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Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates

Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates

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Mass Spectrometry Basics


Quadrupole ion filter

Ion beam


POint ion collector



Figure 30.1

Ions of different rnlz values pass sequentially in time through the quadrupole mass filter to reach an in-line, single-point

ion collector.

Magnetic sector ion analyzer




Figure 30.2

Ions of differing m/z values are dispersed by a magnetic sector and reach foci, which are distributed along a focal plane.

Magnetic sector ion analyzer




Ion beam








Figure 30.3

By adjusting the magnetic field. the dispersed ion beam in Figure 30.2 can be moved up or down so that ions of specific

mlz values can be focused at a point ion collector.

instrument. For this last collection of ions in a time domain, resolution depends only on the analyzer.

Alternatively, having dispersed the ions in space (resolved them according to rnJz value), all can

be detected at the same time over a section of space by using an array of single-point detectors

(the focal plane collector in Figure 30.2).

The array system is discussed in Chapter 29. With array detection, resolution of rnJz values

depends both on the analyzer and the collector. Historically, the method for recording ions dispersed

in space was to use a photographic plate, which was placed in the focal plane such that all ions

struck the photographic plate simultaneously but at different positions along the plate, depending

on rnJz value. This method of detection is now rarely used because of the inconvenience of having

to develop a photographic plate.

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap

mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion

cyclotron resonance (lCR), may not use a formal electron multiplier type of ion collector at all;

the ions can be detected by their different electric field frequencies in flight.

Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates


Another form of array is called a microchannel plate detector. A time-of-flight (TOF) mass

spectrometer collects ions sequentially in time and can use a point detector, but increasingly, the

TOP instrument uses a microchannel plate, most particularly in an orthogonal TOF mode. Because

the arrays and microchannel plates are both essentially arrays or assemblies of small electron

multipliers, there may be confusion over their roles. This chapter illustrates the differences between

the two arrays.

Arrays and Microchannel Plates

In both arrays and microchannel plates, the collector consists of a number of single-point ion

detection elements, each of which is a very small electron multiplier. Each element is much smaller

than the normal single-electron multiplier, and lots of them can be arranged close together as a

planar array to cover a large area of space (Figure 30.4). The actual construction of the arrays is

different as will be described below. The front face of the array contains the entrances or openings

for each small detector, into which ions are deposited or collected. A fast-moving ion striking the

entrance to an element starts a cascade of electrons that increases in size as the electrons bounce

offthe walls of the element during their passage to its back end. The back end of each element is

either closed or open. In an array of closed elements, the end of each can be monitored for the

arrival of cascading electrons, signaling the arrival of an ion or ions. In an array with open-ended

elements, a cascade of electrons from any element is collected onto the same backing plate. Ions

arriving anywhere in space over the face of the array are detected; viz., all of the elements are

monitored as one.

There is potential confusion in the use of the word array in mass spectrometry. Historically,

array has been used to describe an assemblage of small single-point ion detectors (elements), each

ofwhich acts as a separate ion current generator. Thus, arrival of ions in one of the array elements

generates an ion current specifically from that element. An ion of any given rnIz value is collected

by one of the elements of the array. An ion of different rnIz value is collected by another element.

Ions of different rnIz value are dispersed in space over the face of the array, and the ions are detected

by m/z value at different elements (Figure 30.4).

An assemblage (array) of single-point electron multipliers in a microchannel plate is designed

todetect all ions of any single rnIz value as they arrive separated in time. Thus, it is not necessary

for each element of the array to be monitored individually for the arrival of ions. Instead, all of

An entrance to an electron

multiplier element




Three ion beams of different m/z values

dispersed in space and entering array elements

Figure 30.4

Idealized face view of a set of small electron multipliers arranged over a plane. Some typical individual multipliers are

shown in later figures.


Mass Spectrometry Basics




Ion beam from

TOF analyze r






Bands of ions of different

m/z values, separated In





on collector

Individual electron multiplier elements

of the assembly of microchannels

Figure 30.5

Bands of ions of different m/z values and separated in time in a broad ion beam traveling from left to right toward the front

face of a microchannel assembly. The ions produce showers of electrons, and these are detected at the collector plate, which

joins all the elements as one assemblage.

the back ends of the detection elements are connected together electronically. Thus, if ions of, for

example, m/z 100 arrive at some time, t, but are spread spatially over the face of the array, they

are all detected simultaneously, even though some may be collected by one element, others by

another element, and so on. There is no discrimination in rn/z value. That separation of ions by

m/z must be effected in time by the analyzer, usually a TOF instrument (Figure 30.5).

To avoid confusion, the word array is now used to describe an assemblage of small singlepoint detectors that remain as individual ion monitoring elements, and the term microchannel plate

is used to describe an assemblage of small single-point detectors, all of which are connected so as

to act as a single large monitoring element. Because ion arrivals in the microchannel plate are

recorded as instantaneous events (the ion current is digitized), this type of detector is called a timeto-digital converter (TOC).

The Elements of Array and Microchannel Plates

Where space is not a problem, it is possible to use a linear electron multiplier having separate

dynodes to collect and amplify the electron current created each time an ion enters its open end.

For array detection, the individual electron multipliers must be very small so they can be packed

side by side into as small a space as possible. For this reason, the design of an element of an array

is significantly different from that of a standard electron multiplier used for point ion collection,

even though its method of working is very similar. Figure 30.6 shows an electron multiplier (also

known as a Channeltron'") that works without using separate dynodes. Because each Channeltron"

Electron multiplier


Secondary electrons


__ Final output

of electrons

ca. 2000 V_ _--.J

Figure 30.6

A typical single microchannel electron multiplier. Note how the primary ion beam causes a shower of electrons to form.

The shower is accelerated toward the other end of the rnicrochannel, causing the formation of more and more secondary


Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates


Electron multiplier


Incident ion

beam ~ ... _..__ .... -

"..".,.,.. ...



Final output of electrons

Figure 30.7

A different form of miniature electron multiplier. The curved shape is used to reduce backscattering of the electrons. The

final output of electrons flows along a wire to an amplifier.

can be made very small, it is suited to array assemblages, in which as many multipliers as possible

can be fitted side by side into a relatively small space. Each Channeltron" element can be made

in a variety of shapes, one of which is shown in Figure 30.7.

For a microchannel plate, the back end of each element is left open, as shown in Figures 30.5

or 30.6, and forms a microchannel. Any electrons emerging from any element are all detected by

the one collector plate.

Array Elements (Ion Mass Range)

Consider two array elements of the ones illustrated in Figure 30.4, and suppose an ion beam has

been dispersed to give ions of mJz values 100 and 101. If the dispersion is correct for the array

size, the ion of mlz 100 will enter one element and, at the same time, the ion of mlz 101 will enter

a second adjacent element. Thus, at this level of dispersion, unit mlz values can be separated.

Simple extrapolation to, say, five ions of different mlz values or ten array elements in a line shows

that several ion mlz values and abundances can be measured simultaneously to give an instantaneous

spectrum. Further extrapolation indicates that more mlz values can be measured if there are more

array elements. However, these extra measurements do not come without cost, and fitting a very

large number of elements into a compact array becomes increasingly difficult. Therefore, a limited

number of array elements is used, say 100, which means that sections of a mass spectrum can be

measured instantaneously, but, if it is required to measure a spectrum spread over several hundred

or thousand mass units, then it must be measured in sections at a time. Frequently, not all of a

mass spectrum is needed, and it may be unnecessary to measure more than one region of the

spectrum, e.g., maybe only the molecular ion region needs to be covered.

Microchannel Elements (Ion Mass Range)

Consider again two detection elements, and suppose an ion beam has been dispersed in time such

that ions of mlz 100 arrive at each of several elements (Figure 30.5). In this TOF mode, the next

ion of mlz 101 has not yet arrived, and the ion of mlz 99 has arrived previously. Although the mlz

ions are dispersed in time over a region of space and strike different elements of the detector, they

are collected and monitored simultaneously because all of the rnicrochannels are electronically

connected. The operation of the microchannel plate is much easier than that of the array because

all the elements are monitored as one at the plate, while each element must be monitored separately

in the array. The microchannel plate detector is tremendously useful for those cases in which ions


Mass Spectrometry Basics

at each m/z value are separated in time by a mass analyzer but can be delivered to the collector

spread out in space. No ions are lost through scanning, and the micro channel plate serves as a very

sensitive detector.

Uses of Array and Microchannel Collectors

Array Collectors

The major advantage of array detectors over point ion detectors lies in their ability to measure both

a range of m/z values and the corresponding ion abundances all at one time, rather than sequentially.

For example, suppose it takes 10 msec to measure one m/z value and the associated number of ions

(abundance). To measure 100 such ions of different mlz values with a point ion detector would require

1000 msec (l sec). For the array detector, the time is still only 10 msec because all the ions of different

m/z values arrive at the collector at the same time. Therefore, when it is important to be able to

measure a range of ion m/z values in a short space of time, the array detector is advantageous.

There are two common occasions when instantaneous measurement of a range of m/z values is

preferable. First, with ionization sources such as those using laser desorption or radionuclides, a pulse

of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the

mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will

be no ions left by the time the scan has completed more than a few microseconds (ion traps excluded).

The array collector overcomes this difficulty by detecting the ions produced all at the same instant

A second use of arrays arises in the detection of trace components of material introduced into

a mass spectrometer. For such very small quantities, it may well be that, by the time a scan has

been carried out by a mass spectrometer with a point ion collector, the tiny amount of substance

may have disappeared before the scan has been completed. An array collector overcomes this

problem. Often, the problem of detecting trace amounts of a substance using a point ion collector

is overcome by measuring not the whole mass spectrum but only one characteristic m/z value

(single ion monitoring or single ion detection). However, unlike array detection, this single-ion

detection method does not provide the whole spectrum, and an identification based on only one

m/z value may well be open to misinterpretation and error.

Microchannel Plate Collectors

A major advantage of microchannel plate detectors over point ion detectors lies in their ability to

measure the abundance of ions of a single rn/z value, which are spread over a region of space.

When used with a TOF analyzer, another major advantage appears. Typically in a TOF instrument,

ions of adjacent m/z values in the ion beam are separated by about 20 to 30 nsec, Over a total

mass range of 0-3000 mass units, all of the ions arrive at the collector within a period of about 30

usee. Therefore, the microchannel plate acquires a mass spectrum in about 30 usee, which on the

human time scale appears to be instantaneous. Thus, like the array detector on a magnetic scanning

instrument, the microchannel plate on a TOF instrument is capable of generating an almost instantaneous spectrum. Additionally, a full range of ions from, say, 0 to 3000 mass units can be pulsed

into a TOF analyzer at the rate of about 30,000 times per second. The microchannel plate acquires

all of these spectra, which can be summed. Thus, in I sec, a TOF/microchannel plate combination

can sum about 30,000 spectra. This speed is a great advantage for examining spectra that contain

spurious peaks of occasional electronic noise or that have been generated in a small period of time,

as with laser-assisted ionization. Other uses of the microchannel plate are described in Chapter 20,

"Hybrid Orthogonal Time-of-Flight Instruments."

Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates



Amultipoint ion collector (also called the detector) consists of a large number of miniature electron

multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector

can be an array, which detects a dispersed beam of ions simultaneously over a range of rn/z values

and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate

collector detects all ions of one mlz value. When combined with a TOF analyzer, the microchannel

plate affords an almost instantaneous mass spectrum. Because of their construction and operation,

microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly

useful for situations in which ionization occurs within a very short space of time, as with some

ionization sources, or in which only trace quantities of any substance are available. For such fleeting

availability of ions, only multipoint collectors can measure a whole spectrum Or part of a spectrum

satisfactorily in the short time available.



Time-to- Digital

Converters (TDC)


Point and array ion collectors are described in Chapters 28 and 29, respectively. The multipoint

ion collector, in which many thousands of very-small-diameter microchannels are packed closely

together, is a further development of the array. This arrangement gives a large number of ion

collectors within an area large enough to encompass the cross-sectional area of a typical ion beam

in a mass spectrometer. Unlike the array collectors, in which each Channeltron" is separately

monitored so as to distinguish between m1z values, the microchannel plates simply record total ion

arrivals and are particularly suited to time-of-flight (TOF) analyzers. Wherever an ion arrives at

the front face of the microchannel plate, it results in a release of electrons to a positively charged

electrode (back plate). The resulting electric current is passed to a suitable recording device. Ions

in a TOF analyzer arrive sequentially at times that are proportional to m/z. The arrival events for

each rnJz (pulses) are recorded using electrical signals that are already digitized. Since the collector

takes ion arrivals spread over time and converts these events into discrete (digital) electrical pulses,

it is called a time-to-digital converter (TOC). This chapter focuses on the operation of a TOC.

Measurement of m/z Ratios by Time-af-Flight Instruments

Measurement of m/z ratios by TOF instruments is covered in detail in Chapter 26 and described only

briefly here. After acceleration through an electric potential difference of V volts, ions reach a velocity

vgoverned by the equation v =(2zeY/m)O.5, in which e is the electronic charge. Note that the velocity

is inversely proportional to the m/z value of an ion. If the ions travel a distance d before being

detected, the time t needed to travel along the TOP flight tube is given by t = d1v and m/z becomes

proportional to t2 (Figure 31.1). Thus, mass measurement resolves itself into the accurate measurement of the times needed for ions to travel along the flight tube. This timing must be done electronically because normal clocks cannot measure the times accurately enough for flight tubes of reasonable

length (""1.0 m). As an example, for a potential difference of 1000 Ys, a mass of 100, and a charge

of z = I, ions would take 22.8 usee to travel the length of the TOF flight tube, and a mass of 101

would take 22.9 usee, If mass 100 is to be differentiated (resolved) from 101, then the timer must

be able to measure accurately time differences of at least 0.1 usee, TDCs can effect such accurate



Mass Spectrometry Basics


Electrode (V volts)



0-' - - - - - - - - - - - - - - - - - -..,


Drift region (d)


















Drift time

t - to

Drift velocity, v = dI(t - toJ

m1z = 2eV/v2


Figure 31.1

Upon acceleration through an electric potential of V volts, ions of unknown mlz value reach a velocity v (= fZzeV/m]05).

The ions continue at this velocity (drift) until they reach the detector. Since the start (to) and end (t) times are known, as

is the length d of the drift region, the velocity can be calculated. and hence the mlz value can be calculated. In practice,

an accurate measure of the distance d is not needed because it can be found by using ions of known mlz value to calibrate

the system. Accurate measurement of the ion drift time is crucial.

time measurement and provide a very convenient method of recording mass spectra from a TOP

instrument, particularly as the signal coming from the converter is not analog but is already digitized.

Ions in a TOF analyzer are temporally separated according to mass. Thus, at the detector all

ions of anyone mass arrive at one particular time, and all ions of other masses arrive at a different

times. Apart from measuring times of arrival, the TOC device must be able to measure the numbers

of ions at anyone m/z value to obtain ion abundances. Generally, in TOF instruments, many pulses

of ions are sent to the detector per second. It is not unusual to record 30,000 spectra per minute.

Of course, each spectrum contains few ions, and a final mass spectrum requires addition of all

30,000 spectra to obtain a representative result.

Multichannel (Microchannel) Plate Array

The mode of action of a single Channeltron'" (a miniaturized version used as one element of a

microchannel plate array) is shown in Figure 31.2. A fast-moving ion striking the front end of a

single microchannel causes a number of secondary electrons to be ejected. These electrons are

accelerated by an electric field lying the length of the microchannel element and, after a short

distance, strike its walls further along from their point of origin. Each electron impact on the

Channeltron" walls causes several electrons to be ejected, with each initial secondary electron

producing several more electrons. This process of producing more electrons continues along the

length of the microchannel element, with further wall collisions leading to a cascade of many

electrons, which emerge as a burst (pulse) from the other end of the microchannel. The pulse of

electrons crosses to a positively charged metal plate collector and is detected as an electrical current.

Thus, each time an ion arrives, an electrical pulse (electrical digit) is produced. A succession of

ion arrivals yields a corresponding series of electrical pulses. As noted above, these ion arrival

events are already digitized and can be stored directly in computer memory (discussed in the

following section). Thus, each ion event is transformed into an electrical digital pulse, and this

process gives rise to the term time-to-digital converter (TDC).

The front opening of such a microchannel element has a diameter of only a few microns, but it

is only one element of a whole multichannel array (Figure 31.2). Whereas the orifice to one microchannel element covers an area of only a few square microns, an array of several thousand parallel

elements covers a much larger area. In particular, the area covered by the array must be larger than

Time-to-Digital Converters (TDC)


Initial ion collision

with wall

Production of



One element of

microchanel array

Electrons arrive

at backing plate

Potential difference for

acceleration of electrons






to timer

Figure 31.2

Diagram illustrating one element of a microchannel array. In a typical array, this Channeltrorr''-like tube is just one of thousands

of similar ones, each only a few microns in diameter. All microchannel elements end a short distance from a common backing

plate. An ion entering the front end of an array element and striking the wall produces secondary electrons. These in tum are

acceleratedalong the tube and strike the wall. Each secondary electron produces several more secondary electrons. The process

is repeated along the tube until a shower of electrons leaves its end and crosses to the backing plate. The arrival of these

electrons constitutes a pulse of electric current, which is recorded by the TDC timer. The ion arrival time (an event) is recorded.

the cross-sectional area of the ion beam that is detected so that all ions are recorded, Figure 31.3

shows a small representative section of a multichannel array, together with the backing plate.

Timing of Electrical Pulses Resulting from Ion Arrivals at the

Microchannel Plate Collector

As shown above, when an ion arrives at the microchannel array it releases a cascade of electrons

onto the back plate. This cascade constitutes an electrical pulse from the microchannel plate. which

Small section of

microchanel array



event timer



Figure 31.3

Diagrammatic representation of a small section of a microchannel array, with its backing plate. One element of the array is

shown in greater detail in Figure 31.2. Arriving ions can enter anyone of the array elements to produce an electric current at

thebacking plate. Since the latter is common to all of the array elements, it is immaterial which element of the array is involved

becausethe signal is recorded by the common plate. Thus, arrival of an ion at any point on the array (an event) results in the

sending of an electrical signal to the IDC timer (IDC converter). This point is the ion arrival time (t) shown in Figure 3 I.I.


Mass Spectrometry Basics



Timing 'clock' ----..

pul se







....- Time bins

....- Event







After clock pulse




1 :0


Event pulse

Clock pulse






























Initial state









_________________________ ••



Event time = 5 x 0.3 = 1.5 ns

Figure 31.4

A clock pulse travels along a series of electronic gates (time bins). The time taken to cross each gate is about 0.3 nsec.

Passage of the pulse through a gate is recorded by drawing off a part of the electrical signal from the clock pulse - viz.,

part of the signal is sent to the event recorders, which switch from a 0 state to a 1 state. In this example, after passing

through five gates, the elapsed time from the start of the clock pulse is 1.5 nsec, Arrival of an ion (an event or hit) causes

a new pulse to be sent to all of the event recorders which resets the bins affected by the clock pulse. The clock pulse

continues to travel along the time bins, changing the event recorders from 0 to I. Where the event has occurred is marked

by a change of state of the event recorders. Before the timers are reset, the recorded event is stored in memory for later use.

is used for timing ion arrival events. Each electronic pulse resulting from an ion arrival travels to

and is recorded by a time bin. A series of connected time bins can be regarded as a long chain of

electronic gates through which a clock signal passes. The time for the signal to pass through each

gate is about 3 x 10 10 sec (0.3 nsec). Thus, as shown in Figure 31.4, if a gate timing pulse is

regarded as having started at time zero (t = 0), then the time taken for the pulse to pass through

100 bins is 30 nsec (two = 3 X 10-8 sec and two - to = 3 X 10-8 sec).

Anyone bin can be electronically distinguished from the next one, and therefore the bins can

be used like the tick of a standard clock. Each bin serves as one tick, which lasts for only 0.3 nsec.

By counting the ticks and knowing into which bin the ion pulse has gone, the time taken for the

ion to arrive at the detector can be measured to an accuracy of 0.3 nsec, which is the basis for

measuring very short ion arrival times after the ions have traveled along the TOF analyzer tube.

Each ion arrival pulse (event) is extracted from its time bin and stored in an associated computer

memory location.

An explanation of the scheme for storage of the event signals requires an extra degree of

complexity. The bins have two states (on or off, digitally 0 or 1). Before the timing pulse begins,

all bins are set to the 0 state, which changes to a 1 state when the timing pulse arrives. When an

ion arrival event occurs, the clock timing signal will have already traveled through some of the

bins; viz., it will have passed through some of them in sequence, setting their states from 0 to 1.

When the ion signal (a hit) arrives, it is sent to all of the time bins simultaneously, and the bins

will be affected in different ways (Figure 31.4). Those bins through which the timing signal has

already passed will have been reset to the "1" state but, immediately in front of the timing signal,

the next bin will still be set to the original "0" state. Those bins already affected by the timing

signal will he set back to 0 when the "hit" signal arrives. Thus, all of the bins up to the event signal

will be in the 0 state, but those after the event signal will be in the 1 state. After the timing signal '

has passed through all of the bins, they are examined. All those bins that have not recorded a hit

Time-to-Digital Converters (TDC)


are regarded as empty, but the first bin that was set to 1 by a combination of hit and timing pulses

must be the one into which the "hit" signal went. Thus, an electronic inspection of the bins reveals

that the "0/1" state of the bins changes at one of them, and this fact is stored in pennanent memory.

Since the position of any bin in the series is known relative to the starting bin, the time at which

the "hit" signal from an ion arrival affected the bins can be measured to an accuracy of about 3 x

10- 10 sec. After the event has been noted, all bins are reset to their 0 states before the next timing

signal travels along them ready for the next ion arrival. A typical clock timer runs at about 3.6

GHz; viz., it ticks every 0.28 x 10-9 sec (every 0.28 nsec).

In a simplistic way, the bins can be regarded as a series of empty receptacles and an ion arrival

(an event) results in one bin being filled. By looking into the line of bins, the full one can be found.

Rather than discussing tiny fractions of a second, let it be supposed that it takes I second to pass

from one bin to the next in a series of 1000 bins. The total time needed to pass through them all

is 1000 sec. If only bin number 603 is found to be full, then the timing of the event that filled it

must have occurred 603 seconds after the examination began. All empty bins are ignored, but the

full bin is emptied and noted. Thus, the bins are returned to their original empty state, after which

the timing begins all over again in preparation for the next ion event.

Each bin is connected to a memory location in a computer so that each event can be stored

additively over a period of time. All the totaled events are used to produce a histogram, which

records ion event times versus the number of times anyone event occurs (Figure 31.5).With a

sufficiently large number of events, these histograms can be rounded to give peaks, representing

ion m/z values (from the arrival times) and ion abundances (from the number of events). As noted

above, for TOF instruments, ion arrival times translate into m/z values, and, therefore, the time and

abundance chart becomes mathematically an rn/z and abundance chart; viz., a normal mass spectrum

is produced.

Ion Abundances and Dead Time

A mass spectrum is a chart of ion abundances versus mJz values. It is shown above that the TDC

measures ion arrival times, which are converted directly into m/z values. Notionally, the number

ofions arriving at the detector at anyone m/z value is equal to the number of events recorded (one































------ Time (ns)

--Time (ns)


Figure 31.5

Ion arrival events are stored additively in memory locations (Figure 31.4), which also provide the times of arrival. After

recording data for some time, the memories are examined to produce a histogram (a) of the number of events versus arrival

limes. The histogram shown relates to ions of one particular mlz value arriving at the detector at slightly different times.

From the histogram, a peak shape is produced (b), the centroid of which gives the mean arrival time and, therefore, the

mlz value (Figure 31.1). The area of the peak gives the total number of events and therefore the abundance of the ions.

Similar procedures are used for all other ions of other mlz values in the mass spectrum.

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Comparison of Multipoint Collectors (Detectors) of Ions: Arrays and Microchannel Plates

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