Download Multi Pixel Photon Counters (MPPC) as an Alternative to APD in PET

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 3, JUNE 2010
Multi Pixel Photon Counters (MPPC) as an
Alternative to APD in PET Applications
A. Nassalski, Member, IEEE, M. Moszyński, Fellow, IEEE, A. Syntfeld-Każuch, Member, IEEE,
T. Szczȩśniak, Member, IEEE, Ł Świderski, Member, IEEE, D. Wolski, T. Batsch, and J. Baszak
Abstract—The performance of multi pixel photon counters
(MPPC) of 3 mm 2 3 mm size, with 14400 and 3600 pixels, were
studied by means of the signal from a laser light pulser and
using the 3 mm 2 3 mm 2 20 mm LSO pixel scintillator. Special
attention was paid to measure number of fired pixels, generated
by the light of pulser and that of the LSO crystal, using a direct
method of a comparison of the light peak position in the pulse
height spectrum with that of the single photoelectron. The tests
of the LSO crystal showed 1550 6 80 fired pixels per MeV in
the MPPC with 14400 pixels assuring a good linearity of the
response up to about 1 MeV energy of gamma rays absorbed in
the LSO crystal. Energy resolution of 14.8% for 662 keV gamma
rays from 137 Cs source and a time resolution of about 850 ps for
511 keV annihilation quanta were limited by a rather low number
of the fired pixels compared to the number of photoelectrons in
photomultipliers.
Index Terms—Detector linearity, multi pixel photon counters
(MPPC), scintillation detection.
I. INTRODUCTION
ACING the challenge to introduce a solid state photodetector into different fields of photon detection, from medical to high energy physics applications, a lot of efforts have
been made to develop new multi pixel photon counters (MPPC).
The MPPC is a solid state photodetector also known as the silicon photomultiplier (SiPM). It is a photon-counting device consisting of multiple avalanche photodiode pixels connected in
parallel and operating in the Geiger mode. When photons enter
a pixel while it operates in Geiger mode, the pulse output from
the pixel is constant regardless of the number of photons. This
means that each pixel provides only the information whether or
not it received one or more photons. The output signal from the
MPPC is proportional to the number of excited pixels. Details
of the operational characteristics of a SiPM can be found in the
literature [1]–[7].
Nowadays the manufacturers’ efforts are focused on improvement of overall photon detection, effective area, amount of active pixels, detection efficiency as well as reduction of dark
F
Manuscript received October 18, 2008; revised April 02, 2009, July 23, 2009,
and October 08, 2009; accepted October 08, 2009. Date of current version June
16, 2010.
A. Nassalski, M. Moszyński, A. Syntfeld-Każuch, T. Szczȩśniak, Ł.
Świderski, D. Wolski, and T. Batsch are with the Soltan Institute for Nuclear
Studies, PL 05-400 Otwock-Swierk, Poland (e-mail: [email protected]).
J. Baszak is with Hamamatsu Photonics Deutschland GmbH, Polish Office,
PL 02-525 Warsaw, Poland (e-mail: [email protected]).
Digital Object Identifier 10.1109/TNS.2010.2044586
count rate and capacitance of the MPPC detectors [5]. Unfortunately, most of these features because of their contradictory
nature can’t be realized in one device.
Recently Hamamatsu has introduced on the market a
3 3 mm MPPC with 3600 and 14400 pixels, better fitted
to make tests with scintillators of the size assuring acceptable
detection efficiency for gamma rays.
This paper will focus on two aspects in characterization of
the MPPC detectors, first using a laser diode pulser and in the
second part, a study with LSO (Lu SiO :Ce) pixel crystal coupled to the MPPC. A considerable effort has been made to measure the amount of detected light as estimated by the number of
fired pixels (comparable to the photoelectron number in other
detectors) and the pulse height resolution. In scintillation tests
the measurements covered the determination of the energy resolution, non-proportionality of the light yield and the time resolution for 511 keV annihilation quanta for both types of the
3 3 mm Hamamatsu MPPC detectors.
Effects relating to the cross-talk and afterpulses in the MPPC
were not studied in this work, because they were completely
masked by a high noise level.
II. EXPERIMENTAL DETAILS
All the studies were done using two Hamamatsu MPPC of
3 mm 3 mm size, S10362-33-025C and S10362-33-050C,
consisting of 14400 and 3600 pixels, respectively. Two samples
of each device were tested. In the first part of the study, the
MPPCs were illuminated by a red laser diode type HL6501MG
from Opnext, Japan. In the second part of the study, a detector
consisting of a 3 mm 3 mm 20 mm LSO pixel crystal, coupled to both types of MPPC detectors, was analyzed. The LSO
crystal was coated with Teflon tape and coupled to the MPPC
with silicone grease. The main MPPC parameters, according
to the manufacturer data, are summarized in Table I, and the
main physical properties of the LSO scintillator are collected
in Table II.
In all the measurements, the MPPC signal was sent to the
charge sensitive preamplifier [11] and then to the Ortec 460
Delay Line Amplifier, selected to reduce a shift of the baseline at
the amplifier output caused by a high rate of noise pulses in the
MPPC. A PC-based multichannel analyzer (MCA) Tukan 8 K
[12] recorded energy spectra. Peak positions and their full width
at half maximum (FWHM) were obtained by applying Gaussian
fits. In the analysis, the FWHM of the peaks was not corrected
for the saturation effect in the MPPC.
0018-9499/$26.00 © 2010 IEEE
NASSALSKI et al.: MULTI PIXEL PHOTON COUNTERS (MPPC) AS AN ALTERNATIVE TO APD IN PET APPLICATIONS
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TABLE I
MAIN PARAMETERS OF THE HAMAMATSU MPPC USED IN THE STUDY
Fig. 1. Separated single photoelectrons obtained with the S10362-33-025C
MPPC detector.
TABLE II
MAIN PROPERTIES OF LSO SCINTILLATOR
III. RESULTS
A. MPPC Performance
A dark counts pulse height spectrum of the MPPC
S10362-33-025C is shown in Fig. 1. The MPPC dark noise
spectrum was measured at 69 V bias, corresponding to 1 V
over-voltage, and at the temperature in the laboratory of about
20 C. A very high counting rate of dark noise pulses shifts
up the base line at the output of the amplifier introducing
a positive offset in the spectrum that degrades pulse height
resolution. Thus, the position of the single photoelectron peak
was corrected for this offset, which was determined from the
calibration based on the single and double photoelectron peaks.
It was used further, as the reference, to measure the number
of the fired pixels for the light flashes from the laser pulser or
LSO crystal.
The MPPC response to the light pulse from the laser diode
is presented in Fig. 2, as observed at a digital scope (Tektronix
TDS5034B). The laser was triggered by the short pulse of 3 ns
(FWHM) producing light pulse of 6 ns (FWHM), and monitored
by the very fast PMT (R5320), characterized by a rise time of
the single photoelectron pulse of 700 ps.
The rise time of the MPPC response to the laser pulse is defined by the shape of the light pulse, while its fall time is the
effect of the large capacitance of the device. This 320 pF capacitance, together with the 50 ohms load resistor at the input of
the scope forms a low pass filter with the loading time constant
of 16 ns. It is confirmed at the lower panel of Fig. 2 displaying
the MPPC output signal recorded with an additional 50 ohms
resistor at the anode of the MPPC. The fall time is shorter by
about factor of 2.
The reported shape of the MPPC output pulse is different than
that observed and predicted for MPPC devices with small capacitance. The small MPPCs characterize by about 10 times lower
capacitance and then, show two components of the fall of the
output signal [13], distinguished however, due to a much lower
loading integration time constant, below 2 ns. A direct comparison of the response of 1 1 mm and 3 3 mm detectors is
presented in [5], confirming results of Fig. 2.
In the next part of the study, tests of the linearity of the output
signal were performed. Since each pixel detects only whether
or not, one or more photons have entered the detector, the total
number of the fired pixels does not directly correspond to the
number of detected photons. If two or more photons are triggering one pixel then the photon detection linearity degrades
because the number of incident photons becomes larger than the
number of fired pixels. This is reflected in (1), which presents
the relation of the number of fired pixels as a function of the
number of detected photons for a given number of pixels in the
MPPC [2]
(1)
where
:
number of excited pixels,
:
total number of pixels,
: number of incident photons,
PDE:
photon detection efficiency.
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 3, JUNE 2010
Fig. 2. S10362-33-025C MPPC output pulse, the light pulse from laser, as seen by the fast PMT (R5320) and the electrical pulse used to trigger the laser. Upper
panel presents output signal on 50 ohms loading resistor at the input of a scope, while at the lower panel the second 50 ohms resistor was placed at the anode of
the MPPC. Note a faster fall time due to a shorter time constant at the MPPC output.
The photon detection efficiency, PDE, is the product of the
quantum efficiency (QE), the fill factor or geometrical factor
(GF) and the probability that an incoming photon trigger a
breakdown (Pt) [2]
(2)
The number of detected photons is described, in turn, by
(3)
Fig. 3 presents the measured number of the fired pixels versus
the number of detected photons, calculated, following (1) and
(3). It reflects the deviation from the linear dependence represented by the solid line, calculated for the infinite number of
pixels in the MPPC. Error bars are within the point size. Both
MPPCs of 14400 and 3600 pixels show non-linear behavior.
However it is clear that the range of linearity is more extended
in case of the MPPC with a higher number of active pixels.
In the next experiment the MPPC was illuminated by light
pulses from the laser of different intensities and then their pulse
height spectra were stored at MCA. For each position of the
peak, the corresponding number of the fired pixels was determined in relation to the single photoelectron peak and its respective FWHM, expressed in the number of the fired pixels
was obtained from the Gaussian fit. The results are presented in
Fig. 4, where the plot of the measured FWHM versus the square
root of the measured number of fired pixels is presented. The
nonlinear response of MPPC is reflected by a deviation of the
FWHM of the continuous line calculated from Poisson statistic,
for a perfectly linear photodetector. In the case that two or more
NASSALSKI et al.: MULTI PIXEL PHOTON COUNTERS (MPPC) AS AN ALTERNATIVE TO APD IN PET APPLICATIONS
Fig. 3. Number of excited pixels versus the number of detected photons, for
both 14400 and 3600 pixels Hamamatsu MPPC, error bars are within the point
size.
Fig. 5. Energy spectra of singe photoelectron and cesium source (
sured for LSO crystal with 14400 pixels Hamamatsu MPPC.
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Cs), mea-
TABLE III
RESULTS OF LSO GAMMA-RAY SPECTROMETRY MEASUREMENTS
Fig. 4.
FWHM of the light peaks versus the square root of the fired pixels.
photons are triggering one pixel, the photon detection linearity
lowers because the number of incident photons is larger than the
number of excited pixels. Fig. 4 reflects clearly an influence of
nonlinear response of MPPC for larger light signals.
Fig. 3 presents in fact, the illustration of the nonlinear behavior of the MPPC depending on the number of pixels since it
is based on the theoretical (1). In turn, Fig. 4 confirms a good
accuracy of the measured number of fired pixels, well related to
the width (FWHM) of the light pulser peak.
B. Gamma-Ray Spectrometry
In this part of the study, the MPPC signal was sent to the
charge sensitive preamplifier [11] and then to the Ortec 460
Delay Line Amplifier operated with 0.25 s integration time
constant. The energy spectra of the single and double photoCs source are presented in
electrons and gamma rays of a
Fig. 5. It allows measuring the number of the fired pixels by
the method used in the scintillation detectors based on the PMT
light readout [14].
In this method the number of photoelectrons is measured
directly by comparing the position of the full energy peak of
-rays detected in the crystal to the position of the single photoelectron peak. The observed positions of the single and double
photoelectrons was used to correct for the offset introduced into
spectrum by a high counting rate of noise pulses.
Table III summarizes the performance of the detectors when
coupled to an LSO scintillator in comparison to the Photonis XP2020Q PMT and the Hamamatsu S8550 APD array
[15]. Since the pixel size in the S8550 APD array is only 1.8
mm 1.8 mm, the tests were done with a 2 mm 2 mm 15
mm LSO crystal.
In the third column, the number of photoelectrons is shown
for the tested 3 3 20 mm LSO crystal. Note a lower number
of photoelectrons for the 14400 pixel detector in comparison
to the 3600 pixels detector, also confirmed by a poorer energy
resolution, collected in the fourth column. It confirms a lower
fill factor quoted for this MPPC by the manufacturer.
To compare the MPPC performance two types of photodetector were selected: a commonly used Photonis photomultiplier
XP2020Q, characterized by a photocathode blue sensitivity of
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 3, JUNE 2010
TABLE IV
LIGHT OUTPUT OF THE LSO PIXEL CRYSTAL AND PHOTON DETECTION
EFFICIENCY OF MPPC PHOTODETECTORS
10.1 A/lmF (where F stands for filtered blue sensitivity) [16]
and a Hamamatsu S8550 APD array, previously tested by the
author in [15]. Note the observed by a factor of two and four
higher photoelectron and electron-hole pair numbers for PMT
and APD, respectively, than that of the MPPC and significantly
better energy resolution.
The measured photoelectron number with the XP2020Q
PMT allows for estimation of the photon detection efficiency
of the tested MPPC devices for the LSO pixel crystal. First
the light output of the LSO pixel was calculated assuming the
integral quantum efficiency of the XP2020Q, following [14],
see Table IV. For the MPPCs the procedure was reversed and
the PDE was calculated based on the measured light output of
the LSO crystal.
The estimated PDEs of about 10% is comparable to that measured in [3] for SiPM of comparable size of pixels. It could be
improved by a larger over-voltage, however, the bias voltages of
both MPPC was very close to those recommended by the manufacturer, see Table I, corresponding to 1 V of over-voltage. In
the case of the MPPC 050C, the estimated PDE could be affected partly by a nonlinear response for 662 keV peak in LSO,
see Fig. 7 below. A low number of the fired pixels in the MPPCs
is also confirmed by a poor energy resolution, see Table III.
In the Fig. 6, the non-proportionality characteristics of the
LSO pixel crystal, as measured with the MPPC, with 14400 and
3600 pixels, in comparison to the XP2020Q PMT, are presented.
It confirms that the LSO light signal is within the linear response
of the MPPC up to about 1 MeV gamma ray energy in case
of the 14400 pixels detector. The bending down of the curve
at about 1 MeV is common for the MPPC and XP2020 PMT,
characterized by the excellent linearity of the current pulse up
to 100 mA. This effect is characteristic for the pixel crystals
and it is related to a reduced light output of high energy gamma
rays, probably because of a larger light attenuation. In case of the
3600 pixels MPPC detector, the lower number of active pixels
limits the linearity of the detector, below 500 keV gamma ray
energy.
Fig. 7 presents the energy resolution measured with the LSO
crystal as a function of incident -rays energy. The curves
are linear in a double logarithmic scale. The MPPC detectors
present the same behavior of the energy resolution in the whole
range of energies. The energy resolution measured with the
PMT is superior in the whole measured energy range. It is
evidently associated with a larger number of photoelectrons
produced at the PMT photocathode than that of the MPPC.
Fig. 6. Non-proportionality characteristics for LSO crystal measured with
Hamamatsu MPPC 14400 and 3600 pixels detector in comparison to the
XP2020Q PMT.
Fig. 7. Energy resolution (FWHM) characteristics for LSO crystal measured
with Hamamatsu MPPC 14400 and 3600 pixels detector in comparison to the
XP2020Q PMT.
C. Time Resolution Study
To check the usefulness of the MPPC detector for PET instrumentation, the coincidence time resolution of the tested detectors for 511 keV annihilation quanta was measured using the
same LSO pixel crystal. As the reference detector, a truncated
conical BaF crystal 25 mm in diameter and 15 mm high was
used. The crystal was coupled to the Photonis XP20Y0 QDA
PMT equipped with a screening grid at the anode. Its time resops for 511 keV, as reported in [17].
lution was equal to
The timing measurements were performed with a Na -source.
The energy windows in the side channels of the detectors were
set at the 511 keV full energy peaks.
In the time setup, the Ortec 935 four channels Constant Fraction Discriminator (CFD) was used. Note that the CFD was used
in the leading-edge mode with a short circuit of shaping delay
and its threshold was set just above the noise level. The output
NASSALSKI et al.: MULTI PIXEL PHOTON COUNTERS (MPPC) AS AN ALTERNATIVE TO APD IN PET APPLICATIONS
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Fig. 9. LSO light pulse at the output of MPPC with 3600 pixels, as observed
at a digital scope (Tektronix TDS5034B).
Fig. 8. Time resolution spectra for LSO crystal measured with Hamamatsu
MPPC 14400 and 3600 pixels detector in comparison to the XP2020Q PMT.
TABLE V
TIME RESOLUTION OF THE LSO SCINTILLATOR
signals from the CFD channels provided start and stop input signals for the Ortec 567 Time-to-Amplitude Converter (TAC). The
TAC output was recorded by a PC-based multichannel analyzer
Tukan 8 K [12]. The output signals were gated by the full energy
peaks of 511 keV -rays. The time spectra are shown in Fig. 8
and the results of the time resolution measurements are given in
Table V.
In the third column the number of photoelectrons (fired pixels
for MPPC) corresponding to the 511 keV peaks are listed. The
time resolution corrected for the contribution of the reference
detector (128 ps) is presented in the fourth column. The last
column shows the time resolution of the tested detectors normalized to the number of photoelectrons. It is not done for the
APD, since the measured time resolution is mainly determined
by the signal-to-noise ratio at the input of a fast discriminator.
The best time resolution was measured with the XP2020Q
PMT, three times better that those measured with the MPPCs. It
follows, first of all, a factor two larger photoelectron number obtained with the PMT. However, the normalized time resolution
to the photoelectron number, presented in the last column, reflects also a higher speed of the PMT in comparison to the tested
MPPCs. This is the effect of a large time constant of 16 ns due to
the 50 ohms load resistor and the MPPC capacitance of 320 pF.
For the LSO light pulse, with the decay time of 40 ns, it leads
to the rise time of the output pulse of the MPPC of about 20 ns
[16]. It is confirmed in Fig. 9, below, which presents the LSO
light pulse at the output of the MPPC 050C with 3600 pixels.
The rise time of 20.6 ns, seen in Fig. 9, confirms well an
earlier prediction. Thus, a 1 mm 1 mm MPPC is preferable
in this respect, due to 9 times lower capacitance. In the tested
MPPC, the speed of the LSO pulse could be largely improved by
a drastic reduction of the load resistor, below 10 ohms [17]. Both
tested MPPCs give comparable time resolution, which follows
measured numbers of fired pixels. It is well reflected in the same
values of normalized time resolution.
The strength of SiMPs in the fast timing is their excellent time
resolution for 1 photoelectron of about 100 ps [2]. However,
for slow decaying scintillators, like LSO, the time resolution is
more governed by the decay of the light pulse than the time jitter
of photodetector [17]. Thus, the high number of photoelectrons
is more important than a low time jitter [19]. A low time jitter of
MPPC will be reflected in timing with fast light pulses of plastic
scintillators or Cherenkov radiation.
However, the MPPCs are superior in comparison to the APD
array. The measured time resolution is more than twice better
than in case of APD arrays. It is the result of dark noise of the
APD array which limits mainly the time resolution.
The presented timing study of the tested detectors, confirmed
a future possibility to introduce the proposed detectors in PET
applications.
IV. CONCLUSION
Multi pixel photon counters are novel type of solid state
photodetectors with significant advantages such as a high gain,
fast timing, compactness and insensitivity to magnetic field. All
of them make MPPC very attractive in application to nuclear
medicine and high energy physics.
The present studies showed that the new Hamamatsu
S10362-33-025C/050C MPPC could be easily applied in nuclear medicine, in particular in detectors for positron emission
tomography, as an alternative to APD arrays. A large number of
pixels in the detector assure a good linearity of the response to
the light from LSO pixel crystals and a good energy resolution.
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 3, JUNE 2010
A slow output pulse of MPPC for the LSO crystal, associated
with a high capacitance of the devices, limits the time resolution; however, it is still superior in comparison to APDs.
A considerable effort was done to determine the number of
fired pixels and its relation to the pulse height and time resolutions. The estimated photon detection efficiency for the light
from LSO crystal showed a value of about 10%, typical for silicon photomultipliers.
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