Detectors and pre-amplification

Microchannelplate Detector Assemblies

Photo of 40mm dectector assembly

Kore offers dual micro channel plate-based (DMCP) assemblies for ion detection over large areas, based on either 40mm or 25mm diameter plates. The Kore DMCP detector assembly is typically used as the ion detector in time of flight mass spectrometers (though is not limited to TOF-MS), and the large active area gives high sensitivity and large fields of view in spectrometers with long flight tubes.

The choice of plate size is determined by the expected size of the ion beam at the detector position.

Features

The DMCP consists of a matched pair of active micro-channelplates in a 'chevron' configuration with anode. Burle Long Life™ channelplates are employed to achieve good time resolution together with high gain, to allow for ion counting applications and long lifetime. The complete assembly comes ready to be bolted on to a standard Conflat vacuum flange and is bakeable.

In Kore TOF-MS systems, the detector is mounted on a conflat flange for direct insertion into Kore flight tubes at the correct position and angle. In TOF-MS instruments without floating liners (such as SIMS instruments) the detector is fitted with an integral earth shield, with a front entrance grid. This confines the electrostatic field resulting from the detector voltages and prevents it from affecting ions in the field-free region. A second grid brought out to its own feedthrough allows simple energy filtering to be realised. In TOF-MS instruments with floating liners, an entrance electrode with grid is mounted in front of the DMCP, and connects to the field-free potential.

The detector front, rear and anode connections are brought out on separate feedthroughs so that the detector may be operated in floating mode for some modest post-acceleration (max. feedthrough voltage 5kV). The ability to operate with the anode floated away from ground allows both negative and positive ions to be detected with high efficiency.

An optional pre-amplifier is also available from Kore to provide analogue and/or ion counting outputs in a single package that mounts directly to the flange via a quick release system for baking. This unit also provides a high voltage resistor network to correctly set the various Channleplate voltages from as little as one external high voltage supply.

Specifications

Maximum excitation voltage "FRONT" to "BACK" 2000V
Bias current range @ 2000 Volts 8 - 79 µAmp
Typical anode bias with respect to "BACK" 200V
Minimum charge gain (at 2000V) 4 x 106 Minimum
Pore diameter 10 µm
Pore spacing 12 µm
Bias angle 8°
Maximum voltage on any pin ±5000V
Feedthrough type Pins see drawing
(suits Kore pre-amp unit)
Grids (shield and filter) 90% transmission 0.050" pitch
Grid spacing earth grid to filter grid 12 mm
Grid spacing filter to detector front 6.5 mm

Typical performance (Detector assembly)

Oscilloscope screen shot and measuring scheme

The left side of the graphic above shows a screen shot from a LeCroy 9361 digital oscilloscope (input bandwidth 300MHz) used to test the detector assembly. On the right is a schematic of the test setup used. The 100 Ω load seen by the MCP anode matches that used in the Kore pre-amp unit (see below). Note that the pulse width is probably limited by the oscilloscope input.

Options

  1. Substitute SHV connectors for pin feedthroughs: may be useful for those not wishing to add the Kore pre-amplifier module, who instead wish to connect cables directly to the flange.

  2. Extended dynamic range (EDR) MCP plates: may be useful in circumstances where large average signal currents are expected. As a general rule, for linear operation, one should avoid taking an average signal current greater than 1/10 of the plate bias current.

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Flange mounting analogue/digital detector pre-amplifier

Schematic of pre-amp connections Photo of pre-amp mounted and in use

The analogue/digital pre-amplifier is designed to mount directly to the flange of the floatable, large area detector and provide signal amplification and 50 ohm cable drive for all time-of-flight applications. The connections are made by plugging the circuit board directly onto the vacuum feedthrough, to allow fast disassembly from the flange for baking. The unit includes high voltage biasing and smoothing to set detector voltages, suitable for positive and negative ion detection, well decoupled to ground at the flange. Two high voltage sockets (SHV) accept the float and detector excitation voltages from an external supply (for example Kore's TOF-MS Voltage Controller). In addition a third SHV socket is connected internally to the filter grid pin for convenience of connection.

Features

Typical performance

Oscilloscope screen shot of pre-amp outputs

This oscilloscope (300MHhz input bandwidth) screen shot shows both the analogue (red) and digital (blue) outputs of the pre-amp in response to a single ion arrival at the detector. Note that they are displayed at different sensitivities (analogue - 20 mV/div; digital - 0.5 V/div).

Detailed description

Black diagram of pre-amplifier box

The pre-amplifier input is coupled to the detector anode via a high voltage capacitor so that signals may be gathered with the anode up to 5 kVolt away from ground. It includes a high speed analogue gain stage (20x rise time <1.5nsec)) capable of driving 50 ohm cable, followed by a fast comparator stage that can provide digital output pulses at ECL standard levels (suitable for example to drive the Kore TDC histogramming unit) delivered, again, down 50 ohm cable. A pre-set potentiometer can be used to adjust the comparator threshold level. Either output may be used alone, with the other terminated, or both may be used simultaneously.

A third BNC socket is provided which provides a proper ECL termination (50 ohms to -2 Volt) for convenience. For example, if the digital output is not needed, then it may linked to the termination socket to ensure proper termination and eliminate any noise associated with unwanted signal reflections. A second common use arises if the ECL signals are to be examined on a conventional oscilloscope with a high input impedance. The ECL output would be led to a tee piece at the 'scope input and then back to the ECL termination socket on the pre-amp unit.

A measure of protection is provided at the input of the amplifier by a very fast diode, primarily to speed up the recovery from excessively large input signals that often occur in a time-of-flight experiment. For example an initial intense burst of electrons from a laser based experiment. However, inevitably with research equipment, unusual experiments may lead to unintended high voltage flash-overs. Perfect protection from these is incompatible with very high-speed performance so to minimise the pain associated with such events the semiconductor components on the pre-amplifier board are mounted in ultra low profile sockets and can therefore be changed relatively easily.

The pre-amplifier is powered by +5 and -5 Volt DC inputs at a single DIN connector. These are compatible with the DC power output socket provided with the Kore TDC Histogramming unit. A single cable assembly, provided with the pre- amplifier unit, is therefore all that is required to use the two products together.

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10kV Post-accelerated High Mass Detector For TOF-MS Instruments

ETP have discontinued their large area, 10kV post-accelerated discrete dynode detector, owing to low turnover. This detector has been fitted to two Kore TOF-MS systems.

Kore has come across a number of clients for whom such a detector might be useful. These would be clients seeking to measure high mass species created at, or near, ground potential. In order to be able to fulfil this requirement for 10kV post-acceleration on a discrete dynode detector, Kore has now designed, built and tested its own version of this detector. It has recently been shipped to a U.K. customer (January 2008), and is now available as a product from Kore.

For large organic molecules, such as those produced in MALDI experiments, 20kV post acceleration is achieved by having the sample at 20kV and the detector at ground potential. In many experiments however it is not possible or convenient to have a sample at high potential to achieve such high post-acceleration potentials. Furthermore, for large non-organic clusters, there are calculations for MCP detectors predicting that a modest post-acceleration energy of 10kV is capable of producing very worthwhile increases in conversion efficiency, compared to having no post-acceleration. For instance, 80% detection efficiency is predicted for Cr clusters of ~5,000 m/z when post-accelerated to 10keV, compared to ~2% for clusters of the same m/z post-accelerated to 2keV I. S. Gilmore and M. P. Seah, Int. J. Mass Spectrom. 202 (2000) 217.

By having a conversion dynode at -10kV, it is possible for ions of low flight energy to be accelerated in the last short section of their flight and gain a high momentum as they strike the conversion dynode.

In this detector embodiment, the rear of the multiplier chain is held at ground potential (signal is at ground potential), and the front of the multiplier stack therefore is raised to whatever gain is required. For instance, in a relatively new detector, the conversion dynode would be at -10kV, the front of the multiplier at -2.1kV and the rear of the multiplier at ground. As the detector ages with time, the front potential would be raised to be more negative, e.g. -2.5kV (and so on).

Seen to the left is the new detector. The conversion dynode is hand-polished, and floated to -10kV. The conversion dynode is also unusual because of its large opening of 25mm x 22mm. The entrance is gridded with high transmission mesh (91%)

The detector is mounted on a 150mm OD CF flange with feedthroughs for the 10kV post-acceleration voltage, the front of the multiplier and the signal (rear of multiplier).

For applications where the TOF-MS does not have a lined flight tube, it is possible to house the detector inside an enclosed earthed shield. Details upon enquiry.


We recommend the use of a Kore close-coupled pre-amplifier that has been designed to fit onto the detector flange.

The signal produced by the detector is still of low amplitude, and so an extra stage of amplification is required to boost the signal to a level that can be detected by suitable electronics. This is why we supply detectors with matched pre-amplifiers. These pre-amplifiers are very fast and are 'close-coupled' which means that they need to be mounted directly onto the detector flange.

The pre-amplifier is designed to mount directly to the flange of the detector and provide signal amplification and 50 ohm cable drive for all time-of-flight applications.

Seen above is the pre-amplifier housing and the separate 10kV SHV feedthrough that brings in the post-acceleration voltage.

The connections are made in such a way as to allow fast disassembly from the flange for baking. Pre-amplifier power (±5V) needs to be supplied to a 3-pin DIN connection.

Features:

A measure of protection is provided at the input of the amplifier by a very fast diode, primarily to speed up the recovery from excessively large input signals that often occur in a time-of-flight experiment. However, inevitably with research equipment, unusual experiments may lead to unintended high voltage 'flashovers'. Perfect protection from these is incompatible with very high-speed performance, so to minimise the pain associated with such events the semiconductor components on the pre-amplifier board are mounted in ultra low profile sockets and can therefore be changed relatively easily

This pre-amplifier can have either digital or analogue outputs. Please specify which you would like at the time of ordering.

Prices February 2008

Detector on 150mm OD CF flange + close-coupled pre-amplifier £4,750
Detector on 150mm OD CF flange (only) £3,550
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Last updated: 02 June 2008 20:04

© Kore Technology Limited 2006