Angular Correlation Distribution Research Articles

Experiments on the Competitive Double-Gamma (γγ/γ) Decay

The recent observation of the competitive double-γ decay process is presented. It is a second-order electromagnetic decay mode. The 662-keV decay transition from the 11/2− isomer of 137Ba to its ground state has been studied. The emission of a single 662-keV γ quantum is a factor of 5 × 105 more likely than the simultaneous emission of two γ quanta instead of one. The observed angular correlation and energy distribution of these rare coincident double-γ quanta are well described by a dominant M2 – E2 and a sub-dominant E3 – M1 contribution to the double-γ decay branch.

First observation of the competitive double-gamma (γγ/γ) decay process

First observation of the competitive double-γ decay process is presented. It is a second-order electromagnetic decay mode. The 662-keV transition from the 11/2− isomer of 137Ba to its ground state proceeds at a fraction of 2 x 10−6 by simultaneous emission of two γ quanta instead of one. The angular correlation and energy distribution of coincident γ quanta are reproduced by a dominant M2-E2 and a minor E3-M1 contribution to the double-γ decay branch. The data are well accounted for by a calculation in the Quasiparticle Phonon Model.

First observation of the competitive double-gamma (“γγ/γ”) decay process

First observation of the competitive double-γ decay process is presented. It is a second-order electromagnetic decay mode. The 662-keV decay transition from the 11/2− isomer of 137 Ba to its ground state proceeds at a fraction of 2 × 10−6 by simultaneous emission of two γ quanta instead of one. The observed angular correlation and energy distribution of coincident γ quanta are well described by a dominant M2 – E2 and a minor E3 – M1 contribution to the double-γ decay branch. The data were well accounted for by a calculation using the Quasiparticle Phonon Model.

Measurements of electrons from heavy-flavour hadron decays in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Heavy-flavour hadrons, i.e. hadrons carrying charm or beauty quarks, are a well-suited probe to study the Quark–Gluon Plasma (QGP) in relativistic heavy-ion collisions. For this reason, measurements of electrons from heavy-flavour hadron decays have been performed in pp, p–Pb and Pb–Pb collisions at the LHC with the ALICE detector. Results for the nuclear modification factors ( R pA and R AA ) support a final-state energy loss of heavy quarks in central Pb–Pb collisions and, in semi-central collisions a positive elliptic flow coefficient v 2 of electrons from heavy-flavour hadron decays was observed. Furthermore, a double-ridge structure was observed in the measured two-particle angular correlation distribution, triggered by heavy-flavour decay electrons, in high-multiplicity p–Pb collisions relative to low-multiplicity p–Pb collisions and to pp collisions.

Measurements of heavy-flavour decay leptons in pp, p-Pb, and Pb-Pb collisions with ALICE

We present measurements of electrons and muons originating from the semi-leptonic decays of heavy-flavour hadrons at central and forward rapidities with ALICE in pp ( = 2.76 and 7 TeV), p-Pb ( = 5.02 TeV), and Pb-Pb ( = 2.76 TeV) collisions. The measured pT-differential production cross sections in pp collisions, the nuclear modification factors in p-Pb and Pb-Pb, and the v2 measurements in Pb-Pb collision are disgusted. In addition, the angular correlation distribution between heavy-flavour decay electrons and charged particles, measured in p-Pb collisions, is shown. The results are compared to their corresponding theoretical predictions.

Azimuthal angular correlations between heavy-flavour decay electrons and charged hadrons in pp collisions at s=2.76TeV in ALICE

In ultra-relativistic heavy-ion collisions heavy quarks (charm and beauty) are produced in hard scattering processes and thus carry relevant information on the properties of the created hot and dense medium. Heavy-flavour hadrons can be measured using electrons from their semileptonic decay channels. The relative contribution of charm and beauty hadrons can be estimated from the study of the near side azimuthal angular correlations between these electrons and charged hadrons. We present the measurement of azimuthal angular correlations between electrons and charged hadrons in pp collisions at 2.76 TeV measured with ALICE at the LHC. Electrons are identified using the Electromagnetic Calorimeter and the Time Projection Chamber and the charged hadrons are identified using the Time Projection Chamber. The azimuthal angular correlation distributions from PYTHIA simulations are used to extract the relative contribution from B-hadron decays to the yield of electrons from heavy-flavour decays up to $p_\textrm{t}$ = 10 GeV/c.

K−absorption at rest in nuclei followed bypΛemission

$p\,\Lambda$ emission in coincidence following $K^-$ absorption at rest in nuclei is studied using quantum mechanical scattering theory and nuclear wave functions. $K^-$ absorption is assumed to occur on two protons in the nucleus. In the formalism, emphasis is put on the study of the final state interaction (FSI) effects of $p$ and $\Lambda$ with the recoiling nucleus. We include elastic scattering and single nucleon knock-out (KO) channels in the FSI. Calculations are presented for the $^{12}$C nucleus, using shell model wave functions, and without any extra mass modification of the $K^-\,pp$ system in the nucleus. Calculated results are presented for the angular correlation distribution between $p$ and $\Lambda$, their invariant mass distribution and the momentum spectra of $p$ and $\Lambda$. These results are compared with the corresponding experimental measurements \cite{agnello}. With only elastic scattering FSI included, the angular correlation distribution and the momentum spectra are found to be in good accord with the corresponding measurements. With full FSI the calculated $p\,\Lambda$ invariant mass distribution is found to have two peaks, one corresponding to the elastic scattering FSI and another to single nucleon KO FSI. The KO peak agrees fully, in position and shape, with the peak observed in Ref. \cite{agnello}. The peak corresponding to elastic scattering FSI does not seem to exist in the measured distribution. Considering that such a two peak structure is always seen in the inclusive ($p$, $p^\prime $) and ($e$, $e^\prime $) reactions in nuclei at intermediate energies, absence of the elastic scattering peak in the $p\,\Lambda$ reaction is intriguing.

Studying the relative bottom contribution via heavy-quark decay electron measurements in STAR

We report measurements of the azimuthal angular correlation distribution of heavy-quark decay electrons and open charmed mesons in pp collisions at \(\sqrt{s}=200\) GeV in the STAR experiment at RHIC. This measurement in combination with current theoretical model calculations allows to extract the relative bottom contribution to the heavy-quark decay electrons, which is important for the interpretation of the observed strong suppression of the high-p T electron yield in central Au+Au collisions.

Continuum spectroscopy of Borromean two-neutron halo nuclei

Energy and angular correlation distributions of the three fragments in $^{6}\mathrm{He}$ breakup on $^{208}\mathrm{Pb}$ at a collision energy of 240 MeV/nucleon are discussed within the microscopic four-body distorted wave model and compared with experimental data. The nuclear structure of the ground state and low-energy three-body continuum of $^{6}\mathrm{He}$ is calculated by the method of hyperspherical harmonics within the three-body cluster model. Reflections of the fundamental permutation symmetry of the halo neutrons in angular and energy correlations are pointed out. The calculations describe the experimental data for fragment correlations near breakup threshold rather well, and the physics is contained in a few elementary modes; but with increasing excitation energy of $^{6}\mathrm{He}$, some striking deviations from experimental distributions are encountered. Possible reasons for this are discussed.

The effects of positron binding and annihilation mechanisms in biomolecules on PET resolution

The angular correlation distribution of electron-positron annihilation photons is presented for polar biological molecules, and the associated noncollinearity effect on resolution in positron emission tomography (PET) is discussed. The image resolution in PET is known to be limited by a number of factors such as radioactive decay statistics, attenuation, scatter, or random coincidence of annihilation photons. This paper deals with the resolution limits on the side of the measured system. Our calculated distribution of annihilation momentum belongs to the intrinsic resolution factors together with the positron range in biological tissue; the other factors are due to the interactions along the photon path from the tissue to the detector. Here we demonstrate that the angular distribution of annihilation trajectories is not necessarily Gaussian, especially when the positron temporarily binds in a molecular dipole moment field before annihilation. The theoretical framework treats the electron and positron on the same footing by using the multireference single- and double-excitation configuration interaction algorithms applicable also to other bound-state systems. Our results form a subset of parameters relevant to the statistical image reconstruction algorithms, which may eventually increase the sensitivity of PET imaging in clinical setting

Depth-selective 2D-ACAR studies on low- k dielectric thin films

Depth-selective 2D-ACAR investigations on ordered mesoporous silica thin films provide direct evidence that para-positronium ( p-Ps) created deep in the films can escape through a network of interconnected pores. The depth dependence of the escape fraction and of the average kinetic energy of non-thermally excited p-Ps is in quantitative agreement with Monte Carlo modeling, assuming classical collisions of p-Ps with the pore walls. The model provides insight in the shape of the angular correlation distributions and their sensitivity to, e.g., the effective wall mass M s and pore dimensions.

Molecular dynamics simulations of ultra-thin Cu nanowires

To study the properties of ultra-thin copper nanowires (NWs), we have simulated several copper NWs using classical molecular dynamic simulations. As the temperature increases, copper NWs were transformed into structures with the lowest surface stresses and the lowest surface energy, cylindrical shapes with {1 1 1}-like surface. As the thickness of copper NW increases, the temperature achieving the breaking and the structural transition of the NW also increases. The investigations on angular correlation and radial distribution functions were shown that ultra-thin {1 1 1} NWs were more stable than that of {1 0 0} NWs. The vibrational frequency of NWs was different to that of bulk around 3 THz and above 8 THz. The structural properties of cylindrical multi-shell NWs were greatly different from that of face-centered cubic.

A new detector for measuring three-dimensional momenta of charged particles in coincidence

A detector system for the study of the dissociation dynamics of molecular ions is described. The measurements of the time of flight and of the hit coordinates of the charged fragments on the detector are transformed into the three momentum components along Cartesian coordinates. The detector uses 2×32 independent channels which makes possible the simultaneous detection of several particles and a spatial resolution of 0.65 mm on a 40 mm diameter channel plate. The low dead time of this detector (≃1 ns) enables us to study the dissociation of homonuclear molecules and also the angular correlation and energy distribution between two electrons of very low kinetic energies. As an illustration of its possibilities, results obtained in the dissociation of charged molecular ions will be considered to illustrate the capacity of this detector for the identification of dissociation channels, the accurate kinetic-energy distribution, and the investigation of the dissociation kinematics.

Momentum distributions of electron-positron pairs annihilating at vacancy clusters in Si

We report calculations of momentum densities of electron-positron pairs annihilating at various vacancy clusters in Si. The densities integrated along one direction, i.e., those corresponding to the spectra measured by the two-dimensional angular correlation of the annihilation radiation method, are shown to be isotropic if the positron is captured by a small vacancy cluster. The densities integrated along two directions correspond to the one-dimensional angular-correlation distribution or the Doppler-broadened annihilation radiation line shape. The characteristic narrowing of these one-dimensional distributions at vacancy clusters is demonstrated. The line-shape parameters $S$ and $W$ describing mainly the annihilations with valence and core electrons, respectively, are calculated and compared quantitatively with experimental data. We find a systematic increase in the $S$ and a decrease in the $W$ parameter as the size of the vacancy increases.

Angular correlation studies of positron annihilation in the noble gases

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

Angular-correlation distribution of positron-electron annihilation in GaAs.

Angular correlation of positron annihilation radiations has been measured along the [100], [110], [111], [211], and [221] crystallographic axes in GaAs using the long-slit geometry. A pseudopotential calculation is carried out to understand the experimental data. There is good agreement between experimental and theoretical data. The structures seen in the low-momentum region of the data are interpreted in terms of the \ensuremath{\sigma} and \ensuremath{\pi} bonds of the ${\mathit{sp}}^{3}$ hybridized bonding orbitals. The experimental data when compared with the calculated data reveal the presence of electron-positron many-body correlation effects.

Angular correlation distribution of electron-positron annihilation in graphite

Using the empirical pseudopotential method for the electron wavefunction and the variational positron wavefunction, the authors have calculated the one- and two-dimensional angular correlation distribution of electron-positron annihilation (ACAR) as well as the electron momentum distribution (EMD) for graphite. The calculated ACAR as well as its anisotropy are in good agreement with experiments. Due to the presence of the positron, the calculated ACAR shows stronger anisotropy than the corresponding EMD, which is in agreement with the experimental data. It is shown that the layer structure of graphite is more clearly exhibited by the ACAR than the EMD. The remaining discrepancy between the presently calculated ACAR distribution and the experimental spectra is discussed and it is argued that it originates from the quality of the single-crystal sample used in the experiments and the electron-positron many-body interaction which has not been included in their theoretical treatment.

Wide-angle and out-of-plane correlations in 7Li fragmentation

Angular correlation distributions have been measured for the break-up of 70 MeV 7Li into the α + t channel following interaction with 12C and 120Sn targets. Large area silicon strip detectors were used to measure efficiently both in-plane and out-of-plane correlations. The yield of the fragments is found to spread over a wider angular range than would be expected for sequential decay of the projectile. These wide-angle yields are considered to arise from a rapid break-up of the projectile in the nuclear field of the target. The strong in-plane yield suggests that the final channel fragments are simply being deflected by the target nucleus rather than by an absorptionre-emission process.

A novel formalism for calculating positron annihilation characteristics

The authors propose a novel formalism to calculate the positron annihilation characteristics in solids in which the positron-electron product (PEP) wavefunction is obtained directly from a wave equation. From this PEP equation it is easy to see that the most structure-dependent term, the Coulomb potential, present in the individual electron and positron wave equations is cancelled exactly. The remaining potential is a considerably smoother and weaker function than the Coulomb potential. In a qualitative analysis, one can then easily explain some typical but apparently paradoxical features in earlier treatments. As an illustration of the PEP formalism the authors have calculated the bulk angular correlation distribution of a positron annihilating in Al metal. Within the new formalism the effect of the crystalline potential (symmetry) and of the many-body interaction on the angular correlation distribution can be calculated in a phenomenological way. The results are in good agreement with experiments, even as regards the finer details, and compare well with the conventional formalism. The advantages and shortcoming of the new formalism are discussed.

Positron Annihilation in Si and Ge

AbstractA computational technique is developed to evaluate electron and electron—positron momentum densities in silicon and germanium and the use of these to provide means of interpreting the k‐space densities obtained by experimentalis's using the two‐dimensional angular correlation of positron annihilation radiation technique (2D ACAR). As a consequence of the good agreement observed in the momentum densities and the LCW folded data obtained for silicon and germanium the independent particle approximation (IPM) coupled with the use of electron pseudo‐wavefunctions represent an excellent starting point for a systematic many‐particle description of the process, since significant deviations from the experimental angular correlation distribution can be ascribed to the electron—positron and electron—electron interactions.