Beta-delayed gammas and QEC-values in the decays of150m,152mHo and149Dy

^{150m, 152m}Ho, and¹⁴⁹Dy were produced by fusion-evaporation reactions of⁴⁰Ar with targets of natural indium and¹¹⁴Cd. Theβ-decays of the isotopes were reinvestigated with emphasis on high-excitation daughter states. The decay energies for selectedβ-transitions were measured from electron-capture to positron-decay ratios. Corrections forγ-feeding from higher-lying states were taken into account. The following Q_EC-values were derived for^150mHo,^152mHo, and^149mDy of 6819 ( _?100 ⁺¹¹⁷ , 6263 ( _?80 ⁺⁸⁹ ), and 3914 ( _?52 ⁺⁷⁶ ) keV, respectively.     

Decay energies of the low-spin states of147Tb and148Tb

Using¹¹²Cd(⁴⁰Ar,pxn) reactions andγ-ray spectroscopy, the EC/β ⁺ and EC(K)/β ⁺ probability ratios were determined for individualβ-transitions in the¹⁴⁷Tb(1.6 h)→¹⁴⁷Gd and¹⁴⁸Tb(60 min)→¹⁴⁸Gd decays. Under assumption of applicability of theoretical ratios for allowedβ-decays on the first-forbidden nonunique transitions studied here, the experimental ratios were converted into information on decay energies. Combining the results with the known masses of^147,148Gd andQ _α values of neutron-deficient holmium-to-lutetium isotopes, mass excesses could be determined for¹⁴⁷Tb,¹⁴⁸Tb, low-spin¹⁵¹Ho, low-spin¹⁵²Ho,¹⁵⁵Tm,¹⁵⁶Tm and¹⁵⁹Lu.     

β +-endpoint measurements near100Sn and146Gd

With a summation-free ⁺-endpoint spectrometer the ⁺-decay energies of¹⁰⁴Sn,¹⁴⁷Tb,^{148, 149}Dy,¹⁴⁹Ho,¹⁵⁰Er, and¹⁵¹Tm were remeasured with improved accuracy. Combined with known proton and alpha decay energies, the resulting Q_EC values lead to atomic masses of very neutron-deficient isotopes including nuclei beyond the proton drip-line such as¹⁰⁹I and¹¹³Cs. Furthermore, the Gamow-Teller-strength of the even-even nuclei¹⁰⁴Sn,¹⁴⁸Dy, and¹⁵⁰Er is reevaluated with reduced uncertainty.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

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.     

Protonh 2 11/2 and octupole excitations in148 66Dy82 and149 66Dy83

The yrast states of¹⁴⁸Dy and¹⁴⁹Dy have been studied by γ-ray and conversion electron measurements in (α, xn) and (¹⁶O,xn) reactions on enriched¹⁵²Gd and¹³⁵Ce targets. Level schemes to above 4 MeV for the two nuclei are reported. The πh ² _11/2 spectrum identified in¹⁴⁸Dy and the πh _11/2 effective chargee _eff=1.52±0.05e, derived from the measuredE2 transition rate between the (πh ² _11/2) 10⁺ and 8⁺ states, are discussed and compared with results for other two-particle nuclei. The yrast cascades in¹⁴⁸Dy and¹⁴⁹Dy continue above the (πh ² _11/2) 10⁺ and πh ² _11/2 vf _7/2) 27/2^? states by ～ 1 MeVE1 transitions de-exciting the lowest members of octupole multiplets built on these states. The energy shifts for the observed members of the πh ² _11/2 × 3^? multiplet are analyzed in terms of twoparticle-phonon exchange coupling using an empirical coupling strength extracted from the one valence particle nucleus¹⁴⁷Tb. The dominantvf _7/2×3^? character of low-lying 13/2⁺ isomers in¹⁴⁹Dy and otherN=83 nuclei is emphasized.     

Precise atomic masses of147Eu,147Gd,and151Tb derived from their decay properties

Theβ-decay energies of¹⁴⁷Eu,¹⁴⁷Gd, and¹⁵¹Tb were determined by usingγ-spectroscopical methods. The comparison of experimental with calculatedK-capture probabilities yielded theQ _EC values 1.690( _?16 ⁺²¹ )MeV and 2.203( _?13 ⁺¹⁹ )MeV for¹⁴⁷Eu and¹⁴⁷Gd, respectively. By measuring the ratio of positron decay to electron capture for two branches in¹⁴⁷Eu decay, the decay energiesQ _EC=1.702(13) MeV andQ _EC=1.709(18)MeV were derived. Also fromEC/β ⁺ ratios the valuesQ _EC=2.225(75) MeV for¹⁴⁷Gd, andQ _EC=2.566(12)MeV for¹⁵¹Tb were obtained. Earlier discrepancies in the mass adjustment of these isotopes were removed. In course of the present studiesγ-decay properties of¹⁴⁷Eu and¹⁴⁷Gd were reinvestigated.     

Seniority isomerism in theN]=82 isotone152Yb; Favouredβ transitionsπh 11/2→vh 9/2

The reaction⁵⁸Ni→⁹⁶Ru has been investigated withγ spectroscopic methods and pulsed-beam techniques. A level scheme of the magic nucleus¹⁵²Yb has been established. The half-life of the isomeric 10⁺ state was found to be 39±5 μs. The result is discussed in the frame-work of the seniority scheme. Theory and experiment show excellent agreement for the half-lives of the 10⁺ states in theN=82 isotones¹⁴⁸Dy,¹⁵⁰Er and [¹⁵²Yb. A level scheme of¹⁵¹Tm, consisting of ten excited states, has been established.β-decay schemes of [¹⁵²Yb (3.2±0.3 s),¹⁵²Tm^(m) (8.0±1.0 s) and¹⁵¹Tm (3.8±0.8 s) have been derived. The logft value of theβ transition¹⁵²Yb (0⁰)→¹⁵²Tm (1⁺) has been determined to be about 3.4. It was found that theft values of the favouredβ transitions 0⁺→1⁺ in theβ decays of [¹⁴⁸Dy,¹⁵⁰Er and¹⁵²Yb are almost inversely proportional to the numbers of protons out of theh _11/2 subshell.     

Particle plus octupoleM2/E3 isomers and high-spin particle-hole states in147Tb and148Tb

The level structures of theN=82 andN=83 nuclei¹⁴⁷Tb and¹⁴⁸Tb have been studied by means of (α, 8n) and (α,7n) reactions induced by 68 to 110 MeVα particle bombardments of¹⁵¹Eu targets. In-beam conversion electron measurements have established that isomers withT _1/2=4.8(6)ns in¹⁴⁷Tb andT _1/2=22(1)ns in¹⁴⁸Tb decay byM2+E3 transitions to the ground states. The measuredB(E3) values show that the isomeric states arise from the coupling of the valence nucleon(s) to the¹⁴⁶Gd core octupole. Particlephonon coupling in these nuclei and in the one-neutron nucleus¹⁴⁷Gd is discussed and compared with well known cases involving the²⁰⁸Pb core. The higher lying yrast states in the two Tb nuclei are described as shell-model particle-hole excitations using empirical single particle energies and nucleon-nucleon interactions.     

High spin states in148Tb

The level scheme of¹⁴⁸Tb has been extended to an excitation energyE _x =14.0 MeV and a spinI≈ 38? in two experiments with the OSIRIS Comptom suppressed detector array at the HMI Berlin using the reactions¹²²Sn(³¹P,5n)¹⁴⁸Tb at 160 MeV and¹²⁰Sn(³¹P,3n)¹⁴⁸Tb at 152 MeV.     

Occupancy of single-proton orbitals in148Gd

The (³He,d) and (t, α) reactions on a radioactive Z=64¹⁴⁸Gd target were used to populate the single-proton states in¹⁴⁹Tb and¹⁴⁷Eu. In¹⁴⁹Tb the data establish the five single-proton states of the 51 to 82 major shell. The proton occupancies deduced from these experiments suggest that in the¹⁴⁸Gd ground state there are on average 1.8(3) proton pairs above the Z=64 shell gap.     

TheN=83 nucleus149Dy from Gamow-Teller decay of its 11/2− and 1/2+ 149Ho parents

A high-sensitivityγ-spectroscopic study of the¹⁴⁹Hoπh _11/2 andπs _1/2 β-decays using mass separated sources has located dominant 0⁺ → 1⁺ GT decay strength associated with decay of pairedh _11/2 protons, leading to 3п-states in the¹⁴⁹Dy daughter nucleus. In theirγ-decay low-lying¹⁴⁹Dy levels characteristic of anN=83 nucleus are excited. They include theνf _7/2,νp _3/2,νh _9/2 andνp _1/2 single particle- and theνs ₁ ^2/?1 andνd ₃ ^2/?1 two-particle one-hole states, as well as the νf _7/2 × 3^? andνf _7/2 × 2⁺ particle-phonon multiplets. A synopsis is given of these excitations in theN=83 isotones from¹⁴⁹Nd to¹⁵³Yb. The¹⁴⁹Dy GT decay strength is discussed in terms of the¹⁴⁷Tb₈₂ and¹⁴⁸Dy₈₂ decays. The strength function results are also compared with independent¹⁴⁹Ho 11/2^? decay data from the literature based on totalγ-ray absorption measurements.     

New results in the decay of150Ho and152Tm

A new isotope¹⁵²Tm (T _1/2=5.2±0.6 s) and a new isomeric state in¹⁵⁰Ho (T _1/2=88±15 s) were observed with the on-line isotope separator ISOCELE II at Orsay. The decay of¹⁵²Tm is similar to the one of the high spin state (9⁺) in¹⁵⁰Ho. The new¹⁵⁰Ho isomer, probably with a low spin (2 or 3), decays mainly to the 2⁺ level in¹⁵⁰Dy.     

High spin states in147Tb

The level scheme of¹⁴⁷Tb has been extended to 116 levels and 198 transitions in two experiments with the OSIRIS Compton suppressed detector array at the HMI Berlin with the reactions¹²²Sn(³¹P, 6n)¹⁴⁷Tb at 160 MeV and¹²⁰Sn(³¹P, 4n)¹⁴⁷Tb at 152 MeV. Evidence for a new high spin isomer atE=7664.1 keV with a life-time of the order of 2 ns has been found.     

Very proton rich nuclei withN≈82

Postaccelerated⁵⁸Ni and⁶⁰Ni beams from the Munich MP tandem have been used to study the compound reactions⁵⁸Ni→⁹³Nb,⁹²Mo,⁹⁴Mo and⁶⁰Ni→⁹²Mo. The final nuclei¹⁴⁸Er,¹⁵⁰Er and¹⁴⁷Ho have been investigated with in-beamγ-spectroscopic methods, such as excitation functions,γγ coincidences, bunched-beam and pulsed-beam techniques. Level schemes of¹⁴⁸Er,¹⁵⁰Er and¹⁴⁷Ho have been established with the following level energies, spins, parities and half-lives:¹⁴⁸Er: 646.6 keV 2⁺; 1,524.0(4⁺); 2,254.3(5^?); 2,526.9(6⁺); 2,537.3(7^?), 2,706.0(7^?, 8^?); 2,784.4(8⁺); 2,915.5(10⁺) 13±3 μs¹⁵⁰Er: 1,578.9 keV 2⁺; 1,786.4 3^?; 2,261.2(5^?); 2,634.1(7^?); 2,734.3(8⁺); 2,798.3(10⁺) 2.7±0.2 us; 4,001.9; 4,491.8; 4,886.0; 5,222.7¹⁴⁷Ho: 765.4; 1,737.2; 2,355; 2,656.6 The level scheme of¹⁵⁰Er has been discussed in the framework of the seniority scheme. Perfect agreement with the experiment has been found for the predictions of this scheme for the half-lives of 10⁺ states inN=82 isotones. This is the first nuclear region where the predictions of the seniority scheme have been observed without strong perturbation.     

Decays of millisecond isomers in odd-oddN=81 nuclei146Tb,148Ho and150Tm

Following reactions induced by 245 MeV⁶⁰Ni ions, isomers have been identified in the odd-oddN=81 isotones¹⁴⁶Tb,¹⁴⁸Ho and¹⁵⁰Tm with half-lives of 1.18(2), 2.35(4) and 5.2(3) ms, respectively. Their decays have been characterized byγ-ray spectroscopy. The isomers are interpreted as (πh _11/2 νh ₁ ^2/?1 ) 10⁺ states decaying to members ofπh _11/2 νd ₃ ^2/?1 andπh ₁₁ ^2/?1 multiplets. The observed level spectra and M1 branching ratios are found to be in good agreement with shell model predictions based on empirical and Schiffer-Trueπν ^?1 residual interactions.     

Production of149gTb in deep inelastic transfer reactions: An approach to the angular momentum of fragments

The excitation functions for deep inelastic reactions in which two to six charges are transferred from⁴⁰Ar and⁶³Cu ions to rare earth targets have been measured using activation techniques, the observed radionuclides being^{150, 151}Dy and^149gTb. From the comparison of the curves relative to^149gTb and those relative to^{150, 151}Dy, it was deduced that the low spin isomer^149gTb was produced with significant probability for low incident energies. Using data from (heavy ions,xn) reactions, it was possible to attribute this production to the deexcitation of Tb fragments formed in deep inelastic transfers with angular momenta lower than 9?. This result is in good agreement with the angular momentum calculations performed under the hypothesis that the initial angular momentum window leading to deep inelastic reactions is situated between the critical angular momentum for fusion and that corresponding to grazing collisions. As far as Cu induced reactions are concerned, both hypothesis of rolling and sticking are consistent with the experimental data. For Ar induced reactions, the results indicate that the stage of sticking is not reached when the incident energy is lower than 200 MeV. Nuclear Reactions:^{142, 145}Nd,^{144, 148}Sm,¹⁵⁴Gd(Ar,X)^{151, 150}Dy,^149gTbE=160–280 MeV; measuredσ(E).¹⁴⁴Nd,¹⁴⁸Sm,¹⁵¹Eu,¹⁵⁴Gd(Cu, X)^{151, 150}Dy,^149gTb.E=280–420 MeV measuredσ(E). Enriched targets. Deduced angular momentum of Tb fragments.     

Beta decay energies and nuclear masses of148Ba,148La and151Pr

Theβ-decay energy of the neutron-rich isotopes¹⁴⁸Ba,¹⁴⁸La and¹⁵¹Pr has been measured at the Institute Laue-Langevin (ILL). The¹⁴⁸Ba- and¹⁴⁸La-nuclei were produced using a high-temperature ion source at the OSTIS separator of this institute, whereas samples of¹⁵¹Pr were obtained from the thermal fission of²³⁹Pu, used as a target in the mass separator LOHENGRIN. At both instruments,βγ-coincidences were measured with a plastic scintillator telescope and a Ge(I)-detector, together withβ-singles andγ-singles spectra. Theβ-spectra emitted in coincidence with 44γ-lines in the decay of these nuclei have been evaluated. From their endpoint energies, the followingQ _β -values have been obtained:Q _β (¹⁴⁸Ba)=5115±60keV;Q _β (¹⁴⁸La)=7255±55 keV;Q _β (¹⁵¹Pr)=4170±75 keV. Using these results, the nuclear massesA of these nuclei can be calculated with a relative error ΔA/A?5·10^?7. In addition, the two-neutron separation energies and the mass excesses have been derived; the results are compared with recent predictions of theoretical mass calculations.     

Gamow-teller strength and particle-phonon states in149Dy from decay of its 11/2− and l/2+ 149Ho parents

Studies of the¹⁴⁹Ho 11/2^? and 1/2⁺ GT-decays using on-line mass separation have established in the N=83¹⁴⁹Dy nucleus the single neutron excitations below 1.8 MeV and members of the νf_7/2×3^? and νf_7/2×2⁺ particle-phonon multiplets. Quantitative analysis of the GT-decay strength to¹⁴⁹Dy 1qp- and 3qp-states is made in terms of the strength values measured in¹⁴⁹Dy₀ and¹⁴⁷Tb_11/2? decay.     

States of167Ho from the decay of neutron rich nuclide167Dy

Theβ ^?-decay of ₆₆ ¹⁶⁷ Dy produced through the fast neutron reaction¹⁷⁰Er(n, α)¹⁶⁷Dy has been investigated by using several kinds of detectors and a high-capacity two-parameter recording system. The half-life andβ ^?-decay energy of¹⁶⁷Dy were determined to beT _1/2 = 6.20 ± 0.08min andQ _β-=2.35±0.06, respectively. The observed level scheme of ₆₇ ¹⁶⁷ Ho (completely unknown previously) contains 12 states, among them a 6.0±0.1 μsM2 isomer at 259.3 keV. On the basis of theoretical and systematic considerations combined with multipole determinations, the following Nilsson model assignments are proposed for the lowest states of¹⁶⁷Ho: 0 keV (7?/2 [523]), 259.3 keV (3+/2[411]), 319.8 keV (5/2 3+/2[411]), 392.5 keV (1+/2[411]), 410.0 keV (3/2 1+/2[411]), 569.7 keV (3?/2{7?/2[523], 2⁺}). Theβ-decay proceeds mainly to the proposed gamma-vibrational state at 569.7 keV with an anomalously low logft value 5.4, indicating similarity between the microscopic structures of this state and the famous ¦K ₀?2¦ gamma vibration of¹⁶⁵Ho.     

A new method to test virtual photon spectra

Millisecond isomers in¹⁴⁶Tb,¹⁴⁸Ho and¹⁵⁰Tm, observed in⁶⁰Ni reactions on³⁹Y,⁹⁰Zr, and⁹²Mo targets, are interpreted as (πh_11/2 vh_11/2 ^?1) 10⁺ isomers predicted by the shell model. Their decays are characterized.