G.W. Fraser

The operation of microchannel plates at high count rates

Abstract The measured count rate characteristics of a large number of microchannel plate electron multipliers are compared with the predictions of a universal, paralysable-counter model. Individual plate resistances for the experimental study lie in the range 27–2450 MΩ. The gain behaviour of single channel plates is shown to differ from that of multi-stage MCP detectors. The measured dependence of multi-stage multiplier recovery time on illuminated area is interpreted in terms of inter-channel coupling and of changes in conduction current during plate operation. Our findings have significance for the calibration of particle and photon spectrometers and for the development of future channel plate detectors with extended dynamic range.

The direct detection of thermal neutrons by imaging microchannel-plate detectors

Abstract We show, theoretically and experimentally, that it is possible to directly detect thermal neutrons using an imaging microchannel-plate (MCP) electron multiplier, if the lead-oxide glass from which the plates are fabricated contains lithium. This development may make available to neutron physics the high degree of spatial precision (⪡ 100 μ m FWHM resolution) currently attainable in the many other fields where MCPs are employed. The consequences of incorporating boron into microchannel-plate glass are also considered.

The X-ray energy response of silicon (B): Measurements

Abstract In this, the second part of a detailed study of the interaction of soft X-rays with silicon, we summarise the results of a large number of experiments on charge coupled devices (CCDs), carried out both in our laboratory and at the Daresbury Synchrotron Radiation Source (SRS). Measurements of the energy variation of the W parameter and of the Fano factor F are in substantial agreement with the predictions of the model developed in Part (A) of the study [G.W. Fraser et al., Nucl. Instr. and Meth. A 350 (1994) 368]. The consequences of using a Gaussian pulse height distribution model in the experimental determination of F are discussed. Variations in X-ray event morphology (i.e. the frequency distribution of single-, two-, three-pixel events) across the silicon K edge are described. Measurements of CCD quantum detection efficiency Q (counts/photon) showing XAFS (X-ray absorption fine structure) modulation in the vicinity of the Si K edge are compared with calculations based upon new, experimentally-determined linear absorption coefficients for Si, SiO 2 and Si 3 N 4 . Finally, the X-ray photoyield from silicon is described, both experimentally and theoretically.

The gain, temporal resolution and magnetic-field immunity of microchannel plates

Abstract The variation of microchannel plate (MCP) pulse gain, pulse transit time and transit time spread with channel diameter and length-to-diameter ratio is described, using a variable-spacing dynode model of the electron avalanche which accounts for the effects of wall charge saturation. The analysis includes a description of the response of a single microchannel to the prompt arrival of N electrons, thus providing a microscopic basis for the empirical laws currently used to describe the gain of two- or three-stage MCP multipliers. Preliminary results of microchannel plate gain fatigue “lifetests” conducted in software are described. Finally, an expression is derived for the gain of a microchannel plate, as a function of bias voltage, in the presence of a strong axial magnetic field. Here, as throughout the paper, the results of calculation are compared with experiment.

Calculation of the output charge cloud from a microchannel plate

Abstract The imaging performance of microchannel plate (MCP) detectors of photons and charged particles depends on the angular and energy distributions of the electrons in the output charge cloud. Measurements of these distributions are sparse and do not cover the full range of MCP operating parameters (input current, bias voltage, electrode design, channel length and channel diameter). Our two dimensional Monte Carlo model of the avalanche process in a single microchannel yields the output energy distribution of electrons (EDOE) and angular distribution of electrons (ADOE) for arbitrary channel geometries. The model has been validated by comparison with the available experimental data and used to examine the (a) effects of current saturation and (b) novel output electrode geometries.

MICROCHANNEL PLATE OPERATION AT HIGH COUNT RATES - FURTHER-STUDIES

In this paper, we extend and refine our previous model of the operation of microchannel plate (MCP) electron multipliers at high count rates [Fraser et al., Nucl. Instr. and Meth. A306 (1991) 247]. By analysing structured UV images obtained with a position-sensitive MCP detector, we confirm in detail the “pore bleaching” hypothesis of Anacker and Erskine [Rev. Sci. Instr. 62 (1991) 1246].

X-ray focusing using square-pore microchannel plates First observation of cruxiform image structure

Abstract Soft X-ray (0.28 and 1.74 keV) images produced by the focusing action of a planar, square-pore microchannel plate (MCP) are described. These images contain for the first time the three-component cruxiform structure expected from the studies of Angel [Astrophys. J. 233 (1979) 364] and Chapman et al. [Rev. Sci. Instr. 62 (1991) 1542], and so indicate a high degree of geometric regularity within and between the ∼ 46000 active channels in the MCP structure. The angular resolution of the central “true” focus is ∼ 10 arc min FWHM.

UV AND XUV QUANTUM DETECTION EFFICIENCIES OF CSI-COATED MICROCHANNEL PLATES

Abstract Quantum efficiency measurements are presented for CsI-coated microchannel plates in the waveband 25–200 nm. The influence of exposure to a humid atmosphere on the efficiency of the CsI-photocathode has been studied in great detail. Due to the very high susceptibility of CsI to humidity, particularly for wavelengths longer than 140 nm, the best quantum efficiencies have been obtained for a CsI-coated channelplate continuously stored in vacuum. The results are interpreted on the basis of photoelectron emission theory.

Dark noise in microchannel plate X-ray detectors

Abstract We present a detailed study of dark noise sources in microchannel plate (MCP) X-ray detectors. Previously postulated noise mechanisms are critically reviewed. Noise measurements carried out in the light of the review are then reported. The sea-level background count rate in two-stage MCP detectors is shown to have two principal components. The first — variable with position across the MCP and decaying with time under vacuum in a manner dependent on the plate history — is attributed to outgassing of the channel plate structure. The second — isotropic, independent of detector bias voltages, time and temperature — is shown to be consistent, in magnitude and in terms of its output charge spectrum, with the results of beta decay from the radioactive potassium content of the multiplier lead glass. Based on this identification of noise sources, prospects for the production of ultralow-noise MCP detectors for imaging X-ray astronomy are discussed. Measurements of noise reduction by coincidence methods using a partitioned anode are described.

On the experimental determination of the Fano factor in Si at soft X-ray wavelengths

Abstract We have investigated the experimental determination of the Fano factor in silicon using a low-noise CCD detector. We have tested the hypothesis of Fraser et al. (Nucl. Instr. and Meth. A 350 (1994) 365) that the distribution of secondary electrons generated by low-energy X-ray interactions is not normally distributed, leading to an asymmetry in the electron number distribution with energy. This in turn, leads to systematically low values of the Fano factor when derived using a traditional analysis of the energy resolution function. Based on measurements taken at the Daresbury Synchrotron Radiation Source (SRS) we find that monoenergetic energy-loss distributions are indeed non-Gaussian to a similar degree predicted by Fraser et al. (1994). The Fano factors determined from a probability analysis of the pulse height data are typically (0.155±0.002) which are significantly different from the value of (0.143±0.001) derived from a Gaussian decomposition of the energy resolution function. The results are in excellent agreement with the theoretical values of ∼0.158 derived from the photoionization theory of Fraser et al. (1994). Lastly, we show that for practical detection systems, failure to correct for a finite energy threshold can also lead to an underestimate in the derived value of F —by as much as ∼5% in the present case.