Download R11-24 Simulation and Experimental Results on Monolithic

Simulation and Experimental Results on
Monolithic CdZnTe Gamma-Ray Detectors
Eric Gros d’Aillon, Marie Claude Gentet, Guillaume Montémont, Jacques Rustique, Loick Verger
Abstract— Monolithic CdZnTe detectors are promising for
medical and small animal imaging because of a good energy
resolution that allows both multi-drug diagnostic and scatter
event rejection. Optimizations of electrode geometry and detector
thickness are used to improve detection efficiency as well as
energy resolution for various material transport properties, with
the ULYSSE simulator. As an example, for a 5 mm thick detector
with (µτ)e =3. 10-3 cm²/V, a 1.8 mm pitch gives the best simulated
energy resolution on the conventional spectrum at 122 keV.
Nevertheless, even with an optimized detector, electronic
correction methods contribute to reduce tailing. Correction using
cathode and anode signals have been tested with HPBM CdZnTe
and THM CdTe:Cl detectors. The appropriate method to correct
energy seems to depend on material. This paper also presents
experimental results on charge sharing and loss.
Index Terms—Gamma-ray detector, CZT, CdTe:Cl, ULYSSE
simulation, correction methods, charge sharing
I
I.
INTRODUCTION
n the field of gamma ray detection for medical and small
animal imaging, pixelated CdZnTe (CZT) semiconductors
are promising detectors.
CZT performances are given by energy resolution,
efficiency and spatial resolution. They are measured with
Conventional Spectrum (CS) but also with Bi-Parametric
Spectrum (BPS) that allows correction methods. For an
interaction, the pulse height depends on the photon energy
deposition and on the detector response: the Charge Induction
Efficiency (CIE). So, the spectrometry performance are
directly related to the CIE. CIE depends on physical
parameters such as transport properties of materials ((µτ)e,
(µτ)h) that give the charges mean free path and on geometric
parameters like electrode size and pitch and detector thickness
that influence the weighting potential, i.e. the charge induction.
As CIE is a function of interaction position in the detector,
mainly the depth, it is admitted that energy can be corrected
with interaction depth measurement.
We have simulated the detector to adapt the electrode pitch
and detector thickness (i.e. detector geometry) with typical
Manuscript received may 17, 2004
The authors are with LETI – CEA Recherche technologique, CEA Grenoble,
17 rue des Martyrs, F 38054 Grenoble, cedex 9, France (corresponding author
[email protected])
material transport properties to achieve a good energy
resolution on CS. We have tested different energy correction
methods and studied the charge sharing and charge loss
between pixels on both THM and HPBM material.
II. SIMULATION STUDIES
Simulations have been made with a three-dimensional
model of a semiconductor gamma-ray detector: ULYSSE [1].
ULYSSE takes into account the gamma ray and charge
collection physical phenomena involved in the detection
process and models the readout electronic response and noise.
It computes in 3D the electric field, the weighting field and the
CIE (using a finite element method) and the CS and the BPS
(using a Monte Carlo method). We use ULYSSE on
monolithic detectors to understand the effect of the different
parameters and to optimize it. To have a resolute CS, CIE must
be as constant as possible through the material. CIE decreases
on anode side due to incomplete charge induction at an anode
distance of the same order of magnitude as anode pitch.
Electron trapping decreases CIE especially on cathode side
where the drift is longer. So, in order to have a constant CIE,
the small pixel effect that depends on the ratio pitch on
thickness and the material transport properties (related to the
(µτ)e product) must be adapted. All simulations have been
made with a collimated 57Co gamma source. As there are
quickly trapped in CZT, hole participation was neglected. To
reduce parameters number, noise was neglected.
A. Charge transport properties effect
Given that the electric field cannot be increased to avoid
breakdown and to limit detector noise, the µτ product gives the
mean free path.
We modeled a standard CZT pixelated geometry (Fig. 1)
(the detectors dimensions are 10 mm x 10 mm x 5 mm, with
16 anodes 2 mm x 2 mm in area, 2.5 mm pitch). The bias is
500 V. We have modeled the response of the detector for four
classical electron µτ products: (µτ)e =1.10-3, 2.10-3, 3.10-3 and
4.10-3 cm²/V. Fig 1 shows the CIE, the CS and the BPS for
these four mean free paths (i.e. 1 cm, 2 cm, 3 cm and 4 cm at
500 V). Without correction, the best resolution on CS
(FWMH) is not obtained with the material with the highest
(µτ)e but the one that is adapted to the pixel dimension (in this
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example, (µτ)e = 2 10-3 cm²/V). A low (µτ)e (10-3 cm²/V) is
not enough to have a good CS because of the electrons
trapping. In this case, a photon which interacts near the
cathode induces less charge on the anode than a photon which
interacts in the middle of the detector, leading to a large peak
on CS. Even with material having a good (µτ)e, there is still a
tailing due to the geometrical effect that decreases energy
resolution. A very good agreement must be found between
charge transport properties and electric field. This agreement
may be impossible to find if the material is not homogeneous.
Correct energy with interaction depth allows working with all
the transport properties and reduces tailing as previously
shown on HPBM monolithic CZT detectors [1].
B. Detector Geometry effect
The small pixel effect is related to detector geometry. We
use ULYSSE to optimize the resolution on CS for different
transport properties and pixel pitches. Fig. 2 shows the
evolution of energy resolution measured on CS when thickness
or pitch varies. As an example, for a 1.2 mm pitch detector
with (µτ)e =3.10-3 cm²/V, the better energy resolution
measured on CS is obtained with a 4.5 mm thick detector.
With a 5 mm thick detector, a 1.8 mm pitch detector gives the
best result.
Thus, detector geometry must be adapted to transport
parameters in order to have a good energy resolution on CS.
However, not all the geometric parameters can be chosen only
to improve energy resolution but also for system
specifications. IT may be impossible to find an optimum if the
material properties are inhomogeneous. That is why electronic
correction may be advantageously used with optimized and
non-optimized geometries.
III. EXPERIMENTAL RESULTS
A. Description of the test bench
We use a test bench dedicated to the CZT monolithic
detector study described in [2]. It contains 17 complete readout
channels (from preamplifier to electronic board), one per each
anode and one for the cathode. For each interaction, the 17
electrodes pulse heights and rise times are simultaneously
recorded, that allows data post treatment. Readout channels
have been calibrated and the electronic noise of has been
measured by injecting current directly on the preamplifiers
through a capacity. The mean total electronic noise is 0.7 %
rms on signal pulse height and 1.4 % rms on signal rise time.
We work with 10 mm x 10 mm x 5 mm HPBM CZT and THM
CdTe:Cl crystals with 4x4 pixels, 2 mm pad size and 2.5 mm
pitch. A collimated 1 mm 57Co gamma source illuminates the
detectors.
We use this test bench to study electronic correction
methods to enhance energy resolution on CS, charge sharing
and charge loss between pixels, and material quality effect.
B. Spectroscopy: correction methods
It has been shown that energy resolution can be significantly
improved by empirically compensating the hole trapping,
given the depth of interaction. Determining the photon
interaction depth using the cathode signal or the anode rise
time is well established [2, 3]. Due to the uniform electric
field, the electron drift duration is proportional to the
interaction depth. Due to the cathode uniform weighting field,
the charge induced on the cathode is proportional to the
interaction depth, the energy deposit and the trapping. Even if
theoretically, cathode pulse rise time, cathode pulse height and
anode pulse rise time lead to the same depth information, these
methods are not equivalent. Leakage courant is larger on the
cathode due to its larger area, so electronic noise to. On the
other way, noise on anode rise time measurement is due to the
difficulty to measure the beginning of electron drift, especially
for strong pixel effect.
A comparison on electronic correction methods has been
made with two HPBM detectors (named D1 and D8) and two
THM detectors (named A1 and A2). Resistivity has been
measured with I(V) (ρ=3.5 1010 Ω.cm for D1 and D8 and ρ=1
109 Ω.cm for A1 and A2) and µτ product with alpha particles (
(µτ)e = 3 10-3 cm²/V for D1 and D8 and (µτ)e = 2 10-3 cm²/V
for A1 and A2). BPS are shown in fig. 3 for THM detectors
and fig 4 for HPBM detectors. On these figures, the fourth
BPS is cathode pulse height vs. cathode rise time. The
correlation between pulse height and interaction depth appears
with all BPS, but noise is more important for tested THM
detectors due to lower resistivity. We studied also a correction
using cathode rise time and anode rise time sum which we
expect to be less noisy because signals sum is linear and noise
sum is quadratic.
Table 1 contains the average FWMH energy resolution
obtains at 122 keV without correction, and for three
corrections methods: with anode rise time (1), cathode and
anode rise time sum (2) and cathode pulse height (3). It
appears clearly that tested HPBM detectors have better energy
resolution than tested THM detectors thanks to higher
resistivity that decreases leakage current.
With tested HPBM CZT detectors, corrections using
cathode is slightly better than correction using anode rise time,
perhaps because cathode signal is easier to measure. With
tested THM CdTe:Cl detectors, correction using anode rise
time gives better results, mainly due to leakage current noise
on cathode. As an example of corrected spectra fig. 5 shows
CS, BPS, corrected BPS and corrected CS. The two detectors
have the same geometry, the bias is more important on HPBM
detectors (400 V) than on THM detectors (300 V) because of
higher resistivity. The CIE difference due to electron trapping
on cathode side appears clearly on uncorrected BPS. These
two BPS can be corrected. The gain on energy resolution is
obvious for both detectors. However, the noise on THM
detector is not corrected so energy resolution is strongly better
for HPBM detector. It must be notice that the two tested THM
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detectors are more homogeneous than the two tested HPBM
detectors. All 16 anodes rise time vs. pulse height BPS are
shown in Fig. 6 for A1 and fig. 7 for A2. It can be seen that
pixel effect is stronger for center pixel than for side pixels. The
two figures are very similar because the two detectors are very
homogeneous. It can be seen that for one anode (on the bottom
right) the rise time is larger than for others. It is due to rise
time gain on the associated electronic board which is larger
than for others.
To conclude, one can correct energy on both tested THM
and tested HPBM material using interaction depth. Method to
measure interaction depth depends on material properties. It is
obvious that this choice depends also on detector geometry.
For larger detectors, noise on the cathode will increase. For
smaller anode, stronger pixel effect will made anode rise time
more difficult to measure.
C. Charge sharing
The phenomenon of charge sharing has been studied by
other groups for CZT detectors [4]. It can lead to bad energy
measurement or wrong localization of interaction. The test
bench helps us to quantify and localize the charge sharing in
our detectors. The collimated gamma source is moved above
the detector. For each interaction, all anodes signal pulse
heights are measured. If two or more pulse heights are superior
to a threshold, the charge was shared between pixels. The
threshold is arbitrarily fixed above the noise. For each source
position, the amount of charge collected by only one pixel and
the amount of interaction for which the charge is shared
between at less two anodes are monitored (fig. 8 and fig. 9 for
THM detectors with a 0.25 mm step and fig. 10 and fig. 11 for
HPBM detectors with a 0.50 mm step).
Regardless of the source position, there are always
interactions for which charges are collected by neighboring
pixels and interactions for which charge are shared. It can be
seen that, for these detectors, charge sharing is more important
between edge pixels than between center pixels. When the
source is localized between two pixels, charges are shared for
approximately up to 50 % of the measured interaction. The
average ratio of interaction for which the total charge is shared
is 15 % in all the detector area for both THM and HPBM
detector.
It must be notice that the difference between the pulse
height of event collected on only one anode and the sum of the
two pulse heights of events shared between two anodes is
lower than the noise, that means that the possible charge loss
between pixels is lower than the noise.
IV. CONCLUSION
However, detector geometry cannot only be chosen to
compensate transport properties but may be adapted to system
requirement. Pitch must be adapted to spatial resolution needed
and thickness to wished efficiency. We show that electronic
correction with interaction depth correct geometric effect and
trapping for different (µτ)e on both THM CdTe:Cl and HPBM
CdZnTe. For the presented geometry (2.5 mm pitch, 5 mm
thick), measuring interaction depth with cathode signal shows
the best result for tested CdZnTe and with anode rise time
shows better results for tested CdTe:Cl. Nevertheless, after
interaction depth correction, energy resolution is strongly
better for tested HPBM detectors because of its higher
resistivity so lower leakage current. Therefore, resistivity
seems to be the key point for spectrometry. However, biparametric spectra are more homogeneous on tested THM
detectors, the comparison between two detectors shows that
they are very similar. Therefore, THM material can be a good
choice for applications where the stress is laid on homogeneity
but no on energy resolution.
The next studies will be experiments on the geometry
modeled (pitch variation) and on other types of material to
validate the simulation results. Further investigations will be
made on charge sharing.
V. REFERENCES
[1]
[2]
[3]
[4]
F. Mathy, A. Glière, E. Gros d’Aillon, P. Massé, M. Picone, J.Tabary and
L. Verger, “A Three-dimensional model of CdZnTe Gamma-Ray
Detector: Experimental Validation”, IEEE Trans. Nucl. Sci., submitted
for publication.
L. Verger, P. Ouvrier-Buffet, G. Montémont, J. Rustique and C. Riffard,
« Performances of a new CdZnTe portable spectrometric system for high
energy applications », IEEE Nucl. Sci. Symp. Conf. Record, Portland,
USA, 2003
Z. He, G. Knoll, D. Wehe, R. Rojeski, C. H. Mastrangelo, M. Hammig,
C. Barrett and A. Uritani “1-D position sensitive single carrier
semiconductor detectors”, Nucl. Intstr. And Meth. A 380 p228-231, 1996
E Kalemci, J.L. Matteson “Charge splitting among anodes of a CdZnTe
strip detector”, Proc. SPIE, vol 4141, pp. 235-242, 2000
BPS
1 D CIE
ano de
cathod e
2 mm
CS
5 mm
We have shown that ULYSSE helps us to adapt detector
geometry with transport parameters in order to have good
energy resolution as well as good detection efficiency on
conventional spectra.
2. 5 mm
Fig. 1. CIE, BPS and CS for different µτe products. Computation without
electronic processing nor noise modeling. The small pixel effect is the same
for all these spectra but electron trapping decreases CIE on the cathode side for
poor µτe. The BPS curve off the is due to this trapping
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Fig. 2. Evolution of energy resolution (fwmh) on CS with detector
thickness and pitch for two mean free paths: 1 cm (left) and 3 cm (right).
TABLE 1
ENERGY RESOLUTION MEASURED ON CS (FWMH) (MEAN AND STANDARD
DEVIATION OF 16ANODES) WITHOUT CORRECTION (0), WITH ANODE RISE TIME
CORRECTION (1), WITH ANODE AND CATHODE RISE TIME SUM (2) AND WITH
CATHODE PULSE HEIGHT (3) FOR THM CDTE:CL (A1 AND A2) AND HPBM
CZT (D1 AND D8).
A1
A2
D1
D8
Non
corrected
9.5 +- 1.3 %
9.0 +- 0.8 %
4.3 +- 0.9 %
4.2 +- 1.2 %
1
2
3
6.8 +- 0.5 %
6.7+- 0.4 %
3.3 +- 0.4 %
2.9 +-0.7 %
7.4 +-1.5 %
7.0 +-1.4 %
2.9+- 0.4 %
2.7 +- 0.6 %
7.7 +- 1.9 %
8.5 +- 0.9 %
2.9 +- 0.4 %
2.6 +- 0.6 %
Fig. 4. Typical HPBM BPS. For graph 1 to 3, X-coordinate is anode pulse
height. Ordinate is anode rise time (1), cathode and anode rise time sum
(2) and cathode pulse height (3). On graph 4 is plotted cathode rise time
vs. cathode pulse height. Bias 400V. Irradiation with 57Co and 1 mm
collimator
Fig. 3. Typical THM BPS. For graph 1 to 3, X-coordinate is anode pulse
height. Ordinate is anode rise time (1), cathode and anode rise time sum (2)
and cathode pulse height (3). On graph 4 is plotted cathode rise time vs.
cathode pulse height. Bias 300V. Irradiation with 57Co and 1 mm collimator.
Fig. 5. Comparison between THM CdTe:Cl detector (A1 : left, bias 300V)
and CdZnTe THM detector (D8 : right, bias 400V). From top to bottom are
shown CS, BPS (cathode on anode pulse height ratio vs. anode pulse height),
corrected BPS and corrected CS (1 mm collimator). It can be seen that both
THM and HPBM spectra can be corrected but the energy resolution measured
on CS is better for tested HPBM material because of higher resistivity
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Fig. 6. 16 Rise Time vs. pulse height BPS of each anodes of A1 detector
(57Co, bias 300V, 1 mm collimator).
Fig. 9. Amount of interaction for which the charge was collected by two
pixels in function of gamma source position. THM CdTe:Cl detector. 300V
bias. 1 mm collimator. The black squares represent the anodes. It can be seen
that there is more charge sharing near the edge of the detector than on the
center.
Fig. 7. 16 Rise Time vs. pulse height BPS of each anodes of A2 detector
(57Co, bias 300V, 1 mm collimator).
Fig. 8. Amount of interactions for which the whole charge was collected
by only one pixel. THM CdTe:Cl detector. 300V bias. 1 mm collimator. The
black squares represent the anodes. The color represents the detector
efficiency. Anode can be seen with this efficiency
Fig. 10. Number of interaction for which the all charge was collected by
only one pixel. HPBM CdZnTe detector. 400V bias. 1 mm collimator. The
black squares represent the anodes. The color represents the detector
efficiency. Anode can be seen with this efficiency
Fig.11. Amount of interaction for which the charge was collected by two
pixels in function of gamma source position. HPBM CdZnTe detector. 400V
bias. 1 mm collimator. The black squares represent the anodes.
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