2p - Microchannel Plate (MCP) Detector
2p
-Detector is multi-detector, based on microchannel plate detectors. It can be used for detection of electrons, ions of both polarity and photons in wide variety of experimental research. Both energy and angular distribution of different particles can be measured as well as the correlation between these quantities.This new type, 2p-steradian multi-detector originally was developed as low energy electron spectrometer for probing highly charged ion (HCI) surface interaction. This interaction gives rise to the emission of a large number of slow electrons. For example, the impact of a single Th71+ ion on a gold target produces a mean number of 250 "slow" electrons (E<60eV).
2p detector made of 16 independent detection units, made of two multichannel-plates (MCP), located on a half sphere of 60-mm radius surrounding the solid target. The 16 detectors are held from their backside on an R = 85mm hemispherical frame (fig. 1). Three additional holes are bored in this frame: one hole for beam injection at normal incidence and two opposite holes located in the equatorial plane for beam injection at grazing angle allowing the incident and the reflected ions to enter and exit the hemisphere. The field free half sphere between the target and detectors is materialized by an equatorial plane and a so-called inner spherical grid located one mm ahead of the detectors input area.
The individual detection units have been designed to minimize the dead area for maximum solid angle efficiency. At the entrance side, the detector has a 34-mm ring leaving 31-mm active area on the first MCP. To achieve such compact design, all connections have been located on the backside. The two MCP, mounted in chevron, are separated by a 10 mm foil acting as a spacer. This technique, called micrometer gap allows to simplify the assembly while maintaining good pulse amplitude distribution. During separate testing of the MCP units, a pulse amplitude resolution (FWHM/position of maximum) up to 35% has been achieved. In this case, it could be possible to distinguish between single and multiple hits on the same MCP below the 10-20 ns dead time of the time-of-flight electronics. The entrance ring is insulated from the MCP by sapphire balls and may receive a grid to accelerate or repel charged particles. The collector is made of a single metal piece but may be changed to a 3-piece collector allowing higher "granularity" of the 2p system. The collector disk is decoupled from high voltage by a 50 mm thick
Kapton disk (built in capacitor) to allow the collector to be biased by + (or -) 5 kilovolts. Each detector unit has five high voltage connections, two built-in capacitors and a 50W cable for the signal. This strategy allows independent biasing of each detector as well as independent bias voltage of the input and grid voltages, but ends up with a rather complex wiring of almost 100 cables and twice as much connectors under ultra-vacuum.
At run time, any number of units may then be optimized for detection of photon/ electron and negative ions or photons/positive ions. The entrance grid can be used to repel slow electrons and negative ions with a detector biased positive or negative (allowing only photons to be detected). Apart from the channel plates, all parts (more than 70 per detector) have been specially machined out of non-magnetic UHV compatible material.
Each detection unit is terminated by a built-in 50W connector whose signal and coaxial shield are coupled by high voltage capacitors to the collector and the 2nd MCP output respectively. An UHV 50W cable (Kapton/cu) drives the signal to a CF 100 flange bearing 19 double sided SMA feed through. The (DGMO) discriminators specially designed by the IPNO (Institut de Physique Nucleaire-Orsay) for MCP detectors offers decisive advantages. They are:
- rather insensitive to "low frequency" noise (below 100 kHz)
- very sensitive to the MCP signal since the threshold can be as low as 3 mV, which allow increased dynamic range.
- constant fraction discriminators, working in the ideal case of a 100% fraction to take full benefit of the nice symmetrical shape of the MCP signal, An ultimate time resolution below 20 ps has been measured
- they include a built-in linear amplifier which picks up the signal at high impedance and regenerate the entrance signal at 50 W allowing simultaneous pulse amplitude distribution measurements or direct view of the triggering signal on the scope.
- As a result only the threshold has to be adjusted as well as the NIM output width.
There are 16 independent detector units and discriminators. All other electronic devices are designed for 16 channels. The NIM signal out of the discriminator is converted into ECL signal and directly sent to the corresponding channel of the multi-hit (x16) multi-channel (x16) multi-event (x32) numerical Time to Digital Converter LeCroy 1176. This new generation of TDC may be operated both in common start or common stop mode. It records simultaneously all the stops on the 16 channels with a ns accuracy over a full range of 65536 ns (for a price, which competes with traditional TAC). There is no dead time for hits in different channels but 20 ns are needed before the same channel can accept a new hit. The pulse height distributions of all units are also recorded in coincidence with the first stop of each channel; the regenerated analogue signal out of the DGMO discriminators is sent to a 24 channel preamplifier card (Lecroy 2724) and to a "charge to digital" converter (16 channels "Phillips 7176" QDC with individual gates). The position sensitive detector uses standard preamplifier, pulse shaping electronics and ADC to record the position information. The time signal is taken from the output of the second MCP and may be used to stop the TDC after an adjustable delay.
The TDC has been taken in a VME configuration allowing direct high speed 40 Mb/s readout by a VME computer card (Motorola MVME 167 based on a 68040 processor operated under VxWorks). This allows easy ethernet link and file sharing with UNIX workstation, HP 715-33 in our case.
References:
Morosov V. A., Kalinin A., Szilagyi Z., Barat M. and Roncin P.,
"2p spectrometer: A
new apparatus for the investigation of ion surface interaction"
Rev. Sci. Inst., 67, 2163 (1996) download
P. Roncin, J. Villette, J. P. Atanas, and H. Khemliche
"Energy Loss of Low Energy Protons on LiF(100): Surface Excitation and
Mediated Electron Emission"
Phys. Rev. Lett. 83, 864 (1999) downloan
Villette J, Barat M, Roncin P
"Calibration of a multiple microchannel plate detectors system by
alpha-induced secondary electrons"
Rev. Sci. Inst., 71, Iss 6, pp 2367-2370 (2000) download
J. Villette, A. G. Borisov, H. Khemliche, A. Momeni, and P. Roncin
"Subsurface-Channeling-Like Energy Loss Structure of the Skipping Motion on
an Ionic Crystal"
Phys. Rev. Lett. 85, 3137 (2000) download
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