1 Geant4 to Simulate Moderater Thickness to thermalize fast 1 M ev Neutrons

Transcription

1 Syed F. Naeem, hw-13, Phy Geant4 to Simulate Moderater Thickness to thermalize fast 1 M ev Neutrons 1.1 Introduction Nuclear reactors emit fast neutrons and a fundamental question in the design of neutron sheilding is how far from a point or target source does a neutron of some initial energy E 0 could be moderated to some lower energy E [1]. Generally, high energy neutrons are classified with energies above E 100MeV, whereas fast neutrons are between few hundred kev to ten s of MeV. Neutrons are known as slow or thermal neutrons if they have an energy of about E = 1 ev at the room temperature. Between 0.1eV 100keV, 40 where nuclear resonance reactions occur, neutrons are referred to as epithermal neutrons, these epithermal neutrons are of principle interest for fissionable material detection for security purposes [2]. Geant4 simulation could be implemented to track thermal neutrons emitted from the moderater. In order to find the appropriate thickness of the moderater, number of collisions induced by the incoming neutrons are determined, where, Σ collisions = ln ( ) E 0 E 1 + (A 1)2 2A ln ( A 1 A+1 ) (1) Σ collisions =number of collisions, E 0 =incident neutron energy, E =target neutron energy, A =Atomic number of the moderater. Mean-free-path (λ p ) is the probability per unit distance of travel that an electronic collision takes place in the target material [3]. λ p is defined as, where, λ p = 1 ρσ T (cm) (2) C-1

2 ρ =density of target material ( g cm 3 ), σ T =total neutron cross section. The total neutron capture cross section (σ T ) for Hydrogen (H) at 1MeV is 4.26b [4]. Once mean-free-path length and number of collisions are known than thickness of the moderater ( x moderater ) can be calculated by multiplying equations (1) and (2), x moderater = σ T λ p (3) 1.2 Methods and Materials A liquid hydrogen (H) target of 59.68cm following the calculation of equation (3) is used in this simulation to moderate incoming fast 1MeV neutrons. Following process was added in the Physicslist program to capture events produced by the incoming neutrons: if (particlename == neutron ) { //neutron //G4LElastic* elasticmodel = new G4LElastic(); G4NeutronHPorLElastic* elasticmodel = new G4NeutronHPorLElastic(); // define process to handle elastic scattering G4HadronElasticProcess* hadelastproc = new G4HadronElasticProcess(); // register the model for eleastic scatterin hadelastproc > RegisterMe(elasticModel); // add the elastic scattering process to the process manager pmanager > AddDiscreteProcess(hadElastProc); pmanager > AddProcess(new G4StepLimiter, -1,-1,3) } C-2

3 Following simulation was obtained with four runs of incident neutrons of 1 MeV as shown in Figure. 1. Figure 1: Incident fast neutrons of 1MeV being thermalized by the liquid hydrogen moderater. 1.3 Results and Conclusion Geant4 simulation gives pretty good approximation to thermalize fast incoming 1MeV neutron through liquid hydrogen target (58.69cm) as shown in the following table: C-3

4 Calculated Σ collisions G4 Simulated Σ collisions Table 1: Calculated and simulated number of collisions required to thermalize 1MeV neutrons in LH 2 moderater. SteppingV erbose.cc file was modified to fetch the number of collisions required to thermalize the fast 1MeV neutrons and the results are presented in Table-2: //PDG ID For neutrons = 2112 if(ftrack >GetDefinition() >GetPDGEncoding() == 2112 && ftrack >GetVolume() > == Target && ftrack >GetTrackID() == 1 ) { //G4cout outfile << ftrack >GetCurrentStepNumber() << << ((ftrack >GetKineticEnergy()) * ) << ev << ftrack >GetPosition().x() << << ftrack >GetPosition().y()<< << G4endl; } C-4

5 Step No. E neutron after collision (ev ) Table 2: G4 simulated E n (ev ) per collision required to thermalize 1MeV neutrons in LH 2 moderater. References [1] Nellis, W. J., Slowing-down distances and times of 0.1 to14 MeV neutrons in hydrogenous materials, Lawrence Livermore Laboratory, University of California, Livermore, CA 94550, 1976 [2] Leo. W. R., Techniques for Nuclear and Particle Physics Experiments, New York: Springer-Verlag, 1994 [3] Turner, J. E., Atoms, Radiation, and Radiation Protection, New York: Wiley and Sons, Inc, 1995 [4] Rinard, P., Neutron Interactions with Matter, Los Alamos National Lab (LANL) [5] Knoll. G. F., Radiation Detection and Measurements, John Wiley and Sons, Inc., 2000 [6] LBL/Lund ENSDF Viewer, C-5