Low Energy Electromagnetic Processes in P Nieminen ESAESTEC

Low Energy Electromagnetic Processes in P. Nieminen (ESA-ESTEC) http: //www. ge. infn. it/geant 4/low. E/ 20 February 2002 Geant 4 Users' Workhsop, SLAC 1

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

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

Electron and photon processes Energy cut offs · Geant 3. 21 · EGS 4, ITS 3. 0 · Geant 4 “standard models” Photoelectric effect Compton effect Bremsstrahlung Ionisation (d rays) Multiple scattering · Geant 4 low energy models 20 February 2002 Geant 4 Users' Workhsop, SLAC 10 ke. V 1 ke. V 250 e. V 4

Cosmic rays, jovian electrons X Ray Surveys of Solar System Bodies Solar X-rays, e, p Geant 3. 21 ITS 3. 0, EGS 4 Courtesy SOHO EIT Induced X ray line emission: indicator of target composition (~100 m surface layer) 20 February 2002 Geant 4 C, N, O line emissions included Geant 4 Users' Workhsop, SLAC 5

Features of electron and photon models · Validity range from 250 e. V to 100 Ge. V · Elements Z=1 to 100 · Data bases: EADL (Evaluated Atomic Data Library), EEDL (Evaluated Electrons Data Library), EPDL 97 (Evaluated Photons Data Library) from LLNL, courtesy Dr. Red Cullen. A version of libraries especially formatted for use with Geant 4 available from Geant 4 distribution source. 20 February 2002 Geant 4 Users' Workhsop, SLAC 6

Processes included: …in preparation: · · · · · Auger effect · Positrons Compton scattering Photoelectric effect Rayleigh effect Pair production Bremsstrahlung Ionisation Atomic relaxation Polarised processes 20 February 2002 New physics Geant 4 Users' Workhsop, SLAC 7

OOAD Technology as a support to physics Rigorous adoption of OO methods openness to extension and evolution Extensive use of design patterns Booch methodology 20 February 2002 Geant 4 Users' Workhsop, SLAC 8

Calculation of total cross sections where E 1 and E 2 are respectively the lower and higher energy for which data (s 1 and s 2) is available. Mean free path for a given process at energy E, with ni the atomic density of the ith element contributing to the material composition 20 February 2002 Geant 4 Users' Workhsop, SLAC 9

Compton scattering · Energy distribution of the scattered photon according to Klein Nishina formula multiplied by scattering functions F(q) from EPDL 97 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 EPDL 97. Rayleigh effect · Angular distribution: F(E, )=[1+cos 2( )] F 2(q), where F(q) is the energy dependent form factor obtained from EPDL 97. 20 February 2002 Geant 4 Users' Workhsop, SLAC 10

Gamma conversion · 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 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 2002 Geant 4 Users' Workhsop, SLAC 11

Photons 20 February 2002 Geant 4 Users' Workhsop, SLAC 12

Electron bremsstrahlung F(x) obtained from EEDL. At high energies: Continuous energy loss 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) Gamma ray production 20 February 2002 Geant 4 Users' Workhsop, SLAC 13

Electron ionisation · The d electron production threshold Tc is used to separate the continuous and discrete parts of the process · Partial sub shell cross sections ss 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 d ray are considered · The resulting atomic relaxation treated as follow on separate process 20 February 2002 Geant 4 Users' Workhsop, SLAC 14

Electron ionisation Bs is the binding energy of sub shell s Continuous energy loss Value of coefficient A for each element is obtained from fit to EEDL data for energies available in the database d electron production 20 February 2002 Geant 4 Users' Workhsop, SLAC 15

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 2002 Geant 4 Users' Workhsop, SLAC 16

Atomic relaxation Domain decomposition leads to a design open to physics extensions 20 February 2002 Geant 4 Users' Workhsop, SLAC 17

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water Fe water Photon attenuation coefficient Comparison with NIST data Standard electromagnetic package and Low Energy extensions 20 February 2002 Geant 4 Users' Workhsop, SLAC 19

Thorax slice CT image 6 MV photon beam Siemens KD 2 Courtesy LIP and IPOFG-CROC (Coimbra delegation of the Portuguese Oncology Institute) 20 February 2002 Geant 4 Users' Workhsop, SLAC 20

Polarised Compton Scattering x The Klein Nishina cross section: x h 0 Where, h 0 : energy of incident photon h : energy of the scattered photon : angle between the two y polarization vectors 20 February 2002 Geant 4 Users' Workhsop, SLAC O h a f A z C 21

Angular distribution of scattered radiation composed of two components: ’|| and ’^ with respect to AOC plane ’|| x ’ b x h O x ’^ A C distribution obtained with the class 20 February 2002 Geant 4 Users' Workhsop, SLAC 22

Test of the distribution: a) Low energy b) High energy The distribution function is: where and = h / h 0. Low energy: h o << mc 2 => h h o => =1 => a=0 the distribution reduces to the Thompson distribution => the probability that the two polarization vectors are perpendicular is zero. 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 2002 Geant 4 Users' Workhsop, SLAC 23

Results Scalar product between the two polarization vectors for three different energies. Upper histograms: Low polar angle Lower histograms: High polar angle 100 ke. V 1 Me. V 10 Me. V These distributions are in agreement with the limits obtained previously. 20 February 2002 Geant 4 Users' Workhsop, SLAC 24

Hadron and ion processes Variety of models, depending on energy range, particle type and charge Positive charged hadrons Bethe Bloch model of energy loss, E > 2 Me. V 5 parameterisation models, E < 2 Me. V based on Ziegler and ICRU reviews 3 models of energy loss fluctuations Positive charged ions 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 Effective charge model Nuclear stopping power Scaling: 0. 01 < b < 0. 05 parameterisations, Bragg peak based on Ziegler and ICRU reviews b < 0. 01: Free Electron Gas Model Negative charged hadrons Parameterisation of available experimental data Quantum Harmonic Oscillator Model 20 February 2002 Model original to Geant 4 Negative charged ions: required, foreseen Geant 4 Users' Workhsop, SLAC 25

HERMES X-Ray Spectrometer on Mercury Planetary Orbiter PIXE from solar proton events 20 February 2002 Geant 4 Users' Workhsop, SLAC 26

Hadrons and ions Open to extension and evolution Physics models handled through abstract classes Algorithms encapsulated in objects Transparency of physics, clearly exposed to users 20 February 2002 Geant 4 Users'sets Workhsop, SLAC Interchangeable and transparent access to data 27

Hadron and ion low energy e. m. extensions Low energy hadrons and ions models based on Ziegler and ICRU data and parameterisations 20 February 2002 Geant 4 Users' Workhsop, SLAC Barkas effect: models for antiprotons 28

Proton energy loss in H 2 O Ziegler and ICRU parameterisations 20 February 2002 Geant 4 Users' Workhsop, SLAC 29

Application examples l Five advanced examples developed by the Low. E EM WG released as part of the Geant 4 Toolkit (support process) X ray telescope l g-ray telescope l Brachytherapy l Underground physics & radiation background l X ray fluorescence and PIXE l fluorescence Full scale applications showing physics guidelines and advanced interactive facilities in real life set ups Fe lines Ga. As lines Extensive collaboration with Analysis Tools groups 20 February 2002 Geant 4 Users' Workhsop, SLAC 30

Conclusions · A set of models has been developed to extend the Geant 4 coverage of electromagnetic interactions of photons and electrons down to 250 e. V, and of hadrons down to < 1 ke. V · Rigorous software process applied · Wide user community in astrophysics, space applications, medical field, HEP, in the U. S. , Europe, and elsewhere · Modularity of Geant 4 enables easy extensions and implementation of new models · Further low energy electromagnetic physics developments and refinements are underway 20 February 2002 Geant 4 Users' Workhsop, SLAC 31

Useful links · http: //www. ge. infn. it/geant 4/low. E/ · http: //www. llnl. gov/cullen 1/ · http: //www. icru. org/pubs. htm 20 February 2002 Geant 4 Users' Workhsop, SLAC 32