UNIVERSITY OF LJUBLJANA

FACULTY OF MATHEMATICS AND PHYSICS

DEPARTMENT OF PHYSICS

Seminar

Microchannel Plate Photomultiplier

Author: Matija Dmitrovič Petrič

Supervisor: prof. dr. Peter Križan

Abstract

Microchannel plate photomultiplier (MCP - PMT) is considered to be the most promising

candidate for photon detectors which are required by the Cherenkov detector. This seminar

describes the basic structure of MCP - PMT and its characteristics.

Ljubljana, march 2008

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Table of Contents

1. Introduction ......................................................................................................................................... 2

2. Cherenkov radiation ............................................................................................................................ 3

3. Microchannel Plate Photomultiplier .................................................................................................... 4

3.1 Structure of MCPs ......................................................................................................................... 5

3.2 Structure of MCP-PMTs ............................................................................................................... 7

4. Characteristics of MCP-PMTs ............................................................................................................ 7

4.1 Gain characteristics ....................................................................................................................... 7

4.2 Time characteristics ....................................................................................................................... 9

4.2.1 Rise/fall times ......................................................................................................................... 9

4.2.2 Transit time ............................................................................................................................. 9

4.2.3 TTS (transit time spread) ...................................................................................................... 10

4.3. Dark current ................................................................................................................................ 10

4.4 Magnetic characteristics .............................................................................................................. 10

5. Multianode MCP-PMTs .................................................................................................................... 12

6. References: ........................................................................................................................................ 14

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1. Introduction

One of the unsolved theoretical questions in physics is why the universe is made chiefly of

matter, rather than consisting of equal parts of matter and antimatter. The Big Bang should

have produced equal amounts of matter and antimatter, so in time there should have been total

cancellation of both. In other words, protons should have cancelled with anti-protons,

electrons with positrons, neutrons with anti-neutrons, and so on for all elementary particles.

This would have resulted in a sea of photons in the universe with no matter. Since this is quite

evidently not the case, after the Big Bang, physical laws must have acted differently for

matter and antimatter.

The most plausible explanation for this is the CP violation in particle physics. CP is the

product of two symmetries: C for charge conjugation, which transforms a particle into its

antiparticle, and P for parity, which creates the mirror image of a physical system. The strong

interaction and electromagnetic interaction seem to be invariant under the combined CP

transformation operation, but this symmetry is slightly violated during certain types of weak

decay.

One experiment that investigates this violation is the Belle experiment at the High Energy

Accelerator Research Organisation (KEK) in Tsukuba, Japan. For the Belle particle

identification system upgrade a new Cherenkov detector is being considered. It requires

photon detectors that work in high magnetic fields of about 1.5 T, have pad size of few mm

and timing resolution around 50 ps. The most promising candidate is a microchannel plate

photomultiplier (MCP - PMT). In this seminar I will describe how Cherenkov detector works,

and why MCP PMT has all of the above advantages.

Figure 1: Typical MCP – PMT is a few cm big

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2. Cherenkov radiation

In a medium such as water, the speed of light is considerably less than the speed of light in

vacuum (c). In a medium with refractive index n the velocity of light is vlight = c/n. If the

refractive index is greater than one it is possible for a particle to travel through water (nwater

= 1.3) or other media at a speed faster than the speed of light in that media. When a charged

particle does so (v > vlight), a faint radiation is produced from the medium. The charged

particle excites the water molecules which then return to their normal state emitting photons

of blue light. Because the particle is moving faster than the speed of light in water, it can

trigger a cascade of photons which are in phase with each other and can constructively

interfere to form the visible blue glow. The light propagates away in a cone forward of the

region where the interaction took place. The half angle θ of the cone can easily be derived in

terms of the velocity v of the particle by looking at where wave fronts emitted from the track

of the particle constructively interfere.

cosθ = vlight/v = c/(vn)

The emitted radiation is called Cherenkov radiation after P. Cherenkov who received the 1958

Nobel Prize for Physics with Igor Y. Tamm and Ilya M. Frank for their investigation of the

phenomenon. The Cerenkov angle is zero at the threshold velocity for the emission of

Cerenkov radiation. The angle takes on a maximum as the particle speed approaches the speed

of light. It is possible to detect the Cherenkov radiation as it forms circles on a surface and can

be used to measure the speed and direction the particle was travelling in. It is therefore a very

useful means of studying the products of particle collisions and cosmic rays.

The equivalent phenomenon in acoustics is the formation of the acoustical shock wave

generated by a body moving with supersonic velocity.

Figure 2: Identification of particles by measuring Cherenkov angle and momentum for two

different refractive indexes

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Cherenkov radiation is commonly used in experimental particle physics for particle

identification. One could measure (or put limits on) the velocity of an electrically charged

elementary particle by the properties of the Cherenkov light it emits in a certain medium. If

the momentum of the particle is measured independently, one could compute the mass of the

particle by its momentum and velocity, and hence identify the particle.

The most advanced type of a detector is the RICH, or ring imaging Cherenkov detector,

developed in 1980s. In a RICH detector, a cone of Cherenkov light is produced when a high

speed charged particle traverses a suitable gaseous or liquid medium, often called radiator.

This light cone is detected on a position sensitive planar photon detector, which allows

reconstructing a ring or disc, the radius of which is a measure for the Cherenkov emission

angle.

Figure 3: Cherenkov radiation from a charged particle nearly at the speed of light moving in a

medium with index of refraction n > 1

3. Microchannel Plate Photomultiplier

MCP-PMT is a planar component used for detection of photons, especially of blue light. It is

closely related to an electron multiplier, as both intensify single particles or photons by the

multiplication of electrons via secondary emission. MCP-PMTs, photomultiplier tubes that

incorporate an MCP in place of the conventional discrete dynodes, offer wide-bandwidth

measurements down to the picosecond level as well as low-light-level detection at the photon

counting level.

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3.1 Structure of MCPs

The MCP consists of a two-dimensional array of a great number of glass capillaries

(channels) bundled in parallel and formed into the shape of a thin disk (Figure 4). MCPs are

typically 2 mm thick. Each channel has an internal diameter ranging from 6 to 100 microns

with the inner wall processed to have the proper electrical resistance and secondary emissive

properties. Accordingly, each channel acts as an independent electron multiplier. Being

accelerated by the electric field created by the voltage VB (usually between -1000 and –3600

V) applied across both ends of the MCP, these secondary electrons bombard the channel wall

again to produce additional secondary electrons (Figure 4). This process is repeated many

times along the channel and as a result, a large number of electrons are released from the

output end.

Figure 4: Schematic structure of an MCP

PHOTOELECTRONS

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Figure 5: Schematic of a single channel of an MCP showing the mechanism of secondary

electron production

MCPs are quite different in structure and operation from conventional discrete dynodes and

therefore offer the following outstanding features:

- High gain despite compact size (104 – 10

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- Fast time response (< 100 ps)

- Two-dimensional detection with high spatial resolution

- Stable operation even in high magnetic fields (up to 2 T)

There are various types of detectors that utilize the advantages offered by MCPs, for example

image intensifiers for low-light-level imaging, fast time response photomultiplier tubes that

incorporate an MCP (MCP-PMTs), position-sensitive multianode photomultiplier tubes,

streak tubes for ultra-fast photometry, and photon counting imaging tubes for ultra-low light

level imaging.

Photoelectron

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3.2 Structure of MCP-PMTs

Figure 6 shows the cross section of a typical MCP-PMT. This MCP-PMT consists of an input

window, photocathode, dual MCP, and anode. The photoelectrons emitted from the

photocathode enter the channels of the MCP and impinge on the inner wall where they are

multiplied by means of secondary emission. This process is repeated along the channels, and

finally a large number of electrons are collected by the anode as an output signal. The

photocathode to MCP distance is approximately 2 millimetres, forming a close-proximity

structure. Two MCPs are stacked to obtain sufficient gain. A thin film called "ion barrier" is

usually formed on the photoelectron input side of the MCP in order to prevent ions generated

inside the MCP from returning to the photocathode.

Figure 6: Cross section of a typical MCP-PMT

4. Characteristics of MCP-PMTs

4.1 Gain characteristics

The gain of an MCP-PMT depends on the number of MCPs incorporated in the tube. Figure 7 shows

the typical gain versus supply voltage characteristics of an MCP-PMT.

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Figure 7: Typical gain of an MCP-PMT (using a two-stage MCP of 6 µm channel diameter)

The gain (µ) of an MCP is determined by the length-to-diameter ratio 𝛼 (= 𝐿 / 𝑑) of a channel, and

approximated as follows:

µ = 𝑒𝐺𝛼

where G is the secondary emission characteristics called the gain factor. This gain factor is an inherent

characteristic of the channel wall material and is a function of the electric field intensity inside the

channel.

In general, a higher gain can be obtained as α is made greater, though the gain rising point moves to

the higher supply voltage side. However, if the gain becomes higher than 104, noise begins to increase

significantly due to ion feedback effects, which causes a serious problem. To avoid this, α is usually

selected to be around 40 so that a single MCP provides a gain of about 104 at 1 kV supply voltage.

As shown in Figure 7 above, a higher gain can be obtained from a two-stage MCP-PMT. This gain

level enables photon counting measurements.

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4.2 Time characteristics

The signal pulse can broaden during the multiplication process from the photocathode to the

anode. This is due to the emission-angle distribution and initial-velocity distribution of

photoelectrons and secondary electrons, as well as the effects of the focusing lens. In a MCP-

PMT, a strong electric field is applied parallel from the photocathode to MCPin and the

MCPout to anode, so that the emission-angle distribution and initial-velocity distribution of

photoelectrons can be almost ignored. Furthermore, since MCP is used in place of

conventional dynodes, the electron transit time in the secondary electron multiplication

process is very short, allowing a dramatic improvement in the transit time spread. Due to

these features, the MCP-PMT offers time response characteristics that are the best among

currently available photomultiplier tubes

4.2.1 Rise/fall times

The rise and fall times of an MCP-PMT are evaluated from the output waveform when the

MCP-PMT detects a light pulse whose width is sufficiently short compared to the time

response of the MCP-PMT. These parameters are especially important when observing the

waveform of ultra-short pulsed light. Figure 8 shows an actual waveform obtained with an

MCP-PMT.

Figure 8: Pulse response waveform of MCP-PMT

4.2.2 Transit time

The transit time is the time delay between the input of a light pulse at the photomultiplier tube

and the appearance of the output pulse from the photomultiplier tube.

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4.2.3 TTS (transit time spread)

When one photon enters the MCP-PMT, the photocathode converts it into an electron which

travels to the anode while being multiplied. The transit time of an electron bunch differs

depending on each input photon. The distribution of this transit time is referred to as the

transit time spread or TTS. This TTS is an important parameter, especially in the time-

correlated photon counting technique where the measurement of timing is of prime

consideration.

Figure 9: Transit time spread fitted with double Gaussian function. The timing distribution

has a main peak with σ of about 40 ps and characteristically long tail due to the backscattering

of photoelectrons. Contributions of electronics and laser to the width of the main peak are σ ≈

15 ps and σ ≈ 12 ps respectively.

4.3. Dark current

As with normal photomultiplier tubes, the dark current of MCP-PMT greatly depends on the

photocathode type and operating temperature. In particular, the dark current of a multialkali

photocathode with enhanced red sensitivity are relatively high at room temperatures, so MCP-

PMTs using such a photocathode may need to be cooled during operation.

4.4 Magnetic characteristics

The following points are essential for good magnetic characteristics:

- The distance between the photocathode, dynodes and anode should be shortened to

minimize the electron transit distance.

- The electrodes should be designed to apply a parallel electric field from the photocathode to

the anode so that the secondary electrons do not converge but travel in parallel to the tube

axis.

- A high electric field intensity should be applied.

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Because of the small dimensions and correspondingly high electric filed strengths, the MCP-

PMT meets all the above requirements and provides superior magnetic characteristics. The

extent of the effect of a magnetic field on the output depends on the direction of the magnetic

field with respect to the MCP axis. In magnetic fields parallel to the tube axis, the MCP-PMT

can operate at up to 2.0 T, but in magnetic fields perpendicular to the tube axis, the output

drops drastically if fields exceed 70 mT (Figure 10 and 11).

Figure 10: Typical magnetic characteristics of an MCP-PMT when in magnetic fields parallel

to tube axis.

Figure 11: Typical magnetic characteristics of an MCP-PMT when in magnetic fields

perpendicular to tube axis.

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5. Multianode MCP-PMTs

The previous sections mainly discussed MCP-PMTs having a single anode. A variety of

MCP-PMTs with independent multianodes have been developed and put to practical use.

These multianode MCP-PMTs offer simultaneous, two-dimensional (or one-dimensional)

detection as well as fast response speed and low-light-level detection.

Figure 12: 4-channel multianode MCP-PMT

Surface response of PMTs is fairly uniform. Multiple counting is observed at pad boundaries

due to charge sharing effect. Slice of 2D distribution shows uniform response within the pads,

short range cross-talk due to charge sharing and long range photoelectron backscattering

cross-talk (Figure 13)

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Figure 13: Individual pad count rates as a function of position across 4 channels in linear

(upper) and logarithmic (lower) scale.

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6. References:

J. L. Wiza, Microchannel plate detectors, Nuclear Instruments and Methods 162 (1979), 587 - 601

S. Korpar et al., Timing and cross-talk properties of BURLE multi-channel MCP PMTs

Hamamatsu Photonics, Photomultiplier tubes

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html

http://en.wikipedia.org/wiki/Cherenkov_radiation

http://commons.wikimedia.org/wiki/Image:Cherenkov2.svg

http://en.wikipedia.org/wiki/Micro-channel_plate

http://hea-www.harvard.edu/HRC/mcp/mcp.html

http://belle.kek.jp