1

Light CollectionOnce light is produced in a scintillator it

must collected, transported, and coupled to some device that can convert it into an electrical signal (PMT, photodiode, …)

There are several ways to do this Plastic light guides

2

Light Guides

Isotropic light emission

3

Light Guides

Even for total internal reflection over all angles, if x1 >> x2 there will be substantial light loss

11

22

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sinx

xsin

sinx2sinx2

xx says theoremsLiouville'

divergenceangular sin

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4

Wavelength Shifters

Liouville’s theorem can be beat by decreasing the energy of the photons

Wavelength shifter can be used To collect light from large areas and

transport it to a small PMT area To better match the PMT sensitivity To bend the light path

5

Wavelength Shifters Wavelength shifting bars

Wavelength shifting fibers

6

ATLAS Tile Calorimeter

ATLAS Tile Calorimeter

7

ATLAS Tile Calorimeter

ATLAS Tile Calorimeter

8

ATLAS Tile Calorimeter

ATLAS Tile Calorimeter

9

Outer Reflectors

Usually the scintillator and light guide are wrapped/enclosed with an outer reflector Measurements at 440 nm,

10

Photon DetectorsOnce light is produced in a

scintillator we need to convert it into an electronic signal Vacuum based (this lecture)

Photomultiplier tubes (PMTs) Semiconductor (later lectures)

Photodiodes, APDs, SSPM, CCDs, VLPCs, …

Hybrid Vacuum+semiconductor

Gas based (TEA, TMAE) For Cerenkov detectors

11

Photon DetectorsWe’ll be interested mainly in the visible

region today

12

Photon DetectorsThe main principle used is the

photoelectric effect which converts photons into electrons (photoelectrons)

Important quantities characterizing the sensitivity are the quantum efficiency and radiant sensitivity

nm

WmASQE

SN

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/1240%

powerincident

ntphotocurre and %

13

Photomultiplier Tubes (PMTs)

14

WindowsBorosilicate typical

15

PhotocathodesThe important process in the

photocathode is the photoelectric effect Photons are absorbed and impart energy to

electrons Electrons diffuse through the material losing

energy Electrons reaching the surface with sufficient

energy (> W) escape

Alkalai metals have a low work function e.g. bialkali is SbKCs

AG EEWhE

16

PhotocathodesQE of bialkali PMT’s

17

Photocathodes

As you can see from the graph, the maximum QE is about 25% for current bialkali Photoelectron emission is isotropic

50% to first dynode, 50% to window Transmission losses

Bialkali photocathodes are ~40% transmissive

0.5 x 0.4 ~ 0.2

18

Energy ResolutionIn gamma ray spectroscopy and

other applications, the energy resolution is an important quantity

One contribution to the energy resolution is the statistical variance of the produced signal quanta

In the case of a PMT, the energy resolution is determined by the number of photoelectrons arriving at the first dynode

19

Energy Resolution

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20

Electron Focusing

21

Dynode StructureThe dynode structure multiplies the

number of electrons Process is similar to photocathodes but here

the incident radiation is electrons

22

Dynode StructureThere are a variety of dynode structures

including some that are position sensitive

23

Dynode Structure

of 4-6 for most dynode materialsAnd typically there are 10-14 stages

(dynodes)

electronsincident ofnumber

electronssecondary ofnumber

6

10

10

4

gain

G

G

G N

24

Dynode StructureTypical instantaneous current?

Assume 103 photons at the photocathode

Then there are 2.5x102 electrons at the first dynode

Then there are 2.5x108 electrons at the anode

And collected in 5ns gives a peak current of 2.5x108 x 1.6 x 10-19 / 5 x 10-9 = 8 mA Of course the average current is much

smaller

25

Dark CurrentA small amount of current flows in the

PMT even in completely dark stateCauses of dark current include

Thermionic emission from photocathode and dynodes

Leakage current (ohmic leakage) between anode and other electrodes

Photocurrent produced by scintillation from glass or electrode supports

Field emission current Cosmic rays, radioactivity in glass envelope,

radioactivity (gamma) from surroundings (cement)

Dark current increases with increasing supply voltage

26

PMT Gain and HV Supply

regulated wellbemust supply HV theThus

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27

PMT Gain and HV SupplyTypical gain versus high voltage curveRule of thumb is V=100 gives G=2

28

PMT BaseA voltage divider network is used to

supply voltage to the dynodes Typical supply voltage is 2kV

The manufacturer usually supplies a circuit diagram and often sells the accompanying base

29

PMT BaseThe HV supply must be capable of

providing a DC current (to the divider network) as well as average and peak signal currents Typical signal current ~ 20 mA Typical average current ~ 20 A

It is possible that at high rates that the HV supply cannot provide enough current to the last dynodes and hence the PMT voltage will “sag” Additional charge can be supplied by using

capacitors or transistors

30

PMT Base Using capacitors or transistors to supply charge

31

Magnetic ShieldingV between the dynodes is ~100-

200VLow energy electrons traveling from

dynode to dynode can be affected by small magnetic fields (e.g. earth B ~ 0.5 G) Effect is largest for head-on type PMT’s

when the magnetic field is perpendicular to the tube axis

A magnetic shield (e.g. mu-metal) is used to reduce gain changes from magnetic fields

32

Magnetic Shielding

33

PMT’sThere are a wide range of PMT types and

sizes From Hamamatsu catalog

34

ATLAS Tile Calorimeter PMT

35

Light Guides

Liouville’s theorem Phase space is conserved

Phase space density of photons cannot be increased

You can’t make a tapered light guide without losing light

36

Light Guides

So for a maximum output angle 2 the input angle 1 is limited

Even for complete TIR, if x1 >> x2 there will be substantial light loss

11

22

2211

2211

sinx

xsin

sinx2sinx2

xx says theoremsLiouville'

divergenceangular sin

coordinate e transvers

guidelight a in photona for element space phasea Consider

nn

pp

np

x

37

Light GuidesTransport light by total internal reflection

(TIR)

The air gap between scintillator and wrapping is important

Maximum angle for TIR at light guide output is

For light that does escape the light guide it can be recaptured using specular (Al foil) or diffuse (Tyvek) reflection

nn

nextc

1sin

c 902

38

Light Guides

39

Light GuidesAn example

Many people use Tyvek but one should do studies for each specific application

40

Light GuidesThere will be some light loss even in the

case of equal dimensions

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1.5n and xfor

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angles taper smallfor sin1sin

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1

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21

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x

nx

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c

cc

c

41

Wavelength ShifterLiouville’s theorem

Phase space is conserved Phase space density of photons cannot

be increased You can’t make a tapered light guide

without losing light

One can get around Liouville’s theorem by using a wavelength shifter such as BBQ Light is absorbed and subsequently

emitted (isotropically) at a longer wavelength

42

Wavelength Shifter