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heavy ion collisions at intermediate energy ( 20 - 100 mev / nucleon ) feature several mechanisms , such as fusion , deep inelastic collisions ( dic ) and direct reactions present at low energy , or incomplete fusion and multifragmentation , characteristic of higher energies @xcite . through heavy - ion collisions , very excited nuclei can be produced offering the possibility to study nuclei far from their ground state conditions . the investigation of the thermodynamical properties of nuclear matter , such as temperature , density , excitation energy ,
can then be carried out .
there is intense theoretical and experimental research surrounding the debate on the presence of a liquid - gas phase transition at the origin of the multifragmentation process @xcite-@xcite . at low excitation energies ,
the decay of hot nuclei follows the predictions of the statistical model for a compound nucleus , and the deexcitation occurs by binary sequential decays @xcite,@xcite . for systems excited at higher energy around 3
mev / nucleon @xcite , the multifragmentation channel opens which is interpreted as a manifestation of a liquid - gas phase transition @xcite .
many experimental clues have been collected recently @xcite,@xcite on multifragmentation and its relation to phase transition . nonetheless the question is still debated .
disentangling binary sequential decay from multifragmentation close to the threshold should help to better define the thermodynamical properties mentioned above .    in this aim
we will study the decay of hot nuclei formed in the @xmath5 reaction at @xmath2 mev / nucleon @xcite . using the backtracing technique @xcite-@xcite
the data will be compared with the predictions of two codes : gemini @xcite for binary sequential decay , and the statistical multifragmentation model ( smm ) @xcite for multifragmentation .
the reaction dynamics was simulated by boltzmann - nordheim - vlasov ( bnv ) transport model simulations which describe the evolution of the one - body distribution function according to the nuclear mean field and including the effects of two - body collisions with the test - particle method @xcite , @xcite .
we used a soft equation of state ( @xmath6 mev ) and a free nucleon - nucleon collision cross - section @xmath7 .
the time evolution of the density contour in the reaction plane for various impact parameters was simulated @xcite .
those simulations @xcite predict the formation of a single source by incomplete fusion for small impact parameters ( @xmath8 fm ) . for larger impact parameters the mechanism showed a binary character with formation of two sources , quasi - projectile ( qp ) and quasi - target ( qt ) . in this paper we will study the decay of incomplete fusion and qp sources .
collisions of @xmath0 ions at 30 mev / nucleon with a @xmath9 thick @xmath1 target were studied at the ganil facility using the indra multidetector .
indra is constituted by 324 independent telescopes on 16 rings : the telescopes covering the polar angular range from @xmath10 to @xmath11 are comprised of an ionization chamber , a 300 @xmath12 m thick silicon detector and a csi(tl ) scintillator .
those covering the range from @xmath11 to @xmath13 are comprised of two layers : an ionization chamber and a csi(tl ) crystal @xcite-@xcite .
the reaction @xmath14 was measured during a campaign of experiments for which indra was coupled to the chimera ( @xcite - @xcite ) first ring , which covered the angular range from @xmath15 to @xmath10 , and some indra modules in the horizontal plane were replaced by silicon strip detectors .
however , for the present reaction study , none of these additional detectors were used .
indra detects charged particles and fragments with an efficiency close to @xmath16 of the whole solid angle and a high granularity in order to reduce double counting ( down to @xmath17 ) .
the identification thresholds are low ( @xmath18 mev / nucleon ) .
indra can measure ion charge and energy in a wide range and it can resolve masses up to z=4 .
the energy calibration of the silicon detectors and ionization chambers for heavy fragments was obtained by elastic scattering of ar , ni and xe beams on a gold target at incident energies from 7 to 9 mev / nucleon . for the scintillators
the energy calibration was accomplished with secondary beams of hydrogen and helium isotopes at different energies .
the charge identification was realized by means of a seven parameter fit of the @xmath19-@xmath20 matrices @xcite which well reproduces the form of the lines for each atomic number , @xmath21 .
unit charge resolution was obtained for all nuclei produced in this reaction .
the data for the @xmath5 at 30 mev / nucleon were recorded with an acquisition trigger requiring at least 5 fired telescopes .
the calculated reaction cross section is @xmath22 barns @xcite .
the total measured cross - section , calculated from the target thickness , integrated beam current , and total number of recorded events , corrected for dead time , is @xmath23 barns .
the lower experimental value is due to the lack of detection at forward angles ( @xmath24 ) and , above all , to the acquisition trigger condition which eliminates the most peripheral collisions . for this reaction ,
the available center of mass energy is @xmath25 mev and the projectile velocity is @xmath26 cm / ns . the grazing angle is @xmath27 .
in order to select well - detected events , we used the @xmath28 correlations ( see figure  [ mom ] ) , the total parallel momentum versus the total charge on an event by event basis . for the analysis , only the events with a total parallel momentum greater than @xmath29 of the beam momentum were kept , @xmath30 mev / c .
the events were sorted as a function of two observables correlated with the impact parameter @xcite : @xmath32 , the total transverse energy , and @xmath31 , the total charged particle multiplicity .
three regions are defined according to cuts made perpendicular to the ridge line of this correlation , which we refer to as t1 , t2 and t3 ( see figure  [ molty ] ) .
the cuts were made in order to retain approximately the same number of events in each region .
events in the t1 region correspond to less dissipative ( peripheral ) collisions , while those in t3 correspond to more dissipative ( central ) collisions .
table  [ expsorg ] shows the corresponding average values of the total transverse energy , the total multiplicity , the residue ( the biggest fragment ) velocity and charge in the three regions .       in perspective one should understand why the alpha particle multiplicity is underestimated by gemini
this could be connected to the dependence of the level density parameter @xmath33 from the excitation energy ( @xcite,@xcite ) . in the present work ,
the lestone parameterization was used , but there are different formulae describing this dependence of @xmath33 that could be used alternatively in the backtracing .    in order to better enhance the differences between the two models , an attempt could also be done using the backtracing of dynamical observables , more sensitive to the coulomb effects @xcite .
one should however recall that some of the dynamical observables ( like velocities for example ) are mass dependent .
this addresses the problem of the choice of the mass parameterization previously evoked . on this subject
, we may hope to gain some useful insight by upgrading in the mass identification of existing @xmath34 devices like chimeraps or by new generation projects like fazia ( four 4@xmath35 a z ion array )  @xcite .
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fazia website : http://fazia.in2p3.fr
we analyzed the @xmath0 on @xmath1 reaction at @xmath2 mev / nucleon in the aim of disentangling binary sequential decay and multifragmentation decay close to the energy threshold , i.e. @xmath3 mev / nucleon . using the backtracing technique applied to the statistical models gemini and smm we reconstruct simulated charge , mass and excitation energy distributions and compare them to the experimental ones . 
 we show that data are better described by smm than by gemini in agreement with the fact that multifragmentation is responsible for fragment production at excitation energies around @xmath4 mev / nucleon .    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    
 ,    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    -0.15 cm    backtracing technique , multifragmentation , binary sequential , excitation energy , smm , gemini 25.70.-z 25.70.pq 24.10.pa