GB2415086A

GB2415086A - Charge detector array

Info

Publication number: GB2415086A
Application number: GB0412686A
Authority: GB
Inventors: Keith Birkinshaw; David Langstaff
Original assignee: Aberystwyth University
Current assignee: Aberystwyth University
Priority date: 2004-06-08
Filing date: 2004-06-08
Publication date: 2005-12-14
Also published as: GB0412686D0; EP1766658A2; WO2005122213A3; WO2005122213A2

Abstract

A charge detector array for recording signals from a microchannel plate (MCP) in which a synchroniser circuit (14) is provided in respect of each of a plurality of channels, interposed between a respective charge amplifier and discriminator unit 12 and a counter 16, for monitoring output pulses generated in respect of the channel of interest and one or more channels adjacent thereto, and for only causing an output pulse in respect of the channel of interest to be counted if it is generated before any output pulses generated in respect of the adjacent channels, and/or its duration is greater than that of any output pulses generated in respect of the adjacent channels, thereby enhancing the resultant image.

Description

1 24 1 5086 CharEe Detector Array This invention relates generally to a

charge detector array and, more particularly, to an integrated array detector for recording signals from a microchannel plate (MCP).

The microchannel plate (MCP) is a versatile and powerful detector for energetic particles such as ions, electrons and photons with energies ranging from UV to X-ray.

The impact of an ion, electron or photon on the front face of the MCP gives rise to secondary electrons inside the body of the MCP which results in an avalanche multiplication process leading to a pulse of up to 107 electrons emerging from the rear face of the MCP. By the use of a suitable collection system for these electrons, the position of the original impact on the front surface of the MCP may be determined to within a few microns.

Integrated array detectors are well known devices for recording signals from a microchannel plate (MCP). By collecting the electrons as they emerge from the rear face of the MCP on a plurality of electrodes and arranging for the arrival of these electrons to cause an electronic counter circuit associated with each electrode to operate, it is possible to form an image of charged particles or ionising radiation arriving at the front face of the MCP.

Referring to Figure 1 of the drawings, consider, for example, the charge sensitive detector array described in the reference by D.P. Langstaff (2002), "An MCP-based detector array with integrated electronics", International Journal of Mass Spectrometry 215 ( 1-3): 1-12, which comprises a number of custom silicon integrated circuits mounted on a ceramic substrate in proximity to the rear face of an MCP.

Each detector chip contains 192 channels, or pixels, on a pitch of Mom. Each channel has a metal anode 100 to collect the electrons as they emerge from the MCP, a charge sensitive amplifier 102 to produce a digital signal in response to the electron pulse and an 8-bit counter 104 associated with it to accumulate the counts as they arrive; and circuitry to read out the data sequentially from all channels in the array.

The circuit is designed such that an arbitrary number of detector chips may be abutted together on a substrate behind the MCP, allowing for long focal plane detectors to be built, limited only by the size of MCPs available. The outputs from the counters 1 04a r on the detectors is presented via an 8-bit port and is read into a PC by means of a simp] einterface 106.

It will be appreciated that a charge sensitive detector array of this type finds application in the fields of electron energy spectrometry, mass spectrometry and other

fields.

A problem with array detectors of the type described above, and similar types, is that a single charged particle arriving at the front face of the MCP may cause a number of adjacent channels on the integrated array detector to trigger. This is due to spreading of the charge cloud as it passes through and exits the MCP assembly. This multiple triggering of channels causes a degradation of image quality resulting in a loss of sharpness in the image (see, for example, Edgar, M.L., Kessel, R., Lapington, J.S. and Walton, D.M. (1989), "Spatial Charge Cloud Distribution of Microchannel Plates", Review of Scientific Instruments 60(12):3673-80 and Lapington, J.S. and Edgar, M.L.

(1989), "The Size and Spatial Distribution of Microchannel Plate Output Electron Clouds", Proceedings of the Society of Photo-Optical Instrumentation Engineers 1159:565-75).

It is therefore an object of the present invention to overcome the problems outlined above, and provide a detector array in which occurrences of false triggering of detector channels located toward the edge of a charge cloud emerging from a microchannel plate, is at least reduced so as to reduce the above-mentioned degradation in image quality and the resultant loss of sharpness in the image.

In accordance with the present invention, there is provided a detector array for recording signals from a microchannel plate, said detector array comprising a plurality of channels, each channel having associated therewith collection means for collecting electrons emerging from said microchannel plate, sensor means for sensing the charge collected in respect of a respective channel, means for triggering the generation of an output pulse only if said charge is greater than a trigger value, and counter means for counting output pulses, said detector array further comprising means for monitoring the generation of output pulses in respect of a channel of interest and one or more channels substantially adjacent thereto and only causing an output pulse to be counted in respect of said channel of interest if a) a respective output pulse is generated prior to the generation of an output pulse in respect of said one or more adjacent channels, and/or b) the duration of a respective output pulse is greater than that of one or more output pulses generated in respect of respective said one or more adjacent channels.

Also in accordance with the present invention, there is provided a method for recording signals from a microchannel plate, the method comprising providing a detector array comprising a plurality of channels, collecting, in respect of each channel, electrons emerging from said microchannel plate, sensing the charge collected in respect of a respective channel, means for triggering the generation of an output pulse only if said charge is greater than a trigger value, and counting output pulses, the method further comprising monitoring the generation of output pulses in respect of a channel of interest and one or more channels adjacent thereto and only causing an output pulse to be counted in respect of said channel of interest if a) a respective output pulse is generated prior to the generation of an output pulse in respect of said one or more adjacent channels, and/or b) the duration of a respective output pulse is greater than that of one or more output pulses generated in respect of respective said one or more adjacent channels.

Preferably, said means for monitoring is arranged and configured to only cause an output pulse to be counted in respect of said channel of interest if a) an output pulse is generated in respect of said channel of interest prior to the generation of an output pulse in respect of any of said one or more channels substantially adjacent thereto, and/or b) the duration of an output pulse generated in respect of said channel of interest is greater than that of an output pulse generated in respect of any of said one or more channels substantially adjacent thereto.

For the avoidance of doubt, it will be appreciated that the term "substantially adjacent" is intended to cover not only channels immediately adjacent the channel of interest, but also a channel having only a small number of channels between it and the channel of interest. Thus, for example, the channel of interest and one or more alternate channels adjacent thereto could be monitored.

Thus, the solution proposed by the present invention in respect of the above- mentioned problems uses the fact that a channel at the centre of a charge cloud emerging from the microchannel plate (MCP) will collect more electrons than a channel towards the edge of the charge cloud. The result of this is that the sensor means (comprising, for example, a charge amplifier and discriminator) associated with the channel at the centre of the charge cloud will reach its discrimination level (i.e. the charge as a result of collected electrons will reach some predetermined threshold value) and hence produce an output pulse before a channel towards the edge of the charge cloud. Likewise, the output pulse from such a central channel will last for longer due to the increased length of time for the larger charge to be conducted away, allowing the channel to return to its quiescent condition.

The monitoring means beneficially comprises a synchroniser circuit interposed between the sensing means and the counting means in respect of each of the plurality of channels. Each synchroniser is beneficially arranged and configured to take inputs from the respective channel and one or more channels adjacent thereto and only produce (or cause to be produced) an output pulse if the channel of interest "triggers" (i.e. the charge resulting from electrons collected at the channel exceeds the trigger value) before the one or more adjacent channels.

The degree of charge spreading in the MCP, and hence the number of adjacent channels liable to false triggering, is dependent upon the operating conditions of the MCP. For optimal performance, the number of channels considered in each synchroniser circuit should match the charge spreading in the MCP. In order for this to be achieved, in accordance with a preferred exemplary embodiment of the present invention, the synchroniser circuits provided in respect of the channels of the detector array may comprise means (possibly in the form of Sating or the like) for allowing the number of channels adjacent to the respective channel of interest to be monitored, to be adjusted by means of a 'Width' control.

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiment described herein.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a single-chip array detector circuit according to

the prior art;

Figure 2 is a schematic block diagram of an array detector according to an exemplary embodiment of the present invention, incorporating a synchronizer circuit; and Figure 3 is a schematic gate level diagram of a 7-way, variable width synchroniser circuit for use in a detector array according to an exemplary embodiment of the present invention.

Referring to Figure 2 of the drawings, a portion of an array detector with on-chip image enhancement according to an exemplary embodiment of the present invention, comprises a plurality of channels, in respect of each of which is provided a collector anode 10 and a charge amplifier and discriminator unit 12. Also provided in respect of each channel is a synchroniser (in this exemplary embodiment, a 7-input, variable width synchronizer) 14 and a counting means 16.

Thus, the circuit illustrated in Figure 2 of the drawings takes inputs from the channel of interest 20 (i.e. the channel with which it is actually associated) and (in this case) three adjacent channels 22 on either side of the channel of interest 20, and only causes an output pulse to be generated (and, therefore, counted) if the charge amplifier and discriminator unit 12 associated with the channel of interest 20 is triggered before those associated with the adjacent channels 22. It will be appreciated that, in practice, each channel on the detector array would be furnished with such a synchronizer circuit 14 (and counting means 16).

An exemplary realization of the synchronizer circuit 14 itself is illustrated schematically in Figure 3 of the drawings, in which it can be seen that this particular realization is constructed from identical NAND gates. It will be appreciated that, although the illustrated system uses 7-way synchronizers to consider the outputs from the three adjacent channels on each side of the channel under consideration (with which the respective synchroniser is associated), the proposed scheme could be relatively easily extended to consider 4 or more adjacent channels on either side of the channel of interest by extending the variable width synchroniser circuit using a synchroniser width control mechanism 24 (see Figure 2).

Thus, a unique feature of the present invention is the use of a synchroniser circuit or the like in order to enhance the output from a multi-channel detector. The fact that the number of channels taken into account in respect of the synchronizing process can be varied to account for different operating conditions of the MCP is also a significant feature of the above-described exemplary embodiment of the invention.

It should be noted that the above-mentioned embodiment illustrates rather than limits the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In particular, it will be appreciated that in the embodiment described above, the number of inputs received by the synchroniser circuit is 7, i.e. the channel of interest and three adjacent channels on either side of the channel of interest; however, in an alternative embodiment, any number of channels on one or both sides of the channel of interest may be considered and this may comprise an equal number on either side of the channel of interest, or it may not, as required.

In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS: 1. A detector array for recording signals from a microchannel

plate, said detector array comprising a plurality of channels, each channel having associated therewith collection means for collecting electrons emerging from said microchannel plate, sensor means for sensing the charge collected in respect of a respective channel, means for triggering the generation of an output pulse only if said charge is greater than a trigger value, and counter means for counting output pulses, said detector array further comprising means for monitoring the generation of output pulses in respect of a channel of interest and one or more channels substantially adjacent thereto and only causing an output pulse to be counted in respect of said channel of interest if a) a respective output pulse is generated prior to the generation of an output pulse in respect of said one or more substantially adjacent channels, and/or b) the duration of a respective output pulse is greater than that of one or more output pulses generated in respect of respective said one or more adjacent channels.
2. A detector array according to claim 1, wherein said means for monitoring is arranged and configured to only cause an output pulse to be counted in respect of said channel of interest if a) an output pulse is generated in respect of said channel of interest prior to the generation of an output pulse in respect of any of said one or more channels substantially adjacent thereto, and/or b) the duration of an output pulse generated in respect of said channel of interest is greater than that of an output pulse generated in respect of any of said one or more channels substantially adjacent thereto.
3. A detector array according to claim 1, wherein the monitoring means comprises a synchronizer circuit interposed between the sensing means and the counting means in respect of each of said plurality of channels.
4. A detector array according to claim 2, wherein each synchroniser is arranged and configured to take inputs from the respective channel and one or more channels adjacent thereto and only cause an output pulse to be counted if it is generated in respect of said channel of interest before any output pulses generated in respect of the one or more substantially adjacent channels.
5. A detector array according to claim 2 or claim 3, wherein the synchronizer circuits provided in respect of the channels of the detector array comprise means for allowing the number of channels adjacent to the respective channel of interest to be monitored, to be adjusted.
6. A method for recording signals from a microchannel plate, the method comprising providing a detector array comprising a plurality of channels, collecting, in respect of each channel, electrons emerging from said microchannel plate, sensing the charge collected in respect of a respective channel, means for triggering the generation of an output pulse only if said charge is greater than a trigger value, and counting output pulses, the method further comprising monitoring the generation of output pulses in respect of a channel of interest and one or more channels substantially adjacent thereto and only causing an output pulse to be counted in respect of said channel of interest if a) a respective output pulse is generated prior to the generation of an output pulse in respect of said one or more substantially adjacent channels, and/or b) the duration of a respective output pulse is greater than that of one or more output pulses generated in respect of respective said one or more substantially adjacent channels.
7. A method according to claim 5, further comprising selecting a number of adjacent channels to be monitored based on a measure of charge spreading of said microchannel plate.
8. A detector array substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.
9. A method for recording signals from a microchannel plate, the method being substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.