Transcript phys586-lec04

Light Collection
 Once 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
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Light Guides
Isotropic light emission
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Light Guides
Consider a phasespace elementfor a photonin a light guide
T hecanonically conjugatevariablesare taken tobe
x  transverse coordinate
p  n sin   angular divergence
Liouville's theoremsays Dx1Dp1  Dx 2 Dp2
2Dx1n sin 1  2Dx 2 n sin  2
Dx 2
sin  2 
sin 1
Dx1
 Even for total internal reflection over all
angles, if Dx1 >> Dx2 there will be
substantial light loss
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Wavelength Shifters
Liouville’s theorem can be beat by
decreasing the energy of the photons
Wavelength shifter can be used
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
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
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Wavelength Shifters

Wavelength shifting bars
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Wavelength shifting fibers
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ATLAS Tile Calorimeter
ATLAS Tile Calorimeter
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ATLAS Tile Calorimeter
ATLAS Tile Calorimeter
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ATLAS Tile Calorimeter
ATLAS Tile Calorimeter
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Outer Reflectors
Usually the scintillator and light guide
are wrapped/enclosed with an outer
reflector

Measurements at 440 nm,
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Photon Detectors
Once 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
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Photon Detectors
 We’ll be interested mainly in the visible region
today
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Photon Detectors
 The 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
N pe
photocurrent
QE% 
and S 
N
incident power
S m A/ W 
QE%  1240
 nm
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Photomultiplier Tubes (PMTs)
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Windows
 Borosilicate typical
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Photocathodes
 The important process in the photocathode is
the photoelectric effect
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Photons are absorbed and impart energy to
electrons
Electrons diffuse through the material losing energy
Electrons reaching the surface with sufficient energy
(> W) escape
E  h  W  EG  EA
 Alkalai metals have a low
work function

e.g. bialkali is SbKCs
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Photocathodes
 QE of bialkali PMT’s
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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
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Energy Resolution
In 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
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Energy Resolution
 E    N 

E
N
the probability distribution governing
the number of photoelectronsis theP oissondistribution
v n e 
f (n, ) 
; n is number, is mean
n!
 N 
N
1
so


N
N
N
for 3000electronsarrivingat thefirst dynode thisis
 E 
E
1

 2%
3000
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Electron Focusing
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Dynode Structure
 The dynode structure multiplies the number of
electrons

Process is similar to photocathodes but here the
incident radiation is electrons
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Dynode Structure
 There are a variety of dynode structures
including some that are position sensitive
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Dynode Structure
number of secondary electrons
d
number of incident electrons
 d of 4-6 for most dynode materials
 And typically there are 10-14 stages (dynodes)
gain G  d
N
G  4
10
G  106
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Dynode Structure
Typical instantaneous current?
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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
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Dark Current
 A small amount of current flows in the PMT
even in completely dark state
 Causes of dark current include
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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
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PMT Gain and HV Supply
gain G  d  kV
N
N
d
dG
NG
N 1
 NkVd 
dVd
Vd
dG
dVd
dVa
so
N
N
G
Vd
Va
for N  10
dG
dVa
 10% only if
 1%
G
Va
T hus the HV supply must be well regulated
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PMT Gain and HV Supply
 Typical gain versus high voltage curve
 Rule of thumb is DV=100 gives DG=2
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PMT Base
 A 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
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PMT Base
 The HV supply must be capable of providing
a DC current (to the divider network) as well
as average and peak signal currents
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Typical signal current ~ 20 mA
Typical average current ~ 20 mA
 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
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PMT Base
 Using capacitors or transistors to supply charge
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Magnetic Shielding
DV between the dynodes is ~100-200V
Low 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
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Magnetic Shielding
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PMT’s
 There are a wide range of PMT types and sizes

From Hamamatsu catalog
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ATLAS Tile Calorimeter PMT
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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
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Light Guides
Consider a phasespace elementfor a phot onin a light guide
x  transverse coordinate
p  n sin   angular divergence
Liouville's theoremsays Dx1Dp1  Dx 2 Dp2
2Dx1n sin 1  2Dx 2n sin  2
Dx 2
sin  2 
sin 1
Dx 1
 So for a maximum output angle 2 the input angle
1 is limited
 Even for complete TIR, if Dx1 >> Dx2 there will be
substantial light loss
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Light Guides
 Transport light by total internal reflection (TIR)
next 1
sin  c 

n
n

The air gap between scintillator and wrapping is
important
 Maximum angle for TIR at light guide output is
2    90  c

 For light that does escape the light guide it can
be recaptured using specular (Al foil) or diffuse
(Tyvek) reflection
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Light Guides
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Light Guides
 An example
 Many people use Tyvek but one should do
studies for each specific application
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Light Guides
 There will be some light loss even in the case
of equal dimensions
sin  2   sin   90   c 
sin  2   cos   c   1  sin 2    c 
sin  2   1  sin 2  c  for small taperangles
1
sin  2   1  2
n
thensin 1  
Dx 2
1
1 2
Dx1
n
for Dx1  Dx2 and n  1.5
sin 1   0.75
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Wavelength Shifter
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
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
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Wavelength Shifter
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