Search for superdense nuclei of Rb and Cs among the products of 8 GeV proton interactions with Ta

Experimental searches for β-active superdense nuclei of Rb and Cs among the products of 8 GeV proton interactions with Ta are described. The method of selective off-line mass-separation with scanning of the β-activity distribution on the collector was used. According to π-condensation theory, the long-lived anomalous nuclei are expected to be more neutron-deficient than the usual ones. Moreover, their mass numbers may be significantly nonintegral measured on the normal mass scale, because of unusual binding energy. These predictions guided the present search. No anomalous nuclei were found. The upper limit for Rb nuclei in the range 69 ? A ? 78 is 10^?5 with respect to the yield for ⁸¹Rb; for Cs in the range 112 ? A ? 122 the limit is (5–10) × 10^?6 of the yield for ¹²⁷Cs. These estimations are based on the assumption that the half-life of superdense nuclei equals 5 h.     

Mass-spectrometric measurements of exotic Rb,Cs and Fr isotopes

Direct mass measurements of exotic nuclei have been performed with a double-focussing mass spectrometer on line with an isotope separator. We report here on mass measurements with improved accuracy of exotic isotopes of Rb and Cs and on the first measurement of the masses of ¹¹⁶Cs and ¹⁴⁸Cs.     

ISOLTRAP: A tandem penning trap mass spectrometer for radioactive isotopes

The masses of over sixty short-lived Rb, Sr, Cs, Ba, Fr and Ra isotopes have been measured at the on-line mass separator ISOLDE at CERN/Geneva by determination of their cyclotron frequency in the tandem Penning trap system ISOLTRAP. Resolving powers exceedingm/m(FWHM)=10⁶ and accuracies of typically m/m=10^–7 could be achieved. ISOLTRAP can also act as an isomer separator, as proven for the cases of⁸⁴Rb and⁷⁸Rb.     

Nuclear spins of76Rb and119Cs byβ radiation detected optical pumping

An optical pumping apparatus has been attached to the reconstructed ISOLDE on-line mass separator at CERN in order to obtain nuclear data of isotopes far fromβ stability. As first results the spins of⁷⁶Rb (T _1/2=36.8 s) and¹¹⁹Cs (T _1/2=37.7 s) have been determined byβ radiation detected optical pumping (β-RADOP) to beI=1 andI=9/2 respectively.     

Production of Cs and Fr isotopes from a high-density UC targets with different grain dimensions

A UC target material of 11.3±0.5 g/cm^3 uranium density with the grain size of 20 and 5μm manufactured in a form of pills by the method of powder metallurgy has been tested on-line within the temperature range of 1800-2100 ^° C . The mass of uranium exposed to the beam was 4-7g. The yields and release rates of Cs and Fr isotopes produced by fission and spallation reactions of ²³⁸U by 1GeV protons have been measured. The yields of Cs and Fr isotopes obtained from the tested target materials have been compared, including yields of very short-lived Fr isotopes with half-lives down to 1ms. Temperature-resistant materials (porous graphite and tantalum foil) have been used for the internal-container construction, which holds the UC target pills inside a tungsten external container heated by the resistant heating. The fastest release and the highest efficiency for short-lived isotopes have been obtained for the targets with the internal container manufactured from the tantalum foil. Results of on-line tests of a big mass target (730g of 5μm grain UC target material) have been discussed.     

On-line production of Rb and Cs isotopes from uranium carbide targets

A series of on-line mass separation experiments have been performed at the IRIS facility to measure the yield and release of Rb and Cs neutron-rich isotopes produced by fission reaction of ²³⁸U. A 1 GeV proton beam was used to bombard uranium carbide targets with the densities of 11 g/cm³ and 1.5 g/cm³ held at temperatures in the range (2000-2230) ^°C. The release curves of Rb and Cs long-lived isotopes were measured from both kinds of targets. The overall production efficiency was determined making use of experimentally measured cross-sections of that isotope production. Comparison of the experimental yields of Rb and Cs isotopes with the calculated ones after corrections for losses due to finite release times suggests that the diffusion is the dominating process reducing the efficiency for short-lived isotopes. When normalized to the same thickness, an enhancement for the high-density rod target of the measured isotope yields is observed when going far from stability. This is possibly explained by the reactions induced by secondary neutrons. A significant odd-even effect with higher yields of Cs even neutron isotopes has been observed, confirming a similar effect obtained in earlier experiments.     

Target mass correction on the 3He polarized structure function

The diffusion-effusion model has been used to analyse the release and yields of Fr and Cs isotopes from uranium carbide targets of very different thicknesses (6.3 and 148 g/cm²) bombarded by a 1 GeV proton beam. Release curves of several isotopes of the same element and production efficiency versus decay half-life are well fitted with the same set of parameters. Comparison of efficiencies for neutron-rich and neutron-deficient Cs isotopes enables separation of the contributions from the primary (p + ²³⁸U) and secondary (n + ²³⁸U) reactions to the production of neutron-rich Cs isotopes. A rather simple calculation of the neutron contribution describes these data fairly well. The FLUKA code describes the primary and secondary-reaction contributions to the Cs isotopes production efficiencies for different targets quite well.     

The excited states of Kr and Rb isotopes in the 78 ≦ A ≦ 85 mass region

The excited states of the ^{78, 80, 81}Kr and ^{80, 81, 83}Rb nuclei were investigated in radio-active decays of the respective Rb and Sr isotopes. The level schemes were established and for a number of levels the spin-parity assignments suggested on the basis of the log ft values and γ-branching ratios, as well as in some cases angular-correlation results. The structures of excited states of the odd Kr, odd Rb, and doubly even Kr isotopes are compared and discussed in the framework of existing theoretical approaches.     

Experimental indication of the onset of nuclear deformation in neutron-rich Sr isotopes at mass 98

A partial decay scheme for 0.1 sec ⁹⁸Rb has been deduced from γ-singles, γ-multispectra ana γγ coincidence measurements taken at the OSTIS mass-separator system at the Institut Laue-Langevin. The low-lying levels of ⁹⁸Sr follow the energy level systematics of the even-AN = 60 isotones. The onset of nuclear deformation in even-A neutron-rich Sr isotopes appears to occur at mass 98, as the first 2⁺ level energy drops from the nearly constant value of about 800 keV for masses 90–96 to 144 keV at mass 98. Energy level systematics indicate that a transition in the nuclear structures of the more neutron-rich nuclei near mass 100 occurs rather sharply at neutron number N = 60.     

Preparing a journey to the east of 208Pb with ISOLTRAP: Isobaric purification at A = 209 and new masses for 211-213Fr and 211Ra

With the Penning trap mass spectrometer ISOLTRAP, located at ISOLDE/CERN, preparatory work has been performed towards mass and decay studies on neutron-rich Hg and Tl isotopes beyond N = 126 . The properties of these isotopes are not well known because of large isobaric contamination coming mainly from surface-ionised Fr. Within the studies, production tests using several target-ion source combinations were performed. It was furthermore demonstrated around mass number A = 209 that the resolving power required to purify Fr is achievable with ISOLTRAP. In addition, masses of several isobaric contaminants, ^211-213Fr and ²¹¹Ra , were determined with a three-fold improved precision. The results influence masses of more than 20 other nuclides in the ²⁰⁸Pb region.     

Calculation of nucleon densities in calcium,nickel, and molybdenum isotopes on the basis of the dispersive optical model

The radial distributions of proton and neutron densities in the even–even isotopes ^40?70Cа and ^48?78Ni and the analogous distributions of neutron densities in the even–even isotopes ^92?138Mo were calculated on the basis of the mean-fieldmodel involving a dispersive optical potential. The respective root-mean-square radii and neutron-skin thicknesses were determined for the nuclei under study. In N > 40 calcium isotopes, the calculated neutron root-mean-square radius exhibits a fast growth with increasing N, and this is consistent with the prediction of the neutron-halo structure in calcium isotopes near the neutron drip line.     

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

⁸⁷Rb and ³⁹K nuclear magnetic resonance (NMR) spectra of RbKSO₄ single crystals were measured at room temperature. ⁸⁷Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/T₁, and the spin–spin relaxation rate, 1/T₂, were measured as a function of temperature. The values of 1/T₁ for the ⁸⁷Rb and ³⁹K nuclei were found to increase with increasing temperature, and 1/T₁ was determined to be proportional to Tⁿ. Therefore, for the ⁸⁷Rb and ³⁹K nuclei, Raman processes with n=2 are more significantly in nuclear quadrupole relaxation than direct processes.     

Half-lives,neutron emission probabilities and fission yields of neutron-rich rubidium isotopes in the mass regionA=96 toA=100

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of²³⁵U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms¹⁰⁰Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP _n of⁹⁹Rb and of the fission yields of⁹⁷Rb,⁹⁸Rb, and⁹⁹Rb were performed.     

Alpha decay studies of tellurium,iodine, xenon and cesium isotopes

Very neutron-deficient isotopes in the trans-tin region were produced by reactions of 290 MeV ⁵⁸Ni ions on ⁵⁸Ni targets, and studied by on-line mass separation at GSI. Twelve α-emitters — ^107–110Te, ^110–113I, ^111–113Xe, ¹¹⁴Cs — were identified. Systematics of Q_α values and reduced α-widths for this new island of α-emission are discussed.     

Microscopic aspects of identical bands in 78Sr and 78Rb

Recent investigations of the shape transition and shape coexistence phenomena dominating the structure of the even-even N ? Z nuclei in the A ? 80 mass region are extended to the odd-odd nucleus ⁷⁸Rb. Special attention is paid to the structure of some “identical” bands which have been recently identified in ⁷⁸ Sr and ⁷⁸Rb. The ground band of⁷⁸ Sr and the yrast as well as excited negative parity bands in ⁷⁸Rb are studied within the EXCITED VAMPIR approximation using complex Hartree-Fock-Bogoliubov transformations in a relatively large model space. The results are compared with the available experimental data. The emerging picture reveals the role played by the strong quadrupole deformation on the appearance of identical bands as well as the influence of the shape coexistence on their evolution. Predictions for the electromagnetic and alignment properties of the bands are presented.     

Half-lives and delayed neutron emission probabilities of short-lived Rb and Cs precursors

Half-lives and delayed-neutron emission probabilities (P _n ) of short-lived Rb and Cs precursors in the mass chains 94–98 and 143–147 were measured. Sources of isotope separated nuclides were obtained from the on-line mass-separator OSTIS installed at the Grenoble high-flux reactor. A newP _n -value of (25.4±3.2)% is given for the (214±30) ms¹⁴⁷Cs; theP _n -values of nine alkali precursor nuclides were redetermined: (2,730±20) ms⁹⁴Rb with (9.7±0.5)%, (377 ±6)ms⁹⁵Rb with (8.6±0.5)%, (197±5)ms⁹⁶Rb with (12.5±0.9)%, (171±4) ms⁹⁷Rb with (25.2±1.8)%, (114±13)ms⁹⁸Rb with (18.4±2.9)%, (1,765±30)ms¹⁴³Cs with (1.74 ±0.12)%, (1,000±10)ms¹⁴⁴Cs with (2.95±0.25)%, (616±20) ms¹⁴⁵Cs with (12.2±0.9)%, (325±10)ms¹⁴⁶Cs with (13.2±0.8)%. The results are compared with the existing data, and theP _n -values are discussed within some simple model predictions.     

Estimates of the bound-state QED contributions to the g-factor of valence ns electrons in alkali metal atoms

The bound-state QED corrections to g-factors are partly calculated and partly estimated for the ns valence electrons in the atoms K, Rb, Cs, Ba⁺ and Fr. It is shown that these corrections should be taken into account in the comparison with experimental data.     

Study of the124Xe and126Xe structure

The level structure of¹²⁴Xe and¹²⁶Xe is studied on the basis ofβ ⁺ decay of¹²⁴Cs and¹²⁶Cs. The Cs isotopes were produced in the Xe(p, n) Cs reactions at a proton energy ofE≈9.5 MeV. Decay schemes are proposed for these nuclei. The available experimental data are compared with the predictions of various models.     

Alpha decay of new neutron deficient gold,mercury, and thallium isotopes

In fusion evaporation reactions of a⁹²Mo beam with targets of neutron deficient Rb — Mo isotopes very neutron deficient isotopes of elements between Au and Po have been produced. The new isotopes^{173, 174}Au,^{175, 176}Hg, and¹⁷⁹Tl were identified by alpha spectroscopy. The mass excess value of¹⁷⁶Hg could be linked to known values of theN?Z=16 chain. The location of the new isotopes with respect to the proton drip line is discussed. A new high energyα transition of (7.20±0.02) MeV andT _1/2=(1.4±0.5) ms has been found in the reaction⁹²Mo+⁸⁹Y→¹⁸¹Tl^* at an excitation energy of 37 MeV. It is tentatively assigned to isotopes produced in 2-particle evaporation channels.     

Beta-decay energies and nuclear masses of very neutron-rich Rb and Cs isotopes

Theβ-endpoint energies of very neutron-rich Rb and Cs isotopes with mass numbers 94≦A≦98 and 142≦A≦146, respectively, have been measured with a plastic scintillatorβ-telescope at the on-line mass separator OSTIS. From these,Q _β-values and mass excesses are calculated and compared with the results obtained in direct mass determinations.