Abstract
The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.
Highlights
The pulsed low energy positron system [1] (PLEPS) at the high intensity neutron induced positron source [2] (NEPOMUC) at the Munich research reactor FRM-II is a very powerful user facility to perform depth-dependent high-quality positron annihilation lifetime spectroscopy (PALS) measurements
In the 2D-AMOC setup the two almost collinearly emitted gamma quanta from the annihilation process are registered with a photomultiplier tube (PMT) to measure the lifetime of the positron in coincidence with a Germanium detector to measure the longitudinal momentum of the electron
The electron cloud produced in the channels of the MCP image intensifier is collected on a so-called resistive screen anode which is capacitance-coupled to a commercial position-encoding delay-line anode (RoentDek GmbH) placed outside the MCP image intensifier
Summary
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 J. U Ackermann, W Egger, P Sperr, B Löwe, L Ravelli, G Kögel, G Dollinger and O Jagutzki
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