J. Kern

The 112Cd nucleus: A laboratory for the study of collective excitations

Abstract The level structure of the even-even 112 Cd nucleus was investigated by means of the 110 Pd(α, 2nγ) reaction spectroscopy. On the basis of the γγ-coincidences measured using three Compton-suppression spectrometers, 151 transitions (out of 190 lines assigned to 112 Cd) were placed in the decay scheme. Most of the 80 observed excited states were grouped into 6 well-developed collective bands. The level scheme was extended by 43 new levels. The systematic use of the excitation function slopes and of the side-feeding intensities, together with the results gained from the angular distribution allowed us to assign the spins and the parities to nearly all observed levels. The collective states, including in particular those of the ground-state, intruder, quasi-γ and octupole bands, and their electromagnetic properties were investigated in the framework of the spdf 1BA-1 model with configuration mixing of normal and intruder states.

Search for nuclei exhibiting the U(5) dynamical symmetry

Abstract The level energy spectra of a set of 26 nuclei presenting a vibrational structure were analyzed using a simple U(5) prescription in the IBM-1 framework. Twelve of these nuclei are relatively well fitted and present level patterns in accordance to the model predictions. They are thus confirmed or possible U(5) nuclei. The analysis suggests that they present extensive multiphonon structures. This implies the survival of such structures at higher energies. The experimental B(E2) values between multiphonon states are shown to agree with the finite N dependence predicted by the IBM. It is also observed that the 21+ energies are quite generally about 15% lower than expected from the global nuclear properties.

Study of 110Cd by the 108Pd(α, 2nγ) reaction

Abstract The gamma-ray transitions following the 108Pd(α, 2n) reaction have been observed using different semiconductor detector systems, including a Compton suppression spectrometer. Approximately 200 transitions assigned to 110Cd were identified in the energy range 120 to 1600 keV. About 2.5 × 107 γγ-coincidence events were recorded and the angular distributions of most transitions were analyzed. The 110Cd level scheme was extended by 28 new levels. With the angular distribution results and a systematic use of the excitation slopes and of the side-feeding intensities (ESSI-method), the spins and parities of nearly all observed levels were determined. The results regarding the ground-state, intruder and octupole bands are interpreted in the framework of the s-p-d-f interacting boson model in which intruder mixing is explicitly included. The predictions of strong configuration mixing are also examined in detail in the neutron-proton interacting boson model framework. We shortly discuss possible non-collective, broken-pair excitations in a qualitative way.

Systematic study of the yrast band in the doubly-odd 158,160,162,164,166Tm nuclei☆

Abstract High-spin rotational states of 158,160,162,164Tm were investigated by in-beam γ-spectroscopy using the (14N, 4n) and (14N, 6n) reactions. The strongly-aligned band π[ h 11 2 ]ν[ i 13 2 ] has been observed in all these nuclei. Our experimental results, together with the data obtained by another group on 166Tm, are interpreted in the frame of a cranking-model analysis. The behaviour of the moment of inertia versus rotational frequency and the existence of a triaxial shape is discussed.

SPECTROSCOPIC QUADRUPOLE MOMENT OF 23Na FROM MUONIC X-RAYS+

Abstract With the crystal spectrometer at muon channel I at SIN we measured the 3 d 5 2 −2 p 3 2 transition in muonic 23Na. The h.f.s. of the 2 p 3 2 level was resolved and the spectroscopic quadrupole moment of the nuclear ground state determined: Q = 0.1006 ± 0.0020 b. Using a collective particle-rotor model description of 23Na which includes small ΔK = 1 admixtures in the wave functions, we find agreement between our measured spectroscopic quadrupole moment and the one derived from measured B(E2) values. Combining the quadrupole moment Q with the quadrupole coupling constant measured in the electronic atom of 23Na, we derive the electric field gradient at the nucleus for two different electronic states: 〈dE/dz〉 = 0.1185(32)ea−30 for the 3 2 P 3 2 state, 〈dE/dz〉 = 0.0410(27)ea−30 for the 4 2 P 3 2 state. The field gradient for the 3 2 P 3 2 state is compared with recent many-body calculations of the hyperfine interaction in alkali atoms.

Vacuum polarization test and search for muonhadron interactions from muonic X-rays:: Crystal-spectrometer experiments

Abstract We have measured the wavelengths of the 3 d 5 2 −2 p 3 2 and the 3 d 3 2 -2 p 1 2 X-ray transitions in μ - 24 Mg, - 28 Si and - 31 P with the bent-crystal spectrometer at the SIN muon channel. The X-rays are measured relative to the wavelengths of the 84 keV and the 63 keV γ-rays of 170 Tm and 169 Yb which have recently been calibrated to about 1 ppm. The measured X-ray wavelengths λ exp are compared with theoretical values λ th , as obtained from QED calculations. The relative difference, averaged over all six measured transitions, is λ exp −λ th λ th = (2 ± 8) × 10 −6 This result corresponds to a test of the vacuum polarization effect in QED of (0.6 ± 2.4) × 10 −3 . Assuming the QED calculations to be correct, we can use the result to put limits on additional muon-nucleon interactions (as required by gauge theories). If such an interaction is mediated by a scalar, isoscalar boson with a mass smaller than 1 MeV, the coupling constant is found to be g N g μ 4π = (−4 ± 17) × 10 −9 Alternatively, we can deduce from our experiments the most accurate direct value to date for the negative muon mass, m μ - = 105.65906(91) MeV .

A Compton suppression spectrometer (CSS) for in-beam gamma-ray spectrometry

Abstract It is shown that the peak-to-background ratio in complex gamma-ray spectra, such as those observed in the study of (HI, x n) type reactions, cannot be improved by the use of larger detectors to the same extent as the nominal peak-to-Compton ratio ( P / C ) given for a simple spectrum with very few transitions. A Compton suppression spectrometer (CSS) is described which provides an important improvement of the background level. As an additional bonus several parasitic lines, e.g. the annihilation line, are strongly attenuated. Examples from a study of the 165 Ho(α, 2n) 167 Tm reaction are presented.

Study of 124Te by the 122Sn(α,2nγ) reaction and by the decay of 124I

Abstract In-beam γ-ray spectroscopy and the decay of 124 I were used to study the nuclear structure of 124 Te. On the basis of γγ-coincidences, a level scheme was constructed with over 100 levels and 170 transitions. The use of excitation functions and angular distributions for the (α, 2n) reaction, and the log( ft ) values for the decay, allowed spin assignments for a large number of the levels to be made. The resulting level scheme was interpreted in the framework of the interacting boson model and the particle-core coupling model.

Spectroscopic quadrupole moments of 25Mg and 27Al from muonic X-rays

Abstract With the crystal spectrometer at Muon Channel I at SIN we measured the 3d 5 2 -2p 3 2 X-ray transitions in muonic 25Mg and 27Al. The h.f.s. of the 2p 3 2 was resolved and the spectroscopic quadrupole moment of the nuclear ground states determined: Q( 25 Mg ) = +0.201 ± 0.003 b , Q( 27 Al ) = +0.150 ± 0.006 b . We find agreement between the intrinsic quadrupole moment value derived from Q( 25 Mg ) with those calculated from measured B(E2) values. This indicates that for 25Mg the strong-coupling limit of the rotational model with an axially symmetric charge distribution is realized in the lower part of the ground-state rotational band. We combine the quadrupole moments Q with the quadrupole coupling constants measured in electronic atoms of 25Mg and 27Al and derive the electric field gradients at the nucleus (in units of e · a0−3, where a0 is the Bohr radius): 〈 d E d z 〉 = 0.339± 0.005 for the 3P2 state of 25Mg, 〈 d E d z 〉 = 0.54± 0.02 for the 2P 3 2 state of 27Al. From Hartree-Fock calculations of the 2P 3 2 state of Al we derive values for the Sternheimer correction factor R and compare them with calculated values.

Electromagnetic properties of high-K rotational bands in the odd-odd 160Ho nucleus☆

The electromagnetic properties of levels up to spin 19ħ in the yrast band and 9ħ in the ground-state band of the odd-odd nucleus 160Ho have been measured with standard in-beam γ-ray spectroscopy using the 159Tb(α, 3n) reaction. For the yrast band, the experimental B(M1, I → I − 1)/ B(E2, I → I − 2) ratios increase sharply for rotational frequencies larger than 0.25 MeV, while observed energies do not show any sudden change. This effect suggests a change in the nature of the wave functions. The results are interpreted in terms of the particle-rotor model, cranking shell model and compared to the available data on the neighbouring odd-A nuclei.