IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 14, JULY 15, 2013
1309
Fast Timing Silicon Photomultipliers for
Scintillation Detectors
Jung Yeol Yeom, Ruud Vinke, Nikolai Pavlov, Stephen Bellis, Liam Wall, Kevin O’Neill,
Carl Jackson, and Craig S. Levin
Abstract— A new silicon photomultiplier is fabricated for fast
timing applications by SensL Technologies Ltd. This new family
of silicon photomultiplier, herein referred to as fast SPM devices,
is fabricated with a third terminal that has a low output
capacitance to improve timing performance. Two fast SPMs
(an N-on-P type and a prototype P-on-N type) are assessed for
energy and timing performances in scintillation detectors. When
coupled with L(Y)SO:Ce crystals, the optimal energy resolutions
for the 511 keV photon peak are 13.7% and 13.1%, whereas
coincidence resolving times (CRTs) of 184 ± 5 and 157 ± 3 ps
are attained with 2 × 2 × 3 mm3 crystals for the N-on-P and Pon-N devices, respectively. For longer crystals (3 × 3 × 20 mm3 ),
more relevant for positron emission tomography, the CRTs are
298 ± 9 and 234 ± 6 ps for the two SPM types, respectively, a
significant improvement from standard SPM devices.
Index Terms— Silicon photomultipliers, timing resolution, scintillation detector, PET.
S
I. I NTRODUCTION
INCE it was first reported that applying a high electric
field across a uniform p-n junction can cause an avalanche
multiplication of carriers upon impact by an incoming photon
[1], semiconductor photosensor technologies have developed
into the silicon photomultiplier (or Geiger-mode avalanche
photodiode, solid state photomultiplier, etc) as known today.
A silicon photomultiplier consists of an array of microscopic
avalanche photodiodes (microcells) operated in Geiger-mode
and produces an output proportional to the number of excited
microcells (and thus to the number of incident photons). These
new photosensors have a fast response time, high gain, high
quantum efficiency, single photon detection capability, and
they are an attractive alternative to PMTs for their compact,
rugged size and insensitivity to magnetic fields. Silicon photomultipliers are increasingly being used in areas where timing
Manuscript received January 23, 2013; revised April 16, 2013; accepted
May 9, 2013. Date of publication June 12, 2013; date of current version
June 26, 2013. This work was supported in part by the Stanford Dean’s
Postdoctoral Fellowship, and in part by the Netherlands Organization for
Scientific Research and under Grant NIH-NIBIB R21EB014405.
J. Y. Yeom and R. Vinke are with Molecular Imaging Program at Stanford
and Department of Radiology, Stanford University, Stanford, CA 94305 USA
(e-mail: yeomjy@stanford.edu; rvinke@stanford.edu).
N. Pavlov, S. Bellis, L. Wall, K. O’Neill, and C. Jackson are with
SensL Technologies Ltd., Cork, Ireland (e-mail: npavlov@sensl.com;
sbellis@sensl.com; lwall@sensl.com; koneill@sensl.com; cjackson@
sensl.com).
C. S. Levin is with the Department of Radiology, Physics, Electrical
Engineering and Bioengineering, Stanford University, Stanford, CA 94305
USA (e-mail: cslevin@stanford.edu).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2013.2264049
Fig. 1. Circuit schematic of a Fast SPM architecture (N-on-P type) detailing
the capacitive coupling of several microcells directly onto the third output
(Fast Terminal) with readout connections in parentheses. The diode symbol
represents an individual microcell while the arrows depict incident light
photons. No loss in fill factor or performance degradation is observed with
the inclusion of the third electrode. If the Fast Terminal is not connected to
output circuitry, these devices operate as standard 2-terminal (anode, cathode)
SPMs.
precision on the order of picoseconds is necessary, such as
time-of-flight (TOF) positron emission tomography (PET) [2],
high energy physics [3], biomedical applications [4], etc.
Recently, a novel silicon photomultiplier (Fast SPM) architecture with an additional electrode (Fast Terminal) for fast
timing applications has been developed [5]. The Fast Terminal
is a third electrode in addition to the two electrodes in standard
silicon photomultipliers (Fig. 1). This capacitively coupled fast
output, with an output capacitance of about one-twentieth of
the standard device, is less affected by signal shaping with
the input impedance (e.g. 50 ohm) of subsequent readout
electronics [6] and is thus able to produce a much faster
signal rise time for improved timing performance without the
need for custom designed readout electronics with low input
impedance. In this letter, we assessed the performance of these
new Fast SPM devices for applications that employ scintillatorbased radiation detectors such as ToF PET.
II. E XPERIMENTAL M ETHOD AND S ETUP
Two types of Fast SPMs have been fabricated — an N-onP device (M-series, model no: MicroFM-30035-SMT) with a
peak photon detection efficiency (PDE) at long wavelengths
and a prototype P-on-N device (B-series, referenced here
1041-1135/$31.00 © 2013 IEEE
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 14, JULY 15, 2013
Fig. 2. Photograph of the N-on-P type (left) and P-on-N type (right) Fast
SPMs. The P-on-N type is a prototype in a non-optimized package for tests.
TABLE I
T YPICAL S PECIFICATIONS OF THE FAST SPM S U SED IN T HIS L ETTER
N-on-P type
(M-Series)
3 × 3 mm2
Parameters
Active area
Microcell size /
number
Pixel fill factor
Peak wavelength
(λ p )
PDE at (Vop , λ p )
Gain (Vop , 20o C)a
Dark count rate
(Vop , 20o C)
35 µm / 4774
P-on-N type
(B-Proto)
3 × 3 mm2
35 µm / 4774
64%
64%
500 nm
420 nm
20%
5.1 × 104
37%
∼105
6 MHz
6 MHz
a Readout from Fast Terminal
as B-Proto) with a peak PDE at shorter wavelengths [7].
Fig. 2 is a photograph of the two devices while typical
specifications are summarized in Table I.
The energy resolution and timing resolution (coincidence
resolving time, CRT) of scintillators placed “head-on” has
been assessed with the setup shown in Fig. 3. Each SPM was
read out with an radiofrequency (RF) power amplifier (Minicircuits RAMP-33LN+) and digitized with a fast oscilloscope
(Agilent DSO90254A) to acquire raw signal waveforms for
offline processing.
For this letter, polished cerium doped lutetium (yttrium)
oxyorthosilicate scintillator crystal [L(Y)SO] — most widely
used crystal in commercial PET scanners for, among other
factors, their high light yield, fast response time and high
stopping power [8] were coupled to the Fast SPMs with optical
grease (Bicron BC-630). The specifications of crystals tested
are given in Table II. The long crystal (3 × 3 × 20 mm3 )
is a common size used in PET for effectively stopping the
511 keV gamma photons, while the shorter crystal (3 × 3 ×
5 mm3 ) provides insight into the intrinsic timing resolution of
a detector pair by minimizing light propagation variance within
the crystal. As Fast SPMs are sealed with a protective epoxy
layer (200 - 300 µm), crystals with smaller cross-sectional area
(2 × 2 × 3 mm3 ) to maximize the light collection efficiency
have also been evaluated. A reflector that enables better timing
performance for each of the respective crystal geometries has
been selected [9] and all measurements were carried out at
room temperature.
III. R ESULTS
A. Pulse Response
The response of Fast SPMs has been assessed with a fast
pulse laser (50 ps, 650 nm). For this particular study, the
Fig. 3. Setup for timing measurements. The detector pair detects gamma
photons that are read out with the amplifiers (A). The outputs are sent to
the oscilloscope where only coincidence events are digitized. For the energy
measurements, data are acquired in singles mode.
TABLE II
D IMENSIONS AND R EFLECTORS OF S CINTILLATION C RYSTALS T ESTED
Crystal
LYSO: Ce
LYSO: Ce
LSO: Ce
Size
(mm3 )
3×3×5
3 × 3 × 20
2×2×3
Reflector
Teflon
3M ESR+
Teflon top
Teflon
Fig. 4. Typical laser pulse response acquired from a N-on-P type Fast SPM.
The gain (area under curve) is smaller but the response is significantly faster
than that of standard SPM as shown in the rise/fall times (10% - 90%).
output from the Fast Terminal was connected directly to the
oscilloscope (50 ohm impedance) without a preamplifier. The
10% - 90% rise and fall times (recovery time) of both types
were about 1 ns. Fig. 4 shows a digitized output from the
Fast terminal of the N-on-P device (M-Series) and that from
a standard SPM.
B. Energy Resolution
The energy resolution was calculated by integrating digitized signals of a 3 × 3 × 5 mm3 LYSO crystals coupled
to the Fast SPMs and irradiated with a Na-22 source. As the
signal output from silicon photomultipliers can saturate due
to the limited number of microcells, the energy spectrum has
been corrected for this saturation and Fig. 5 shows the Na-22
energy spectrum with a P-on-N type Fast SPM (B-proto).
The overvoltage (voltage over breakdown voltage) was
varied and the corrected energy resolution of the 511 keV
peak is plotted in Fig. 6. The energy resolution improves
YEOM et al.: FAST TIMING SPMs FOR SCINTILLATION DETECTORS
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TABLE III
O PTIMAL CRT S O BTAINED AND G AUSSIAN F ITTING E RROR (2σ )
Crystal Size
(mm3 )
CRT(ps
FWHM)
[Overvoltage]
Fig. 5. Linearity corrected energy spectrum of the Na-22 radiation source
at 4 V overvoltage (voltage over breakdown voltage). Also embedded in the
figure are the pictures of the Fast SPM and scintillator (3 × 3 × 5 mm3
LYSO) used.
Fig. 6. Energy resolution of the Fast SPMs. The background from the Lu-176
radioactivity and from Compton scattered gamma photons (fig. 5) was taken
into account and subtracted during the Gaussian fit of the 511 keV photopeaks
for energy resolution calculation. Error bars indicate 95% confidence intervals.
with increasing bias voltage due to the increasing PDE but
degrades after an optimal point (4 V overvoltage) as the noise
contribution becomes more dominant. The energy resolution
at the optimal voltage were 13.7% and 13.1% for the N-on-P
and P-on-N device, respectively.
C. Coincidence Resolving Time (CRT)
The timing performance measurement was performed with
a Ge-68 source and coincidence events with energies greater
than ∼ 450 keV were digitized. A time pickoff method as
described in [10] to minimize the noise floor of the oscilloscope and correct for the baseline shift induced by the
dark counts was performed on coincident signal waveforms,
after which the triggering levels were swept to acquire the
optimal trigger points and the resultant timing spectrum was
fitted with a Gaussian curve to compute the CRT. For each
detector/scintillator pair, measurements were taken at various
overvoltages and the best CRT values are presented. The
lowest CRTs have been attained with the 2 × 2 × 3 mm3
LSO:Ce crystals and were 184 ± 5 ps and 157 ± 3 ps for the
N-on-P and P-on-N devices, respectively. For longer crystals
(3 × 3 × 20 mm3 ), more relevant for positron emission
tomography (PET) scanners, the CRTs were 298 ± 9 ps and
234 ± 6 ps for the two SPM types respectively. The computed
CRT values are summarized in Table III.
2×2×3
3×3×5
3 × 3 × 20
N-on-P
(M-series)
184 ± 5
[4 V]
247 ± 10
[4 V]
298 ± 9
[4 V]
P-on-N
(B-Proto)
157 ± 3
[4.5 V]
183 ± 4
[5 V]
234 ± 6
[5 V]
IV. C ONCLUSION
These newly developed Fast SPMs from SensL have shown
significant improvement (> 50 ps) in timing resolution over
standard SPM [5], making them attractive for fast timing
applications, such as ToF PET. Especially, the prototype P-onN (B-proto) Fast SPM, with a higher PDE in the blue visible
light spectrum, matches well with the emission wavelength of
L(Y)SO scintillator crystals, leading to the better energy and
timing performances. These results will likely improve slightly
with better packaging due to lower pin inductance and thinner
epoxy seal for improved light collection. However, while the
P-on-N Fast SPM is preferred for ToF PET detectors with
L(Y)SO, the selection of the SPM type in general will depend
on the wavelength of incoming photons.
The reason for the improvement in timing performance of
these devices over standard SPMs can explained as follows.
The timing variance (σt ) of a signal pulse is associated with
its noise voltage (σv ) and slope at the trigger point (dv/dt), as
expressed in the equation below [11].
σv
σt = !
(1)
dv
dt
Therefore, for a given scintillator and photosensor, high bandwidth and low-noise electronics are required to maximize
the timing performance. However, as silicon photomultipliers
inherently suffer from a high capacitance(∼ 500 pC for a
3 × 3 mm2 standard SPM) that shapes the output pulse
depending on the impedance of the readout electronics, custom
preamplifiers with low input impedance (< 20 ohm) are
often designed using discrete components or CMOS ASIC
to mitigate this effect. This, however, inevitably compromises
noise performance. With these new Fast SPMs, the low output
capacitance allows the use of extremely low-noise, high-speed
commercial amplifiers like monolithic microwave integrated
circuit (MMIC) amplifiers widely used in wireless/cellular
applications despite having relatively high 50 or 75 Ohm
characteristic impedance to facilitate signal transmission. With
these amplifiers, which can be easily implemented in most
cases, both variables (noise and slope) in equation (1) are
optimized.
In addition to the improvement in timing performance, there
are several advantages associated with the new Fast SPMs:
1) The fast rise/decay times significantly decrease the duration of dark counts, thereby reducing dark count-induced
signal pileup. 2) Due to the lower output capacitance (capacitive loading effect), these Fast SPMs would be more stable when read out with high-speed electronics. 3) Standard
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 14, JULY 15, 2013
CMOS-compatible manufacturing processes, which avoid proprietary process steps, enable cost-competitive fabrication at
standard silicon foundries.
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