R&D on the Geant4 Radioactive Decay Physics

1. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 1 R&D on the Geant4 Radioactive Decay Physics Joint International Conference on Supercomputing in Nuclear Applications + Monte Carlo 2010 Steffen Hauf, Markus Kuster, Philipp-M. Lang, Maria Grazia Pia, Zane Bell, Dieter H.H. Hoffmann, Georg Weidenspointner, Andreas Zoglauer 0 Credit: CNES, NASA

2. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 2 Introduction • Radioactive decay simulation as part of a larger MC code is important for a variety of applications • Examples of Geant4 dosimetry • Biophysics • Medical physics • Accelerator physics (i.e. LHC) • Manned space mission (i.e. ISS, Moon, Mars) • Unmanned probes (i.e. JIMO), observatories (i.e. IXO) • National Security • … • We plan to use Geant4 to estimate the prompt and delayed background for future (X-ray) detectors (IXO WFI). • The low uncertainty levels needed require a thoroughly validated radioactive decay simulation and support for long term activation 0

3. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 3 Introduction • Geant4 radioactive decay simulation originally developed as part of ESA contract. • Uses tabulated data to obtain decay parameters (half-life, branching, levels, intensities). • These data are stored in ASCII-files, but the database does not include reference information. • After decay nucleus and decay products are delegated to other Geant4 processes (photo-deexcitation). Other processes RadDecay Ground state

4. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 4 Introduction: Issues and Status • Issue: tabulated data is poorly referenced • Solution: new database based on current available ENSDF data •  Combined effort with international Nano5 team to create common Geant4 data model. • Status: retrieval code is implemented, need to finalize data format for use in Geant4. Data files used for our simulations have been updated in old format • Issue: only sporadic validation of results* • Solution: comparison with experiments for variety of isotopes •  gamma spectroscopy at Oak-Ridge Laboratories •  activation and decay experiment at GSI Phelix Laser • Status: gamma spectroscopy validation is ongoing, laser experiment in proposal phase • Issue: no native support for long term activation •  current implementation can bias decay times, this removes particles from MC •  MEGALib and Cosima have addressed this issue, include these concepts for general use • Status: early code version implemented, no validation or extensive testing done yet *usually includes adjusting detector efficiency to fit measurement i.e. Hurtado et al., 2003, Sahin and Ünlü 2009

5. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 5 Data Source • Explored in Nano5: common Geant4 data model • Should include references to data origin • Should allow generic unit testing • Should be „easy“ to update • Common superstructure but adaptable for physics process needsStatus: • Retrieval code for ENSDF is completed, can quickly be adapted to produce data files in new format • Isotopes used in our simulations have already been updated Deviation of energy levels in keV in Geant4 database compared to ENSDF • Comparison between current ENSDF and current Geant4 database shows inconsistencies

6. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 6 Experimental VerificationSimple and Self Consistent Approach • Simple and self consistent approach at Oak Ridge Labs: • Measure gamma spectrum of isotopes with a HPGe detector. • Self consistent means: we do not tweak the detector's geometrical properties or efficiency or source properties to fit our simulated data. • What we know: • measurement time • isotope • measured activity of isotope at a given date • detector background • detector geometry (uncertainty does exist for electron transport during readout, i.e. dead layer at entry window) • detector efficiency • detector area and volume • source geometry Measured so far: 2 2Na, 5 4Mn, 5 6Mn, 5 7Co, 6 0Co, 1 3 7Cs, 1 3 3Ba source position Experimental setup source Geant4 geometry

7. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 7 Experimental ValidationSimple and Self Consistent Approach Current best fit simulated gamma spectrum (black) of 1 3 7Cs compared to measurement (gray) peaks have different offsets Qualatively good, but offsets of up to 100% not tolarable • Fitted gaussians with underlying continuum to measured (blue) and simulated(red) peaks. • Ideally they should be at same position and have same height. • Possible solution is to model detector response so that peaks fit → this is what we do not want to do 50% deviation 20% deviation 100% deviation

8. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 8 Effects of Geant4 Version and Physics Settings Gamma spectrum of 1 3 7Cs for different Geant versions compared to experimental data(gray) • Tested: G4.9.1, 4.9.1-ref04, G4.9.2 Low Energy EM physics • Spectra are very similar for all Geant4 versions tested • Differ in particle species produced • (see next slide) • All versions show deviation from experiment of ~20% in continuum • True for other isotopes as well • Errors on following slides below 10% Goodness of Fit (Kolmogorov-Smirnov and Anderson-Darling) GoF tests: all OK at 90% CL

9. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 9 Effects of Geant4 Version and Physics Settings Spectrum of particles deposting energy in detector for 1 3 7Cs and different Geant versions (Low Energy) • Tested: G4.9.1, 4.9.1-ref04, G4.9.2 Low Energy EM • Same physics parameters are used for every G4 version. • Total spectrum is similar (previous slide) but contribution particles strongly differ. This needs to be investigated. • Not critical for this validation because we compare total spectrum with experiment, but would be confusing if constituents of spectrum are of interest. • For the total spectrum one must add the gamma energies to the energy deposited by each electron. electron spectrum of 4.9.1 and 4.9.2 gamma spectrum of 4.9.1 and 4.9.2

10. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 10 Effects of Geant4 Version and Physics Settings • Tested: G4.9.1, 4.9.1-ref04, G4.9.2 Std EM • Same physics parameters are used for every G4 version. • Total spectrum is similar (previous slide) contributions stay the same. • But total spectrum does not compare as well as Low Energy EM physics • For the total spectrum one must add the gamma energies to the energy deposited by each electron. Spectrum of particles deposting energy in detector for 1 3 7Cs and different Geant versions (Std) Goodness of Fit (Kolmogorov-Smirnov and Anderson-Darling) GoF tests: all OK at 90% CL

11. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 11 Effects of Detector Geometry Introduction of a „dead“ layer at detector entrance side (i.e. 1 3 7Cs) and placing the detector in a room sensitive dead peak vanishes source Anode continuum improves area of curved field lines Goodness of Fit (Kolmogorov-Smirnov and Anderson-Darling) All errors below 1%

12. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 12 Effects of Detector Geometry • Most energy is deposited in front part of HPGe crystal and at outer and anode edge. • In real world detector this area will have strongly curved electric field lines causing non-trivial charge transport and collection • Dead layer of of 1mm (previous slide) shows best fit with measurement. This is extremely thick compared for instance to the ~0.02mm determined by [Hurtado et. al, 2003] • Possible explanation: we see combination of dead layer effects and effect of non-trivial charge transport at detector front • Next steps: • model charge transport • and verify using different detector with other read out characteristics Energy deposit per volume in detector (divided into radial and long. volumes) detector front

13. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 13 Effects of Detector Geometry • Majority of energy is deposited in frontal detector regions • In these regions modelling of charge transport and collection (in semiconductors) is important and non-trivial • Geant4 is currently not able to model charge collection and transport • Selectively modifying detector efficiency for these regions is possible but conflicts with original goal of self-consistent simulation • We will still consider this as a temporary „work around“ to see if it produces desired results (work in progress) Cumulative energy deposition with respect to location in detector entrance window detector rear

14. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 14 Experimental Verification (IXO Use Case) Using the PHELIX Laser to Verify Activation and Decay Physics Target Normal Sheath Acceleration • Space based detectors like IXO WFI prone to background due to secondary particles produced by cosmic rays and decays in material activated by cosmic rays • Graded Z shield is a possibility to reduce flourescence lines in background • Contains high-Z materials with large activation cross-sections • Can be simulated with Geant4, but validation with experiment needed • Proton beam produced by laser via Target Normal Sheath Acceleration has similar spectral shape as cosmic ray spectrum and covers wide energy range (0-40 MeV) M. Schollmeier, PhD Thesis TU Darmstadt 2008 M. Roth TU Darmstadt Parameters Laser intensity: Proton flux: Pulse duration: pico seconds

15. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 15 Experimental Verification Using the PHELIX Laser to Verify Activation and Decay Physics Comparison between cosmic and laser accelerated proton spectrum • Laser acceleration offers self-consistent verification environment: • Proton spectrum measured using radiochromatic films during shot • After shot: gamma spectroscopy of target evidences activated isotopes and decay products • Verifies activation and decay part of simulation • We know: • Proton beam spectral shape and intensity (20% error) • Target composition • Time between shot and gamma spectroscopy • Geometry of gamma detector Optical density analysis of radiochromatic films to determine beam spectrum

16. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 16 Laser Accelerated Protons for Testing Activation and Decay Simulation Target RC- Stack • Test measurements showed promising results regarding spectral shape of proton beam and gamma spectroscopy of target • Beam time proposal for additional 18 shots at GSI PHELIX laser was submitted • Stacks are halfed: one half holds the target, the other side holds radiochromatic films and copper absorbers • After shot: gamma spectroscopy in HPGe detector • Simulation in Geant4 using GPS to emit protons with measured spectral properties HPGe Target p+

17. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 17 Issue: Geant4-native Long Term Activation Material composition at t=0 activation due to incident particles/radiation/decay n steps Material composition at t=tn Material composition at t=t f i n a l Work in progress!

18. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 18 Conclusion and Outlook • Code for updating radioactive decay database using data available in ENSDF was developed and can be adapted to future data models which will include references. Reasearch for other data libs shows that new data models can result in improved code speed (M. Han: Physics data management tools: computational evolutions and benchmarks, Session G3) • Experimental validation using self-consistent HPGe detector setup shows that Geant4 can qualitively model radioactive decays, but reducing deviation between measurement and simulation to below current 20-100% currently requires fudging detector efficiency. This is undesirable. • The introduction of a dead layer into the detector geometry improves the way the simulation models the measured data. Needs to be seen as the effect of a thin dead layer in real detector and region with non-trivial efficiency at detector front • Implementation of charge collection and transport could solve this issue. This feature is important for very broad range of detectors. • It was shown that simulation results are stable for Geant4.9.1 and G4.9.2 but spectrum of energy depositing constituents greatly changes. This is worrying and the reasons for this need to be investigated.

19. 08.10.10| TU Darmstadt | Institut für Kernphysik | Steffen Hauf | 19 Conclusion and Outlook • Proposed experiment at PHELIX laser will allow study of activation and decay processes using a proton beam with a spectrum similar to the cosmic ray spectrum. • Validating Geant4 simulation with this setup is of great importance to our IXO work. • We can test our shielding concept in a controlled enviroment. • Results will be very useful for missions with similar shielding concept. • We have started studying native long term activation in Geant4. This is still in the very early stages.Overall conclusion: For the Geant4 radioactive decay simulation to be fully useful in estimating the properties of planned detectors, validation and code advances are a must. Our current results within the Nano5 team look very promising, but much work still has to be done and is justified by the wide range of applications similar to our IXO work.