Fission and gamma-ray decay of the238U shape isomer

The decay of the²³⁸U superdeformed shape isomer has been reinvestigated by detecting for the first time simultaneously the fission and the gamma-back decay. An electrostatic deflection system has been used to transport the^238mU recoils, produced in a²³⁸U(d, pn) reaction with a pulsed beam of 18 MeV deuterons, in front of a detector set-up consisting of three ion-implanted solidstate detectors and a Ge(Li) gamma-detector. The gamma-back decay has been measured in coincidence with conversion electrons of the 2⁺ 0⁺ transition deexciting the first rotational state in²³⁸U. Two gamma-transitions of 2.513 MeV and 1.878 MeV have been observed with half-lives consistent with the result obtained for the decay by delayed fissionT _1/2=(298±18) ns.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Search for the lowest 2+ mixed-symmetry state in102Ru

TheK-conversion coefficients of some 2_i ⁺ 2_f ⁺ transitions in¹⁰²Ru have been measured and their electric monopole components have been deduced. The intensity of theE0 component in the 2₃ ⁺ 2₁ ⁺ transition supports a possible interpretation, based on gamma decay, of the 2₃ ⁺ level as the lowest mixed-symmetry state. Negative parity has been assigned to the level at 2261 keV.We would like to thank Mr. M. Ottanelli for his continuous, important technical support. One of the authors (D.C.) thanks the INFN for hospitality and financial support during this work.     

High spin states in102Ag

High spin states are populated in¹⁰²Ag using the reaction¹⁶O+⁸⁹Y with projectile energies in the energy range 60–80 MeV. Gamma multipolarities are inferred from DCO ratios and coincidence relationships are established among the gamma rays assigned to¹⁰²Ag. A comprehensive level scheme is constructed with four band systems, two of positive parity and the other two of negative parity. The present study extended spins upto 19^– in the main negative parity band. A new band is populated in the present work and is similar to the one observed in¹⁰⁶Ag.The authors wish to thank and record their appreciation to Dr. S.K. Parthasaradhi, VECC, Calcutta for preparing the target used in this experiment. The authors gratefully acknowledge useful discussions with Dr. Rakhesh Popli. The authors record their grateful thanks to Prof. A.P. Patro and Prof. G.K. Mehta, former and present directors of the Nuclear Science Centre, for their interest in this work, helpful discussions and providing excellent facilities. The authors also thank Dr. S.K. Datta of the Nuclear Science Centre for his continued interest and helpful discussions and accelerator staff of N.S.C. for their efficient services in delivering the beam. Grateful thanks are also due to the University Grants Commission for providing financial support.     

Beta decay of very neutron-rich110Mo studied at the new IGISOL facility

The decay of the new activity¹¹⁰Mo (t_1/2=0.30(4).s) has been observed at the new IGISOL separator. Multiscaled singles,--t and -y-t coincidences were recorded. The decay scheme suggests I=2⁺ for the ground state of the daughter nucleus¹¹⁰Tc. Three 1⁺ levels are fed with logft values below 5, indicating no drastic change among Mo and Ru decays at the middle of the neutron shell. This experiment confirms the expectation that the new IGISOL facility will allow the identification of one or two new more neutron-rich isotopes per element in this region.Supported by the Academy of Finland.     

Picosecond lifetime measurements in147Pm

Half-lives of excited levels in¹⁴⁷Pm have been measured using BaF₂ scintillators. New values of t_1/2 are 2420(10), 43(5), 22(9), 17(10) and 17(10) ps for the 91.1, 410.5, 489.2, 531.0 and 685.9 keV level, respectively. E2 enhancement factors of -transitions have been deduced.     

Conversion electron measurements in the124Laβ-decay. Spin and parity assignments in124Ba

Low-lying energy levels in¹²⁴Ba have been investigated from¹²⁴La-decay by using He-jet technique. Spin and parity assignments have been deduced from internal conversion electron measurements. A-vibrational band based on a second 0⁺ state has been identified. Experimental level energies have been compared with IBFA-2 calculations.Deceased in September 1990     

Excited states of111I and the observation of a 21 ns isomer

Proton rich nuclei close to ¹⁰⁰ Sn have been investigated inbeam using the NORDBALL detector array. A beam of 270 MeV⁵⁸ Ni was used to bombard a ⁵⁴ Fe target. Reaction channel separation was achieved with a 4 charged particle multi-detector set-up together with a 1 neutron detector wall placed in the forward direction. Gamma-ray transitions belonging to the ¹¹¹ I nucleus were identified for the first time. A level scheme constructed from --particle-coincidence analysis is proposed. The level structure is discussed within the framework of the shell model and the systematics of the heavier odd iodine nuclei.     

M1-transitions and the mutual orientation of the unpaired nucleons in odd-odd nuclei

Experimental lifetimes and branching ratiosB(M1)/B(E2) are used to determine reduced M1-transition probabilities between collective-band levels withI=1 in^{102, 104, 106}Ag and in the odd-odd isotopes of La, Pr, Pm and Tl. The extractedB(M1) values are compared with calculations in the framework of the quasiclassical approximation and the model axial rotor+two quasiparticles. The reduced M1-transition probabilities along with the magnetic moments were found to depend on the mutual orientation of the angular momenta of the unpaired nucleons in the odd-odd nuclei.The authors would like to thank the staffs of Cyclotron Laboratory of P.T.I. in St. Petersburg and the Nuclear Moments Laboratory of I.N.R. in Kiev for assistance in the experiments. One of us (A.I.L.) wishes to thank the Sektion Physik of the Ludwig Maximilians Universität München for its hospitality and financial support.     

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.     

The deformedN=60 nucleus 41 101 Nb

The ^– decay of¹⁰¹Zr has been investigated at the fission-product separators JOSEF and LOHENGRIN. The half-life of¹⁰¹Zr has been determined to 2.5(1) s and a level scheme for¹⁰¹Nb has been established from ray singles as well as X/3- and — coincidence measurements. Conversion coefficients for transitions in¹⁰¹Nb and level half-lives between 10 ps and 2 ns have been determined. Three rotational bands are identified among the low-lying levels with band heads at 0 keV, 206 keV and 208 keV. The bands are probably based on the Nilsson configurations [422 5/2⁺], [301 3/2^–] and [303 5/2^–], respectively. The deformation has been determined to _q=0.40(4) and 0.41(8) for the ground state band and the band based on the 206 keV level from the half-lives of the first and second excited members of these bands. This shows that the rapid onset of deformation at N=60 which is typical for the A=100 region of neutron-rich nuclei, takes also place in the Nb isotopes. Nilsson model calculations describe the experimental data well, especially the several determined transition probabilities including those for E1 transitions from the 206 and 208 keV band heads to the ground state.Dedicated to Prof. Dr. P. Armbruster on the occasion of his 60th birthday     

Electromagnetic moments of the 7+ rotational bandhead in116Sb

The lifetimes of low-lying negative parity yrast levels have been measured in¹³¹La using the recoil distance Doppler shift method. The nuclear reaction employed was¹¹⁶Cd+¹⁹F at a beam energy of 80 MeV. The data were analysed using the differential decay curve method. The E2 transition strengths for negative parity yrast states and the energy spectra are well described by coupling a fermion to a-soft core in the frame of the interacting boson fermion approximation model.We gratefully acknowledge discussions with R.F. Casten. One of us (N.V.Z.) is grateful to Universität zu Köln for its hospitality. This work was supported by the Bundesministerium für Forschung und Technology — Germany under contract 06OK143 and by Institute of Atomic Physics — Romania under contract 62-91-1.     

Confirmation of the superdeformed band in192Pb

High-spin states in¹⁹²Pb were populated via the reaction¹⁷³Yb(²⁴Mg,5n) at a beam energy of 140 MeV, and the resulting-rays were deteced using GAMMASPHERE. A previously observed superdeformed band in¹⁹²Pb has been confirmed.     

Study of the yrast band in the doubly odd156Tm

The yrast band in the doubly odd¹⁵⁶Tm nucleus was studied through¹⁴⁴Sm(¹⁹F,2p5n)¹⁵⁶Tm reaction at beam energy of 105MeV. Several high-spin states of¹⁵⁶Tm were identified and the highest spin of the band with configuration7/2^–[523] v1/2⁺[660] could be built up to spin 25. The level structure shows the onset of a non- or weak collectivity which generally appears at neutron number of 87 in neutron-deficient rare-earth nuclei.This work was supported in part by the Fok Ying Tung Education Foundation and the Natural Science Fund of China.     

New investigation of the decay of the high-spin isomer in 151Er

The decay of the T _1/2 = 420 ns isomer in ¹⁵¹Er has been reinvestigated. The multipolarities of the decaying transitions have been established by measuring the electron conversion coefficients. An I ^π = 67/2⁻ assignment is proposed for this isomer with a π[h ₁₁^2/4 d ₃^2/1 d ₅^2/−1]⊗ν[f _7/2 h _9/2 h ₁₁^2/−1] configuration.     

Excited states in the141Gd nucleus

Excited states above the 11/2 isomer have been identified for the first time in the neutron deficient ¹⁴¹Gdnucleus. The level sequence built on the 11/2^– isomeric state was constructed using -ray spectroscopy methods. The new data are compared with level patterns in neighbouring nuclei.     

On the newly proposed levels in152Gd and152Sm

It is shown that there is no evidence for the existence of levels at 1312.41, 1485.67 and 1698.34 keV in¹⁵²Gd and at 1436.65 and 1681.56 keV in¹⁵²Sm, which were recently claimed to be observed in the decay of¹⁵²Eu(3^–, 15 y).Work supported in part by the Natural Science Fund of China and in part by the Department of Science and Technology of China Nuclear Energy Industry Corporation     

Can strong electric dipole transitions in deformed rare earth nuclei be explained by the macroscopic behavior of the octupole phonon?

StrongE1 transitions of greater than 10^–3 Weisskopf units occur in many octupole states in the deformed rare earth region. It is shown using the droplet model that the electric dipole moment resulting from the macroscopic behaviour of the octupole phonon cannot by itself account for the observedE1 strengths, and it is observed that this result is consistent with the proposal of Donner and Greiner and of Zilges von Brentano and Richter that admixtures of the giant dipole resonance into the low energy octupole states are responsible for the fastE1 transitions. It is also suggested that calculations similar to those performed by Egido and Robledo forN 92 nuclei may be able to reproduceE1 transitions inN=94–104 nuclei.Supported by the National Science Foundation and the State of Florida     

High-spin states of odd-odd156Ho

High spin states in the nucleus¹⁵⁶Ho have been studied via the fusion-evaporation reaction¹⁴⁰Ce(¹⁹F,3n)¹⁵⁶Ho at the beam energy of 82 MeV.- coincidences, E, I and excitation functions have been measured. With two rotationally-aligned bands identified as [h_11/2] [i_13/2], we found two new bands with a signature splitting, whose structure is believed to arise from either [h_9/2] or [f_7/2] coupled to [h_11/2]. Our placement of rays belonging to these new bands suggests that the previously known level scheme should be corrected. We also found many new low-energy rays in the low excitation energy. As compared to¹⁵⁴Ho, the present nucleus with two more neutrons becomes more collective, and shows well-observed rotationally-aligned bands with a clear signature splitting.Communicated by B. Herskind     

Rotational damping motion in nuclei

The recently proposed model to explain the mechanism of the rotational motion damping in nuclei is exactly solved. When compared with the earlier approximative solution, we find significative differences in the low excitation energy limit (i.e. ). For the strength functions we find distributions going from the Wigner semicircle through gaussians to Breit-Wigner shapes.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday. Work supported in part by the DFG and Cicyt, project PB88-0177     

Yrast excitations in129Te

Using an 11.4MeV/u¹³⁶Xe beam of the UNILAC accelerator of GSI and a^natW 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.