Upload
others
View
13
Download
0
Embed Size (px)
Citation preview
University of Palermo
Dipartimento di Fisica e Chimica
Compound Semiconductors and
Digital Pulse Processing Techniques
for Radiation Detection
Leonardo Abbene
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Outline
General principles on radiation detection
X-ray and gamma ray spectroscopy
General characteristics of spectrometers
X-ray and gamma ray interactions with matter
Demands from modern spectrometers
Technological detection improvements: new materials
and new electronics
New semiconductor materials (CdTe, CdZnTe)
Digital Pulse Processing (DPP) Electronics
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Radiation detection (1)
Ionizing radiation….
Charged radiation
Uncharged radiation
• Electrons e Positrons
• Heavy charged particles (alpha,
protons, fission products,…)
• Neutrons
• Electromagnetic radiation
(X-rays, gamma rays)
We will focus our attention on X-ray and gamma ray detection!!!
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Radiation detection (2)
Interaction with matter….
Charged radiation
Main processes
Loss of energy Deflection from its incident direction
• Inelastic collision with atomic electrons
• Elastic scattering from nuclei
• Cherenkov radiation
• Bremsstrahlung
• Nuclear reactions
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Radiation detection (3)
Interaction with matter….
Uncharged radiation
Main processes
• Photoelectric Absorption
• Compton Scattering
• Pair production
. Rayleigh scattering
• Thompson scattering
• Nuclear scattering
• Delbruck scattering
Neutrons
• Elastic and Inelastic scattering
• Neutron capture
• Nuclear reactions
• Fission
• Hadron showers
Photons
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Radiation detection (4)
Types of detectors….
We will focus on Spectrometers !!!
· position sensing (tracking)
· energy measurement (spectrometers, calorimeters)
· timing
· particle identification
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
GOAL
Energy Input Counting Rate
Spectrometer Requirements
Total absorption of radiation
Capability to produce electrical signals
Electrical signals proportional to incident photon energy
X-ray and gamma ray spectroscopy (1)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray spectroscopy (2)
Main functions of spectrometers….Detector and Electronics
1. Radiation deposits energy in a detecting medium
• The absorbing medium will be chosen to optimize energy loss (high density, high Z).
• Ionization produces a given charge in the absorbing medium
ion-electron pairs in gas electron-hole pairs in semiconductors
In a spectrometer, the charge must be proportional to the absorbed energy !!!
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Direct conversion:
Radiation ionizes atoms in
absorber, creating mobile charges
(ionization chambers).
X-ray and gamma ray spectroscopy (3)
2. Energy is converted into an electrical signal,
either directly or indirectly.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Indirect conversion:
Radiation excites atomic/molecular
states that decay by emission of light,
which in a second step is converted
into charge. (scintillation detectors)
X-ray and gamma ray spectroscopy (4)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Main functions of spectrometers….
3. Amplification of electrical signals
a) by electronics
b) secondary multiplication within the detectors (proportional
chambers, photomultipliers)
4. Shaping of electrical signals
Shortening the pulses and filtering to enhance the signal-to-noise ratio
5. Digitizing the pulse height
By using ADCs to create energy histograms (energy spectra)
X-ray and gamma ray spectroscopy (5)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Typical detection system for X-ray spectroscopy
Induced current
Integration and
Amplification Filtering and
Amplification
Pulse Height
histogram
(energy spectrum)
Direct detection…..
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (1)
Interaction of photons in matter is dramatically different from that of
charged particles :
1) X rays and gamma rays are many times more penetrating than charged
particles
2) A beam of photons is NOT degraded in energy but only attenuated in intensity
Beer’s law:
I(x) = Ioexp [-μ(E). x]
Intensity of a photon beam decreases with
distance into material, but the energy of
individual photons remains the same.
μ is the linear attenuation coefficient
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (2)
100 5 101 5 10210-1
100
101
102
103
104
photon energy, keV
/,
cm
²/g
Calcio Ferro Piombo
30 keV photons
Calcium: μ/ρ = 4 cm2/g
Iron: μ/ρ = 8 cm2/g
Lead: μ/ρ = 30 cm2/g
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.00
100
200
300
400
500
600
700
800
900
1000
1100
I (x
)
Thickness (mm)
Calcium
Iron
Lead
30 keV
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (3)
Main interaction processes….
• Photoelectric Absorption
• Compton Scattering
• Pair production
Less important processes
• Rayleigh scattering
• Thompson scattering
• Delbruck scattering
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (4)
Photoelectric Absorption …..
35,4 ,μ EZricphotoelect
Photoelectric absorption involves the
interaction of an incident photon with an
inner shell electron in the absorbing atom
that has a binding energy similar to but
less than the energy of the incident
photon. The incident x-ray photon
transfers its energy to the electron and
results in the ejection of the electron from
its shell (usually the K shell) with a kinetic
energy equal to the difference of the
incident photon energy and the electron
shell binding energy.
Ek = h - We
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (5)
Compton Scattering…..
1,μ EZCompton
Compton scattering is an inelastic interaction
between an x-ray photon of energy that is
much greater than the binding energy of an
atomic electron (in this situation, the electron
is essentially regarded as “free” and
unbound).
Partial energy transfer to the electron causes
a recoil and removal from the atom at an
angle. The remainder of the energy, is
transferred to a scattered x-ray photon with a
trajectory of angle relative to the trajectory
of the incident photon.
cos1/1
cos1/2
2
'
cmh
cmhhhhE
e
e
e
)cos1(1
h'h
2
mc
h
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (6)
Pair production…..
Photon energy greater than 1.02 MeV
interacts with nucleus and conversion of
energy to e+ e- charged particles; e+
subsequently annihilates into two 511-keV
photons
22 cmhEE eee
2Zpair
Probability of interaction increases with increasing energy, unlike other processes
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (7)
Rayleigh scattering….
Photon interacts with bound atomic
electron without ionization; photon is
released in different direction without loss
of energy
No energy absorption occurs;
photons mainly scattered in forward
direction
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
100 5 101 5 102 5 103 5 104 510-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
photon energy, keV
/,
cm
²/g
Rayleigh Fotoelettrico Compton Pair Totale
Iron
PairComptonphotRayleigh tot
X-ray and gamma ray interactions with matter (8)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
X-ray and gamma ray interactions with matter (9)
Relative importance of the three interaction processes:
Z and Energy dependence
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (1)
Sensitivity
Capability to produce a useful signal for a given type of radiation
Detector Response
In spectrometers, is defined as the relation between the radiation energy and
the charge collected.
A linear detector response is needed in spectrometers.
Efficiency
Fraction of events emitted by a source which is registered by the detector
(intrinsic and geometrical efficiency)
To characterize a spectrometer….
e.g. generating a signal charge major than that of noise.
Choosing the proper detecting material to ensure sufficient counts in the
spectra
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (2)
Mode of Operation
All spectrometers operate in pulse mode. Detector is designed to record each
photon that interacts in its absorption medium.
Each photon generates a
given amount of electric
charge within the active
volume
A current will flow for a
time equal to the
collection time. The area
is the generated charge.
Detectors in pulse mode operation will analyze each current pulse.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (2b)
Why integration of each current pulse?
The magnitude and duration of each current pulse may vary depending on the
depth of interaction of photons.
Current pulses generated by
photons with the same
energy:
• Different heights of the
current pulses
• But..same area (charge)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
40 60 800
500
1000
1500
2000
Co
nte
gg
i
Energia (keV)
IDEAL
Spectrum
Monoenergetic
photons
Dirac delta
function
40 60 800
500
1000
1500
2000
Co
nte
gg
i
Energia (keV)
Gaussian
function
REAL
Spectrum
The function mainly depends on:
• interaction mechanisms
• detector material and geometry
• electronic noise
General characteristics of spectrometers (3)
Response function
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (4)
Response function
Changes from interaction mechanisms …..
Photoelectric Compton
Pair Absorption medium should
enhance photoelectric
interaction!!
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
A. The Full-energy photopeak. This
peak represents the heights of the
pulses that arise from photoelectric
interactions in the detection
medium.
General characteristics of spectrometers (5)
Response function
A realistic example…..
B. Compton Background Continuum.
These pulses come from interactions
involving only partial photon energy
loss in the detecting medium.
C. The Compton Edge. This is the region of the spectrum
that represents the maximum energy loss by the incident
photon through Compton scattering. This corresponds to
a collision between the photon and the electron, where
the electron moves forward and the gamma-ray scatters
backward through 180°
D. Backscatter Peak. This peak is
caused by gamma rays that have
interacted by Compton scattering in
one of the materials surrounding the
detector.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (6)
Energy resolution
It is the most important parameter for a spectrometer, that quantifies the broadening
of the photopeak for a monoenergetic source.
It represents the detector capability to resolve the energy photopeaks in the energy
spectra.
40 60 800
500
1000
1500
2000
Co
nte
gg
i
Energia (keV)
FWHM
(Full Width at Half Maximum)
355.22ln22GaussE
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (6)
Energy resolution
2222
elettrraccstattot EEEE
Fluctuation of the
number of ionizations
(Statistic noise)
Fluctuations due to
incomplete charge
collection
Fluctuations due
electronic noise
energyionizationmeanw
wEw
EwFWHM
w
ENPoisson
w
EN
Stat
355.2355.2
;
For Spectrometers
(full absorption)
Fano correction is needed
wEFw
EFwFWHM Stat
355.2355.2
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (7)
Energy resolution
2222
elettrraccstattot EEEE
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (8)
Energy resolution
Typical values
Indirect Detection
Direct Detection
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (9)
Dead Time
The minimum time that must separate two events in order to record them as two
separate pulses. The time that a system is busy processing a pulse.
Dead Time Distortions
The measured output
counting rate (OCR) is
lower than the Input
counting rate (ICR)
The typical counting
statistics (Poisson) is
distorted
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (10)
Dead Time Models
Generally, dead times are classified into two main categories…..
m = measured rate in the spectrum
n = true rate
= dead time
ablenonparalyzn
nm ;
1
Non-Paralyzable
Non-paralyzable dead time (also known as non-extendable, non-cumulative or
type I) is produced at each time an event is recorded and any arrival event from
the recorded time to the dead time period will not be recorded.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (11)
m = measured rate in the spectrum
n = true input rate
= dead time
Non-Paralyzable
n
m
m/n is generally termed throughput of a system
throughput curves
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (12)
Paralyzable
eparalyzablnnm ;exp
In the paralyzable case, dead periods
are not always of fixed length
In paralyzable model (also known as extendable, cumulative or type II), each
arrival event, whether recorded or not, produces a dead time and any new
arrival event with a delay less than the dead time from the previous arrival
event will not be recorded.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
General characteristics of spectrometers (13)
Dead Time
To avoid ambiguity (paralyzable case), it is important to maintain a maximun
throughput of 40% (m/n*100). The maximum of the blue curve is at n= 1/
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Modern Spectrometers
Demands…..
• Near room temperature operation (energy resolution < 10 % 60 keV and
high detection efficiency)
• High rate capabilities (> 100 kcps)
• Multi-parameter analysis, i.e. a system should provide, besides ICR and energy
spectrum, additional experimental parameters for each event:
(i) the event arrival time (e.g. for coincidence/anticoincidence measurements and dead
time correction)
(ii) the pulse shape, i.e. the peaking time (e.g. for detector performance enhancements,
photon tracking or particle identification)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Technological Solutions
Materials…..
• Compound Semiconductors.
Compound semiconductors were first investigated as radiation detectors in 1945 by Van
Heerden, who used AgCl crystals for detection of alpha particles. The great advantage of
compound semiconductors is the possibility to produce materials with a wide range of
physical properties (band gap, atomic number, density), making them suitable to several
applications.
Electronics…..
• Digital Pulse Processing (DPP) approaches.
Digital systems are based on the direct digitizing and processing of detector signals.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Materials
Semiconductor Detectors…….
• 3 eV for semiconductors
• 30 eV for gas
• 300 eV for scintillators coupled to photomultipliers
Small mean ionization energy w
wEFw
EFwFWHM Stat
355.2355.2 Better energy resolution !!!
High density
Development of compact systems!!!
Semiconductors with high Z and wide bad gap
Development of room temperature systems !!!
Detection of hard X-rays (> 15 keV)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Silicon detectors
Among the semiconductor devices, silicon (Si) detectors are the key
detectors in the soft x-ray band (<15 keV). Si–PIN diode detectors and
silicon drift detectors (SDDs), operated with moderate cooling by means of
small Peltier cells, show excellent spectroscopic performance and good
detection efficiency below 15 keV.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Germanium detectors
Germanium (Ge) detectors are unsurpassed for high resolution
spectroscopy in the hard X-ray energy band (>15 keV) and gamma energy
band (> 200 keV) and will continue to be the first choice for laboratory-
based high-performance spectrometers.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
New semiconductor materials
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
There has been a continuing desire for the development of room temperature detectors with
compact structure having the portability and convenience of a scintillator but with a
significant improvement in energy resolution. To this end, numerous high-Z and wide band gap
compound semiconductors have been exploited. Compound semiconductors are generally
derived from elements of groups III and V (e.g. GaAs) and groups II and VI (e.g. CdTe) of the
periodic table. Besides binary compounds, ternary materials have been also produced, e.g.
CdZnTe and CdMnTe.
Compound Semiconductor Detectors
materiale Si Ge
density ( g/cm3 ) 2.33 5.33 5.32 6.20 5.78 6.40
GaAs CdTe Cd0.9Zn0.1Te HgI2
atomic number (max) 14 32 33 52 52 80
band-gap ( eV ) 1.12 0.67 1.43 1.44 1.57 2.15
w ( eV ) 3.62 2.96 4.20 4.43 4.64 6.50
τe (cm2 / V ) > 1 > 1 8 · 10-5 3 · 10-3 3 · 10-3 3 · 10-4
τh (cm2 / V ) 1 > 1 4 · 10-5 10-4 10-4 4 · 10-5
resistivity ( · cm) 104 50 107 109 1010 1013
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
CdTe and CdZnTe (1)
Among the compound semiconductors, Cadmium Telluride (CdTe) and Cadmium Zinc
Telluride (CdZnTe) attracted growing interests in the development of x-ray detectors.
Due to their high atomic number, high density, and the wide band gap, CdTe and CdZnTe
detectors ensure high detection efficiency, good room temperature performance and are very
attractive for x-ray and g ray applications.
Difficulties in producing detector-grade materials and in growing chemically pure and
structurally perfect crystals are the critical issues of CdTe and CdZnTe detectors.
CZT are usually grown by using the high pressure
Bridgman (HPB), low pressure Bridgman (LPB),
vertical Bridgman and THM methods. The supply
of spectrometer grade CdZnTe is limited to a small
number of companies: eV Products (USA), Imarad
(Israel), Eurorad (France) and Redlen
Technologies (Canada).
CdTe are usually grown by the THM method and
doped with Cl to compensate background
impurities and defects, resulting in high resistivity
p- type materials. Supplies of spectrometer grade
CdTe crystals are offered by a few companies:
Imarad (Israel), Eurorad (France) and Acrorad
(Japan).
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
materiale Si Ge
density ( g/cm3 ) 2.33 5.33 5.32 6.20 5.78 6.40
GaAs CdTe Cd0.9Zn0.1Te HgI2
atomic number (max) 14 32 33 52 52 80
band-gap ( eV ) 1.12 0.67 1.43 1.44 1.57 2.15
w ( eV ) 3.62 2.96 4.20 4.43 4.64 6.50
τe (cm2 / V ) > 1 > 1 8 · 10-5 3 · 10-3 3 · 10-3 3 · 10-4
τh (cm2 / V ) 1 > 1 4 · 10-5 10-4 10-4 4 · 10
resistivity ( · cm) 104 50 107 109 1010 1013
CdTe and CdZnTe (2)
Advantages…….high Z
High detection efficiency and high photoelectric
interaction probability high Z
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Compton
Photoelectric
CdTe and CdZnTe (3)
Del Sordo et al... Sensors 9, (2009) 3491.
Linear attenuation coefficients for photoelectric absorption and Compton
scattering of CdTe, Si, HgI2, NaI and BGO
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
CdTe and CdZnTe (4)
L. Abbene et al... CdTe detectors, in Comprehensive Biomedical Physics, (2014) Elsevier.
Total and photoelectric efficiency for 1-mm-thick CdTe detector compared
with Si and Ge.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
CdTe and CdZnTe (5)
Efficiency of CdTe detectors as function of detector thickness at various
photon energies.
Del Sordo et al... Sensors 9, (2009) 3491.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
materiale Si Ge
density ( g/cm3 ) 2.33 5.33 5.32 6.20 5.78 6.40
GaAs CdTe Cd0.9Zn0.1Te HgI2
band-gap ( eV ) 1.12 0.67 1.43 1.44 1.57 2.15
w ( eV ) 3.62 2.96 4.20 4.43 4.64 6.50
τe (cm2 / V ) > 1 > 1 8 · 10-5 3 · 10-3 3 · 10-3 3 · 10-4
τh (cm2 / V ) 1 > 1 4 · 10-5 10-4 10-4 4 · 10
resistivity ( · cm) 104 50 107 109 1010 1013
CdTe and CdZnTe (6)
Advantages…….wide band-gap
Low leakage current even a room temperature ( < nA) Wide band-gap
atomic number (max) 14 32 33 52 52 80
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
-200 -150 -100 -50 0 50 100 150 200-5.0x10
-10
-4.0x10-10
-3.0x10-10
-2.0x10-10
-1.0x10-10
0.0
1.0x10-10
2.0x10-10
3.0x10-10
4.0x10-10
5.0x10-10
Cu
rre
nt
(A)
Cathode Bias Voltage (V)
T = 25 ° CCZT detector
Au/CZT/Au
0.5 mm thick
CdTe and CdZnTe (7)
CZT detectors with quasi-ohmic contacts
ensure very low leakage currents even a room
temperature ( < nA)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Drawbacks…defects
CdTe and CdZnTe (8)
Presence of defects and impurities in the crystals acting as trapping
centers
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
drawbacks
CdTe and CdZnTe (9)
Position of the ionization energy levels of native defects, impurities, and
defect complexes in semi-insulating CdZnTe.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
drawbacks
CdTe and CdZnTe (10)
The major processes that determine the electron and hole trapping,
detrapping, and recombination lifetimes.
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
materiale Si Ge
density ( g/cm3 ) 2.33 5.33 5.32 6.20 5.78 6.40
GaAs CdTe Cd0.9Zn0.1Te HgI2
Z (max) 14 32 33 52 52 80
band-gap ( eV ) 1.12 0.67 1.43 1.44 1.57 2.15
w ( eV ) 3.62 2.96 4.20 4.43 4.64 6.50
τe (cm2 / V ) > 1 > 1 8 · 10-5 10-3 10-3 3 · 10-4
τh (cm2 / V ) 1 > 1 4 · 10-5 10-4 10-4 4 · 10-5
resistivity ( · cm) 104 50 107 109 1010 1013
Poor mobility lifetime products Reduction of the charge collection efficiency
Drawbacks……poor charge transport properties.
CdTe and CdZnTe (11)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
materiale Si Ge GaAs CdTe Cd0.9Zn0.1Te HgI2
band-gap ( eV ) 1.12 0.67 1.43 1.44 1.57 2.15
w ( eV ) 3.62 2.96 4.20 4.43 4.64 6.50
τe (cm2 / V ) > 1 > 1 8 · 10-5 10-3 10-2 -10-3 3 · 10-4
τh (cm2 / V ) 1 > 1 4 · 10-5 10-4 10-4 4 · 10-5
resistivity ( · cm) 104 50 107 109 1010 1013
Difference between the transport
properties of the holes and the electrons Distortions in the spectra (hole tailing)
Drawbacks……..poor hole transport properties
CdTe and CdZnTe (12)
density ( g/cm3 ) 2.33 5.33 5.32 6.20 5.78 6.40
Z (max) 14 32 33 52 52 80
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Drawbacks…hole tailing
CdTe and CdZnTe (13)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Generated charge ……Planar detectors
CdTe: Photons of 60 keV
Charge (E/w)·e = q
Charge ~ 2· 10-15 C
1 2
Typical response function of CdTe/CZT detectors (1)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
+ + + + +
+ + +
-q
Q1
Q2
Vo +
i2(t)
(1)
(2) dQ2
Induced Charge│Q1│+│Q2│= q
Positive charge + q (holes) Negative charge – q (electrons)
Generated Charge (E/w)·e = q
+ q
Q1
Q2
Vo +
i1(t)
dQ1
(1)
(2)
x
P L
- - -
- - -
Anode
Anode
Cathode Cathode
Typical response function of CdTe/CZT detectors (2)
Generated charge ……Planar detectors
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Induced Charge on the electrodes???
Shockley-Ramo Theorem…and
the concept of a weighting potential.
)()( 00 if xxqQ
The weighting potential is defined as the potential that would exist in the detector
with the collecting electrode held at unitary potential, while holding all other
electrodes at zero potential.
The weighting potential is for a specific electrode is obtained by setting the
potential of the electrode to 1 and setting all other electrodes to potential 0.
Note that the electric field and the weighting field are distinctly different.
· The electric field determines the charge trajectory and velocity
· The weighting field only depends on geometry and determines how charge
motion couples to a specific electrode. Only in 2-electrode configurations are the
electric field and the weighting field of the same form (planar detectors).
Typical response function of CdTe/CZT detectors (3)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Weighting Potential
L
xz 1z00 zz
Typical response function of CdTe/CZT detectors (4)
Induced charge ……Planar detectors
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Typical response function of CdTe/CZT detectors (5)
Induced charge ……Planar detectors
Induced Charge (no trapping) )()()( ,, tQtQtQ eindhindind
hr
h
hr
hind
ttxL
Ne
E
xttxtx
L
Ne
tQ
,0
0,0
,
])([
)(
er
e
er
eind
ttxLL
Ne
E
xLttxtx
L
Ne
tQ
,0
0,0
,
][
])([
)(
Electron contribute
Hole contribute
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Typical response function of CdTe/CZT detectors (6)
Induced charge ……Planar detectors
tr,h= x0/ hE
Qe=N·e·(L-x0)/L
Qh=N·e·(x0/L)
t
N·e
tr,e=(L-x0)/ eE
)(tQind
NezNezNeQ )1)(()0)((
holes
electrons (t > tr,h e t > tr,e )
Induced Charge (no trapping)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Typical response function of CdTe/CZT detectors (7)
Induced charge ……TRAPPING
E
xL
eeE
x
hheindhindind
eehh eL
Ee
L
EeNQQQ
00
11,,
eNQQQ eindhindind ,,
Hecht Equation
IDEAL Induced charge will only depend on N and therefore on the photon energy.
The charge also depends on the incoming photon interaction position!!!
t
eQtQ
0)(REAL: Uniform Trapping
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
In/CdTe/Pt planar
2x2x1 mm3
(Amptek, USA)
LOW-RATE SPECTROSCOPIC PERFORMANCE
Energy (keV)
22.1 59.5 122.1
Energy
resolution
FWHM (%)
T = - 20 °C
2.5 1.3 1.6
Typical response function of CdTe/CZT detectors (8)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
The output signals from charge sensitive preamplifiers (i.e. CSP
waveform) are sampled and digitized by ADC and then processed by using
digital algorithms (DPP firmware).
Digital Electronics (1)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
As widely recognized, the digital approach gives many benefits against
the analog one, among which:
(i) possibility to implement custom filters and procedures, which are
challenging to realize in the analog approach
(ii) stability and reproducibility (insensitivity to pick-up noise as soon
as the signals are digitized)
(iii) the possibility to perform multi-parameter analysis for detector
performance enhancements and new applications.
Digital Electronics (2)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
• L. Abbene, G. Gerardi. NIM A 654 (2011) 340. • L. Abbene, et al., JINST 8 (2013) P07019
General purpose method for both Real-Time and Off-Line analysis !!
• G. Gerardi, L. Abbene,. NIM A 768 (2014) 46. • L. Abbene, et al. NIM A 730 (2013) 124.
Digital Electronics (3)
Custom DPP system
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Digital Electronics (3a)
Digitized Preamplifier Waveform (red) and Pulses after
digital shaping (black)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Digital Electronics (4)
Arrival time of the events….
Experimental measurement of the Time-Interval Distribution (TID)
of the events
0.0 5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
2.5x10-6
3.0x10-6
3.5x10-6
4.0x10-6
e7
e8
e9
e10
e11
e12
e13
e14
ICR = 2.2 Mcps
calculated from exponential fitting
Ag-target X-ray source
30 kV
Ln
[C
ou
nts
]
Time (s)
0 10 20 30 40 50 60 70 800
500000
1000000
1500000
2000000
2500000
3000000
Ag-target X-ray source
30 kV
ICR from TID
Ph
oto
n C
ou
nti
ng
Ra
te (
cp
s)
Tube Current (A)
Poisson Process Non- linearity is less than 0.5%
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Pulse shape analysis …..reduction of trapping effects…..
Bi-parametric techniques (Pulse Height and Peaking time)
Since both peaking time and pulse height
are correlated with the depth of
interaction, one can use these
measurements to improve spectroscopic
performance. Such methods are termed
bi-parametric. In practice, it is achieved
by plotting peaking time as a function of
pulse height, from which one can
generate a set of correction factors.
Digital Electronics (5)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of trapping effects…..
Bi-parametric techniques
(Pulse Height and Peaking time)
L. Abbene et al…. NIM A 654 (2011) 340–348.
Digital Electronics (6)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of trapping effects…..
Bi-parametric techniques (Pulse shape discrimination)
L. Abbene et al…. NIM A 654 (2011) 340–348.
Spectral improvements
but with a reduction of
the counts in the
spectra (> 90%)!!!!!
Digital Electronics (7)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of trapping effects…..
Bi-parametric techniques (Pulse shape correction)
L. Abbene et al…. NIM A 654 (2011) 340–348.
Digital Electronics (8)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of trapping effects…..
Bi-parametric techniques (Pulse shape correction)
L. Abbene et al…. NIM A 654 (2011) 340–348.
no reduction of photon
counts!!!!
Digital Electronics (9)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of pile-up effects…..
Pile-up rejection
L. Abbene et al…. NIM A 654 (2011) 340–348.
Pulse shape discrimination can also be used to
minimize peak pile-up events, i.e. overlapped
preamplified pulses within the peaking time. Because
the shape (peaking time)of a peak pile-up pulse differs
from that of a pulse not affected by pile-up, analyzing
the measured spectra at different peaking time regions
(PTRs) in the peaking time distribution is helpful to
reduce peak pile-up.
Digital Electronics (10)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Reduction of pile-up effects…..
Pile-up rejection
L. Abbene et al…. NIM A 654 (2011) 340–348.
Results by using digital
techniques
Digital Electronics (11)
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Applications (1)
Focal plane detectors for
X-ray telescopes (1 - 100 keV)
Portable systems for mammographic
X-ray spectroscopy (1- 40 keV)
astrophysics
medical physics
PET (511 keV)
gamma camera (140 keV)
energy resolved detectors for
diagnostic medicine (1-140 keV)
Detectors for radioactive
isotopes
Homeland security
XRF, Compton techniques
cultural heritage
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
Applications (2)
X-ray colour imaging in diagnostic medicine (1 -140 keV)
High contrast
Quantitative analysis (the gray-scale pixel
values in traditional X-ray images are not
quantitative but qualitative)
Reduction of patient dose
“Compound Semiconductors and Digital Pulse Processing Techniques for Radiation Detection’’
References
-G. F. Knoll, Radiation Detection and Measurement, 3rd Edition, Wiley 2000.
- K. Debertin and R.T. Helmer - “Gamma and X-Ray Spectrometry with
Semiconductor Detectors ” – North Holland Publishers (Amsterdam 1988).
- Del Sordo S., Abbene L., Caroli E., Mancini A. M., Zappettini A., Ubertini P. “Progress in the Development of CdTe and CdZnTe Semiconductor Radiation
Detectors for Astrophysical and Medical Applications ’’ Sensors, 9, pp. 3491-
3525, (2009)
- Abbene L. and Del Sordo S. “CdTe detectors ’’ Comprehensive Biomedical
Physics, Elsevier (2014).