for a determination of the time between the _q feeding of a level of interest and its depopulation. A value of _q =0.38 (1) for the deformation parameter has been deduced from the half-lives of the first and second excited states. The results of Particle-Rotor model-calculations with the Nilsson parameters=0.084 and=0.28 support the assignment of the Nilsson orbital [532] 5/2^– to the ground state of¹⁰⁵Mo.The authors acknowledge with pleasure fruitful discussions with Profs. W.R. Phillips, I. Ragnarsson and O.W.B. Schult.     

Spin polarization in the photoemission from the 5d-core levels of a thallium film

A thick unordered Tl film has been deposited on Pt(111). Photoemission with photon energies between 20 eV and 27 eV reveals two peaks due to emission from the 5d _3/2-and the 5d _5/2-levels of Tl. The measurements are performed with circularly polarized radiation. Spin analysis allows separation of the photoelectron intensityI into its partsI ₊ with spin polarization vector parallel andI _– antiparallel to the photonspin. For thed _3/2-and thed _5/2-level the ratioI ₊/I _– turns out to be approximately 31 and 11.8 respectively. These values are close to the values expected ford-level photoionization of free atoms if spin orbit interaction in the final states is neglected. A slight crystal field splitting of 0.2 eV is observed for thed _5/2-level. The data are compared with results for the photoemission from Pb films [1] and data for the photoionization from thed-levels of Hg [2].     

DieK-ElektroneneinfangwahrscheinlichkeitPK beim Zerfall von139Ce

TheK-Shell Electron-Capture ProbabilityP _K for the Electron Capture of¹³⁹Ce has been measured using the coincidence method. The detectors used have been a NaJ(Tl)-detector with Be-window for the Lanthan-X-ray, a Ge(Li)- and a 76.2 mm Ø×76.2 mm-NaJ(Tl)-detector for the 166 keV--ray. The escape-effect of the Lanthan-X-ray in the NaJ(Tl)-detector was investigated. The experimental results areP _{K K}= 0.726±0.010 andP _K =0.801±0.034 using _K=0.906±0.026.     

Investigation of new neutron-rich gold isotopes203Au and205Au

Using a²⁰⁸Pb beam of the UNILAC accelerator at GSI we produced the new isotopes²⁰³Au and²⁰⁵Au and identified them by measuring their-decay properties. Their half-lives were found to be 60(6)s and 31(2)s, respectively, and theQ-value of²⁰³Au was determined to beQ =2040(60)keV. Nuclear structure information of^203,205Hg was derived from this measurement. Furthermore, production cross-sections of^203,205Au are given and compared to those obtained in other experiments.This work has been supported by the Federal Ministry for Research and Technology (BMFT) under contract number 06 Gö 451-1/2     

Gamow-Teller beta decay of the very neutron-deficientN=50 nuclide98Cd

For the first time, detailed decay-spectroscopic investigations were performed for the very neutron-deficientN=50 nuclide⁹⁸Cd. The⁹⁸Cd activity was produced in spallation reactions between 600 MeV protons and a natural tin target, yielding a⁹⁸Cd beam intensity of 10 to 60 atoms/s at the collector of the ISOLDE massseparator. By means of-ray and conversion-electron spectroscopy, 19 transitions were found to follow the ⁺/EC decay⁹⁸Cd⁹⁸Ag. The transitions at 61 and 107 keV were shown to beM1(+E2) andE2, respectively, and the⁹⁸Cd half-life was measured as 9.2±0.3 s. TheQ _EC value of⁹⁸Cd is determined semiempirically and is compared to model predictions together with the measuredQ _EC values of the neighbouring cadmium isotopes^100,102Cd and theN=50 isotones⁹²Mo,⁹⁴Ru, and⁹⁶Pd, taken from the literature. The newly established decay scheme of⁹⁸Cd includes 9 excited states of⁹⁸Ag. Four states at 1691, 1861, 2164, and 2544 keV are directly fed by 0⁺ 1⁺ Gamow-Teller beta transitions with a summed strength of 3.5 _–0.7 ^+0.8 . This value corresponds to 25±5% of the strength predicted for the GT transformation of a g_9/2 proton (in⁹⁸Cd) into a g_7/2 neutron (in⁹⁸Ag) by the extreme single-particle shell model. The GT-strength splitting and quenching, observed for⁹⁸Cd, are compared with the corresponding data for lighter even-even N=50 isotones, and are discussed with reference to the predictions of more sophisticated nuclear models. We find that only in some cases it is possible to explain qualitatively the observed GT strength distribution and its total magnitude without renormalizing the free-neutron value of the axial-vector coupling constant.Dedicated to Prof. P. Armbruster on the occasion of his 60th birthday     

Nuclear orientation study of the decay of204BiFe

The decay of²⁰⁴Bi nuclei (I =6⁺, T_1/2=11·22 h) oriented in an iron host was investigated on the JINR low-temperature nuclear orientation facility SPIN. The orientation parameterB ₂=1·17 (6) was obtained from the analysis of six prominent E1 gamma-transitions. From the measured normalized intensities of the gamma-rays observed some 70 values of multipole mixing ratios for the gamma-transitions in²⁰⁴Pb nucleus were determined for the first time. The spins 6, 6, 5 and 4 could be uniquely assigned to the²⁰⁴Pb negative parity levels at 3891·5 keV, 3768·4 keV, 3301·5 keV and 2338·2 keV, respectively. The spin-parity assignments of the levels at 4183·8 keV, 4094·2 keV, 3782·0 keV, 2506·9 keV and 2065·1 keV were confirmed as 6^–, 6^–, 5^–, 5^– and 5⁺, respectively. For the level at 3105·1 keV spin-parity 5^– was suggested and spinparity 7^– of the level at 2696·4 keV was called in question. The possible placements of the gammatransitions 3 1351·7 keV and 1353·4 keV in the decay scheme is discussed. The reorientation parameters for the long-living levels at 2264·2 keV (T _1/2=0·45 s) and 1273·9 keV (T _1/2= =265 ns) were determined asG ₂=0·41 (14) andG ₂=0·60 (17), respectively. For the isomeric level at 2185·7 keV (T _1/2=67·2 min) the value ofG ₂=0·88 (49) was proposed.The authors would like to express their thanks to T. I. Kracíková and M. Trhlík for the valuable discussions in the course of the evaluation of the experimental data.     

139Sm,a new nuclide of relevance for the systematics of theN=77 isomers

The ground state and an isomeric state of¹³⁹Sm have been identified witht _1/2 ^m =9.5 ± 1.0 s andt _1/2 ^g =2.6 ± 0.3 min. The isomeric decay scheme is presented. It extends the chain of isomers withN=77. The distance between thed _3/2 andh _11/2-levels reaches a maximum forZ=60. This maximum is much more pronounced than expected from previous theoretical considerations and from systematic trends in theN=81 and 79 chains.     

Spin and parity of isomeric and ground states of the doubly-odd nucleus184Au

Low-spin states in doubly-odd¹⁸⁴Au have been populated in the ⁺/EC decay of¹⁸⁴Hg produced by bombarding¹⁸⁴Sm targets with an 185 MeV⁴⁰Ar beam. Radioactive Hg nuclei were transported by a He jet system. A new level scheme has been established from --t and X--t coincidence measurements. Spin and parity values I=5⁺, I =2⁺ and I=3^– were assigned to the ground state and the two isomeric states of¹⁸⁴Au, respectively. The structure of these states is discussed using B(E1) values.     

Electric quadrupole moments of radioactive Pt isotopes

Nuclear orientation experiments were performed on¹⁸⁹Pt (I=3/2^–; T_1/2=11 h),¹⁹¹pt (I=3/2^–; T_1/2=2.8 d) and^195mpt=3/2^–; T_1/2=4.0 d) in an Os single crystal at temperatures down to 10 mK. Preliminary results for the quadrupole splitting _o=e²qQ/h are +48 (9), +39 (6), and –87 (12) MHz for¹⁸⁹ptOs,¹⁹¹ptOs, and¹⁹⁵mptOs, respectively. Preliminary results for the quadrupole moments are –0.8 (4), –0.6 (3), and +1.4 (7) b for¹⁸⁹pt,¹⁹¹pt, and¹⁹⁵mpt, respectively. In addition, the quadrupole subresonance structure of¹⁸⁹pt and¹⁹¹pt in Fe was measured with quadrupole-interaction resolved NMR-ON spectroscopy. The results for the magnetic hyperfine splitting _M=¦g_NB_HF/h¦ and _Q are:¹⁸⁹ptFe: _M=277.9 (1) MHz; _Q=+1.39 (5) MHz.¹⁹¹ptFe: _M=320.17 (3) MHZ; _Q=+1.39 (3) MHz. The ratio of the quadrupole moments of¹⁹¹pt and¹⁸⁹pt is deduced to be Q (¹⁹¹Pt/Q^l89pt)=+1.00 (4). The negative quadrupole moments of¹⁸⁹pt and¹⁹¹pt indicate a predominantly oblate shape of the ground states.     

Nuclear structure study of the neutron deficient cadmium isotopes100, 101, 102Cd

The neutron-deficient nuclei^100,101Cd were identified for the first time in — beam following the reactions⁵⁸Ni +^46,48Ti and⁵⁸Ni +⁵⁰Cr at 230 MeV and 245 MeV bombarding energy of the⁵⁸Ni beams. The-decay of the isotopes^100,101,102Cd was studied inp, pp,n, nn, andn prompt and delayed coincidence spectra. Isomers were found with spin and half life {ie1-01}=8⁺, T_1/2=52(5) ns in¹⁰⁰Cd, {ie1-02}=(19/2⁺), T_1/2=4.6(4) ns in¹⁰¹Cd and {ie1-03}=8⁺, T_1/2=56(4) ns in¹⁰²Cd. In a PAD experiment theg-factor of the {ie1-04}=8⁺ isomer in¹⁰⁰Cd was measured to be g=1.24(6). Using the reaction³⁶Ar +⁷⁰Ge at 135 MeV energy of the³⁶Ar beam, theg-factor and the quadrupole moment of the {ie1-05}=8⁺ isomer in¹⁰²Cd were measured to be g=1.29(3) and Q=87(10) fm², respectively. The spectroscopic results are discussed within a shell model approach using a¹⁰⁰Sn core, which accounts well for the electromagnetic properties of the proton aligned isomers. The structure of the spherical neutron states, dominated by the close lying vd_5/2, g_7/2 orbitals, and the gradually developing quadrupole collectivity are reproduced by the shell model calculations.The authors gratefully acknowledge fruitful discussions with K. Heyde and are indebted to him for communicating unpublished theoretical results. The help of N.A.F.M. Poppelier and A.G.M. van Hees in running the RITSSCHIL code is very much appreciated.     

Determination of the unsaturated losses and of the saturation intensity in the KrCl excimer laser

The unsaturated losses , and the saturation intensityI _s, were measured in an uv-preionized KrCl laser for optimized He and Ne based laser gas mixtures. The measurements were made as a function of the specific power loading and of the total pressure of the laser mixtures. Higher values for andI _s were found for the Ne-based laser mixture than for the He-based mixture. At 45 kV charging voltage and at 355 kPa of total pressure we measured =0.053 cm^–1 andI _s =9.4MW/cm² for the Ne based mixture, and =0.035 cm^–1 andI _s =5.1MW/cm² for the He based mixture.     

A mini transmissionγ-ray Compton polarimeter and measurements of the sign of magnetic moments of radioactive nuclei

A mini transmission-ray Compton polarimeter was developed for the measurement of the sign of magnetic moments of radioactive nuclei with low-temperature nuclear orientation. The signs of the magnetic moments of the following isotopes were determined:¹⁹³Os [=+0.7297(16) _N];^191m Ir [=+6.20(9) _N];¹⁹²Ir [=+1.924(10) _N];¹⁹⁴Ir [=+0.39(1) _N];^195m Pt [=–0.605(15) _N].Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday