Geant4 in Underground Physics: Simulations, Advancements, and Applications

1. Geant4 and the Underground Physics Community Luciano Pandola INFN, Laboratori Nazionali del Gran Sasso for the ILIAS JRA1 and N3 Monte Carlo groups Geant4 Workshop Hebden Bridge, UK 13-19 September 2007

2. Many underground experiments are now using Geant4 as the base toolkit for their Monte Carlo simulations Advantages of Geant4: flexibility; robustness and variety of physics models (also specific in the low- and high-energy domains); regular improvements/releases; support; transparency Geant4 in the low-background community Geant4 is widely used in the field, but (my opinion) it still needs dedicated development and validation to be fully considered as THE “reference” Monte Carlo code The EU underground laboratories and experiments are coordinated in the ILIAS project A working group is devoted to background studies: simulations of various types of background in underground laboratories; comparison of the results from MC codes with eachother and with experimental data (validation) Geant4 is also considered Geant4 Workshop – Hebden Bridge

3. Neutrinoless bb decay: GERDA, Majorana (within MaGe), COBRA, CUORE,... Dark matter detection: Zeplin-II/III, Drift, Igex, Warp, Edelweiss, ArDM, Xenon10/100, CRESST, Lux, Elixir, Eureca,... Solar neutrinos: Borexino, ... Experiments using Geant4 Coordination within ILIAS Geant4 Workshop – Hebden Bridge

4. Geant4 applications in this field Experiment backgrounds internal detector radioactivity rock radioactivity m-induced neutron production shielding and veto systems Geant4 is uniquely suited for integrated simulations of underground and low-background detectors (e.g. dark matter) Optics Photon generation Light collection Detector response Scintillation Ionisation Simulated Data Visualisation Run-time analysis Input to data analysis software Calibration Neutrons Gammas A dedicated advanced example (underground_physics) is released with Geant4 (ZEPLIN experiment) Geant4 Workshop – Hebden Bridge

5. Low energy electromagnetic extensions • precise tracking of low-energy leptons and hadrons • precise energy and angular spectra • atomic de-excitation (e.g. fluorescence x-rays) • High energy muons interactions & showers • neutron, hadron and isotope production Hot topic What’s critical for underground exp? • Low-energy (~MeV) neutrons • precise tracking of neutrons and nuclear recoils • emission of g-rays from (n,n’g) and (n,g) interactions • Radioactive decay • very precise decay schemes (low-branching channels) Geant4 Workshop – Hebden Bridge

6. Importance of EM processes Precise description of EM processes of g-rays and e± down to low energies crucial for all simulation of low-background experiments: neutrinoless bb decay, direct dark matter searches, neutrino experiments, ultra-low background g-spectroscopy Most applications also require simulation of fluorescence x-rays (e.g. sum peaks in 0nbb decay experiments or in g spectroscopy) Precise description of proton/a-particle/nucleus EM processes crucial for several experiments (mainly 0nbb) plot by R. Johnson degraded a-particles can penetrate thin dead layers and be a background source required spatial precision for a-particles EM interactions in these applications < 0.1 mm Geant4 Workshop – Hebden Bridge

7. Geant4 provides dedicatedLow Energy EM models electrons, positrons andg-rays down to 250 eV Based on EPDL97, EEDL and EADL evaluated data libraries shell effects NIST data Penelope Hadron, anti-pandionmodels Attenuation coeff. (cm2/g) The physics content of the Penelope code has been re-engineered into Geant4: alternative set of models (w/ atomic effects). Possible thanks to the OO-technology of Geant4 Low-energy electromagnetic physics - I The low-energy models include atomic effects, as fluorescence x-rays and Auger electrons. K-shell PIXE also available Models are continuosly tested and maintained; new ones are developed Geant4 Workshop – Hebden Bridge

8. G4 LowE NIST Phys. Rev. 102 (1956) 1598 Data: Shimizu et al, Appl. Phys. 9 (1976) 101 320 nm G4Standard Al slab E = 20 keV IEEE Trans. Nucl. Sci. 52 (2005) 910 data + simulation 1040 nm Energy (MeV) photon attenuation electron transmission bremsstrahlung Low-energy electromagnetic physics - II Geant4 EM physics models (“standard” and “low energy”) for g-rays validated in a systematic and quantitative way K. Amako et al., IEEE Trans. Nucl. Sci. 52 (2005) 910 Similar work in progress for e- and ion EM models Geant4 Workshop – Hebden Bridge

9. 60Co source & NaI detector nucl-ex/0701005 simulation average deviation ~5% data 60Co source & segmented Ge detector Low-energy electromagnetic physics - III After validation of each single EM model, it is necessary to validate the whole set of models with experimental use cases (namely, full simulation of experimental setup & measurement) Can be done by experimental groups with their own applications  coordination with the Geant4 validation effort Geant4 Workshop – Hebden Bridge

10. Hottest topic in our Monte Carlo community increasing literature Araujo et al., NIM A 545 (2005) 398 Kudryavtsev et al., NIM A 505 (2003) 688 Wang et al., Phys. Rev. D 64 (2001) 013012 Mei and Hime, Phys. Rev. D 73 (2006) 053004 Carson et al., NIM A 546 (2005) 509 Pandola et al., NIM A 570 (2007) 549 Marino et al., arXiv:0708.0848 .... Araujo et al., NIMA 545 (2005), 398 Kudryavtsev et al., NIMA 505 (2003) 688 Wang et al., Phys. Rev. D 64 (2001) 013012 See other dedicated talks Scholl, Lindote, Horn, Iguaz Muon-induced neutrons and isotopes Precise description of propagation of cosmic ray muons and of the induced production of neutrons and isotopes is a critical issue for all underground exp’s (especially DM) Geant4 Workshop – Hebden Bridge

11. ensured by the data-driven approach of the NeutronHP models. Database (from ENDF/B-VI) for elastic and inelastic scattering, capture and fission (both XS and FS) Drawback: database files are missing for some isotopes and/or for some interaction channels. In some cases, only natural composition available Lemrani et al, NIM A, 560 (2006) 454 Good agreement between Geant4 and MCNP-X for applications of our interest (shielding design) Low-energy neutrons - I Precise tracking of fast (MeV) neutrons down to thermal energy very important for the simulation of nuclear recoils (dark matter) and of induced g-rays (0nbb, spectroscopy) Geant4 Workshop – Hebden Bridge

12. Low-energy neutrons - II Other side of the coin for the NeutronHP models: there are some long-standing bugs that are relevant for our applications, and some necessaryimprovements/developments... Bug #821: in some cases, missing residual nucleus after inelastic scattering, e.g. Ge(n,2n). Potentially relevant for dark matter. Reported 12-Dec-2005 Bug #526: wrong g-lines produced by inelastic scattering. Fixed with the new G4NDL3.11 database (at least for Ge)  Potential non-conservation of energy in G4NeutronHPCapture: recoil nucleus is generated only if only one g-ray is emitted. G4NeutronHPCapture does not produce metastable states of the daughter nucleus: missing de-excitation g-lines. Relevant for 0nbb See also S. Scholl, talk at IDM 2006 Geant4 Workshop – Hebden Bridge

13. 2.5 MeV neutrons on 40Ar, = 79.4 deg Inelastic scattering. G4 gives Erec = 21 keV at any angle. Should be 72 keV at 79.4 deg Elastic scattering (Erec = 100 keV at this angle) scattered neutron neutron detector  neutron beam Ar target Recoil energy (MeV) Low-energy neutrons - III Bug #675: wrong kinematics of the recoil nucleus after inelastic interactions (missing Lorentz boost). Reported 10-Apr-2004. Now deferred to end 2007 In Geant4, the recoil energy after inelastic scattering does not dependon thescattering angle. Elastic scattering is ok Geant4 Workshop – Hebden Bridge

14. Radioactive decay - I In several applications of ultralow-level g-spectroscopy and 0nbb decay it is crucial to take into account summing effects in g-ray cascades Easiest way: set the nucleus at rest as “primary” and let Geant4 simulate the final state according to the database (based on the ENSDF) Extremely precise and reliable for most nuclei (e.g. 134Cs). Also fluorescence x-rays are generated in decays Courtesy of D. Budjas (MPIK) Geant4 Workshop – Hebden Bridge

15. Bug #952 (from Geant4 8.x): no more than one conversion electron per cascade is allowed. Spoils the agreement with the ToI for some nuclei (133Ba) and low-energy lines g-rays from the decay of 133Ba 7.0: 120.1% 9.0: 85.6% ToI: 121.4% traced & fixable 7.0: 33.7% 9.0: 42.8% ToI: 34.1% Energy (keV) Radioactive decay - II Again, other side of the coin a few problems and some welcome improvements Branching ratio of very weak lines set artificially to 10-5 (may be a problem for specific simulations, 60Co) x-ray yield is generally under-estimated. Maybe because the code does not handle more than one atomic vacancy at the same time (wish for future): it would be nice to have angular correlation in g-cascades Geant4 Workshop – Hebden Bridge

16. Conclusions • Geant4 is widely used in underground physics • A dedicated Monte Carlo group within the EU project ILIAS ensure coordination of experiments and comparison with data • Experiments have some specific requirements in terms of physics and funcionalities of Geant4 • Status of the EM models is satisfactory for low-bck applications, both in performances and validation • Validation effort ongoing also within low-bck experiments • Interactions of cosmic ray muons and neutron production is the hottest topic in low-bck Monte Carlo community • High-precision neutron models work fine, though there are some relevant bugs for low-bck applications • (Wish for the future): it would be nice to have in Geant4 neutron production physics, e.g. (p,n) and (a,n) • Radioactive decay also works fine, with some possible improvements Geant4 Workshop – Hebden Bridge