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The domain of nuclei situated far from the line of b stability has always been an arena of numerous experimental pursuits and a testing ground for new theoretical models. With radioactive beams on the horizon, the physics of nuclei with an excess of neutrons or protons has become one of the focal points of nuclear physics. These nuclei define the very limits of nuclear existence and will be susceptible to phenomena associated with low binding energy, such as halos, skins, or mixing between bound and continuum states. The proton separation energy decreases with decreasing neutron number. Proton-rich nuclei, which have a negative proton separation energy and are, thus, situated beyond the proton drip-line, can spontaneously emit protons. The proton decay rate is governed by the energy and orbital angular momentum of the emitted proton. It also depends on the wave function of the proton-decaying state, which is determined by the shape of the nuclear potential and by residual interactions between valence nucleons. Most of the known proton emitters have decay rates consistent with the assumption of a spherical potential. The recent experimental

The majority of experiments performed during the 2-year long stay of GAMMAS-PHERE at the Argonne National Laboratory aimed to study proton-rich nuclei far from the line of stability at and beyond the proton drip-line. A high reaction channel selectivity was required to assign in-beam {gamma}-ray transitions to weakly populated exotic nuclei in the presence of background from strong reaction channels. In many of the experiments this was achieved by using the Argonne fragment mass analyzer to separate heavy-ion fusion-evaporation reaction products from scattered beam and disperse them according to their mass-over-charge-state ratio. For medium mass and heavy a and proton emitters the Recoil-Decay Tagging method was implemented. In-beam {gamma}-ray transitions were observed in several proton emitters between Z=50 and Z=82. Among others, rotational bands were assigned to {sup 141}Ho and {sup 131}Eu. A quadruple deformation of {beta}=0.25(4) was deduced for the ground state in {sup 141}Ho from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2{sup {minus}} [523] and 1/2{sup +}[411] configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations indicates, however, that {sup 141}Ho may have significant hexadecapole deformation and could be triaxial.

The entry distribution in angular momentum and excitation energy for the formation of {sup 254}No has been measured after the {sup 208}Pb( {sup 48}Ca, 2 n) reaction at 215 and 219 MeV. This nucleus is populated up to spin 22({Dirac_h}/2{pi}) and excitation energy (greater-or-similar sign)6 MeV above the yrast line, with the half-maximum points of the energy distributions at {approx}5 MeV for spins between 12({Dirac_h}/2{pi}) and 22({Dirac_h}/2{pi}) . This suggests that the fission barrier is (greater-or-similar sign)5 MeV and that the shell-correction energy persists to high spin. (c) 2000 The American Physical Society.

The level structures of several Ru isotopes near $A=104$ have been studied in the fission of the compound nucleus ${}^{197}\mathrm{Pb},$ formed in the ${}^{24}{\mathrm{Mg}\mathrm{}+}^{173}\mathrm{}\mathrm{Yb}$ reaction at 134.5 MeV. A sequence of five transitions, observed in coincidence with known transitions in the ${}^{86}\mathrm{Sr}$ and ${}^{87}\mathrm{Sr}$ complementary fragments, has been assigned to ${}^{105}\mathrm{Ru}.$ Comparison with the neighboring odd-mass Ru isotopes supports assignment of this sequence as built on the ${h}_{11/2}$ excitation in ${}^{105}\mathrm{Ru}.$ The positive-parity band built on the ${7/2}^{+}$ state of ${}^{103}\mathrm{Ru}$ has been extended above the backbending induced by the alignment of two ${h}_{11/2}$ neutrons. In addition, the ground-state bands of ${}^{104}\mathrm{Ru}$ and ${}^{106}\mathrm{Ru}$ have been extended to higher spins.

Excited states in the nucleus ${}^{191}\mathrm{Pb}$ have been investigated by in-beam \ensuremath{\gamma}-ray spectroscopic techniques using the Gammasphere array. The reaction ${}^{173}\mathrm{Yb}{(}^{24}\mathrm{Mg},6n)$ at a beam energy of 134.5 MeV was used to populate states of ${}^{191}\mathrm{Pb}.$ The level scheme of ${}^{191}\mathrm{Pb}$ has been extended up to 5.1 MeV excitation energy and spin $45/2.$ The lower part (below $\ensuremath{\approx}2.3\mathrm{MeV}$) presents the general features common in all odd-mass Pb isotopes, i.e., spherical states built on the ${13/2}^{+}$ isomer, with bandlike structures that follow weak-coupling expectations. In the upper part of the level scheme two dipole bands are observed with bandhead energies of $\ensuremath{\sim}2.512$ and 2.291 MeV. A comparison of dipole band 1 with the strongest negative-parity dipole bands in the heavier odd-mass Pb isotopes reveals striking similarities. The spherical states are interpreted as the coupling of the ${i}_{13/2}$ neutron hole to the yrast states in the even-mass ${}^{192}\mathrm{Pb}$ core. The dipole bands are interpreted as $M1$ cascades which arise from the coupling of two protons in high-$j$ orbitals above the $Z=82$ gap to neutron holes in the ${i}_{13/2}$ orbital.

Excited states in the proton-rich $N=84$ isotones ${}_{72}^{156}{\mathrm{Hf}}_{84},\phantom{\rule{0.3em}{0ex}}{}_{73}^{157}{\mathrm{Ta}}_{84}$, and ${}_{74}^{158}{W}_{84}$ were observed using $^{102}\mathrm{Pd}(^{58}\mathrm{Ni},\phantom{\rule{0.3em}{0ex}}\mathit{xp}2n)$ reactions at 270 MeV. \ensuremath{\gamma} rays were detected with the Gammasphere array of Compton-suppressed Ge detectors coupled with the Argonne fragment mass analyzer and were assigned to individual reaction channels using the recoil-decay tagging method. Prompt \ensuremath{\gamma}-ray cascades were associated with the \ensuremath{\alpha} decay of both the ground state and the ${8}^{+}$ isomeric state in $^{156}\mathrm{Hf}$, the ${h}_{11/2}$ state in $^{157}\mathrm{Ta}$, and the ${8}^{+}$ isomeric state in $^{158}\mathrm{W}$. The level schemes constructed for $^{156}\mathrm{Hf},^{157}\mathrm{Ta}$, and $^{158}\mathrm{W}$ are compared with these of lighter $N=84$ isotones and are discussed within the framework of the shell model.

Superdeformed excitations in $^{195}\mathrm{Pb}$ were populated via the $^{174}\mathrm{Yb}$($^{26}\mathrm{Mg}$,$5n$) reaction, and \ensuremath{\gamma}-ray spectroscopy measurements were made with Gammasphere. Analysis of the intensities of the discrete transitions between superdeformed states indicates that the decay out occurs from the favored band of the lowest signature pair. The quasicontinuous spectra associated with the decay of superdeformed bands in $^{195}\mathrm{Pb}$ have been extracted for both signatures. The onset of \ensuremath{\gamma}-ray intensity and the normalized \ensuremath{\gamma}-ray intensity of the quasicontinuum decay spectrum are used to set a lower bound of 2.5 MeV at $\ensuremath{\sim}11\ensuremath{\hbar}$ for superdeformed excitation energy relative to normal deformed excitations in $^{195}\mathrm{Pb}$.

Proton-decay studies led in recent years to the observation of about 20 new proton emitters. Among others, first two cases of highly deformed proton emitters, namely 131Eu and 141Ho, were reported [1]. Also, the first case of proton-decay fine structure was found in 131Eu [2]. In this work the studies of the deformed proton emitters 131Eu and 141Ho were extended to their excited states using in-beam spectroscopic methods. The experiments were performed at the Argonne National Laboratory using the GAMMASPHERE array of Ge detectors coupled to the Fragment Mass Analyzer. Partial results from this study has been published in [3]. There were no data on excited states in 131Eu and 141Ho prior to this work. Nothing is known on their immediate neighbors. The quadrupole deformation calculated by Moller and Nix for 141Ho is about β2 = 0.29. According to the proton-decay rate calculations [4] half-lives of the two know proton-decaying states in 141Ho are consistent with the 7/2−[523] and 1/2+[411] Nilsson orbitals at this deformation. 131Eu is expected to be even more deformed with β2 = 0.33. Its proton-decay rate and branching ratio to the 2+ state are consistent with the 3/2+[411] orbital.

The last five years have witnessed a rapid increase in the volume of data on proton decaying nuclei. The path was led by decay studies with recoil mass separators equipped with double-sided Si strip detectors. The properties of many proton-decaying states were deduced, which triggered renewed theoretical interest in the process of proton decay. The decay experiments were closely followed by in-beam {gamma}-ray studies which extended one's knowledge of high-spin states of proton emitters. The unparalleled selectivity of the Recoil-Decay Tagging method combined with the high efficiency of large arrays of Ge detectors allowed, despite small cross sections and overwhelming background from strong reaction channels, the observation of excited states in several proton emitters. Recently, in-beam studies of the deformed proton emitters {sup 141}Ho and {sup 131}Eu have been performed with the GAMMASPHERE array of Ge detectors and the Fragment Mass Analyzer at ATLAS. Evidence was found for rotational bands in {sup 141}Ho and {sup 131}Eu. The deformations and the single-particle configurations proposed for the proton emitting states from the earlier proton-decay studies were confronted with the assignments deduced based on the in-beam data. It should be noted that the cross section for populating {sup 131}Eu is only about 50 nb, and it represents the weakest channel ever studied in an in-beam experiment.

The ground-state band of the Z=102 isotope 254No has been identified up to spin 14, indicating that the nucleus is deformed. The deduced quadrupole deformation, β=0.27, is in agreement with theoretical predictions. These observations confirm that the shell-correction energy responsible for the stability of transfermium nuclei is partly derived from deformation. The survival of 254No up to spin 14 means that its fission barrier persists at least up to that spin.

High-spin states of several isotopes in the mass A ≈ 100 region have been investigated following the fission of compound nuclei formed in three heavy-ion fusion-evaporation reactions. New sequences have been assigned to 91Sr, 107Ru, and 113, 115, 117, 119Cd isotopes using the complementary fission fragment technique. The ground-state bands have been extended to higher spins in 114, 116, 118, 120Cd. The level schemes of 88, 90, 92Sr and 101Tc have been considerably enriched. The plethora of results demonstrates the efficiency in which spectroscopic information can be collected on nuclei near the line of stability or slightly neutron-rich from studies of fission fragments following fusion-evaporation reactions. The prompt γ-ray intensity distributions of the fragments following the fission of the 197Pb compound nucleus are reported. The empirical distributions as a function of Z and A are not reproduced by expected yields of symmetric fission. Rather these distributions reflect the selectivity of Gammasphere to detect collective nuclei and to study high-spin excitations in fission fragments.