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\begin{document}

\begin{frontmatter}

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\title{Comparison of a silicon photomultiplier to 
a traditional vacuum photomultiplier\thanksref{gluex}}

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\author[Physics]{V.D.~Kovaltchouk},
\author[Physics]{G.J.~Lolos\corauthref{cor1}},
\author[Physics]{Z.~Papandreou},
\author[Science]{K.~Wolbaum}

\address[Physics]{Department of Physics, University of Regina, Regina, SK, S4S 0A2, Canada}
\address[Science]{Faculty of Science, University of Regina, Regina, SK, S4S 0A2, Canada}

\thanks[gluex]{This paper reports on research carried out on behalf of the 
GlueX/Hall~D collaboration at Jefferson Lab in Newport News, VA USA}
\corauth[cor1]{Corresponding author's e-mail: George.Lolos@uregina.ca}

\begin{abstract}
  Silicon photomultiplier devices (SiPM) were investigated as a
  possible front-end detector system for the electromagnetic barrel
  calorimeter of the GlueX Project at Jefferson Laboratory, USA, and
  compared against a traditional two inch vacuum photomultiplier tube.
  The SiPM has gain and timing resolution comparable to that of a PMT,
  requires a simple electronic circuit and is not sensitive to
  magnetic fields. These attributes allow us to conclude that it is
  feasible to use the SiPM as a front-end detector for this
  calorimeter.
\end{abstract}

\begin{keyword}
% keywords here, in the form: keyword \sep keyword
silicon photomultiplier \sep scintillating fiber \sep barrel calorimeter
% PACS codes here, in the form: \PACS code \sep code
\PACS 29.40.Gx \sep 29.40.Mc \sep 42.81.Cn \sep 85.60.Dw
\end{keyword}
\end{frontmatter}

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\section{Introduction}

The goal of the GlueX/Hall~D experiment \cite{cdr,dzierba,lolos} at
Jefferson Laboratory in Newport News (Virginia, USA) is to search for
gluonic excitations manifested in exotic hybrid vector mesons with
masses up to 2.5~GeV/c$^2$ using linearly polarized photons of
8$-$10~GeV energy.  The decay products of the produced mesons must be
identified and measured with good resolution and with full acceptance
for all decay angles. As such, the GlueX detector system must have the
capability of measuring the emission angles and energies of neutral
particles and the four-momenta of charged particles with good overall
resolution to allow for a complete kinematic reconstruction of events
and subsequent partial wave analysis~\cite{lolos}.

The electromagnetic barrel calorimeter (BCAL) is a crucial GlueX/Hall~D
detector subsystem. BCAL will be positioned immediately inside the
super-conducting solenoid, which constrains its outer radius to be
90~cm, and will be approximately 4 m long and 25 cm thick.  The
calorimeter will be constructed using at least 4~m long scintillating
fibers (SciFi), 1~mm in diameter, embedded in a lead matrix of
successive layers of lead (0.5~mm thick) and SciFi's.  The readout
system has to collect the scintillation light -- with peak emission in
the range 350$-$500~nm -- from about 3.2~m$^2$ of active surface of
the barrel calorimeter. As a result, the front-end detector and the
readout system must achieve an energy resolution as good or better
than $\frac{5\%}{\sqrt{E(GeV)}}$~\cite{kloe}, and adequate timing
resolution. For example, the requirement of 10 cm spatial resolution
translates into a timing resolution of $\sigma$=150~ps for a Gaussian
fit. The anticipated count rate of events is about $10^6$~events/s
within the magnetic field created by the superconducting solenoid with
a central value greater than 2~Tesla.

Traditional vacuum photomultiplier tubes (PMT), hybrid photo-diodes
(HPD) and silicon photomultipliers (SiPM) have been
investigated~\cite{kvd} as possible front-end detectors, whereas the
HPD and the PMT have proven unsuitable, the most promising results
were obtained with the SiPM.

\section{Silicon Photomultiplier}

Although single-pixel Geiger mode devices (Avalanche Photo Diodes -
APDs) were developed in the mid sixties, the SiPM is a novel type of
APD~\cite{dolgoshein}.  It is a promising device for our application
in GlueX, since it is insensitive to magnetic fields, has a high gain
($\sim 10^6$), good quantum efficiency, provides excellent timing
resolution ($\sim 120$~ns for single photo electron detection) and a
fast risetime (sub-ns), achieves good dynamic range ($\sim
10^3$/mm$^2$), and does not suffer from nuclear counter effects when
operated in Geiger mode.  Finally, it has a solid performance at room
temperature (in contrast to VLPCs) and does not exhibit any serious
radiation damage effects, other than perhaps from
neutrons~\cite{renker}.

The SiPM is a multi-pixel photo diode with a large number of micro
pixels (500-1500 each with a typical size of 20$-$30~$\mu$m) joined
together on a common substrate and under a common load.  The
photodiode has a multi-layer structure with different doping levels.
As a result, within the thin depletion region between $p^+$ and $n^+$
layers, a very high electric field of about 5$\times$$10^5$~V/cm is
created, with the right conditions for a Geiger discharge mode to take
place.  The operational bias voltage is 10$-$20\% higher than
breakdown voltage, with typical supplied bias voltage of 50$-$60~V.
The diode is reverse biased to increase the field in the depletion
region.

The pixels are electrically decoupled from each other and operate in a
Geiger mode, limited by the charge accumulated in the pixel capacitor.
The Geiger discharge ceases when the voltage drops to values below the
breakdown value due to the external resistor on each pixel.  Each
pixel detects and amplifies the charge from a photon, independently of
all other pixels, and its signal does not depend on the impinging
number of quanta that fire the pixel in a Geiger mode.  The total
number of pixels defines the dynamic range of the photodetector.

The gain for a single pixel is determined by the accumulated charge in
the capacitor of the pixel (typically $C_{pixel}=100$~pF) and depends
on an overvoltage: $Q = C_{pixel} \cdot
(V_{bias}-V_{breakdown})$, where $V_{bias}$ and $V_{breakdown}$ are
the supplied bias and breakdown voltages, respectively. Due to its
high gain, the electronics noise of the SiPM is negligibly small and
the main source of the noise is the dark rate.

The single pixel gain is approximately $10^6$, roughly the same order
of magnitude as that of a traditional PMT.  While each pixel operates
digitally as a binary device -- because all SiPM pixels work together
on a common load and there is a large number of pixels -- the output
signal is a sum of the signals from all pixels registering a
``hit''.  Thus, the SiPM, as whole, is an analogue detector that can
measure the incident light intensity.  The distribution of the voltage
across the depletion depth of 4$-$6~$\mu$m is such that for only a
fraction of the depth ($\sim$1$-$2$~\mu$m) the former exceeds a value
sufficient for Geiger discharge creation, and therefore, the Geiger
discharge is contained within this limited region. As a result, its
duration is very short, a few hundreds of ps, resulting in typical
rise times of 1~ns.

The SiPM's photon detection efficiency, $\eta$, is given by $\eta = QE
\cdot \varepsilon$, where $\eta$ is the photon detection efficiency,
QE is the quantum efficiency and $\varepsilon$ is a geometrical
factor.  The latter is a ratio of the sensitive area (as defined by
the total active pixel area) to the total area.  This value does not
depend on wavelength and is a constant for each SiPM.  The devices
used in our particular tests had geometrical factors of 0.3/1.0 for
all SiPM's/PMT's investigated, respectively.  The quantum efficiency
is a function of incident wavelength of light and has a maximum in the
500$-$600~nm region, decreasing monotonically for lower wavelengths.
The efficiency, $\eta$, for the SiPM's is about 20\% for
$\lambda=550$~ nm, a value considerably larger than that of a 2''
Burle 8575 PMT at this wavelength (5\%).  However, for $\lambda$=400
nm, the SiPM has a lower value of $\eta$ than a PMT, and as a result
is more sensitive in the green-yellow region of wavelength than the
PMT, an attribute that has practical implications since scintillators
exist for the UV-blue and green-yellow spectrum.

The SiPM's used in our tests were developed and produced by the Moscow
Engineering and Physics Institute (MEPHI) in cooperation with a
state enterprise (PULSAR).  The specific SiPM's had 1000 pixels in each
detector covering the 1~mm$^2$ sensitive area and the supplied bias
voltage was 50$-$60~V. Although the geometrical factor for these SiPMs
was 0.3, efforts are underway at MEPHI/PULSAR to increase this to as
much as 0.7.  Competitors at the University of Obninsk in cooperation
with a private firm (CPTA)~\cite{valeri} claim that their device has
50\% higher photon detection efficiency in the green region and fewer
constraints on mechanical performance.  Moreover, the Obninsk/CPTA
team is carrying out R\&D towards the manufacturing of $3 \times
3$mm$^2$ SiPM active area wafers and is also investigating the
construction of larger areas by connecting SiPMs in a matrix
configuration~\cite{valeri}.

\section{Tests with a Nitrogen Plasma Discharge Tube}

The purpose of this work was the investigation of SiPM's as front-end
detectors for the barrel calorimeter readout system, as defined by the
requirements of the GlueX/Hall~D Project.  Specifically the SiPM's were
investigated under the following two conditions:

\begin{itemize}
  \item Detection of incident light of high flux intensity, where
  about 200$-$500~SiPM pixels registered a hit, but the signal was not
  saturated.
  \item Detection of light of lower photon flux intensity in which
  case only few pixels registered a hit.  This regime corresponds to
  a few-photon-counting condition.
\end{itemize}

\subsection{High Photon Flux Tests}

An Optitron nanosecond broad spectrum optical pulse radiator (Model
NR-1A) with a Nitrogen Plasma Discharge Tube\footnote{Optitron
Inc. 23206 Normandie Ave. \#8, Torrance, CA 90502, USA.} was used as a
source of light for the SiPM investigation under high photon flux
conditions.  The light pulses had a 1~ns rise time and a few ns pulse
duration, and were measured with the SiPM as well as with a 2''~Burle
PMT (model 8575).  A clear, pure fused silica fiber of 5~m length was
used to transport the light from the Optitron plasma discharge tube to
the sensitive surface of the SiPM.  The light intensity emitted from
this tube could be varied within a broad range of amplitudes and was
monitored by the PMT.

Pulses from the SiPM and the PMT were measured with a Tektronix
TDS-5104 digital oscilloscope, as shown in Fig.~\ref{fig:pulses}.
The supplied high voltage to the PMT was 1900~V while the bias for the
SiPM was 55.5~V.  The signals from the SiPM and PMT were each split in
two, with one branch fed to an ADC (LeCroy 2249A) and the other to a
Constant Fraction Discriminator (CFD Tennelec TC455) whose output was
then fed to a TDC (LeCroy 2228A).  In this measurement, the trigger
signal from the plasma discharge unit acted as the ``start'' signal
for the TDC unit.

\begin{figure}[ht]
\begin{center}
\includegraphics*[width=2.7in]{Fig1}
\caption{\label{fig:pulses} The shape of pulses from SiPM (left) 
     and PMT (right). A light flasher (Optitron Inc.)  was used as a
     light source.  The horizontal and vertical scales
     are 20~ns/div and 100~mV/div, respectively.}
\end{center}
\end{figure}

The detected signal amplitude for the SiPM was $\sim$300~mV,
corresponding to $\sim$200$-$300~pixels registering a hit.  This
performance rendered the use of an external amplifier unnecessary.
Moreover, the signal from the SiPM exhibited no saturation as was
concluded from the linear dependence of its ADC amplitude versus light
intensity.  Whereas the timing distributions have a similar structure
with risetimes of 1~ns and 4~ns for the SiPM and PMT, respectively,
the former has a $\sigma$ that is less than half of the latter's:
$\sigma = 140$~ps vs. 375~ps.

\subsection{Low Photon Flux Tests}

In order to investigate the energy resolution of the SiPM, we measured
the pulse amplitude distribution under low photon flux conditions
employing the Optitron unit and a neutral-density attenuation filter
that reduced the light to 1\% of its initial value.

The amplitude of the signals in the SiPM, in this case, was
~5$-$10~mV, and this necessitated the use of a fast amplifier (LeCroy
612A).  Under such conditions it was not possible to eliminate
completely the noise pick-up from electronic equipment present in the
area, especially the noise originating from the plasma discharge unit.
While RF (pick-up) noise was present in all measurements that utilized
the plasma discharge unit, this noise was negligible under high photon
flux conditions.  However, when only a few pixels generate output,
even low amplitude noise contributes to the overall resolution.
Nevertheless, the pulse amplitude distribution shown in
Fig.~\ref{fig:lowlight} exhibits five well separated peaks
corresponding to single photon detection and good separation for
emission of up to five photoelectrons.  The 2'' Burle 8575 PMT cannot
resolve individual photo-electron peaks.

\begin{figure}[ht]
\begin{center}
\includegraphics[width=3.5in]{Fig2}
\caption{\label{fig:lowlight} SiPM pulse height spectrum for low intensity 
  light pulses.}
\end{center}
\end{figure}

\section{SiPM as a front-end detector for a 4-m scintillating fiber}

Next, we evaluated the performance of the SiPM used as a front-end
detector for light signals produced by minimum ionization particles
traversing a 4-m-long Kuraray SCSF-81 single-clad SciFi.  Fibers with
similar parameters will be used in the barrel calorimeter for
GlueX/Hall~D detector system.  The schematic diagram of the
experimental setup is shown in Fig.~\ref{fig:electronics}.

\begin{figure}[ht]
\begin{center}
\includegraphics[width=4.3in]{Fig3}
\caption{\label{fig:electronics} The electronics diagram of the 
  experimental setup using a 4-m scintillating fiber coupled to a SiPM
  and PMT.}
\end{center}
\end{figure}

Kuraray SCSF-81 SciFi's have an emission spectrum range of
400$-$550~nm, peaking at 437~nm, with a 2.4~ns scintillation decay
time and an attenuation length of $\sim$3.5~m.  The fiber had a 1~mm
outer diameter and the cladding thickness was 3\% of the diameter.
One end of the fiber was connected to the SiPM and the other to the
PMT.  Both ends of the SciFi were polished to increase the light
transmission and a special arrangement was used to preserve the
optical connection between the fiber and the sensitive surfaces of the
PMT and the SiPM.  The fiber was in direct contact with the surface of
the PMT window, while it had a 0.3$-$0.5~mm air gap between the end of
the fiber and the sensitive surface of the SiPM in order to prevent
damage to the SiPM.  It should be noted that the Burle 8575 PMT has a
25\% efficiency at the peak of the Kuraray emission wavelength while
the SiPM's efficiency for that region is about 15\%.  As a result, for
the the same light intensity from the scintillating fiber, the SiPM
exhibits a photon detection efficiency that is 60\% of the PMT's.  The
comparison in efficiency between the two devices is summarized in
Table~\ref{tab:eta}.

\begin{table}[htbf]
\begin{center}
\begin{tabular}{|c|l|c|r|r|}
\hline
$\lambda$ & Device & $\epsilon$ & $QE$ & $\eta$ \\ 
\hline 
550~nm    &  SiPM  &  0.3  & 60\%  & 20\% \\ 
          &  PMT   &  1.0  &  5\%  &  5\% \\ \hline
437~nm    &  SiPM  &  0.3  & 45\%  & 15\% \\
          &  PMT   &  1.0  & 25\%  & 25\% \\ 
\hline
\end{tabular}
\caption{\label{tab:eta} Detection efficiency, $\eta = QE
\cdot \varepsilon$, for the SiPM's and PMT's.}
\end{center}
\end{table}

The scintillating fiber was excited using a $^{90}$Sr($^{90}$Y) beta
source with 2280~keV maximal and 935~keV average energy of beta
particles.  The ionization source had 0.1~$\mu$Ci activity. The
difference in the distributions of ionization energy loss in the
scintillating-fiber core, as calculated by a Monte-Carlo simulation
for the triggering beta particles and compared to the minimum
ionization particles, is only a few percent on average~\cite{okusawa}.

An aluminum collimator for beta particles was used to improve the
signal to noise ratio.  The distance between the ionization source and
the fiber was 0.5 cm.  A ``trigger'' detector was placed under the
fiber and the ionization source and it registered the beta particles
passing through the scintillating fiber.  The signals from the PMT's
were split into two paths.  One, after a delay, was directed to an ADC
for amplitude spectrum measurement.  The second path passed through a
constant fraction discriminator (Ortec CFD 455) set to minimize the
``walk'' correction of the signals from the PMT's.  The coincidence
(LeCroy 465) between the two PMT's signals created a ``start'' signal
for the Camac ADC/TDC units.  The integration time window for the TDC
unit was set to 120 ns.

The signal from the SiPM was amplified by a fast 12-channel amplifier
(LeCroy 612A) that has a 200~MHz bandwidth, fast rise time,
DC-coupling, 1~mV stability at the output, 0.1\% integral linearity
and insures a faithful reproduction of signal shape after
amplification.  The gain factor was set to 10.  One output pulse of
the amplifier was digitized by a ADC 2249A unit.  The second was put
through a CFD en route to a TDC unit.

The TDC and ADC spectra were accumulated and analyzed to extract the
dependence of the detected light and timing resolution as a function
of distance of the source from the respective readout end of the
fiber.  The mean values of the distributions were used in the
calculation of the attenuation length and are plotted in
Fig.~\ref{fig:adcdependence}.

\begin{figure}[h]
\begin{center}
\includegraphics[width=2.7in]{Fig4a}~
\includegraphics[width=2.7in]{Fig4b}
\caption{\label{fig:adcdependence} The dependence of the scintillating 
  light yield with distance from the SiPM is shown for the PMT (left)
  and the SiPM (right).}
\end{center}
\end{figure}

The experimental data were fit with an exponential curve, $y=I \cdot
\exp(-x/L)$, where $I$ is the amount of light produced at the
interaction point, $L$ is the attenuation length and $x$ is the
distance to the ionization source.  The ratios of the mean values for
the two identical positions of an ionization source are larger for the
PMT in comparison with the SiPM.  As a result, two different
attenuation lengths were obtained for the same fiber,
$L_{SiPM}=251$~cm and $L_{PMT}=146$~cm for the SiPM and the PMT data,
respectively, stemming from the difference in the spectral sensitivity
of these two devices.  The SiPM is more sensitive to longer values of
$\lambda$ where the transmission loss for the Kuraray fiber is lower.
Therefore, the SiPM ``realizes'' a longer attenuation length compared
to the PMT.  Obviously, the SiPM-fiber combination provides a clear
advantage over the PMT-fiber one, in applications where long fibers
must be used.

The TDC peak location (the mean value of the Gaussian fit) was plotted
versus the distance from the front-end detector and is displayed in
Fig.~\ref{fig:tdcdependence}.  The slopes of the linear fits for the
SiPM and the PMT agree with each other within the error of
measurement, and are equal to 1.32$\pm$0.01~ch/cm and
1.33$\pm$0.01~ch/cm for the SiPM and the PMT experimental data,
respectively.  The TDC conversion factor was 47~ps/ch. The calculation
of the velocity of light propagation gives $v=(1.60 \pm 0.03) \times
10^{8}$~m/s, a value that agrees with the Kuraray SciFi specifications
sheet.

\begin{figure}
\begin{center}
\includegraphics[width=2.7in]{Fig5a}~
\includegraphics[width=2.7in]{Fig5b}
\caption{\label{fig:tdcdependence} The time propagation of the scintillating 
  light along the fiber. The left (right) graph corresponds to the PMT
  (SiPM) experimental data.}
\end{center}
\end{figure}

The timing resolution is an important factor in the determination of
the position of a particle traversing the fiber, and is presented in
Fig.~\ref{fig:timingsigma} as a function of the distance of the
ionization source from each front-end detector.  The start signal was
provided by the coincidence unit between the trigger detector and the
front end PMT.  The data presented in Fig.~\ref{fig:timingsigma} have
not been corrected for the time jitter of the trigger detector.
Additionally, the quoted timing resolution for the SiPM combines the
intrinsic SiPM resolution plus the jitter connected with the LeCroy
612 amplifier.

\begin {figure}
\begin{center}
\includegraphics[width=2.7in]{Fig6}
\caption{\label{fig:timingsigma} The timing resolution as a function 
  of the distance from the SiPM detector.}
\end{center}
\end{figure}

The smallest values of sigma were 1.5~ns and 1.1~ns for the SiPM and the
PMT, respectively, corresponding to the minimal distance between the
ionization source and the front-end detector.  The resolution
increases/decreases monotonically as the ionization source moves
away/towards the front-end detector.

Finally, the timing resolution depends on the number of detected
photoelectrons.  The average number of photoelectrons detected for the
closest position of the ionization source from each front-end detector
was $\sim$3$-$5 for the SiPM and $\sim$5$-$8 for the PMT.  The
resultant timing resolutions for the SiPM and for the PMT were
comparable.
    
\section{Conclusions}

The properties of a SiPM working in Geiger limited mode have been
measured and compared with a standard 2'' vacuum PMT.  The measurement
with the nitrogen plasma discharge unit shows that the SiPM can
achieve better time and energy resolutions under high photon flux.  To
evaluate the possibility of using the SiPM as a front-end detector for
an electromagnetic calorimeter readout system, we measured the ADC/TDC
spectra from the SiPM for 4 m scintillating fiber irradiated by
$^{90}$Sr beta source. Coupled to the performance attributes of SiPMs,
the results of these investigations demonstrated that SiPMs satisfy
the basic requirements for such an application. The only outstanding
issue that remains is the determination of the coupling choice between
the calorimeter's scintillating fibers and the SiPMs, something that
may be accomplished with the use of intermediate wavelength shifter
fibers, as attempted for the TESLA experiment~\cite{dolgosheintalk}.

This experiment was supported in part by NSERC (Canada) and Jefferson
Lab (USA).  The Southeastern University Research Association (SURA)
operates the Thomas Jefferson National Accelerator Facility for the
U.S. Department of Energy under contract DE-AC05-84ER40150.\\[1ex]

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