1. The Double Chooz Monte Carlo (selected topics !) Dario Motta (Irfu/SPP) Anatael Cabrera (APC)

2. Performance Backgrounds Detector Monte Carlo

3. The Double Chooz Monte Carlo ● Scintillation and optical model ● PMT geometry and optics ● Read-out System simulation ● User-friendly interfaces for event generation ● Data-Base handling Geant4-based, detailed detector description, however several extensions :

4. Optical Model Note: Geant4 does not “understand” molecules ! ➔ No native optical micro-model Data-Base material optical properties Data-Base material optical properties Run-time optical model Run-time optical model Guidelines: ● Flexibility ● Detailed micro-physical model modeled as in: D. Motta, et al., NIMA 547 (2005), 368-388 Tunable composition for all sub-volumes

5. Optical model : light emission Fluor and WLS choice => Emission & Re-emission spectra (≠) Primary (before any interaction) emission spectrum depends on Fluor -> WLS energy transfer: ● radiative ● non radiative ● both channels Fluorimetric measurements says energy transfer mode Example : PPO -> Bis-MSB mostly radiative (especially true @ low Bis-MSB concentrations)

6. Scintillation time profile DC Measurements 4-exp fit Geant4 allows only 2 time constants (fast + slow)

7. Optical model : light attenuation Partial attenuation lengths calculated by using : ● Molecular extinction coefficients (DC spectrophotometric measurements) ● Concentrations in the mixture Target scintillator

8. Optical model : wavelength-shift Partial re-emission probabilities calculated by using : ● Partial att. length (see above) to get the partial absorption probabilities ● Molecular re-emission yields (from literature and fluorimetric measurements)

9. Optical model : the PMT Faithful geometry through the GLG4sim “ThorusStack” class (non G4-native) Hamamatsu 10'' Internal photon tracking

10. PMT optical model : definitions q θ R (l,q) T (l,q ) QE (l,q) med ium A (l, q) CE (x pmt ) QE (l,0) air ≠ QE (l,q) medium QE (l,0) air is the quantum efficiency typically measured with standard techniques ● QE( l,q ) medium Probability for pe emitted in vacuum ● A( l,q ) - QE( l,q ) medium is lost ● R( l,q ) reflects and must be tracked ● T( l,q ) is transmitted into the PMT and must be tracked ● DE( l,q ) = QE( l,q ) medium x CE(x)

11. Optics of an absorbing thin film The experimental input for the model (D. Motta & S. Schönert, NIMA 539 (2005), pp. 217-235) + photocathode thickness, which is quite typical : 20 - 30 nm

12. PMT Model ● A( q,l ) ; R( q,l ) and T( q,l ) calculated for any impinging photons QE( q,l ) medium = QE( 0,l ) air x [A( q,l )/A(0,l ) air ]  QE/A = const Relevant QEMeasured QECalculatedAssumed l = 410 nm

13. PMT Model : CE(x) photocathode ● Simplified 1d model : CE = CE(r), normalized to maximum ● Complete 2d model DE = DE ( q,f ) discussed by Anatael ● Convenient to make an accurate CE simulation in post-processing : (PEs killed by CE do not any longer need Geant4 !)

14. Thank you ! Yes, this is the end