1. Advanced Neutron Spectrometer (ANS) Geant4 Simulations
John Watts
University of Alabama Huntsville/CSPAR
May 27,2014
(256)
Team
Marshall Space Flight Center/NASA
Dr. Mark Christl, Joey Norwood, Chris Dobson
Oak Ridge Associated Universities
Dr. Mohammad Sabra

2. ANS Geant4 Simulations
Introduction
A Geant4 application has been developed to simulate the response of the Advanced Neutron Spectrometer (ANS) and to investigate approaches to meeting the instrument requirements over the proposed energy range (0.1 to 100 MeV). The configuration shown in Figure 1 consisted of a volume of scintillator-doped vinyltoluene 10 inches long with a 2x2 inch readout photomultiplier tube (PMT) on each end. The ends are 4x4 inches, and the center is 5 x5 inches. An array of 120-micrometer diameter lithium glass fibers are embedded in the plastic scintillator. The arrays has a 1 mm pitch with 72x72 fibers which yields a mass fraction of 3% in the array region. The photomultiplier tubes count the optical photons produced by scintillation as energetic charged secondary particles deposit energy in the scintillation plastic or glass fibers. These are created when the primary neutrons undergo elastic and inelastic scatter and neutron capture.

3. ANS Geant4 Simulations
Figure 1 ANS diagram

4. ANS Geant4 Simulations
Neutron Capture
For the embedded lithium glass scintillator, neutron captures are identified by the energy released and then deposited by secondary particles. The fibers are composed of 20% Li2O-15% MgO-64.4%SiO2-0.6%Ce2O3 with the lithium being 95% 6Li. Neutrons are captured in the glass which has different scintillation characteristics in terms of spectral output and timing from vinyltoluene scintillator. The capture interaction here is
𝑛+ 6 𝐿𝑖 →𝛼+𝑡𝑟𝑖𝑡𝑜𝑛
where the triton energy is MeV and the α energy is MeV.
To reach energies where this interaction is probable the neutron must be at near thermal energies having lost energy through elastic scattering. In vinyltoluene (C10H11) most of the energy is lost through elastic scatters with the hydrogen or less frequently the carbon. The ions produced interact, depositing their energy and generating scintillation photons. The approach in ANS is to identify the unique light pulse associated with a neutron capture event and use the total light produces previously in the event as a measure of primary neutron energy deposited in the detector volume.

5. ANS Geant4 Simulations
Code Structure
 Geant4.10.p01 with multithreading enable was used for the simulations. The physics list was QGSP_BIC_HP with the addition of optical processes.
A GDML description of the ANS geometry and the appropriate materials was constructed for input into a Geant4 simulation tailor to transport neutron and describe the response of the various parts of the detector in detail. A key part of the simulation is the description of the optical characteristics of the scintillator material, the scintillator volume surface characteristics, and the optical coupling between the scintillator and the photomultiplier tube. In Geant4 optical photons are treated a separate particle from x-rays and gamma particles. They can be affected by several processes, but in this simulation only scintillation, Cerenkov, and attenuation processes are included. To create and transport them, tables of the index of refraction and attenuation length in the optical materials are required together with the scintillation yield, scintillation time constants, and Birks parameters. The characteristic of the optical interface between the scintillator volume with the world and with the PMT must also be defined. These are largely based on data from a Bicron data sheet[1] for their BC-400 plastic scintillator and reference 2 for the scintillating glass.

6. ANS Geant4 Simulations
Code Structure
For the simulations show here a beams of neutrons at a selected set of energies were fired at normal incidence on the center of the positive Z face of the detector. (The center of the world coordinates is at the center of the plastic scintillator volume and the PMT’s are on the positive and negative Z faces.)
For each event a number of parameter were scored. These are tabulated in a ROOT format file for analysis and plotting using the g4tools header package. After the Geant4 simulation generates a set of events, analysis code written in C++ ROOT macros is used to impose various cut and plot the detector response and statistical measures.
Figure 2 shows the mean energy response from 10 ev to 200 MeV. (The number of events was 107.)

7. Figure 2 Mean energy response for an ANS detector.

8. Figure 3 Number of neutron captures per incident neutron for an ANS detector.
Note that the number of captures per event is low especially at higher energies because the neutron scatter length is relatively large compared to the detector size. Thus most neutrons escape the detector volume before undergoing a capture. This means that a large number of events must be simulated to get good statistics.

9. ANS Geant4 Simulations
Results
Geant4 allow control over the order in which secondary particles are simulated through a stack and staging mechanism. To improve the efficiency of the simulation a StackingAction class was used where:
First, the primary neutron and any secondary neutrons were transported while other secondaries were put on a suspended stack.
Then, in the second stage event process is continued only where one or more of the neutrons had a capture process.
If there were no captures the event is killed.

10. ANS Geant4 Simulations
Results
Figures 4, 5, 6, 7, 8, and 9 show the energy deposition in the detector volume for events that had one or more neutron captures. Energy deposited by the neutron capture secondaries is excluded.
The sharp peak (red) at the initial neutron kinetic energy is due to neutron that deposited most of their energy through elastic scatters in the detector plastic.
The peak (green) at 5.3 MeV in Figure 6 (10 MeV) and 15.5 MeV in Figure 7 (19.9 MeV) are due to neutron breakup of a carbon atom by the reaction
𝑛 𝐶 → 9 𝐵𝑒 +𝛼
The binding energy is ~4.5 MeV.
The peak (blue) at 12.7 MeV in Figure 8 is due to the reaction
𝑛 𝐶 →𝑛+3𝛼
The binding energy is ~7.3 MeV.
The peak (pink) at 14 MeV in Figure 9 is due to the reaction
𝑛 𝐶 → 11 𝐵 +𝑝
The binding energy is ~16 MeV.
Event from these colored peaks were tagged and their photon response plotted separately in the photon response plots shown later.

11. Figure 4 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 10 eV before neutron capture.

12. Figure 5 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 1 MeV before neutron capture.

13. Figure 6 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 10 MeV before neutron capture.

14. Figure 7 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 19.9 MeV before neutron capture.

15. Figure 8 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 20 MeV before neutron capture.

16. Figure 9 Fraction events versus energy deposited in the ANS detector volume for an incident neutron energy of 30 MeV before neutron capture.

17. Figure 10 Optical photon response of the ANS detector versus incident neutron energy
The detector energy response is measures by counting the optical photons emitted through scintillation with PMTs coupled to the end faces of the plastic scintillator volume. (Scintillation photons produced during capture are excluded.) The mean detector optical photon response versus incident neutron energy is shown.

18. ANS Geant4 Simulations
Results
Figures 11, 12, 13, 14, and 15 show the optical photon response distribution for energies 1, 10, 19.9, 20, and 30 MeV for the ANS detector. Photons from before the capture are scored. (No photons are produced before capture for at 10 eV because there is not enough kinetic energy available.) The colored curves are for the specific identified peaks in the energy deposit graphs shown previously.

19. Figure 11 ANS detector optical photon response distribution for 1 MeV neutrons before neutron capture.

20. Figure 12 ANS detector optical photon response distribution for 10 MeV neutrons before neutron capture.

21. Figure 13 ANS detector optical photon response distribution for 19
Figure 13 ANS detector optical photon response distribution for MeV neutrons before neutron capture.

22. Figure 14 ANS detector optical photon response distribution for 20 MeV neutrons before neutron capture.

23. Figure 15 ANS boron-doped detector optical photon response distribution for 30 MeV neutrons before neutron capture.

24. Neutron Time to Capture
ANS Geant4 Simulations
Neutron Time to Capture
When neutron enter the detector they will not have a capture interaction until they have loss most of their energy and reached near thermal energies where the capture cross section is large. The time require to thermalize defines how far back in time one must look to see the photons produced during this energy deposition. Figures 16 shows the distribution of time to capture at a neutron energy of 1 MeV. The time for 95% capture was 45 microsecond over all energies.

25. Figure 16 ANS lithium-glass detector time to capture for 1 MeV neutrons.

26. Comparison of CPU time using MPI versus Multi-threading for the ANS Geant4 Simulation
The operating system used was Scientific Linux 6.4 with gcc and open MPI. The machine had two 8-core Xeon ES-2650 cpu’s running at 2 GHz with MB of memory. There were 16 MPI processes or 16 threads.

27. ANS Geant4 Simulations
References
Bicron, “Bicron Organic Brochure”.
M. Bliss, et al., “Relationship between Microstructure and Efficiency of Scintillating Glasses”, Mat. Res. Soc. Proc. Vol. 348, pp , 1994.