Quantar Technology Mepsicron-II tm Series Single-Photon Imaging Detector System

Transcription

1 Quantar Technology Mepsicron-II tm Series Single-Photon Imaging Detector System MODEL 2601B TECHNICAL DESCRIPTION, OCT 2007 Examples of typical applications are: # Low-light multichannel spectroscopy and imaging (Raman) # Fluorescence and luminescence spectroscopy/imaging # Single-photon time-resolved spectroscopy and imaging # Optical emission microscopy for microelectronics # Astronomy and space science # X-ray and neutron imaging with fiber optics/scintillators # Biological and medical imaging Similar UHV-vacuum-compatible systems are also available from Quantar Technology for direct detection and imaging of single atomic charged-particles (electrons, positrons, ions) and extreme-uv and soft X-ray photons in vacuum environments. General Description The Model 2601B Mepsicron-II -tm Single-Photon Imaging Detector System is a high-performance, ultra-low-light-level digital electronic camera /optical detector system. It is designed for demanding, ultra-low-light scientific imaging and spectroscopy applications in the spectral range from below 180 nm to beyond 900 nm. This detector combines the most desirable features of two fundamentally different types of traditional optical detectors into one system. The Mepsicron-II offers the exceptional ultralow-light sensitivity, low-noise, high signal-to-noise ratio and time-resolved capabilities of a digital, single-photon-counting photomultiplier (PMT) combined with the full X-Y positionsensitivity of a 1-D or 2-D array detector. Although simpler in practice, the system effectively operates much like a 23 mm diameter array of more than 125,000 microscopic (50 x 50 micron) single-photon-counting PMT's, each capable of detecting single photons with an extremely high signal-to-noise ratio. Spectra and images are created by digitally summing successive single-photon digital events. These unique and powerful capabilities have resulted in the Mepsicron-II tm system being successfully used worldwide in a range of specialized applications in physical, chemical and biological science where improved ultra-low-light or timeresolved imaging performance is required compared to more conventional detectors such as CCD's or single-channel (nonimaging) PMT's. Key Features # Ultimate, true single-photon-counting for superb ultra-low light sensitivity with high signal-to-noise. Exhibits far lower detector noise than CCD cameras. # Spectral range from below 180 nm to above 900 nm. # Exceptionally low detector dark count, typically less than 10-5 counts/sec/pixel in 2-D mode at -25 deg C. No CCD- type readout noise. # Large 23 mm circular active area enables maximum light collection from experiments. # High spatial resolution, typically better than 55 microns FWHM across full 23 mm active area.. # Virtually unlimited time-integrated dynamic counting range with digital counting avoids pixel count saturation. No on-chip signal storage is required. # Remarkable 100 psec FWHM single-photon time-resolution available (optional subsystem) combined with single-photon imaging for time-resolved spectroscopy and mapping, such as time-resolved fluorescence imaging. # Real-time XY photon display, low cosmic- ray sensitivity, simple operation, user-friendly. # Complete, integrated and factory-tested detector system with optional PC-based data collection and display systems. Reliable operation and dependable support.

2 System Performance Advantages Today, several converging technology trends are placing new demands on high-performance, low-noise scientific detector systems for use in ultra-low-light applications. First, more applications involve ultra-low photon count rates. The increasing use of inherently low-photon-yield probe and spectroscopy techniques, the prevalence of smaller emitting areas associated with smaller sample and device geometries, and the need to avoid sample damage with many materials of interest by limiting incident flux levels, has led to the need for detectors that will operate satisfactorily at very low, true single-photon count rates without losing signals in detector background noise. Second, there is a need to make more measurements, faster. For this, position-sensitive imaging capability (1D or 2D) is increasingly required, both for parallel multichannel spectroscopy (which can reduce data acquisition times in spectroscopy by orders of magnitude over single channel detectors) and parallel multipixel 2D full imaging applications generally. This has led to the increased use of array detectors such as 1-D photodiode arrays and 2-D CCD detectors. Finally, there is increasing interest in tracking dynamic, timeresolved processes in many areas of science and technology ranging from solid-state physics and semiconductor processing to biological science. This has led to a need for imaging detectors which can provide, as an option, high-resolution (sub-nanosecond) time-resolved, single-photon measurements combined with photon XY spatial position. Quantar Technology's Model 2601B Mepsicron-II (Multiple- Element, Position-Sensitive Imager with Time) Single-Photon Imaging Detector System is designed to meet these kind of specific application needs and combines these capabilities together in a commercially available, state-of-the-art detector system. Ultimate single-photon sensitivity. The detector operates in a true, single-photon-counting mode, with a threshold discriminator, resulting in high noise-rejection. The user can view single photons arriving in the spectra or image, one at a time, detected with high statistical S/N ratio. Ultra-low detector dark count. In many applications, detector-generated noise can mask low-count rate signals of interest. In the Mepsicron, the only source of background count rate is thermally-generated dark count from the photocathode and MCP multpliers. This dark count is extremely low on a per pixel basis, primarily due to the very small effective photocathode area associated with each pixel. Exact dark count depends on the specific type of photocathode selected (more red-spectrum sensitive types have higher dark count), and is typically in the exceptionally low range of 10-5 counts/pixel/second (with 1024 x 1024 pixel digitization), orders of magnitude below other imaging detectors. In many applications, this dark count is so low it is ignored, and rarely requires dark count subtraction procedures required with other detectors. The system uses a thermoelectrically-cooled (TE) detector head, and no LN is required. There is no additive readout-associated count noise contribution from each readout as is found with other detector types such as CCD's, even further enhancing the suitability of the Mepsicron for ultra-low-light applications. Compared to CCD s, readout noise is zero. High signal-to-detector noise ratio. The combination of single-photon counting and ultra-low detector noise background results in extremely high signal-to-detector noise ratios, often statistically averaging 10,000:1 or more for a single detected photon in a pixel. This enables truly quantum-noise limited measurements, where the ultimate, quantum-limited statistical fluctuations in the photon signal itself ultimately limit the accuracy of the measurement rather than detector noise, even at very low photon count rates. Spectral sensitivity from EUV to near-ir. Optimized for maximum quantum detection efficiency in various spectral regions, photocathodes are available with responses ranging from below 180 nm (solar blind) through the visible to the near IR region (900+ nm). Excellent UV sensitivity to 180 nm (and below with MgF faceplates in place of the normal fused-silica faceplate) is possible without resorting to problematic and uncharacterized techniques such as backthinning or use of special wavelength luminescent converter coatings. Solar-blind (visible insensitive) photocathode types are available for spacescience and related applications. Spatial Resolution. The detector can be used either in a large-area, one-dimensional mode or a full 2-D (XY) imaging mode. Spatial resolution is typically better than 50 microns FWHM (guaranteed less than 62 microns) in both the X and Y dimensions. Virtually unlimited time-integrated dynamic range. Each photon is detected and processed as a single-event, and the position stored in an external digital RAM memory channel corresponding to a specific X-Y spatial position (pixel) and displayed on the monitor of the optional data system. Integrated counting range is limited only by the depth of the external counting memory. Therefore, there are no on-chip full-well saturation problems during long integrations due to either signal or dark count, as are incurred with integrating type detectors such as CCD s. Furthermore, because external digital memory accumulates the photon data, there is no on-chip pixel crosstalk; this means one pixel can have an extremely large number of accumulated counts while an adjacent pixel can have few or none, without interfering effects. High photon count rate readout electronics. The pulse processing electronics used to decode the spatial position is designed for the lowest possible dead-time per detected photon, consistent with good position accuracy. The system can accurately process sequential photon events up to global (entire image) maximum rates of 50,000 to 100,000 detected events per 2

3 second (assuming worst-case random, Poisson-arrival photon statistics), depending on the spatial pixel digitization desired (256, 512 or 1024 channels) per axis (rates up to 1,000,000 detected counts per second can be accommodated in special high-speed versions). The detector is therefore useful over an instantaneous dynamic range of 10 7 to 10 8, from a minimum set by the very low background dark count rate per pixel to the maximum permissible count rate. Lower maximum count rate limitations can occur for incoming photons concentrated in geometrically small, spatially localized areas such as point source images, due to MCP localarea gain saturation, a characteristic of all MCP-intensified devices including MCP-intensified CCD s. Large Active Detector Area. The defined opticallysensitive detector area diameter is 23 mm circular (415 mm 2 ). This larger area often means greater ability to collect more light from an experimental apparatus such as a spectrograph, microscope or lens collection system, while still maintaining excellent spatial resolution. Better than 100 picosecond FWHM single-photon time resolution. The ability to measure and correlate single-photon time-of-arrival (relative to a reference event such as a laser pulse or other excitation) together with the single-photon XY spatial coordinates is an extremely unique and powerful feature of this detector. In such a setup, auxiliary equipment, very similar to that used for conventional single-photon time-correlated measurements is used for the timing channel. This includes a timing pickup from the MCP electron multipliers, fast amplifier, constant-fraction discriminator and high-resolution time-to-digital converter (TAC/ADC). This digital representation of photon time-ofarrival is then written to a unique 3-parameter PC-based histogramming data system which preserves the X-Y-T data set for each individual photon event. Time-resolved spectra and images can then be reconstructed (summed) from a large number of these single-photon events, even with softwareselected specific ranges of spatial position or time if desired. Low cosmic ray sensitivity. Unlike other types of detectors such as CCD s which are highly sensitive to destructive image blurring and pixel saturation due to naturally occurring cosmic rays and residual radioactivity in detector packaging, the Mepsicron is relatively insensitive to such events (if detected at all, such events typically result in only a single count in a single pixel and often are eliminated completely by upper level discriminators). This advantage makes very long integrations possible (hours and tens of hours), which are not practical with more conventional detectors because of the number of such spurious events that are recorded that are difficult to distinguish from true signal. Real-time XY photon display. The Mepsicron offers the unique feature of a real-time photon image display on any analog X-Y display (such as an X-Y oscilloscope), simultaneous with actual data accumulation. An intensified bright dot is momentarily placed on the screen of the display for each detected photon, in its correct spatial position. With multiple photons, this forms a photon image that can assist substantially in optical alignment, focusing, diagnostics and monitoring of data accumulation. Other detector types offer the ability to view the image either before or after readout, but typically not during actual data accumulation. Ease of use. The 2601 is an inherently simple detector system and is extremely easy to use. There are few user adjustments required in operation. A special monitoring circuit (Overcount Trigger Module) automatically electro-optically gates the imager by switching the photocathode-to-mcp voltage in the event a preset photon count rate limit is exceeded. This minimizes the possibility of sensor head damage due to excessive light input levels (which is possible with any MCP-intensified type device). A front-panel meter monitors count rate, MCP gain and system deadtime loss. Front-panel controls can provide image edgegating to enable manual selection of the electronically-active image detection area, or exclude undesired spatial areas (for example, areas where no active signal is present) and thereby further reduce background. Fully integrated system. The system is fully integrated, tested and complete. It includes the sealed-tube Mepsicron-II tm sensor head; a water-assisted (optionally air-assisted) thermoelectrically-cooled housing, controller and liquid flow sensor to cool the sensor head to minus 25 degrees C; complete readout electronics; and a high-voltage power supply/hv divider network with protection circuit to bias the MCP stages. It is ready to interface to your experimental apparatus. Optional PCbased data acquisition systems are available, for 1-D and 2-D multichannel spectroscopy, 2-D X-Y imaging and 3-parameter (X-Y-T) time-resolved measurements. Count-Rate Limits. The Mepsicron detector system is designed specifically for low photon-count-rate cw applications where the noise level of other types of detectors (due to dark count, readout noise, cosmic-ray background or other noise sources) is excessively high for the application, and/or where single-photon time-resolution combined with position-sensitive imaging is desired. The maximum count rate limitation results from the minimum processing time required between successive detected photons (4 to 10 microseconds depending on spatial digitization, 400 nsec in specialized high-speed version). This minimum period enables the pulse-processing electronics to recover and the position determination and digitization to be sequentially completed for each photon. The detector is designed to detect single photons one-by-one and not in simultaneous bunches. The detector images satisfactorily with continuous count rates up to 10 5 (10 6 in high speed version) detected photons per second (note maximum incident photon flux levels can be higher than these limits because the photocathode quantum detection conversion efficiency is less than 100%). Dead-time losses will 3

4 be relatively high at maximum count rates. Detected count rates above these levels are not recommended with the Mepsicron due to excessive dead-time counting losses. The use of integrating type detectors such as CCD s are suggested in those cases. In some pulsed-source applications, two or more photons at different spatial positions can arrive in bunches at the detector simultaneously or near-simultaneously, which can result in the rejection of both events or position errors. Use of the Mepsicron in such applications is not recommended. However, in many low-light pulsed applications with higherrepetition-rate pulsed sources and low photon yields per pulse, one or fewer photons are detected per excitation pulse, which is often fully compatible with the Mepsicron capabilities. 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 10 7 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 electronlens-focused intensifiers (versus electrostatically, proximityfocused devices such as the Mepsicron with close photocathodeto-mcp spacing), the Mepsicron is virtually free of electronlens-induced image distortion. Readout Electronics. The electronic readout system includes a separate four image-channel preamplifier module and rackmountable 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 Cross-section, Mepsicron-II Imager with TE-cooling modules # Fast, look-ahead, pulse-pileup rejection circuits 4

5 # 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 redspectrum 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). Airassisted 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 lownoise 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 Spectratrak tm (1-D dimensional, counts versus channel number) and Imagtrak tm (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 multidimensional 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, Basic 2601 System Functional Diagram Shown With Optional Time-Resolved Modules 5

6 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. 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. 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-lowlight detection capability is essential to successful Quantar Technology Mepsicron and Mepsicron-II are trademarks of Quantar Technology Inc. QUANTAR TECHNOLOGY INCORPORATED 2620A Mission Street, Santa Cruz, CA Tel: FAX: MktPC: WP8l/2600dsr Rev Oct 2007 Mktg: 2600desr1.wpd 6