Γγ Coincidence Research Articles

First information about the level structure of 91Ru

The present work presents, for the first time, the yrast level scheme of 91Ru. The level structure was investigated via the fusion-evaporation reaction 58Ni(40Ca, α2pn)91Ru at beam energies of 187 MeV using in-beam γ-ray spectroscopy techniques. The NORDBALL detector system including charged particle and neutron selection was used. The decay scheme was constructed on the basis of Compton suppressed γγ coincidences, intensity relations and measured γ-ray anisotropies. A number of positive as well as negative parity yrast levels was found with excitation energies up to 7.5 MeV and 9.6 MeV with maximum spin values of (41/2+) and at least 45/2− respectively. The experimental results are discussed in terms of shell-model configurations and the systematics of the structure of the vg9/2−3 configuration.

Thermal-neutron-capture studies on 135Ba

The nucleus 135Ba has been studied via primary, secondary γ-rays, γγ coincidences following thermal-neutron capture. A level scheme was established up to 3.7 MeV consisting of 41 levels including 114 γ-transitions. The neutron binding energy was determined to be 6971.78(17) keV. The total thermal cross section and isomer production of the 11 2 − state were determined to be σ(tot) = 1.4(4) and σ( 135Ba) = 0.11(2) barn. The level energies, E2 and M1 transitions and electromagnetic moments were compared with model predictions of the interacting boson-fermion model (IBFM).

Nuclear structure and high-spin states of 137Pr

Levels in 137Pr were populated in the 126Te( 14N, 3n) and 122Sn( 19F, 4n) reactions and the subsequent radiation was studied using in-beam γ-ray spectroscopy methods including γ-ray excitation-function, angular-distribution, γγ( t) coincidence and γ( t) measurements. A level scheme with new states up to spin 35 2 belonging to 137Pr is given. The lifetime of the 11 21 state at 563.4 keV has been determined as T 1 2 = 2.66±0.07 μ s . The calculation of low-lying levels in 137Pr performed in IBFM has been compared to experimental data.

High spin states in66,68Ge

High spin states of66,68Ge have been investigated at the FN Tandem accelerator of the University of Koln via the reactions40Ca(32S,α2p,4p)66,68Ge at a beam energy of 100 MeV and58Ni(16O,α2p)68Ge at 65 MeV. The OSIRIS spectrometer with 12 escape suppressed Ge detectors was used to measure γγ coincidences and γ-ray angular distributions. In66Ge (68Ge) 33 (22) new levels were found and 63 (62) new γ-transitions were placed in the level scheme. Both nuclei show a rather complicated but similar excitation pattern, ruled by the interplay of quasiparticle and collective degrees of freedom. The results are compared to the recently published EXVAM calculations for68Ge.

Nuclear excitation and prompt fission in muonic238U

A study of muonic238U has been performed in a combined (μ−,γ f) and (μ−,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔEb=(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.

Investigation of shape changes in 87Mo

The level scheme of 87Mo was investigated with the reaction 58Ni( 32S, 2pn) at projectile energies of 110–130 MeV and the reaction 40Ca( 50Cr, 2pn) at 170 MeV. Extensive spectroscopic information was collected including γγ and neutron-gated γγ coincidences as well as coincidences with identified 87Mo residues. Level lifetimes were studied with the RDDS and DSA methods. The angular distributions of low-lying transitions were measured. The results indicate a sudden transition from single-particle-like to collective behavior which is caused by g 9 2 particle alignment. The level scheme was constructed up to 12 MeV excitation energy and a probable spin of 40 2 h ̵ .

Level structure of 191Os: Coexisting oblate and prolate configurations

An extensive level scheme for the transitional nucleus 191Os has been constructed on the basis of (n, γ), (n, e−) and (d, t) reaction data. γ-rays following thermal capture and average resonance capture (ARC) were detected with pair spectrometers. Secondary γ-ray spectra were obtained with the GAMS bent crystal spectrometers. The high precision of the resultant energies was the critical factor in developing a level scheme on the basis of Ritz combinations. γγ coincidences were obtained from both primary and secondary transitions. Conversion electrons, detected with the BILL spectrometer, gave multipolarity information. The (d, t) reaction measured with a Q3D magnetic spectrometer gave additional information on the hole states in 191Os. A discussion of the resulting level scheme briefly reviews the fragmentation of negative-parity strength but focuses mostly on a rare example of an anti-aligned decoupled set of coexisting positive-parity states. For these, triaxial particle-plus-rotor calculations are carried out. Reasonable agreement is obtained and the calculations also explain the unusually strong population in (n, γ) of this set of mostly medium spin states.

Positron age-momentum correlation studies of defects and positronium by an MeV positron beam

A monoenergetic positron beam (6 × 10 4 e + s −1) in the energy range of 0.5 MeV ⩽ E ⩽ 6.5 MeV ( ΔE E ≈ 10 −4 FWHM) has been installed at the Stuttgart Pelletron accelerator. Besides experiments on e + e − scattering (Bhabha scattering) and e + e − annihilation-in-flight experiments, β + γ positron-lifetime measurements in solids and melts have been performed. Substantially increased count rates and virtually eliminated background are among the advantages of the beam-based β +γ technique over the conventional γγ coincidence technique. Measurements under difficult conditions (e.g., in high-temperature melts) are simplified. Time-resolved information on the evolution of the positron states (e.g., trapping of positrons at defects) and a sensitive detection of positronium formation are obtained by measuring the lifetime of a positron together with the energy of one of the annihilation quanta in a triple-coincidence setup (age-momentum correlation). In contrast to the conventional γγE age-momentum-correlation technique, the beam-based β + γE technique permits high count rates and almost completely eliminates the deterioration of the measured spectra by pileup effects and by random coincidences.

Inelastic and transfer reactions in 92Mo+255 MeV 60Ni collisions studied by γγ coincidences

For the 92 Mo+255 MeV 60 Ni system, inelastic and few-nucleon transfer events populating non-collective states of moderately high spin have been studied by γγ coincidence measurements. Besides the strong inelastic scattering channel, twelve transfer processes were identified, ranging from 1 n to 2α transfer; typically, cross coincidences between the γ-rays from both products were observed. Potential spectroscopic applications are indicated.

The high-spin structure of 202Po and 204Po and systematical features of the even polonium isotopes

The high-spin structure of 202,204Po was studied using the 196,198Pt(12C, 6n) and 204Pb(α, 4n) reactions at a beam energy of 100 and 52-54 MeV, respectively. In-beam conversion electron and γ-ray spectroscopy measurements including γγ(t) coincidence, γ(t) and angular distributions were performed. The level scheme of 202Po was extended to spin 21 h and the excitation energy 5.5 MeV while levels in 204Po were observed at 5.9 MeV with a spin of 20 h. The previously observed isomeres in 202Po and 204Po were confirmed and in addition to these a Jπ = 13− level with the half-life 9(3) ns was observed in 204Po at 3387.3 keV. The expected configuration of this state is πh9/22 ⊗ vf5/2-1 i13/2-1. A shell-model description of the observed excited states is given.

The Stuttgart positron beam, its performance and recent experiments

A monoenergetic MeV positron (e +) beam, with a flux at present of 6 × 10 4 e +/s in the energy range of 0.5 to 6.5 MeV, has been installed at the Stuttgart Pelletron accelerator. The stabilization and the absolute calibration of the energy E is monitored by a Ge detector with real-time feedback; a relative energy stability of Δ E E ⩽ 10 −4 is obtained. So far, e +e − scattering and annihilation-in-flight experiments for investigating the low-energy e +e − interaction as well as β + γ positron lifetime measurements in condensed matter have been performed. The advantages of the β + γ method compared to the conventional γγ coincidence technique have been demonstrated. Recently, triple-coincidence positron “age-momentum correlation” measurements have been carried out on fused quartz. A brief account is given on the development of a “positron clock” aiming at a substantial improvement of the time resolution of the β + γ positron lifetime measurements.

Inelastic and transfer reactions in 92Mo+255 MeV 60Ni collisions studied by γγ coincidences

Inelastic and transfer reactions in 92Mo+255 MeV 60Ni collisions studied by γγ coincidences

Loss of collectivity along the yrast line in 158Er

Lifetimes of yrast states from spin 14 through 40 have been measured in 158Er. The Doppler-shift recoil-distance method was used in conjunction with the multi-Ge detector array HERA, and γγ coincidences were analyzed. The collectivity of the nucleus drops from 200 W.u. to 110 W.u. in the region of the second backbend. By spin 38 it has decreased further to 25 W.u., consistent with a band termination.

Investigation of the high-spin structure of 200,202Pb

The yrast structure of 200,202Pb was studied using the 200,202Hg(α, 4n) reactions at E α = 53 MeV. The de-excitation of the excited states was studied using conventional in-beam γ-ray spectrosopic methods including γγ( t) coincidence, pulsed-beam γ( t) and angular distribution measurements. Levels with a possible spin up to 21 ħ were observed in both nuclei. In addition to the previously known isomers, a 16 + isomeric state at 4069.8 + δ keV with T 1 2 = 5(1) ns and a 19 − isomeric state at 4932.0+ ε keV with T 1 2 = 67 (5) ns were observed in 200Pb. A new 19 −isomeric state at 5241.9 + S keV with T 1 2 = 107 (5) ns was also observed in 202Pb. The g-factors of the isomeric states were determined using the time differential perturbed angular distribution technique and the results are g = −0.094 (7) and g = −0.099 (3) for the 19 − states in 200Pb and 202Pb, respectively. The measured g-factors indicate a predominantly four-quasineutron configuration ν f 5 2 −1 i 13 2 −3 for the 19 − isomeric states.

Levels in 109Rh

The levels in 109Rh have been investigated via the γ-rays following the β− decay of 34.5 s 109Ru. The ruthenium activity was produced in the thermal neutron-induced fission of 149Cf and separated chemically from the fission product mixture with the on-line centrifuge system SISAK. The emitted γ-rays were studied by γ singles and γγ(t) coincidence measurements. In addition, the 110Pd(d, 3He) reaction was investigated at Ed = 50 MeV to study the proton-hole states in 109Rh. From the comparison of the measured angular distributions with DWBA calculations l-transfers and spectroscopic factors were deduced. The level scheme of 109Rh that was obtained is discussed and compared with neighbouring nuclei and with recent calculations in the framework of the interacting bosonfermion model (IBFM-1). An interpretation of the observed candidates for intruder states as a rotational band built upon the [431] Nilsson configuration is proposed.

Single particle radiation between high spin states in 147Gd

Transitions above the T 1 2 = 550 ns, 8.59 MeV isomer in 147Gd have been studied using the ( 30Si, 5n) reaction. Results from γγ coincidence, angular distribution and recoil distance measurements are combined to establish a level scheme up to 16.9 MeV and I ∼ 79 2 . Single particle configurations are assigned on the basis of the deformed independent particle model. The single particle nature of the highest spin states and the apparent lack of collectivity is discussed.

Measurements of multipolarities in 227Ra as tests of evidence for stable octupole deformation

Multipolarities of ~30 transitions in 227Ra have been established by measuring conversion electrons following the β − decay of 227Fr. For this purpose a “mini-orange”-type electron spectrometer has been constructed. The 227Fr isotopes were produced by the ISOLDE on-line separator at the CERN Synchro-cyclotron. Internal conversion coefficients were obtained from singles spectra and also from simultaneous γe − and γγ coincidence measurements. The new results support the placement of levels and transitions in the earlier level scheme but require changes in the previously assigned parities for four of the levels. Also, one E0 transition was identified. The results are consistent with previous interpretations for most of the levels that have been used to argue in favour of a small permanent octupole deformation for 227Ra.

Collective states in 230Ra fed by β − decay of 230Fr

The γ-rays following the β − decay of 230Fr have been investigated by means of γ-ray singles including multispectrum analysis and γγ coincidence measurements using Ge(Li) detectors. The half-life of 230Fr was measured to be 19.1 ± 0.5 s. Most of the observed transitions could be placed in a level scheme comprising 23 new excited states of 230Ra, ten of them grouped into the K π = 0 + ground-state band, the K π = 0 − band with its 1 − state at 710.9 keV and a K π = 2 + γ-vibrational band with its head at 734.8 keV. It is concluded that 230Ra is a better rotator than the lighter radium isotopes, and has no ground-state octupole deformation.

Low- and medium-spin states in 204Po

Excited states in 204Po were populated in the reaction 204Pb( 3He, 3n) at beam energies 23.0 and 27.6 MeV. The subsequent de-excitation was studied using in-beam spectroscopic methods including γ-ray excitation function, γ-ray angular distribution, γ-RF timing, conversion-electron and γγ coincidence measurements. Yrast states up to J π = 13 − and E exc = 3440 keV, and mediumspin states with spins 4–8 kh and energies in the range 2.0–2.5 MeV were observed. The half-life of the previously reported J π = 8 + isomer was remeasured to T 1 2 = 158(2) ns , the J π = 9 − state at 2227 keV was observed to have a half-life of 15.6(5) ns, and the half-life of the yrast 11 − state at 2621 keV was determined to be T 1 2 = 3.6(2) ns . The systematic features of even- A Po isotopes, including excitation energies and transition rates, are discussed.

Levels in 107Rh

The levels in 107Rh have been investigated via the γ-rays following the β − decay of 3.75 min 107Ru. The ruthenium activity was produced in the thermal neutron-induced fission of 239Pu and separated chemically from the fission product mixture with the on-line centrifuge system SISAK. The emitted γ-rays were studied by γ singles and γγ( t) coincidence measurements. In addition, the 108Pd(d, 3He) reaction was investigated at E d = 50 MeV to study the proton-hole states in 107Rh. From the comparison of the measured angular distributions with DWBA calculations l-transfers and spectroscopic factors were deduced. The obtained level scheme of 107Rh is discussed and compared with neighbouring nuclei and with recent calculations in the framework of the interacting boson-fermion (IBFM-1) model.