Detectors:
Things you should know
Rob Lewis
Medical Imaging, University of Saskatchewan
Medical Imaging and Radiation Sciences, Monash
University
Scott Automation and Robotics

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Electrons

AmplifierMany e-Signal
Processing

ImagesSpectraConverter

Photons

WindowDetector Chain of Events

Computer
AnalysisDetection Mechanisms■ There are many means of detection. All require
the interaction of photons/electrons 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

+
+ ++
+ +

φ
-

-

h
1  cos 
l 
m0c

e-

An Example Detector

EchidnaIonisation Chamber+ePhotons+ ions-+Gas Volume■ Approx. 1 e- ion pair per 30eV
deposited (symbol often w)
Bias ■ IF w, density and gas known, it
is possible to convert current toflux■ Flux measurement can beaffected by several factors♦ Recombination of e- and ions● Higher voltages required at higher rates
since more carriers♦ Diffusion losses due to ion and egradient● Helped by higher voltages, small gap♦ Electron losses when e- directly reachanode● Helped by larger gap■ Pressure, temperature andhumidity affect density

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

Signal = 1
Noise = 0.01

1

Detector output

0.8

PhotonSignal

0.6

Threshold■ In this case 17
photons hit detector
but only 16 recorded■ Illustrates a problem
with counting♦ Pulse pair resolution

0.4

0.2
0
-0.2
0

100

200

Time

300

400

500

SNR = 1
4Photon■ Black dots show
when photons arrive■ Impossible to decide
where to set
threshold■ Counting detectors
require very high
SNR

Signal3

Detector output

2
1

0
-1
-2

Noise = 1
Signal = 1

-3
-4

0

100

200

Time

300

400

500

Counting and Integrating■ Usually SNR is insufficient for counting andwe have to accumulate many photons orparticles before the signal becomesmeasurable

Counting & Integrating SNR =10018

Photon16

Counting
14
Integrating

Detector output

12

17 photons hit detector

108Noise = 0.01
Signal = 1

6
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

300
Time

400

500

Geiger Tube and Proportional Counter
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■ Which he invented in…..

■ 1968 …….24 years
publication to prize!

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)Integrating Detectors■ ModeSaturationlevel♦ Measures deposited energy at end of
integration period(“fog level”)♦ Dead time between frames♦ 220 keV phts = 140 keV photon
i.e. Cannot perform simultaneous
spectroscopy and positioning♦ Examples: Image plates, CCDs

Dynamic range♦ High input flux capability♦ Read noise dominates at low signal

Output signal■ Characteristics

Noise
level

Input flux

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

Dynamic range■ Characteristics

Output signal

arrives. Only active pixels read
Saturationlevel

Input flux

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
Perfect detector

SNRinc  N inc

 N inc  SNR 2 inc

Imperfect detector SNRNonideal  Ninc
We can define Nphotons that describes real SNR
NEQ  SNR2 Nonideal
Ratio of NEQ to Ninc is a measure of efficiency
NEQ SNR2 Nonideal
DQE 

N inc
SNR2 inc
Note that DQE is f(spatial and spectral frequencies)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

Willard S. Boyle & George E. Smith■ Which they invented
in…..

Bell Laboratories
Murray Hill, NJ, USA■ 1969 …….40 years
publication to prize!

CCD

Nobel prize inphysics 2009
"for the invention of
an imaging
semiconductor circuit– the CCD sensor"

Charge Coupled Device
Photons
in

V1

V2
V3
Si O2
Siliconsubstrate

Columnisolation
Electronhole pair
Depletion
regionSignalelectrons■ 1 optical photon = 1 electron hole pair

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■ Pixels have limited capacitance (well depth) ~104105 e■ This limits dynamic range for direct detection of
x-rays♦ 10keV photon creates ~ 3000e- so saturation = ~ 10 photons♦ 1 optical photon = 1e-■ Clock speed is restricted by line capacitance andcharge transfer efficiency♦ Well depth and 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 charge transfer efficiency ( line capacitancebecomes very high)Shutter required

Agilent S2 CCD with Smart Sensitivity■ The S2 CCD detectors
employ groundbreaking
Smart Sensitivity Control,
which tunes detector
sensitivity to match thestrength of the data
observed. Similar to ISO
settings in digital
photography, this selects
the widest dynamic rangeor the highest sensitivity,
as needed, to maximize
your data quality.

Complimentary Metal-Oxide
Semiconductor (CMOS)
Read Amplifiers
Row Drivers
Bias supply■ 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 has had higher sensitivity andlower noise■ Modern techniques mean that the differences are
small■ 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Front and Back Illumination

http://www.sony.net/SonyInfo/News/Press/200806/08-069E/index.html

Human Eye

http://webvision.med.utah.edu/imageswv/Sagschem.jpeg

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

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

Well depth = 2×105
So dynamic range ~100
Phosphor coupled 1:1 to sensor
Phosphor gain >> 1
Optics Gain < 1

TFT Flat panel Detector

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

Bias Line

Row
line

TFT
SwitchOne
pixelPhotodiode

Read Amplifiers

Row Drivers
Bias supply

Big CMOS

Specifications

Specifications

Resolution is NOT pixel size

Resolution is NOT pixel size

Sampling
1
0.8
0.6
0.4
0.2
0
-0.2

0

0.2

0.4

0.6

0.8

-0.4
-0.6
-0.8

1

1.2
Sampling 5Hz
f=10Hz
f=5Hz
Sampling 5Hz
f=2.5Hz-1■ Shannon’s Theorem and Nyquist Criterion♦ The highest frequency that can be ‘measured’ is HALF the sampling frequency

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=1Hz

-0.6

Sampling 5Hz

-0.8

f=6Hz
f=11Hz-1■ 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, the very best spatial resolution that you can expect (inthe absence of noise) is 200μm■ In any real system > 200μm■ And that is all assuming NO DISTORTIONS!!

Spatial distortion x = f(y)ESRF Imageintensifierdetector

Gaps
Spec

0.2mm max

Worst gap

2.97mmPixels in gaps

513922
5.45%

Overlaps

SpecificationsElectronic Rolling Shutter

https://en.wikipedia.org/wiki/Rolling_shutter

Specifications

Data Rates■ 120 LVDS pairs at 200Mbps = 24Gbps■ Interfaces♦ USB2:♦ USB3:♦ USB3.1:♦ SATA III:
♦ Thunderbolt 2:

480 Mbps
5 Gbps
10 Gbps???
6 Gbps
10 Gbps■ Disk write speeds♦ HDD:♦ SSD:

200 Mbps
530 Mbps■ So 45 SSDs required to store data!!!!

Specifications

Dark Current■ Dark current is the signal produced underzero illumination■ Dark noise is a measure of the fluctuations
in dark current■ Dark noise sets the minimum detectablesignalDark Currents
Flat and Dark Correction
For each image, two correction images must
be recorded.
1. A flat field (uniform illumination of thedetector)
2. A dark image (no irradiation of detector)
Both must be recorded with the same
exposure time as the original image sincedark current is a function of exposure time.
Then apply the following correction

250

200
Set 1

150

100
0

100

200

300

400

500

600


image  dark 
Corrected  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

dpiX Flashscan 30 PaxScan 4030

Flashscan 30 - Performance
Mar Image Plate

Flashscan-30

tint=30s

tint=190s

Specifications

Sensitivity■ 550nm photon 𝐸 = ℎ𝑐 𝜆 = 3.6×10-10 nJ■ So 1 nJ equates to 3×109 photons■ 1 cm2 = 3305785 pixels■ So 1 nJ/cm2 = 908 phts / pixel■ Sensitivity is 255 DN/nJ/cm2■ 1 LSB = 0.3 optical phts / pixel■ But what does this mean for x-rays ?

Sensitivity■ Ideal sensitivity is 1 DN per x-ray photon
detected■ Many factors beside chip sensitivity♦ Phosphor efficiency●Conversion x-rays to light (1 photon/30eV typical)●Ability for light to escape phosphor♦ Transfer efficiency from phosphor to chip, lens, FOT●Lenses poor f1.4 = 10% transmission (no reflections)● F1.0 = 20%●FOT better but distortions

Geometric Distortion x ≠ f(y)
250

0.12

Vertical

0.1
0.08

Fit Y

0.04
0.02

100
0

Residuals (mm)

0.06
150

-0.02

50

-0.04

0

50

100

150
Position (mm)

-0.06

120.7

200

0.08

250

Horizontal

120.65

0.06

120.6

0.04
Fit X

120.55

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

Residuals (mm)0Measured Position (mm)Measured Position (mm)200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

We would like sensitivity to be a constant, neither
varying in time or position

Specifications■ All these are interrelated

Specifications

Quantisation Error

Analogue in

S&H

Sampled signal

ADC

Digital out■ Quantisation errors are ±½ LSB■ Quantisation errors look likerandom noise and can be treatedas suchhttp://www.dspguide.com/ch3/1.htm

ADC Resolution and Noise■ Max noise is ±½ LSB■ Standard deviation 𝜎 =

1
√12

𝐿𝑆𝐵

nBits

8

10

12

16

nLevels

256

1024

4096

65536

Q error (%)

0.113

0.028

0.007

0.0015

Dynamic Range (dB)

48

60

72

96

■ More bits = usually slower and more expensive■ Number of levels sets max dynamic range■ Dynamic range is max signal divided by min signal■ Often expressed in dB where dB = 20 log10 (Ratio)

Specifications

Dynamic Range■ dB = 20 log10 (Ratio)■ Ratio = 10dB/20■ Intra scene 67.0dB = 2339
■ Inter scene 75.4dB = 5888

■ Why the difference?

Image Lag
Flashscan 30

Fraction of initial differe nce remaining

100.00%10.00%

1.00%

0.10%
0

2

4

6
Time (Minutes)

8

10

12

Specifications

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Intensity Test
1

5

9

2

6

10

3

7

11

4

8

12

Graded Absorber Comparison
Mar Image Plate

ESRF-Thompson IIT / CCD

Daresbury MWPCCollagen 100s Exposure
MWPC

Image
Plate
100000
Proportional Counter
Image Plate

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
Proportional Counter

Intensity (Photons/mm^2)

Image Plate

1000

100

10

1
0

20

40

60

80

Position (mm)

100

120

140

160

Collagen 0.3s Exposure
MWPC

Image
Plate
100
Proportional Counter

Intensity (Photons/mm^2)

Image Plate

10

1

0.1

0.01
0

20

40

60

80

Position (mm)

100

120

140

160

Specifications

Well Depth■ 23000e- will saturate with 8 10keV photons■ Indirect detection is crucial to increase
dynamic range■ Reduce number of electrons per x-ray
photon.. but how much?

Radiation Damage■ Indirect detection is important for anotherreason……
■ Medipix♦ Damage occurred at 40Gy or 1.3×1010pht/mm2 in thereadout 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.

Specifications

Dynamic Range and SNR■ Datasheet DNR = 75.4dB or 5888
■ Datasheet SNR = 43.0dB or 141 !!!!
■ Why the difference?
■ DNR = Max signal / Detector Noise= 23000/3.2 = 7186 = 77.1dB■ SNR = Max signal / Sum of all noise= 23000/√23000 = 151.6= 43.6dB♦ Nb/ Poisson statistics apply to electrons as well as photons■ Difference between my calculation and their values isthat they use FWC of 20000

Real SNR
120
100

1 pixel2 pixel5 pixel
10pixelIntensity

80
60
40
20

0
-20
Position■ Usually better to have signal in fewer
pixels becasue each pixel has its own noise

Spectroscopic Detectors

Rainbow Lorikeets

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

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. σ = √N
■ Reality is somewhere in between so we introduce Fano factor F2■ Fano factor is defined asFN2
where σ is the variance and N 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

Counting still sort after
■
■
■
■
■

Xe-based gaseous avalanche detector
Active area: 14 x 14 cm2
Number of pixels: 2048 x 2048
Pixel Size: 68µm x 68µm
Global Counting Rate
♦ Maximum: 1.6 Mcps
♦ Linear part (10% deviation fromlinearity): 0.9 Mcps

■ Local Counting Rate
♦ Max per point-like reflection: 250 kcps
♦ Linear part (10% deviation from linearity): 160
kcps■ Background < 5 cps per whole area
■ Maximum Dynamic Range
109x sqrt(collection time in seconds)■ Radiation Hardness
1012 X-rays/mm2 (1016 photons in total)■ Accidental Irradiation Intensity
No limit

Bruker VÅNTEC-2000

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

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 fordetectors!

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

PILATUS3X Detector
■
■
■
■
■
■
■

Pilatus has a 20 bit counter for each pixel
Count rates up to 10 Mcts/sec/pixel
No readout noise
No dark current
Room temperature operation
Region of interest readout
Built of modules

PILATUS 3X CdTe Detector
Flat Panel

Pilatus3X CdTe

ESRF ID15A, 46.3keV, 100ms exposure

PILATUS3X Detector

Combine Imaging and Spectroscopy

X-ray

Detector

HPD

ADC

Preamp

101

1001

E=23
X=145
Y=2371FPGA GDAQ

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

CdZnTe Spectral Resolution
with Te inclusions

without Te inclusions

The Problem of Multiple Scatters
E0

E2

E1

Eesc

■ We would like to know E0
but…..
■ E0=E1+E2+Eesc■ We don’t know Eesc
■ E1 and E2 are separate events■ So we must be able toassociate multiple energy
deposits as single input photon■ We must also minimise Eesc
■ Not a simple problem!

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Other Issues■ In addition to detector performance metrics
such as♦ Spatial resolution, Spectral resolution, etc.

■ Often we need to measure function not form
■ Requires that…
♦ The detector respond to triggers♦ Be able to synchronise with other systems measuringmultiple parameters♦ Do things like phase contrast

Whole Breath Lung Morphology

S Dubsky, A Fouras et al

4D PIVS Dubsky, A Fouras et al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

■ High Speed Digital Cameras 16, 14, 12, 10 & 8-bits cameras, What
does it really mean?
♦ http://www.motionvideoproducts.com/MVP%20papers/16%20bit%20High%20Spee
d%20Digital%20Cameras.pdf■ The Scientist and Engineer's Guide to Digital Signal Processing,
Steven W. Smith♦ http://www.dspguide.com/■ Proceedings of the International Conferences on position sensitive
detectors■ IEEE Nuclear Science Symposia