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emission of light fragments ( lf ) from nuclear reactions is an interesting open question .
different reaction mechanisms contribute to their production ; the relative roles of each , and how they change with incident energy , mass number of the target , and the type and emission energy of the fragments is not completely understood .
none of the available models are able to accurately predict emission of lf from arbitrary reactions . however , the ability to describe production of lf ( especially at energies @xmath4  mev ) from many reactions is important for different applications , such as cosmic - ray - induced single event upsets ( seus ) , radiation protection , and cancer therapy with proton and heavy - ion beams .
the cascade - exciton model ( cem ) @xcite version 03.03 and the los alamos version of the quark - gluon string model ( laqgsm ) @xcite version 03.03 event generators in the monte - carlo n - particle transport code version 6 ( mcnp6 ) @xcite , describe quite well the spectra of fragments with sizes up to @xmath0he across a broad range of target masses and incident energies ( up to @xmath1  gev for cem and up to @xmath2  tev / a for laqgsm ) .
however , they do not predict well the high - energy tails of lf spectra heavier than @xmath3he .
most lf with energies above several tens of mev are emitted during the precompound stage of a reaction .
the current versions of the cem and laqgsm event generators do not account for precompound emission of these heavier lf .
the aim of our work is to extend the precompound model in the codes to include such processes , leading to an increase of predictive power of lf - production in mcnp6 .
this entails upgrading the modified exciton model currently used at the preequilibrium stage in cem and laqgsm .
it will also include expansion and examination of the coalescence and fermi break - up models used in the precompound stages of spallation reactions within cem and laqgsm .
extending our models to include emission of fragments heavier than @xmath3he at the precompound stage already gives preliminary results with much better agreement with experimental data .
our models consider that a reaction begins with the intranuclear cascade , referred to as the inc .
the incident particle or nucleus ( in the case of laqgsm ) enters the target nucleus and begins interacting with nucleons , scattering off them and also often creating new particles in the process . the incident particle and all newly created particles
are followed until they either escape from the nucleus , are absorbed , or , for nucleons , reach a threshold energy ( roughly 10 - 30 mev ) and are then considered  absorbed " by the nucleus .
the preequilibrium stage uses the modified exciton model ( mem ) to determine emission of protons , neutrons , and fragments up to @xmath3he from the residual nucleus .
we discuss the mem in more detail below .
this stage can have a highly excited residual nucleus undergoing dozens of exciton transitions and particle emissions .
the preequilibrium stage ends when the residual nucleus is just as likely to have a @xmath5 exciton transition as a @xmath6 exciton transition .    in the evaporation stage
, neutrons and protons in the residual nucleus can  evaporate , " either singly or as fragments .
the cem evaporation stage is modeled with a modification of the furihata s generalized evaporation model code ( gem2 ) @xcite , and includes light fragments up to @xmath7 mg .    during and after evaporation
, the code looks to see if we have an isotope that has @xmath8 and is fissionable . if it is , and there is fission , then the code also allows evaporation from the fission fragments .
there are two models that are not directly parts of this progression : coalescence and fermi break - up .
the inc stage only emits neutrons , protons , and pions ( and other particles , when using laqgsm at high energies ) , so the coalescence model 
coalesces " some of the inc neutrons and protons into larger fragments , by comparing their momenta .
if their momenta are similar enough then they coalesce .
the current coalescence model can only coalesce up to @xmath3he fragments , the same as the preequilibrium stage .
the fermi break - up is an oversimplified multifragmentation model that is fast and accurate for small atomic numbers ; in the current cem model it is used when any residual nucleus or fragment has a mass number less than 13 .
the mem used by cem and laqgsm @xcite calculates @xmath9 , the emission width ( or probability of emitting particle fragment @xmath10 ) as @xmath11 where the partial transmission probabilities , @xmath12 , are equal to @xmath13 eq .
( [ lambdatransmission ] ) describes the emission of neutrons and protons . for complex particles ,
the nuclear level density @xmath14 becomes more complicated and an extra phase - space factor @xmath15 must be introduced : @xmath16    eq .  ( [ gammabeta ] ) for @xmath15 is a only a rough estimation that we improve by parameterizing it over a mesh of residual nuclei energy and mass numbers in our codes @xcite .
as the mem uses a monte - carlo technique to solve the master equations describing the behavior of the nucleus at the preequilibrium stage ( see details in @xcite ) , it is very easy to extend the number of types of possible lf that can be emitted during this stage .
we generalize the mem to consider the possiblity of emission up to 66 types of nucleons and lf , up to @xmath7 mg . as a starting point , for the inverse cross sections , coulomb barriers , and binding energies of all lf we use the approximations adopted by gem2 @xcite .
extending the fermi break - up model to include heavier lf ( up to @xmath17 ) allows us to use it for nuclei with @xmath18 , yielding increased accuracy for reactions with light targets .
below are examples of calculations by cem ( with the expanded fermi breap - up model ) compared to experimental data @xcite .
c only . ]
be target . ]
results from the extended fermi break - up model achieve good agreement with experimental results for these light targets .            expanding the mem to include heavier lf ( up to @xmath7 mg ) yields increased accuracy for several reactions we investigate .
figure  [ p200alli ] compares new simulations from our expanded model with data by machner et al .
@xcite for 200 mev p + @xmath19al .
we also find that the integral spectra for n , p , d , t , @xmath20he , and @xmath3he ( not shown ) , are not significantly impacted by this lf emission expansion .
figure  [ p1200ni ] compares our results for 1200 mev p + @xmath21ni with new data by budzanowski et al .
@xcite .
similar results for different lf spectra are obtained for several other reactions ( see , e.g. , @xcite ) .
our results indicate that expanding the mem to include lf preequilibrium emission significantly increases accuracy of the high - energy spectra compared to experimental data .
extending the cem model to include emission of light fragments ( lf ) heavier than @xmath3he ( up to @xmath7 mg ) in the preequilibrium stage results in significantly improved accuracy compared to experimental data for several reactions , especially in the high - energy tails of the spectra .
future work includes finding a global parametrization for @xmath22 , incorporating the expanded event generators into mcnp6 , adding coalescence of heavier fragments , further exploring fermi break - up , and upgrading the evaporation model .
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the ability to describe production of light fragments ( lf ) is important for many applications , such as cosmic - ray - induced single event upsets ( seus ) , radiation protection , and cancer therapy with proton and heavy - ion beams . 
 the cascade - exciton model ( cem ) and the los alamos version of the quark - gluon string model ( laqgsm ) event generators in the lanl transport code mcnp6 , describe quite well the spectra of fragments with sizes up to @xmath0he across a broad range of target masses and incident energies ( up to @xmath1  gev for cem and up to @xmath2  tev / a for laqgsm ) . 
 however , they do not predict the high - energy tails of lf spectra heavier than @xmath3he well . 
 most lf with energies above several tens of mev are emitted during the precompound stage of a reaction . 
 the current versions of our event generators do not account for precompound emission of lf larger than @xmath0he . 
 the aim of our work is to generalize the precompound model to include such processes , leading to increased predictive power of lf production . 
 extending the model in this way provides preliminary results that have much better agreement with experimental data .