Detectors for Synchrotron
Radiation
Rob Lewis
Monash University

Factors Limiting Science■ Detectors are an oftneglected but crucialpart of an experiment■ They often limit thescience

X-rays and their Interaction with Matter

Beam
Sample Time
5%15%
Detector
54%

Otherfactors
11%

Courtesy SRRC, Taiwan

The beam
15%

Scientist’s View of Detector

The Truth!

Detection process

Input

Input

OutputOutputResultResultPage 11

Detection Mechanisms

Albert Einstein■ There are many means of detection. All require
the interaction of photons with matter■ Examples include

Nobel prize inphysics 1921

♦ Gas ionisation● Photons produce electrons and ions which are then detected● E.g. Ion chambers, proportional counters

Photoelectric Effect"for his services to
Theoretical Physics,
and especially for his
discovery of the lawof the photoelectriceffect"♦ Photoelectric effect● Photons eject electrons from a solid creating a current which is measured● E.g.. Beam monitors

Germany and Switzerland
Kaiser-Wilhelm-Institut(now Max-Planck-Institut)
für Physik
Berlin-Dahlem, Germany
1879 - 1955

♦ Generation of electron hole pairs● Photons produce electrons and holes in a semiconductor which are thendetected● E.g.. CCDz

Photoelectron

e+
+ ++
+ +

♦ Fluorescence, scintillation and F centres● Photons produce prompt fluorescence or F centres● E.g. Image plates and Scintillation counters-♦ Chemical effect

X-ray photon λ1● Photons create a chemical change such as dissociating Ag halide● E.g. Film

Arthur Holly Compton

Nobel prize inphysics 1927
"for his discovery of
the effect named
after him"

Compton Effect

University of Chicago
Chicago, IL, USA
1892 - 1962

X-ray photon
λ2 > λ1
X-ray
photon λ1

K fluoresence
X-ray E~Ek-

What are we trying to do?
■ We are usually trying to determine the effect of thesample on the beam
■ i.e. we are looking for a DIFFERNCE■ We therefore need to measure the beam both before andafter the sample

+
+ ++
+ +

Input beamφ

Detector
Input beam Measured beamSampleOutput beam

Detector--h(1− cosϕ )m0ce-■ There are many means of detection. All require theinteraction of photons with matter■ How can we do that without the detector changing thebeam?

An Example Detector

Ionisation Chamber
Gas Volume+
e-Photons+ ions
-

Bias

+

Δλ =

■ Very simple device■ Approximately 1 e- ion pairper 30eV deposited■ Important that recombination
low as possible♦♦

Higher voltages required at higher
rates since more carriers
Diffusion losses caused byseparation of carriers minimisedby higher voltages■ Ion chambers are sensitive
pressure and temperature

Echidna

Page 22

Field Variation

Operation regions of gas filled detectors
Pulse amplitude

αCathodeThreshold forgas multiplication.
Typically 106 Vm-1
10kVcm-1

Ionisationchambers

E
Geiger-Mueller
regionIon
saturation

Proportional
regionFielldE(r) ~ 1/r
Proportionalcounters

Limitedproportional
regionApplied Voltage

n is number of charges
x is distance
α is the first Townsend coefficientdn= α dxnn ( x ) = n ( 0) eα xDistance from wire

Microstrip Variants

Avalanche & Proportional Counter
Increasing
electric
field

Positive
ions

Typical anode width
10 microns

X-Ray photon

Initialionisation
Gas Volume
Electronavalanche
Gain >> 1

Micro Dot

Electrons
out

Anode
Electrons

Counting and Integrating

Counting & Integrating
1.2■ If there is sufficient signal produced by theinteraction of a photon or a particle in thedetector then it is possible to operate thedetector as a counter1

Detecttor output

0.8

PhotonSignal

0.6
0.4
0.2
0

Noise = 1
Signal = 0.01

-0.2
0

100

200

Time

300

400

500

Page 33

Counting & Integrating

Counting and Integrating4■ Usually this is not true and we have toaccumulate many photons/particles before
the signal becomes measurable

PhotonSignal3

Detecttor output

2
1
0
-1
-2
-3

Noise = 1
Signal = 1-40Counting & Integrating

100

200

Time

300

400

500

Counting & Integrating

18

30
Photon16
14Photon25

Counting
Integrating

Counting
Integrating

20

Detecttor output

Detecttor output

12
10
8
6

Noise = 0.01
Signal = 1

4

15
10

Noise = 1
Signal = 1

5
0

2
-5

0
-2

-10
0

100

200

Time

300

400

0

500

100

200

Time

300

400

Integrating Detectors

Photon Counting Detectors■ Mode■ Mode♦ Measures deposited energy at♦ Detects every photon as it

end of integration period♦

Outpuut signal♦♦

signal (“fog level”)
Dead time between frames
2×20 keV phts = 1×40 keV
photon i.e. Cannot perform
simultaneous spectroscopy andpositioning
Examples: Image plates,
CCDs■ Characteristics♦ Quantum limited, Detector
noise often negligible
g g♦ No dead time between frames♦ Can measure position andOuttput signal

arrives. Only active pixels read

■ Characteristics♦ High input flux capability♦ Read noise dominates at low

500

energy simultaneously♦ Limited input flux capability♦ Examples: Prop counters,

Input flux

Input flux

Scintillators

Page 44

Dark Signals & Fog

Types of Detectors

Intensity

Real Signal

Measured signal
overtakes backgroundsignalTotal background

Dark Signal

Fog level / Digitisation Threshold

Crimson Rosella and King Parrot

Time

X-ray Film

Intensifying ScreensFilm■ Active Ingredient♦♦♦♦

Small crystals of silver halide ~1.0 - 1.5μm
Typically 90-99% silver bromide and 1-10% silver
iodide.
Suspended in the gelatin of the film emulsion.
Crystals have a cubic lattice with many point
defects and free silver ions

■ An intensifying screen converts x-ray
energy into light energy■ X-rays are absorbed by the phosphor■ The phosphor becomes excited &fluoresces emitting UV and/or visible
light■ For every x-ray photon absorbed,
hundreds of light photons are emitted■ The
Th use off intensifyingi t if i screens
inevitably means that certain degreeof unsharpness will be introduced
into the image in comparison to nonscreen film

■ Exposure♦♦♦♦

A photon liberates an electron from a bromide ion
The electron travels until trapped at a defect
A free silver ion is attracted to the negative charge
andd combinesbi(is(i reduced)d d) to formf
an atom off
metallic silver (which is optically black).
The single silver atom acts as an electron trap for
another electron which then attracts another atomof silver which is then reduced to metallic silver.
This process continues while the exposure to light
continues.

X-Ray in

Screens■ Conventional film is usually coated withemulsion on only one side, radiographic film
is usually double coated (on each side of thebase) to be used with intensifying screens.

Charge Coupled Device

CCD Readout
Photons in

Photons
in

0V

V1

+V

StoredChargeSi substrate

V2

0V

0V

SiO2
insulator

0V

+V

+V

0V

0V

+V

+V

0V

V3
Si O2
Siliconsubstrate
Columnisolation
Electronhole pair
Depletion
regionSignalelectrons

Clock rows intoline readout
sectionReadout line

Page 55CCD Readout

CCDs■ Charge is moved from pixel to pixel by clocking■ Each pixel has a limited capacitance (well depth)
typically 104-105 e■ This limits dynamic range for direct detection

62mm♦ 10keV photon creates ~ 3000e- so saturation = ~ 10 photons■ Speed of clocking is restricted by line capacitanceand charge transfer efficiency♦ Size of CCD restricted by this■ Noise can be reduced by cooling■ Amplifier usually on chip

Although sizes > 50mm are available, the read speed is slow topreserve low noise and cte ( line capacitance becomes very high)Shutter required♦ Heats up that part of chipCCD detectors

TV detector with IIT

Direct detection
Gain ~ 2000e- / 8keV x-ray

Phosphor
Gain >> 1

Phosphor coupled with reducing optics to CCD
Phosphor gain >> 1
Optics Gain << 1

OpticsGain < 1

OpticsGain < 1

Image
Intensifier
Gain >> 1

CCD/SIT
Gain > 1

Phosphor coupled 1:1 to CCD
Phosphor gain >> 1
Optics Gain < 1

X-Ray in

Optical Photons
Electrons

X-Y Flat bed Scanner

Computed RadiographyRadiography-Image Plate
Exposure
Creation of F
centres
Gain >> 1

Scanning
Stimulationof PSL
Gain < 1

Collectionof PSL
Gain < 1

Phosphor Plate

PMT
Amplification
Gain > 1

Fibre optic light guide

He Ne Laser

Galvanometer mirror

Photomultiplier tube

Blue Filter
F-theta correcting mirror

Distributed Light Collection

Page 66

TFT Flat panel Detectora-Si:H TFT arrayshv

Indirect Conversion
scintillator orconverter

Amorphous silicon
substrate
Pixel active
area

Pixel readout
electronics

Needle diameter

6μm
μ

hv

Direct Conversion+ +scintillator orconverter

Amorphous silicon
substrate
Pixel active
area

CMOS Imagers

a-Si:H Array dpiX - Flashscan 30
Data Line

Bias Line

Row
line

Pixel readout
electronics

TFT
Switch■ Based on standard manufacturing
technology♦ Electronics integrated onto same chip as image
sensor

♦ Gd2O2S - Kodak Lanex® Fine
■ Rad Eye 1 from Radicon♦ 1024x512 48 micron pixels♦ 24.6 x 49.2 mm2 active area
♦ MTF good to 10 lp/mm♦ Tileable on 3 sides♦ Dark current ~4000e-/s = 10 x CCD room

One
pixe
l

Photodiode

Read Amplifiers

Row Drivers

temp♦ 10000:1 dynamic range♦ 4.5 frames/s■ Cheap

Bias supply

Medipix 1

Sardine
■
■
■
■
■
■
■
■
■
■
■

Square pixels of 170µm side-length
64 x 64 pixels per chip
Sensitive to positive charge
Leakage current compensation at
the input
Maximum count rate: ~2 MHz
1 comparator per pixel with 3 bits
fine tuningg
Minimum threshold ~1500e-(~5.5
keV deposition in a Si sensor)
15 bit counter per pixel (32767)
single events
Variable acquisition time
Read-out time: 384µs @ 10 MHz
Active area of the chip: ~1.2 cm²■ Mo X-ray tube +30 µm Mo filter 25 kV,
10 mAs,■ Detector stepped for
one detector width in X
and half a detector
width in Y
■ Aquisition
A i iti time
ti perimage: 500 ms.
■ No image correction
was used (raw data!).
■ The thickness of the
fishbones corresponds
roughly to the pixel
size.

http://medipix.web.cern.ch/MEDIPIX/

Page 7
7

Lysozyme PILATUS

PILATUS 6M Detector■ 1° rotation■ 2s exposure,■ E=12 keV■ 7 detector positions■ Flatfield correctedPaul Scherrer Institut

Detector Group

Ch. Brönnimann, E. Eikenberry, B.Schmitt, M. Naef, G. Hülsen(SLS); R. Horisberger, S. Streuli (TEM); Ch. Buehler (LOG); F.
Glaus (LMN); M. Horisberger (LNS)

Spectroscopic Detectors

Paul Scherrer Institut

Detector GroupElectron multipliers & Scintillators
Scintillator

Optical

X-raysIncreasingly +ve

Channeltron is a similar with
distributed dynode
Increasingly +ve

Micro-channel plates are
mutlichannel channeltrons
with each channel being anelectron multiplier.

Rainbow Lorikeets

SPring--8 128 channel Ge strip
SPringMulti Channel Spectoscopic Detectors■ Ge♦ 55.5×50.5×6mm■ Strips♦♦♦♦

Canberra Ultra-LEGe detector

Number
WidthInterstrip
Length

128
300µm
50µm
5mm

■ Readout♦ Single channel♦ 32 channels

WRULEAD (Windowless, Retractable, Ultra Low
Energy Array Detector) works down to 300eV

100ns
3.2ms

■ Max expected count rate

Multichannel devices up to 30 channels at 3×105 cts
s-1 channel-1 have been built♦ 14kcps

Page 88

Scintillator vs Germanium

Spectral Resolution■ Average number of carriers, N = E/w
where w is energy to create electron hole/ion pair
■ Poisson statistics σ = 1/√N= (E/w) -½ = (w/E)½■ ΔE/E fwhm
f h
= 2.355σ
2 355= 2.355(w/E)½■ For Ge, w= 3eV so at 10keV ΔE/E ~ 4%
■ For NaI, w= 30eV so at 10keV ΔE/E ~ 13%

Things to Look Out For

Fano Factor■ If all energy from photon or particle were converted into carriers
there would be no variance■ Poisson statistics assume only a small fraction of energy goes into
charge creation■ Reality is somewhere in between so we 2introduce Fano factor FσF=■ Fano factor is defined as
μ2where σ is the variance and μ is the mean number of carriers■ For a Poisson process, the variance equals the mean, so F = 1■ Examples♦

Si: 0.115
Ge: 0.13
GaAs: 0.10
Diamond: 0.08

■ Observed relative variance = F x Poisson relative variance
Crocodile

Efficiencies
1

Gas thickness
Windows

1cm
100μm

0.9

1

ESRF TV Detector
Thompson IIT & CCD

0.9

0.8

Be
Al

0.7

Polimide
Polyropylene

0.6

Ar
Kr

05
0.5

Xe

1.15

0.8

1.1

0.7

1.05

0.6
1

05
0.5

p

TransmmissionResponse to Uniform Illumination

0.95

0.4

0.4

0.3

0.3

0.85

0.2

0.2

0.8

0.1

0.1

0.9

0

0
0

5000

10000

15000

20000

25000

0
30000

Energy (eV)Page 99Gaps

Graded Absorber Comparison
Spec

0.2mm max

Worst gap

2.97mmPixels in gaps

513922
5.45%

Mar Image Plate

Spatial distortion

ESRF-Thompson IIT / CCD

Daresbury MWPC

IPlate Single Peak PSF
1.E+00ESRF Imageintensifierdetector

1.E-01

Intensity

1.E-02

1.E-03

1.E-04

1.E-05

1.E-060

Parallax Broadening

5
Position (mm)10Parallax Effect
Image Plate

Gas Proportional Counter

Input

Position
Detection
VolumeOutputPosition

Page 1010Daresbury High Pressure MWPCRAPID2 SAX WAX

1 atmosphere

HeliumMicroGap
Quadrant
Detector

Xenon

SamplePressure Regulation
SystemForce on 28 x 28 cm window at 5 bar = 4 tonnes
Force on window of 1 x 1 cm at 5 bar = 5 kg

MicroGap
CurvedDetector

5 atmospheres

RAPID
electronics

SampleWAXS detector - GMSD

Geometric Distortion
250

0.12

Vertical

0.1
200

0.08

Fit Y

Residuals (mm)Measured Position (mm)

0.06

150

0.04
0.02

100
0

-0.02

50

-0.04

0

50

100

150
Position (mm)

-0.06

120.7

200

250

0.08
Horizontal

120.65

0.06

120.6

Measured Position (mm)

0.04
Fit X

120.55

0.02

120.5

0

120.45

-0.02

120.4

-0.04

120.35-0.06
0Overlaps

Residuals (mm)

0

50

100

150
Position (mm)

200

250

Dark Currents

250

200
Set 1

150

100
0

100

200

300

400

500

600

Flat and Dark Correction
For each image, two correction images
must be recorded.
1. A flat field (uniform illumination of
the detector)
2. A dark image (no irradiation of
detector)
B th mustt beBothb recorded
d d with
ith theth same
exposure time as the original image
since dark current is a function of
exposure time.
Then apply the following correction
Corrected =(image − dark )( flat − dark )Page 1111

Dark Current

Pixels above the 0.2 photons pix-1 specification

Subtraction of dark images30

Top row
Middle row
Bottomrow

25

20

Difference (ADUs)

1000
800
600
400
200

15

10

0
10
5432
9876
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74

Number failing 2 measurements 5-2000s
Mean
44764
0.47%
Min
40822
0.43%
Max
48706
0.52%
nb. 14300 pixels not common to both

5

0
0

500

1000

1500

2000

2500

3000

3500

-5

-10
Position (pixels)

Flashscan 30 - Image Lag

Radiation Damage (Medipix)■ Damage occurred at 40Gy or
1.3×1010pht/mm2 in the readout chip
■ At 13 keV photon energy

Fraction of initial d ifferre nce rem ainin g

100.00%♦ Strong diffraction spots typically 105 phts/s or 106

10.00%

phts/mm2/s● Damage requires ~ 8hours exposure♦ Direct beam (1010–1013 photons/mm2/s)● Damage in less than a second.

1.00%

0.10%
0

2

4

6
Time (Minute s)

810dpiX Flashscan 30 PaxScan 4030

12Flashscan 30 - Performance
Mar Image Plate

Flashscan-30

tint=30s

tint=190s

Page 1212Electronics Issues

Amplification
Rf

Cf

Cin

Rg

■ In almost all cases we require
some form of amplification■ Interaction of pre amplifier
with detector is critical for
performance of system■ Most important element is the
input, often a FET■ Noise is the major issue■ Voltage mode♦♦

Thermal or Johnson Noise♦

Shot Noise● Brownian motion of electrons● No current flow required

Output α input voltage
Effect of Rf dominates Cf■ Current mode♦♦♦

Output α input current
Low input impedancev 2 = 4kTRdf● Fluctuations in current■ Charge mode
Koalas♦♦Albino Kookaburra

Equivalent Noise Charge

Noise Dependence
2
⎡ kT
qI
kT (Cin ) ⎤τ + e Dτ +ENC 2 = e 2 ⎢⎥4
2 g mτ ⎥⎦⎢⎣ 2 R f■ Low noise is no use if signal is low■ Introduce ENC which is that signal charge that will produce the same output as
the RMS noise

Where
■ k
■ T
■ e
■ Rg
■ gm
■ τ
■ Cin

2
⎡ kT
kT (Cin ) ⎤eIτ + Dτ +ENC 2 = exp(2 )⎢⎥4
2 g mτ ⎥⎦⎢⎣ 2 Rg

=
=
=
=
=
=
=

i 2 = 2qe I df

Output α input charge
Cf dominates Rf

Boltzman’s constanttemperature
the electronic charge
Load resistance and/or feedback resistancetransconductance of input FET. (Links current in to voltage out)
Rise time of amplifier
input / stray and feedback capacitance■ τ optimum at2⎡⎤
kT 2 g m⎥ Cin⎢⎣ (kT 2 R f ) + (qe I D 4 ) ⎥⎦

τ opt = ⎢■ Choosing optimum τ gives best noise performance but may not befast enough■ We often have to sacrifice energy resolution for speed■ Note that ENC is directly related to energy resolution■ FWHM(keV) = 2.355×10-3 ENC/ew where w is the energy per electron

Optimum τ⎡⎛ kT ⎞ ⎛ eI ⎞⎛ kTC 2 ⎞⎤2in ⎟⎟ + ⎜ D ⎟⎜ENCmin = 2 exp(2) ⎢⎜⎜⎟⎥⎜⎟⎣⎢⎝ 2 Rg ⎠ ⎝ 4 ⎠⎝ 2 g m ⎠⎦⎥2■ Rg as large as possible ~ 1010Ω■ ID (leakage) as small as possible♦ For Ge coolingg is vital■ Low T is good
■ Cin as small as possible (note that thisincludes Cf)■ gm as large as possible but this affects Cin

Optimum Spectral Resolution■ Low capacitance♦ Small planar < 1pF■ Low leakage currents■ Maximise Rg and/or Rf♦ Remove altogether so Rf = ∞■ Use optical reset■ Can improve FWHM by
20%

Cf
Detector

Bias
LEDPage 13
13

Shannon’s Theorem or Nyquist Criterion

Synchrotron Detectors■ If the input is not band limited to frequencies less than ωs/2, thenaliasing will occurs at frequencies ω1±n where;■ ω1 = original signal frequency, ωs = sampling frequency, n = aninteger■ A synchrotron source is used primarily
when sensitivity is an issue

1

0.125

0.8

0.925

0.6

1.725

♦ Signal too weak
♦ Time resolution too poor
♦ Sample too small

Sampling

0.4
0.2■ More intensity can help this but…
■ It places a major strain on detectors and

0
-0.2
-0.4
-0.6
-0.8

Flux is a major issue!

-1
1

2

3

4

5

6

7

8

Dead TimeEDR Detector for Powder Diffraction
Standard detector

Standard Detector

Counter

Non-Paralysable

EHT

Modified Detector

Paralysable

Low pass
filterCurrent
amplifier♦Detected rate

P(n, t ) =Rd = P(0,τ ) = Ri e − Riτ

e − Rit (Ri t )
n!

Ideal

n

2 .1061.5.106

Intensity (cps)

Rd = Probability of getting no event within τ of an event
Probability of n events in time t is

Saturation count rate: 3 MHz
Linear region: up to 2 MHz
Electronic background: 0.7 cps
Energy range: 4-25 keV

2 .106

Real■ Paralysable
♦

•
•
•
•

Modified
detector

Ideal

Fraction of time detector is dead = Rd τ
Live time is therefore = 1- Rd τ
Input rate = Ri= Rd/(1- Rd τ)

Saturation count rate: 1 MHz
Linear region: up to 300 kHz
Electronic background: 0.2 cps
Energy range: 4-25 keV

Modified detector

Window
discriminator

Pole-zerocancellation

EHT

3 .106■ Non-paralysablep y♦Counter

YAP photomultiplier

Ri=input rate, Rd=detected rate, τ dead time

♦
♦
♦

•
•
•
•

Window
discriminator

Current
amplifier

YAP photomultiplier

1.106

Intensity (cps)0■ The highest frequency that can be measured is twice the sampling
frequency■ If you have 100μm pixels, ideal PSF > 200μm

Standard
detector

1.106

5.105

0

Real

0
0.0

0.2

0.4

0.6

1.43

0.8

1.44

1.45

Gas
Ar
Iso C4 H10(OCH3)2 CH2Ar
Iso C4 H10
Ar
CH4
Ar
CO2
Ar

Ions
(OCH3)2 CH2+
(OCH3)2 CH2+
(OCH3)2 CH2+
Iso C4 H10+
Iso C4 H10+
CH4+
CH4+
CO2+
CO2+
electrons

Mobility (cm2 V-1 s-1)
1.51
0.55
0.26
1.56
0.61
1 87
1.87
2.26
1.72
1.09
~1000

1

Input rate
1.2E+60.9

2.8E+50.8

2.5E+3

1.48■ As rate rises♦ Spectral resolution
deteriorates

0.7
0.6

♦ Note also the K escape

0.5

feature

0.4
0.3
0.2
0.1
0
0

For 1kV across 1cm.

1.47

Spectral Peak Shift vs Rate
Normaalised Counts

Ion Mobility

1.46

ω(deg.)

Slit size (mm)Electrons take 1ms

50

100

150
200
Channels

250

300

Ions take ~1ms!

Page 1414

Detector Considerations■ Intensity Measurement♦ Uniformity across device♦ Ageing, radiation damage♦ Dynamic Range♦ Linearity of Response♦ Stability■ Spatial Measurement♦ Spatial Resolution♦ Spatial Distortion♦ Parallax

A Universal Specification?

■ Energy Measurement♦ Spectral Resolution♦ Linearity of Response♦ Uniformity of Response♦ Stability■ Time Measurement♦ Frame Rate
♦ Photon Time Resolution■ Others♦ Size and weight♦ CostWombatCounting Statistics

Performance Measure - DQE■ Photons are quantised and hence subject to probabilities■ The Poisson distribution expresses the probability of anumber of events occurring in a time periodnk e−n■ If the expected number is n thenP(n, k ) =k!
■ The mean of P(n,k) is n
■ The variance of P(n,k) is n
■ The standard deviation is √n■ Fractional error = (√n)/n=1/√n■ As n increases, uncertainty and noise decrease∴ N inc = SNR 2 inc

Perfect detector

SNRinc = N inc

Real detector

SNR Non −ideal < N inc

Can define Nphotons that describes real SNR
NEQ = SNR 2 Non −ideal

Ratio of this to Ninc is a measure of efficiency
DQE =NEQ SNR 2 Non −ideal
=
2
N inc
SNRincNote that DQE is f(spatial and spectral frequencies)

DQE Comparison

Effect of Peak Width

DN-5 beam
2.6μGy120

60%Flat Panel100
1 pixel80

400 film

50%

2 pixel5 pixel
10pixelLight sensitive screen

60

D
DQE

Inte
ensity

40%

40

30%
20%

20

10%
0

0%

-20

0

Position

0.5

1

1.5
2
2.5
Frequency (lp/mm)

3

3.5

4

Page 15
15

To Count or Not to Count

Specifications

Tasmanian Devil

WombatCollagen 100s Exposure

Collagen 10s Exposure

MWPC

MWPC

Image
Plate

Image
Plate
10000

100000

Proportional
Counter

Proportional
Counter

Intensity (Photons/mm^2)

Intensity (Photons/mm^2)

10000

1000

100

1000

100

10

1

10
0

20

40

60

80

100

120

140

0

160

20

40

60

80

100

120

140

160

Position (mm)Position (mm)Collagen 0.3s Exposure

Cornell PAD (Integrating)■ Rapid Framing Imager
♦
♦
♦
♦
♦

MWPC

Image
Plate

15×13.8mm2 active area
150µm square pixel
Storage for 8 frames
Selectable Tint down to 1µs
Deadtime < 1µs

Intensity (Photons/mm^2)

Proportional
Counter

10

1

0.1

0.01
0

20

40

60

80

100

120

140

160

Position (mm)

Sol Gruner, Cornell

Page 16
16

The Furture

Diesel Fuel Injection Movie■ Injection♦♦♦♦■ Movie
♦
♦
♦
♦
♦
♦

Supersonic injection 1350psi Cerium added
Chamber 1atm SF6
108-109 X-rays/s/pix (6keV)
1.1ms Pulse

LengthFrame length
Dead time
168 frames (21 groups of 8)
Average 20× to improve S/NSequence

1.3ms
5.13µs2.56µs / frame
5×104 images

A. MacPhee et al, Science (2002) 295, 1761-1763

Pixel Array Detector

A Detector System

A.
B.

X-ray

Detector

HPD
ADC

Preamp

C.
D.
E.
F.

101

1001

E=23

A

FPGA GDAQ

B

X=145

C

Top electrode
Pixellated
semiconductor
Collection electrodes
Bump bonds
Input electrode
Pixellated ASIC

D

E

F

Y=2371Counting Pixel Detector Problems

Available Compound Semiconductors■ High power consumption■ Predominately CdZnTe,
CdTe and GaAs.
■ II-VI materials CdTe and
CdZnTe cover a suitable
range of band gaps:♦ 1.44 eV (CdTe), 1.57 eV♦ Cooling■ Number of connections♦ Multiplexing♦ Read out time significant■ Limited number of bits in counter(CdZnTe, 10% Zn),
(CdZnTeZn) 11.64
64 eV(CdZnTe, 20% Zn)♦ Dynamic range issues for diffraction♦ 15bits @ 1Mcps input rate = 30ms frame♦ Read time can be significant● Fast read > high power■ Technology not yet good enough for microsecond
framing■ Resistivity of CdZnTe ishigher than CdTe, hence
lower dark current, higher
spectroscopic resolution■ Poor hole transport requires
electron-sensitive detectors
Paul Sellin, Surrey

Page 1717

Absorption Efficiency of Semiconductors

CdZnTe Spectral Resolution
with Te inclusions

without Te inclusions

Paul Sellin, Surrey

References■ Delaney CFG and Finch EC♦ Radiation detectors. Physical Prnciples and Applications,
Clarendon Press, Oxford 1992, ISBN 0 19 853923 1■ Knoll GE♦ Radiation Detection and Measurement, John Wiley and Sons
1989

■ Proceedings of the 6th International Conference on
position sensitive detectors♦ Nuclear Instruments and Methods in Physics Research A513
(2003)■ IEEE Nuclear Science Symposia

Semiconductors

Readout Strategies■ Imaging♦

Massively parallel♦

X-Y Interpolating♦

Sequential● Position derived from individual pixel● Highly parallel: 2000×2000 pixels = 4 million channels!!!
● Suitable for counting and integrating systems● Pixel array detectors● Position derived from measuring signals● Moderately parallel: 2000×2000 pixels from few hundred channels● Only suitable for counting systems● MWPCs e.g. RAPID● Position derived from point in sequence● Not really parallel● Only really suitable for integrating systems● CCDs, Image plates■ Spectroscopic♦

Can only add more channels for speed

Page 1818

Signal Levels
Energy perelectronhole pair,w (eV)

Stage 1signal @10keV

Scintillators - Basic Properties
Stage 2
Minimum N
Transfer to @ 10keV
electrongainStage n0 noisegain

Signal(e-)

Gas Ionisation
Argon

24.4

410e-

1

410

105

4×107

Xenon

20.8

481e-

1

481

5×104

2.4×107

S lid State
Solid
S
Silicon3.62

2760e-

1

2760

1

2.8×103

Germanium

2.963380e-

13380

1

3.4×103266 photons

0.1

30

105

3×106
2×105
5×105

Fluorescence or scintillation
NaI(Tl) + PMT
Gd2O2S + IIT

500 photons

0.04

20

104

BaFBr:Eu2+

75 F centres

0.07

5

105

Light O/P
[photons/keV]

Decay Time[ns]

Emis. Wavelength[nm]

Density[g/cm3]

NaI(Tl)

38

250

415

3.7

CsI(Tl)

54

1000

550

4.5

BaF2

10

0.7/630
fast/slow

220/310
fast/slow

4.9

LaCl3(Ce)

49

28

350

3.8

LaBr3(Ce)

66

16

380

5.1

FWHM energy resolution at @ 662 keV
NaI(Tl)LaCl3
LaBr3
CdZnTeΔE/E ~ 6%
ΔE/E ~ 4%
ΔE/E ~ 3%
ΔE/E ~ 2% (after correction for carrier recombination)

Tortoise and Hare?
■ Accelerators currently 1013-1014 photons tosample■ New machines e.g. XFEL, TESLA♦ 1025 photons to sample!!!

■ Detectors♦ Currently 107-108♦ In 10 years…………

■ Hare shows no sign of slowing down
■ Tortoise is not catching up

Page 1919