Post on 20-Dec-2015
20 February 2002 Geant4 Users' Workhsop, SLAC 1
Low-Energy Electromagnetic Processes in
P. Nieminen (ESA-ESTEC)
http://www.ge.infn.it/geant4/lowE/
20 February 2002 Geant4 Users' Workhsop, SLAC 2
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 EnergyPhysics
20 February 2002 Geant4 Users' Workhsop, SLAC 4
Electron and photon processesEnergy cut-offs
Geant3.21 10 keV EGS4, ITS3.0 1 keV Geant4 “standard models”
- Photoelectric effect 10 keV- Compton effect 10 keV- Bremsstrahlung 1 keV- Ionisation (-rays) 1 keV- Multiple scattering 1 keV
Geant4 low-energy models 250 eV
20 February 2002 Geant4 Users' Workhsop, SLAC 5
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 2002 Geant4 Users' Workhsop, SLAC 6
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 2002 Geant4 Users' Workhsop, SLAC 7
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 2002 Geant4 Users' Workhsop, SLAC 8
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 2002 Geant4 Users' Workhsop, SLAC 9
Calculation of total cross sections
12
1221
/log
/loglog/logloglog
EE
EEEEE
where E1 and E2 are respectively the lower andhigher energy for which data (1 and 2) is available.
iii nE
1
Mean free path for a given process at energy E, withni the atomic density of the ith element contributingto the material composition
20 February 2002 Geant4 Users' Workhsop, SLAC 10
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+cos2F2(q), where F(q) is the energy-dependent form factor obtained from EPDL97.
20 February 2002 Geant4 Users' Workhsop, SLAC 11
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 2002 Geant4 Users' Workhsop, SLAC 12
PhotonsPhotons
20 February 2002 Geant4 Users' Workhsop, SLAC 13
Electron bremsstrahlung
s
T
eV
T
Ts
MAX
MAX
C
dtdt
d
dtdt
d
TT
1.0
s
T
eV
T
eVs
MAX
C
dtdt
d
dtdt
dt
Tdx
dE
1.0
1.0
Continuous energy loss
Gamma ray production
T
tx
x
xF
dt
d ,
275.01 xxxF
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 2002 Geant4 Users' Workhsop, SLAC 14
Electron ionisation
The -electron production threshold Tc 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 2002 Geant4 Users' Workhsop, SLAC 15
Electron ionisation
s
T
eV
T
Ts
MAX
MAX
C
dtdt
d
dtdt
d
TT
1.0
s
T
eV
T
eVs
MAX
C
dtdt
d
dtdt
dt
Tdx
dE
1.0
1.0
s
s
BT
Btx
x
xPC
dt
d
,2
Continuous energy loss
-electron production
x
Ag
xx
xxggxxP
1
1
111
22
2/12 g
Value of coefficient A for each element is obtained from fit to EEDL data for energies available in the database
Bs is the binding energy of sub-shell s
20 February 2002 Geant4 Users' Workhsop, SLAC 16
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 Geant4 Users' Workhsop, SLAC 17
Domain decomposition leads to a design open to
physics extensions
Atomic relaxationAtomic relaxation
20 February 2002 Geant4 Users' Workhsop, SLAC 18
20 February 2002 Geant4 Users' Workhsop, SLAC 19
Photon attenuation coefficient
Comparison with NIST data
Standard Standard electromagnetic package
and Low EnergyLow Energy extensions0.01 0.1 1 10
0.01
0.1
1
10
100
1000
Geant4 LowEn NIST
/ (
cm 2
/g) i
n iro
n
Photon Energy (MeV)
Fe
0.01 0.1 1 10-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
14
16
Delta = (NIST-G4EMStand) / NIST
Delta = (NIST-G4LowEn) / NIST
Del
ta (
%)
Photon Energy (MeV)
water water
20 February 2002 Geant4 Users' Workhsop, SLAC 20
Courtesy LIP and IPOFG-CROC (Coimbra delegation of the Portuguese Oncology Institute)
6 MV photon beamSiemens KD2
Thorax sliceCT image
20 February 2002 Geant4 Users' Workhsop, SLAC 21
Polarised Compton Scattering
2
0
020
220 cos42
h
h
h
h
h
hr
4
1
d
d
The Klein-Nishina cross section:
Where,h0 : 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 2002 Geant4 Users' Workhsop, SLAC 22
Angular distribution of scattered radiation composed of two components: ’
and ’with respect to AOC plane
’
’
CO
Ah
’
x
22
0
020
220 cossin2
h
h
h
h
h
hr
2
1
d
d distribution obtained with the class
20 February 2002 Geant4 Users' Workhsop, SLAC 23
Test of the distribution:
a) Low energy b) High energy
Low energy: ho << mc2 => h ho => =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 / h0.
High energy: small => h ho => 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.
2cos1
baba
P 4,21
ba
20 February 2002 Geant4 Users' Workhsop, SLAC 24
ResultsScalar product between the two polarization vectors for three different energies.
Upper histograms: Low polar angle
Lower histograms: High polar angle
100 keV 10 MeV1 MeVThese distributions are in agreement with the limits obtained previously.
20 February 2002 Geant4 Users' Workhsop, SLAC 25
Hadron and ion processesVariety 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 hadronsPositive charged hadrons
Positive charged ionsPositive charged ions
Negative charged hadronsNegative 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
ion
ppppionion m
mTTTSZTS ,2
26Geant4 Users' Workhsop, SLAC20 February 2002
HERMES X-Ray Spectrometer on
Mercury Planetary Orbiter
PIXE from solar proton events
20 February 2002 Geant4 Users' Workhsop, SLAC 27
Algorithms encapsulated in
objects
Physics models handled through abstract classes
Interchangeable and transparent access to data sets
Hadrons and ionsHadrons and ions Open to extension and evolution
Transparency of physics, clearly exposed to users
20 February 2002 Geant4 Users' Workhsop, SLAC 28
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 2002 Geant4 Users' Workhsop, SLAC 29
Proton energy loss in H2O
Ziegler and ICRU parameterisations
20 February 2002 Geant4 Users' Workhsop, SLAC 30
Application examples
Five advanced examplesadvanced examples developed by the LowE EM WG released as part of the Geant4 Toolkit (support process)
Extensive collaboration with Analysis Tools groups
Brachytherapy Underground physics & radiation background X-ray fluorescence and PIXE
X-ray telescope -ray telescope
Full scale applications showing physics guidelines and advanced interactive facilities in real-life set-ups
GaAs linesFe lines
fluorescence
20 February 2002 Geant4 Users' Workhsop, SLAC 31
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 2002 Geant4 Users' Workhsop, SLAC 32
Useful links
http://www.ge.infn.it/geant4/lowE/ http://www.llnl.gov/cullen1/ http://www.icru.org/pubs.htm