The invention relates generally to the field of electromagnetic signal detection and, more particularly, to signal detection using photon-counting detectors.
Microchannel plates (MCP) have found a wide range of application as photon-counting or particle-counting detectors for science, industry and medicine. In general, these detectors provide excellent sensitivity and dynamic range. However, MCP detectors are limited in their counting rate capability by the physics of an electric recharging process within the detector. The functionality of a MCP detector is such that an incident particle at the input of the MCP causes an avalanche multiplication process that leaves the MCP channels with a local net positive charge. This charge must be neutralized by current conduction through the walls of the MCP channels before the next avalanche discharge can occur. The time for this neutralization, i.e., the recharge time, is given by:
xcfx84xe2x88x9dRchCch
where Rch and Cch are the resistance and capacitance, respectively, of the MCP channels. At local count rates higher than about 0.3/xcfx84 the microchannels do not have time to recharge after an avalanche and the gain strongly decreases.
Since the permittivity and geometry of the device effectively fix the capacitance of the channels, use of the detector at high count rates requires that the channel resistance be decreased. Special microchannel plates that are doped for high conductivity are available for high count rate applications. However, a fundamental difficulty arises with the use of these devices. Because microchannel plates are constructed of semiconducting glass with a negative temperature coefficient of resistance, as the temperature of the channels increases their resistance decreases. The power that is dissipated in the MCP channel by the dark current is given by:   P  =            V      2                      R        ch            ⁡              (        T        )            
where V is the voltage applied to the MCP (which must be fixed to maintain a constant gain) and T is the temperature of the device. As can be seen, at high count rates, local heating of the MCP channel occurs which reduces the channel resistance. This, in turn, further increases the dark current power dissipation. Thus, a positive feedback effect can continue to drive up the power and temperature of the MCP. Above a certain threshold, thermal runaway can occur in which the MCP channels are locally melted, resulting in the destruction of the MCP.
Because of the thermal runaway effect, high conductivity MCPs are typically limited to operation at counting rates of less than 105 counts/mm2/sec. The counting rate can be increased to a counting rate on the order of 106 counts/mm2/sec if a cooling plate is attached directly to the face of the MCP. However, doing so significantly reduces operational flexibility. The counting rate limitations on MCPs make their use in photon-counting impractical for a number of important high flux applications such as x-ray crystallography, medical diagnostic imaging and electron microscopy. Thus, analog imaging techniques are currently used for these applications. However, these methods are necessarily limited in sensitivity and dynamic range. Therefore, it would be desirable to have a detector with gain characteristics similar to conventional MCPs but with the ability to count at much higher rates.
In accordance with the present invention, an energy conversion apparatus is provided that uses a porous matrix of dielectric material intermingled with a metallic conductor. In the preferred embodiment, the matrix is an electron multiplication apparatus and has a zero or slightly positive temperature coefficient of resistance, and therefore remains thermally stable at high count rates. The material also exhibits a high secondary electron emissivity, as is required for an effective electron multiplier. In the preferred embodiment, the dielectric material has a large bandgap that allows warm electrons to travel for long distances through the material lattice without energy loss via electron-electron scattering. Because of the large bandgap, the dielectric, if used alone, would have a very low electron conductivity. In such a case, the dominant conductivity in this material would be thermally activated ion conductivity, which would result in a negative temperature coefficient. However, the matrix of the present invention uses high electron conductivity fragments intermingled with the dielectric material. This results in a material having significant quantum tunneling electron conductivity to prevent a negative temperature coefficient of resistance, and thermal runaway is thereby avoided.
The general components of an electron multiplication device using the matrix layer of the present invention include a conductive cathode and a conductive anode, with the matrix material located between them. A voltage source provides a voltage differential across the anode and cathode, resulting in an electric field in the region of the matrix layer. The matrix material, in general, is a porous combination of a dielectric material interspersed with fragments having a relatively high electrical conductivity. In the preferred embodiment, the dielectric is a material with a high secondary electron emissivity. For example, in one embodiment of the invention the dielectric is a metal oxide, such as an alkaline earth oxide, while in another the dielectric is an alkali halide. Preferably, the dielectric material is made up of particles having an average length of one to five microns. The conductive fragments are preferably a relatively inert metal, and have an average length of less than one micron. Also, the matrix preferably has pores with an average length of between five and ten microns.
In the preferred embodiment, the device also comprises a conductive material in contact with the side of the matrix layer toward the anode of the device. Typically, an air gap exists between the matrix layer and the anode, and the conductive material resides in conductive contact with the matrix. In one particular embodiment, the conductive material is a mesh that provides an electrical return to the cathode. This electrical contact between the cathode and the opposite side of the matrix prevents a polarization of the matrix layer and a corresponding reduction of the net electric field within the matrix layer to zero. Such a polarization would otherwise inhibit the production of secondary electrons within the matrix layer.
Fabrication of the matrix material can be done in different ways. In the preferred embodiment, a substrate is provided, and is located in the vicinity of an oxidizable metal and the relatively inert metal. The two metals are first degassed, and are then both vaporized such that they are codeposited on the substrate, interspersed in a porous layer. Use of an inert gas atmosphere during the vaporization stage provides the desired pores in the deposited layer. The porous, bimetallic matrix is then baked in an oxidizing atmosphere so as to oxidize the oxidizable metal. This produces a metal oxide dielectric with high secondary electron emissivity, interspersed with the fragments of high electron conductivity. The matrix is then located between the anode and cathode of the desired device, the cathode preferably serving as the substrate as well. In an alternative embodiment, the layer is formed by first combining the oxidizable material and the conductive material with an evaporable host material. The host material may be, for example, a combination of amyl acetate and magnesium carbonate. The combination of the host material and two matrix materials are applied to the substrate to a desired thickness, and the host material is then heated and decomposed, leaving behind the matrix layer.