SILICON PHOTOMULTIPLIERS IN PARTICLE PHYSICS:
POSSIBILITIES AND LIMITATIONS
B. DOLGOSHEIN
Moscow Engineering and Physics Institute, Kashirskoe shosse 31
Moscow, 115409, Russia
ON BEHALF OF SIPM COLLABORATION*
The current status of the Silicon Photomultipliers (SiPM’s) – the limited Geiger mode
devices with gain of 106 and single photon sensitivity  is described. Using such a SiPM
features as low noise, excellent photon counting capability and very good timing
resolution the applications of SiPM’s in particle physics are discussed. The advantages
and limitations of this relatively new technique are experimentally studied and future
perspectives of the development are considered.
1.
Silicon Photomultiplier (SiPM) description and performance
1.1. Background
The Silicon Photomultiplier (SiPM) is relatively young and progressively
developing photodetection technique [1,2,3 and also 4], which allows to obtain
the intrinsic gain for single photoelectron at the level of 10 6, the value close to
that of vacuum Photomultipliers. Such a large gain, which confirms the name
“Photomultiplier”, became achievable due to the fact that the SiPM operates in
limited Geiger mode in contrast with well known Avalanche Photodiodes
(APD’s), operating in proportional mode. Proportional mode of the APD
operation can achieve a typical gain at the level of 102 only (in exceptional cases
104 [5] ) due to relatively large value of the holes ionization coefficient in silicon
– in contrast for instance with gas proportional counters, where the positive ions
ionization coefficient in gas is negligibly small and much larger gas gain (up to
106 - 107 ) is possible.
The limited Geiger mode operation of the silicon photodetectors is well
known for at least 30 years (see for instance excellent review papers [6,7]). The
Single Photon Avalanche Diode (SPAD) described in [6,7] is just a single small
(20-200 m size) pixel, operating with bias voltage of 10-15% higher than the
*
SiPM Collaboration: P. Buzhan, B. Dolgoshein, A. Ilyin, V. Kantserov, V.
Kaplin, A. Karakash, P. Komissarov, E. Popova, V. Tikhomirov (MEPhI,
Moscow)
L. Filatov, S. Klemin (PULSAR, Moscow)
breakdown voltage. A Geiger discharge, initiated by photoelectron is limited
either passively – by quenching resistor or actively – by special quenching
electronics. The gain of 108-109 for single photoelectron has been achievable for
the SPAD; however this device is just very small size single photon counter that
is one bit binary (yes/no) device, not capable to measure the light intensity.
1.2. SiPM: main features and characteristics
The Silicon Photomultiplier (SiPM) is multipixel silicon SPAD type photodiode
with many micro pixels joined together on common silicon substrate and
working on common load (see Fig. 1.). The typical size of each pixel is 20-30
m, the typical pixel number is about 103 mm-2. The pixels should be decoupled
from each other in order to minimize the interpixel crosstalk and work as
independent photon microcounters.
Figure 1. Schematic view of multipixel Silicon Photomultiplier.
This decoupling is realized by:
Polysilicon quenching resistors for each pixel, which limit Geiger discharge
and at the same time decouple pixel from pixel electrically.
 Specially designed boundaries between pixels in order to prevent the
electrical coupling due to interpixel currents in silicon itself. Unfortunately
these boundaries occupied some SiPM’s area, so the total sensitive pixels
area covers geometrically only part of total one (geometrical package).
There is another type of interpixel coupling – “optical crosstalk”. Its origin is
related to the photons created in Geiger discharge with intensity of 10 -5 photons

per one electron [8]. These photons, generated in fired pixel, can initiate the
discharge in adjacent pixel(s).
Operational SiPM bias voltage is 10-15% higher than the breakdown
voltage, so each SiPM pixel operates in Geiger mode limited by individual
polysilicon resistor. Provided that all pixels operate as independent
microcounters one pixel Geiger signal is determined by the charge accumulated
in pixel capacitance Cpixel: Qpixel= CpixelV= Cpixel(VbiasVbreackdown) , where
overvoltage V value is about of few volts, Cpixel is typically of 50 fmF, so Qpixel
is about of 300 fmC or 2106 electrons.
The SiPM signal doesn’t depend on the number of primary carriers (because
of Geiger mode), therefore each pixel detects the carriers created by photon or
ionization particle or thermally with the same response signal (~10 6 electrons),
that is the nuclear counter effect for SiPM is negligibly small.
Due to the large number of independent photon microcounters the SiPM
acquires a new capability compared to the SPAD ─ the measure of light
intensity, which is proportional to the number of pixels fired by photons.
Actually SiPM is semidigital and semianalogue at the same time: each pixel
operates as a binary device, but the SiPM on the whole is an analogue detector,
which can measure the light intensity within the dynamic range, determined by a
finite number of pixels (~103 mm-2).
As the SiPM depletion region is small (~2 m) and operating electric field is
very high ((23)105V/cm with carrier drift velocity of ~107 cm/s), the Geiger
discharge is extremely short and SiPM signal is intrinsically very fast (see Fig.
1).
1.3. SiPM photon counting capability
The SiPM pulse height spectrum from low intensity light flash produced by light
emission diode (LED) source is shown in Fig. 2 (room temperature) [3] in
comparison with the same spectra from Hybrid Photodiode HPD (room
temperature) and Visible Light Photon Counter VLPC (temperature 6.5K). It is
possible to distinguish each photoelectron contribution even for more intensive
light pulses with mean photoelectron (or pixel fired) number of 46 (Fig. 2d).
An excellent SiPM single photoelectron (pixel) resolution (Fig. 2) is
determined by several reasons
 A good pixel to pixel uniformity (gain variation).
 Negligible contribution of electronics noise (it can be estimated from
pedestal width as 0.1 electron) due to high SiPM gain.
 Very low contribution of Excess Noise Factor (ENF), connected with
Geiger discharge development (it can be estimated as 1.05), in contrast
with large ENF for Avalanche Photodiode case (a typical value is 23) due
to fluctuation of avalanche development process [11].
a)
b)
HPD
c)
d)
Figure 2. SiPM (a) photon counting capability compared to VLPC (b) [9] and HPD (c) [10]. SiPM
pulse height spectrum (d) for more intensive light burst with mean photoelectron number of 46.
1.4. SiPM gain and photon detection efficiency
One can see in Fig. 3 the comparison of photon detection efficiencies [3] for
vacuum and silicon devices. For PMT, APD and HPD the photon detection
efficiency is actually a quantum efficiency QE, whereas for SiPM it is less than
that due to geometrical packing factor geom=sensitive area/total area; the shape
of photon detection efficiency curves for SiPM and APD (or another silicon
photodetector) is rather similar (Fig. 3). The SiPM photon detection efficiency is
for the time being at the level of PMT QE for blue light and larger for yellow-red
region, that is important for some applications (for instance for the usage of
wave length shifter (WLS) fibers ─ see below, Sec. 2.1.1 and 2.1.2).
The photon detection efficiency depends on overvoltage V [3] (see Fig. 4),
because the probability of Geiger mode discharge is overvoltage dependent. At
the same time the single pixel gain increases linearly with overvoltage (as
mentioned above) proportionally of accumulated charge CpixelV.
Wavelength, nm
Figure 3. Comparison of photon detection
efficiency for PMT [12], APD [11] and
SiPM [3].
Figure 4. SiPM gain and photon detection
efficiency vs overvoltage.
The sensitivity of SiPM gain and efficiency on temperature and bias voltage
is quite important from practical point of view. The variation of temperature or
bias voltage leads to variation of single pixel gain and SiPM efficiency. For
instance, the gain variation of 3% (for the SiPM gain of 2106) may be
determined by temperature change of about 7 or bias voltage 0.15V (or 310-3
Vbias) [3]. This variation is quite comfortable practically in contrast with much
larger sensitivity of APD’s [11].
1.5. The timing by SiPM
The SiPM is intrinsically very fast due to very small width of depletion layer and
extremely short time of Geiger type discharge development (less than 500 ps).
Moreover after Geiger discharge each pixel is recovered with a typical time of
order CpixelRpixel30 ns; in some cases the single pixel recovery time of 10-15 ns
can be achieved (see below, Sec. 2.1.1).
The timing properties of the SiPM can be seen in Fig. 5 [3] where the single
photoelectron (single pixel) timing resolution is shown. Timing by SiPM has
been measured using very low intensity fast laser pulse (40 ps FWHM) [3]. The
SiPM single photoelectron timing resolution including the laser pulse and
electronics contribution is 123 ps FWHM, which corresponds the intrinsic SiPM
single photoelectron timing resolution of FWHM0 100 ps for photons absorbed
in depletion region. In addition, the dependence of timing resolution on
Nphotoelectrons as FWHM=FWHM0/(Nphotoelectrons)1/2 has been shown to be valid up
to Nphotoelectrons of 100.
300
Number of events
250
MIP
efficiency
200
95%
90%
25 pixels fired
a)
150
100
50
0
50
100 150 200 250 300 350 400 450 500 550 600
100
Dark rate, Hz
b)
10
1
expected cosmic
ray level
0,1
50
100 150 200 250 300 350 400 450 500 550 600
Threshold, channels
Figure 5. Single photoelectron timing
resolution for SiPM.
Figure 6. Pulse height distributions detected
by scintillator+WLS fiber+SiPM for MIP
signal from -source Sr90 (a), SiPM dark
rate (b).
It should be noted that such a single photoelectron timing resolution can be
achieved using just a leading edge discriminator because the single
photoelectron (single pixel) pulse is very stable (see Fig. 2).
1.6. Summary of SiPM features
Let’s count the arguments in favor of SiPM:
 High gain (106)
 Low electronics noise, low excess noise factor
 Excellent photon counting capability
 Very low charge particle sensitivity (negligible nuclear counting effect)
 Very good timing (100 ps)
 Small recovery time
 Good temperature and voltage stability
 Insensitivity to magnetic field
 Low bias voltage (50 V)
 Low power consumption (50 W/mm2)
 Compactness
 Room temperature operation
 Simplest electronics
 Relatively low cost (low resistivity Si, relatively simple technology)
There are of course some SiPM drawbacks and limitations. They will be
considered below in Sec. 2 together with discussion of possible particular
applications.
2.
SiPM: fields for Applications in Particle Physics
The main field for SiPM applications in Particle Physics is Low Light Level
(LLL) detection. We consider in more detail three main directions.
2.1. LLL scintillation readout
2.1.1. Scintillator+WLS fiber+SiPM readout application for Calorimetry
The plastic scintillator with wavelength shifter (WLS) readout is very attractive
in case of the necessity to read out a very large number of scintillators with a
small room available and also the need to perform in high magnetic field. One
out of examples of such technique is TESLA Hadron Tile Calorimeter [13]. This
very high granularity Calorimeter has to operate in magnetic field of 4 Tesla and
consists of more than million of plastic scintillator tiles with a size of a few cm,
located in between of 2 cm iron plates and read out by WLS fibers. The R&D of
the TESLA Tile HCAL has been carried out in framework of CALICE
Collaboration [14]. Using such an advantage of the SiPM as compactness, the
SiPM is embedded just in the body of scintillator tile with a size of 50505
mm3 [15]. In Fig. 6a one can see the pulse height spectrum of -electrons from
Sr90 source for such a tile+sci fiber+SiPM.
The system of such a tiles (99 pieces) interleaved by 2 cm of iron has been
build by CALICE Collaboration. Those 99 tiles were grouped in 11 layers with
33 tiles matrix and this detector (called MINICAL) has been tested in DESY
electron test beam with energy from 1 to 6 GeV [15].
Electromagnetic shower longitudinal profile (4 GeV electron in MINICAL)
can be seen in Fig. 7; note the MIP peak from noninteractined electron at
Figure 7. MINICAL based on SiPM: 4 GeV electron shower longitudinal profile.
MINICAL level 1, which is clearly seen due to a good SiPM pulse height
resolution. Another advantage of SiPM – the gain of 106 – allowed us do not
make use of any preamplifiers in this test beam measurements: just 30 m of cable
between SiPM and ADC LeCroy2249A. Fig. 8 shows the linearity of MINICAL
energy response; shower signal has been obtained as a sum of individual tile
signals normalized on MIP signal for each tile and corrected by SiPM nonlinear
response (see below). The MINICAL energy resolution is also shown in Fig. 8 in
comparison with GEANT based MC simulations taking into account only
physical sampling fluctuations of the shower energy. One can see from Fig. 8
that MINICAL based on SiPM does not deteriorate somehow the EM shower
energy resolution.
There are some limitations of the usage of Sci+WLS fiber+SiPM read out
for tile calorimeters:
First, the limited number of pixels leads to nonlinearity of SiPM signal for
number of photoelectrons/tile greater than number of pixels. Fig. 9 shows such a
nonlinear response of SiPM for short laser light pulse of 40 ps. However when
light pulse is produced by real MINICAL Sci+WLS system, (~15 ns FWHM, see
Fig. 10), the SiPM response saturates much more slowly: at the level of ~2000
pixels for SiPM with 1024 pixels (compare Fig. 9 with Fig. 10); this means that
effective recovery time for each pixel is very small (~10 ns) and each pixel is
fired approximately twice in average during Sci+WLS signal.
200
20
150
15
100
10
b)
50
5
0
0
0
1
2
3
4
5
6
Energy, GeV
Figure 8. The linearity response (a) and
energy resolution (b) of MINICAL/SiPM
detector.
Energy resolution, %
25
a)
Exp data
MC data
Number of pixels fired
Energy Deposition, MIP's
250
1000
100
576
1024
4096
10
1
1
10
100
1000
10000
Number of photoelectrons
Figure 9. Nonlinear response of SiPM’s
with different number of pixels. Light signal
is produced by fast laser (40 ps).
Figure 10. Nonlinear response of SiPM with 1024 pixels for light signal (15 ns FWHM)
corresponding to real MINICAL scintillator+WLS system response (insertion).
Е0=47,6
6
10
5
Е=53В (kpixel=19,7*10 )
5
5
Number of photoelectrons
Е=50,5В (kpixel9,5*10 )
Dark rate, Hz
10
4
10
3
10
2
no crosstalks
10
1
10
0
1
2
3
4 5 6 7
Threshould, ph.e.
8
9
Threshold, phe
Figure 11. SiPM (1024 pixels, room
temperature) dark rate for different SiPM
gains.
10 11
Coordinate along Sci bar, cm
Figure 12. SiPM pulse height in number of
photoelectrons vs MIP particle coordinate
along the scintillation bar (2.5 cm  1 cm 
2 m). Lower curves correspond the
individual SiPM from both ends of the bar,
upper one is a sum of both SiPM’s.
Notwithstanding this nonlinearity of SiPM response, there is no impact on
energy resolution (see Fig. 8) for EM shower with energy up to 6 GeV (~1200
photoelectrons/tile in shower maximum, SiPM with 1024 pixels).
Another limitation factor in Sci+WLS fiber+SiPM read out for TESLA Tile
HCAL is SiPM dark rate at threshold of 90-95% of MIP detection. This is
important because the TESLA HCAL is assumed to be calibrated using cosmic
muons without any triggering, so SiPM dark rate has to be of order of cosmic
muons rate (see Fig. 6b). The SiPM dark rate as function of threshold is
determined by optical crosstalk between SiPM pixels (see above, Sec. 1.2),
which in turn depends on SiPM gain (see Fig. 11). The requirement to have a
low dark rate limits the SiPM gain (at the level of 10 6) and efficiency at the level
10-12% for room temperature in case of TESLA Tile HCAL application.
2.1.2 Scintillator+WLS fiber+SiPM readout: application for scintillator
strip tracking
The SiPM’s can be used also for another type of scintillation systems  long (2-4
m) strips with transverse size (1-2) cm – for muon tracking in collider
experiments or for large neutrino experiments. Here SiPM’s can compete
successfully with multianode PM’s thanks their relatively low cost.
Fig. 12 shows the test results [16] obtained with such a scintillation strip
(length of 2 m, with a size of 12.5 cm), both ends of which were equipped by
two SiPM’s (1024 pixels, 11 mm size). Scintillator bar “Vladimir” type was
wrapped by 3M reflecting paper; the WLS fiber (Kuraray Y-11, 1 mm in
diameter) was positioned in the rectangular groove along the bar side of 2.5 cm
 2 m.
Fig. 12 shows the average values of scintillation signals from cosmic muons,
which transverse of 1 cm scintillator bar. The SiPM pulse height from each end
of the bar depends on muon coordinate along the bar, whereas the sum of both
SiPM signals is rather stable and equal of about 18-20 photoelectrons. Such a
signal looks quite promising for applications just today.
2.1.3. SiPM application for scintillator fiber readout
The SiPM looks also attractive for readout in scintillator fiber trackers due to
compactness, low noise and relatively low cost. Fig. 13 demonstrates the MIP
particle (electrons with energy  1.5 MeV from Sr90 -source) detection by
means SiPM (576 pixels, 11 mm2 size) which read out the signal from
multicladding Kuraray scintillator fiber SCSF-3HF (1500)M with core diameter
0.94 mm, emission peak at 530 nm, decay time of 7 ns and attenuation length >
4.5 m [3].
SiPM signal-to-noise ratio at room temperature looks almost as good as for
VLPC (T=6.5K) [17] and much better than for APD (room temperature) [18]
because of very low electronic noise and negligible contribution of surface
leakage current.
The possible SiPM limitation for fiber tracking readout is relatively low
photon detection efficiency (15% for measurements in Fig. 13), which can be not
enough for thinner scintillator fibers.
2.2. SiPM for a single photon counting: possible application for EUSO
(Extreme Universe Space Observatory) Experiment [19]
EUSO Experiment is a good example, where the SiPM usage could be very
effective. EUSO is a large optical telescope, located on the International Space
Station at the 400 km distance from Earth Surface. It looks backwards at Earth
atmosphere which is used as a target with an air mass of 21012 tons for a study
of Extremely High Atmosphere Showers with energy of more than 1019-1020 eV
(more than Greisen-Zatsepin-Kuzmin  GZP limit) and High Energy Neutrino
Astronomy.
Figure 13. SiPM application for scintillator fiber MIP detection (at room temperature), see text.
EUSO telescope consists of the system of Fresnel lenses (2.5-3 m in
diameter) with focal surface, containing of (2-3)105 photodetectors with a size
of 4-5 mm. Actually EUSO telescope is the calorimeter-type detector, which
measures the energy of Atmospheric Showers by detection of the fluorescent
light of atmospheric nitrogen. Such a telescope has to detect not only shower
energy (by measuring of number of photons – typically 100/shower) but also
the location and arrival time of each photon, that is a 3D position of the shower.
In other words EUSO is gigantic TPC type calorimeter; its performance depends
crucially on photodetector properties (efficiency, dark noise and timing). The
main requirements to photodetector are:
 good single photon counting capability
 sensitivity in 300-400 nm range with photon detection efficiency 30-40% or
more
 timing at the level of 10 ns (corresponds to a few meters of spacing
accuracy using light propagation time – for typical transfer size of the
shower of hundreds meters)
 low single photoelectron dark rate (less than night sky rate  1MHz /
photodetector)
 compactness, low weight, low power consumption
The SiPM’s could be a good candidate for EUSO telescope provided that
some limitation existing nowadays will be surmounted, namely:
1. Photon detection efficiency for 300  400 nm region is too low (a few %,
see Fig. 3) – needs to be improved by increase of pixel packing efficiency
and the change of SiPM topology.
2. SiPM’s with size of 11 mm2 is only carefully studied for the time being.
Larger size SiPM’s fabrication shows worse yield presumably due to
unperfectness of the Si substrate – needs to use the better quality Si wafers.
3. Single photoelectron (single-pixel) dark rate for SiPM size of 11 mm2 is
too high ((23) MHz) for EUSO application and should to be decreased by
factor of 100. There are two components of dark rate ─ thermal one, which
decreases very rapidly with temperature, and electric field assisted
component [20], which contributes significantly because of high electric
field of ~3·105V/cm. Therefore the dark rate not more then night sky rate is
needed for EUSO experiment and the SiPM’s with 4x4 (5x5) mm2 will
require apparently a working temperature about -30ºC.
2.3. SiPM application for fast single photon timing
Let us consider the possible SiPM application for a new generation of
Cherenkov DIRC (Detector of Internal Reflected Cherenkov light) detectors.
Such a detectors are considered for future High Luminosity B-factories: DIRC
upgrade for BaBar [21] and Time of Propagation (TOP) detector for BELLE
experiment [22]. The timing better than 100 ps is required for such a devices.
For BaBar DIRC such a timing is needed for rejection of background hits
and to reduce the chromatic aberrations, which limit the Cherenkov angle
measurement accuracy and DIRC Particle Identification capability.
For TOP counter the Cherenkov angle is extracted directly from space-time
image of the light cone and timing of about 50 ps is required together with
position sensitivity of a few mm.
The SiPM has met this timing requirements needed (Fig. 5) although there is
some unsolved problems, which are look like limitations, which we had for
EUSO experiment, namely:
 SiPM dark rate (<300 kHz/few mm2 size is needed)
 Photon detection efficiency (better than 30% in wide range of wavelengths
is desirable)
3.
SiPM’s : perspectives of the developments
The Si Photomultiplier is a rapidly developing technique, which has not reach its
best parameters for the time being. Nevertheless, already now the SiPM has a
good chance to be used for next generation of the experiments in Particle
Physics, especially for next generation of High Luminosity Colliders (fast
calorimetry and scintillation tracking, subnanosecond timing etc.). However
some important SiPM parameters have to be improved, such as:
 Photon detection efficiency ─ up to 30-50% for wide wavelength region.
 Dark rate has to be reduced at least one order of magnitude.
 Optical crosstalk has to be reduced at least by one order of magnitude.
 Size of the SiPM’s has to be increased up to a few mm.
The SiPM Collaboration (MEPhI+PULSAR(SiPM producer)) is planning to
come as near as possible to SiPM parameters needed for most important
applications in particle physics (and other fields) in one-two years. The main
directions of the developments are:
 Improvement of SiPM technology process (better purity Si substrate, better
gettering process, production yield increase etc.).
 Improvement of SiPM structure (pixel topology, geometrical packing
efficiency, optical isolation between pixels, etc.)
Acknowledgments
All results obtained by SiPMs Collaboration during a few years have became
possible thanks to the strong support of DESY directorate (especially of Prof.
R.Klanner) and also due to ISTC grant No 1275-99, Alexander von Humboldt
Foundation Research Award (IV, RUS 1066839 GSA) and INTAS grant No
YSF150-00.
We thank also our colleagues from ITEP (Moscow) and from DESY for
fruitful collaboration during the MINICAL test.
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