Ahmed S.N. Physics and Engineering of Radiation Detection

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366 Chapter 6. Scintillation Detectors and Photodetectors

the outer surface of the ﬁsh-tail light guide as shown in Fig.6.4.3(b). However one

could also opt for enclosing the ﬁsh-tail light guide in a separate reﬂecting enclosure

as depicted in Fig. 6.4.3(a).

Radiation

Incident

Scintillation Photons

Radiation

Incident

Reflecting Surface

Light Guide

(b)

(a)

Photodetector

Light Guide

Photodetector

Scintillator

Reflecting Surface

Figure 6.4.3: Two types of hybrid light guides. (a) A ﬁsh-tail

light guide is enclosed in a container having highly polished

reﬂecting inner surface. (b) The outer surface of a ﬁsh-tail

light guide has been covered with a reﬂecting paint.

The surface of the reﬂecting part of a hybrid light guide must be extremely

smooth and highly polished. Although the absorption of photons in such a surface

can not be totally avoided but careful construction can help minimize the loss.

6.5 Photodetectors

The photons produced by scintillators can be detected by a number of means, the

most notable of which are photomultiplier tubes (PMTs) and photodiodes (PDs).

6.5. Photodetectors 367

Both of these detector types have their own pros and cons. for example photo-

multiplier tubes have sensitive mechanical structures that are prone to damage in

mechanically unstable environments while the photodiodes are made of semiconduc-

tor materials and are therefore mechanically stable. On the other hand the response

time of PMTs is much smaller than photodiodes and are therefore preferred in timing

applications. Of course these are only two parameters and the choice of a photode-

tector is in reality a compromise between a number of factors including cost. We will

learn about these factors as we discuss these two types of detectors in the following

sections.

6.5.A Photomultiplier Tubes

Photomultiplier tubes are sensitive devices that are capable of converting light pho-

tons into a very large number of electrons. The basic building blocks of a com-

plete PMT are a photocathode, an electron multiplication structure, and a readout

electrode. The photons incident on the photocathode get converted into electrons

through the process of photoelectric eﬀect. The electron is then made to acceler-

ate and strike a metallic structure called dynode, which results in the emission of

more electrons. The newly produced electrons are again accelerated towards another

dynode, where even more electrons are produced. This process of electron multi-

plication continues until the electrons reach the last dynode where the resulting

current is measured by some electronic device. This process is graphically sketched

in Fig.6.5.1.

Before we go on to the discussion of individual components of a PMT, let us

have a look at the characteristics that make PMT based photodetectors desirable

for radiation detection. The most important of these characteristics are listed below.

 High sensitivity.

 Good signal-to-noise ratio.

 Fast time response.

 Large photosensitive area.

The reader should bear in mind that here we are not comparing PMTs with pho-

todiodes. With rapidly developing technology, new photodiode detectors are now

being developed that match or even surpass these and other properties.

A.1 Photocathode

We learned about the process of photoelectric eﬀect in Chapter 2 and saw that some

particular materials emit electrons when they absorb photons of energy above a

certain threshold. This threshold energy is called work function and is characteristic

of the material. The material itself is called photocathode. If we assume that no

energy is lost during this process then the energy of the emitted photon would simply

be equal to the diﬀerence between the photon energy and the work function of the

material, that is

− Φ, (6.5.1)

368 Chapter 6. Scintillation Detectors and Photodetectors

Dynode

Vacuumed Enclosure

Photons

Photocathode

Electrons

Figure 6.5.1: Working principle

of a typical side-on type PMT.

The light photons (e.g. com-

ing from a scintillator) produce

electrons through the photoe-

mission process in the photo-

cathode. These electrons are

focused on to the ﬁrst dynode

where they produce secondary

electrons, which then move to-

wards the second dynode and

produce more secondary elec-

trons and so on. An actual

PMT may contain 10 or more

such dynodes. The ampliﬁed

signal is measured at the ﬁnal

dynode or anode.

where λ is the wavelength of the incident light and Φ is the energy threshold or

work function of the material. For a given material, this expression can be used to

determine the maximum detectable wavelength (see example below).

Example:

A material having a work function of 2 eV is used to convert photons into

electrons, which are then detected by a photomultiplier tube. Compute the

maximum wavelength of the photons it can convert into electrons.

Solution:

The maximum wavelength can be obtained by setting E

=0inequation

6.5.1, which simply means that all of the incident energy has been used in the

conversion process and the electron has not taken away any of the photon’s

6.5. Photodetectors 369

energy. Hence we have

max

−Φ

⇒ λ

max



6.63 × 10

−34



2.99 × 10



2 × 1.602 × 10

−19

=6.187 × 10

−7

= 618.7 nm.

Looking at equation 6.5.1 it is apparent that choosing a material with low work

function is advantageous in terms of delivering energy to the outgoing electrons.

However this is not the only criterion for selecting a photocathode for use in a

PMT. for example if the PMT is to be used to detect scintillation photons then

the most important factor is the conversion eﬃciency of the material at the most

probably wavelength of the scintillation light. If the material does not have high

enough eﬃciency at that wavelength then even a very low work function would not

matter much. Since the realization of this problem, a number of extensive stud-

ies have been carried out to ﬁnd the optimum photocathode materials at diﬀerent

scintillation wavelengths. Most of these eﬀorts have gone into understanding the

spectral response of the materials. By spectral response we mean the eﬃciency of

the production of photoelectrons as a function of light wavelength. This eﬃciency

is generally referred to as photocathode quantum eﬃciency and is simply deﬁned as

the ratio of the number of emitted photoelectrons to the number of incident photons,

that is

QE =

, (6.5.2)

where N

is the number of electrons emitted and N

is the number of incident

photons. Quantum eﬃciency can also be expressed in terms of more convenient

quantities, such as incident power and photoelectric current. To do that we ﬁrst

note that the incident power can be deﬁned as

= n

hν, (6.5.3)

where n

represents the number of photons of frequency ν incident on the detector

per unit time. The expression for quantum eﬃciency can then be written as

QE =

/hν

hν

. (6.5.4)

Here n

is the number of photoelectrons ejected per unit time and I

= en

is the

photoelectric current. This equation can be used to determine the photoelectric

current for a particular value of the incident power (see example below).

370 Chapter 6. Scintillation Detectors and Photodetectors

Example:

A photocathode produces a current of 20 nA when exposed to light of

wavelength 510 nm. Determine the photoelectric current if the wavelength

of light is changed to 475 nm such that the incident power remains the

same. Assume that the material has the same quantum eﬃciency at the two

wavelengths.

Solution:

Equation 6.5.4 for photoelectric current can be written as

=(QE)

= Kλ,

where λ is the wavelength of the incident light and we have lumped together

all the constant terms into one parameter K. This constant can be eliminated

by writing the above equation for the two wavelengths and then dividing one

with the other. Hence the required current is

pe,2

= I

pe,1

=20

475

510

=18.6 nA. (6.5.5)

Fig.6.5.2 shows a typical spectral response curve of a photocathode. Here the

quantum eﬃciency of the photocathode is plotted against wavelength of incident

light. The interesting thing to note here is that the curve has a plateau where the

variation in eﬃciency is not very large. As soon as one goes beyond this plateau on

either side, the eﬃciency decreases rapidly. Therefore to build a good scintillation

detector with a PMT, one should ensure that the spectrum of scintillation light has

a peak somewhere in the middle of this plateau. There are some materials that have

very short plateaus as well and using those in PMTs is generally not a good idea

unless the scintillation spectrum is also narrow and has clearly deﬁned peak that

occurs at or near the peak of the spectral response curve.

As noted above the particular requirements of the system drive the choice of a

photocathode in a PMT. Since there are a host of scintillation materials available

having their unique scintillation spectra, therefore eﬃcient detection of those pho-

tons with a PMT would require the use of photocathode materials with matching

spectral response characteristics. Then there are also applications where the photons

to be detected are not coming from a scintillator. That is, a PMT is a photodetec-

tor that can be used to detect photons no matter from where they are coming. for

example the use of PMTs for detecting Cherenkov photons has recently gotten a lot

of attention in highly sensitive large scale neutrino detectors. In essence, a PMT is a

versatile photodetector that can be used to detect photons in virtually any environ-

ment provided it is equipped with a photocathode having the appropriate spectral

response.

6.5. Photodetectors 371

Wavelength ( )nm

300

200

400 500 600 700

800

QE (%)

Figure 6.5.2: Quantum ef-

ﬁciency of a typical pho-

tocathode as a function of

wavelength of incident pho-

tons.

Photocathodes can be used in essentially two diﬀerent modes: transmission and

reﬂection. In transmission mode the photocathode is semitransparent to allow trans-

mission of photoelectrons (see Fig.6.5.3(a)). Such a photocathode is constructed by

depositing a very thin layer of the material on the inside of the photon entrance

window. Since most of the photoelectrons are emitted in the direction of travel of

the incident photons therefore it is called transmission photocathode. Most PMTs

are constructed with this type of photocathodes. There are also some photocathode

materials that have high quantum eﬃciencies but very poor transmission properties.

These opaque materials are used to construct the so called reﬂection photocathodes

as shown in Fig.6.5.3(b). A reﬂection photocathode is made by depositing a thin

layer of the material on a metal electrode inside the PMT.

Photocathode

Photons

Dynode

Photoelectrons

Photons

Photocathode

(a)

(b)

Figure 6.5.3: (a) Semitransparent photocathode

used as a transmission photoemission device in

a head-on type PMT. (b) Reﬂection type photo-

cathode. Such photocathodes are generally used

in circular type PMTs.

372 Chapter 6. Scintillation Detectors and Photodetectors

A host of photocathode materials have been identiﬁed with varying characteristics

and spectral responses, some of which are listed below.

 AgOCs: This is one of the most widely used photocathode materials. It has a

photoemission threshold of 1100 nm and a peak quantum eﬃciency at around

800 nm. The working range of this material is in the near infrared range of the

photon spectrum. The main disadvantage of AgOCs is its very low quantum

eﬃciency, which has a peak of less than 1%. It is mainly used as a transmission

photocathode.

 GaAs(Cs): This material has a spectral response that ranges from ultraviolet

to 930 nm. This broadband response makes it suitable for used with a wide

range of scintillators without the need of a wavelength shifter. In most instances

it is used in the transmission mode. Since the quantum eﬃciency of GaAs(Cs)

is temperature dependent with a peak at very low temperatures, it is sometimes

operated at very low temperatures.

 InGaAs(Cs): This material has greater sensitivity in the infrared range and

higher signal-to-noise ratio in 900-1000 nm range than GaAs(Cs).

 SbCs

: This is one of the earliest used photocathode materials. It is still very

popular amongst manufacturers as its spectral response ranges from ultravio-

let to the visible region with a peak quantum eﬃciency of around 20%. The

photoemission threshold of SbCs

lies at around 700 nm and has a peak at ap-

proximately 400 nm. Since it has very poor transmission capabilities therefore

it is generally used as a reﬂection photocathode.

 Bialkali Materials: The bialkali materials such as SbRbCs and SbKCs are

the most widely used of all photocathode materials due to their high sensitiv-

ities to blue light generated by NaI scintillators. The reader might recall that

NaI is the most popular scintillator for radiation detection. The sensitivity

to blue light is not the only reason for their popularity, though. These ma-

terials also have high quantum eﬃciencies with peaks of just less than 30%.

Another advantage is their good stability at elevated temperatures. Some bial-

kali materials can be used at a temperature as high as 175

C. A common

bialkali material SbKCs has a photoemission threshold of about 700 nm and

maximum quantum eﬃciency of 28% at around 400 nm.

 Multialkali Materials: These materials have very wide spectral response ranging

from ultraviolet to near infrared, making them highly suitable to be used with

a number of diﬀerent scintillators. Their main disadvantage is high thermionic

emission of electrons even at room temperature and therefore external cooling

is generally required. NaKSbCs is a common multialkali.

 CsTe, CsI: These materials are sensitive to photons in ultraviolet region only

and are therefore not very widely used.

A.2 Electron Focusing Structure

Since photocathodes have low quantum eﬃciencies therefore each electron produced

is important and should be collected by the ﬁrst dynode. This requires the use of an

6.5. Photodetectors 373

electron focusing structure to guide the photoelectrons to the dynode. The reason

is that the photoelectrons are produced in all directions with varying energies and

can easily go astray if they are not directed properly. An ideal electron focusing

structure has the following two distinct characteristics.

 It directs the electrons from the photocathode to the ﬁrst dynode such that

all of them have the same transit time regardless of their initial energy. This

is a very important requirement specially for tubes that are to be used in fast

timing applications.

 It is able to focus all the photoelectrons produced in the photocathode.

Of course such strict requirements can be fulﬁlled only by an ideal focusing structure.

However with a carefully designed structure one can achieve an electron collection

eﬃciency of 80% or better. One thing that should be pointed out here is that not

all PMTs have very stringent focusing requirements. Some PMT structures, as we

will see later, have large ﬁrst dynodes and are therefore able to collect most of the

photoelectrons even without a focusing structure.

Focusing of electrons on to the ﬁrst dynode depends on two factors: the geometry

of the focusing structure and the electric ﬁeld intensity in the space between the

photocathode and the ﬁrst dynode. The ﬁeld intensity is generally made non-uniform

and has a higher value near the photocathode to minimize the dependence of electron

transit time on the initial electron velocity.

A.3 Electron Multiplication Structure

Earlier we saw that the quantum eﬃciency of a typical photocathode ranges between

10% and 30%. An eﬃciency of 10% means that for every 10 photons only one photo-

electron is getting out of the photocathode. For low to moderate photon ﬂuxes this is

a problem since the resulting electron ﬂux may not be suﬃcient to constitute a mea-

surable current (see Example below). Hence to obtain a measurable signal and good

signal-to-noise ratio the electron yield must somehow be increased. Now since the

photocathode materials have their own physical and engineering limitations and can

not be made more eﬃcient, therefore the electron yield must be increased by some

other process. This is accomplished in a PMT through the electron multiplication

structure. The basic idea is to utilize the process of electron ejection from certain

metals when they are bombarded by electrons. Such metals eject more electrons

than the incident electrons provided the incident electrons have high enough energy.

This implies that the electrons must ﬁrst be accelerated as well. This may sound

complicated but in actuality it can be done through a simple structure as shown in

Fig.6.5.1. This PMT has 5 dynodes and a readout electrode. Each of these dynodes

is kept at a higher potential than the preceding dynode to direct and accelerate the

electrons towards itself. The result of these successive accelerations and secondary

emissions is a cascade of electrons ﬂowing down the dynode chain. Consequently

the initial small number of photelectrons gets multiplied into a very large number

at the ﬁnal electrode. To collect these electrons another metallic structure called

anode is placed near the ﬁnal dynode. The electron current at the anode is passed

on to the associated electronics for further processing.

374 Chapter 6. Scintillation Detectors and Photodetectors

Example:

A NaI scintillator is bombarded by ionizing radiation that results in the

deposition of 1.5 MeV of energy per second. The scintillation photons thus

produced are detected by a PMT. Determine the current at the ﬁrst dynode

of this PMT. Assume that 15% of photons are lost before reaching the

photocathode, which itself has a quantum eﬃciency of 20%. NaI produces

about 40,000 photons per MeV.

Solution:

The number of electrons produced by the photocathode per unit time can be

estimated from

=(N

)(η)(QE),

where N

represents the number of scintillation photons produced per unit

time, η is the eﬃciency of the process of transfer of photons to the photocath-

ode, and QE is the quantum eﬃciency of the photocathode. The number of

photons being produced per second in NaI is

=(40, 000)(1.5) = 60, 000 s

−1

Hence the number of photoelectrons being produced per second is

=(60, 000)(0.15)(0.2)

=1, 800 s

−1

Since each electron carries a unit electrical charge, the current at the ﬁrst

dynode is given by

I =(1, 800)



1.6 × 10

−19



−1

=2.9 × 10

−15

A. (6.5.6)

It is evident that it will be quite a diﬃcult to measure this current of about

0.3 fA and also have a good signal-to-noise ratio. The reader should note

that even when we started with a scintillator that had one of the highest light

yields, the current at the ﬁrst dynode turned out to be extremely small. Of

course in this case the incident photon ﬂux was also very low, which refers to

the operation of the detector in low radiation ﬁelds. In high radiation ﬁelds

the photoelectric current could be several tens of nanoseconds. In very low

radiation ﬁelds, obtaining a measurable requires that the signal multiplication

structure of the PMT has a high enough gain. for example a PMT having a

gain of 10

would amplify the 0.3 fA signal to about 3 pA, which may still be

too low for reliable measurements.

Every dynode in the electron multiplication structure does not have the same

shape. for example the ﬁrst dynode is specially designed to maximize the collection

of photoelectrons that are already low in numbers. The structure of other dynodes

depends on their particular locations. The focusing requirements for dynodes are

also very stringent to avoid unnecessary loss of electrons. These requirements include

not only proper designing of the dynodes but also their placement. Due to these

6.5. Photodetectors 375

issues the performance of a PMT is susceptible to mechanical jitters, which may

lower the ability of one or more dynodes to focus or direct the electrons. Loss of

electrons leads to signal deterioration and possibly non-linearity and therefore the

sensitive mechanical structure of a PMT is one of its major disadvantages. However,

as we will see later, some PMT designs are less prone to mechanical instabilities and

should therefore be preferred if the environment is mechanically unstable.

The electron multiplication structure shown in Fig.6.5.1 is by no means a standard

for PMTs. In fact, PMTs are manufactured in many diﬀerent geometries. The

choice of the geometry is mainly application dependent. In the following we will

look at the most common of the PMT geometries and discuss their advantages and

disadvantages.

 Linear Focused Type: This is perhaps the most famous type of electron

multiplication structure (see Fig.6.5.4). Generally it is used in the head-on

type of PMT. The distinguishing feature of this design is its extremely fast

time response, which is achieved by positioning the dynodes to minimize the

electron transit times. At each multiplication stage the electrons are focused

by high electric ﬁeld between the carefully placed dynodes, leading to a very

fast response time. This design is susceptible to external magnetic ﬁeld.

Incident

Photons

otocat

Dynode Chain

Vacuu

ass

osu

Focussing Structure

Electrons

Anode

Figure 6.5.4: Linear focused type PMT.

 BoxandGridType: This is a somewhat simpliﬁed version of the linear

focused type PMT. Such a structure is made of several quarter cylindrical

dynodes installed in succession as shown in Fig.6.5.5. Since the collection area

of the ﬁrst dynode is larger than in the linear focused type therefore its focusing

requirements are a bit relaxed. Although this simple design looks identical to

the linear focused type but it lacks the progressive focusing of electrons between

the dynodes. Hence it has slower response time and is therefore not a popular

choice for most applications. Like linear focused type, this design also suﬀers

from magnetic ﬁeld dependent sensitivity.

 Circular Cage Type: This is a very compact structure with dynodes arranged

in a circular fashion (see Fig.6.5.6). This type of PMT has a fast response time

and is therefore suitable for use in timing applications. Although most of

the circular cage type PMTs have side-on type structure but this geometry

can be used to construct head-on type PMTs as well. The compactness of