Quantar Technology
Mepsicron-II tm Series Single-Photon Imaging Detector System
MODEL 2601B TECHNICAL DESCRIPTION, OCT 2001
System Component Modules and Operation
Imager Sensor Head. The Mepsicron-II tm Imager is a microchannel-plate (MCP) type, XY position-sensitive photomultiplier PMT imager. The detector head is a permanently vacuum-sealed assembly. The front section of the imager resembles a conventional proximity-focused image intensifier and an MCP-PMT. In operation, single incoming photons are incident on the integral photocathode, generating photoelectrons (as indicated by the radiant QE%). Under the influence of a strong electric field, these photoelectrons impact the proximity-focused wafer-type MCP electron multiplier, spaced closely to the photocathode (no electron lens is used). The position of each incoming photon is thereby replicated, photon-by-photon, on the MCP surface. Various photocathode types are available to match application needs.
The near-noiseless charge amplification of the multi-MCP stack (equivalent of 3 normal thickness MCP's) cause the single photoelectron to avalanche into a charge cloud consisting of over 107 electrons, with its X-Y spatial position preserved. This device differs fundamentally from a conventional image intensifier. First, the gain is considerably higher, using multiple MCP's rather than a single MCP and secondly, there is no phosphor screen used, thus avoiding problems related to optical conversion efficiency, gain variations, blooming and decay lag typical of phosphor screen readouts. Instead, the charge cloud is directly incident on a patented, proprietary charge-division x-y position encoder located inside the imager tube behind the MCP stage.
The spatial position is recovered, by the readout electronics, to typically better than 55 microns FWHM. The X and Y coordinates of each successive photon are then histogrammed in an external RAM digital memory and transferred to an optional, PC-based data system for display and analysis. See System Block Diagram for details.
Through this process, 1-D spectra and 2-D spectra and images are created, fully digital in nature, developed from a summation of single-photon counted events.
The charge-division X-Y position encoder is totally passive and thus is not subject to long-term electron-irradiation damage common to semiconductor-type sensors. Unlike some electron-lens-focused intensifiers (versus electrostatically, proximity-focused devices such as the Mepsicron with close photocathode-to-MCP spacing), the Mepsicron is virtually free of electron-lens-induced image distortion.
Readout Electronics. The electronic readout system includes a separate four image-channel preamplifier module and rack-mountable controller/analyzer module, all optimized for operation in a system of this type
The preamp module consists of a matched set of four low-noise charge-sensitive preamplifiers, followed by optimized pulse shaping amplifiers, and processes the low-level signals directly from the x-y encoder. Only events exceeding a preset lower threshold discriminator are counted, thus eliminating much of the background noise of other detector approaches. Simultaneous double events and other high-gain events are typically eliminated from processing by an upper-level discriminator and pile-up rejection circuits.
The controller/analyzer module (called a position analyzer) provides a number of electronic functions including:
- A rapid ratio calculation to recover the x and y spatial position of each photon.
- Lower and upper-level pulse discriminators
- Fast, look-ahead, pulse-pileup rejection circuits
- Metering (count rate, MCP gain and system dead-time) and edge-gating circuits (to enable selection of a rectangular electronic active area smaller than the optical active area.
- Fast, 130 MHz clock-rate, highly-linear, Wilkinson-type ADC's to provide up to 10-bit (1024 spatial channels) digitized spatial positions on each axis, X and Y.
- Other functions, including low-voltage power supplies, pulse-height analysis for setup, and certain data system functions.
Cooled PMT Housing/Controller. A thermoelectric (TE) cooler provides a sub-ambient environment for the entire imaging PMT. This reduces residual thermally-generated dark count from the photocathode, especially important for red-spectrum sensitive photocathode types. Heat is removed from the thermoelectric head pump elements by user-supplied cooling water (minimum of 10 gal/40 liters per hour). Air-assisted options are available in place of water-assisted cooling. A double-pane insulating window is included.
The PMT cooled housing, containing the imager, can be mounted on the users experimental apparatus using various threaded hole patterns in the front mounting surface of the housing (mating surfaces are O-ring sealed for light-tightness).
HV Divider/Supply. The system includes an adjustable low-noise high-voltage power supply and resistive divider network to generate the bias voltages needed to operate the imager. Also included is an electronic monitoring circuit that electronically shutters the imager tube photocathode from the MCP's, should the detected count rate exceed a preset level. This provides a substantial level of protection of the imager against permanent damage caused by excessive input light exposure.
System Cables, Integration and Test. All necessary power and signal cables are provided. Each system is factory-integrated and tested to ensure performance to guaranteed specifications. It is ready to attach to the users experimental equipment.
Data Acquisition and Display Systems. Several PC-based data systems are available, depending on the application.
The Quantar Technology 2200 Series Spectratraktm (1-D dimensional, counts versus channel number) and Imagtraktm (2-D dimensional, number of counts versus x and y spatial position) MCA-type data systems provide efficient acquisition, display and analysis of photon-event data. This software is easy to use and customized to the detector system.
Other PC-based multiparameter data collection and display systems available from Quantar Technology offer increased flexibility for both one and two dimensional (XY) imaging applications, and in addition, offer unique and powerful multi-dimensional histogramming capabilities to handle single-photon, time-resolved data as a third simultaneous parameter combined with imaging (XYT for each photon event).
High-Resolution, Photon Timing Modules. Signal processing electronics used with the Mepsicron -II system for single-photon time-resolved applications are similar to those used for conventional PMT time-resolved experiments. These include an MCP timing pickup circuit, wide bandwidth amplifier, constant-fraction discriminator and time-to-digital (TAC-ADC) converter. Contact Quantar Technology regarding availability of these modules (see Model 2601B Option 080 Time-Resolved Subsystem Technical Data Sheet).
Warranty. The system is covered by a 12-month limited factory warranty. Special warranty terms apply to the imager tube. Contact Quantar Technology for details.
The sale, use and manufacture of intensified charge-division type detectors and related systems are covered under one or more US patents owned by or exclusively licensed to Quantar Technology Inc.
Applications
The 2600 Series Systems are being successfully used in a wide range of ultra-low-light applications in research and analytical laboratories worldwide, where high performance, ultra-low-light detection capability is essential to successful measurements and experiments. Applications range from astronomy to solid state physics to biological science. For many applications, the principal distinguishing feature is simply the extremely high signal-to-noise ratio achievable in ultra-low photon flux applications in spectroscopy and XY imaging due to of the extremely low Mepsicron detector noise. However, increasingly, the unique merging of high single-photon time-of-arrival resolution (sub-100 picosecond FWHM time resolution is achievable with auxiliary equipment) combined with the imaging, position-sensitive capability of the Mepsicron-II detector is of paramount importance to users and enables experiments to be successful that were previously not possible or were impractical.
You are invited to contact Quantar Technology to discuss specific applications in detail.
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