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aerogels are materials with density and refractive index in the region between gases and liquids or solids .
already some time ago , cantin et al .
@xcite proposed that cherenkov radiation from silica aerogels could be used for detection of particles .
besides particle detectors like for example tasso @xcite , such threshold counters found applications also in other fields @xcite . with improved manufacturing techniques , aerogels of higher transparency
i.e. less rayleigh scattering became available , permitting their consideration as radiators in ring imaging cherenkov ( rich ) counters @xcite .
ypsilantis and seguinot @xcite proposed a combined aerogel+gas , mirror - focused rich counter for the lhc - b experiment at cern .
the hermes team @xcite constructed and operated such a ring imaging detector at desy .
the present paper reports on experimental investigation of an aerogel based rich detector not requiring mirrors i.e. of the proximity focusing type .
such a detector is being considered in connection with a possible upgrade of the belle particle identification system at kek @xcite .
initial tests of the apparatus with cosmic rays were reported recently @xcite .
the present paper describes measurements and results obtained with the @xmath1 beam at kek .
a beam particle - pion , muon or electron - traversing the apparatus is signaled by two @xmath2  @xmath3 scintillation counters which determine the time of arrival .
two co@xmath4 gas cherenkov counters produce signals only upon the passage of electrons so these signals could be used either to select or to exclude electrons .    the aerogel radiator and the position sensitive , single photon detector are contained in a light tight box ( fig .
[ fig1 ] ) , of which the entrance and exit sides each have a multiwire proportional chamber for measuring the track of the incident particle .
these @xmath2  @xmath3 mwpc s , with 15 @xmath5 m diameter , gold - plated tungsten anode wires at 2  mm pitch and with 90% ar + 10% ch@xmath6 gas flow , are read out by delay lines on the x and y cathode wires .    after passing through the entrance mwpc ,
the charged particle hits the aerogel radiator in which it emits cherenkov photons .
measurements have been made mainly with 2  cm thick aerogel slabs of @xmath7 , @xmath8 and @xmath9 , produced by the method described in @xcite .
the position sensitive detector of cherenkov photons is situated 17 - 29  cm downstream of the aerogel , depending on the refractive index value of the specific aerogel .
the detector is a @xmath10 array of 16 channel multianode photomultiplier tubes ( hamamatsu pmts type r5900 - 00-m16 with borosilicate window @xcite ) at 30  mm pitch . the sensitive surface of the m16 pmt is divided into 16 ( @xmath11 ) channels , each covering @xmath12 mm@xmath13 .
it follows that only 36% of the detector area is occupied by the photosensitive channels , the rest being dead space .
the photon detection system and the aerogel radiator tile may be rotated around an axis perpendicular to the beam direction , enabling measurements of angular acceptance i.e. measurements of the number of detected cherenkov photons as a function of the charged particle incident angle .
the pmts are plugged into voltage divider boards inside the light tight box with signals passing through connectors to the readout system located outside the box .
the pmt anode signals are first discriminated and then recorded by camac multihit , multichannel tdcs , for which the common stop is provided by the scintillation counter signals .
the tdc information is stored for later analysis in a personal computer .    as only 196 readout channels were available for the 576 pmt anode outputs , only part of the system
could be read out with the 4.5  mm pixel size . however , by connecting 4 ( @xmath14 ) anodes to one readout channel , the entire system could be read out with 9  mm pixel size .
pion given by the measured track position.,width=340 ]    a few typical events are displayed in fig .
[ fig2 ] . from the photon hit position and the measured direction of the incident charged particle , the cherenkov angle is calculated .
accumulated distributions of hits , depending on their cherenkov angles , are plotted in figs .
[ fig3 - 0 ] and [ fig3 ] .
peaks and rings , corresponding to pions , muons and electrons , are clearly visible .
signals from the gas cherenkov counters may be used for either selecting or excluding electrons .
fitting these distributions with gaussian peaks and linear backgrounds yields the average values and standard deviations of the measured cherenkov angles .
the main contributions to the resolution in cherenkov angle as determined from a single photon ( standard deviations of the peaks in distributions of figs .
[ fig3 - 0 ] and [ fig3 ] ) come from pixel size and from uncertainty in the emission point . for normal incidence of tracks the first contribution
could be estimated as @xmath15cos@xmath16 , where d is the pixel size , @xmath17 is the cherenkov angle and @xmath18 is the distance from aerogel to detector .
the second contribution is @xmath19 sin@xmath20cos@xmath21 , where @xmath22 is the aerogel thickness .
the uncertainty in the track direction is negligible at 3  gev/@xmath23 , but increases at lower momenta .
the error due to dispersion in the radiator ( chromatic error ) should also be negligible , but contributions could arise due to possible non - uniformities of the aerogel ( position variations in refractive index ) , non - flat aerogel surface , forward scattering of photons etc .    the measured cherenkov angle resolution , i.e. the standard deviation of peaks in distributions of photon hits versus the value of aerogel refractive index , is shown in fig .  [ fig4 ] for 3  gev/@xmath23 pions .     pion beam .
full symbols correspond to the measured values , empty ones to estimates of contributions from pixel size and emission point uncertainty only .
x is the radiator - to - photon - detector distance , l is the radiator thickness and d is the photon detector pixel size.,width=340 ]    different data points in the figure refer to different values of parameters such as the radiator thickness , the radiator - to - photon - detector distance and the photon detector pixel size .
the measured values are represented by full symbols , with different symbol shapes indicating different combinations of parameter values . using the above expressions for the contributions of pixel size and emission point uncertainty and summing them in quadrature , one obtains estimates for the resolution , represented with empty symbols in fig .
it may be seen from the figure that such estimates give a good approximation to the measured resolution .
pion beam .
l is the aerogel radiator thickness .
full symbols correspond to the measured values , empty ones to estimates.,width=340 ]     ( open symbols ) and @xmath7 ( full symbols ) . squares and circles
correspond to different granularities of the photon detector .
the curves are fits to corresponding data.,width=340 ]    the other important parameter of a rich counter is the number of detected photons per incident charged particle .
this is usually parametrised as @xmath24 , where @xmath25 is the cherenkov angle , l is the radiator thickness and @xmath26 is a figure of merit depending on the radiator and system transparency , geometrical acceptance of photons ( area and angle ) , quantum efficiency , photoelectron collection efficiency etc . due to rayleigh scattering ,
the aerogel transparency has a strong wavelength dependence in the region of r5900-m16 pmt quantum efficiency , so one may expect a sensitivity of the number of detected photons on the particular aerogel sample i.e. on the production procedure . the number of detected photons per cherenkov ring is shown in fig .
[ fig5 ] for 3  gev/@xmath23 pions .
first one notices that the number of photons does not increase with refractive index as may be expected for @xmath27 particles ; @xmath28 sin@xmath29 .
then it is also obvious that the 4  cm thick aerogel radiator does not produce two times as many photons as does the 2  cm thick aerogel . and finally we see that the 2.8  cm thick aerogel tile @xcite , produced in a different process @xcite , yields more cherenkov photons than the 4  cm thick aerogel @xcite . that a higher refractive index of the aerogel sample does not necessarily produce more photons , is observed also in fig .  [ fig6 ] .
although the threshold for @xmath8 is reached , as expected , at lower particle momenta than for @xmath7 , the saturated number of detected photons per cherenkov ring is more or less the same for both radiators .
the above discrepancies can be well understood , and have been estimated from the known aerogel attenuation lengths and the response of the counter ( fig .
[ fig5 ] ) .
the attenuation lengths at 400  nm for the samples of fig .
[ fig5 ] are 36  mm , 15  mm and 7  mm for the aerogel samples with @xmath7 , @xmath8 and @xmath9 , respectively , and 36  mm for the 28 mm thick novosibirsk sample with @xmath8 @xcite .    it has been already noted by the hermes group @xcite , that a loss of cherenkov photons occurs at the side wall boundaries between adjacent aerogel tiles .
we have confirmed this finding by measuring the number of photons on the cherenkov ring as a function of the distance of the charged particle impact point from the boundary between two tiles . the measurement is shown in fig .
[ fig7 ] , where a dip is seen at the tile boundary @xmath30  mm . in order to eliminate other geometrical factors , like for example
the acceptance of the photon detector , the measured yield was normalized to the yield obtained with one tile covering the entire range .
the result clearly indicates the reduction of yield when the charged particle is closer than about 5  mm to the boundary of a 2  cm thick @xmath8 aerogel tile .
it is worth noting that a simple model , where all photons hitting the boundary between the two tiles get lost , accounts for most of the observed dependence .
we have constructed and tested a proximity focusing rich detector module with aerogel as radiator and multianode pmts as position sensitive detectors of individual cherenkov photons .
the measured values of the basic parameters i.e. the single photon cherenkov angle resolution and the number of photons detected per cherenkov ring , look promising .
the resolution is in relatively good agreement with estimates based on pixel size and emission point uncertainty .
the number of detected photons , however is sensitive to the particular aerogel used .
it seems that these differences are due to rayleigh scattering , which reduces the aerogel transparency mainly in the wavelength region of maximal photocathode sensitivity .
photomultipier tubes with a higher fraction of active area , possibly combined with a light collection system , consisting of lenses or light guides , are expected to increase the number of detected cherenkov photons by reducing the dead space of the photon detector .
the increase in photon yield , however , is in the latter case at the expense of an increase in the effective pixel size , so a compromise , optimizing the final resolution of the charged particle cherenkov angle should be found .
the information obtained from the results of the present tests suggests that a proximity focusing aerogel rich as required by the belle particle identification upgrade is feasible , so investigations of optimal detector parameters are being continued .
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using aerogel as radiator and multianode pmts for photon detection , a proximity focusing cherenkov ring imaging detector has been constructed and tested in the kek @xmath02 beam . 
 the aim is to experimentally study the basic parameters such as resolution of the single photon cherenkov angle and number of detected photons per ring . 
 the resolution obtained is well approximated by estimates of contributions from pixel size and emission point uncertainty . 
 the number of detected photons per cherenkov ring is in good agreement with estimates based on aerogel and detector characteristics . 
 the values obtained turn out to be rather low , mainly due to rayleigh scattering and to the relatively large dead space between the photocathodes . a light collection system or a higher fraction of the photomultiplier active area , together with better quality aerogels are expected to improve the situation . 
 the reduction of cherenkov yield , for charged particle impact in the vicinity of the aerogel tile side wall , has also been measured .    
 cherenkov counters , aerogel , multianode pmts , belle spectrometer .