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Shallow ion implantation and rapid thermal annealing (RTA) was used to modify the optical properties of strained InGaAs/GaAs quantum wells (QWs). After RTA, QW exciton energies, determined from peak positions of the photoluminescence spectra, shifted significantly to higher energies in the implanted areas, whereas they remained basically unaffected in the unimplanted regions. The magnitudes of the energy shifts depend on the well width, RTA temperature and ion implantation fluence. The shifts were interpreted as arising from modification of the shapes of the as-grown QWs due to diffusion of In out of the well material. This process is enhanced by diffusion of vacancies generated near the sample surface by ion implantation. QWs with compositions near the critical thickness exhibit different behaviour from that of fully pseudomorphic layers, due to the presence of dislocations in these layers. [ABSTRACT FROM AUTHOR]

The effect of rapid thermal annealing (RTA) on the shapes of GaAs/AlGaAs quantum wells (QWs) has been investigated by monitoring exciton energies using low temperature photoluminescence and photoluminescence excitation spectroscopies. After RTA, large changes in exciton energies were observed only in regions of the samples in which excess surface vacancies were generated, either by capping with a thin layer of SiO or by low-energy ion implantation. These changes were interpreted as resulting from modifications of the shapes of the as-grown QWs from abrupt or square to gradual (rounded) due to enhanced interdiffusion of well/barrier atoms. For single QWs there was an increase in exciton energy whose magnitude depended on the width of the well, its distance from the surface of the wafer, the annealing temperature and the total number of surface vacancies available. From studies of coupled QWs, there was clear evidence of asymmetry in the heterostructure after RTA. Although both techniques of vacancy generation yield substantial QW shape modifications, the ion implantation technique has the advantages of being highly reproducible and of being compatible with any material system. [ABSTRACT FROM AUTHOR]

Presents a study that discussed the conductivity enhancement in polydiacetylene crystals due to chemical doping and ion implantation was measured and analyzed in relation to their spin concentration and photocurrent decay. Information on polydiacetylenes; Structural and optical information obtained through infrared and visible reflectivity spectra; Photocurrent decay time measurements.

Presents a study which utilized electroreflectance spectroscopy to investigate a GaAs/Ga[sub0.77]Al[sub0.23]As coupled double quantum well (CDQW) system as a function of externally applied electric field. Detection of intra- and inter-well exciton transitions; Lineshape consideration for modulated reflectivity; Effects of electric fields on CDQW.

A new experimental setup for low-energy Coulomb excitation experiments was constructed in a collaboration between the National Superconducting Cyclotron Laboratory (NSCL), Lawrence Livermore National Laboratory (LLNL), and the University of Rochester and was commissioned at the general purpose beam line of NSCL’s ReA3 reaccelerator facility. The so-called JANUS setup combines γ -ray detection with the Se gmented G e A rray (SeGA) and scattered particle detection using a pair of segmented double-sided Si detectors (Bambino 2). The low-energy Coulomb excitation program that JANUS enables will complement intermediate-energy Coulomb excitation studies that have long been performed at NSCL by providing access to observables that quantify collectivity beyond the first excited state, including the sign and magnitude of excited-state quadrupole moments. In this work, the setup and its performance will be described based on the commissioning run that used stable 78 Kr impinging onto a 1.09 mg/cm 2 208 Pb target at a beam energy of 3.9 MeV/u. [ABSTRACT FROM AUTHOR]

The γ -ray tracking array GRETINA was coupled to the S800 magnetic spectrometer for spectroscopy with fast beams of rare isotopes at the National Superconducting Cyclotron Laboratory on the campus of Michigan State University. We describe the technical details of this powerful setup and report on GRETINA's performance achieved with source and in-beam measurements. The γ -ray multiplicity encountered in experiments with fast beams is usually low, allowing for a simplified and efficient treatment of the data in the γ -ray analysis in terms of Doppler reconstruction and spectral quality. The results reported in this work were obtained from GRETINA consisting of 8 detector modules hosting four high-purity germanium crystals each. Currently, GRETINA consists of 10 detector modules. [ABSTRACT FROM AUTHOR]

Focuses on the use of magnetophotoluminescence excitation spectroscopy to study GaAs-AlGasAs coupled double quantum wells. Technique used to measure the hole Landau levels; Photoluminescence excitation spectrum at zero magnetic field; Effects of the magnetic field.

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the T z = ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T = 2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f 7 / 2 shell, and present a stringent test of the shell-model prescription. [ABSTRACT FROM AUTHOR]

Proton capture on the excited isomeric state of 26Al strongly influences the abundance of 26Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic 26Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical 26mAl(p,γ)27Si reaction. These constraints demonstrate that the rate is at least a factor ∼8 lower than previously expected, indicating an increase in the stellar production of 26Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of 26Al. [ABSTRACT FROM AUTHOR]

Energy differences between isobaric analogue states have been extracted for the A = 79 , 79Zr/79Y mirror pair following their population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. These are the heaviest nuclei where such measurements have been made to date. The deduced mirror energy differences (MED) are compared with predictions from a new density-functional based approach, incorporating isospin-breaking effects of both Coulomb and nuclear charge-symmetry breaking and configuration mixing. [ABSTRACT FROM AUTHOR]

The neutron-rich nuclei in the N=28 island of inversion have attracted considerable experimental and theoretical attention, providing great insight into the evolution of shell structure and nuclear shape in exotic nuclei. In this work, for the first time, quadrupole collectivity is assessed simultaneously on top of the 3/2- ground state and the 7/2- shape-coexisting isomer of 43S, putting the unique interpretation of shape and configuration coexistence at N=27 and 28 in the sulfur isotopic chain to the test. From an analysis of the electromagnetic transition strengths and quadrupole moments predicted within the shell model, it is shown that the onset of shape coexistence and the emergence of a simple collective structure appear suddenly in 43S with no indication of such patterns in the N=27 isotone 45Ar. [ABSTRACT FROM AUTHOR]

The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in 21O, finding τ 1 / 2 + = 420 − 32 + 35 (stat) − 12 + 34 (sys) ps. This result together with the deduced level scheme and branching ratio of several γ -ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of B (E 2 ; 1 / 2 + → 5 / 2 g. s. +) = 0.71 − 0.06 − 0.06 + 0.07 + 0.02 e2 fm4, derived from the lifetime of the 1 / 2 + state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces. [ABSTRACT FROM AUTHOR]

New experimental data obtained from γ-ray tagged one-neutron and one-proton knockout from 55 Co are presented. A candidate for the sought-after T = 1, Tz = 0, Jπ = 6+ state in 54 Co is proposed based on a comparison to the new data on 54 Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full fp-model space employing charge-dependent contributions, and isospin-symmetry arguments. Furthermore, possible isospin-symmetry breaking in the A = 54, T = 1 triplet is studied by calculating the experimental c coefficients of the isobaric mass multiplet equation (IMME) up to the maximum possible spin J = 6 expected for the (1f7/2)-2 two-hole configuration relative to the doubly magic nucleus 56 Ni. The experimental quantities are compared to the theoretically predicted c coefficients from LSSM calculations using two-body matrix elements obtained from a realistic chiral effective field theory potential at next-to-next-to-next-to-leading order (N³ LO). [ABSTRACT FROM AUTHOR]

Angle-integrated cross-section measurements of the 56Ni(d , n) and (d , p) stripping reactions have been performed to determine the single-particle strengths of low-lying excited states in the mirror nuclei pair 57Cu−57Ni situated adjacent to the doubly magic nucleus 56Ni. The reactions were studied in inverse kinematics utilizing a beam of radioactive 56Ni ions in conjunction with the GRETINA γ -array. Spectroscopic factors are compared with new shell-model calculations using a full pf model space with the GPFX1A Hamiltonian for the isospin-conserving strong interaction plus Coulomb and charge-dependent Hamiltonians. These results were used to set new constraints on the 56Ni(p , γ)57Cu reaction rate for explosive burning conditions in x-ray bursts, where 56Ni represents a key waiting point in the astrophysical rp -process. [ABSTRACT FROM AUTHOR]

Background: In the late stages of stellar core collapse just prior to core bounce, electron captures on medium-heavy nuclei drive deleptonization. Therefore, simulations require the use of accurate reaction rates. Nuclei with neutron number near N=50 above atomic number Z=28 play an important role. Rates presently used in astrophysical simulations rely primarily on a relatively simple single-state approximation. In order to improve the accuracy of the astrophysical simulations, experimental data are needed to test the electron-capture rates and to guide the development of better theoretical models and astrophysical simulations. Purpose: The purpose of the present work was to measure the Gamow-Teller transition strength from 86Kr to 86Br, to derive the stellar electron-capture rates based on the extracted strengths, and to compare the derived rates with rates based on shell-model and quasiparticle random-phase approximation (QRPA) Gamow-Teller strengths calculations, as well as the single-state approximation. An additional purpose was to test the impact of using improved electron-capture rates on the late evolution of core-collapse supernovae. Method: The Gamow-Teller strengths from 86Kr were extracted from the 86Kr(t,³He+γ) charge-exchange reaction at 115MeV/u. The electron-capture rates were calculated as a function of stellar density and temperature. Besides the case of 86Kr, the electron-capture rates based on the QRPA calculations were calculated for 78 additional isotopes near N=50 above Z=28. The impact of using these rates instead of those based on the single-state approximation is studied in a spherically symmetrical simulation of core collapse just prior to bounce. Results: The derived electron-capture rates on 86Kr from the experimental Gamow-Teller strength distribution are much smaller than the rates estimated based on the single-state approximation. Rates based on Gamow-Teller strengths estimated in shell-model and QRPA calculations are more accurate. The core-collapse supernova simulation with electron-capture rates based on the QRPA calculations indicate a significant reduction in the deleptonization during the collapse phase. Conclusions: It is important to utilize microscopic theoretical models that are tested by experimental data to constrain and estimate Gamow-Teller strengths and derived electron-capture rates for nuclei near N=50 that are inputs for astrophysical simulations of core-collapse supernovae and their multimessenger signals, such as the emission of neutrinos and gravitational waves. [ABSTRACT FROM AUTHOR]

Excited states of the neutron-rich isotopes 42,44S and 41,43P have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA γ-ray tracking array to extract inelastic-scattering cross sections. Deformation lengths of the 21+ excitations in 42,44S have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements Mn/Mp for the 21+ excitations in these nuclei. The present results for 41,43P(p,p') are used to compare two shell-model interactions, SDPF-U and SDPF-MU. As in a recent study of 42Si, the present results on 41,43P favor the SDPF-MU interaction. [ABSTRACT FROM AUTHOR]

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 02+ state in 32Mg populated in a two-proton removal reaction. We confirm the 02+→21+γ-ray transition of 32Mg and constrain the 02+ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 02+ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 02+ state, indicating the intruder dominance of this state. [ABSTRACT FROM AUTHOR]

A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus 42Si--going beyond earlier comparisons of excited-state energies--is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying 42Si(2+1) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich 42Si with a one-proton removal reaction from 43P projectiles at 81 MeV/nucleon. The measured cross sections to the individual 42Si final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited 0+ states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in 42Si is proposed to be the (0+2) level. [ABSTRACT FROM AUTHOR]