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the recent development of micro - pattern - gas - detectors ( mpgd ) allows an extended field of application for detectors with gas multiplication . for time - projection - chambers ( tpc ) the readout with gas - electron - multipliers ( gem ) @xcite has been demonstrated in conjunction with an unconventional readout option with high pixelation using the medipix2 chip @xcite .
the timepix  @xcite and medipix2 is investigated the first time with 5 gev electrons from desy ii .
they have the virtue of negligible multiple scattering . an external tracking with si - telescope
provides information on the track position which is used for resolution determination and for the drift velocity measurements using a timepix chip .
-0.5 cm [ stack ]    a cross sectional view of the triple gem plus medipix2 and timepix detector with an electron beam crossing the drift region is shown in fig.1 ( left part ) .
the drift volume of 6 mm thickness and 10@xmath010 @xmath2 in size serves for charged track detection .
the drift field is about 1.1 kv / cm .
three cern - produced gems of the same area as the drift volume with 70@xmath3 m holes of 140@xmath3 m pitch are arranged in a stack above the readout plane with 2 mm ( 2 - 2 - 1 ) or 1 mm distances ( 1 - 1 - 1 ) for the transfer gaps , the latter provides a collapsed setup in order to study possible effects on the spatial resolution . with the triple gem stack gas amplification of @xmath410@xmath5 can be achieved with ar / co@xmath6 and he / co@xmath6-mixtures for the high resolution detection of minimum ionizing particles .
the medipix2 and timepix chip is positioned at a distance of 1 mm from the last gem exposed to a field of e@xmath7 = 4.0 kv / cm .
the advantage of a gem setup is the robust operation , since the fields of the gas multiplication region ( @xmath4 70 kv / cm ) are well shielded insige gem holes , see @xcite .
the medipix2 and timepix chip has a surface of 14@xmath014 mm@xmath8 and a pixel size of 55@xmath055@xmath1 .
it is positioned close to the border of the gem stack , see fig .
1 ( right part ) .
the remaining surface of the gems is covered with 24 anode pads of 2@xmath9@xmath2 size for monitoring purposes .
the gas tight box contains the gems , the resistor chain , the timepix and medipix2 chip and the readout electronics of the pads .
+ the readout of the medipix2 and timepix is done with muros2 using the fast shutter option .
the thresholds for the pixels used were 990e@xmath10 and 830e@xmath10 for the medipix2 and the timepix , respectively .
the electron beam is defined by trigger scintillating counters of 1@xmath01.5 @xmath2 in size and a si - telescope with 3 planes of strips ( two planes allow measurement of the x - coordinate in front and behind the gem plus medipix2 and timepix , and one plane is used for the y - coordinate measurement in front of the detector ) . the effective readout pitch of si - telescope is 50@xmath3 m .
the timepix has a clock , which is distributed throughout the entire chip .
a register on each pixel counts the number of clock cycles in a way depending on the chosen mode of operation . for each pixel
this mode can be set individually . in time - mode
the cycles are counted from the point when the signal crosses the threshold till a common stop by the gate signal ( = `` fast shutter '' ) .
the other mode `` time - over - threshold '' records the clock cycles as long as the pulse is above the threshold .
the maximum number of counts in this measurement is limited by the chosen gate width of @xmath11 , which is about 600 counts at a given clock frequency of 48mhz .    in a special configuration `` mixed mode ''
every other pixel switches the time and tot mode in checker board fashion .
this results in 1/2 of all pixels are of time - type and another 1/2 of the tot - type .
this allows a proximity information of both time and tot .
typically , 8 - 9 non - separated clusters per track were observed , see fig .
2 ( left part ) . images with double tracks were not considered in the current analysis .
the following three clustering methods were used , where last two can separate the overlapping clusters using the charge deposition information ( tot or mixed modes ) resulting in everage 10 - 11 clusters per event :    * `` contiguous areas '' method leaves overlapping clusters not separated .
that method is applicable for any timepix mode .
the result is a lower number of clusters depending on the effective threshold settings and therefore not covered here * `` saddle point '' method separates the contiguous clusters making a line for secondary maximum in a projection transverse to track .
the line divides the merged clusters at the saddle point * `` island '' method joins the adjacent pixels with a nonzero tot values into clusters in the way that a pixel is joined into the cluster of its neighbor with a highest tot value .
the procedure is repeated for each pixel to produce a unique assignment of pixels to clusters .
it results in a separation in two spacial coordinates available compared with the above case    -0.1 cm [ sigma0 ]    the evaluation follows basically @xcite : a cluster is rejected if it contains less than 9 hits .
the noise is removed beyond the cluster region .
two different methods of the straight line fits to the centers of the clusters result in an inbiased estimation of the standard deviation .
residuals of the fit to all cluster centroids ( n ) and a fit to ( n-1 ) centroids resulting in a residual of the exempt cluster which is permutated over all clusters .
these residuals enter into two gaussian fits which produce @xmath12 .    the left part of fig .
2 shows a measured dependance of the resolutions versus the drift space for ar / co@xmath6 . in the drift region to separate the lateral diffusion within the drift space from the intrinsic gem plus timepix and medipix2 resolution the y - coordinate information of the external telescope is used .
the following parametrisation is applied for the fit : @xmath13 .
the @xmath14 equal to 24.9 @xmath15 0.4 results from the fit for ar / co@xmath6 .
the results of spatial resolution @xmath14 and @xmath16 corresponding to different clustering methods for ar / co@xmath6 and he / co@xmath6 are summarised in tab .  2 .
the comparison with heed simulations  @xcite is also given in the table .
& gas & @xmath14 & @xmath16 & @xmath14 & @xmath16 + `` island '' & ar / co@xmath6 & 24.9 @xmath15 0.4 & 521 @xmath15 9 &  &  + & he / co@xmath6 & 29.4 @xmath15 0.5 & 660 @xmath15 14 &  &  + `` saddle & ar / co@xmath6 & 18.4 @xmath15 2.7 & 467 @xmath15 36 & 15.2 @xmath15 3.8 & 726 @xmath15 41 + point '' & he / co@xmath6 & 27.1 @xmath15 4.9 & 547 @xmath15 78 & 19.4 @xmath15 4.0 & 989 @xmath15 54 +    [ table ]    the agreement between two clustering methods is found to be good for both @xmath14 and @xmath16 values . it is found that simulations considerably overestimate the slope @xmath16 comparing to the measurements for both gases .
the slope depends on the size of merged clusters because a higher number of electrons per detected cluster than for an ideally resolved cluster is present .
this leads to a reduction of the effective transverse diffusion through the active volume and reduces the slope parameter .
based on this observation it is possible that the discrepancy of the monte carlo with respect to the slope is present , since the average number of detected clusters lower by a factor of 2 than expected from simulations  @xcite .
we would like to thank the eudet project for its financial support and the medipix collaboration for supporting us work with readout software and hardware , especially with respect to the timepix chip used for the very first time in an experiment .
we would also like to thank michael campbell , erik heijne , xavier llopart and fabio sauli for stimulating discussions and a lot of valuable advices .
nucl.instr.meth.a386(1997)531 a.bamberger et al.,``readout of gem detectors using the medipix2 chip '' , nima573 , 361 - 370(2007 ) x. llopart , see this conference m.hauschild , http://hausch.cern.ch/hausch/medipix.html
this contribution investigates a prototype of a tpc readout with a highly pixelated cmos asic , which is an option for charged particles tracking of the ilc . 
 a triple gem stack was joined with a timepix and medipix2 chip ( pixel size of 55@xmath055@xmath1 ) and its readout properties were investigated with 5 gev electrons . 
 the spatial resolution of the cluster center reconstruction was determined as a function of drift distance using different cluster alhoritms and compared with monte carlo predictions .