Position sensitive detectors in (e,2e) coincidence measurements

Abstract

The two outgoing electrons from (e,2e) scattering processes are selected by two 180° electrostatic analyzers whose energy dispersed output spectra are detected by microchannel plates followed by resistive anodes. These detectors permit the identification of all multiple coincident events occurring within the dispersal width of the analyzers. It was found that the time difference between the two detectors for correlated events is strongly dependent on the energy of the incident electrons, which consequently causes the time coincidence window to move systematically. This behavior has been accounted for by a model that determines the time taken by an electron to go around a hemispherical analyzer. A description is given of a data collection system that can compensate for the shift in the coincidence spectra and that records all multiple coincidence events with an increase in the overall coincidence count rate of about two orders of magnitude relative to conventional non position sensitive detectors without position sensing. The performance of the system is shown for an (e,2e) spectrum of argon.

(e,2e) spectroscopy

“…This is usually accomplished by reducing the coincidence time window (AT). Schemes have recently been put forward to deal with the variation in flight times of electrons through electric-deflection energy analyzers (Volker and Sandner, 1983;Best and Zhu, 1985;Hayes et aL, 1988;Lower and Weigold, 1989). Fast rise-time amplifiers and constantfraction discriminators can be used to reduce electronic time dispersion.…”

“…As many as 49 points on the momentum density can be measured at once. Weigold and co-workers (Cook, McCarthy, Stelbovics, and Weigold, 1984;Cook, Mitroy, and Weigold, 1984;Lower and Weigold, 1989;Weigold, 1990) and Williams and his collaborators (Hayes et aL, 1988) have taken a different approach by developing a multiplexed apparatus that permits an entire binding energy spectrum to be observed at a single value of momentum. This apparatus uses two spherical, electrostatic-deflection-type energy analyzers in a symmetric-noncoplanar geometry.…”

Dual toroidal photoelectron spectrometer for investigating photodouble ionization in atoms and molecules

“…II E. The distribution of random events is uniform and we have found no need for a noise antenna to remove electrical pickup. 30,31 For each coincident event the TAC output, ⌬t, and the (x,y) coordinates from both detectors are digitized and stored. In the subsequent analysis, radial filtering is applied to remove the electrons outside the sharp image ͑see Fig.…”

Invited Article: An improved double-toroidal spectrometer for gas phase (e,2e) studies

“…The spectrum shows coincidence counts as a function of t 1 − t 2 , where t 1 and t 2 , respectively, represent arrival times for electrons comprising each coincident electron pair, after correction has been made for the electron energy-dependent flight-time variations through the analyzers. 3,44 Evident is a prominent peak ͑2.9 ns FWHM͒, corresponding to electrons derived from common ionization events, superimposed on a background due to electrons derived from separate ionization events. Subtraction of these background events is performed in software using standard statistical techniques.…”

“…Reductions in timing resolution degrade the statistical quality of acquired ͑e ,2e͒ data due to increased levels of measured background. 3 While the recorded electron arrival positions can be used to correct for the energydependent variation of flight times through the analyzer, 3,44 no correction can be made for the variations in the trajectories of electrons of the same entrance energy E a but different entrance angles ␣ a , as they focus to the same location on the detector.…”