Low Energy Electromagnetic Physics

1. Low Energy Electromagnetic Physics Maria Grazia Pia INFN Genova Maria.Grazia.Pia@cern.ch on behalf of the Low Energy Electromagnetic Working Group http://www.ge.infn.it/geant4/lowE/

2. Compton scattering Rayleigh scattering Photoelectric effect Pair production Bremsstrahlung Ionisation Polarised Compton + atomic relaxation fluorescence Auger effect following processes leaving a vacancy in an atom In progress More precise angular distributions (Rayleigh, photoelectric, Bremsstrahlung etc.) Improved PIXE Development plan Driven by user requirements Schedule compatible with available resources Overview of physics • in two “flavours” of models: • based on theLivermore Library • à laPenelope

3. Stéphane Chauvie Susanna Guatelli Vladimir Ivanchenko Francesco Longo Alfonso Mantero Barbara Mascialino Petteri Nieminen Luciano Pandola Sandra Parlati Luis Peralta Andreas Pfeiffer Maria Grazia Pia Pedro Rodrigues Simona Saliceti Andreia Trindade Paolo Viarengo Stefano Agostinelli Henrique Araujo Pablo Cirrone Giacomo Cuttone Maria Catarina Espirito Santo Franca Foppiano Stefania Garelli Patricia Goncalves Alex Howard Ana Keating Susanne Larsson Jakub Moscicki Michela Piergentili Alberto Ribon Giovanni Santin Bernardo Tome Low Energy Electromagnetic Physics Advanced Examples

4. Activities • Recent physics developments • Testing • Open issues and concerns • Plans

5. Processes based on the Livermore Library • Physics models are stable • Small improvements in parameterisations • Main effort on systematic validation • Photon cross sections, stopping powers, fluorescence: done • Next round (already started): final state distributions • Feedback from users • Complementary to our own validation • Major project for the future: design iteration • Needs stable physics models, sound physics and regression testing system • Performance is an issue • Will be addresses together with the design iteration, based on quantitative evaluations

6. Recent development Penelope processes

7. Processes à la Penelope • Physics models by F. Salvat et al., implemented in a FORTRAN Monte Carlo code • the physics models have been specifically developed and a great care was devoted to the low energy description (atomic effects, etc.) • the (declared) lower limit is 100 eV • The whole physics content of the Penelope Monte Carlo code has been re-engineered into Geant4 (except for multiple scattering) • processes for photons: June 2003 release, for electrons: December 2003 release (Luciano Pandola) • Alternative approach w.r.t. the processes based on the Livermore Library

8. Gamma conversion Mean free path in Si e- angular distr. 5 MeV g on Si log (mfp/cm) Livermore-based Penelope Livermore-based Penelope log (Energy/MeV) Angle (degrees)

9. Rayleigh scattering Mean free path in Pb Angular distr. 100 keV g on Pb log (mfp/cm) Low Energy Penelope Low Energy Penelope log (Energy/MeV) Angle (degrees)

10. Photoelectric effect The cross sections are read from the database Interfaced with Livermore-based atomic relaxation Mean free path in Cu Mean free path in water log (mfp/cm) log (mfp/cm) Low Energy Penelope Low Energy Penelope log (Energy/MeV) log (Energy/MeV)

11. Compton scattering Analytical parametrisation for the cross section The model also predicts which atomic level is ionised  fluorescence generation (not present in Livermore-Compton) e- angular distr. 1 MeV g on water Mean free path in water log (mfp/cm) Low Energy Penelope Low Energy Penelope Angle (degrees) log (Energy/MeV)

12. Bremsstrahlung (electrons) genergy spectrum f(Z,Eel)  database (as in G4LowEnergyBremsstrahlung, but 32 points instead of 15) Also the angular distribution is data-driven Recent developments by V. I. not shown here g angular distr. 1 MeV e- on Pb g energy distr. 1 MeV e- on Pb Low Energy Penelope Low Energy Penelope Relative g energy Angle (degrees)

13. Bremsstrahlung (positrons) It is assumed: g(Z,E)  parameterised correction function, independent from the genergy W Mean free path in water log (mfp/cm) Electrons Positrons log (Energy/MeV) The g energy spectrum and the angular distribution are the same as for electrons, only the cross section changes

14. Validation • Relative comparison LowE-Livermore/Penelope only for curiosity • helpful to understand effects of different modeling approaches • and to identify software bugs! • Validation against reference data • LowE-Livermore and Penelope processes both subject to the same validation process • See EM Validation talk by Barbara on Tuesday

15. New branch of development Precise angular distributions

16. Bremsstrahlung Angular Distributions Three LowE generators available in GEANT4 6.0 release: G4ModifiedTsai, G4Generator2BS and G4Generator2BN G4Generator2BN allows a correct treatment at low energies (< 500 keV)

17. Bremsstrahlung Angular Distributions Open issues and news • Large initialization time for G4Generator2BN (see Physics Manual for details) • use of pre-calculated data (reduces initialization time to zero) • introduced in Geant4 6.1 • Switching mechanism between different generators • design iteration for final state needed • time scale for re-implementation and test under discussion

18. Photoelectric Angular Distributions • Current status of photoelectric angular distributions in GEANT4: • G4 LowE and LowE PENELOPE processes • The incident photon is absorbed and one electron is emitted in the same direction • as the primary photon • G4 Standard (à la GEANT3) • The polar angle of the photoelectron is sampled from an approximate Sauter-Gavrila • cross-section (for K-shell) • PENELOPE • The polar angle is sampled from K-shell cross-section derived from Sauter • The same cross-section is used for other photoionisation events • EGSnrc: Controlled by a master flag IPHTER • IPHTER = 0(similar to G4 LowE) • IPHTER = 1(Sauter distribution valid for K-shell) Both assume that azimuthal angle distribution is uniform (no polarisation)

19. Photoelectric Angular Distributions in progress • Sauter formalism is valid for light-Z, K-shell photoelectrons and non-polarised photons • In progress: use a more generalized approach based on Gavrila theory • Valid for all elements • For photoelectrons emitted from K and L shells also includes the effect of the polarisation of the incident photon This enhancement is of significance importance for the design of experiments that aim to measure the polarisation of X-rays emitted from black holes and neutron stars

20. New development PIXE

21. PIXE • Calculation of cross sections for shell ionisation induced by protons or ions • Based on a theoretical model for the calculation of cross sections • M. Gryzinski, Two-Particle Collision. I. General Relations for Collisions in the Laboratory System,   Phys. Rev. vol. 138, no. 2A, 19 April 1965 • M. Gryzinski, Two-Particle Collision. II. Coulomb Collisions in the Laboratory System of Coordinates, Phys. Rev. vol. 138, no. 2A, 19 April 1965 • Implementation in Geant4 in 2002 • Verified to be intrinsically inadequate • New data-driven model • based on evaluated data library by Paul & Sacher, 1989 (compilation of experimental data complemented by calculations from EPCSSR model by Brandt & Lapicki) • Incident proton energy between 5 KeV and 500 MeV • Elements from C to U • Generation of X-ray spectrum based on EADL • Uses the common de-excitation package

22. Fit to Paul & Sacher data library; results of the fit are used to predict the value of a cross section at a given proton energy allow extrapolations to lower/higher energy than data compilation First iteration, Geant4 6.2 (June 2004) The best fit is with three parametric functions for different groups of elements 6 ≤ Z ≤ 25 26 ≤ Z ≤ 65 66 ≤ Z ≤ 99 Second iteration, Geant4 7.0 Refined grouping of elements and parametric functions, to improve the model at low energies PIXE – Cross section model

23. Regression deviation Residual deviation Total deviation Quality of the PIXE model • How good is the regression model adopted w.r.t. the data library? • Goodness of model verified with analysis of residuals and of regression deviation • Multiple regression index R2 • ANOVA • Fisher’s test • Results (from a set of elements covering the periodic table) • 1st version (Geant4 6.2): average R2 0.998 • 2nd version (Geant4 7.0): average R2 improved to 0.999 at low energies • p-value from test on the F statistics < 0.001 in all cases Test statistics Fisher distribution

24. PIXE: status and future developments • First implementation for protons, K-shell • Geant4 6.2 • preliminary model • Second iteration • improved model currently under test • Geant4 7.0 • Third iteration: protons, L-shell • time scale subject to availability of resources • Fourth iteration: ions, K-shell • compilations of cross-sections limited to K-shell

25. Ongoing... • Regular maintenance and improvements in many areas • improved, precise calculation of range for hadrons and ions • extension of parameterised models for hadrons up to ~8 Mev • improved treatment for some materials (i.e. graphite) • etc. • The physics testing project helped fixing bugs, identifying small problems, improving many details • Thanks to systematic and quantitative tests

26. Lower energy extensions • Old requirement (still from RD44) • Recurrent in interactions with users • Last User Workshop: requirement to reach ~10 eV • We are interested (of course) • We are aware it is difficult • An entire new set of phenomena to take into account • Are there any data available? How good are they? • Project for lower energy extensions currently explored together with ESA • More at next Geant4 Workshop…

27. Current major activity Validation See Barbara’s talk on Tuesday

28. Main concern: SPI • Software Process Improvement • Huge investment in SPI • Excellent response from most of the WG • Many young collaborators “grown” to appreciate rigorous SP • But also occasional spells of SPD… • Coding without design • Maintaining “private” code, dropping regular maintenance • Undocumented test cases • Physics/algorithm documentation lies in the mind of individual developers • Difficult change management • etc. • Peer reviews • We would like to do more… • Very much needed: code review • Limitation: time availability and geographical spread …but not worse than the rest of Geant4!

29. Summary • Validation • Major effort, with significant results • Highlighted some problems (solved or solutions planned) • Triggered various physics improvements • Sound testing system essential for planned design iteration • Recent developments • Penelope re-engineering • Precise angular distributions (Bremsstrahlung, photoelectric) • PIXE • Outlook • Lower E requirements to be addressed • Validation of existing models • Design and code reviews • SPI (continuous education to a rigorous software process)