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Validation of the bremssrahlung process

Validation of the bremssrahlung process. IV Workshop on Geant4 physics validation Susanna Guatelli, Luciano Pandola, Maria Grazia Pia, Valentina Zampichelli. Genova, July 18 – 20, 2005. Now e - /e + EM processes to be taken into account.

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Validation of the bremssrahlung process

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  1. Validation of the bremssrahlung process IV Workshop on Geant4 physics validation Susanna Guatelli, Luciano Pandola, Maria Grazia Pia, Valentina Zampichelli Genova, July 18 – 20, 2005

  2. Now e-/e+ EM processes to be taken into account more difficult to find reference data and to disentangle effects (because of the continuous part) For bremmsstrahlung, 3 sets of models: G4eBremsstrahlung G4LowEnergyBremsstrahlung G4PenelopeBremsstrahlung 3 alternative angular distributions: Tsai, 2BS, 2BN Validation of EM physics The sets of models for g‘s have already been extensively and successfully validated K. Amako et al., in press on IEEE Trans. Nucl. Scie. Luciano Pandola

  3. Photon (energy, θ) θ Z axis Preliminary test: comparison of final states Check the final state (= energy and angular distribution) generated by the different bremsstrahlung models Create an e- of given energy (70 keV) in a given material (Al)  force the bremmstrahlung interaction, i.e. the call to the PostStepDoIt() 70 keV e- in Al cut = 0.5 mm energy and polar angle of the emitted photon Notice: test of the shape of ds/dqdW, not of the absolute value Luciano Pandola

  4. One step back: check the Penelope implementation in G4 The final state (= energy and angular spectrum) generated by G4PenelopeBremsstrahlung exactly matches (as it has to!) the one from the original Penelope (Fortran) 70 keV e- in Al 70 keV e- in Al Geant4 Fortran Geant4 Fortran Energy (MeV) cosq Luciano Pandola

  5. Energy distributions Standard Low Energy Penelope Penelope Low Energy Penelope-FORT Energy (keV) Energy (keV) Photon energy distributions are very similar for all the three models available in Geant4 Luciano Pandola

  6. Angular distributions Penelope Standard Low Energy (default) Penelope TSAI (def) 2BS 2BN Angle (deg) Angle (deg) Angular distribution of photons is strongly model-dependent Luciano Pandola

  7. θ Second test: validation Beam of 70-keV e- incident on a thin slab of Al (94 mm) Photon (energy, θ) test51 Electrons andd-rays are absorbed while bremsstrahlung photons can be transmitted 70 keV e- yield, energy and polar angle of theemitted photons Z axis cut = 0.5 mm = 990 eV All physics processes of e- (ionisation, bremsstrahlung, multiple scattering) and gcome into the play Luciano Pandola

  8. The reference data Transmitted energy spectrum at two different emission angles for four materials (Al, Pb, W, Ag) Absolute yields are reported (= photons/primary), though with a strange normalization The absolute bremsstrahlung cross section can be tested R. Ambrose et al., Nucl. Instr. Meth. B 56/57 (1991) 327 Luciano Pandola

  9. Simulation results: backscattering geant4-07-00-ref03 Penelope Standard Low Energy (TSAI) Al Ka(1.4 keV) Penelope Standard Low Energy Energy (keV) theta (deg) The backscattered radiation is mainly Kax-ray of Al. At low energy ionisation dominates over bremmstrahlung! Luciano Pandola

  10. Simulation results: transmission geant4-07-00-ref03 Penelope Standard Low Energy (TSAI) Penelope Standard Low Energy Energy (keV) theta (deg) The photons yield is small (0.8 · 10-3 photons/electron)  large statistics to be generated (work in progress) Luciano Pandola

  11. What does come out...? Preliminary Penelope Low Energy - TSAI 45 deg direction Intensity/Z (eV/sr keV) photon direction 70 keV e- 45 deg geant4-07-00-p01 Photon energy (keV) Simulation limited by statistics (Penelope = 400 M events generated, Low Energy = 250 M). Yield slighly lower Luciano Pandola

  12. Preliminary Relative comparison... Low Energy - TSAI Penelope Intensity/Z (eV/sr keV) Intensity/Z (eV/sr keV) Photon energy (keV) Photon energy (keV) Relative comparison (45 deg direction). Shapes of the spectra are in good agreement Luciano Pandola

  13. What does come out...? (II) Preliminary 90 deg direction Penelope Low Energy - TSAI Intensity/Z (eV/sr keV) photon direction 70 keV e- 90 deg geant4-07-00-p01 Photon energy (keV) Strong limitation by statistics (in the cone: 18 events/1M primary). Yield seems slightly underestimated Luciano Pandola

  14. Preliminary Relative comparison... (II) Low Energy - TSAI Penelope Intensity/Z (eV/sr keV) Intensity/Z (eV/sr keV) Photon energy (keV) Photon energy (keV) Relative comparison (90 deg dir). The shapes ~agree Role of angular distribution has to be understood Luciano Pandola

  15. From the original paper it is impossibleto understand theexperimental details or the systematics problems related with the experimental systematics? our misunderstanding of the reference data? Short discussion preliminary The result have to be taken as Caveatsconcerning the absolute yield:   The simulated spectra have large statistical errors  Other angular distributions (e.g. 2BN) may be more appropriate in the range of energy of interest Luciano Pandola

  16. generate larger statistics and check for the Standard process take into account the alternative angular distributions understand better the reference data (mainly sistematics!) and the simulation results repeat the test with other target materials (and compare with reference data)  agreement vs. Z look for other experimental reference data Next steps to be done work in progress This is a Luciano Pandola

  17. Conclusions We are testing the G4 alternative bremsstrahlung models Relative comparison: shape of spectra agrees with data To do: statistically quantify the agreement It is possible to do an absolute comparison for energy and angular differential cross sections with reference data Absolute yield agrees as order-of-magnitude (OK) Quantitative comparison requires extra work ( in progress) to understand data (systematics) and to simulate larger statistics (also alternative angular distributions) Luciano Pandola

  18. Backup slides Luciano Pandola

  19. Preliminary Relative comparison... Low Energy - TSAI (+ 13%) Penelope (+ 23%) Intensity/Z (eV/sr keV) Intensity/Z (eV/sr keV) Photon energy (keV) Photon energy (keV) Relative comparison (45 deg direction). Shapes of the spectra are in good agreement Luciano Pandola

  20. Preliminary Relative comparison... (II) Low Energy - TSAI (+ 40%) Penelope (+ 26%) Intensity/Z (eV/sr keV) Intensity/Z (eV/sr keV) Photon energy (keV) Photon energy (keV) Relative comparison (90 deg dir). The shapes ~agree role of angular distribution has to be understood Luciano Pandola

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