1. Recent developments in the simulation of organic scintillation detectors with GEANT4
A. R. Garcia, E. Mendoza and D. Cano-Ott
Nuclear Innovation Unit – CIEMAT, Spain
R. Nolte
Physikalisch-Technische Bundesanstalt (PTB), Germany

2. GEANT4 Limitations of GEANT4 to describe breakup reactions
Monte Carlo simulation toolkit developed at CERN for High Energy Physics.
Also used in Nuclear Physics and a wide variety of applications.
Very flexible → allows for simulation complex experiments and detections sytems, e.g.: MONSTER.
GEANT4
The High Precision neutron model in GEANT4 (G4NeutronHP) does not always provide a complete description of breakup reactions:
Multistep
n+12C n’+ 12C*  α+8Be  2α
n+12C α + 9Be* n’+8Be  2α
Step Step 2
ONLY the first step described.
No gamma cascade to GS if residual nucleus left in excited state.
Direct
n+12C n’+3α
Energy-angle distribution data OR n-body phase space distribution (if data are not available).

3. NRESP Completion of 12C(n,n′3α) reaction description in GEANT4
Monte Carlo simulation code developed at the Physikalisch-Technische Bundesanstalt (PTB), Germany, to study the response of organic scintillation detectors to fast neutrons between and 20 MeV.
NRESP
Accuracy describing experimental response functions of BC501A and NE213 detectors to monoenergetic neutrons with an accuracy better than 2%.
Multistep breakup model for the 12C(n,n’3α) reactions incorporated from NRESP7.1 into GEANT p01 (G4NeutronHP), i.e., at each step:
Relativistic kinematics.
Isotropic angular distribution of first reaction product in the center-of-mass system.

4. Nuclear data in GEANT4 and NRESP
Both GEANT4 and NRESP take neutron data from ENDF, yet from different releases.
Angular distribution data for the 12C(n,α)9Be reaction also taken from NRESP.
ENDF/B-VII.1
n + 12C → n’ + 12C* → α + 8Be → 2α
n + 12C → α + 9Be* → n’ + 8Be → 2α

5. Verification against NRESP
Equivalent simulations performed with NRESP7.1, GEANT p01 as released (std) and modified to incorporate angular distributions for the C(n, α)9Be reaction and multistep breakup model for the 12C(n,n’3α) reactions (mod).
Standard physics list “Shielding” used with GEANT4:
Standard electromagnetic + High Precision neutron models.
Production cuts for protons and heavy ions set to 0 mm → production of recoils.
Electron production cut set to 0.7 mm → no electrons produced.
Same results obtained with other physics lists using the same neutron models and cuts.

6. Threshold for 12C(n,n’3a) reactions is 8. 81 MeV
Threshold for 12C(n,n’3a) reactions is 8.81 MeV. So effect of using angular distributions for 12C(n,a)8Be reaction can be better observed for 8 MeV neutrons.
Also observed for 10 MeV neutron since cross sections for 12C(n,n’3a) reactions are negligible at this energy.
8 MeV
10 MeV

7. 12 MeV
14 MeV
16 MeV
19 MeV

8. Validation against Time-Of-Flight measurements
Time-Of-Flight (TOF) measurements at the PTB Ion Accelerator Facility (PIAF).
Quasi-monoenergetic neutrons produced by inducing the D(d,n)3He reaction in a gas target with a deuterium beam delivered by the variable energy isochronous cyclotron CV28.
Gas-in/gas-out measurements and apropriate TOF window.
Pulse shape analysis performed on each signal from the photomultiplier anode to separate neutron and γ events to produce pulse height spectra.
Start signal provided by the cyclotron.

9. 8 MeV
10 MeV
12 MeV
14 MeV

10. Conclusions
The high precision neutron model of GEANT4 (G4NeutronHP) modified to incorporate from NRESP7.1 multistep breakpup models for 12C(n,n’3α) reactions and angular distribution data for the 12C(n,α)9Be reaction.
Significant improvement to the capacity of GEANT4 to study the response of organic scintillation detectors to neutrons in the energy range from 0 to 20 MeV by providing it with a complete description of neutron induced α production reactions on carbon.
The results have been verified against simulations with NRESP and validated against Time-Of-Flight measurements performed at PTB.
These results are part of a more comprehensive work (to be published in short) where the preformance of GEANT4 with different evaluated nuclear data libraries (ENDF/B-VII.0, JEFF-3.2, JENDL-4.0 and CENDL-3.1) is also investigated.
This work has potential applications beyond organic scintillation detectors in other types of fast neutron detectors where carbon reactions require an accurate description, e.g., diamond detectors.
Work is in progress to extend these results to any multistep breakup reaction.