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t2k ( tokai - to - kamioka )  @xcite is a long baseline neutrino oscillation experiment . in t2k
, an intense muon neutrino beam is produced with a proton synchrotron in j - parc facility and sent to the massive super - kamiokande detector 295  km away .
the main goals of t2k are a sensitive search for the @xmath0 appearance from @xmath1 , which is related to the yet - unmeasured neutrino mixing angle @xmath2 , and precise measurements of neutrino oscillation parameters @xmath3 and @xmath4 . in order to achieve the aimed precision , good understanding of the neutrino beam properties and neutrino - nucleus interaction
are indispensable .
the near detector ( nd280 ) complex is placed at about 280  m from the proton target to provide this information .
the t2k - nd280  @xcite consists of several sub - detectors with specific and complimentary functions .
as the basic elements for the particle detection , most of detectors use the plastic scintillator read out via wavelength shifting ( wls ) fibers .
this is a widely used technique , especially in recent accelerator neutrino experiments  @xcite . in those experiments , multi - anode pmts ( mapmts )
have been used as the photosensor . for t2k ,
mapmt is not suitable because some of the detectors have to operate under a magnetic field of 0.2  t provided by a dipole magnet originally built for ua1 experiment at cern , and also because the available space is very limited .
we selected the multi - pixel photon counter ( mppc ) as the photosensor for nd280 and started the development in cooperation with hamamatsu photonics and kek detector technology project  @xcite .
for nd280 , 64,000 mppcs are used in total .
after three years of study , we developed an mppc that satisfied our requirements and started the mass production in february 2008 . because it was the first time that the mppcs are used on such a large scale , it was necessary to develop a method to characterize each device for quality assurance purpose . in order to characterize a large number of mppcs
, we have developed a system that simultaneously measures 64 mppcs at kyoto university . using this test system ,
we have measured 17,686 mppcs used for two sub - detectors of nd280 , called ingrid  @xcite and fgd  @xcite .    in this paper ,
the design of the test system , testing procedure and summary of measured performances of t2k - mppc are presented .
the multi - pixel photon counter ( mppc ) is a new photodetector manufactured by hamamatsu photonics , japan  @xcite .
an mppc consists of many ( 100 to @xmath51000 ) small avalanche photodiodes ( apds ) , each with an area of either 25@xmath625 , 50@xmath650 or 100@xmath6100  @xmath7m@xmath8 , in an area of 19  mm@xmath8 .
each apd micropixel independently works in limited geiger mode with an applied voltage a few volts above the breakdown voltage ( @xmath9 ) .
when a photoelectron is produced , it induces a geiger avalanche .
the avalanche is passively quenched by a resistor untegrated to each pixel .
the output charge @xmath10 from a single pixel is independent of the number of produced photoelectrons within the pixel , and can be written as @xmath11 where @xmath12 is the applied voltage and @xmath13 is the capacitance of the pixel .
the overvoltage , @xmath14 is the parameter that controls the performance of mppc as we will see later .
it is known that @xmath15 is dependent on the temperature with a coefficient of @xmath16  mv / k  @xcite . combining the output from all the pixels , the total charge from an mppc
is quantized to multiples of @xmath10 and proportional to the number of pixels that underwent geiger discharge ( `` _ _ fired _ _ '' ) .
the number of fired pixels is proportional to the number of injected photons if the number of photons is small compared to the total number of pixels .
thus , the mppc has an excellent photon counting capability as long as the number of photo - electron does not approach the total number of pixels in the device .    for an mppc , the operating voltage
@xmath12 is a few volts above the breakdown voltage and well below 100  v. the pixel capacitance @xmath13 is on the order of 10100  ff , giving a gain of 10@xmath1710@xmath18 .
these features enable us to read out the signal from the mppc with simple electronics .
in addition , because the thickness of the amplification region is a few @xmath7 m , an mppc is insensitive to the magnetic field and has a fast response .
the photon detection efficiency ( pde ) of an mppc is expressed as a product of three effects : @xmath19 the geometrical efficiency @xmath20 represents the fraction of active area in a micropixel .
based on a measurement  @xcite , @xmath20 is about 0.6 for an mppc with 50@xmath650  @xmath7 m pixel size .
the quantum efficiency of the apd , qe , depends on the wavelength of photon and is typically 0.70.8 for the wavelength of 400500  nm , the range of current interest .
the probability of inducing a geiger discharge when a photoelectron is generated , @xmath21 , depends on @xmath22 .
the last factor introduces the @xmath22 dependence of pde .
the dark noise of mppc at the room temperature is dominated by the geiger discharge induced by thermally generated electron - hole pairs .
because it is amplified with the identical process , a dark noise pulse can not be distinguished from the signal induced by the external photon irradiation , although the dark noise is mainly at the single photoelectron level in absence of the cross - talk effect described below .
the dark noise rate of mppc is proportional to the area . for 50  @xmath7 m pixel type mppc ,
the dark noise rate per active area is about 300500  khz / mm@xmath8 at 25  with a gain of 7.5@xmath23 .
there are two known processes which give an additional charge to the original signal ; the cross - talk between neighboring micropixels and afterpulse .
the origin of the cross - talk is presumed to be optical photons emitted during avalanche  @xcite which enter neighboring micropixels and trigger another geiger discharge .
it gives an additional charge output at the same time as the original geiger discharge .
the afterpulse of mppc is considered to be due to delayed release of carriers trapped at lattice defects , giving a time - correlated but delayed charge output to the original signal  @xcite .    in order to evaluate the mppc performance ,
it is important to measure key parameters such as @xmath15 , pde , dark noise rate , cross - talk and afterpulse probabilities together with their dependence on the temperature and applied voltage .      a picture of the mppc for the t2k - nd280 ( s10362 - 13 - 050c ) is shown in fig .
[ fig : mppc ] and the major specifications is summarized in table  [ tab : spec ] .
we use the 1.0  mm diameter kuraray y11(200)ms wls fiber for the nd280 detector .
the sensitive area of the mppc is enlarged from 1@xmath61  mm@xmath8 of those on catalogue ( s10362 - 11 - 050c ) to 1.3@xmath61.3  mm@xmath8 , so that we can minimize the light loss at the optical contact with a simple coupler .
the size of apd pixel is 50@xmath650  @xmath7m@xmath8 and the number of apd pixels  .
the large pde of mppc results in a light yield sufficiently large to safely reject the dark noise , which is mainly at a single photoelectron level .
in addition , thanks to the pulsed neutrino beam timing , the random dark noise has only little effect for the neutrino event reconstruction .
.specifications of t2k - mppc(s10362 - 13 - 050c ) . [ cols="^,^,^",options="header " , ]
for the t2k near neutrino detectors , we have developed the multi - pixel photon counter ( mppc ) with 1.3@xmath61.3  mm@xmath8 active area and with 667 50  @xmath7 m pitch pixels ( s10362 - 13 - 050c ) . in total ,
about 64,500 mppcs were produced for t2k from february 2008 to february 2009 .    at kyoto university , we have successfully developed the system and technique to characterize a large number of mppcs , and tested 17,686 mppcs .
the performance of mppcs has been confirmed to satisfy our requirements .
the failure rate is found to be about 0.5% .
the authors are grateful to the solid state division of hamamatsu photonics for providing us test samples during the development .
we appreciate useful suggestions from the members of t2k nd280 photosensor group .
we thank f.  retiere for careful reading and helpful comments on this manuscript .
this work was supported by mext and jsps with the grant - in - aid for scientific research a 19204026 , young scientists s 20674004 , scientific research on priority areas `` new developments of flavor physics '' , and the global coe program `` the next generation of physics , spun from universality and emergence '' .
the development of mppc and the test system is also supported by the kek detector technology project .
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we have developed a multi - pixel photon counter ( mppc ) for the neutrino detectors of t2k experiment . 
 about 64,000 mppcs have been produced and tested in about a year . in order to characterize a large number of mppcs 
 , we have developed a system that simultaneously measures 64 mppcs with various bias voltage and temperature . 
 the performance of mppcs are found to satisfy the requirement of t2k experiment . in this paper 
 , we present the performance of 17,686 mppcs measured at kyoto university .    
 multi - pixel photon counter , geiger - mode apd , photodetector , characterization , quality assurance 29.40.wk