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

April Luciano Pandola2 Validation of EM physics The sets of models for  ‘s have already been extensively and successfully validated K. Amako et al., in press on IEEE Trans. Nucl. Scie. 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

April Luciano Pandola3 Preliminary test: comparison of final states Check the final state (= energy and angular distribution) generated by the different bremsstrahlung models Z axis Photon (energy, θ) θ 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  m energy and polar angle of the emitted photon Notice: test of the shape of d  d  dW, not of the absolute value

April Luciano Pandola4 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) Energy (MeV) cos  70 keV e - in Al Geant4 Fortran Geant4 Fortran 70 keV e - in Al

April Luciano Pandola5 Energy distributions Energy (keV) Standard Low Energy Penelope Photon energy distributions are very similar for all the three models available in Geant4 Penelope Low Energy Penelope-FORT

April Luciano Pandola6 Angular distributions Angle (deg) Angular distribution of photons is strongly model-dependent Penelope Standard Low Energy (default) Penelope TSAI (def) 2BS 2BN

April Luciano Pandola7 Second test: validation Beam of 70-keV e - incident on a thin slab of Al (94  m) yield, energy and polar angle of the emitted photons Z axis 70 keV e- Photon (energy, θ) θ Electrons and  -rays are absorbed while bremsstrahlung photons can be transmitted cut = 0.5  m = 990 eV All physics processes of e - (ionisation, bremsstrahlung, multiple scattering) and  come into the play test51

April Luciano Pandola8 The reference data The absolute bremsstrahlung cross section can be tested 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 R. Ambrose et al., Nucl. Instr. Meth. B 56/57 (1991) 327

April Luciano Pandola9 Simulation results: backscattering The backscattered radiation is mainly K   x-ray of Al. At low energy ionisation dominates over bremmstrahlung! Energy (keV) theta (deg) Penelope Standard Low Energy Penelope Standard Low Energy (TSAI) geant ref03 Al K  (1.4 keV)

April Luciano Pandola10 Simulation results: transmission The photons yield is small (0.8 · photons/electron)  large statistics to be generated (work in progress) Energy (keV) theta (deg) Penelope Standard Low Energy Penelope Standard Low Energy (TSAI) geant ref03

April Luciano Pandola11 What does come out...? Simulation limited by statistics (Penelope = 400 M events generated, Low Energy = 250 M). Yield slighly lower 45 deg direction Penelope Low Energy - TSAI Photon energy (keV) Intensity/Z (eV/sr keV) 70 keV e - photon direction 45 deg geant p01

April Luciano Pandola12 Relative comparison... Relative comparison (45 deg direction). Shapes of the spectra are in good agreement Intensity/Z (eV/sr keV) Photon energy (keV) Penelope Low Energy - TSAI Intensity/Z (eV/sr keV)

April Luciano Pandola13 What does come out...? (II) Strong limitation by statistics (in the cone: 18 events/1M primary). Yield seems slightly underestimated 90 deg direction Photon energy (keV) Intensity/Z (eV/sr keV) 70 keV e - photon direction 90 deg Penelope Low Energy - TSAI geant p01

April Luciano Pandola14 Relative comparison... (II) Relative comparison (90 deg dir). The shapes ~agree  Role of angular distribution has to be understood Intensity/Z (eV/sr keV) Photon energy (keV) Penelope Low Energy - TSAI

April Luciano Pandola15 Short discussion From the original paper it is impossible to understand the experimental details or the systematics problems related with the experimental systematics? our misunderstanding of the reference data? The result have to be taken as Caveats concerning 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   

April Luciano Pandola16 Next steps to be done This is a generate larger statistics and check for the Standard process take into account the alternative angular distributions repeat the test with other target materials (and compare with reference data)  agreement vs. Z understand better the reference data (mainly sistematics!) and the simulation results look for other experimental reference data

April Luciano Pandola17 Conclusions We are testing the G4 alternative bremsstrahlung models 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) Relative comparison: shape of spectra agrees with data To do: statistically quantify the agreement Quantitative comparison requires extra work (  in progress) to understand data (systematics) and to simulate larger statistics (also alternative angular distributions)

April Luciano Pandola18 Backup slides

April Luciano Pandola19 Relative comparison... Relative comparison (45 deg direction). Shapes of the spectra are in good agreement Intensity/Z (eV/sr keV) Photon energy (keV) Penelope (+ 23%) Low Energy - TSAI (+ 13%)

April Luciano Pandola20 Relative comparison... (II) Relative comparison (90 deg dir). The shapes ~agree  role of angular distribution has to be understood Intensity/Z (eV/sr keV) Photon energy (keV) Penelope (+ 26%) Low Energy - TSAI (+ 40%)