Jonathan S. Lapington

The Bragg Crystal Spectrometer for SOLAR-A

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fexxvi and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (λ/Δλ) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.

Position sensitive anodes for MCP read-out using induced charge measurement

Abstract We investigate the method of an indirect detection of a MCP charge avalanche projected onto a resistive layer (G. Battistoni, et al., Nucl. Instr. and Meth., 202 (1982) 459). If the sheet resistance is favourable one can detect the charge cloud by the capacitive coupling to an anode structure a few millimetres behind the layer. The anode structure can be, for example, a wedge-and-strip electrode pattern (M. Unverzagt, Diplomarbeit, Universitat Frankfurt 1992, private communication) as it is used for directly collecting the electron avalanche from a MCP. Detection of the induced charge is beneficial in several respects. Firstly, image distortions produced by secondary electron mediated charge redistribution are eliminated. Secondly, the noise component due to quantized charge collection, commonly referred to as partition noise, is not present. In addition, the dielectric substrate can function both as an element of the vacuum enclosure and HV insulator, making the electrical connections easily accessible and the pattern operable at ground potential, independently of detector operating voltages. This technique can be used to simplify the electronic design requirements where varying high voltages are required at the detector input face such as plasma analysers, etc. It also has application in the manufacture of intensifier tubes (J. Barnstedt, M. Grewing, Nucl. Instr. and Meth., these proceedings) where the inclusion of a readout pattern inside the intensifier body with associated electrical feed-throughs can prove problematic. We will present data on the performance of such detection geometries using several types of charge division anode, and discuss the advantages compared with the “traditional” charge collecting method.

Spatial charge cloud distribution of microchannel plates

We describe an experiment to measure the spatial charge distribution produced by a microchannel plate intensifier with a new type of charge division readout, the ‘‘split strip’’ anode. This anode is a modified strip and strip anode which determines both the amount of charge deposited on each half of the anode and the centroid position for each event. We present experimental measurements of microchannel plate charge cloud distributions for a variety of detector operating conditions. We find that, as a first‐order approximation, one can assume the charge cloud to be azimuthally symmetric. Additionally, the charge cloud remains virtually unchanged from event to event and pore to pore. The general form of the radial distribution is best described by the sum of two exponential components whose scale and relative weights vary with detector operating conditions. The central component of the distribution is three to six times smaller than the outer, or ‘‘wing’’ component. Typically most of the charge is in the ce...

The Design and Manufacture of Wedge and Strip Anodes

We describe the Wedge and Strip Anode (WSA), a two-dimensional position sensitive charge read-out system. Two new optimised WSAs have been developed, giving improved position resolution over previous WSAs for a given charge gain. We show that due to the effective absence of partition noise, WSAs outperform resistive disk anodes at all charge levels. We present measurements of the performance of these optimsed read-outs. Finally, we have designed and built a software controlled WSA manufacturing facility, which can produce any pattern of up to 400 × 500 mm2 with an absolute accuracy of ± 2.5¿m.

Optimisation of Wedge and Strip Anodes

We describe the Wedge and Strip Anode (WSA) position read-out system and compare the performance of currently used WSAs with that of Resistive Disk Anodes (RDAs). We show that for small systems the WSA can outperform the RDA while for larger systems the RDA cannot be used at all due to size limitations. We discuss noise in WSAs and present a comprehensive optimisation scheme based on a model for the WSA capacitance [1] which is capable of reducing the electronic noise by a factor of up to 6 compared with current WSAs. We present capacitance, noise and position resolution measurements of several WSAs with active diameters of up to 200mm. We describe a simple technique for the manufacture of WSAs and use this to produce WAs on thin metallised polymer film as an alternative noise reduction method. We discuss a special low noise application of WSAs to spectroscopy. The conclusion is reached that in most cases RDAs can be advantageously replaced by WSAs.

Microchannel Plate Pore Size Limited Imaging with Ultra-Thin Wedge and Strip Anodes

We describe a detector employing a curved channel microchannel plate (MCP) in front of an optimised, thin substrate wedge and strip anode (WSA) imaging readout. We obtain a position resolution better than 20¿m FWHM allowing the images of individual MCP pores to be distinguished. The collimation provided by a single pore is used to probe the fundamental limitations in resolution of the WSA. Finally, having experimentally verified the limitations imposed by electronic and partition noise, we describe a detector which will produce 20¿m FWHM resolution over a 50mm diameter imaging area.

High-Resolution Spectroscopy of G191-B2B in the Extreme-Ultraviolet

We report a high-resolution (R = 3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme-ultraviolet band 220-245 A. A low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.

THE SPATIAL EXTENT OF GAIN DEPRESSION FOR MCP-BASED PHOTON DETECTORS

We describe a technique to measure the spatial extent and magnitude of gain depression in the quiescent pores surrounding an active region of an MCP and present preliminary results. We find that significant gain depression occurs up to 1.5 mm from the center of the active region. The magnitude of the reduction in gain decreases linearly with radius and is proportional to the count rate. We evaluate two possible mechanisms for gain depression. We find that the pore deactivating model in which the electric field from the discharging of a pore distorts the electric fields in surrounding pores is more successful than the transfer of charge from quiescent pores.

THE EFFECTS OF SECONDARY ELECTRON EMISSION ON THE OPERATION OF POSITION SENSITIVE ANODES

Abstract Secondary electron emission plays a crucial role in the operation of position sensing anodes used in imaging microchannel plate (MCP) intensifiers. Secondary electrons emitted from the anode strongly influence both the size and shape of the collected charge cloud distribution and also mediate charge redistribution between the anode electrodes. The performance of position readouts using charge division can be severely limited by these mechanisms. We describe experiments designed to study the secondary electron emission process from a charge division position readout. An intermediate grid between the MCP and anode is used to establish an electric field above the anode to control the trajectories of the secondaries without disturbing the operation of the MCP. Tuning of the grid and anode voltages allows the measurement of a variety of physical processes occurring within the detector. We present measurements of the secondary emission coefficients of anode and grid materials, and show how both the initial spatial and energy distribution of primary electrons from the MCP may be measured. The usefulness of controlling secondary electrons to improve imaging performance is demonstrated.

A novel imaging readout with improved speed and resolution

We describe a novel, charge centroiding, position readout, the spiral anode (SPAN) which combines excellent spatial resolution with very high count rate performance. SPAN is a planar structure of six electrically isolated electrodes. The ratio of the charges collected by the electrodes determines a two-dimensional position. The novel design allows the spatial resolution to be an order of magnitude better than the charge measurement accuracy. We present results from a prototype detector consisting of a microchannel plate stack in conjunction with the SPAN readout and discuss opportunities for the use of the SPAN readout with other photon counting detectors.