\(e^+e^-\) Pair Spectrometer for Studying the Internal Pair Creation in \(^{8}\)Be at LNL-INFN

Level scheme of 194Ir

Levels of 155Eu from thermal neutron capture

Radiative corrections to bremsstrahlung and pair-production cross sections have been detected experimentally by measuring the thin-target bremsstrahlung spectrum of 5-GeV electrons with a pair spectrometer. Deviations of up to 5% from lowest-order calculations have been observed and found to agree with predictions for radiative corrections given by Mork and Olsen.

Pulse height defect due to electron interaction in the dead layers of Ge(li) γ-Ray detectors

Spectroscopy of the ODD-ODD nucleus 76As and its supersymmetric description

We calculate the spatial correlation of electrons and positrons emitted by internal pair conversion of Coulomb excited nuclei in heavy-ion collisions. The alignment of the nucleus results in an anisotropic emission of the electron-positron pairs that is closely related to the anisotropic emission of \ensuremath{\gamma} rays. However, the angular correlation in the case of internal pair conversion displays additional features which provide the possibility for a deeper understanding of the nuclear structure. Our results are of particular interest for the electron-positron coincidence experiments currently analyzed at GSI (Darmstadt) and at Argonne. \textcopyright{} 1996 The American Physical Society.

Recently, we measured the e$^+$e$^-$ angular correlations in internal pair creation (IPC) for the M1 transitions depopulating the 17.6 and 18.15 MeV states in $^8$Be, and observed a peak-like deviation from the predicted IPC for the 18.15 MeV transition, but not for the 17.6 MeV one. To the best of our knowledge no nuclear physics related description of such deviation can be made. The deviation between the experimental and theoretical angular correlations is significant and can be described by assuming the creation and subsequent decay of a boson with mass $m_0c^2$=16.70$\pm0.35 $(stat)$\pm 0.5 $(sys) MeV. The branching ratio of the e$^+$e$^-$ decay of such a boson to the $\gamma$ decay of the 18.15 MeV level of $^8$Be is found to be $5.8\times10^{-6}$ for the best fit. The data can be explained by a 17.6 MeV vector gauge boson X that is produced in the decay of the excited state to the ground state, and then decays to e$^+$e$^-$ pairs. In the present work we re-investigated the e$^+$e$^-$ pair correlation in the 17.6 MeV transition of $^8$Be in which some smaller deviation was also expected. The branching ratio of the $e^+e^-$ decay of such a boson to the $\gamma$ decay was found to be $(2\pm2)\times10^{-6}$.

A detailed discussion of the radiative corrections to internal pair creation is given, including the effect of all internal bremsstrahlung. Numerical results are presented for the 6⋅04 MeV zero-zero transition in 16 O, for which the effects of the Coulomb field, of the nuclear structure and of the non-Coulomb forces are shown to be negligible. The total correction amounts to a decrease of 0⋅7% in the total pair emission, and varies quite slowly with the angle between the pair, except near θ = 0° (due to the interaction between the pair–the number of pairs with separation < 10° is increased by 2⋅3%) and θ = 180° (an increase of 5% in pair emission due to pairs accompanied by a hard photon).

We present the first energy and angle resolved measurements of e+e- pairs emitted from heavy nuclei (Z>=40) at rest by internal pair conversion (IPC) of transitions with energies of less than 2MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76MeV E0 transition in Zr90 (Sr source) and the 1.77MeV M1 transition in Pb207 (Bi source) have been investigated experimentally using the essentially improved set-up at the double-ORANGE beta-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher beta- and gamma background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the e+e- pairs, the angular correlation of the emitted pairs was measured. In the Zr90 case the correlation could be deduced for a wide range of energy differences of the pairs. The Zr90 results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in Pb207 is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results.

The angular distribution of electrons and positrons emitted in internal pair conversion is calculated. Coulomb-distorted waves are used as electron wave functions. Nuclear transitions of various multipolarities L>0 and of magnetic (ML) and of electric (EL) type are considered as well as E0 conversion. Analytical expressions for the angular correlation are derived, which are evaluated numerically assuming a finite extension of the nucleus and, for the EL and ML conversion, also in the point-nucleus approximation. The calculated angular correlations are compared with results obtained within the Born approximation and, for the E0 case, with experimental data.

Electron spectrometer for electric monopole (E0) transition studies in nuclei

The motion of electrons and positrons was modeled in a spectrometer based on a constant magnetic dipole and their subsequent detection in position-sensitive silicon detectors. The electron and positron emission energy and angle resolutions were obtained. The spectrometer is a part of setup for studying rare intranuclear processes in light nuclei with the production of internal conversion pairs.

A campaign of Coulomb-excitation experiments to study the electromagnetic structure of \(^{110}\)Cd was performed using beams of \(^{14}\)N, \(^{32}\)S, and \(^{60}\)Ni. The use of various reaction partners enables disentangling the contributions of individual electromagnetic matrix elements involved in the excitation process, yielding, among others, a precise determination of the lifetime of the 2\(^+_2\) state in \(^{110}\)Cd. Abstract Published by the Jagiellonian University 2025 authors

We study the angular correlation of electrons and positrons which are emitted in internal pair conversion (IPC). This process is not only of interest in nuclear physics where our work supplements the early calculations of Rose [1] who employed the plane wave Born approximation. Since in different experiments at the Gesellschaft für Schwerionenforschung (GSI) two collaborations (ORANGE, EPOS) [2] have measured narrow peak structures in electron and positron spectra, the various pair creating processes have to be considered in more detail. The spectra are recorded in heavy-ion collisions with energies near the Coulomb barrier. The origin of these narrow lines is presently not understood although various explanations have been proposed [3].KeywordsAngular CorrelationNuclear ChargeBorn ApproximationCoulomb BarrierAnisotropy FactorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Data on internal pair conversion in widely different nuclear transitions with energies above 12 MeV have an apparent excess in pairs around 9 MeV/ invariant mass. While the deviations individually may be circumstantial, in combination they appear to point to an overall anomaly.