MCNP6 simulation of light and medium nuclei fragmentation at intermediate energies
aa r X i v : . [ nu c l - t h ] A ug MCNP6 simulation of light and mediumnuclei fragmentation at intermediateenergies
Stepan G. Mashnik and Leslie M. Kerby , Los Alamos National Laboratory, Los Alamos, NM 87545, USA University of Idaho, Moscow, Idaho 83844-4264, USA
Abstract
Fragmentation reactions induced on light and medium nuclei byprotons and light nuclei of energies around 1 GeV/nucleon and be-low are studied with the Los Alamos transport code MCNP6 andwith its CEM03.03 and LAQGSM03.03 event generators. CEM andLAQGSM assume that intermediate-energy fragmentation reactionson light nuclei occur generally in two stages. The first stage is theintranuclear cascade (INC), followed by the second, Fermi breakupdisintegration of light excited residual nuclei produced after the INC.CEM and LAQGSM account also for coalescence of light fragments(complex particles) up to He from energetic nucleons emitted duringINC. We investigate the validity and performance of MCNP6, CEM,and LAQGSM in simulating fragmentation reactions at intermediateenergies and discuss possible ways of further improving these codes.
Fragmentation reactions induced by protons and light nuclei of energiesaround 1 GeV/nucleon and below on light target nuclei are involved indifferent applications, like cosmic-ray-induced single event upsets (SEU’s),radiation protection, and cancer therapy with proton and ion beams, amongothers. It is impossible to measure all nuclear data needed for such applica-tions; therefore, Monte Carlo transport codes are usually used to simulate1mpacts associated with fragmentation reactions. It is important that avail-able transport codes simulate such reactions as well as possible.The Los Alamos Monte Carlo transport code MCNP6 [1] uses by defaultthe latest version of the cascade-exciton model (CEM) as incorporated inits event generator CEM03.03 to simulate fragmentation of light nuclei atintermediate energies for reactions induced by nucleons, pions, and photons,and the Los Alamos version of the quark-gluon string model (LAQGSM)as implemented in the code LAQGSM03.03 (see [2] and references therein)to simulate fragmentation reactions induced by nuclei and by particles atenergies above ∼ . He, and He via final-state interactions among emittedcascade nucleons, already outside of the target. The subsequent relaxationof the nuclear excitation is treated in terms of an improved version of themodified exciton model of preequilibrium decay followed by the equilibriumevaporation/fission stage of the reaction. But if the residual nuclei afterthe INC have atomic numbers with
A <
13, CEM and LAQGSM use theFermi breakup model to calculate their further disintegration instead of us-ing the preequilibrium and evaporation/fission models. Thus, for targetswith
A <
13, reactions are assumed to occur only in two stages.The “standard” version of CEM and LAQGSM account for possible mul-tiple emission of only n, p, d, t, He, and He during the preequilibrium stageof reactions (see Ref. [2]). Their latest, “F”, version (see Refs. [3, 4, 5])considers a possibility of preequilibrium emission of light fragments (LF)heavier than He, up to Mg. It also simulates coalescence of LF heavierthan He, up to A = 7, in CEM03.03F (see [3, 4, 5]), and up to A = 12, inLAQGSM03.03F (see [6]).In recent years, MCNP6, with its CEM and LAQGSM event generators,has been extensively validated and verified (V&V) against a large variety ofnuclear reactions on both thin and thick targets (see, e.g. Refs. [3] - [8] andreferences therein). In Ref. [3], it was tested specifically on fragmentationof light nuclei at intermediate energies. Here, we present a few results fromour recent work [3] and investigate further the performance of MCNP6,CEM, and LAQGSM in simulating fragmentation reactions at intermediateenergies and discuss possible ways of further improving these codes. Results and conclusion
Figs. 1, 2, 3, and 5 show examples of fragmentation reactions on light nucleisimulated by our codes. Figs. 6 and 4 shows examples for medium targets, Ca and nat
Ag. Many more similar results, their discussion, and usefuldetails can be found in Refs. [3] - [9] and references therein.
10 100 1000 T p (MeV) -3 -2 -1 C r o ss s e c t i on ( m b ) O(p,X) C Exp. data: nat
OExp. data: OCEM03.03, A
Fermi =12CEM03.03, A
Fermi =16CEM03.03, A
Fermi =14MCNP6, A
Fermi =12
Figure 1: Excitation function for theproduction of C from p + O calcu-lated with CEM03.03 using the “stan-dard” version of the Fermi breakupmodel ( A F ermi = 12) and with cut-off values for A F ermi of 16 and 14, aswell as with MCNP6 using CEM03.03( A F ermi = 12) compared with experi-mental data, as indicated. Experimen-tal data are from the T16 Lib compila-tion [10] (see details in [3]).
T (MeV) -6 -5 -4 -3 -2 -1 d σ / d T / d Ω ( m b / M e V / s r)
200 MeV p + Al → Li + ...
20 deg x 10
45 deg x 10
60 deg x 10
90 deg x 10110 deg x 1CEM03.03CEM03.03.F
Figure 2: Comparison of experimental Li spectra at 20, 45, 60, 90, and 110degrees by Machner et al. [11] (sym-bols) with calculations by the unmod-ified CEM03.03 (dashed histograms)and results by CEM03.03.F (solid his-tograms), as indicated (see more detailsin [3]).
Our results indicate thatMCNP6 using CEM03.03 andLAQGSM03.03 simulates fragmen-tation reactions on light andmedium-light nuclei at intermediate energies well, in a satisfactory agree-ment with experimental data. The recent “F” version of codes (see Refs.[3] – [6]) is even better, as it allows us to describe emission of energetic LFfrom practically arbitrary reactions.However, MCNP6 is not yet ready to predict well heavy fragments fromreactions with heavier nuclei, with mass numbers A ∼
500 1000 1500 2000 2500 3000 3500 p t (MeV/c) -6 -5 -4 -3 -2 -1 E d σ / d p ( m b / s r / G e V / c - )
800 MeV/A Ne + Ne → t + ... Exp. data: 30 deg (x1)45 deg (x10 -1 )60 deg (x10 -2 )90 deg (x10 -3 )130 deg (x10 -4 )LAQGSM03.03LAQGSM03.03, coal1LAQGSM03.03, coal2 Figure 3: Comparison of measured[12] t spectra at 45, 60, 90, and 130degrees from 800 MeV/nucleon Ne + Ne with calculations by LAQGS03.03using its “standard” version of the co-alescence model ( p = 0 .
108 GeV/cfor t and He; dotted lines) and withmodified values of p labeled in legendas “coal1” (dashed lines) and “coal2”(solid lines), as indicated in legend anddiscussed in detail in Ref. [3].Figure 4: Comparison of experimen-tal data by Green et al. [13] (cir-cles) for the production of Li at anangle of 60 ◦ from the reaction 480MeV p + nat Ag, with results by stan-dard CEM03.03 (dot-dashed line) fromCEM03.03F without coalescence ex-pansion (solid line) and CEM03.03Fwith coalescence expansion (dashedline) (see details in [5, 19]).
One way to approach this problemwould be to employ after the INCstage of reactions a fission-likesequential-binary-decay model, like the code GEMINI by Charity et al. [16]to describe the compound nuclear decay. In our case, this means sep-arately merging CEM and LAQGSM with GEMINI. Actually, we alreadyhave done so more than a decade ago, producing the “G” versions of CEMand LAQGSM we had at that time (see, e.g. , Ref. [18] and referencestherein).Another way to address this problem is to implement in CEM andLAQGSM the Statistical Multifragmentation Model (SMM) by Botvina etal. [17]. Thus, we would consider multifragmentation as a mode competitiveto evaporation of particles and light fragments, when the excitation energy E ∗ of a compound nucleus produced after the preequilibrium stage of a re-action is above a certain value, E ∗ tr , e.g., E ∗ tr = 2 × A MeV, as we did inthe “S” versions of CEM03.01 and LAQGSM03.01 (see, e.g., Ref. [18] andreferences therein).As of today, neither the “S” nor the “G” versions of CEM and LAQGSMhave been implemented in MCNP6. We plan to incorporate them in ourvent generators used by MCNP6 after we tune several parameters in SMM
T (MeV) -6 -5 -4 -3 -2 -1 d σ / d T / d Ω ( m b / M e V / s r) C → He (35 o ) + ... Figure 5: He spectra (at 35 ◦ ) from1.2/1.9/2.5 GeV p + C measuredby M. Fidelus of the PISA collabora-tion [14] (symbols) with calculations byMCNP6 using CEM03.03 (see details in[5]). and GEMINI, that are essentialin chosing the excitation energy(or temperature) of nuclei whenreaction mechanisms change from“usual evaporation”, to binary de-cays described by GEMINI, and/orto multifragmentation simulated with SMM (see details in [19]). -2 B C N O -3 F Ne Na Mg -4 Al Si P S -2 Cl Ar K Ca Neutron excess N-Z C r o ss s e c t i on ( m b ) Figure 6: Measured cross sec-tions for Ca fragmentation on Be at 140 MeV/nucleon [15]compared with LAQGSM03.03predictions (see details in [9]).
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