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the sipm@xcite is a promising avalanche photodiode ( apd ) variant consisting of 576 silicon micro pixels per square mm of detector surface .
each pixel is a @xmath5 independent photon micro - counter operating in limited geiger mode with gain of @xmath6 .
all sipm pixels are connected to the common load , so the output signal is the sum of all signals .
thus a proportional signal is created by the sum of the digital micro - apd ( pixel ) signals .
the main features of sipm are low excess noise factor , low bias voltage ( 50v ) , and excellent timing ( 30 ps for 10 photoelectrons ) .
sipms have a low excess noise factor comparable to the hybrid photodiode ( hpd)@xcite because the gain mechanism relies on counting how many of the micro - apds have fired .
the sipm noise is high at room temperatures@xcite , but is reduced significantly when operated at cryogenic temperatures .
the photon detection efficiency is similar to a pmt , but comes from the product of a higher quantum efficiency ( qe ) multiplied by the ratio of sensitive area to the total detector area .
it is thus well - suited to a purely solid state solution to lxe scintillation detection .
liquid xenon is a very good scintillation material for various applications of particle detection@xcite .
the columbia group , in particular , is interested in development of a liquid xenon time projection chamber ( tpc ) for compton imaging of mev sources in high energy astrophysics @xcite , and more recently , in the development of a dual phase ( liquid / gas ) tpc for the direct detection of dark matter particles in the form of wimps ( weakly interacting massive particles)@xcite . in all these applications ,
efficient detection of the lxe scintillation is key to the sensitivity of the detector and its minimum energy threshold .
the wavelength of liquid xenon scintillation light is centered around 178 nm @xcite , making its detection a challenge .
currently , the tpc for the xenon dark matter search@xcite uses compact metal channel pmts to detect both primary and proportional scintillation light produced by wimp nuclear recoils .
these pmts ( r9288 and r8520 ) , produced by hamamatsu photonics co. , have typical quantum efficiencies of 20% at liquid xenon temperature ( @xmath7 ) .    in the development phase of the xenon tpc
, we have tested alternative uv photon sensors in liquid xenon , including multi - channel plate pmts ( mcp - pmt ) and large area avalanche photodiodes ( laapds ) .
the sipm is another very promising device , which can detect a small amount of light with a very good single photoelectron detection capability .
here we summarize our first attempt to detect liquid xenon scintillation light with a small sipm immersed in liquid xenon .
the lxe detector used for the test of the sipm is the same one used for testing different light sensors , including the hamamatsu metal channel pmt@xcite and the laapd@xcite .
the detector consists of a 6 cm diameter stainless steel electrode with a radioactive alpha source @xmath1am deposited in its center .
the @xmath8 sipm ( type z , serial number 217 ) was mounted on a teflon support plate facing the source plate .
a blue led was also mounted on the teflon plate .
the distance between source and sipm was 4.7 mm .
the detector was pumped down to a vacuum level of @xmath9 and baked out for 36 hours before filling with lxe .
xe gas , purified through a saes getter , was condensed in the detector vessel , cooled by a bath of liquid nitrogen and alcohol mixture at @xmath10 .
fig.[fig : nevis ] shows a schematic of detector , gas system and electronics .
a scope trace of the blue led signal as detected by sipm is shown in fig .
[ fig : signal ] .
the scintillation photons , absorbed by the sipm , produce photoelectrons which are consequently amplified inside the silicon via a geiger mode avalanche .
the sipm electrical signal is fed into a charge sensitive pre - amplifier , followed by an ortec 450 shaping amplifier .
a test pulse generator is used to calibrate electronics chain system .
the great advantage of a sipm is that it is self - calibrating , since its single photoelectron sensitivity can be used for calibration . the resulting low amplitude part of the @xmath2-source spectrum is shown in fig .
[ fig : lowamp ] .
note the excellent resolution ( low excess noise factor ) that allows up to 11 photoelecton peaks to be clearly distinguished .
each single photoelectron peak is fitted using a gaussian function .
the fitted mean value is plotted versus peak number ( number of photoelectrons ) in fig .
[ fig : calibr ] , clearly showing a linear behavior .
am particle scintillations .
low amplitude part of the spectra.,scaledwidth=90.0% ]     is subtracted.,scaledwidth=90.0% ]      we have estimated the sipm gain @xmath11 with a calibrated test pulse signal as follows :    @xmath12    where @xmath13 and @xmath14 are the test pulse amplitude and channel number in the mca spectrum , respectively .
@xmath15 is the capacitance of the charge sensitive pre - amplifier .
@xmath16 is number of channels corresponding to a single photoelectron and @xmath17 is the elementary electronic charge .
@xmath18 is the gain of the amplification system used in this measurement .
based on these values , the sipm gain is estimated to be around @xmath3 at the operation voltage of 52 v.      in fig . [
fig : alpha ] the @xmath1am @xmath2-source energy spectrum , measured with the sipm bias voltage at 52 v , is shown .
am particle scintillations .
fit - gaussian function.,scaledwidth=90.0% ]    since the sipm signal is calibrated in units of photoelectrons , the average number of photoelectrons detected from lxe scintillation light induced by @xmath1am @xmath2 particles is determined to be 55 p.e .
the total number of scintillation photons striking the sipm can be calculated using the energy of @xmath2 particle ( 5.48 mev ) , the average energy needed to produce single scintillation photon in lxe ( 19.6 ev for @xmath2 particle @xcite ) , and the geometrical acceptance of the sipm detector . for the detector geometry used in the tests , the average number of scintillation photons produced by one 5.48 mev @xmath2 particle striking the sipm surface is @xmath19 = 1006 photons .
thus the measured photon detection efficiency is @xmath20 .
the quantum efficiency of the sipm ( qe ) can be calculated as @xmath21 , where @xmath22 is the active area ratio of the device .
assuming @xmath22 = 0.254@xcite , we infer a @xmath23 = 22@xmath4 including the probability of initiating the geiger avalanche .
a silicon photomultiplier was tested for the first time in lxe to detect its scintillation light at @xmath24 = 178 nm , at an operation temperature at @xmath10 .
a high quantum efficiency of 22% has been demonstrated .
large arrays of sipms offer a promising solid state photodetector approach for reading out lxe detectors in applications ranging from @xmath25-ray astrophysics to particle physics and medical imaging .
the authors wish to express their gratitude to prof . b.dolgoshein and dr .
e.popova for the donation of sipm samples and their assistance in understanding sipm behavior .
this work was supported in part by grants from the national science foundation to the columbia astrophysics laboratory ( grant no .
phy-02 - 01740 ) and by the department of energy ( hep grant no .
94er40823 ) to the university of minnesota .
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we have studied the feasibility of a silicon photomultiplier ( sipm ) to detect liquid xenon ( lxe ) scintillation light . 
 the sipm was operated inside a small volume of pure lxe , at -@xmath0 , irradiated with an internal @xmath1am @xmath2 source . 
 the gain of the sipm at this temperature was estimated to be @xmath3 with bias voltage at 52 v. based on the geometry of the setup , the quantum efficiency of the sipm was estimated to be 22@xmath4 at the xe wavelength of 178 nm . 
 the low excess noise factor , high single photoelectron detection efficiency , and low bias voltage of sipms make them attractive alternative uv photon detection devices to photomultiplier tubes ( pmts ) for liquid xenon detectors , especially for experiments requiring a very low energy detection threshold , such as neutralino dark matter searches .    
 ,    ,    ,    sipm , liquid xenon , dark matter