G. de Angelis

Investigations of 159-163Dy using Incomplete Fusion Reactions

The isotopes 159–163Dy have been investigated using the incomplete fusion reactions 7Li → 158,160Gd at beam energies of 8 MeV/u. The γ-rays were detected in the GASP spectrometer in coincidence with the fast charged particles detected in the ISIS silicon ball. The level schemes of all five isotopes could be amply extended. The most important result was the observation of both the ground state and Stockholm bands in 162Dy up to high spin covering the band crossing region. The crossing in this nucleus is shifted to a higher frequency in comparison to the lighter Dy isotopes. The interaction strength between the two bands could be determined with high precision and with this result, a full oscillation of the interaction strength from one node to the next within an isotopic chain could be observed for the first time.

Isospin Symmetry Along The N=Z Line In The sd Shell

Excited states have been studied in sd‐shell nuclei following the 16O (70 MeV) + 24Mg (400 μg/cm2) fusion‐evaporation reaction. The GASP spectrometer in conjunction with the charged‐particle detector ISIS and the Neutron ring allowed the detection of the γ‐rays in coincidence with evaporated light particles. New data on the mirror pairs A=31 and A=35 have been obtained. In particular, the comparison between the level schemes of 35Ar and 35Cl has confirmed the importance of the electromagnetic spin‐orbit term, which explains the large Mirror Energy Difference values. Evidence of isospin mixing can be deduced from the E1 transitions.

Lifetimes and electromagnetic transition strength in 157Dy

Excited states in 157Dy have been studied by γ-γ coincidence measurements via the reaction 124Sn(36S,3n) at a beam energy of 155MeV. Lifetimes of the relatively lower-spin states in 157Dy were measured by means of the Recoil Distance Dopplershift technique in the coincidence mode. The experiment was performed at the Laboratori Nazionali di Legnaro with the GASP array and the Cologne plunger device. With the same setup a Doppler- shift attenuation (DSA) lifetime measurement was performed for the higher spin states. The Differential decay-curve method was applied for the lifetime determination.

Electromagnetic transition strengths in 155Dy

Lifetimes of excited states in 155Dy were measured by means of the Recoil Distance Doppler-shift technique in the coincidence mode. The experiment was performed at the Laboratori Nazionali di Legnaro with the GASP array and the Cologne plunger using the reaction 124Sn(36S,5n)155Dy at a beam energy of 155 MeV. The Differential decay-curve method was applied for the lifetime determination. The measured transition probabilities in 155Dy and the energy spectrum are compared to the predictions of the Particle plus rotor model. The comparison indicates slightly different quadrupole deformations characterizing the low-lying one-quasineutron bands which may point to a shape coexistence.

Lifetime measurements in light barium isotopes

The transition strenghts for the low—lying levels of 122,124Ba have been determined analysing the Recoil Distance Doppler Shift data by the Differential Decay Curve Method. The trend of normalised B(E2) reveals a drop in correspondance of the 81+ state. This feature is also present in 122Ba: entering the backbending region the interplay between collective and quasiparticle degrees of freedom have to be carefully taken into account.

Proton‐neutron interaction and the relative isomer masses in 148Tb

Proton‐neutron states have been investigated in the odd‐odd nucleus 14865Tb83 with Li‐induced in‐beam γ‐ and e−‐experiments and αγ‐coincidence measurements of 152Ho decay. The results establish the three complete πu2009+1νf7/2 multiplets, and in addition a number of multiplet members involving a proton hole and octupole excitations. The (πh11/2νh9/2)u20091+ attraction is determined as −1.36 MeV.

Study of multi‐nucleon transfer reactions with light nuclei

Multi-nucleon transfer reactions are useful tools to populate exotic nuclei, particularly the neutron-rich ones. In this view, two different experiments have been performed employing a stable ( 22 Ne) and a radioactive ( 24 Ne) beam, both impinging on a 208 Pb target. The first reaction has been studied using the CLARA-PRISMA-DANTE set-up at Laboratori Nazionali di Legnaro (Legnaro-Italy), while the second reaction was performed at Ganil (Caen-France) employing a SPIRAL radioactive beam of 24 Ne. In this case recoils and coincident γ rays were detected with the VAMOS-EXOGAM set-up. The data show that MNT reactions can selectively populate states of different nature and, therefore, are a good tool to study nuclear structure further away from stability.

VARIETY OF COLLECTIVE MOTIONS IN THE MIDDLE OF THE 1F7/2 SHELL

A variety of collective phenomena is proven to occur at the middle of the 1f7/2 shell, which can be microscopically described by Shell Model calculations. For this purpose a detailed spectroscopy as well as precise DSAM lifetime measurements were performed. The cases of 48,49,50Cr are illustrated.

Magnetic dipole bands in 82Rb, 83Rb and 84Rb

We have studied the isotopes 82 Rb 45 , 83 Rb 46 and 84 Rb 47 to search for magnetic rotation which is predicted in the tilted-axis cranking model for a certain mass region around A=80. Excited states in these nuclei were populated via the reaction 11 B+ 76 Ge with E=50 MeV at the XTU tandem accelerator of the LNL Legnaro. Based on a γ-coincidence experiment using the spectrometer GASP we have found magnetic dipole bands in each studied nuclide. The regular M1 bands observed in the odd-odd nuclei 82 Rb and 84 Rb include B(M1)/B(E2) ratios decreasing smoothly with increasing spin in a range of 13 − ⩽J π ⩽16 − . These bands are interpreted in the tilted-axis cranking model on the basis of four-quasiparticle configurations of the type π(fp)πg 9/2 2 νg 9/2 . This is the first evidence of magnetic rotation in the A≈80 region. In contrast, the M1 sequences in the odd-even nucleus 83 Rb are not regular, and the B(M1)/B(E2) ratios show a pronounced staggering.

Double and triple octupole excitations in the A≈150 region

The results of recent experiments performed at L.N.L. on multiple octupole excitations in the N=84 isotones are reviewed. In particular, the results on B(E3) strengths in 146Sm are discussed and expectations for higher-order octupole excitations are compared with the present experimental situation.