Secondary Electron Emission by Bruce Darrow Gaither

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Electron Multiplication through Secondary Electron

Emission

By Bruce Darrow Gaither

In this era of energy shortages we have all daydreamed about owning a device which

would take the energy that we have and multiply it. Imagine how happy we would be if

you could simply plug in a device which would double your electricity. Numerousresearchers, writers and inventors have sought to do just that.

But scientists have cautioned that the law of conservation of energy dictates that energy is

never created nor destroyed, only converted from one form to another. None of theseschemes, they say, would ever work. They are just perpetual motion machines.

The purpose of this book is to discuss advances in electronics and materials sciencewhich have made things possible which were not contemplated when the laws of

Thermodynamics were postulated decades ago.

Secondary electron emission is a well-known process. It is that effect which causes

additional electrons to be emitted when a substance is bombarded by a stream of

electrons. This secondary emission effect was discovered a century ago, and it has found

application in a variety of devices which are in use today.

Advances in electronics and the development of new materials have revealed new

methods and substances which make this secondary electron emission even moreeffective. In fact, today the impossible is possibleone electron at a time.

If a beam of electrons is aimed at a target electrode coated with a given substance thenelectrons are emitted from that target. The number of electrons emitted from the target

which has been bombarded is compared to the number of primary electrons in the

original electron beam. The electrons emitted after bombardment by primary electrons

are called secondary electrons.

The materials propensity to emit electrons after bombardment is called the secondary

electron emission coefficient. That is expressed as the number of secondary electronsdivided by the number of primary electrons.

A secondary electron coefficient of less than 1 means that the substance does not emit asmany electrons as it is bombarded with. A coefficient of greater than 1.0 means that the

substance emits more secondary electrons than bombarded it

We are interested in this book in those materials which exhibit a high coefficient of

secondary electron emission. The goal is to perfect a device which will emit more


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One process which is common in devices which employ secondary electron emission is

that of multiple impacts upon this emissive target material. The great inventor, PhiloFarnsworth, was the first to devise methods to facilitate these multiple impacts upon

emissive materials. He called his devices multipactors because of the multiple impacts

they made with secondary electron emissive material. Thus, if a target electrode had a

secondary electron emission coefficient of 2 then the number of primary electrons woulddouble when they hit the target electrode. If there were two successive impacts then the

primary electrons would double, and then that total would double againor be four timesthe original primary electrons put into the device.

If the primary electrons were somehow sent through a series of 8 target electrodes thenthe multiplication factor would become astronomical, and each of the impacts would

result in an exponential increase of electrons based upon the coefficient of secondary

electron emission from that material.

So various devices were designed and perfected to make the primary electrons impact

numerous electrodes one after another. One branch of these devices is employed byphotomultiplier tubes. Many of these devices are capable of multiplying the primaryelectrons one hundred million times. Thus minute electric currents can be sensed and

multiplied so that they can register on scientific equipment. But numerous other

configurations and devices are in use today.

One method is to bounce the electrons off of two opposing electrodes over and over again,

like a game of ping pong. Another configuration would be to have the electrons strike

electrodes arranged inside a circular tube so that they impact coated electrodes over andover again. A third method is that of forming a cascade of specially-coated electrodes and

having the primary electrons bounce off off each successive electrode until they all come

out the end.

Another genre of devices are called channel devices. In these designs the primary

electrons are sent down a waveguide or tunnel of some sort and the entire length of thedevice is coated with the emissive materials. The electrons keep bouncing off the walls of

these guides until they reach the end and the repeated impacts result in a high

multiplication of the primary electrons.

One of the axioms of electricity is that current will not conduct very well when exposed

to the atmosphere because the gas acts as an insulator. Therefore most secondary electron

emission devices were made in the form of vacuum tubes. The electricity goes throughthe vacuum without loss and then the impacts upon emissive material have the desired

result.

However secondary emission and multipactors have been made into semiconductors and

chips. These use the process of avalanche multiplication in many instances, where the

electrons hit the emissive substance and are then multiplied and pass through a solid statestack of materials. Sometimes the semiconductors include a tiny vacuum space and they

act in the same way as a vacuum tube.


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However there is a snag or two for the use of secondary emission in chips. The firstproblem is space charge. That is the effect of completely filling up a given space with

electrons. One you saturate the evacuated space with space charge then an equilibrium

state is achieved and the primary electrons will no longer multiply as desired. As you

might guess, the greater the area the more electrons will fit into the space before thespace charge saturates the area. The space charge, then, has been found to diminish as to

the 4/3 power of the area of the evacuated space. This means that, for instance, if youtriple the size of vacuum space then that would result in 3x 4/3 power= 12/3 power (or

the 4th power). Then a tripling of space would end up in shrinking the space charge by the

4th

power. The bottom line is that bigger is better.

The second drawback to micro multipactors is that the vacuums must be higher than in

vacuum tubes, and this is hard to achieve. Also many devices use sharp points as

electrodes because more electrodes will emit from sharp points than from blunt shapes. Inthe micro world though the sharpness of the sharp point has to be correspondingly

sharper. The finer the point on the electrode the harder it is to fabricate and fit into thelayered semiconductor devices.

What this book hopes to achieve is not the simple multiplication of electrons to provide

light or brightness but to generate electricity on a larger scale. The aim is not to build agenerator station for thousands of people but to scale the multipactor devices to work

with individual appliances and vehicles. Thus the size and rated capacities of the

components in the proposed multipactors must be designed to be in the range of home

current up to the amount of voltage and current required to power an electric car.

At this point the discussion of secondary electron emission must include some of the

math and physics. Dont let your eyes glaze over. Everybody knows a little bit aboutelectricityand it is pretty simple. But there is a hazy horizon on the amount of

knowledge of the basics of electricity. The terms are VOLTAGE, AMPERAGE and

POWER. The easy rule of thumb is that VOLTAGE x AMPS = POWER.

You need to throw in the RESISTANCE into this formulabut for now we will stick

with VOLTSxAMPS=POWER.

OKso we will calculate one AMP. An Ampere involves the amount of charge, which

is calculated in terms of a COULOMB. A Coulomb is 6.24151 1018 electrons.

So the process of secondary electron emission results in a lot of electrons.

The secondary electrons are not moving very much after they are multiplied.So they have low voltagebut they DO have AMPERAGE because of the presence of

lots of electrons.

The purpose of this analysis is to point out that we have low volts and high amps from

secondary emission. When you remember volts x amps = power then you can see that we

have to have just a high enough voltage to meet the requirements of modern electricaldevices.


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There are numerous well-known devices which can act as VOLTAGE MULTIPLIERS.These devices will increase the voltage, but only at the expense of a proportional

decrease in the amperage.

The end product of these multipactors can be made usable, therefore, by running thesehigh amperage currents through a voltage multiplier. You just fine-tune the voltage

multiplier to give the right mix of volts and amps.

In short, secondary electron emission creates NEW electrons. We put the new electrons

to use by stepping up the voltage to required levels.

In this way the laws of conservation of energy are negotiated. The multipactor creates

new electrons and THEN the energy is converted from one form into another. But there is

indeed a creation of new electrons in multipactors.

I will leave it to the scientists and inventors in their respective papers and patents todescribe the manner in which the secondary electrons are created and how themultipactor devices are designed.

The point of my analysis is simply that the multipactors will create new electrons and thenew electrons can be made usable through voltage multipliers.

One of these voltage multipliers is a Cockroft-Walton circuit. Modern electronics has

manufactured numerous cheap transistor devices that you could get at Radio Shack orelectric supply houses. A Cockroft-Walton circuit is simply a ladder of diodes and

capacitors (pennies apiece) which double the voltage at each step of the ladder. So a

multi-step ladder creates a multiple doubling of the original voltage. Some of the olderdesigns apply a step-up transformer to do the same thing.

A simple Cockroft-Walton multiplier would look like thisdiagram:

So we see that there is a problem with voltage in secondary electron emission. The

inventors have figured out a method to use voltage to their advantage in the multipactor

devices. They apply the rule that opposite charges attract. This accelerates the cloud ofsecondary electrons so that they will impact the next target with its emissive coating.

The electron is a negative charge. So the inventors manipulate the sluggish cloud of

negative charge by providing a positive electrode to put it into motion.


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Some designs will make the next electrode one with a positive charge, so when theprimary electrons strike the first target and are multiplied then the cloud of secondary

electrons is accelerated right up to the second electrode coated with emissive materials.

When they make a long chain or cascade of these target electrodes then they give each of

them a successively higher positive charge so that the ever-increasing cloud of secondaryelectrons is accelerated one step at a time in the desired direction.

Other designs use an electrode which is positioned between the first impact target and the

second and they give that intermediate electrode a positive charge to accelerate the cloud

of secondary electrons in the desired direction. This intermediate electrode might be inthe form of a screen or grid or a tube. The positive charge, in every instance, attracts the

opposite charge on the electrons and they are suddenly accelerated through the holes in

these intermediate electrodes and then the electrons continue with their increased voltage

until they impact the coated electrode. This step may be repeated again and again.

The positive charge on these attracting electrodes is often provided by using Cockroft-Walton circuits. So either a single or a multi-step CW circuit may be used to multiply aninitial small current to give a charge bias of increasing strength to a series of attracting

electrodes. Oftentimes the CW circuit contains taps which tap the current at a certain

step in that multiplying step ladder. The step would then have one voltage level to applyto the attracting electrode, and then the next step would have a higher voltage which

could be tapped at that level and applied to the next attracting electrode, and so on.

Going back the purpose of this analysis again: we are trying to get as many electrons aspossible out of the multipactor. So the gameplan is to select the coating material for

electrodes which has the highest secondary electron emission coefficient. Then the

voltage at which the primary electrons must be accelerated to achieve the optimalsecondary emission must be applied. The spatial requirements are important too because

we want the right angle and the right depth for the impact zone. So we get the highest

electron multiplication at each step. Then we take that level of electron multiplication andexponentially multiply it by the number of impacts in the multipactor device.

Some devices, as aforestated, simply bounce the electrons back and forth between two

opposed electrodes. In these designs the electrons are moving at the speed of light, sothey hit the opposite electrode in a known length of time. Then they bounce back to the

original electrode. The desired effect is to have but one cloud of secondary electrons

bouncing back and forth, and not a lot of different clouds. Therefore the two electrodesare given opposite charges, positive and negative, and these charges are sequentially

reversed so that the electron cloud always moves away from the first electrode after they

have been multiplied and then toward the target electrode for more multiplication. Sincewe know the distance between the two electrodes and because the speed of light is known,

then we can determine the FREQUENCY at which the electric charge is reversed on

these electrodes. So, take the speed of light and divide it by the distance between theelectrodes. Say, 186,000 miles per second divided by 6 inches.


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The resulting frequency is in the range of billions of cycles per second.

There are modern oscillator chips which cost pennies which can do that.

The point here is that we take the secondary electron coefficient, and lets say that this is

2 for the sake of argument. Then we apply the frequency of the impacts on these emissive

electrodesand that is perhaps one billion times per second. In this example we wouldthen obtain 2 to the one billionth power!

Are you beginning to get the picture?

If we make the device the right size so that the space charge does not saturate the vacuumthen we can generate sufficient electrons so that we can step up the voltage and step

down the amperes to achieve the desired power characteristics for our electric appliance

or motor.

For the purposes of our last example we have a secondary emission coefficient of 2, or a

doubling of the primary electrons at each impact with the electrode with the emissivecoating. But what if the secondary emission coefficient were 10or 100or even 1000?Just apply the math and you can see the possibilities of these multipactors.

Attached to this anthology is one of the latest research papers from Korea wherescientists have obtained a new record for the secondary emission coefficient: 22,000!

Thus reason dictates that the proper coating must be selected for the electrodes. Then the

rest of the components must be selected and positioned so that the size, frequency andangle of impact are optimal.

I think I heard somebody say, Hey, Einsteinit still has to be hooked up to electricity tostart up and to power the attracting electrodes. What about that?

The answer lies in the principle of feedback and self-oscillation. We know that manyoscillators are known to exhibit the characteristic of self-oscillation. Once you get them

going then they tend to keep on oscillating on their own. This process works in

multipactor-oscillators. It just takes a little electricity to get them started and then the

internal processes take over and they self-oscillate, producing electrons without the inputof outside electricity.

Many electronic devices apply the principle of feedback, especially in audio devices. Wecan remember Jimi Hendrix hitting a note on his guitar and then holding the guitar in

front of his amplifier. The amps sound creates a feedback loop with the guitar and a

sound is created which is self-sustaining without the additional input of playing anothernote. Numerous transistors work with feedback loops to take the electrical output of the

device and split that output and send part of it back to the original input where it is again

amplified. So the coupling of the output to the input wires is what is required.


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So using either feedback or self-oscillation or both a multipactor device can be fabricated

so that it will have self-sustaining output of electrons.

That still leaves us the positive bias charge that is placed upon the attracting electrodes to

accelerate those sluggish clouds of secondary electrons.

Again, we simply split the output signal and loop part of it back to the accelerating

electrodes, and this is the positive charge remember. So the negative charge goes back tofeedback the input and the other loop goes to the voltage multiplier. A Cockroft-Walton

multiplier can be either positive or negative in chargeyou simply reverse the

connection between the diodes and capacitors and it multiplies the positive charge.

Therefore, we could use batteries to start up the multipactor and then apply common

electronics components and devices to split the output and loop it back to the input and

bias the positive electrodes. Then the battery can be shut off, and even recharged whilethe multipactor runs on self-sustaining current.

That guy who used Einsteins name like a dirty word again wants to voice his opinion,Hey, genius, this stuff is a bunch of hooey! How do we know this would work?

How do we know?Because of TELEVISION.

These multipactor devices were invented by Philo Farnsworth when he invented

television. Just one glance at this super-egghead fellow should give you the answer. This

guy was a super-brain and he just NEEDED to have special vacuum tubes to strengthenthe broadcast signal of television from remote locations to make the picture tubes bright

enough to seeso he simply invented multipactors to multiply that weak input signal.

Check out that cranium!


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If these multipactors work then why didnt Farnsworth take over the whole world?

The reason is related to the laws of business and not the laws of physics. PhiloFarnsworth saw the value of television and his multipactors but he had an independent

streak which caused him to form his own Farnsworth Television company with which he

intended to put RCA and GE out of business. Instead they put Farnsworth out of businessby using monopoly tactics. But Philo Farnsworth applied his principles based upon

secondary electron emission to the point that he invented a nuclear fusion reactor before

he was through.

The heyday of vacuum tubes was filled with imitators of every sort. There is even an

International Patent classification which contains only Farnsworth Tubes.

Since Farnsworths day the vacuum tube was supplanted by the Japanese transistor and

then the Silicon Valley semiconductor chip. Nobody makes vacuum tubes anymore and

the vacuum tube multipactor concepts have been lost in the world of microelectronics.But even today secondary electron emission is applied in the plasma television sets where

scores of little holes and dots are brightened by electron multiplication. Other areas such

as scintillation counters and electron detectors and night vision goggles use the process,often in the solid-state configuration. There exists an offshoot applying vacuum tubes

the sector called PHOTONICS which use vacuum tubes to multiply light into electronic

signals.

As stated above, there are several basic methods of achieving multiple impacts of

electrons.


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The BACK AND FORTH method:


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and the CIRCULAR ACCELERATOR method:

Secondary Emission Coefficients

These graphs and excerpts were developed over a period of time. The more ancient the

research the lower the coefficients. As newer and newer materials were invented and

tested there is a general trend toward higher and higher coefficients. I would respectfullycall your attention to the source material in the following sections for detailed analyis of

the methodology and results of individual studies and devices with various emissive

materials.

Attention should be paid to the voltage required to obtain a certain coefficient ofsecondary electron multiplication. The graphs are not in parallel so they are slightlydifferent pictures. But they should give a general idea of how much electron

multiplication could be obtained by a particular substance.


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The following chapters will discuss individual studies and patents. Some of theseresource documents contain excellent discussion of historical development of the

secondary electron emission devices. It is of note that secondary electron emission was

first discovered about a century ago, and the first patent for a vacuum tube as applied for

in 1919.

The discussion also includes mention of work factor as an indicator of secondaryemission coefficient. The lower the work factor the higher the coefficient.

Another area of interest is that of negative electron affinity as an explanation forsecondary electron emission. In short, the term affinity implies that a particular substance

either likes or rejects electrons. The materials with negative electron affinity then are

predisposed to not like negatively-charged particles and thus reject them when

bombarded.

Treatises on vacuum tubes have been consulted and quoted in pertinent part. Patents areinserted to this anthology to examine their significance at particular points in time.Various studies on the individual materials exhibiting secondary emission.

Finally, I include several of my own designs for multipactor devices to power electricalappliances and motors.


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The photomultiplier is a very versatile and

sensitive detector of radiant energy in theultraviolet, visible, and near infrared regionsof the electromagnetic spectrum. A schemat-ic diagram of a typical photomultiplier tubeis given in Fig. 1. The basic radiation sensoris the photocathode which is located inside avacuum envelope. Photoelectrons are emit-ted and directed by an appropriate electricfield to an electrode or dynode within theenvelope. A number of secondary electronsare emitted at this dynode for each imping-ing primary photoelectron. These secondaryelectrons in turn are directed to a seconddynode and so on until a final gain ofperhaps 10

6is achieved. The electrons from

the last dynode are collected by an anodewhich provides the signal current that is readout.

PHOTOELECTRONS

Fig. Schematic representation of a photo-multiplier tube and its operation

For a large number of applications, thephotomultiplier is the most practical or sen-sitive detector available. The basic reasonfor the superiority of the photomultiplier isthe secondary-emission amplification thatmakes it possible for the tube to approachideal device performance limited only bythe statistics of photoemission. Amplifica-

tions ranging from 103

to as much as 108

provide output signal levels that are com-patible with auxiliary electronic equipment

without need for additional signal amplifica-tion. Extremely fast time response with risetimes as short as a fraction of a nanosecondprovides a measurement capability in specialapplications that is unmatched by otherradiation detectors.

EARLY DEVELOPMENT

The development (history) of the photo-multiplier is rooted in early studies of secon-dary emission. In 1902, Austin and Starke

1

reported that the metal surfaces impacted bycathode rays emitted a larger number of elec-trons than were incident. The use of secon-dary emission as a means for signalamplification was proposed as early as1919.2 In 1935, Iams and Salzberg

3of RCA

reported on a single-stage photomultiplier.The device consisted of a semicylindrical

photocathode, a secondary emitter mountedon the axis, and a collector grid surroundingthe secondary emitter. The tube had a gainof about eight. Because of its better frequen-cy response the single-stage photomultiplierwas intended for replacement of thefilled phototube as a sound pickup formovies. But despite its advantages, it sawonly a brief developmental sales activitybefore it became obsolete.

Multistage Devices

In 1936, Zworykin, Morton, and Malter,all of reported on a multistagephotomultiplier. Again, the principal con-templated application was sound-on-filmpickup. Their tube used a combination ofelectrostatic and magnetic fields to directelectrons from stage to stage. A photographof a developmental sample is given in Fig. 2.Although the magnetic-type photomultiplierprovided high gain, it had several dif-ficulties. The adjustment of the magnetic

field was very critical, and to change the gainby reducing the applied voltage, themagnetic field also had to be adjusted


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Photomultiplier Handbook

Another problem was that its rather wideopen structure resulted in high dark currentbecause of feedback from ions and lightdeveloped near the output end of the device.For these reasons, and because of thedevelopment of electrostatically focusedphotomultipliers, commercialization did notfollow.

Fig. 2 Magnetic-type multistagereported by Zworykin, Morton, andin 1936.

The design of multistage electrostaticallyfocused photomultipliers required ananalysis of the equipotential surfaces be-tween electrodes and of the electron trajec-tories. Before the days of high-speed com-puters, this problem was solved by amechanical analogue: a stretched rubbermembrane. By placing mechanical models of

the electrodes under the membrane, theheight of the membrane was controlled andcorresponded to the electrical potential ofthe electrode. Small balls were then allowedto roll from one electrode to the next. Thetrajectories of the balls were shown to cor-respond to those of the electrons in the cor-responding electrostatic fields. Workingwith the rubber-dam analogue, both J.R.

of Bell Laboratories and J.A.of RCA devices linear arrays of

electrodes that provided good focusing prop-

erties. Although commerical designs did notresult immediately from the linear dynodearray, The Rajchmann design with somemodifications eventually was, and still is,used in photomultipliers-particularly forhigh-gain wide-bandwidth requirements.

First Commercial Devices

The first commercially successfulphotomultiplier was the type 931. This tubehad a compact circular array of ninedynodes using electrostatic focusing. The

first such arrangement was described byZworykin and Modificationswere later reported by Rajchmann and

Snyder8and by Janes and all of

RCA. The basic electron-optics of the cir-cular cage was thus well determined by 1941and has not changed to the present timealthough improvements have been made in

processing, construction, and performanceof the 931A product.

The success of the 931 type also resultedfrom the development of a much improvedphotocathode, Cs3Sb, reported by Gorlich

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in 1936. The first experimentalmultipliers had used a Ag-O-Csode having a typical peak quantum efficien-cy of 0.4% at 800 nm. (The Ag-O-Cs layerwas also used for the dynodes.) The new

photocathode had a quantum effi-ciency of 12% (higher today) at 400 nm. It

was used in the first 931s, both as aphotocathode and as a secondary-emittingmaterial for the dynodes.

PHOTOEMITTER ANDEMITTER DEVELOPMENT

Photocathode Materials

Much of the development work onphotomultiplier tubes has been concerned

with their physical configuration and therelated electron optics. But a very importantpart of the development of photomultipliertubes was related to the photocathode andsecondary-emission surfaces and their pro-cessing. RCA was very fortunate during the1950s and 60s in having on its staff, prob-ably the worlds foremost photocathode ex-pert, Dr. A.H. Sommer. His treatise onPhotoemissive continues to pro-vide a wealth of information to all

photocathode process engineers.Sommer explored the properties ofnumerous photocathode materials-par-ticularly alkali-antimonides. Perhaps hismost noteworthy contribution was themultialkali photocathode (S-20 spectralresponse). This photocathode,is important because of its high sensitivity inthe red and near infrared; the earlier Cs3Sbphotocathode spectral response barely ex-tends through the visible, although it is verysensitive in the blue where most scintillators

emit.Bialkali photocathodes were also de-

veloped by Sommer and have proven to be


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better in some applications than thephotocathode. Thus, the cathode has been found to be stable at highertemperatures than and, in addition,has a very low dark (thermal) emission. It

has been particularly useful inlogging applications. Another bialkalitocathode, is more sensitive than

in the blue and is, therefore, used byRCA to provide a better match to the

crystals used in scintillation count-ing.

Dynode Materials

The first secondary-emission materialused practically by RCA was the Ag-O-Cs

surface. But with the development of thematerial for photocathodes, it was

found that this material was also an excellentsecondary emitter. Other practical secondaryemitters developed during the early years ofphotomultiplier development were MgO:Cs(often referred to as silver-magnesium)and BeO:Cs (copper-beryllium).

In the early R.E. whileworking at the RCA Laboratories developedhis revolutionary concept of Negative Elec-tron Affinity Electron affinity is theenergy required for an electron at theconduction-band level to escape to thevacuum level. By suitably treating the sur-face of a p-type semiconductor material, theband levels at the surface can be bentdownward so that the effective electron af-finity is actually negative. Thermalized elec-trons in the conduction band are normallyrepelled by the electron-affinity barrier; theadvantage of the NEA materials is that theseelectrons can now escape into the vacuum as

they approach the surface. In the case ofsecondary emission, secondary electrons canbe created at greater depths in the materialand still escape, thus providing a muchgreater secondary-emission yield. In the caseof photoemission, it has been possible toachieve extended-red and infrared sen-sitivities greater than those obtainable withany other known materials. The first prac-tical application of the NEA concept was tosecondary emission.

Simon and Williams

An early paper bydescribed the theory

and early experimental results ofemission yields as high as 130 at 2.5 forG C

Introduction

APPLICATIONS DEVELOPMENT

Astronomy and Spectroscopy

Early applications of the photomultiplierwere in astronomy and spectroscopy.Because the effective quantum efficiency ofthe photomultiplier was at least ten timesthat of photographic film, astronomers werequick to realize the photomultiplier tubesadvantage. Furthermore, because the outputcurrent of the photomultiplier is linear withincident radiation power, the tube could beused directly in photometric andtrophotometric astronomy. The type 1P28, atube similar to the 931 but having anultraviolet-transmitting envelope was par-ticularly useful in spectroscopy. The size and

shape of the photocathode were suitable forthe detection and measurement of line spec-tra and the very wide range of available gainproved very useful.

14

Radar Jammer

A totally unexpected application for thenew photomultiplier tube occurred duringWorld War II. The development of radar fordetecting and tracking aircraft led to thesimultaneous need for

noise sources as radar jammers. Althoughother sources of noise were tried, thephotomultiplier proved to be most suc-cessful. The advantage of the tube was itshigh gain and wide band width (severalhundred MHz). As a noise source the tubewas operated with a non-modulated inputlight source and with high gain. The outputamplifier photoelectric shot noise waswhite and thus indistinguishable fromnatural noise sources. This application ofphotomultiplier tubes resulted in productionof thousands per month compared withprevious production measured in only hun-dreds per year.

Scintillation Counting

A proliferation of photomultiplier designsfollowed the invention of the scintillationcounter shortly after World War II.

15,16The

photomultiplier tubes were designed withsemitransparent photocathodes deposited onan end window which could be coupled

directly to the scintillator. The principalscintillator used, doped with thallium,was discovered by Much ofh d l k h l i li


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tubes during this period was reported byRCA and its competitors in the biannualmeetings of the Scintillation Counter Sym-posium. These symposia were reported fullyin the IRE (and later the IEEE) Transactions

on Nuclear Science beginning with themeeting in Washington, January 1948. Thescintillation counter became the most impor-tant measurement instrument in nuclearphysics, nuclear medicine, and radioactivetracer applications of a wide variety.

Headlight Dimmer

During the RCA collaborated withthe General Motors Company (Guide-LampDivision) on a successful headlight dimmer.The photoelectric headlight dimmer-first

made available only on Cadillacs andOldsmobiles-basically used a tube similarto the but redesigned and tested to theauto manufacturers particular require-ments. The optical engineering problem wasto sense the oncoming headlights or tail-lights being followed without responding tostreet and house lights. Vertical and horizon-tal angular sensitivity was designed to matchthe spread of the high beams of the automo-bile. A red filter was installed in the optical

path to provide a better balance between sen-sitivity to oncoming headlights and tolamps being followed. The device achieved aremarkable success, probably because of thenovelty, and thousands of photomultipliertubes were used. But today, one rarely sees aheadlight dimmer.

Medical Diagnostic Equipment

In recent years two medical applicationshave used large numbers of photomultipliertubes and have spurred further develop-ments and improvements. The gamma

is a sophisticated version of the scin-tillation counter used medically for locatingtumors or other biological abnormalities. Aradioactive isotope combined in a suitablecompound is injected into the blood streamor ingested orally by the patient. Theradioactive material disintegrates and gam-ma rays are ejected from preferential loca-tions such as tumors or specific organs. Alarge crystal intercepts the gamma rays and

scintillates. Behind the crystal aremultiplier tubes, perhaps 19, in hexagonalarray The location of the point of scintilla-

depending upon the individual signals fromeach of the photomultipliers. Counting iscontinued until several hundred thousandcounts are obtained and the organ in ques-tion is satisfactorily delineated. The location

of each scintillation is represented by a pointon a cathode-ray-tube presentation.

The Computerized Axial Tomographic(CAT) scanner was introduced to this coun-try in 1973. The device uses a pencil orbeam of X-rays which rotates around the pa-tient providing X-ray transmission datafrom many directions. A scintillator coupledto a photomultiplier detects the transmittedbeam-as an average photomultiplier cur-rent-and a computer stores and computes

the cross-section density variation of the pa-tients torso or skull. The photomultipliersare1/ or 3/4-inch end-on tubes whichcouple to the scintillator, commonly BGO(bismuth germanate). Each unit is equippedwith as many as 600 photomultipliers.

PHOTOMULTIPLIERS ANDSTATE DETECTORS COMPARED

In some applications either aplier or solid-state detector could be used.The user may make his choice on the basis of

factors such as cost, size, or previous ex-perience. In other applications, the choicemay be dictated by fundamental propertiesof the photomultiplier or the solid-statedetector. A discussion follows of some of thecommon applications favoring one or theother detector with reasons for the choice. Asummary presents the principal considera-tions the user must apply in making a choicein an application for which he requires aphotodetector. This information should be

particularly useful to the designer who is notwell acquainted in this field.

Photomultiplier Features

The photomultiplier is unique in its abilityto interface with a scintillation crystal andnot only count the scintillations but measuretheir magnitude and time their arrival. Mostscintillators emit in the blue and near ultra-violet. This spectral output obviously favorsthe photomultiplier having a photocathodewith high quantum efficiency in the short

wavelength range. On the other hand a sili-con p-i-n diode is relatively poor in this partof the spectrum but does best in the red and


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probably, is the gain of the photomultiplierwhich permits the measurement of the verysmall signals from individual scintillationswith a good signal-to-noise ratio, limitedprimarily by the statistics of the number of

photoelectrons per pulse. Finally, the shortrise time of the photomultiplier using fastscintillators permits time-of-flight measure-ments to be made in nuclear physics.

Although the CAT scanner equipmentalso uses photomultipier tubes to detect thescintillations in bismuth germanate (BGO)crystals, the situation is somewhat differentfrom the scintillation counting applicationsdiscussed above. In the CAT scanner theX-rays produce a broad band of pulse

heights and no attempt is made to single outand detect single scintillation events. Thephotomultiplier is used in an analog mode todetect the level of radiation incident on thecrystal. In the CAT scan operation thetypical machine scans the patient in a fewseconds and the level of irradiance from thecrystal onto the photomultiplier is relativelyhigh so that only a relatively low gain photo-multiplier is required. Furthermore, thespeed of response requirement for the

photomultiplier is relatively modest-per-haps a few hundred microseconds. Still, theprincipal advantage of using a photomulti-plier in this application for the detection ofthe radiant signal is its good signal-to-noiseratio. This ratio is very important to the pa-tient because a reduction in itsnoise ratio would have to be made up forwith an increased X-ray dose. Nevertheless,there is interest and development activityaimed at replacing the photomultiplier withsilicon p-i-n detectors. Two factors could

favor the alternate use of a silicon cell: (1) abetter scintillator (BGO is almost an order ofmagnitude less sensitive than (2) afaster scanning machine (a very desirabletechnological advance because is wouldminimize effects of body motions). Both ofthese factors would result in a largerphotocurrent and could bring the signal levelfor the silicon detector to the point where thefundamental signal-to-noise ratio from theX-ray source would not be degraded. Such

developments may be anticipated becausethe silicon detector would also have the ad-vantage of smaller size and perhaps lower

Introduction

As a result of increasing concern about en-vironment, pollution monitoring is becom-ing another important application for photo-multiplier tubes. For example, in the moni-toring of NOx the gas sample is mixed with

O 3 in a reaction chamber. Ainescence results which is measured using anear-infrared-pass filter and a photomulti-plier having an S-20 spectral response. Al-though the radiation level is very low, NOcan be detected down to a level of 0.1 ppm.The advantage of the photomultiplier in thisapplication is again the high gain and goodsignal-to-noise ratio (the photomultiplier iscooled to 0C to reduce dark-current noise)even though the radiation spectrum is ob-served near the threshold of the S-20 spectralrange.

In another pollution-monitoring applica-tion, is detected down to a level of 0.002ppm. Here, the sample containing SO2 is ir-radiated with ultraviolet and the excited SO2molecules fluoresce with blue radiation thatis detected with a combination of aband filter and photomultiplier. Very weaksignals are detected and again it is the highgain, good signal-to-noise ratio and, in addi-tion, good blue sensitivity which makes the

detection and measurement of small contam-inations of SO2 possible.

Spectroscopy is one of the very early ap-plications for photomultipliers. The widerange of radiation levels encountered isreadily handled by the approximately loga-rithmic gain variation of the photomultiplierwith voltage. At very low signal levels, thesignal-to-noise capability of the photomulti-plier is essential. Because photomultiplierspectral response (with quartz or

transmitting-glass windows) covers the rangefrom ultraviolet to near infrared, thephotomultiplier is the logical choice for spec-troscopic applications, except in the infraredregion of the spectrum.

Photocell* FeaturesBecause of their small size and low cost,

and type photocells are the logicalselection for applications such as automaticexposure control in photographic cameras orvarious inspection and counting require-ments.*Photocell is used here to indicate a photosensitivedevice in which the charge transport takes place througha solid as compared with in which the


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Many p-i-n silicon cells are used in com-bination with lasers or LEDs (light emittingdiodes). Here, one of the principal advantagesof the silicon cell is its good response in thenear infrared out to 1100 nm. In combina-

tion with the Nd:YAG laser emitting at 1060nm, the silicon cell is used widely in laserranging and laser tracking. A similar ap-plication utilizes an LED emitting near 900nm with a silicon cell for automatic rangingfor special camera equipment. Size and in-frared sensitivity are again the importantqualifications.

A rapidly growing application for photo-cells is for fiber-optic communicationsystems. LEDs are coupled to the fibers and

the detector may be a p-i-n diode or, for abetter signal-to-noise ratio, a silicon ava-lanche diode. The qualifying attributes forthe choice of detector are size, near infraredsensitivity, adequate speed of response, andgood signal-to-noise ratio.

Smoke detectors now use large numbers ofLEDs and p-i-n silicon cells. Again size,cost, and infrared sensitivity are the impor-tant qualifications.

Characteristics Comparison Summary

Spectral Response. Photomultipliers canbe obtained with good spectral sensitivity inthe range 200 to 900 nm. Silicon cells haverather poor blue sensitivity, but are excellentout to 1100 nm. In general, then, the photo-multiplier is to be preferred for applicationsinvolving the shorter wavelengths, althoughother factors may override this considera-tion.

Speed of Response. If very fast response isrequired, the photomultiplier is usually the

best choice of a detector. Photomultipliersare available with rise times (10 to 90%) of 1or 2 nanoseconds using a load. Theinherent rise time of silicon cells may be inthe range 10 to 20 nanoseconds, dependingupon the area of the cell. However, becauseof the cells capacitance, the effective risetime is much longer depending upon thechoice of load resistance. For example, witha load resistance, the rise timemay be of the order of 20 microseconds. Afairly large load resistance must be chosen to

maintain good signal-to-noise characteristicsfor the silicon cell. Silicon avalanche photo-diodes can have rise times as short as 2

diode can be of the order of 100, but the sen-sitive area is small-about 0.5 squaremillimeter.

Sensitive Area. Photomultiplier tubes aremade in a variety of sizes so that many dif-

ferent optical configurations can be accom-modated. The largest photocathode areaavailable in commercial RCAplier tubes has a nominal diameter of 5 in-ches and a minimum useful area of 97 squarecentimeters. By way of contrast, the 1/2-inchside-on photomultiplier has a projectedtocathode area of 0.14 square centimeter.Silicon p-i-n diodes are available with sen-sitive areas generally not larger than 1 squarecentimeter; and avalanche silicon cells, 0.005

square centimeter. In many applications, afairly large area is required, e.g., coupling toa cathode-ray tube ora large scintillator.This requirement generally indicates the useof a photomultiplier tube. Silicon cells are atan advantage when the source is small fordirect coupling or for lens imaging.

Temperature. Photomultipliers are gener-ally not rated for operation at temperatureshigher than 75 C. Exceptions aretipliers having a Na2KSb photocathode. This

bi-alkali photocathode can tolerate temper-atures up to 150 C or even higher for shortcycles. In oil-well logging measurements thisconsideration is important. Photocathodesensitivities and gain change very little withtemperature, but dark current does increaserapidly. Dark currents at room temperatureare of the order of 10 ampere at thetocathode and double about every 10 C.Silicon cells are rated from 50 to 80 C.Sensitivities are also relatively independentof temperature. But dark current which maybe 10-7 ampere at room temperature, alsotends to double about every 10 C.

Signal-to-Noise Ratio. At very low lightlevels, the limitation to detection andmeasurement is generally the signal-to-noiseratio. One way of describing the limit todetection is to state the Equivalent Noise In-put or the Noise Equivalent Power

The NEP is the power level into thedevice which provides a signal just equal tothe noise. Most often the bandwidth is

specified as 1 hertz and the wavelength of themeasurement is at the peak of the spectralresponsivity ENI is the same type of specifi


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be luminous flux.For a photomultiplier such as one used for

spectroscopy, the NEP at room temperatureat nm is about 7 x 10-16 watts, or theEN1 is about 7 x 10 lumens. Both

specifications are for a l-hertz banFor a p-i-n silicon photocell, the NEP at 900nanometers may be of the order of 2 x10- watts, or the EN1 of 1.5 x 10-

lumens. Both values are for a l-hertz band-width. Thus, the photomultiplier is clearlysuperior in this category. Also it should bepointed out that the silicon diode must becoupled into a load resistance of about 5megohms in order to avoid noise dominationfrom the coupling resistor. Unfortunately,this large resistance then increases the effec-tive rise time of the silicon device to about100 microseconds. The NEP of a silicon ava-lanche photodiode is about 10-14 watt at900 nanometers or the is 8 x 10 lumens, both for a l-Hz bandwidth. Thelumen in these descriptions is that from atungsten source operating at 2856 K colortemperature. Peak emission for such asource is near 1000 nm and thus closelymatches the spectral peak of the silicondevices.

Gain. A photomultiplier can have a gainfactor, by which the fundamentalcathode signal is multiplied, of from 10

3to

108. Silicon avalanche photodiodes have a

gain of about 100. Silicon p-i-n diodes haveno gain. The high gain of theplier frequently eliminates the need ofspecial amplifiers, and its range of gain con-trolled by the applied voltage provides flex-ibility in operation.

Stability. Photomultiplier tubes are notnoted for great stability although for lowanode currents and careful operation theyare satisfactory. When the light level isreasonably high, however, the very goodstability of the silicon p-i-n cell is a con-siderable advantage. The silicon cell makes aparticularly good reference device for thisreason. In fact, the National Bureau of Stan-dards has been conducting special calibra-tion transfer studies using p-i-n silicondiodes.

REFERENCES

1. H. Bruining, Physics and applicationsof secondary electron emission,Hill Book Co Inc ; 1954)

Introduction

2. J. Slepian, U.S. Patent 1, 450, 265,April 3, 1923 (Filed 1919).3. H. E. Iams and B. Salzberg, The

secondary emission phototube, IRE,Vol. 23, pp. 55-64 (1935).

4. V.K. Zworykin, G.A. Morton, and L.Malter, "The secondary-emission multiplier-a new electronic device, IRE,Vol. 24, pp. 351-375 (1936).5. J .R. Pi erce , Electron-multiplier

design, Bell Lab. Record, Vol. 16, pp.305-309 (1938).6. J.A. Rajchman, Le courant residue1

dans les multiplicateurs delectronstrostatiques, These LEcole PolytechniqueFederale (Zurich, 1938).7. V.K. Zworykin and J.A. Rajchman,

The electrostatic electron multiplier,IRE, Vol. 27, pp. 558-566 (1939).8. J.A. Rajchman and R.L. Snyder, An

electrostatically focused multiplierphototube, Electronics, Vol. 13, p. 20(1940).9. R.B. Janes, and A.M. Glover, Recent

developments in phototubes, RCA Review,Vol. 6, pp. 43-54 (1941). Also, A.M. Glover,A review of the development of sensitivephototubes, IRE, Vol. 29, pp.

413-423 (1941).10. P. Gorlich, Uber zusammengesetzte,durchsichtige Photokathoden, 2. Physik,Vol. 101, p. 335 (1936).11. A.H. Sommer, Photoemiss ivematerials, John Wiley and Sons; 1968.12. R.E. Simon, Research in electronemission from semiconductors, QuarterlyReport, Contract DA 36-039-AMC-02221(E) (1963).13. R.E. Simon and B.F. Williams,Secondary-electron emission, IEEE

Trans. Sci., Vol. NS-15, pp. 166-170(1968).14. M.H. Sweet, Logarithmictiplier tube photometer, JOSA, Vol. 37, p.432 (1947).15. H. Kallmann, Natur u (July1947).16. J.W. and F.H. Marshall, Aphotomultiplier radiation detector,Rev., Vol. 72, p. 582 (1947).17. R. Hofstadter, Alkali halide scintilla-tion counters, Phys. Rev., Vol. 74, p. 100(1948).18. H.O. Anger, Scintillation camera,Rev Sci Instr Vol 29 pp 27 33 (1958)


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PHOTOEMISSION

The earliest observation of a photoelectriceffect was made by Becquerel in 1839. Hefound that when one of a pair of electrodesin an electrolyte was illuminated, a voltage

or current resulted. During the latter part ofthe 19th century, the observation of aphotovoltaic effect in selenium led to thedevelopment of selenium and cuprous oxidephotovoltaic cells.

The emission of electrons resulting fromthe action of light on a photoemissive sur-face was a later development. Hertz dis-covered the photoemission phenomenon in1887, and in 1888 Hallwachs measured thephotocurrent from a zinc plate subjected toultraviolet radiation. In 1890, Elster and

Geitel produced a forerunner of the vacuumphototube which consisted of an evacuatedglass bulb containing an alkali metal and anauxiliary electrode used to collect thenegative electrical carriers (photoelectrons)emitted by the action of light on the alkalimetal.

Basic Photoelectric Theory

The modern concept of photoelectricitystems from Einsteins pioneer work forwhich he received the Nobel Prize. Theessence of Einsteins work is the followingequation for determining the maximumkinetic energy E of an emitted photoelec-tron:

Eq. (1) shows that the maximum energy ofthe emitted photoelectron is propor-tional to the energy of the light quanta

must be given to an electron to allow it to

escape the surface of a metal. For eachmetal, the photoelectric effect is character-

In the energy diagram for a metal shownin Fig. 3, the work function represents theenergy which must be given to an electron atthe top of the energy distribution to raise itto the level of the potential barrier at themetal-vacuum interface.

METAL

FERMIENERGY

Fig. 3 Energy mode/ fora metal showingthe relationship of the work function and theFermi level.

According to the quantum theory, onlyone electron can occupy a particular quan-

tum state of an atom. In a single atom, thesestates are separated in distinct shells; nor-mally only the lower energy states are filled.In an agglomeration of atoms, these statesare modified by interaction with neighboringatoms, particularly for the outermost elec-trons of the atom. As a result, the outerenergy levels tend to overlap and produce acontinuous band of possible energy levels, asshown in Fig. 3.

The diagram shown in Fig. 3 is for a

temperature of absolute zero; all lowerenergy levels are filled. As the temperature isincreased, some of the electrons absorb ther-


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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

European Laboratory for Particle Physics

A SUMMARY OF MAIN EXPERIMENTAL RESULTS CONCERNING

THE SECONDARY ELECTRON EMISSION OF COPPER

V. Baglin, I. Collins, B. Henrist, N. Hilleret and G. Vorlaufer

The secondary electron emission of surfaces exposed to the impact of energetic electrons contributes

significantly to the electron cloud build-up. For the prediction of the consequences of this effect the

measurements of the secondary electron yield carried out at CERN are an important source of

information. New experimental results concerning the total secondary electron yield for very low

primary electron energy (between 5 eV and 50 eV) will be also given in the case of as received copper.

Furthermore the energy distribution of the re-emitted electrons is drastically influenced by the primaryelectron energy. The ratio of the number of reflected electrons to the total number of re-emitted electrons

has been measured and its variation with the primary electron energy will be shown. As a consequence

of these new experimental data, a numerical approximation to express the secondary electron yield as a

function of the primary electron energy will be given for the low incident electron energy region

(E


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1 THE EXPERIMENTAL SYSTEMS

The measurements of the secondary electron yield versus electron energy have been carried out

in two separate systems described previously1,2

. They consist in an electron gun delivering a

focused primary electron beam of energy variable between 45 and 3000 eV. This beam hits thesurface of the sample under normal incidence at a circular spot (approximately 3 mm diameter). A

conical cage, coaxial to the primary beam, collects the secondary electrons. The deflection plates

and lens of the electron gun are driven by a computer program which optimises their settings on adummy sample to maximise the transmitted primary current. For the measurements short pulses

(30 ms) of low intensity (some nA) are used to reduce the electron dose received by the sample

(10 nC/mm2 for a complete energy scan). The vacuum system is bakeable and is evacuated by a

turbomolecular pump. After bake out the pressure reached in the system is in the low 10-10

Torr

region.

In EPA1,3

the second system used was based on the same principle as the laboratory system, the

measurement procedure (beam optimisation, current pulses and data handling) was identical.

Figure 1: Variation of the average secondary electron yield versus electron energy for 25 as received

copper samples

For the energy measurements, a 4 grids hemispherical energy analyser has been used. Its

energy resolution is limited to some eV and the energies given do not take into account any

correction of contact potential between the filament gun and the sample. The dose effect was

studied using various procedures. Initially (1978) the electron dose was delivered to the sample

using the measurement gun. The yield was continuously measured at the bombardment energy.

Because of the rapid destruction of the expensive gun filament, this method was abandoned and a

rustic flood gun was added to the laboratory system allowing to irradiate completely the sample

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

0 500 1000 1500 2000 2500 3000

Ene rgy (e V)


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surface. In EPA the electron dose was accumulated by biasing the sample to a variable positive

voltage that attracted the photoelectrons created by the synchrotron light.

A change in the bias voltage modifies the mean energy of the incident electrons as well as their

amount. As the sample bias voltage changes the collection efficiency of the sample, the photon dose

needed to condition the sample are not directly comparable when the sample bias is modified. The

dose is determined by integrating the current collected on the sample. The main difference between

these two methods to accumulate the dose is that in EPA any artefact linked to the presence of a hot

filament for the production of the impinging electron has been eliminated.

Figure 2: Variation of the average secondary electron yield versus electron energy for copper samples

and energies lower than 30 eV

2 THE VARIATION OF THE SECONDARY ELECTRON YIELD OF COPPER VERSUSTHE PRIMARY ELECTRON ENERGY

Figure 1 shows the mean secondary electron yield (S.E.Y.) measured on 25 as received copper

samples cleaned following the LHC recipe. The average maximum is 2.06 ( = 0.16) for an energyof 271 eV ( = 25). The bars are used to display the spread between the minimum and maximumS.E.Y. at each energy for these 25 samples. The low energy part of the (E) curves was measuredrecently and is displayed in Figure 2 for various electron doses (cf. next section). The two curves

labelled 23/03 AR and 31/01 AR show the SEY as a function of the energy for electron energies

between 4 eV and 30 eV. The SEY at 4 eV lies between 0.6 and 0.8. Below 10 eV the as received

curves tend to indicate a constant value of the SEY, independent of the electron energy.

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20 25 30

Ene rgy (e V)

23-01-01/ A.R.

31-01-01/ A.R.

23-03-01 /5.6 e-4

31-01-01/ 5.6 e-4

31-01-01/ 1.01 e-2

23-03-01 / 9.6 e-3


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3 THE CHANGE OF THE SECONDARY ELECTRON EMISSION WITH THE

INCIDENT ELECTRON DOSE

The variation of the SEY with the electron dose is an effect leading, when not properly taken

into account, to underestimated secondary electron yield. This brought us to modify the

experimental set up and procedure in order to decrease as much as possible the electron dose

received by the sample during the measurements4. In Chamonix X

1it was proposed to take profit

of this effect to obtain the decrease of the copper SEY necessary to operate LHC without electroncloud effect.

Figure 3: The variation of the secondary electron yield with the incident electron dose

In Figure 3, the variation of the SEY of as received copper is plotted against the incident

electron dose for various measurements made using various procedures and under various

experimental conditions:

The curve noted electron gun was obtained in 1979 using an electron gun on a coppersample. It gives the SEY of copper at 500 eV energy versus the electron dose. The

impinging electrons had an energy of 500 eV. The area irradiated by the beam was a circle

of approximately 2 mm diameter.

The curve note delta max EPA gives the variation of the maximum SEYagainst the doseof electron collected with a bias of 99 V (i.e. 99 eV impinging electron energy).

The 2 curves noted delta max 23-03 and delta max 31-01 were obtained this year usinga flood gun irradiating the whole sample with an electron energy of 500 eV.

These curves show that an electron dose between 8x 10-4

and 2x 10-3

C/mm2

is necessary to

reach a SEY lower than 1.3. The minimum of the SEY (close to 1.1) is obtained for an electrondose close to 10

-2C/mm

2.(fully conditioned sample).

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02

D o se ( C / m m 2 )

DELTA MAX 23-03

DELTA MAX 31-01

ELECTRON GUN measured a t 500 eV

DELTA MAX EPA 99V


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In Figure 2 the low energy part of the two later curves (23-03 and 31-01) are also shown. The

reproducibility is very good, the SEY are continuously decreasing below 10 eV and are close to 0.2

at an energy of 4 eV.

4 THE ENERGY DISTRIBUTION OF THE SECONDARY ELECTRONS,

EXPERIMENTAL RESULTS AND CURVES REFLECTED/TRUE SECONDARIES

VERSUS INCIDENT ELECTRON ENERGY

The preceding measurements have shown the peculiar behaviour of the SEY as the primaryelectron energy decreases. These peculiarities could be related to the change in the shape of the

secondary electron energy distribution. This distribution was studied using a 4 grids hemispherical

energy analyser. The energy axis is shifted by an unknown amount (< 3eV) due to the unknown

contact potential between the filament and the sample. It must be emphasised that, because of the

large amount of incident electrons needed for this type of measurement, the data presented here are

related to a copper close to the conditioned state.

Figure 4: Variation of the secondary electron yield versus the primary electron energy for 6 fully

conditioned (10-2

C/mm2) copper

In Figure 5 the energy distribution of secondary electrons emitted by a copper sample are

shown using two normalised axis: the abscissae are normalised to 1 at the incident electron energy,

the ordinates are normalised to 1000 for the maximum emitted intensity. On this graph the

increased importance of the reflected electrons at low primary electron energy is striking when the

curves measured at 10 eV and 550 eV are compared. A second fact, also reported in the literature 5,is illustrated in Figure 6 which shows a slight shift towards lower energy in the position of the

d l k h h i id l i i d

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 500 1000 1500 2000 2500 3000

ENERG Y (e V)

LAB SAMPLE 12

EPA 99 eV

LAB SAMPLE 13

EPA 800 eV

EPA 350 eV

LAB GUN 500 eV

ARGON G.D. COPPER


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The truesecondary electron peak is found around an energy of 2 eV. Using the data partially

presented in Figure 5 the ratio between the number of reflected electrons and the total number of

emitted electrons has been calculated for the five energies considered (10, 30, 100, 300, 550 eV).

This ratio is given in Figure 7 as a function of the primary electron energy.

5 NUMERICAL EXPRESSIONS FOR THE RELATION BETWEEN THE SECONDARY

ELECTRON YIELD AND THE INCIDENT ELECTRON ENERGY

Usual expressions given by M. Furman6

or J.J. Scholtz7

can be used to numerically express thevariation of the true secondary electron yield s with the primary electron energy (Ep). Bothformulae give good fits to the measured curves for a given energy range. The simple expression

given by M. Furman produces a reasonable fit in the low primary energy part (E p< 1000 eV) which

is the most interesting for LHC. In the low energy region (


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Figure 6: Secondary electron energy distribution for copper ( below 10 eV)

Figure 7: The ratio between the reflected and the total number of re-emitted electrons in the caseof copper

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6 8 10

ENERGY (eV)

Ep= 10 eV

Ep=30 eV

Ep= 100 eV

Ep=300 eV

Ep=550 eV

E max 550

Emax 10

1%

10%

100%

0 100 200 300 400 500 600

PRIM A RY ELEC TRO N EN ERG Y

REF/ TOT

FIT II

FIT II LOW ENERGY

EXP FIT


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The numerical value used to fit the experimental data presented in Figure 1 are given in

Table 1.

Table 1: Fit parameters for the true secondary yield (M.Furman formula)

SAMPLE STATE AS RECEIVED FULLY CONDITIONED

MAX 2.03 1.13EMAX 262 318

s 1.39 1.35

To introduce the correction due to the reflected electrons two approximations could be made

depending on the accuracy needed. Of course as it appears in Figure 7, the correction for reflected

electrons is only significant at energy lower than 300 eV and amounts to more than 10% for

energies below 100 eV. A relation7

allows to fit the experimental curve given in Figure 7:

ln(f) = A0 +A1 (ln(Ep +E0 )) +A2 (ln(Ep +E0 ))2 +A3 (ln(Ep +E0 ))

3

To obtain the best fit in the low energy part (below 300 eV), the following constants

has been used:A0 = 20.699890, A1= -7.07605, A2= 0.483547, A3= 0, E0=56.914686

(Curve labelled FIT II low energy).

For use up to higher primary electron energy (2000 eV), the following coefficients

should be used:

A0 = 0.300207076, A1= 0.044915014, A2= -0.155498672, A3= 9.50318 x 10-4

, E0=0

(Curve labelled FIT II)

A simplified exponential relation of the form:

f =R0 exp(Ep/w)

can also be used below 100 eV using the following numerical constants:

R0= .64438713, w=43.2268304. (Curve labelled EXP FIT)

The accuracy of the various approximations can be appreciated from Figure 7.

The various formulae used to account for the reflected electron contribution at low energy

combined with Furmans formula have been checked against the measured value of the total

secondary electron yield (t) in the two cases of as received copper and fully conditioned copper(dose = 10

-2C/mm

2). To calculate t, the following formula was considered:

t = S+ R ,

R = f t t = S+ f t

Hence: t = s 1

1 f( )

The results of the two fits are compared to the experimental results on the two Figures 8 (as

received case) and 9 (fully conditioned case). In both cases the agreement with the measured

secondary electron yield is good between 1000 and 100 eV incident energy. For energies greater

than 300 eV, the contribution of the reflected electrons can be neglected and M. Furman formula

used without correction. The low energy part (Ep < 100 eV) of the two graphs 8 and 9 is expanded

on the two graphs 10 and 11 to compare the results of the two fitting formulae to the experimental

results. Above 20 eV incident energy, both formulae give the same results. Below 20 eV (i.e. when

the reflected contribution accounts for more than 25% of the total number of secondary electrons)

the exponential fit gives increasingly underestimated value.


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Figure 8: The numerical expressions for the various contributions to the secondary electron yield

compared to the experimental measurements in the case of as received copper.

Figure 9: The numerical expressions for the various contributions to the secondary electron yieldcompared to the experimental measurements in the case of fully conditioned copper

C U A S REC EIV ED

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600 800 1000

ENERG Y (e V)

EXPERIMENTAL DATA (AVERAGE ON 25 SAM PLES)

FIT FURMAN

REFLECTED

FIT + REFLECTED

EXP FIT + REFLECTED

C U EXPOSED TO 0 .01 C/ m m 2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 200 400 600 800 1000

ENERGY (eV)

EXPERIMENTAL DA TA

FIT FURMAN

FIT+REFLECTED

REFLECTED

EXP FIT + REFLECTED


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compared to the experimental measurements in the case of as received copper (Ep < 100 eV)


compared to the experimental measurements in the case of fully conditioned copper (Ep < 100 eV)

C U A S REC EIV ED

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 20 40 60 80 100

ENERG Y (e V)

EXPERIMENTAL DA TA (AVERAGE

ON 25 SAMPLES)

FIT FURMAN

REFLECTED

FIT + REFLECTED

EXP FIT + REFLECTED

C U EXPO SED TO 0 .01 C / m m 2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100

ENERGY (eV)

EXPERIMENTAL DATA

FIT FURMAN

FIT+REFLECTED

REFLECTED

EXP FIT + REFLECTED


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6 COMPARISON WITH OTHER PUBLISHED DATA

Measurements of the secondary electron yield of as received copper have been published in

references8,9

. They are compared to the results obtained at CERN for as received copper in

Figure 12. The agreement between CERN measurement and Hopman measurements is good. The

measurements made at SLAC gives a maximum secondary electron yield, which is significantly

smaller than those obtained in the two other labs. The different measurements obtained for a

primary electron energy in the vicinity of 1000 eV are close to each other.A dose dependence curve has also been given in reference8. This curve is compared with a

typical curve obtained at CERN in Figure 13. As noticed in Figure 12, the initial yield is much

smaller than what is measured at CERN and the yield decrease proceeds at a slower rate. The fully

conditioned state was apparently not obtained in that reference8.

Figure 12: The variation of the secondary electron yield versus the primary electron energy as

obtained in three different laboratories

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

0 1000 2000 3000 4000

ENERG Y (e V)

HOPMAN,VERHOEVEN AS RECEIVED OFHC COPPER A pp l.Surf.sci 150,1-

7,(1999)

B. HENRIST AS RECEIVED OFHC COPPER CERN 31/ 01/ 01

R.E.KIRBY OFE AS RECEIVED C OPPER SLAC PUB-8212

R.E.KIRBY PEP-II HER COPPER SLAC PUB-8212


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Figure 13: Comparison of the dose dependence of the secondary electron yield as measured at CERN

and at SLAC

7 ESTIMATION OF THE QUANTITY OF GAS RELEASED DURING THE

CONDITIONING.

During the electron bombardment of a surface, leading to the above mentioned decrease of the

secondary electron yield, a significant decrease of the electron induced desorption yield is also

observed10,11

. The desorption yields data, obtained in a separate system, on an as-received copper

sample, are presented together with the data concerning the secondary electron yield in Figure 14 as

a function of the number of electrons impinging per unit area. To allow a better comparison, both

data are normalised to 1 in the initial non-bombarded state. These data allow the calculation of the

amount of gas released during the conditioning of a copper surface by integrating the product of the

desorption yield and the electron dose.Table 2: Total number of molecules released per unit area during processing for the main desorbed

gases

GASES H2 CH4 CO C2H6 CO2 H2O

QUANTITY (cm-2

) 6x1016

8x1014

8x1015

8x1014

8x1015

3x1014

Figure 15 shows as a function of the total number of molecules released per unit surface area,

the secondary electron yield ratio and the desorption yield ratio for three molecular species. These

were chosen (for the sake of clarity) to represent the main desorbed gases of the two types:

hydrogen and carbon containing gases, (H2, CO and C2H6).

1

1.2

1.4

1.6

1.8

2

2.2

2.4

1.E-09 1.E-07 1.E-05 1.E-03 1.E-01

D O SE ( C / m m 2 )

B. HENRIST CERN 31/ 01/ 01

R.E. KIRBY PEP II HER COPPER SLACPUB-8212


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Figure 14: The variation of the normalised desorption and secondary electron yields with the electron

dose

These data show that the reduction of both the secondary electron yield and the desorptionyield are two processes evolving in parallel during the irradiation of a surface by electrons. The

upper limit of the total number (the mantissa being rounded to the next integer) of molecules

released during the conditioning of a copper surface is given in Table 2 for the main desorbed

gases. Hydrogen is the main released gas, its predominance is established during the initial part of

the conditioning when desorption yields are the highest and hydrogen the most abundant species,

carbon monoxyde and dioxyde come in second position. These results are in good agreement with

those concerning desorption yields published in the reference11

.The total number of molecules

removed from an as-received copper surface during its conditioning is smaller than 1017

molecules

per cm2

i.e. less than 100 monolayers. The number of molecules released per unit surface area

during the conditioning process is given in Figure 16 as a function of the final secondary electron

yield achieved. Although both the desorption yield and the secondary electron yield evolve inparallel under electron bombardment, it should not be concluded that the cleaning of the surface is

the origin of the decrease of the secondary electron yield. For example a clean copper surface (e.g.

in situ glow discharge cleaned) has a secondary electron yield higher than a conditioned surface

(see Figure 4, curve labelled: Argon G.D. Copper).

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1E+12 1E+14 1E+16 1E+18 1E+20

ELEC TRO N DO SE (e - / c m 2 )

0.1

1

NO

RM

ALISEDS

ECO

NDARY

ELECTRO

NY

IELD

H2

CH4

CO

C2H6

CO2

DELTA MAX 31-01

DELTA MAX 23-03


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Figure 15: The normalised desorption and secondary electron yields as a function of the total number

of desorbed molecules

Figure 16: The quantity of desorbed molecules per unit area as a function of the final secondaryelectron yield

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

1.E+18

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

SEC O N DA RY ELEC TRO N YIELD

H2-23/03

CO-23/03

CO2-23/03

CH4-23/03

C2H6-23/03

H2O-23/03

TOT-23/ 03

T0T-31/ 01

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1E+12 1E+13 1E+14 1E+15 1E+16 1E+17

TO TA L NUM BER O F DESO RBED M O LECULES PER UN IT

AREA (mo l/ cm- 2

)

0.1

1.0

SECO

NDARY

ELECTRO

NY

IELDR

ATIO

H2

CO

C2H6

DELTA MAX 31-01

DELTA MAX 23-03

x


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8 CONCLUSIONS

The conditioning of as received copper surfaces has been studied in various experimental set-

ups and coherent results have proved its efficiency to reduce the secondary electron yield. Based on

measurements of the secondary electron energy distribution, a numerical expression has been given

which allows to calculate the fraction of reflected electrons with a good agreement to the

experimental data between 4 and 500 eV. The Furmans expression has been used and the relevant

parameters are given to calculate the true secondary electron yield as a function of the primaryelectron energy. Combining both formulae permits to compute the total number of electrons emitted

when the energy of the incident electrons is given. Using data obtained for electron induced

desorption it has been shown that the quantity of molecules removed from a surface during

processing is smaller than 1017

molecules.cm-2

. Using these data, it is now possible to better

estimate the amount of gas released during the beginning of the LHC operation when multipacting

can occur because of the electron cloud effect. Although the phenomenon of conditioning has been

obtained reproducibly on many samples, in different experimental set-ups, the exact mechanism

leading to this effect is not properly understood. This is of course not a comfortable situation as the

LHC operation at nominal intensity relies on this effect. Further studies are going on to try to

elucidate the main physical parameters responsible for this beneficial effect.

1 V. Baglin, B. Henrist, N. Hilleret, E. Mercier, C. Scheuerlein: Proceedings of the X workshop on LEP-SPS

performance Chamonix , 130, 2000.2 V. Baglin, J. Bojko, O. Grbner, B. Henrist, N. Hilleret, C. Scheuerlein, M. Taborelli: 7th Europen Accelerator

Conference, Vienna, 217-220, 2000.3

V. Baglin, I.R. Collins, O. Grbner, C. Grnhagel, B. Henrist, N. Hilleret, B. Jenninger: Proceedings of the

XI workshop on LEP-SPS performance Chamonix , 141, 2001.4 G. Arnolds-Mayer, N. Hilleret: Advances in Cryogenic Engineering Materials, 28, 611-621, 19825

R. Bindi, H. Lanteri, P. Rostaing, J. Phys. D: Appl.Phys. 13,267, 1980.6 M. A. Furman, CERN LHC Project Report 180, 1998.7 J.J. Scholtz, D. Dijkkamp, R.W.A. Schmitz, Philips J. Res. 50, 375-389, 1996.8 R. E. Kirby, F. K. King SLAC-PUB-8212, 2000.9 H.J. Hopman, J. Verhoeven: Applied Surface Science, 150, 1-7, 1999.10 F. le Pimpec: Thse Universit de Paris VI, 2000.11

J. Gomez-Goni, A.G. Mathewson, J. Vac. Sci.Technol. A 15, 6, 3093-3103, 1997


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Secondary electron emission from magnesium oxide on multiwalledcarbon nanotubes

Won Seok KimBK21 Physics Division and Center for Nanotubes and Nanostructured Composites, SungkyunkwanUniversity, Suwon 440-746, Korea

Whikun Yi, SeGi Yu, Jungna Heo, Taewon Jeong, Jeonghee Lee, Chang Soo Lee,and J. M. Kim

NCRI, Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111,Suwon 440-600, Korea

Hee Jin Jeong, Young Min Shin, and Young Hee Leea)

Department of Physics and Center for Nanotubes and Nanostructured Composites, SungkyunkwanUniversity, Suwon 440-746, Korea

Received 22 March 2002; accepted for publication 10 June 2002

We have investigated effects of electric fields on the yield of secondary electron emission SEEfrom the primary electron bombardment on magnesium oxide MgO covering vertically grownmultiwalled carbon nanotubes MWCNTs . We observe that the yield of SEE increases up to at least22 000 at a special condition. The strong local field generated by the sharp tip of vertically grownMWCNTs accelerates secondary electrons generated by primary electrons. This eventually givesrise to so called Townsend avalanche effect, generating huge number of secondary electrons in a

MgO film. Emission mechanism for such a high SEE will be further discussed with energy spectrumanalysis. 2002 American Institute of Physics. DOI: 10.1063/1.1498492

Secondary electron emission SEE with bombardmentof primary electrons plays an important role in vacuum de-vices. For instance, electron multiplier, microchannel plate,and electron gun require the SEE materials with a high am-plification yield. In general, insulators are good candidatesfor high SEE. The secondary electrons generated by the pri-mary electrons in a magnesium oxide MgO film move tothe surface with relatively weak electronelectron scatter-

ings due to the absence of free electrons in insulator, andfinally escape from the surface if they have enough energy toovercome the work function of the materials.1 Single crystalMgO, for instance, has a SEE yield of about 25 at best.2 Onthe other hand, the porous MgO produces high SEE yield ofabout 1000 under the high electric field.3 However, for agiven yield, it is always desirable to look for the condition inwhich the lowest field is used. Recently a MgO film wasdeposited on randomly oriented carbon nanotube CNTpowder, where relatively large SEE of maximum 15000 at abackbias of 1400 V was obtained.4 Yet the SEE obtained wasstrongly dependent on the sample positions and not repro-ducible either. In this report, we introduce a systematic ap-

proach to reproduce high SEE using MgO on verticallygrown multiwalled carbon nanotubes MWCNTs . We ob-served an unusually high SEE yield of greater than 22 000 beyond the limit of a detector at a backbias of 850 V, whichwas strongly related to the MgO film thickness.

MWCNTs were grown by thermal chemical vapor depo-sition on the Ni-coated Si substrate using a C2H2 gas at650C.5 The average diameter and length of the grownCNTs were 300 and 20 m, respectively. A MgO thin film

was deposited on the vertically grown MWCNTs using elec-tron beam deposition. Figure 1 a shows the scanning elec-tron microscope SEM image of MgO

Documents

Secondary Electron Emission by Bruce Darrow Gaither