2nd SENS-ERA WORKSHOP
on “Advanced sensor systems & networks”
TEI Piraeus, 6-7 December 2012
Simulation studies on the localization of
radioactive sources using a portable
detector based on pixelated CdTe crystals
Zachariadou Katerina et al
TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology
Introduction
Radiation detectors that use gamma-ray imaging technologies
gamma-ray imaging technologies
in order to identify radioactive sources are of great scientific
interest because of their wide range of applications:
Compton imaging
based on the interactions of the emitted
gamma-rays with the detector’s sensitive
elements via the Compton scattering process
 nuclear medicine
 astrophysics
waste monitoring
 counter terrorism
 the growing global interest for accurate detection
of radioactive sources
 rapid advances in detector technologies (both in
terms of material fabrication and electronics)
great impetus to the R&D of
Compton imaging detectors with
enhanced detection capability.
the objective of our research is to evaluate the radioactive source
imaging performance of a Compton imaging detector under development
(COCAE) , based on pixelated CdTe crystals
Outline
DesignFunctionality

The COCAE detector
Principle of Compton Imaging Technique
Monte Carlo simulation procedure

Reconstructed Image Resolution


Two radioactive source discrimination
Algorithms for the localization
of point-like radioactive sources
Source orientation estimation
Source-to-detector distance estimation
COCAE
1. TEI of Halkis (Greece)  pixel electronics
2. Greek Atomic Energy Commission (Greece) Monte Carlo Simulations
3. Institute of Nuclear Physics, Demokritos (Greece)  Monte Carlo Simulations
4. Oy Ajat Ltd (Finland) bump bonding of the pixel electronics
5. Freiburger Materialforschungszentrum (Germany) growth of crystals
6. Universidad Autonoma de Madrid Departemento de Fisica de Materiales (Spain) 
growth of crystals
7. Riga Technical University (Latvia) Shottky diode structures
8. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of
Ukraine (Ukraine)  growth of crystals +fabrication of pixel detectors
9. Chernivtsi Yuri Fedkovych National University (Ukraine) radiation detector
characterization
Web Site: www.cocae.eu
The COCAE detector
COCAE is a portable detector aimed to be used for the accurate
localization & identification of radioactive sources,
in a broad energy range up to 2 MeV
Security inspections at the borders (airports, seaports etc)
At recycling factories
for the detection & determination of the strength of possible
radioactive sources into scrap metals
The COCAE Detector
2D array of radiation
sensing pixels (100x100)
(400μm pitch)
Pixelated 2mm thick
CdTe crystals
bump-bonded in a 2D
array of readout CMOS
circuits (300μm thick)
Principle of Compton
Reconstruction
COCAE exploits the Compton imaging technique to deduce:
 the energy
 the origin (within a cone)
of the incident gamma photons emitted by a
radioactive source
Gamma interactions with matter:
Compton scattering: The direct interaction of a
gamma with an electron of the sensing material
 The energy and the momentum of the scattering
are conserved
Principle of Compton Imaging
Deduce the energy of the incident gamma photons as well as their origin within a cone
Successive interactions of the emitted gamma rays
create overlapping cones and the source location is the
intersection of all measured cones
Radioactive source
source
Projection
imaging plane
rˆ0
θ
rˆ1
uˆ
Ee
Eg
rˆ2
z
y
In principle, three cones are sufficient to reconstruct
the image of a point source.
In practice, due to measurement errors & incomplete
photon absorption, a large number of reconstructed
cones are needed to derive the source location
accurately.
 1

1

cos   1  m0 c

 E g E g  Ee 


2
x
cos  rˆ0  uˆ
The design of the COCAE detector
Crucial parameters that determine the design of the detector:
Detection efficiency
Influence the event
statistics
Strong absorption of
gamma photons
Energy resolution
affects the evaluation
of the compton
scattering angle
The design of the COCAE detector
Detection efficiency
CdTe semiconductor crystals have
higher detection efficiency
(compared to Ge and NaI detectors) due to
the higher atomic number (48, 52) and
density (~6gr/cm3)
To achieve even better efficiency, a
thick CdTe detector of several mm
would be needed
BUT
an increase of the crystal thickness
would deteriorate the detector energy
resolution
(due to incomplete charge collection of
CdTe semiconductors)
Bypass: The instrument is designed as a system of ten 2mm
stacked sensors, instead of one thick mono-crystal.
The design of the COCAE detector
 Energy Resolution
Affects the determination of Compton scattering
angle
•The challenge for the COCAE instrument is to
achieve high energy resolution without the
need of cryogenics
(CdTe semiconductors can be operated at room
temperatures due to their high energy bandwidth )
p-n , Schottky diodes (CdTe, CdZnTe) have been investigated
The energy resolution achieved is better than 1% FWHM @662keV
Monte Carlo Simulation steps
 model the exact detector geometry
in order to ensure an accurate simulation of the real detectors’ performance :
Accurate geometric and physical description of the detector’s passive materials.
 implementation of the correct isotopic composition of all detectors’ materials
 Implementation of all the corresponding cross sections of the particle interactions
MEGAlib Geant4
• Geant4: A toolkit for the simulation of the passage of particles through matter, http://geant4.web.cern.ch/geant4/
 model point-like isotropic γ- sources placed at different distances emitting
a large number of photons (~2x109) in an energy range [60keV - 2000keV ]
output : collection of hits (hit=energy deposition + position of each interaction)
A simulated gamma-ray interacts with the COCAE detector. Three energy depositions are recorded.
Monte Carlo Simulation steps-cont
Event reconstruction
• group together the individual simulated hits into events
• identify their original interaction process
(Compton scattering or photo-effect event)
and the associated information (energy and direction)
Compton sequence reconstruction
(identification of the sequence of Compton interactions)
The incident gamma photon can interact with the detector’s sensitive
materials via multiple Compton scatterings before the scattered
photon is ideally fully absorbed in the detector’s volume.
Monte Carlo Simulation steps-cont
Compton Sequence Reconstruction
the distance between the layers is very small
10 cm
it is impossible to have a timing tag for each hit
2-hit event
The correct time ordering
of the sequence of the
Compton interactions
affects the efficiency of
estimating the incident
photons’ orientation
φ1
φ2
Principle of Compton Reconstruction-cont
if N hits are recorded in the detector, there are N! possible combinations
3-hit event
3 != 6 combinations
The algorithms for the Compton scattering sequence reconstruction identify the hit
ordering by exploiting the kinematical and geometrical information of the event as well
as statistical criteria
Sequence Reconstruction techniquesResults
Overall efficiency
~90% @ E=200keV down to ~70% @E>600keV
cont
Localization
of Point-like Radioactive Sources
Source-to-detector distance estimation
Source orientation estimation
Photo-peak count information from each detecting layer
Quality of reconstructed image
Triangulation technique
Reconstructed Image Resolution
Resolve two sources
tested on a large number of Monte Carlo simulated gamma photons (~2x109) interacting with the
detector’s model,
emitted from point-like sources in an energy range [200keV, 2MeV]
located at various orientation and distances from the detector’s model
Image reconstruction of radioactive sources
The image of a source is
reconstructed by applying the
List Mode Maximum Expectation
Maximization imaging (LM-MLEM)
algorithm
The image of a point-like source is generated
• by projecting each Compton event cone into an imaging plane
• by performing successive iterations on the back-projected images in
order to find the source distribution with the highest likelihood of having
produced the observed data.
Reconstructed Image Resolution
LM-MLEM algorithm, 50 iterations:
z
θ
φ
800keV point-like radioactive source
  26.560   0o
(x,y,z=25,0,50 cm)
COCAE
Reconstructed Image Resolution
z
θ
φ
θ
φ
Reconstructed Image resolution:
COCAE
the combined FWHM of the azimuth (φ) &
inclination (θ) profiles of the source image
  26.560
 0
0
  0.020 0.0030
  27.670 0.0010
Reconstructed Image Resolution-cont
OFF the detector’s symmetry axis
ON the detector’s symmetry axis
less than ~ 4x10-3 sr
~2.5x10-3 sr
(for source-to-detector distances ~50cm)
~0.5x10-3 sr
for point-like sources located at distances
greater than ~1m
Reconstructed Image Resolution-cont
Study the dependence of the detector’s ability to reconstruct images on the
number of events used to reconstruct the image of a radioactive source.
Azimuth (φ) of the source : 180o
minimum number of
triggered events required
to reconstruct the source’s
image :
~ 5x103
Minimum detectable source activity
Given that 5000 events are sufficient to reconstruct the source image and that one photon is
emitted per disintegration)
 given the evaluated total efficiency of the detector (~5-7x10-5 for point-like sources located @
z=120cm emitting photons @E:[ 400keV, 1250keV])
Minimum detectable source activity vs the data acquisition time, for various gamma energies:
For a data acquisition time of 60s the system is able to detect 50 μCi radioactive sources @ z=1,2m
Multiple Source Discrimination
Localization
of Point-like Radioactive Sources
Source-to-detector distance estimation
Source direction estimation
Photo-peak count information from each detecting layer
Quality of reconstructed image
Triangulation technique
Source Direction Estimation
z
θ
φ
COCAE
azimuth & inclination source’s coordinate is estimated within
less than one degree
for inclination angles up to 50o
Localization
of Point-like Radioactive Sources
Source-to-detector distance estimation
Photo-peak count information from each detecting layer
Quality of reconstructed image
Triangulation technique
Source-to-Detector Distance
The Photo-peak Count Information Technique
the estimation of the source-todetector distance (d) is based on the
number of the fully absorbed photons
(via a photoelectric effect) (Ni) in
each detecting layer
1 



sin
2
2









d

i

1
g

k




N i  exp  i  1   j t j  
 j


 k2 

1





sin  2
2
d k 
k2
absorption by the front
layers of the detector
g
k
ratio of the solid angle of the ith
detecting layer over the solid
angle of the first detecting layer.
d
Source-to-Detector Distance
The Photo-peak Count Information Technique
z
θ
φ
COCAE
Higher statistics is necessary in order to
reduce the errors for the case of incident
photon energies > ~1000keV emitted by
sources located at distances > ~1m.
this method can estimate source-to-detector
distances within 2σ up to ~2m,
for incident photon energies up to ~2MeV
Localization
of Point-like Radioactive Sources
Source-to-detector distance estimation
Photo-peak count information from each detecting layer
Quality of reconstructed image
Triangulation technique
Source-to-Detector Distance
The Reconstructed Image Technique
the quality of the image should be better (the FWHM of the x and y-distribution of
the image has the lowest value) if the projection imaging plane is placed on the
real source-to-detector-distance rather than in other distances
Projection
imaging planes
 The image of each point source has
been reconstructed by the LM-MLEM
imaging algorithm (200 iterations) at
various projection imaging planes
Real source
position
cocae
 the combined FWHM of
the x- and y- coordinate
distributions is evaluated
Source-to-Detector Distance
The Reconstructed Image Technique
this method can accurately estimate only the distance of point sources
being in the near field of the COCAE detector (distances up to z=30cm)
Localization
of Point-like Radioactive Sources
Source-to-detector distance estimation
Photo-peak count information from each detecting layer
Quality of reconstructed image
Triangulation technique
The Triangulation Technique
To test the method, two models of the COCAE instrument at a given distance (d) have
been used in order to identify the direction of the photons emitted by point-like sources
located on the first COCAE’s symmetry axis, at a distance of 50cm from its center.
The source position is estimated as
the middle of the minimum distance
vector of the two 3D skew lines defined
by the estimated source directions e1
and e2 and the detector position.
The Triangulation Technique
The method can estimate the position of point-like sources within few centimeters
Summary -I
Functionality-Design

The COCAE instrument
Principle of Compton reconstruction
Monte Carlo simulation studies
Summary -II
 Reconstructed Image Resolution
less than ~4x10-3sr
 Minimum detectable source activity
as a function of the data acquisition time
 60s for 50μCi
 Two Source Discrimination
Summary-III
Direction estimation
Distance estimation
Distance estimation
Distance estimation
evaluated within 1o by source
image reconstruction using the
LM-MLEM imaging algorithm, for
inclination angles up to 50o
can estimate distances
(within 2σ) up to ~2m
for point-like sources of
energies up to 2ΜeV
Photo-peak count information
Quality of reconstructed image
can estimate only distances in
the near field
triangulation
Can estimate the position of pointlike sources within few cm
2nd SENS-ERA WORKSHOP
on “Advanced sensor systems & networks”
TEI Piraeus, 6-7 December 2012
Simulation studies on the localization of
radioactive sources using a portable
detector based on pixelated CdTe crystals
Zachariadou Katerina et al
TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology
Spares
Zachariadou Katerina et al
TEI of Piraeus, General Department of Physics, Chemistry & Materials Technology
International Scientific Conference eRA-7 ,TEI of Piraeus, 27-30 September 2012
Monte Carlo Simulation studies
-cont
The full energy peak originates from two
different types of events:
a) one cluster events (photoelectric effect)
b) sequence of Compton scatterings followed
by a photoelectric interaction
Typical energy deposition
spectrum for 200 keV
incident gamma rays
energy illustrating the
photo-peak and the
Compton plateau
Reminder: only Compton scattering events
that are fully absorbed are useful for the
Compton Imaging Principle technique
 1

1


cos  1  m0 c

E

E

E
g
g
e


2
Compton Sequence Reconstruction
techniques
First step:
Apply Compton kinematics to
reject (if possible)
the one of the two orderings
 1
1 

cos  1  m0 c

E

 g E g  Ee 
2
for incident photon energies below
200keV all of the dual cluster Compton
events in the photo-peak have a unique
ordering
2-cluster event
Sequence Reconstruction techniques- cont
Dual cluster events (DCS)
For handling the ambiguous ordering events, three
algorithms have been evaluated
DCS-A: The sequence with the higher Klein2
Nishina cross-section
the
 Ebe

re2  E g  to
E0
d is assumed
g
2 

  

 sin 
correct one
d
2  E0   E0
Eg


DCS-B: calculates the Klein-Nishina differential
cross-section multiplied to the probability for
absorption via a photoelectric effect and assumes
that the sequence with the higher product
probability is the correct one
DCS-C: the cluster that has the larger energy
deposition is assumed to be the first Compton
scattering
algorithms CDS-B and DCS-C have similar performance, being able to
identify the correct Compton sequence with an efficiency of about 95%
for incident gamma energies above 800keV
Sequence Reconstruction techniquescont
Multiple cluster
events (MCS)
For Multiple cluster events there are N! possible sequences
Eg 2
E0
rˆo
φ0
QF  
φ1
uˆ2
rˆ2
z
cos
i 2
uˆ1 E
e
rˆ1
geo
N 1
rˆ3
cos
 cos
2
2
 cos
kin  

cos  geo
kin
i
i

geo 2
i
i
Compton scatter angles are
calculated by the measured energy
depositions
cos
kin
 1

1


 1  m0 c

E

 g 2 E g 2  Ee 
2
Ideally the quality factor equals zero for the correct cluster sequence
ˆ
 u1of uˆCompton
Compton
scatter
angle
positions
of absorbed.
the
2
events,
in the
casecalculated
where theby
photon
is fully
photons
before and
after
the in
scattering
Although
measurement
errors
result
a quality factor greater than
zero, the correct sequence is still most likely to correspond to its
minimum value
Sequence Reconstruction techniquesResults
Overall efficiency
cont
Monte Carlo Simulation studies
Overall Event Reconstruction efficiency
Point-like source @ z=80cm
~80% @ E=200keV down to ~55% @E>600keV
Reconstructed image resolution
ON axis
Reconstructed image resolution
OFF axis
Monte Carlo Simulation studies
Detecting efficiency
Recorded events in the photo-peak over
the total number of generated events
Sources located @ ~70cm from the first layer of the detector
Multiple Source Discrimination-cont
Two 800keV radioactive sources out of the COCAE detector’s
discrimination limit
Source Direction Estimation
φ
Source placed at :
φ=0ο, θ=44.84ο
θ
azimuth & inclination source’s
coordinate is estimated within
less than one degree
for inclination angles up to 50o
Source-to-Detector Distance
The Photo-peak Count Information Technique
Ni counts
E=800keV


k2

sin 
2
2 







d

i

1
g

k


N i  exp  i  1   j t j   
2


 k

 j



sin 1  2
2 
d

k


1
z=80cm
Layer number
Source-to-Detector Distance
The Photo-peak Count Information Technique
this method can estimate within 2σ
source-to-detector distances up to ~2m,
for incident photon energies up to ~2MeV
Higher statistics is necessary in order to
reduce the errors for the case of incident
photon energies > ~1000keV emitted by
sources located at distances > ~1m.
Source-to-Detector Distance
The Photo-peak Count Information Technique
z
for point-like sources located

θ
evaluate the source direction
azimuth & inclination distributions
of the reconstructed image
φ
COCAE
off the detector’s axis of symmetry:

align the detector with the source,
so the photo-peak count technique can
be applied:


k2

sin 1 
2
2 







z

i

1
g

k


N i  exp  i  1   j t j   
2


 k

 j



sin 1  2
2 
z

k


Monte Carlo Simulation studies
Angular resolution
determined by the FWHM of the
ARM distribution
ARM : the angle difference between
the reconstructed Compton cone and
the actual incident photon direction
known a priori from the Monte Carlo
the ARM distribution has a well-shaped peak
Gaussian and a Lorentz distribution for low
energy photons
the angular resolution is evaluated to be < 40
if the detector energy resolution is around 1%
for the case of 600keV incident photons
Sequence Reconstruction techniquesResults
Efficiency of evaluating the direction of the incident photons
The efficiency of evaluating the direction of the incident
photons at a given energy has been defined as
the fraction of reconstructed events that lie inside an
acceptance interval around the ARM peak
The latter is defined as the interval that
contains the 95.5% (2σ) of all the
events, assuming perfect event
reconstruction.
The ARM acceptance interval varies from
~25o at 200keV down to ~13o at
2000keV.
Point-like source @ z=80cm
cont
The Reconstructed Image Technique
Far field case
PID 350 pixelated CdTe radiation detector
AJAT Oy is a pixelated gamma
ray detector based on CdTeCMOS technology
241Am
The active area of the PID350 covers an area of 4.5 cm
x 4.5 cm and consists of two modules.
Each module has 8192 radiation sensing pixels with
350μm size, thus a PID350 detector consists of 16384
pixels.
PID 350 pixelated CdTe radiation detector
A prototype system has been
assembled as a stack of three
PID350 detectors stacked together
99mTc
(140.5keV)
Able to evaluate the distance of a
gamma ray source with good accuracy
placed in a distance range from 20 cm
up to 100 cm.
Readout ASIC
The ASIC has been manufactured using
the 0.18um MM/RF 1P6M process of UMC.
8 columns x 8 rows (64 pixels)
Its power supply is 1.8V.
The pixel pitch is 400um
the pixel dimension is 267um x 251um.
Output:
• a voltage level proportional to the charge
collected from a single photon
• a voltage level proportional to the time
between the hit and the leading edge of an
external pulse
The two voltage levels are digitized in-pixel
Only pixels with hits are read
Detecting layers
Diode structures
• p-n and Schottky using commercially available CdTe crystals (by ACRORAD) and
CdZnTe crystals prepared within the collaboration up to 75mm in diameter
• Ohmic type CdZnTe detectors using crystals developed within the collaboration
Metal electrode Ni for both Schottky and Ohmic
% FWHM of 662keV photo-peak with CdTe p-n diodes
Best resolution @300-350V
Detecting layers
<1% @ 662keV
For 1mm crystal
FWHM of a Ni/CdTe/Ni Schottky diode at room temperature
National University of Ukraine
and Lashkarikov Institute
Current status
Detectors:
We proceed with Ohmic type CdZnTe and CdTe detectors
Since Schottky diode structures need further development in
order to achieve good yield + Cost considerations , large quantity
The pixelization will be performed in Fribourg using CdTe
commercial + CdZnTe
Electronics:
Two versions have been made
towards the final chip