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since carbon tetraflouride ( cf@xmath0 ) has a wide spectrum of applications , most of its properties are well determined ( see e.g.  @xcite ) .
cf@xmath0 gas has been suggested as a good target for directional  @xcite , spin - dependent dark matter searches  @xcite due to the large spin of the fluorine nucleus and the good ionization - counting properties of the molecule .
however , there is a large discrepancy between existing measurements of electron transport parameters , specifically the diffusion and attachment coefficients , that are of vital importance for the design of low - background particle detectors such as dark matter searches .
detectors that are based on the time - projection - chamber ( tpc ) concept  @xcite require long drift distances of ionization electrons through detector gas in a low electric field .
as a swarm of electrons propagates along an electric field , it spreads transversely and longitudinally , which limits spatial precision of the detector
. current measurements of the transverse spread  @xcite differ by more than an order of magnitude at operating conditions needed for dark matter detection .
primary electrons can also attach to cf@xmath0 molecules ( see @xcite for details ) , which attenuates the electron signal in the detector .
measurements of the attenuation rate also differ by more than an order of magnitude  @xcite . in this work
, we present new measurements of the mean electron energy , that determines the transverse diffusion , and the attentuation coefficient of electrons in cf@xmath0 gas .
the detector ( figure  [ fg::dmtpc10l ] ) is a low - pressure time projection chamber with optical and charge readout .
particles interacting in the sensitive volume of the detector ionize the gas either directly or by creating a charged particle recoil
. the sensitive region of the detector is surrounded by a series of stainless steel rings with inner diameter of 27  cm , outer diameter of 32  cm , and thickness of 1  mm .
the rings are connected with 1  m@xmath1 resistors and separated by nylon washers to keep periodicity at 1  cm .
the last ring in the series is connected via a 2.2  m@xmath1 resistor to a ring holding a grounded mesh and is separated from the mesh by 1.7  cm .
the total height of the drift region is 19.7  cm .
the field uniformity in the drift cage is computed using the finite element method ( fem ) with the scaled transverse field component , @xmath2 , less than 1% .
the cathode is made of a stainless - steel wire mesh with a pitch of 512  @xmath3 m and wire diameter of 31  @xmath3 m .
the grounded electrode is made of a stainless - steel mesh with a pitch of 256  @xmath3 m and wire diameter of 28  @xmath3 m .
the detector consists of two tpcs put back - to - back such that electrons drift toward two amplification planes located in the center of the vessel .
only the top tcp is used in these measurements .    , title="fig:",width=377 ] +    the amplification plane is described in more detail in @xcite .
an amplification gap is created by a copper anode ( + 730  v ) and a grounded mesh separated with 0.53  mm fishing lines that are placed at 2.5  cm intervals .
primary electrons that drift toward the plane start avalanches of electrons and scintillation photons  @xcite .
the detector s primary form of readout is the two - dimensional optical readout of scintillation photons created in the avalanche charge multiplication process .
this scintillation light is collected with a nikon photographic lens with f - stop ratio of 1.2 and a focal length of 55 mm .
the scintillation light is recorded by an apogee u6 camera with a kodak 1001e ccd chip .
in addition to the optical readout , the charge deposited on the anode is also recorded .
we use a proportional preamplifier ( ortec 109pc ) to integrate the charge signal and a shaping amplifier ( ortec 485 ) to make unipolar pulses .
a fast digitizer ( alazar ats860 ) is used to convert pulses into waveforms , which are saved with our event record .
the gas pressure is measured by an inficon pcg400 pressure gauge which utilizes a combination of a capacitance and a pirani gauge .
the spatial resolution is estimated by extracting the track width of 5.486 mev alpha particles from an @xmath4am source .
the observed width of the track along the axis in the readout plane is 300 - 500  @xmath3 m .
the resolution is affected by the number of ccd pixels , @xmath5 merged into a readout bin ( 98@xmath6 ) , the spread of an electron avalanche in the amplification gap ( @xmath7 )  @xcite , and the mesh pitch ( @xmath8 ) .
the measurement itself is affected by the imperfect collimation of the source ( 50  @xmath3 m ) and the straggling of alpha particles through cf@xmath0 gas ( 40  @xmath3 m )  @xcite in a 1.2  mm - long track segment .
however , the dominant contribution to the spatial resolution uncertainty comes from the electron diffusion that is described and measured in the next section .
we measure the energy resolution at 5.9  kev for the charge readout using a fe-55 source .
a background - subtracted spectrum of pulse heights is fitted with a narrow gaussian for the full - absorption peak plus a wide gaussian for the escape peak .
the resolution is extracted from the narrower gaussian and is found to be 10% ; a result which is consistent with fano factor of one  @xcite .
we infer the energy resolution for the ccd readout from the amount of light that alpha particles deposit in track segments of different lengths .
the energy deposited into ionization is estimated from the energy loss and calculated by srim  @xcite .
we find that the resolution at 55  kev is approximately 15% , as shown in figure  [ fg::e reso ] .
fe peak from charge readout , and the remaining points are from the optical readout of tracks from the @xmath4am sources .
[ fg::e reso],width=453 ]
electrons propagating through cf@xmath0 gas experience a spread in transverse direction @xmath10 where @xmath11 is the diffusion constant and @xmath3 is the mobility of electrons , i.e. a constant of proportionality between the drift velocity and the electric field .
the @xmath9 parameter is a measure of the average electron energy , @xmath12 .    in previous measurements of @xmath9 , the spread of electron current
was determined with annular anodes that integrate the swarm current  @xcite or with a small - area proportional counter that scans the swarm profile  @xcite .
these approaches yield @xmath9 values that differ by more than an order of magnitude at electric field to number density ratio @xmath13 .
the large data samples of these measurements suggests that the discrepancy is due to an unknown systematic offset , which motivates a new measurement with a different approach .
in this work , we extract @xmath9 from the width of alpha tracks ( @xmath4am ) that are placed at different drift distances inside the drift cage . a 500  ms ccd exposure with alpha tracks emerging from the five sources
is shown in figure  [ fg::imageoffivesources ] .
the gaps in the tracks are the result of missing light due to the resistive separators which are used to maintain the amplification gap separation .
images are taken without a shutter , so it is possible that some alpha tracks appear during the ccd readout . since the ccd is read out in a horizontal direction , these tracks appear shifted to the left or right .
most of such tracks fall out of the fitting range of interest or appear at wrong @xmath14-positions and can be excluded by requiring that the mean of the light distribution of the track segment agrees with the position of the source .
vertical lines depict a 1.2  mm wide region of interest approximately 3  cm from the sources for measurement of track widths .
the kink in the second track from the top is likely due to a large - angle scattering on gas nucleus .
[ fg::imageoffivesources],width=377 ]    we collect 1000 images with 500 ms exposure for each @xmath16 measurement point .
the cf@xmath0 pressure is 50 - 150  torr , and the temperature is roughly 300  k. in each track we select a 1.2  mm long segment taken 3  cm from a source and project it onto the @xmath14-axis .
the projection is then fitted assuming a gaussian signal and a constant term for background , as shown in figure  [ fg::fittosource ] .
we compute the average width squared , @xmath17 , from the collection of all tracks from a single source that have a yield which is consistent with that of a single alpha track .
the width squared is expected to change linearly with the drift distance , @xmath18 , according to @xmath19 where @xmath20 is the applied voltage in the drift cage of length @xmath21 .
the detector resolution , @xmath22 , and @xmath23 are free parameters that are determined in a fit to measurement points taken at five different drift distances and plotted in figure  [ fg::fit ] .
bin corresponds to 143 @xmath3 m in the image plane .
[ fg::fittosource],width=453 ]     versus @xmath18 for the @xmath24  td data point .
[ fg::fit],width=529 ]    data points are taken at three different pressures ( 50 , 75 and 150  torr ) and various drift voltages ( 1 - 5  kv ) . in regions where @xmath16 values overlap ,
measurements are averaged in 1  td bins of @xmath16 .
results are shown in figure  [ fg::d / mu result ] and listed in table  [ tb::d / mu result ] .
.measured values of @xmath9 .
errors are dominated by systematic uncertainities that are listed in table  [ tb::syst ] .
[ tb::d / mu result ] [ cols=">,>",options="header " , ]     we compare this result with previous measurements in figure  [ fg::eta ] , and we find that and we find that our result is consistent with zero and values from ref  @xcite , but inconsistent with the values from  @xcite .    , width=491 ]    ) as a function of reduced electric field ( @xmath16 ) compared with previous measurements .
[ fg::eta],width=491 ]    the reported value of @xmath25 is affected by the uncertainty of the pressure gauge ( 2% ) and the temperature sensor ( 2% ) .
we make measurements within a few hours of a new fill with cf@xmath0 gas , so the effect of outgasing is expected to be small .
this is corraborated with previously mentioned stability in @xmath9 and detector gain over several days .
we confirm that the uncertainty in the spectrum model has negligible impact on the attenuation measurement by fitting the spectrum around the full - absorption peak with a single gaussian and observing a change in the peak position of less than 1% .
we also neglect the error due to imperfect collimation of x - ray source , as this does not change the mean of the drift length .
we add all systematic errors in quadrature .
we have measured the average energy ( @xmath9 ) and attenuation rate of electrons ( @xmath25 ) drifting in cf@xmath0 gas at operating conditions of low - pressure tpc s ( @xmath26  td ) .
discrepancies in previous measurements of these parameters are an obstacle in designing a large volume tpc that is required for high - sensitivity experiments , such as dark matter searches .
our measurement of the @xmath9 parameter is done by imaging alpha tracks that are placed at different heights in the drift cage .
the results are consistent with measurements in ref .
@xcite , but disagree with ref .
the imaging technique can be applied to measurements of @xmath9 for other gases and mixtures that allow optical readout .
the systematic errors can be reduced by improving detector uniformity and using smaller ccd pixel bins .    the reduced attenuation coefficient ( @xmath25 ) is measured by collecting charge created by a collimated x - ray source that is placed at different drift heights .
we find that the @xmath25 is consistent with zero  a trend predicted by ref .
@xcite , but in disagreement with measurements in ref .
carbon - tetrafluoride ( cf@xmath0 ) is used as a counting gas in particle detectors , but some of its properties that are of interest for large time - projection chambers are not well known . 
 we measure the mean energy , which is proportional to the diffusion coefficent , and the attentuation coefficient of electron propagation in cf@xmath0 gas using a 10-liter dark matter detector prototype of the dmtpc project .    
 dark matter , directional detector , cf4 , carbon - tetrafluoride , tpc , electron diffusion , electron attachment , dmtpc 29.40.cs , 29.40.gx , 95.35.+d