Hadronic Physics II. University of Pennsylvania Geant4 Tutorial 17 May 2011 Dennis Wright. Geant April 2011, CMRP, UOW

Transcription

1 Hadronic Physics II University of Pennsylvania Geant4 Tutorial 17 May 2011 Dennis Wright Geant4 9.4

2 Outline Low energy neutrons Ion-ion collisions Radioactive decay High energy models (QCD strings) 2

3 Low Energy Neutron Physics Below 20 MeV incident energy, Geant4 provides several models for treating neutron interactions in detail The high precision neutron models (NeutronHP) are datadriven and depend on a large database of cross sections, etc. the G4NDL database is available for download from the Geant4 web site elastic, inelastic, capture and fission models all use this isotope-dependent data There are also models to handle thermal scattering from chemically bound atoms 3

4 Geant4 Neutron Data Library (G4NDL) Contains the data files for the high precision neutron models includes both cross sections and final states Data are derived from the following evaluated data libraries Brond-2.1, CENDL2.2, EFF-3, ENDF/B-VI, FENDL/E2.0, JEF2.2, JENDL-FF, JENDL-3, MENDL-2 evaluated libraries are subsets of existing data that have been carefully sifted and examined The format of G4NDL is similar but not identical to that of ENDF difficult job to combine data from all the above libraries into one format 4

5 Evaluated Nuclear Data File-6 (ENDF) ENDF has two meanings data formats and procedures how to write nuclear data files how to use them name of libraries from US nuclear data projects ENDF/B-VI.8 : 313 isotopes, 5 isomers, 15 elements ENDF/B-VII.0 : 400 isotopes Geant4 has not yet migrated to ENDF/B-VII The latest version of G4NDL (3.14) concentrates mainly on translation from ENDF library Geant4 no longer does evaluations of these data 5

6 G4NeutronHPElastic Handles elastic scattering of neutrons by sampling differential cross section data interpolates between points in the cross section tables as a function of energy also interpolates between Legendre polynomial coefficients to get the angular distribution as a function of energy scattered neutron and recoil nucleus generated as final state 6

7 G4NeutronHPInelastic Currently supports 34 inelastic final states + n s (discrete and continuum) n (A,Z) -> (A-1, Z-1) n p n (A,Z) -> (A-3, Z) n n n n N (A,Z) -> (A-4, Z-2) d t... Secondary distribution probabilities isotropic emission discrete two-body kinematics N-body phase space continuum energy-angle distributions (in lab and CM) 7

8 G4NeutronHPCapture Neutron capture on a nucleus produces photons described by either the number of photons (multiplicity), or photon production cross sections Photon spectra are either discrete emission, using either cross sections or multiplicities from data libraries, or continuous, with the spectrum calculated according to tabulated parameters f(e -> E ) = i p i (E) g i (E -> E ) where p i and g i come from the libraries 8

9 G4NeutronHPFission Currently only uranium fission data are available in Geant4 First chance, second chance, third chance and fourth chance fission taken into account Resulting neutron energy distributions are implemented in six different ways as a function of incoming and outgoing neutron energy as a Maxwell spectrum general evaporation spectrum evaporation spectrum energy-independent Watt spectrum Madland-Nix spectrum 9

10 10

11 11

12 Including HP Neutrons in Elastic scattering Your Physics List G4HadronElasticProcess* thenep = new G4HadronElasticProcess; // the cross sections G4NeutronHPElasticData* thenedata = new G4NeutronHPElasticData; thenep->adddataset(thenedata); // the model G4NeutronHPElastic* thenem = new G4NeutronHPElastic; thenep->registerme(thenem); neutmanager->adddiscreteprocess(thenep); 12

13 G4NeutronHPorLE models The high precision neutron models do not cover all elements/isotopes no data data, but no evaluation of data evaluated data, but not included in G4NDL Geant4 application must have a model under all circumstances, otherwise a fatal error appears G4NeutronHPorLE models developed to solved this problem HP models used if there is data if not, LEP (GHEISHA-style) models used elastic, inelastic, capture and fission provided 13

14 Thermal neutron scattering from Chemically Bound Atoms At thermal neutron energies, atomic motion, vibration and rotation of bound atoms affect the neutron scattering cross section, as well as the angular distribution of secondary neutrons The energy loss (or gain) of such scattered neutrons may be different from those from interactions with unbound atoms Original HP models included only individual Maxwellian motion of the target nucleus (free gas model) New behavior handled by model and cross section classes: G4HPThermalScatteringData, and G4HPThermalScattering 14

15 Ion-Ion Inelastic Scattering Up to now we've considered only hadron-nucleus interactions, but Geant4 has five different nucleus-nucleus collision models G4BinaryLightIon G4WilsonArasion/G4WilsonAblation G4EMDissociationModel G4QMD G4Incl/G4Abla Also provided are several ion-ion cross section data sets Currently no ion-ion elastic scattering models provided 15

16 G4BinaryLightIonReaction This model is an extension of the G4BinaryCascade model discussed earlier The hadron-nuclear interaction part is identical, but the nucleus-nucleus part involves: preparation of two 3D nuclei with Woods-Saxon or harmonic oscillator potentials lighter nucleus always assumed to be the projectile nucleons in the projectile are entered with their positions and momenta into the initial collision state nucleons are interacted one-by-one with the target nucleus, using the original Binary cascade model 16

17 G4WilsonAbrasion and G4WilsonAblation A simplified macroscopic model nucleus-nucleus collisions based largely on geometric arguments faster than Binary cascade or QMD models, but less accurate Two models are used together G4WilsonAbrasion handles the initial collision in which a chunk of the target nucleus is gouged out by the projectile nucleus G4WilsonAblation handles the de-excitation of the resulting fragments Based on the NUCFRG2 model (NASA TP 3533) Can be used up to 10 GeV/n 17

18 18

19 G4EMDissociation Model Electromagnetic dissociation is the liberation of nucleons or nuclear fragments as a result of strong EM fields exchange of virtual photons instead of nuclear force Useful for relativistic nucleus-nucleus collisions where the Z of the nucleus is large The G4EMDissociation model and cross sections are an implementation of the NUCFRG2 model (NASA TP 3533) Can be used up to 100 TeV 19

20 INCL/ABLA Ion-ion Model 20

21 G4QMD Model BinaryLightIonReaction has some limitations neglects participant-participant scattering uses simple time independent nuclear potential => imposes small A limitation for target or projectile binary cascade itself contributes energy limitation Solution is QMD (quantum molecular dynamics) model an extension of the classical molecular dynamics model treats each nucleon as a gaussian wave packet propagation with scattering which takes Pauli principle into account can be used for high energy, high Z collisions 21

22 22

23 23

24 Putting QMD in Your Physics List G4HadronInelasticProcess* ioninel = G4HadronInelasticProcess( ioninelastic, G4GenericIon::G4GenericIon()); // Cross sections G4TripathiCrossSection* tripcs = new G4TripathiCrossSection; G4IonsShenCrossSection* shencs = new G4IonsShenCrossSection; ioninel->adddataset(shencs); ioninel->adddataset(tripcs); // Assign model to process G4QMDReaction* theqmd = new G4QMDReaction; ioninel->registerme(theqmd); G4ProcessManager* pman = G4GenericIon::G4GenericIon() ->GetProcessManager(); pman->adddiscreteprocess(ioninel); 24

25 Ion-ion Cross Sections Cross section data sets available from 10 MeV/N to 10 GeV/N Tripathi, TripathiLight (for light nuclei) Kox Shen Sihver These are empirical and parameterized cross section formulae with some theoretical insight G4GeneralSpaceNNCrossSection was prepared to assist users in selecting the appropriate cross section formula 25

26 Ion-ion Cross Sections 26

27 Radioactive Decay Simulates the decay of radioactive nuclei Handles, +, decay, electron capture (EC) and isomeric transition (IT) It is a data-driven model using the Evaluated Nuclear Structure Data Files (ENSDF), containing: nuclear half-lives nuclear level structure for the parent or daughter nuclide decay branching ratios energy of the decay process (Q) If daughter nuclide of a decay is an excited isomer, its prompt de-excitation is handled by G4PhotonEvaprotation 27

28 Biasing Radioactive Decay Analog sampling is the default Many options for biased sampling also available increased frequency of decay within a given time window all branching ratios for a decay can be sampled with equal probability a single parent nucleus can be decayed a specified number of times within an event etc. 28

29 Radioactive Decay in a Physics List G4RadioactiveDecay* rd = new G4RadioactiveDecay; G4ProcessManager* pmanager = G4ParticleTable::GetParticleTable()->FindParticle( GenericIon ) ->GetProcessManager(); pmanager->addprocess(rd); pmanager->setprocessordering(rd, idxpoststep); pmanager->setprocessordering(rd, idxatrest); Only have to do this once, even though there are thousands of isotopes Note: if you want tritium decay, make sure G4Triton is NOT created in your physics list 29

30 QCD String Models For incident p, n,, K Also for high energy g when CHIPS model is connected ~10 GeV < E < ~TeV Model handles: selection of collision partners (hadrons and nucleons) splitting nucleons into quarks and di-quarks formation and excitation of strings string hadronization (how strings produce more hadrons) Damaged nucleus remains. Another Geant4 model must be added for nuclear fragmentation and de-excitation 30

31 String Model Algorithm Build up 3-D model of nucleus Large gamma factor collapses nucleus to 2 dimensions Calculate impact parameter with all nucleons Calculate hadron-nucleon collision probabilities use gaussian density distributions for hadrons and nucleons Sample number of strings exchanged in each collision String formation and fragmentation into hadrons 31

32 Longitudinal String Fragmentation QCD string extends between quarks in hadron and nucleon As string is stretched, it breaks. At each break, a new quark-antiquark pair is inserted according to the ratios u : d : s : diquark = 1 : 1 : 0.27 : 0.1 each quark-antiquark pair represents a new hadron Created hadron gets longitudinal momentum from sampling of fragmentation functions fragmentation functions based on parameterizations of DIS data Transverse momentum distribution taken to be gaussian < p t 2 > = 0.25 GeV 2 32

33 Quark-Gluon String Model One of the Geant4 QCD string models valid from 20 GeV - ~TeV In this model, two or more strings are stretched between the partons (quarks or gluons) within the hadrons technically, it is the cutting of a Pomeron exchanged between the hadrons that forms the strings Parton interaction leads to color coupling of quarks sea quarks included as well as valence quarks 33

34 Fritiof Fragmentation Model Alternative QCD string fragmentation model valid from 3 GeV - ~TeV This model applies at much lower energies due to ability to handle lower string masses Reggeon cascade Model uses a different set of fragmentation functions and relies more on fitted parameters than QGS model 34

35 Diffraction Both QGS and FTF models include diffraction projectile or target both break up into hadrons amount of diffraction is adjusted empirically 35

36 QGS Results at 400 GeV/c p Ta -> + X 36

37 FTF Results at 400 GeV/c p Ta -> + X 37

38 Summary Geant4 provides two high precision neutron models for energies below 20 MeV G4NeutronHP, G4NeutronHPorLE Five ion-ion collision models are available + cross sections BinaryLightIon, QMD, Wilson Abrasion/Ablation, Electromagnetic dissociation, INCL/ABLA Radioactive decay model supports the basic decay modes plus biasing of decays Two QCD string models are available for the highest energies in your physics list QGS, FTF 38