Investigations of very neutron-deficient isotopes below100Sn in40Ca-induced reactions

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 of^{58, 60, 62}Ni and^{63, 65}Cu targets with 4.0 MeV/u ions of⁴⁰Ca. The known properties of the β-delayed proton precursor^95m Pd were confirmed, but a search for direct proton decay of^95m Ag 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 ⁺⁴ s) and¹⁰⁰In. In addition,⁹⁴Pd(9.0±0.5 s) was identified from β-γ-X studies. Individual decay schemes of^95m96Pd and^{96, 98}Ag are reported.     

The Gamow-Teller beta decay of100Cd

Neutron-deficient isotopes of cadmium were produced in thep(600 MeV)+^natSn spallation reaction. The ISOLDE facility provided mass-separated beams of these isotopes. The production yield was 10³ to 10⁴ atoms/s for¹⁰⁰Cd and roughly two orders of magnitude less for⁹⁸Cd. The properties of the¹⁰⁰Cd→¹⁰⁰Ag decay were studied in detail by X-ray,γ-ray and conversion electron spectroscopy. Forty nineγ-transitions were assigned to this decay, and all but five of them were placed in the decay scheme. The half-life was determined to be 49.1±0.5 s. TheQ _EC value of 3890±70 keV was deduced from a comparison of experimental and theoreticalβ ⁺/(EC+β ⁺ probability ratios. Seven 0⁺→1⁺ Gamow-Teller transitions with log ft values between 3.5 and 4.9 were identified. The total (summed) strength is about five times smaller than predicted by the shell model for the transformation of ag _9/2 proton into ag _7/2 neutron. The quenching of the Gamow-Teller strength and other questions of nuclear structure are discussed for the¹⁰⁰Cd decay. The properties of⁹⁸Cd are inferred by extrapolation of^104,102,100Cd data, and some preliminary experimental results on⁹⁸Cd decay are presented.     

First in-beam observation of97Ag — The three-proton-hole spectrum in100Sn

Excited states in⁹⁷Ag have been identified for the first time following the reactions⁴⁶Ti(⁵⁸Ni,α p 2n and⁶⁴Zn(³⁶Ar,p 2n. The population cross sections were 1.0(5) mb and 0.7(3) mb, respectively. Theγ-decay of the seniorityΝ=3 states below theI ^π=(17/2^?) and (21/2⁺) isomers was investigated. In a search for short lived isomers half lives oft _1/2=5(2) ns and 2.5(5) ns were found in theN=50 isotones⁹⁷Ag,I ^π=(17/2^?) and/or (21/2⁺) and⁹⁵Rh,I ^π=(21/2⁺). A new isomer was identified in⁹⁷Pd,I ^π=17/2⁺ witht _1/2=2.3(5) ns. The implication of the three-proton hole spectrum in¹⁰⁰Sn for the empirical residual interaction and shell model predictions is discussed.     

Q EC value and Gamow-Teller strength of the102CdEC/gb+ decay

Within the framework of a systematic study of 0⁺→1⁺ Β-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 of^natSn, and mass-separated samples were prepared by means of the GSI and the ISOLDE on-line mass separators, respectively. Observations of X-rays,γ-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, theΒ ⁺ endpoint energy of the main decay component was measured, yielding aQ _EC value of 2587±8 keV. This very small uncertainty ofQ _EC, combined with the improved knowledge of half-life and decay scheme, allows an accurate determination of the observed Gamow-Teller decay strengthB _Σ (GT)=1.513±0.055 for¹⁰²Cd. This result is discussed in comparison with predictions from model calculations.     

Beta-delayed proton emission around N=50 and the rp-process

Detailed information on the decay modes of very proton-rich heavy nuclei is a prerequisite for p-process nucleosynthesis models. We have therefore measuredβ-delayed protons andγ-rays from mass-separated sources of⁹⁶Ag,⁹⁸Ag and⁹⁸Cd, and obtainedβp branching ratios of 3.7(9)×10^?2, (1.1 _?0.4 ^+0.5 )×10^?5 and <2.5×10^?4, respectively. The consequences of the experimental data for rpprocess calculations and the results from a statistical model are briefly discussed.     

The beta-decay energies of103In and104In

The β⁺ spectra of neutron-deficient nuclei with mass numbers 103 and 104 have been measured, using a semiconductor beta spectrometer. The nuclei were produced by on-line mass separation of reaction products formed by bombarding an enriched Mo target with Ne ions. Values for Q_EC have been deduced. The decay energies are in agreement with systematics. The decay of¹⁰⁴In shows remarkably strong feeding to levels above 5 MeV. In addition, Q_EC values for₁₀₃Cd,₁₀₃Ag,₁₀₄Ag and_104mAg have been determined.     

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.     

Atomic masses of147m, 148m, 149m Tb,148Dy,150m, 152m Ho derived from decay properties

Using nuclear fusion reactions of⁴⁰Ar ions with¹¹²Cd,¹¹⁴Sn and¹¹⁶Sn and subsequentγ-ray spectroscopy, the probability ratios of positron emission and electron capture,β ⁺/EC _k andβ ⁺/(EC+β ⁺), are determined for individualβ-transitions in the decay of^147m ,^148m ,^149m Tb,¹⁴⁸Dy and^150m ,^152m Ho. From comparison with theoretical ratios the followingQ _EC values, given in keV, are derived:^147m Tb, 4.620(60);^148m Tb, 5.730(30);^149m Tb, 3.610(50);¹⁴⁸Dy, 2.680(30);^150m Ho, 6.625(120) and^152m Ho, 6.470(80). The present decay-energy data are compared with earlier measurements and the new information obtained for the mass surface around¹⁴⁶Gd is discussed.     

The mass of the nucleus148Dy

Usingγγ coincidences theEC(K)/β ⁺ ratio for theβ decay¹⁴⁸Dy→¹⁴⁸Tb has been determined.¹⁴⁸Dy has been produced through the irradiation of⁹³Nb with 249 MeV⁵⁸Ni ions. The mass of¹⁴⁸Dy has been deduced. With the help of knownQα values the masses of¹⁵²Er,¹⁵⁶Yb,¹⁶⁰Hf and¹⁶⁴W have been obtained. The experimental masses are compared with different current mass formulae.     

Measurement of the mass of110Pd and114Pd using the (14C,16O) reaction

A measurement of the mass excess of¹¹⁰Pd and¹¹⁴Pd has been made by detecting emergent¹⁶O ions in aQ3D magnetic spectrometer using the¹¹²Cd(¹⁴C,¹⁶O)¹¹⁰Pd and¹¹⁶Cd(¹⁴C,¹⁶O)¹¹⁴Pd reactions at 60 MeV. The results are compared with theoretical predictions.     

Light-ion-induced reactions in mass measurements of neutron-deficient nuclides close to A = 100

A survey of neutron-deficient nuclides which can be produced via proton- and ³He -induced fusion-evaporation reactions in the A = 100 region was made using a Penning trap as a high-resolution mass filter. A comparison of the measured isotopic rates with a statistical model calculation for the proton-induced reactions shows the importance of using the precise binding energy values for the final reaction products. In particular, proton separation energies were found to play an important role in the evaporation process. In addition, accurate masses of 12 nuclides, ^{97-99, 101}Pd , ¹⁰⁰Ag , ^101-105Cd and ^{102, 104}In , were determined with uncertainties of less than 10keV.     

Anregung und Schädigung der Lumineszenz von CaWO4 und ZnS/Ag durch Ionen

The luminescence-response of ZnS/Ag and CaWO₄ to ion-bombardement and the deterioration of luminescence under prolonged irradiation was determined as a function of ion-energyE, ion-massM _i and beam-densityI. The variation of lightoutput with ion-energy is of the formJ ₀=C. (E-E ₀) ⁿ withn=2 (ZnS/Ag) or lower values (CaWO₄). The luminescence-response to ions of various mass was found to decrease generally with growing mass, but to be nearly constant to ions of middle atomic weight (ZnS/Ag). The luminescence-efficiency, caused by ions of energy greater than 5 keV, is independent of beam-density within the whole range studied here (maximum 3 · 10^?7 A · cm^?2), but it diminishes in the caseE=5 keV for values ofI above 6 · 10^?8 A · cm^?2. The deterioration-effect grows, except in the case of He⁺-ions, the lightest ions used here, with ion-energy. It also increases by substituting Ne⁺- for He⁺-ions, but remains nearly independent of mass (CaWO₄) or diminishes with growing ion-mass (ZnS/Ag), if the ions are heavier than Ne⁺-ions. Increasing beam-density leads to a reduced deterioration of ZnS/Ag-luminescence, yet has no influence to that of CaWO₄.     

Target isotope effect in high energy photon production atE/A=10 MeV

A comparison has been made between the production of high energy (E _γ≧30 MeV)γ rays from the bombardment of¹²⁴Sn and¹¹²Sn targets withE/A= 10 MeV¹²C ions. The results are well explained by then-p bremsstrahlung model and do not indicate the need for any new processes.     

Atomic masses of rare earth isotopes145Pm,145Sm and151Gd,derived from decay measurements of145Sm and151Gd

The comparison of experimental with calculated K-capture probabilities yielded the decay energies of¹⁴⁵Sm and¹⁵¹Gd,Q _EC=622(5) and 463(3) keV, respectively. Earlier discrepancies in the mass adjustment of these isotopes were removed and adjusted masses for¹⁴⁵Sm,¹⁴⁵Pm and¹⁵¹Gd derived. In the decay of¹⁵¹Gd five newγ-rays were found and the half-life remeasured to be 129(4) d.     

Results on two-, three-, and four-body events from the100Mo+100Mo and120Sn+120Sn collisions aroundE/A=20 MeV

Events with 2, 3 and 4 heavy-fragments (A≧20) have been detected in the reactions¹⁰⁰Mo+¹⁰⁰Mo atE/A =18.7, 23.7 MeV and¹²⁰Sn+¹²⁰Sn atE/A=18.4 MeV. The experiments were performed with an array of 12 detectors which together covered a large fraction of the forward hemisphere and allowed a high detection efficiency for these events. Masses and energies of all fragments have been reconstructed by means of an improved version of the kinematic coincidence method. The probabilitiesP ₃ andP ₄ of producing 3- and 4-body events were found to depend mainly on the dissipated energy rather than on the bombarding energy, thus indicating that their origin lies more in the decay properties of the excited fragments than in the dynamics of the interaction. Emission of light particles from the composite system is shown to become more relevant with increasing bombarding energy and may explain the drop of theP ₃ andP ₄ curves at high energy losses. Small deviations of theP ₃ andP ₄ curves at 23.7A · MeV from those at lower bombarding energies were used to estimate the amount of a possible pre-equilibrium light particle emission as a function of impact parameter.     

Die Intensität der Bremsstrahlung und der charakteristischenK-Röntgenstrahlung dünner Anoden

The intensities of bremsstrahlung and characteristicK X-rays of thin Al, Mn, Cu, Se, Ag and Sn targets were measured with a scintillation counter. The angle between the 50 keV electrons and the direction of emission was 90°. — The spectral intensity of bremsstrahlung for 20 keV quantum energy was determined in agreement with the theory ofKirkpatrick andWiedmann. The result obtained isi _v=(2.7) · 10^?53.Z ^{2, 0} ergs per steradian, unit frequency interval, bombarding electron and atomper-cm². The cross section ofK-ionisation by electron impact has been found out by measuring the characteristicKX-ray intensity in agreement with known theoretical and experimental values. The result of this work isσ _K=(7.0) · 10^?16 ·Z ^?4,3 cm².     

Temperature dependence of the quadrupole interaction of100Rh in a Cd lattice

The quadrupole interaction frequency |v _q| = |eQV _zz /h|0 = 8.71 ± 0.11 MHz was measured for the 75 keV state of¹⁰⁰Rh in a polycrystalline cadmium matrix by use of the γ-e^? and γ-γ time differential perturbed angular correlation (TDPAC) techniques. The temperature dependence of the electric quadrupole coupling constant for¹⁰⁰Rh in Cd is similar to those of other impurities in Cd metal.     

Present status and future aspects of nuclear structure close to 100Sn

The present status of experimental approach to ¹⁰⁰Sn in the spectroscopy of excited states is landmarked by the T_z=3/2 nuclei between ⁹⁵Pd and ¹⁰¹In and the T_z=1 nuclei ⁹⁴Pd and ⁹⁸Cd. The detection limits with Pre-EUROBALL γ-arrays and ancillary detectors are below the 10^?5 level of the total fusion- evaporation residue cross section. A large scale shell model analysis of the existing data reveals the shell structure at ¹⁰⁰Sn, which shows a remarkable similarity to ⁵⁶Ni. Evidence for an increasing proton-neutron interaction in approaching the N=Z line is deduced from high spin isomers and spherical yrast lines. The effective E2 operator for protons and neutrons and implications for a low lying particle-hole (ph) E2 excitation in ¹⁰⁰Sn are discussed.     

Alpha decay studies of very neutron deficient isotopes of Hf,Ta, W,and Re

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of^{107, 109}Ag,^{nat, 108, 110}Pd, and¹⁰³Rh with⁵⁸Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, andα branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes^161–164Re,¹⁶⁰W,^157–161Ta, and¹⁵⁶Hf could be identified. The measuredQ _α values, half lives, and a branching ratios are discussed. Two new high energyα. transitions of (7,408±10) keV and (7,804±15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively.A tentative mass and atomic number assignment givesA=153–156 andZ=70–72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 10⁵ can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10? for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond¹⁴⁶Gd.