Momentum Distribution Techniques 10 The angular resolution is realized by lead slits(Fig.7).It can be adjusted in the range 0.2 to 5 mrad.The energy resolution of a corresponding Doppler- broadening experiment would range from 0.05 to 1.3 keV.Thus,ACAR has a much better momentum resolution than the Doppler-broadening technique. The source is placed outside the visual field of the detectors.The sample- detector distance amounts to several meters.Because of this large distance, annihilation quanta from only a small solid angle are detected.Thus,much stronger sources compared with conventional positron lifetime and Doppler- broadening measurements are required.In order to minimize the reduction of the counting rate due to the distance of several mm between sample and source,a strong magnetic field of about I T is usually applied to guide the positrons to the sample. The main application of angular correlation of annihilation radiation is the study of the electron structure of the bulk and of defects.The momentum resolution is significantly higher than for Doppler-broadening spectroscopy.A two-dimensional record of the momentum distribution is favorable for comparing with theoretical calculations.ID-ACAR machines were hardly used for semiconductors,since the intensive study of semiconductors set in when 2D- ACAR machines became available.This two-dimensional detection(Fig.8)can be carried out with position-sensitive detectors,such as multi-wire proportional chambers or Anger cameras.The electronics of the 2D-ACAR apparatus filters the coincident events of the two detectors and stores the angular deviation from Py Magnet Sample Slit ●et Magnet】 Coincidence Memory Fig.7.Experimental setup for the measurement of one-dimensional angular correlation of annihilation radiation with a long-slit geometry.Sodium iodide crystals are used as scintillator rods(S1 and S2)in the y direction.The left arm with the detector and the lead collimators is fixed,while the right arm can be turned by the angle .Momentum Distribution Techniques 10 The angular resolution is realized by lead slits (Fig. 7). It can be adjusted in the range 0.2 to 5 mrad. The energy resolution of a corresponding Doppler￾broadening experiment would range from 0.05 to 1.3 keV. Thus, ACAR has a much better momentum resolution than the Doppler-broadening technique. The source is placed outside the visual field of the detectors. The sample– detector distance amounts to several meters. Because of this large distance, annihilation quanta from only a small solid angle are detected. Thus, much stronger sources compared with conventional positron lifetime and Doppler￾broadening measurements are required. In order to minimize the reduction of the counting rate due to the distance of several mm between sample and source, a strong magnetic field of about 1 T is usually applied to guide the positrons to the sample. The main application of angular correlation of annihilation radiation is the study of the electron structure of the bulk and of defects. The momentum resolution is significantly higher than for Doppler-broadening spectroscopy. A two-dimensional record of the momentum distribution is favorable for comparing with theoretical calculations. 1D-ACAR machines were hardly used for semiconductors, since the intensive study of semiconductors set in when 2D￾ACAR machines became available. This two-dimensional detection (Fig. 8) can be carried out with position-sensitive detectors, such as multi-wire proportional chambers or Anger cameras. The electronics of the 2D-ACAR apparatus filters the coincident events of the two detectors and stores the angular deviation from Magnet Magnet Sample e+ Q y g g pz px p y Coincidence Memory Q x Slit Slit S1 S2 Fig. 7. Experimental setup for the measurement of one-dimensional angular correlation of annihilation radiation with a long-slit geometry. Sodium iodide crystals are used as scintillator rods (S1 and S2) in the y direction. The left arm with the detector and the lead collimators is fixed, while the right arm can be turned by the angle Qx