Low Noise Proximity Focused Image Intensifiers

Current Generation II Gallium Arsenide (GaAs) image intensifier tube technology requires that the tube microchannel plate (MCP) component have a thin dielectric coating on the side facing the tube's photocathode component. This protective coating substantially reduces the release from the MCP of ions and neutral species, particularly when the image intensifier is operated. The prevention of MCP outgassing is necessary in order ot prevent the poisoning of the Cs:O surface on the GaAs photocathode. Many authors have experimented with omitting the MCP coating. Such experiments have consistently led to an intensifier with a significantly reduced lifetime, due to contamination of the Cs:O layer on the photocathode. Unfortunately the MCP film acts as a scattering cneter to electron transport within the intensifier and effectively reduces the photoelectron detection efficiency. Substantial enhancement of the image intensifier operating parameters is the motivation for the removal of the MCP film. Removal of the MCP film promises to simplify MCP fabrication and enhance the intensifier parameters related to Electro-Optical performance and image quality. This paper presents results showing for the first time that it is possible to fabricate a long lifetime image intensifier with a single unfilmed MCP and achieve improved imaging and performance characteristics.

We provide a detailed description of a model and its computational algorithm for the secondary electron emission process. The model is based on a broad phenomenological fit to data for the secondary emission yield (SEY) and the emitted-energy spectrum. We provide two sets of values for the parameters by fitting our model to two particular data sets, one for copper and the other one for stainless steel. We also present details of the electron-cloud simulation code POSINST that are relevant to the secondary emission process. This note expands on our previously published article.

Flicker noise with a 1/f frequency dependence has been observed in the secondary emission process. No flicker noise was observed in multiplier phototubes which means that a possible flicker noise component caused by the photoemission or secondary emission process is small in comparison with the shot noise of the tube. The secondary emission flicker noise resistance, measured for Philips EFP60 tubes, increases with increasing primary current and also increases with increasing primary energy. The results are interpreted in terms of a fluctuating work function.

An energy-sensitive superconducting tunnel junction (STJ) detector was used to measure the secondary electron emission efficiency, epsilon(e), for a microchannel plate (MCP) detector bombarded by large (up to 66 kDa), slow moving (<40 km/s) molecular ions. The method used is new and provides a more direct procedure for measuring the efficiency of secondary electron emission from a surface. Both detectors were exposed simultaneously to nearly identical ion fluxes. By exposing only a small area of the MCP detector to ions, such that the area exposed was effectively the same as the size of the STJ detector, the number of ions detected with each detector were directly comparable. The STJ detector is 100% efficient for detecting ions in the energy regime investigated and therefore it can be used to measure the detection efficiency and secondary electron emission efficiency of the MCP. The results are consistent with measurements made by other groups and provide further characterization of the loss in sensitivity noted previously when MCP detectors have been used to detect high-mass ions. Individual molecular ions of mass 66 kDa with 30 keV kinetic energy were measured to have about a 5% probability of producing one or more electrons when impacting the MCP. When ion energy was reduced to 10 keV, the detection probability decreased to 1 %. The secondary electron yield was calculated from the secondary electron emission efficiency and found to scale linearly with the mass of the impinging molecular ion and to about the fourth power of ion velocity. Secondary electrons were observed for primary ion impacts >5 km/s, regardless of mass, and no evidence of a velocity (detection) threshold was observed. Copyright 2000 John Wiley & Sons, Ltd.

Neutron Imager and Flux Monitor Based on Micro Channel Plates (MCP) in Electrostatic Mirror Configuration

Low Light Level (LLL) TV cameras, based on the association between image intensifier (II) tubes and solid state image sensors (CCD), add to the extreme performance of electronic tubes, the large system flexibility offered by CCD sensors (video signal). Various combinations of image intensifier tubes and CCDs are investigated. First generation II/CCD appears suitable for many applications where the light level is not too low (> 10-5 lux on the photocathode), but very low light levels require second or third generation devices. Electron bombarded CCDs, not commercially available yet, gives promising results in laboratory experiments. Their very high gain (1,500 at 10 keV) and its very low fluctuations allow both counting and integrating detection modes. Cooled CCDs exhibit rather poor performance in TV mode but their high detectivity them very suitable for still picture imaging. An original application, with detectivity improved, has been carried out in a two-color LLL TV camera. This prototype is based on two 2nd generation II/CCD systems respectively sensitive to the visible and the near IR wavelength range. It takes advantage of the variation of the spectral reflection coefficient of materials to enhance the contrast by image coloration.

ALD-GCA-MCPs are a technology advancement that affords microchannel plates (MCPs) with significant performance benefits compared to conventional MCP technology. Incom now routinely produces ALD-GCA-MCPs with dimensions up to 20 cm x 20 cm. Notable advancements and benefits over conventional lead-oxide based MCPs are larger size and higher mechanical stability, high and stable gain (>1e4for single MCP @ 1000 V, stable gain to ≥ 1 C/cm2 extracted charge), low dark counts (≤ 0.05 cts/sec/cm2), ~3x lower gamma-ray sensitivity, and the unique ability to individually customize and control the MCP resistance and secondary electron emission (SEE) amplification characteristics. ALD-GCA-MCPs find application in a variety of photon and charged particle counting applications and are particularly suited for applications that require fast timing, high spatial resolution, radiation hardness, and long detector life times, such as Ion-TOF, electron spectroscopies, and a variety of other analytical instruments. They can also be incorporated into low-power, low-background photodetectors suitable for applications in extreme environments such as remote sensing applications in rugged environments, in space flight instrumentation, as well as in high-energy physics and nuclear physics experiments. In this paper we will provide a brief ALD-GCA-MCP technology overview highlighting recent developments that focused on improving the glass capillary array (GCA) substrate, and their implications for large-area photodetectors and spectrometers used in nuclear and high energy physics, space science, and other sensing applications.

Operational properties of channel-plate electron multipliers

Recently developed microchannel plates (MCPs) based on amorphous silicon offer potential advantages with respect to glass based ones. In this context, secondary electron emission (SEE) at very low energies below 100 eV has been studied for relevant materials for these novel devices. The aim of this work was to quantify the low energy electron emission—secondary emission and elastic scattering—from amorphous silicon and alumina and the dependence of the emission energy distribution on the primary electron energy, which was previously unknown. Secondary emission and energy distribution were both modelled and measured using equipment particularly designed for this energy range. The effects of roughness, angle of incidence and surface composition were analysed. We show crossover energies as well as the angular dependence of electron emission from amorphous silicon and alumina, with a maximum experimental emission yield value of 2 and 2.8, respectively, at an incident angle of 75∘. A parameterization for the energy dependence of the emission energy spectrum at low energies was derived. This extensive analysis is fundamental for a comprehensive understanding of the performance of amorphous silicon-based MCP detectors. It provides a complete model for SEE for a detailed description of the detector operation. The present results thus set the basis for a simulation framework, which is an essential element to increase the performance of these detectors and enable further developments.

In order to probe into the halo effect for image intensifiers, we have deduced the fitted formula for the gray value distribution (GVD) of pixels on a centerline through the center point of halo images by the numerical theory of Gaussian curve fitting and analyzed the halo effect with the curve separation method. By the image translation processing, the approximate point spread functions (PSFs) for the halo test device and image intensifiers are obtained respectively. The results show that the GVDs of pixels on the centerline in halo images for the test device and low light level (LLL) image intensifiers all consist of two curves. One curve is the Gaussian distribution curve (GDC) and the other is a straight line with the same gray value. The straight line is regarded as the background noise. After the super second generation (Gen II+), the image intensifier is put in the halo test device. Half the width of GDC for the collected halo image is smaller than that for the third generation (Gen III) one. This research is helpful for exploring the formation mechanism of halo effect and beneficial to promote the development of the LLL night vision technology.

Some fundamental properties of 18-mm-diam gated proximity-focussed microchannel-plate (MCP) image intensifiers used as fast image shutters in the 1 to 10 ns range have been identified and studied. Light pulses (&asymp; 5-ps-wide) from a modelocked dye laser optically sample the gated MCP. Shuttering is achieved by applying a forward-biasing electrical gate pulse to the quiescently reverse-biased photocathode-MCP interface. Variable delay (&asymp; 30-ps jitter) between the gate pulse and the laser pulse permit tracing the MCP&apos;s optical response. Gating speeds, turn-on and turn-off patterns, the asymmetric spatial dependence of the MCP optical response, and resolution effects as functions of gate pulse width and photocathode-MCP bias have been characterized. Shutter times of &ge; 750 ps and &le; 5 1p/mm resolution with the MCP fully on were observed. Variations in the intensity profiles of the phosphorl.s spatial response for uniform photocathode illumination are measured with a calibrated silicon-intensified-target (SIT) focus projection, scan (FPS) television camera and a high-speed video digitizer while photomultipliers (PMTs) monitor the laser pulse and the phosphor&apos;s spatially integrated output intensities. The characterization system, gating and biasing circuits, and experimental results will be presented.

Medium energy (100–300 keV) time-of-flight spectrometry for surface analysis uses the correlated detection of an energetic ion and the secondary electrons emitted as it passes through a carbon foil. When microchannel plates are employed in this detection scheme, a typical mean efficiency of detection of less than 30% is achieved. When instead a surface barrier detector is used to detect the ion, providing simultaneous acquisition of velocity and energy information, certain advantages are realized over the two microchannel plate configuration in the characterization of low level constituents of surfaces. Specifically, energy-discriminated gating of the start pulse was observed to nearly eliminate count rate dependent background in a time-of-flight spectrum. Further reduction in background was obtained by the selective elimination of forward recoil species or backscatters from the substrate. Replacement of the stop microchannel plate by a surface barrier detector has resulted in improved detection efficiency for He, as well as provided a means for further study of the processes which affect time-of-flight spectrometer response, including multiple scattering and secondary electron emission in the start foil. In this publication, we describe the application of this particle telescope to the backscattering analysis of gold on silicon and the forward scattering measurement of hydrogen in a self-supporting carbon film.

For imaging cells with fluorescent probes, an extremely low excitation light level can be used if the microscopy system is equiped with an intensification device. The low light level allows, when observing living cells, preservation of the native state with regard to lysosome compartmentation, electrical inner membrane potential, structure and intrinsic reactivity of chromatinized DNA, cytoplasmic rRNA and mitochondrial (mt)DNA. Following contact with ethidium, cycling cells examined under this condition had fluorescent mitochondria. The fluorescence came from ethidium intercalated in mtDNA, since cycling 03F1° (devoid of mtDNA) cells harbour mitochondria, revealed by potential sensitive fluorescent probes, which did not fluoresce following ethidium incubation. As B-DNA where ethidium intercalation gives rise to enhanced fluorescence, it seems that mtDNA is under B-conformation in cycling and living cells. Moreover, mtDNA seems to stay in B-conformation, since the mitochondrial fluorescence maintains a high intensity for several days.

Effective sensory processing requires matching the gain of neural responses to the range of signals encountered. For rod vision, gain controls operate at light levels at which photons arrive rarely at individual rods, light levels too low to cause adaptation in rod phototransduction. Under these conditions, adaptation within a conserved pathway in mammalian retina maintains sensitivity as light levels change. To relate retinal signals to behavioral work on detection at low light levels, we measured how background light affects the gain and noise of primate ganglion cells. To determine where and how gain is controlled, we tracked rod-mediated signals across the mouse retina. These experiments led to three main conclusions: (1) the primary site of adaptation at low light levels is the synapse between rod bipolar and AII amacrine cells; (2) cellular noise after the gain control is nearly independent of background intensity; and (3) at low backgrounds, noise in the circuitry, rather than rod noise or fluctuations in arriving photons, limits ganglion cell sensitivity. This work provides physiological insights into the rich history of experiments characterizing how rod vision avoids saturation as light levels increase.

Guest Editorial: Shedding new light on the twilight zone