The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.
In this paper we present details of the detector electronics used in measuring the Dopplerbroadened gamma spectra associated with the annihilation of positrons with individual core levels electrons. Gamma spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level electrons. The energy of the gamma rays was measured using a HPGe detector. The electrons were energy selected using a trochoidal energy analyzer and detected using a micro channel plate. The outputs of the electron and gamma detectors were fed to a fast coincidence circuit that was used to gate the multi-channel used to acquire the gamma energy spectrum.
IntroductionThe contribution to the gamma ( ) spectra from annihilation with core electrons has become a subject of interest as the result of recent experiments demonstrating that a chemically distinct core signature makes it possible to obtain element-specific information in positron defect studies [1]. Typically more than 90% of the annihilation events occur with the valence electrons due to the repulsion of positron from the positive core. Since it is not possible to uniquely decompose the spectra into core and valence contributions there is a need for an experimental method to separate the core-electron contribution to the annihilation spectra from the much larger valence contribution. Here we report details of the apparatus used in the first measurement of the Doppler broadened ray energy spectra associated with the annihilation of a positron with a selected, individual, core level [2]. The measurement method relies on the fact that almost all of the annihilation events with the outer core electrons result in the nearly simultaneous emission of an energetic electron resulting from a CVV Auger transition in which one valence electron carries off the energy made available when another valence electron fills the core-hole left by annihilation. Previous measurements have demonstrated that it is possible to detect these annihilation induced Auger electrons with high efficiency and with an energy resolution sufficient to infer the energy-level of the initial core hole [3,4]. Thus, spectra associated with positron annihilation with electrons in a particular core level can be obtained by measuring the energies of 's detected in coincidence with annihilation-induced Auger electrons of the appropriate energy.The Auger coincidence data were acquired using a magnetically guided positron annihilation induced Auger (PAES) system [5] with the addition of a high purity Ge (HPGe) detector, multi-channel analyzer (MCA) and fast coincidence circuitry (see Fig. 1). Each sample was 20mm x 20 mm; cut from high purity (99.999%) Ag, Cu and Au foils. Before loading sample into the sample chamber, each sample was treated by etching in a 48% solution of Hydrofluoric acid followed by rinsing in Acetone and Ethyl alcohol. The beam had a fluence of 2.5 x 10 4 positron per second and was incident on the samples at 12 eV...
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