Detectors for Synchrotron
Radiation
Rob Lewis

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

Beam
Sample Time
5%15%
Detector
54%

Otherfactors
11%
The beam
15%

Scientist’s View of Detector
Detection process

Input

OutputResultThe Truth!

Input

OutputResultDetection Mechanisms■ There are many means of detection. All require
the interaction of photons with matter■ Examples include♦ Gas ionisation● Photons produce electrons and ions which are then detected● E.g. Ion chambers, proportional counters♦ Photoelectric effect● Photons eject electrons from a solid creating a current which is measured● E.g.. Beam monitors♦ Generation of electron hole pairs● Photons produce electrons and holes in a semiconductor which are then detected● E.g.. CCD♦ Fluorescence, scintillation and F centres● Photons produce prompt fluorescence or F centres● E.g. Image plates and Scintillation counters♦ Chemical effect● Photons create a chemical change such as dissociating Ag halide● E.g. Film

Albert Einstein

Nobel prize inphysics 1921
"for his services to
Theoretical Physics,
and especially for his
discovery of the lawof the photoelectriceffect"Photoelectric EffectGermany and Switzerland
Kaiser-Wilhelm-Institut(now Max-Planck-Institut)
für Physik
Berlin-Dahlem, Germany
1879 - 1955

Photoelectron

e+
+ ++
+ +

-

X-ray photon l1-

K fluoresence
X-ray E~Ek

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
l2 > l1
X-ray
photon l1

+
+ ++
+ +

υ
-

-

h
1  cos  
l 
m0c

e-

An Example Detector

EchidnaIonisation Chamber+ePhotons+ ions-+Gas Volume■ Very simple device■ Approximately 1 e- ion pairBias
per 30eV deposited■ Important that recombination
low as possible♦ Higher voltages required at higher
rates since more carriers♦ Diffusion losses caused byseparation of carriers minimised byhigher voltages♦ Plates too close cause electron
losses■ Ion chambers are sensitive topressure and temperature

Operation regions of gas filled detectors
Pulse amplitudeaThreshold forgas multiplication.
Typically 106 Vm-1
10kVcm-1

Ionisationchambers

Proportionalcounters

E
Geiger-Mueller
regionIon
saturation

Limitedproportional
Proportional
regionregionApplied Voltage

n is number of charges
x is distancea is the first Townsend coefficient

dn
 adx
n

n( x)  n(0)eax

Field Variation
CathodeField

E(r) ~ 1/r

Distance from wireAvalanche & Proportional Counter
Increasing
electric
field

Positive
ions

X-Ray photon

Initialionisation
Gas Volume

Electronavalanche
Gain >> 1
AnodeElectrons

ElectronsoutGeorges Charpak

Nobel prize inphysics 1992
"for his invention anddevelopment ofparticle detectors, inparticular themultiwire proportional
chamber"

Multi-wire Proportional Counter
FranceÉcole Supérieure dePhysique et Chimie
Paris, France; CERN
Geneva, Switzerland
b. 1924(in Dabrovica, Poland)Counting and Integrating■ 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 counter■ It’s all about signal to noise ratio!

SNR = 100
1.2
1Detector output

0.8

17 photons hit detector
16 photons detected

PhotonSignal

0.6

Threshold

0.4
0.2
0

Signal = 1
Noise = 0.01

-0.2
0

100

200

Time

300

400

500

SNR = 1
4PhotonSignal3

Detector output

2
1

Threshold

0

-1
-2
-3

Noise = 1
Signal = 1

-4
0

100

200

Time

300

400

500

Counting and Integrating■ Usually SNR is insufficient and we have toaccumulate many photons/particles before
the signal becomes measurable

Counting & Integrating SNR =10018
Photon16

Counting

14

Integrating
Detector output

12

17 photons hit detector
1086Noise = 0.01
Signal = 1

4
2
0
-2
0

100

200

Time

300

400

500

Counting & Integrating SNR = 1
30
Photon25

Counting
Integrating

Detector output

20
15

17 photons hit detector

10

Noise = 1
Signal = 1

5
0
-5
-10
0

100

200

Time

300

400

500

Integrating Detectors■ Mode♦ Measures deposited energy at
end of integration period♦ High input flux capability♦ Read noise dominates at low

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

Output signal■ Characteristics

Input flux

Photon Counting Detectors■ Mode♦ Detects every photon as it■ Characteristics♦ Quantum limited, Detector noise
often negligible♦ No dead time between frames♦ Can measure position andenergy simultaneously♦ Limited input flux capability♦ Examples: Prop counters,
Scintillators

Output signal

arrives. Only active pixels readInput flux

Types of Detectors

Crimson Rosella and King Parrot

X-ray Film

■ 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

■ 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
and combines (is reduced) to form an atom of
metallic silver (which is optically black).
♦ The single silver atom acts as an electron trap for
another electron which then attracts another atom ofsilver which is then reduced to metallic silver. This
process continues while the exposure to light
continues.

■ 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.

Intensifying Screens■ 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 use of intensifying screens
inevitably means that certain degreeof unsharpness will be introduced
into the image in comparison to nonscreen film

Film

X-Ray in

Screens

Willard S. Boyle & George E. Smith
Nobel prize inphysics 2009
"for the invention of
an imaging
semiconductor circuit– the CCD sensor"

Bell Laboratories
Murray Hill, NJ, USA

CCD

Charge Coupled Device
Photons
in

V1
V2
V3

Si O2
Siliconsubstrate
Columnisolation
Electronhole pair
Depletion
regionSignalelectrons

CCD Readout
0V

+V

0V

0V

Photons inStoredChargeSi substrate

Clock rows intoline readout
sectionReadout line

SiO2
insulator

0V

+V

+V

0V

0V

+V

+V

0V

CCD Readout■ 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♦ 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
♦ Heats up that part of chip

CCDs
62mm

Although sizes > 50mm are available, the read speed is slow topreserve low noise and cte ( line capacitance becomes very high)Shutter required

Complimentary Metal-Oxide
Semiconductor (CMOS)■ A readout amplifiertransistor on each pixel
converts charge to voltage■ Allows random access to
pixels, similar to the rowcolumn memory cellaccess in RAM

CMOS vs CCD■ Traditionally CCD higher sensitivity and lower
noise■ Modern lithography means they are now similar■ CMOS sensors can have much more functionality
on-chip than CCDs
♦ On chip image processing, edge detection, noise reduction, andanalog to digital conversion■ CMOS lower power → less heat → less noise

Use with X-rays
Direct detection
Gain ~ 2000e- / 8keV x-ray

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

Well depth = 2×105
So dynamic range ~100
Phosphor coupled 1:1 to CCD
Phosphor gain >> 1
Optics Gain < 1TV detector with IIT
Phosphor
Gain >> 1

OpticsGain < 1

Image
Intensifier
Gain >> 1

OpticsGain < 1

CCD/SIT
Gain > 1

X-Ray in

Optical Photons
ElectronsComputed Radiography-Image Plate
Exposure
Creation of F
centres
Gain >> 1

Scanning
Stimulationof PSL
Gain < 1

Collectionof PSL
Gain < 1

PMT
Amplification
Gain > 1

Blue Filter

X-Y Flat bed Scanner
Phosphor Plate
Fibre optic light guide

He Ne Laser

Galvanometer mirror

Photomultiplier tube

F-theta correcting mirror

Distributed Light Collection

TFT Flat panel Detector

a-Si:H TFT arrayshv

Indirect Conversion
scintillator orconverter

Amorphous silicon
substrate
Pixel active
area

Pixel readout
electronics

Needle diameterhv

Direct Conversion+ +scintillator orconverter

Amorphous silicon
substrate
Pixel active
area

Pixel readout
electronics

6mm

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

Bias Line

Row
line

TFT
SwitchOne
pixelPhotodiode

Read Amplifiers
Row Drivers
Bias supply

PILATUS 6M Detector

Paul 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)PILATUS 6M Detector■

Sensor 5 x 12 = 60 modules
♦
♦
♦

■
■
■

■
■
■
■

2463 x 2527 = 6,224,001 pixels
Area 431 x 448 mm2Intermodule gap x: 7 pixels, y: 17 pixels,
8.4% of total area
Dynamic range 20 bits (1:1,048,576)
Counting rate per pixel > 2 x 106 X-ray/s
Energy range 3 – 30 keV
Quantum efficiency(calculated)
♦

■
■
■
■
■

Reverse-biased silicon diode array
Thickness 320 µm
Pixel size 172 x 172 µm2

3 keV: 80%
8 keV: 99%
15 keV: 55%

Energy resolution 500 eV
Adjustable threshold range 2 – 20 keV
Threshold dispersion 50 eV
Readout time 3.6 ms
Framing rate 12 Hz
Point-spread function 1 pixel

PILATUS 6M Detector■ X-ray diffraction image recorded froma ferritin crystal (energy=16 keV,
distance = 204 mm).

Spectroscopic Detectors

Rainbow Lorikeets

Spectroscopic Detectors■ For quantitative work, most are counting
detectors that measure the size of individualenergy deposits■ Alternative ids the use of filters as in opticalcolour cameras

Electron 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.

Multi Channel Spectoscopic Detectors

Canberra Ultra-LEGe detector

WRULEAD (Windowless, Retractable, Ultra Low
Energy Array Detector) works down to 300eV
Multichannel devices up to 30 channels at 3×105 cts
s-1 channel-1 have been built

SPring-8 128 channel Ge strip■ Ge♦ 55.5×50.5×6mm■ Strips♦ Number♦ Width♦ Interstrip♦ Length

128
300µm
50µm
5mm

■ Readout♦ Single channel♦ 32 channels

100ns
3.2ms

■ Max expected count rate
♦ 14kcps

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= 2.355σ= 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%

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 introduce Fano factor F2
■ Fano factor is defined asFmwhere σ2 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Scintillator vs Germanium

Things to Look Out For

CrocodileEfficiencies
1

Gas thickness
Windows

1cm
100mm

0.9

0.9

0.8

Be
Al

0.7
Transmission

1

Polimide
Polyropylene

0.6

Ar

Kr

0.5

Xe

0.8
0.7
0.6
0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0
0

5000

10000

15000
Energy (eV)

20000

25000

0
30000

Response to Uniform Illumination
ESRF TV Detector
Thompson IIT & CCD
1.15

1.1

1.05

1

0.95

0.9

0.85

0.8

Gaps
Spec

0.2mm max

Worst gap

2.97mmPixels in gaps

513922
5.45%

Graded Absorber Comparison
Mar Image Plate

ESRF-Thompson IIT / CCD

Daresbury MWPC

Spatial distortion
ESRF Imageintensifierdetector

IPlate Single Peak PSF
1.E+00

1.E-01

Intensity

1.E-02

1.E-03

1.E-04

1.E-05

1.E-060

5Position (mm)10Parallax Broadening
Input

Position
Detection
VolumeOutputPosition

Parallax Effect
Image Plate

Gas Proportional Counter

Daresbury High Pressure MWPC
1 atmosphere

HeliumXenon

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

5 atmospheres

RAPID2 SAX WAX
MicroGap
Quadrant
Detector

MicroGap
CurvedDetector

Sample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
100

150
Position (mm)

-0.06

120.7

200

250

0.08
Horizontal

120.65

0.06

120.6

0.04
Fit X

120.55Residuals (mm)50

Measured Position (mm)

0

0.02

120.5

0

120.45

-0.02

120.4

-0.04

120.35

-0.06
0

50

100

150
Position (mm)

200

250

Overlaps

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)
Both must be recorded with the 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 Dark Current

Pixels above the 0.2 photons pix-1 specification

100

800

600

400

200
0

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

0
1
2
3
4
5
6
7
8
9
1
0
1
1
1
2
1
3
1
4
1
5
1
6
1
7
1
8
1
9
2
0
2
1
2
2
2
3
2
4
27
5
2
6
2
2
8
2
9
32
0
3
1
3
3
3
3
4
3
5
3
6
3
7
3
8
3
9
4
0
4
1
4
2
4
3
4
4
4
5
4
6
48
7
4
4
9
5
0
5
1
5
2
5
3
5
4
5
5
5
6
5
7
5
8
5
9
61
0
6
6
2
6
3
6
4
6
5
6
6
6
7
6
8
6
9
7
0
72
1
7
7
3
7
4

Subtraction of dark images30

Top row
Middle row
Bottomrow

25

Difference (ADUs)

20

15

10

5

0
0

500

1000

1500

-5

-10
Position (pixels)

2000

2500

3000

3500

Flashscan 30 - Image Lag
Fraction of initial differe nce remaining

100.00%10.00%

1.00%

0.10%
0

2

4

6
Time (Minutes)

8

10

12

Radiation Damage (Medipix)■ Damage occurred at 40Gy or
1.3×1010pht/mm2 in the readout chip
■ At 13 keV photon energy♦ Strong diffraction spots typically 105 phts/s or 106
phts/mm2/s●Damage requires ~ 8hours exposure♦ Direct beam (1010–1013 photons/mm2/s)●Damage in less than a second.

dpiX Flashscan 30 PaxScan 4030

Flashscan 30 - Performance
Mar Image Plate

Flashscan-30

tint=30s

tint=190s

Electronics Issues

KoalasAlbino Kookaburra

Amplification
Rf

Cf

Cin

Rg
v 2  4kTRf■ Voltage mode

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

♦ Output  input current♦ Low input impedance■ Charge mode

♦ Output  input charge♦ Cf dominates Rf■ In almost all cases we require
amplification■ Amplifier-detector interaction
is critical■ Most important element is the
input, often a FET■ Noise is the major issue♦ Thermal or Johnson Noise● Brownian motion of electrons● No current flow required● White noise♦ Shot Noise● Fluctuations in current● White noisei 2  2qe IfEquivalent Noise Charge■ 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
C
kT
eI
2
in
D
ENC  exp 2



4
2 g m 
 2 Rg

=
=
=
=
=
=
=

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■ Note that ENC is directly related to energy resolution■ FWHM(keV) = 2.355×10-3 ENC/ew where w is the energy per electron

Noise Dependence
2




q
I
kT
C
kT
2
2
e D
in
ENC  e 

42 g m 
 2 R f■ τ optimum at2


kT 2 g m
 opt   Cin kT 2 R f   qe I D 4■ Choosing optimum τ gives best noise performance but may not befast enough■ We often have to sacrifice energy resolution for speed

Optimum τ kT   eI  kTC in    D 
 2 exp( 2) 





 2 Rg   4  2 g m 
2

ENCmin22■ Rg as large as possible ~ 1010Ω
■ ID (leakage) as small as possible♦ For Ge cooling is vital■ Low T is good
■ Cin as small as possible (note that thisincludes Cf)■ gm as large as possible but this affects Cin

Sampling & Aliasing
1
0.8
0.6
0.4

0.2
0
-0.2 0

0.2

0.4

0.6

0.8

1

1.2

-0.4

f=1

-0.6

Sampling 5

-0.8

f=6
f=11

-1

■ Shannon’s Theorem and Nyquist Criterion♦ The highest frequency that can be measured is twice the sampling frequency■ If the input is not band limited to frequencies less than fs/2, then aliasing will
occurs at frequencies f±nfs♦ where f = signal frequency, fs = sampling frequency, n = integer■ If you have 100μm pixels, ideal PSF > 200μm

Synchrotron Detectors■ A synchrotron source is used primarily
when sensitivity is an issue♦ Signal too weak
♦ Time resolution too poor
♦ Sample too small■ More intensity can help this but…
■ It places a major strain on detectors andFlux is a major issue!

Dead Time
Non-Paralysable
Paralysable
Ri=input rate, Rd=detected rate, τ dead time

■ Non-paralysable

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

Real■ Paralysable♦ Rd = Probability of getting no event within τ of an evente  Rit Ri t ♦ Probability of n events in time t is Pn, t  n!
♦ Detected rate
Rd  P0,   Ri e  Ri

Ideal

n

Real

EDR Detector for Powder Diffraction
Standard detector

Standard Detector

Counter

YAP photomultiplier

•
•
•
•

Window
discriminator

Current
amplifier
EHT

Modified Detector

Counter

YAP photomultiplier

Current
amplifier
EHT

Low pass
filterSaturation 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

•
•
•
•

Modified
detector

3 .106

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

2 .1061.5.106

1.106

Standard
detector

Intensity (cps)Intensity (cps)

2 .106

1.106

5.105

0

0
0.0

0.2

0.4
Slit size (mm)

0.6

0.8

1.43

1.44

1.45

1.46

w(deg.)

1.47

1.48

Normalised Counts

Spectral Peak Shift vs Rate
1

Input rate
1.2E+60.9

2.8E+50.8

2.5E+3■ As rate rises♦ Spectral resolution

0.7

deteriorates♦ Note also the K escapefeature

0.6
0.5
0.4
0.3
0.2
0.1
0
0

50

100

150
200
Channels

250300Detector Considerations■ Intensity Measurement♦ Uniformity across device♦ Ageing, radiation damage♦ Dynamic Range♦ Linearity of Response♦ Stability■ Spatial Measurement♦ Spatial Resolution♦ Spatial Distortion♦ Parallax■ Energy Measurement♦ Spectral Resolution♦ Linearity of Response♦ Uniformity of Response♦ Stability■ Time Measurement♦ Frame Rate
♦ Photon Time Resolution■ Others♦ Size and weight♦ CostA Universal Specification?

WombatCounting Statistics■ Photons are quantised and hence subject to probabilities■ The Poisson distribution expresses the probability of anumber of events, k occurring relative to an expected
number, nnk enP(n, k ) k!
■ The mean of P(n, k) is n
■ The variance of P(n, k) is n
■ The standard deviation or error (noise) is √n■ If signal = n, then SNR = n/√n = √n
■ As n increases, SNR improves

Performance Measure - DQE
 N inc  SNR 2 inc

Perfect detector

SNRinc  N inc

Real detector

SNR Nonideal  NincCan define Nphotons that describes real SNR
NEQ  SNR Nonideal
2

Ratio of this to Ninc is a measure of efficiency
2
NEQ SNR Nonideal
DQE 

2
N inc
SNRincNote that DQE is f(spatial and spectral frequencies)Effect of Peak Width120

100
1 pixel2 pixel5 pixel
10pixelIntensity

80

60

40

20

0

-20
Position

DQE Comparison

DN-5 beam
2.6mGy60%
Flat Panel
400 film

50%

Light sensitive screen

DQE

40%
30%
20%
10%
0%
0

0.5

1

1.5
2
2.5
Frequency (lp/mm)

3

3.5

4

To Count or Not to Count

Tasmanian Devil

Collagen 100s Exposure
MWPC

Image
Plate
100000
Proportional
Counter

Intensity (Photons/mm^2)

10000

1000

100

10
0

20

40

60

80

Position (mm)

100

120

140

160

Collagen 10s Exposure
MWPC

Image
Plate
10000

Intensity (Photons/mm^2)

Proportional
Counter

1000

100

10

1
0

20

40

60

80

Position (mm)

100

120

140

160

Collagen 0.3s Exposure
MWPC

Image
Plate
100

Intensity (Photons/mm^2)

Proportional
Counter

10

1

0.1

0.01
0

20

40

60

80

Position (mm)

100

120

140

160

Cornell PAD (Integrating)■ Rapid Framing Imager♦ 15×13.8mm2 active area
♦ 150µm square pixel♦ Storage for 8 frames♦ Selectable Tint down to 1µs♦ Deadtime < 1µs

Sol Gruner, Cornell

Diesel Fuel Injection Movie■ Injection♦♦♦♦

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

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

The Future

A Detector System
X-ray

Detector

HPD

ADC

Preamp

101
1001

E=23
X=145
Y=2371FPGA GDAQ

Pixel Array Detector
A.
B.

C.
D.
E.
F.

A

B

C

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

D

E

F

The Problem of Multiple Scatters
E0■ Need to measure E0■ E0=E1+E2+Eesc
E2
E1

Eesc

■ Must be able to detect
multiple deposits assingle event■ Must minimise Eesc

Counting Pixel Detector Problems■ High power consumption♦ Cooling■ Number of connections♦ Multiplexing♦ Read out time significant■ Limited number of bits in counter♦ 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Available Compound Semiconductors■ 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(CdZnTe, 10% Zn), 1.64 eV(CdZnTe, 20% Zn)■ Resistivity of CdZnTe ishigher than CdTe, hence
lower dark current, higher
spectroscopic resolution■ Poor hole transport requires
electron-sensitive detectors
Paul Sellin, Surrey

Absorption Efficiency of Semiconductors

Paul Sellin, Surrey

CdZnTe Spectral Resolution
with Te inclusions

without Te inclusions

References■ Delaney CFG and Finch EC♦ Radiation detectors. Physical Principles and Applications,
Clarendon Press, Oxford 1992, ISBN 0 19 853923 1■ Knoll GE♦ Radiation Detection and Measurement, John Wiley and Sons
2000

■ Proceedings of the 7th International Conference on
position sensitive detectors♦ Nuclear Instruments and Methods in Physics Research Volume573, Issues 1-2, Pages 1-322■ IEEE Nuclear Science Symposia