20 February 2002Geant4 Users' Workhsop, SLAC1 Low-Energy Electromagnetic Processes in P. Nieminen (ESA-ESTEC)

20 February 2002Geant4 Users' Workhsop, SLAC2 Contents 1.Introduction 2.Electron and photon low-energy electromagnetic processes in Geant4 3.Hadron and ion low-energy electromagnetic processes in Geant4 4.Conclusions

Low-Energy e.m. applications Mineralogical surveys of Solar System bodies Spacecraft internal charging analyses Dark matter search, Fundamental physics Radiation effects analysis in X-and  -ray astrophysical observatories Radiotherapy, brachytherapy Neutrino physics Antimatter experiments High Energy Physics

20 February 2002Geant4 Users' Workhsop, SLAC4 Electron and photon processes Energy cut-offs  Geant keV  EGS4, ITS3.01 keV  Geant4 “standard models” - Photoelectric effect10 keV - Compton effect10 keV - Bremsstrahlung1 keV - Ionisation (  -rays)1 keV - Multiple scattering1 keV  Geant4 low-energy models250 eV

20 February 2002Geant4 Users' Workhsop, SLAC5 X-Ray Surveys of Solar System Bodies Induced X-ray line emission: indicator of target composition (~100  m surface layer) Cosmic rays, jovian electrons Geant3.21 ITS3.0, EGS4 Geant4 Solar X-rays, e, p Courtesy SOHO EIT C, N, O line emissions included

20 February 2002Geant4 Users' Workhsop, SLAC6 Features of electron and photon models  Validity range from 250 eV to 100 GeV  Elements Z=1 to 100  Data bases: - EADL (Evaluated Atomic Data Library), - EEDL (Evaluated Electrons Data Library), - EPDL97 (Evaluated Photons Data Library) from LLNL, courtesy Dr. Red Cullen. A version of libraries especially formatted for use with Geant4 available from Geant4 distribution source.

20 February 2002Geant4 Users' Workhsop, SLAC7  Compton scattering  Photoelectric effect  Rayleigh effect  Pair production  Bremsstrahlung  Ionisation  Atomic relaxation  Polarised processes …in preparation:  Auger effect  Positrons Processes included: New physics

20 February 2002Geant4 Users' Workhsop, SLAC8 OOAD Rigorous adoption of OO methods  openness to extension and evolution Extensive use of design patterns Booch methodology Technology as a support to physics

20 February 2002Geant4 Users' Workhsop, SLAC9 Calculation of total cross sections where E 1 and E 2 are respectively the lower and higher energy for which data (  1 and  2 ) is available. Mean free path for a given process at energy E, with n i the atomic density of the ith element contributing to the material composition

20 February 2002Geant4 Users' Workhsop, SLAC10  Energy distribution of the scattered photon according to Klein- Nishina formula multiplied by scattering functions F(q) from EPDL97 data library.  The effect of scattering function becomes significant at low energies (suppresses forward scattering)  Angular distribution of the scattered photon and the recoil electron also based on EPDL97. Compton scattering Rayleigh effect  Angular distribution:  (E,  )=[1+cos 2  F 2 (q), where F(q) is the energy-dependent form factor obtained from EPDL97.

20 February 2002Geant4 Users' Workhsop, SLAC11  The secondary e - and e + energies sampled using Bethe-Heitler cross sections with Coulomb correction  e - and e + assumed to have symmetric angular distribution  Energy and polar angle sampled w.r.t. the incoming photon using Tsai differential cross section  Azimuthal angle generated isotropically  Choice of which particle in the pair is e - or e + is made randomly Gamma conversion Photoelectric effect  Subshell from which the electron is emitted selected according to the cross sections of the sub-shells. De-excitation via isotropic fluorescence photons; transition probabilities from EADL.

20 February 2002Geant4 Users' Workhsop, SLAC12 Photons

20 February 2002Geant4 Users' Workhsop, SLAC13 Electron bremsstrahlung Continuous energy loss Gamma ray production F(x) obtained from EEDL. At high energies: Direction of the outgoing electron the same as that of the incoming one; angular distribution of emitted photons generated according to a simplified formula based on the Tsai cross section (expected to become isotropic in the low-E limit)

20 February 2002Geant4 Users' Workhsop, SLAC14 Electron ionisation  The  -electron production threshold T c is used to separate the continuous and discrete parts of the process  Partial sub-shell cross sections  s obtained by interpolation of the evaluated cross section data in the EEDL library  Interaction leaves the atom in an excited state; sampling for excitation is done both for continuous and discrete parts of the process  Both the energy and the angle of emission of the scattered electron and the  -ray are considered  The resulting atomic relaxation treated as follow-on separate process

20 February 2002Geant4 Users' Workhsop, SLAC15 Electron ionisation Continuous energy loss  -electron production Value of coefficient A for each element is obtained from fit to EEDL data for energies available in the database B s is the binding energy of sub-shell s

20 February 2002Geant4 Users' Workhsop, SLAC16 Atomic relaxation  EADL data used to calculate the complete radiative and non- radiative spectrum of X-rays and electrons emitted  Auger effect and Coster-Kronig effect under development; fluorescent transitions implemented  Transition probabilities explicitly included for Z=6 to 100  K, L, M, N, and some O sub-shells considered. Transition probabilities for sub-shells O, P, and Q negligible (<0.1%) and smaller than the precision with which they are known  For Z=1 to 5, a local energy deposit corresponding to the binding energy B of an electron in the ionised sub-shell simulated.  For O, P, and Q sub-shells a photon emitted with energy B

20 February 2002Geant4 Users' Workhsop, SLAC17 Domain decomposition leads to a design open to physics extensions Atomic relaxation

20 February 2002Geant4 Users' Workhsop, SLAC18

20 February 2002Geant4 Users' Workhsop, SLAC19 Photon attenuation coefficient Comparison with NIST data Standard Standard electromagnetic package Low Energy and Low Energy extensions Fe water

20 February 2002Geant4 Users' Workhsop, SLAC20 Courtesy LIP and IPOFG-CROC (Coimbra delegation of the Portuguese Oncology Institute) 6 MV photon beam Siemens KD2 Thorax slice CT image

20 February 2002Geant4 Users' Workhsop, SLAC21 Polarised Compton Scattering The Klein-Nishina cross section: Where, h 0 : energy of incident photon h : energy of the scattered photon  : angle between the two polarization vectors y O z x     h h   A C

20 February 2002Geant4 Users' Workhsop, SLAC22 Angular distribution of scattered radiation composed of two components :  ’  and  ’   with respect to AOC plane  ’   ’  CO  A h  ’   x  distribution obtained with the class

20 February 2002Geant4 Users' Workhsop, SLAC23 Test of the distribution: a) Low energy b) High energy Low energy: h o h  h o =>  =1 => a = 0 the distribution reduces to the Thompson distribution => the probability that the two polarization vectors are perpendicular is zero. The distribution function is: where and  = h / h 0. High energy: small  => h  h o => equal to low energy high  : it is possible to demonstrate that b/(a+b) ->0, so in this case the distribution tend to be isotropic.

20 February 2002Geant4 Users' Workhsop, SLAC24 Results Scalar product between the two polarization vectors for three different energies. Upper histograms: Low polar angle  Lower histograms: High polar angle  100 keV 10 MeV1 MeV These distributions are in agreement with the limits obtained previously.

20 February 2002Geant4 Users' Workhsop, SLAC25 Hadron and ion processes Variety of models, depending on energy range, particle type and charge Bethe-Bloch model of energy loss, E > 2 MeV 5 parameterisation models, E < 2 MeV ­ based on Ziegler and ICRU reviews 3 models of energy loss fluctuations ­ Density correction for high energy ­ Shell correction term for intermediate energy ­ Spin dependent term ­ Barkas and Bloch terms ­ Chemical effect for compound materials ­ Nuclear stopping power ­ PIXE included Positive charged hadrons Positive charged ions Negative charged hadrons Scaling: 0.01 <  < 0.05 parameterisations, Bragg peak ­ based on Ziegler and ICRU reviews  < 0.01: Free Electron Gas Model Parameterisation of available experimental data Quantum Harmonic Oscillator Model ­ Effective charge model ­ Nuclear stopping power ­ Model original to Geant4 ­ Negative charged ions: required, foreseen

20 February 2002Geant4 Users' Workhsop, SLAC26 HERMES X-R ay S pectrometer on M ercury P lanetary O rbiter PIXE from solar proton events

20 February 2002Geant4 Users' Workhsop, SLAC27 Algorithms encapsulated in objects Physics models handled through abstract classes Interchangeable and transparent access to data sets Hadrons and ions Open to extension and evolution Transparency of physics, clearly exposed to users

20 February 2002Geant4 Users' Workhsop, SLAC28 Hadron and ion low-energy e.m. extensions Low energy hadrons and ions models based on Ziegler and ICRU data and parameterisations Barkas effect: models for antiprotons

20 February 2002Geant4 Users' Workhsop, SLAC29 Proton energy loss in H 2 O Ziegler and ICRU parameterisations

20 February 2002Geant4 Users' Workhsop, SLAC30 Application examples advanced examples l Five advanced examples developed by the LowE EM WG released as part of the Geant4 Toolkit (support process) Extensive collaboration with Analysis Tools groups l Brachytherapy l Underground physics & radiation background l X-ray fluorescence and PIXE l X-ray telescope  - ray telescope Full scale applications showing physics guidelines and advanced interactive facilities in real-life set-ups GaAs lines Fe lines fluorescence

20 February 2002Geant4 Users' Workhsop, SLAC31 Conclusions  A set of models has been developed to extend the Geant4 coverage of electromagnetic interactions of photons and electrons down to 250 eV, and of hadrons down to < 1 keV  Rigorous software process applied  Wide user community in astrophysics, space applications, medical field, HEP, in the U.S., Europe, and elsewhere  Modularity of Geant4 enables easy extensions and implementation of new models  Further low-energy electromagnetic physics developments and refinements are underway

20 February 2002Geant4 Users' Workhsop, SLAC32 Useful links   