WO2002086944A1

WO2002086944A1 - Voltage divider circuit for an electron multiplier

Info

Publication number: WO2002086944A1
Application number: PCT/AU2002/000384
Authority: WO
Inventors: Robert Paul Gregory
Original assignee: Varian Australia Pty Ltd
Priority date: 2001-04-24
Filing date: 2002-03-28
Publication date: 2002-10-31
Also published as: AUPR455801A0

Abstract

An electron multiplier (12) having a voltage divider circuit (10) for applying predetermined voltages to the dynodes (14-28) of the electron multiplier (12). One of the dynodes (18) is for attenuating an output of the electron multiplier. The voltage divider circuit (10) is made up of two networks, the first of which is a normal resistor chain (32-46) which is connected to the dynodes except the attenuating dynode (18) for applying predetermined biasing voltages to those dynodes. The second network is also made up of resistances (50, 52, 54) and applies an attenuating voltage to the attenuating dynode (18). It furthermore includes a switch (58) which is operative to vary the resistance in the second network by a discrete amount to thereby vary the attenuating voltage in a discrete step. The first network is connected between an extra-high tension voltage (EHT) supply (48) and earth (49) and the second network Is connected between the voltage supply (48) and a second voltage supply (56) which is connected to the dynode (20) following the attenuating dynode (18) for controlling the gain of the following multiplier stages. The use of two voltage sources (48, 56) ensures that the attenuation voltage is variable in discrete steps independently of the pre-determined biasing voltages applied to the remaining dynodes.

Description

VOLTAGE DIVIDER CIRCUIT FOR AN ELECTRON MULTIPLIER

Technical Field The present invention relates to an electron multiplier having a voltage divider circuit for applying predetermined biasing voltages to the electrodes (commonly termed the dynodes) of the electron multiplier, for example for an electron multiplier detector in a mass spectrometer. Hereinafter in this specification the terms "electrode" and "dynode" are used interchangeably. The invention also relates to a voltage divider circuit as such for an electron multiplier and to a circuit a such for applying attenuating voltages to an attenuating dynode of an electron multiplier.

Background Pulse counting electron multipliers can be used to detect a range of energetic particles including ultra-violet, X-ray and gamma ray photons, high energy neutrals and ions. They are widely used as ion detectors in mass spectrometry. Such electron multipliers contain a first electrode (or dynode) that may be biased at a high negative voltage relative to the ion source for ion detection and which emits one or more electrons when struck by an ion. A second electrode (or dynode), biased at a less negative voltage than the first electrode, is struck by electrons emitted from the first electrode and emits more than one electron for each incoming electron. This has the effect of multiplying the electron current as it passes between the first and second electrodes. A third multiplying electrode, biased at a less negative voltage than the second electrode, provides a second stage of multiplication. The process is repeated through several more stages and results in the electron multiplier emitting a detectable pulse of current when an ion strikes the first electrode. The pulse of current is amplified and counted by electronic means as is known.

When mass spectrometers are used for chemical analysis of analytical samples, it is often found that the count rate for one ion of interest differs from the count rate for another ion of interest by many powers of ten. The count rate for any given ion depends ultimately on the concentration in the original sample of the species that gives rise to that ion. As concentrations can vary greatly from sample to sample, so too do the ion count rates that have to be measured. Accordingly, it is desirable that ion detectors for mass spectrometry should be capable of measuring ion count rates that vary by many powers of ten while maintaining a linear relationship between incoming ions and the resulting electrical signal. Detectors having such a capability are said to have an extended dynamic range.

A recently known pulse counting electron multiplier provides an attenuation function to allow a varying proportion of the electrons released when ions strike the first electrode at the entrance of the multiplier to propagate through the full gain stage to the output. This allows very intense input signals to be measured using conventional pulse counting techniques. An example of such an electron multiplier is the DM 169 electron multiplier manufactured and sold by ETP Electron Multipliers of 31 Hope Street, Ermington, New South Wales, 2115, Australia.

However this ETP detector requires the use of a high voltage power supply which is precisely continuously variable, which is expensive. Furthermore, the detector must be calibrated prior to each measurement and such calibration must be conducted for all attenuation voltages to give a continuous calibration curve for each ion species across the full attenuation range of interest. Collection of the data to provide such accurate calibration is very time consuming.

The above discussion of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date established by the present application.

Summary of the Invention

According to the invention there is provided an electron multiplier having a voltage divider circuit for applying predetermined voltages to the dynodes of the electron multiplier wherein at least one of the dynodes is for attenuating an output of the electron multiplier, the voltage divider circuit including: a first network of series connected components connectable to a voltage source and connected to the dynodes except the at least one attenuating dynode for applying predetermined biasing voltages to those dynodes, a second network connectable to a voltage source and connected to the at least one attenuating dynode for applying an attenuation voltage to the at least one dynode, the second network including at least one switch operative to vary the attenuation voltage in discrete steps independently of the predetermined biasing voltages.

The invention includes the voltage divider circuit as such for use with the electron multiplier. It also includes effectively the second network as such for use with an electron multiplier having a built-in resistive biasing network.

Preferably the second network includes resistors and the at least one switch is operative to vary the resistance in the second network by discrete amounts to thereby vary the attenuation voltage in discrete steps.

By virtue of the invention, one or more pre-set and selectable attenuation modes are provided, effectively in a digital fashion. Thus rapid switching between the pre-set attenuating electrode voltages is possible such that when the conventional pulse counting electronics detects an excessive ion count rate at the output, the pre-set attenuation voltages can be switched in until the output ion signal is within allowable limits for processing. Accurate calibration of an electron multiplier according to the invention in a mass spectrometer requires collection of much less data than in the above described prior art.

The invention is also well suited to digital implementation via range switches resulting in simplicity and low cost of implementation.

A feature of the invention is that the switch or switches of the second network is/are operative to vary the attenuation voltage(s) independently of the predetermined biasing voltages applied to the dynodes. This may be ensured by provision of a single voltage supply (herein the terminology "voltage supply" refers to a discrete power supply as such whereas the terminology "voltage source" is intended to encompass a connection, such as to an earth, that provides a potential difference relative to other connections to the circuit) with end terminations of the second network connectable between the same end terminations as the first network. Preferably, however, two voltage supplies are provided for ensuring the independence of the two biasing networks. For example, the first network may be connectable across a first voltage supply with the second network connectable between the first voltage supply and a second voltage supply. The dynodes following the at least one attenuating dynode may then be connected across the second voltage supply for controlling the gain of the following multiplier stages.

The use of two voltage supplies advantageously allows the option of using a voltage other than an earth reference at the output end of the first network of series connected components. Thus the first network of series connected components may be connectable between a first extra high tension (EHT) voltage supply and earth with a second EHT voltage supply (which is more positive than the first EHT voltage supply) connectable to the dynode following the attenuating dynode for controlling the gain of the following multiplier stages, in which arrangement the second network is connectable between the first and second EHT voltage supplies for operation of that network hot to influence the biasing provided by the first network. In this circuit arrangement, the two voltage supplies and the electron multiplier output are referenced to an earth. Alternatively the electron multiplier output may be referenced to an arbitrary third EHT voltage supply, in which case this third voltage supply must be more positive than the second EHT voltage supply which in turn must be more positive than the first EHT voltage supply. This alternative arrangement may also be modified in that the first EHT voltage supply may be replaced by a voltage source such as an earth reference, in which case both the second and third EHT voltage supplies must be positive, with the third EHT supply more positive (that is, of greater magnitude) than the second EHT voltage supply. A still further modification is for the first network to be connectable between the first and third EHT voltage supplies and the second network connectable between the first EHT supply and an earth reference.

Thus the use of at least two voltage supplies for powering a voltage divider circuit of an electron multiplier of the invention provides various options for connection of the first and second networks between those supplies to achieve the result of the switch or switches of the second network being operative to vary the attenuation voltage supplied to the attenuating dynode independently of the predetermined biasing voltages supplied to the other dynodes via the series connected components of the first network.

Preferably the components of the first network are resistors. Alternatively, the components may be resistors and zener diodes, with the zener diodes providing voltage bias for some of the dynodes, for example the dynode following the cathode and the last dynode.

Preferably the second network includes at least two series connected resistors at least one of which has a switch connected in parallel therewith for bypassing the resistor. More preferably the at least two series connected resistors each has a switch connected in parallel therewith for bypassing one or the other or both resistors depending on operation of the switches. This provides an increased number of discrete steps for varying the attenuation voltage.

Preferably the switch or switches of the second network are electronic devices, such as for example opto-isolators to provide isolated logic operation at high speed. Phototransistors or diodes may be used instead of opto-isolators where a light sensitive element is placed in the second (voltage bias) network to act as the switch whilst the light activating source may be remote and connected via a fibre-optic cable. The advantage of this is that the activating electronics may be remote from the detector and its high voltage. Electromechanical relays may alternatively be used, but their operating speed is slower. For a better understanding of the invention and to show how it may be performed, preferred embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Fig. 1 shows typical attenuating functions for a prior art electron multiplier.

Figs. 2 to 5 are electron multipliers having voltage divider circuits according to embodiments of the invention illustrating different voltage source arrangements.

Fig. 6 is an electron multiplier and voltage divider circuit according to a preferred embodiment of the invention that uses opto isolators for switching.

Fig. 7 is an electron multiplier and voltage divider circuit according to a further embodiment of the invention that uses resistors and zener diodes for biasing the dynodes of an electron multiplier.

Fig. 8 shows portion of a voltage divider circuit illustrating buffering of the electron multiplier dynode bias resistors.

Fig. 9 is a graph illustrating performance of the circuit of Fig. 6 in combination with an electron multiplier.

Detailed Description

The graph of Fig. 1 illustrates attenuating functions of a prior art electron multiplier such as the DM169 electron multiplier detector of ETP Electron Multipliers. This electron multiplier includes an attenuating electrode placed close to the front of the detector and is consequently biased at a high voltage. The graphs of Fig.1 are for an electron multiplier whose first electrode is at -2.5 kV and the attenuating electrode is biased from -2.0 to -2.4 kV (see horizontal axis). This gives a powerful attenuation at the output (Ϊ0^"1 to 10^"4 - see vertical axis) but has the disadvantage of requiring very accurate control of the attenuating voltage. Fig. 1 also illustrates that for a given attenuating voltage, the attenuation varies with mass. This is due to the secondary electron yield of the first electrode varying with incoming ion species. Thus a calibration must be obtained prior to measurement for each ion species across the full attenuation range of interest. This requires collection of a lot of data and is thus excessively time consuming.

In Figs. 2 to 8, unless otherwise indicated the same reference numerals are used to denote the same components in the different figures.

Fig. 2 shows a voltage divider circuit 10 in combination with an electron multiplier 12 according to an embodiment of the invention. The electron multiplier 12 (which is merely schematically illustrated in the figures) includes dynodes 14-28 (there may be more or less dynodes than those illustrated), the third one of which, 18, is an attenuating dynode for attenuating an output of the electron multiplier 12, which may be taken across an output resistor 46. The voltage divider circuit 10 includes a first network of series connected resistors 32-46 connected across the dynodes 14-28 except the attenuating dynode 18. Resistor chain 32-46 is a conventional arrangement for biasing the dynodes 14- 28 (except 18) and may be "built in" internally of the electron multiplier 12 or may be externally connectable to the dynodes. Resistor chain 32-46 is connected to an EHT voltage supply 48 which sets the voltage of the first dynode 14 (which is a cathode). The voltage divider circuit 10 includes a first input terminal 47 to which the first network 32-46 is connected and to which the voltage supply 48 is connectable. Voltage supply 48 and resistor chain 32-46 are both referenced to an earth 49 and thus the resistor chain 32-46 applies predetermined biasing voltages to the dynodes 16-28 (except 18) in accordance with their respective resistance values.

A second EHT supply 56 is connected via a second input terminal 55 to the multiplying dynode 20 immediately following the attenuating dynode 18. Instead of an independent second voltage supply 56, the voltage source 56 may be taken from a series junction of a single EHT voltage supply such as 48. Voltage supply 56 controls the voltage and hence the gain of the following multiplier stages 20-28 and may be adjusted to ensure adequate output pulse height for the output electronics (not shown). As the electron multiplier 12 ages, the last multiplying dynodes tend to become deactivated since they operate at the highest current density. This results in decreasing output pulse height. Voltage supply 56 may be increased to restore output pulse height.

The attenuating dynode 18 is biased from a second network of series connected resistors 50, 52, 54 connected across the first and second input terminals 47 and 55 and thus between the voltage supplies 48 and 56, with the dynode 18 connected to the junction between resistors 52 and 54. A switch 58 is connected across resistor 52. This is operative to vary the alternation voltage applied to attenuating dynode 18 to preset values between the voltages of voltage supplies 48 and 56. In the example circuit 10 of Fig. 2, with switch 58 open, attenuating dynode 18 receives the voltage that provides minimum attenuation and so the electron multiplier 12 operates like a conventional pulse counting electron multiplier. When switch 58 is closed to short resistor 52, the attenuating voltage on dynode 18 is rapidly moved to a preset value defined by the magnitudes of the voltages of the EHT voltage supplies 48 and 56 (note that supply 56 is greater, ie more positive, than supply 46) and the resistor network values 50, 52 and 54. By suitable choice of the resistance values of resistors 50, 52 and 54, the attenuation selection may be adjusted to a convenient step. More than three resistors may be provided in the second network with an individual switch connected across two or more of them, respectively, such that the attenuation voltage applied to attenuating dynode 18 may be varied in a number of discrete steps, for example, each step might conveniently reduce the output count rate by two powers of ten.

EHT voltage supply 56 is also referenced to the earth 49 and it will be appreciated that operation of the second network 50-54, 58 of the circuit arrangement 10 to vary the attenuating voltage on dynode 18 substantially does not disturb the predetermined biasing voltages on dynodes 14-28 (except 18) because the first and second networks of the voltage divider circuit 10 are separately sourced, that is, the attenuation voltage on dynode 18 is variable in discrete steps independently of the biasing voltages on dynodes 14-28 (except In Figure 3, the voltage divider circuit 10 of the electron multiplier is the same as that of Fig. 2, except that it is differently supplied via three EHT voltage supplies 60, 62, 64. Thus the output resistor 46 is referenced to an arbitrary positive voltage via an EHT voltage supply 64, with the first network of series connected resistors 32-46 biased via an EHT voltage supply 60 connected to the first dynode (cathode) 14. Another EHT supply 62 is connected to dynode 20 immediately following attenuating dynode 18, and the second network 50, 52, 54, 58 for controlling the bias on attenuating dynode 18 is connected between the EHT voltage supplies 60 and 62. In this circuit voltage supply 64 is greater (more positive) than voltage supply 62 which in turn is greater than voltage supply 60.

Fig. 4 illustrates another voltage supply option for the voltage divider circuit 10 of an electron multiplier in which the output of the electron multiplier 12 is referenced to a high positive EHT voltage supply 66, the dynode 20 following the attenuating dynode 18 is connected to another positive EHT voltage supply 68 (which is less positive than supply 66), the first dynode 14 is connected to a voltage source being an earth 49 and the second network 50, 52, 54, 58 is connected between the earth 49 and the voltage supply 68.

Fig. 5 illustrates yet another voltage supply option for the voltage divider circuit 10 of an electron multiplier 12 in which the dynode 14 (cathode) of electron multiplier 12 is connected to the negative of an EHT voltage supply 70, the electron multiplier output is referenced to a positive high voltage via connection to an EHT voltage supply 72, the dynode 20 following attenuating dynode 18 is connected to a voltage source being an earth 49 and the second network 50, 52, 54, 58 is connected between the supply 70 and the earth 49 connection.

Fig. 6 shows another voltage divider circuit 76 of an electron multiplier 77 according to an embodiment of the invention. This circuit is similar to that of Fig. 2 in that it includes a first network of series connected resistors 32-46 (schematically illustrated as "built into" the electron multiplier) for biasing dynodes 14-28 (except 20) of the electron multiplier 77, except in this embodiment the fourth dynode 20 (and not the third dynode 18) is the attenuating dynode. As in the Fig. 2 embodiment, the first dynode 14 is connected to an EHT voltage supply 48 and the dynode 22 immediately following the attenuating dynode 20 is connected to an adjustable EHT voltage supply 56 for controlling the gain of the following multiplier stages.

The second network of voltage divider circuit 76 of the Fig. 6 embodiment differs from that of the Fig. 2 embodiment in that it includes four series connected resistors 78, 80, 82, 84 connected between voltage supplies 48 and 56 and two switches 86 and 88 connected across resistors 80 and 82, respectively. The biasing voltage for attenuating dynode 20 is taken from the junction between resistors 82 and 84. The switches 86 and 88 allow for the voltage on the attenuating dynode 20 to be adjusted to preset values between the voltage supplies 48 and 56. In the example shown in Fig. 2, with switches 86, 88 open, the attenuating electrode 20 receives the voltage that provides minimum attenuation and so the electron multiplier 77 acts like a conventional pulse counting electron multiplier. Closing one or more switches 86, 88 will rapidly move the attenuating electrode 20 voltage to pre-set values defined by voltage supplies 48, 56 and the resistor network values 78-84. By suitable choice of the second network resistor values 78-84, the attenuation selection may be adjusted to convenient steps, for example each step might reduce the output count rate by two powers of ten.

Isolated switches such as 58 or 86 and 88 and precision resistors such as 50, 52, 54 or 78, 80, 82, 84 are a more cost-effective and simpler means of rapidly and precisely setting the attenuating voltages than, for example, the use of a conventional fast and variable power supply connected to an attenuating electrode as in the prior art.

Preferably the switches 58 or 86, 88 of the second network are opto- isolators to provide isolated logic operation at high speed. As in known, each opto isolator switch 86, 88 comprises an optical transistor 90 which is switchable via light 94 emitted by a light emitting diode 92 when it is energised. The invention is not limited to the use of normally open switches. Different numbers of switches may be used corresponding to other numbers of attenuation ranges. Also a fine attenuation trim switch may be implemented by adding a small offset switch. The second bias network 78-84, 86, 88 may be connected between the supply 48 for the first dynode 14 and any multiplying dynode 22 etc after the attenuating dynode 20. The invention may be applied to electron multiplier detectors employing conversion dynodes by applying the second network of resistors 78-54 between any voltage source more negative than the attenuating dynode 20 and voltage supply 56.

Figure 9 illustrates dynamic performance of a voltage divider circuit 76 like that of Fig. 6 in combination with an electron multiplier 77. The attenuating electrode of an ETP DM169 electron multiplier was switched between -2.05kV and -2.2kV using an opto-isolator within an auxiliary resistor bias network (ie a second network of the invention). The data is from a Varian UltraMass 700 inductively coupled plasma mass spectrometer, continuously measuring an aqueous solution of 100 micrograms per litre of indium (In). A100Hz TTL square wave was driving the opto-isolator during measurement to modulate the sensitivity of the electron multiplier 77. This graph shows that the circuit switched the gain of the electron multiplier by two decades (100000 to 10000000 cps) at a frequency of 100 Hz.

The electron multiplier 12 and voltage divider circuit 96 of Fig. 7 is similar to that of Fig. 2 except that zener diodes 98 and 100 are used in the first network (replacing resistors 32 and 44) to provide voltage bias for some dynodes, in this case dynodes 16 and 28. Thus a first network of series connected components of the invention may comprise only series connected resistors, or series connected resistors including one or more zener diodes.

Fig. 8 illustrates portion of a first network of a voltage divider circuit in which a usual biasing resistor chain R-i, R₂, R₃, 4 includes transistors T-i, T₂, T₃ connected emitter to collector between the resistors R₁-R₄ and the electron multiplier dynodes D-i, D₂. Bases of the transistors T₁-T₃ are connected to the junctions of the resistors Rι-R₄ and each dynode Dι, D₂ is connected to an emitter to collector connection between two transistors. This circuit illustrates a method of buffering the dynode bias resistors R-i-R. of an electron multiplier to enable operation at higher bias current. The resistor network RrR is the primary bias voltage setting means as in a regular resistive bias network whilst the transistors T₁-T₃ enable higher bias current to flow for linear operation at much higher signal currents. Such transistors may be used for part or all of the dynodes.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims

Claims

1. An electron multiplier having a voltage divider circuit for applying predetermined voltages to the dynodes of the electron multiplier wherein at least one of the dynodes is for attenuating an output of the electron multiplier, the voltage divider circuit including: a first network of series connected components connectable to a voltage source and connected to the dynodes except the at least one attenuating dynode for applying predetermined biasing voltages to those dynodes, a second network connectable to a voltage source and connected to the at least one attenuating dynode for applying an attenuation voltage to the at least one dynode, the second network including at least one switch operative to vary the attenuation voltage in discrete steps independently of the predetermined biasing voltages.

2. An electron multiplier as claimed in claim 1 wherein the second network includes resistors and the at least one switch is operative to vary the resistance in the second network by discrete amounts to thereby vary the attenuation voltage in discrete steps.

3. An electron multiplier as claimed in claim 1 wherein the voltage divider circuit includes a first input terminal connectable to a relatively negative voltage source and to which the first dynode is connected, a second input terminal connectable to a relatively less negative voltage source and to which the dynode immediately following the attenuating dynode is connected wherein the relatively less negative voltage source is for controlling gain of the multiplier stages following the attenuating dynode, and wherein the second network is connected between the first and second input terminals.

4. An electron multiplier as claimed in claim 3 wherein the second network includes at least three resistors connected in series between the first and the second input terminals and the attenuating dynode is connected to a junction between the resistor connected to the second input terminal and the adjacent resistor, the at least one switch being connected in parallel across said adjacent resistor and operative to bypass that resistor to vary the attenuation voltage.

5. An electron multiplier as claimed in claim 3 wherein the second network includes at least four resistors connected in series between the first and the second input terminals and the attenuating dynode is connected to a junction between the resistor connected to the second input terminal and the adjacent resistor, and including a plurality of switches, each switch connected in parallel across a respective resistor of the series connected resistors except the resistor connected to the second input terminal, each switch being independently operable to bypass a resistor to vary the attenuation voltage.

6. An electron multiplier as claimed in any one of claims 1 to 5 wherein the at least one switch is, or the plurality of switches are, electronic devices.

7. An electron multiplier as claimed in claim 6 wherein the switch is or the switches are opto-isolators.

8. An electron multiplier as claimed in any one of claims 1 to 3 wherein the components of the first network are resistors.

9. An electron multiplier as claimed in any one of claims 1 to 3 wherein the components of the first network are resistors and zener diodes.

10. An electron multiplier as claimed in claim 3, wherein the first input terminal is connected to the negative of an EHT voltage supply having its positive connected to an earth, the second input terminal is connected to the negative of another EHT voltage supply having its positive connected to the earth and that is less negative than the first EHT supply, and the first network is terminated to the earth.

11. An electron multiplier as claimed in claim 3, wherein the first input terminal is connected to the positive of a first EHT voltage supply, the second input terminal is connected to the positive of a second EHT voltage supply that is more positive than the first EHT supply, and the first network is terminated to the positive of a third EHT voltage supply that is more positive than the second EHT supply, the negatives of the three EHT voltage supplies being earthed.

12. An electron multiplier as claimed in claim 3, wherein the first input terminal is connected to an earth, the second input terminal is connected to the positive of a first EHT voltage supply the negative of which is connected to the earth, and the first network is terminated to the positive of a second EHT voltage supply the negative of which is connected to the earth, the second EHT voltage supply being more positive than the first.

13. An electron multiplier as claimed in claim 3, wherein the first input terminal is connected to the negative of a first EHT voltage supply the negative of which is connected to an earth, the second input terminal is connected to the earth, and the first network is terminated to the positive of a second EHT voltage supply the negative of which is connected to the earth.

14. A circuit for applying attenuating voltages to an attenuating dynode of an electron multiplier, the electron multiplier including a built in resistive chain for applying predetermined biasing voltages from a voltage source to the dynodes of the electron multiplier except the attenuating dynode, the circuit including a resistive network connectable between a voltage source and the attenuating dynode and a switch which is operative to vary the resistance of the resistive network by a discrete amount to thereby vary the attenuation voltage in a discrete step independently of the predetermined biasing voltages.

15. A voltage divider circuit for applying predetermined voltages to the dynodes of an electron multiplier wherein at least one of the dynodes is for attenuating an output of the electron multiplier, the voltage divider circuit including: a first network of series connected components connectable between a voltage source and the dynodes except the at least one attenuating dynode for applying predetermined biasing voltages to those dynodes, a second network connectable between a voltage source and the at least one attenuating dynode for applying an attenuation voltage to the at least one dynode, the second network including at least one switch operative to vary the attenuation voltage in discrete steps independently of the predetermined biasing voltages.