You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
monolithic cmos pixel sensors open new perspectives in fast nano - imaging through single electron direct detection in transmission electron microscopy ( tem ) .
high - voltage electron microscopy , developed and used for high resolution imaging in the late 1970s  @xcite produced advances in spatial resolution , but was abandoned due to the severe displacement damage of the sample .
as the displacement damage threshold is proportional to @xmath2 , there is now much interest in tem of organic samples with energies of 80 - 100  kev , where recent advances in electron optics ensure deep sub - angstrom spatial resolution  @xcite .
for example , the maximum energy transferred by an 80  kev electron to a carbon atom is 15.6  ev , which is below the threshold for knock - on damage .
this makes low energy tem necessary for atomic resolution studies of samples such as single atomic layers of carbon in graphene or carbon nanotubes  @xcite and in biology .
there are two main issues to be considered for imaging with low energy electrons .
the first is the large fluctuations in the energy deposition .
the second is the degradation of the point spread function ( psf ) due to the @xmath3 increase of the electron multiple scattering in the detector .    in an earlier paper  @xcite
, we presented the design of a radiation tolerant cmos pixel cell and investigated the response of 10  @xmath1 m and 20  @xmath1 m pixels to electrons in the energy range 120  kev up to 300  kev for tem . in this letter
we extend that study to lower energies , by investigating the response to 80  kev and 100  kev electrons .
we perform a detailed simulation of the charge deposition and signal formation in the cmos pixel sensor based on the geant-4 program  @xcite using the low energy electromagnetic physics models  @xcite .
the cmos sensor is modelled according to the detailed geometric structure of oxide , metal interconnect and silicon layers , as specified by the foundry .
electrons are incident perpendicular to the detector plane and tracked through the sensor . for each interaction within the epitaxial layer ,
the ionisation point position and the amount of energy released are recorded .
charge collection in the sensor is simulated with pixelsim , a dedicated digitisation module  @xcite , developed in the marlin c++ reconstruction framework  @xcite , originally deployed for the international linear collider particle physics project .
the processor starts from the ionisation points generated along the particle trajectory by geant-4 and stored in lcio format  @xcite .
the pixelsim simulation models diffusion of charge carriers from their production point in the epitaxial layer to the collection diode .
this provides us with full simulation of the response of each individual pixels in the detector matrix , including sensor geometry and electronics noise effects , which can be processed through the same analysis chain as the experimental data .
the simulation has a single free parameter , the diffusion parameter @xmath4 , used to determine the width of the charge carrier cloud .
its value is extracted from data by a @xmath5 fit to the pixel multiplicity in the clusters of 1.5  gev electrons since , at this energy , the multiple scattering contribution to the charge distribution is negligible .
we find @xmath4 = ( 16.3 @xmath0 1.4 )  @xmath1 m , which agrees well with the diffusion length estimated from the doping in the epitaxial layer and the charge collection time  @xcite .
the detector charge - to - voltage conversion is 0.98  kev / adc count or 26.7  @xmath6/adc count at 6.25  mhz readout frequency , obtained by recording the position of the 5.9  kev full energy peak of a collimated 2.2  mci @xmath7fe source .
we use the titan test column at the national center for electron microscopy ( ncem ) to characterise the detector response to 80  kev and 100  kev electrons and validate the simulation . the signal pulse height in a 3@xmath83 matrix around each seed
pixel having a signal - to - noise in excess of 4.5 is shown in figure  [ fig : landau ] for data and simulation .
the broadening of the energy distribution compared to electrons of higher energy is evident , however the ratio of the landau width to the landau most probable value does not increase significantly compared to that for electrons of higher energy .
we estimate the uncertainty on the number of electrons per pixel that can be reconstructed from the measured pulse height in a single pixel . we simulate a flat field illumination by generating multiple electrons hitting each pixel and reconstruct the pixel pulse height .
this accounts for cross - feed between neighbouring pixels due to charge diffusion and multiple scattering .
we determine the number of electrons on each pixel by dividing the simulated pixel pulse height by the average pulse height induced by a single electron and study the distribution of the reconstructed number of electrons as a function of that simulated .
we find that the relative uncertainties on this number scale from 0.17 ( 0.14 ) for 10 @xmath6/pixel to 0.12 ( 0.10 ) for 20 @xmath6/pixel and to 0.08 ( 0.06 ) for 50 @xmath6/pixel at 80  kev ( 100  kev ) , respectively .
these results are comparable to relative uncertainties of 0.15 , 0.11 , 0.07 obtained for 200  kev electrons .    [ cols="^,^ " , ]     [ tab : psfsim ]    we describe the measured pulse height on the pixel rows across the image projected by the wire with a box function having the same width as the measured wire diameter smeared by a gaussian term , which describes the point spread function .
the contrast factor , i.e.  the ratio of maximum to minimum pulse height levels , for the pixels away from the wire shadow and for those exactly below the wire centre , respectively , are set to those observed in data and we perform a simple 1-parameter @xmath5 fit to extract the gaussian width term , which gives the estimation of the psf .
results are given in table  [ tab : psfsim ] .
a good agreement is found between the measurement and the prediction from simulation .
these results are compared with those obtained at higher energies , presented in  @xcite .
it is interesting to observe how the degradation of the psf at decreasing energies , caused by multiple scattering , reaches a plateau around 120  kev .
this is due to the decrease of the electron range with the particle energy , which limits the distance over which charge can be spread .
in fact , we measure a point spread function value for 80 and 100  kev electrons which is compatible with that measured at higher energies , as predicted by the simulation .
this result is quite encouraging for extending the application of cmos pixel sensors to fast tem imaging of organic and biological samples with low energy electrons .
we wish to thank thomas duden , rolf erni and zhongoon lee .
this work was supported by the director , office of science , of the u.s .
department of energy under contract no .
de - ac02 - 05ch11231 .
m.  battaglia _ et al .
_ , nucl .
instrum .
a * 598 * ( 2009 ) 642 [ arxiv:0811.2833 [ physics.ins-det ] . s.  agostinelli _ et al .
_ , nucl .
instrum .
a * 506 * ( 2003 ) 250 .
s.  chauvie , g.  depaola , v.  ivanchenko , f.  longo , p.  nieminen and m.  g.  pia , _ prepared for chep01 : computing in high - energy physics and nuclear , beijing , china , 3 - 7 sep 2001 _    m.  battaglia , nucl .  instrum .
a * 572 * ( 2007 ) 274 .
f.  gaede , nucl .
instrum .
meth .  a * 559 * ( 2006 ) 177 .
f.  gaede , t.  behnke , n.  graf and t.  johnson , in the _ proc . of 2003 conf . for computing in high - energy and nuclear physics _
( chep 03 ) , la jolla , california , 24 - 28 mar 2003 , pp tukt001 , [ arxiv : physics/0306114 ] .
this letter presents the results of a study of the response of a test cmos sensor with a radiation tolerant pixel cell design to 80  kev and 100  kev electrons . 
 the point spread function is measured to be ( 13.0 @xmath0 1.7 )  @xmath1 m at 100  kev and ( 12.1 @xmath0 1.6 )  @xmath1 m at 80  kev , for 20  @xmath1 m pixels . 
 results agree well with values predicted by a geant-4 and dedicated charge collection simulation .    ,    ,    ,    ,    monolithic active pixel sensor , transmission electron microscopy ;