Road to complete Geant4 simulation

1. Road to complete Geant4 simulation Ivan Logashenko James Miller Physics Department, Boston University

2. The simulation project structure • It will be impossible to implement single Geant4 simulation for the whole experiment. At least, the following pieces are necessary: • background simulation (can be coupled with the neutron transport simulation) • He3 simulation during storage time • Light output simulation • UCN storage simulation (uses results of all previous pieces and the field configuration calculated elsewhere) • Although these tasks are independent, it is better if the same computer program is used for them (with different configuration, some pieces switched on/off, etc.). We should use the common detector geometry for all of the tasks.

3. Geometry description • There are several ways in which geometry can be entered into Geant4. The most common is to describe the geometry directly in C++ code (with a few parameters that can be changed during run-time). This is the most convenient way, but it is limited if the system is large. • In our simulation, the most useful approach would be to use GDML files (text files, in which geometry and materials are described in a readable-editable form). Advantages are: • easier to combine geometries from sub-pieces • easier to organize different versions of the geometry • mechanical drawings can be exported to GDML (tricky, but possible) • If this is the approach we are going to use, then each group doing Geant4 simulation, should try to switch to GDML – it will take some effort.

4. Import of the mechanical drawings into Geant4 The drawings are imported the following way: Drawing  STEP Viewer  intermediate files  GDML converter  GDML All imported objects are tessellated, which is not very efficient in simulation. The best strategy is to use manually-edited GDML files, where simple objects are put by hand and complicated ones are imported from the drawings. It is important to use the same naming scheme throughout the experiment. Imported geometry

5. Backgrounds simulation • This is a typical Geant4 task. We should build the experiment geometry, generate neutrons at the entrance with proper distributions and let them go, get captured, etc. The idea is to simulate what particles we get in the central detector at storage time. • The second part of the background simulation is to simulate cosmic rays. Everything is similar, except for the primary generator – we start with the cosmic rays at storage time. • The challenges here are: • to build the whole geometry • to check if the neutron cross-sections and decay models, defined in Geant4, are ok

6. He3 simulation • He3 simulation in Geant4 is tricky, because we have to treat it as a particle or as a material depending on the task • When one simulates He3 diffusion and spin rotation, He3 is just another particle. The usual Geant4 machinery can be used to track He3 atoms one by one. As a result of the simulation, one can get the time-dependent maps of the He3 location (density) and spin orientation. • When one simulates neutron storage, He3 has to be considered as a material (n and He3 cannot be tracked at the same time). The maps generated in the separate He3 simulation, can be used to get the proper probabilities for n-He3 capture.

7. Light simulation In Geant4, the final point of every simulation step is the local energy deposition. We can implement the full simulation of how the energy deposition becomes the PMT signal. But this approach is impractical – too many optical photons to track. A separate simulation can be done in which the optical photons are propagated and the effective function Edep(x,y,z,time,particle)  PMTs signals (time) is constructed. This function can be used for digitization in every simulation task.

8. Ultra-cold neutrons simulation (1) Also a typical Geant4 task (assuming the background, light, and He3 simulations are performed before), but there is a problem: the features of the ultra-cold neutrons (reflections on the material boundaries, gravity) are not implemented in the standard Geant4 distribution. The extension code, Geant4UCN, was developed at PSI, which implements all these features. While Geant4UCN does successfully track UCNs, it has some limitations: UCN is not a neutron (therefore most of the physics processes are not defined), the spin tracking is limited. We have to extend the code, and should decide if we are going to stick with Geant4UCN, or build our own, using Geant4UCN as an example.

9. Ultra-cold neutrons simulation (2) • UCN processes are defined in Geant4UCN by several simple constants (in addition to Fermi potential of material): • The boundary reflection is described by diffuse scattering probability, loss probability and spin-flip probability • The transmittion inside the material is described by loss (constant), absorption (1/v) and elastic scattering cross-sections • Is this approximation good enough and do we know all these constants? • There are no n-He3 capture cross-sections implemented. • There is no downscattering, but it can be implemented in the primary generator.

10. Spin tracking 1 sec 10 sec 100 sec Spin tracking seems to work fine in current version of Geant4 After 100 sec

11. Output data format and data analysis The simulation program will consist of several pieces. There going to be at least several input/output data files. These files can be any format, but it would be very convenient to have the common base for these files. It is very convenient to use ROOT trees as a base, mainly because it would simplify the data analysis. It would be a good idea to store data analysis code (ROOT scripts) in the software repository, close to the simulation code. That helps to build expertise. Eventually, we’ll want the output file from the storage simulation to be in the same format as the raw data.

12. Software repository At the moment, each group develops code independently. Eventually, we want to have a single project, which everyone can use or modify for the particular task. To reach this goal, we have to reach agreement on: • Software repository – a centralized place, where the code is archived and available • Standard (SVN?) • Location • Software standards • Geant4 version • ROOT version • Anything else?

13. Organization It would be very helpful to organize the group of people who develop and analyze simulation. That would be a place to ask questions, discuss the code organization and the technical issues, share the achievements To make the group functional, we’d need all the usual tools: • mailing list • teleconferences (not too often, but regular) • web-site (ideally, a wiki site)

14. Ongoing simulation efforts I’m aware of E. Beise, Maryland ,T.Ito, Los Alamos: Lightguide simulations D. Dutta, Mississippi: incorporating A(n,gamma)A' cross sections into GEANT W. Korsch, C. Crawford, Kentucky: neutron beam line B. Filippone and colleagues, Caltech: simulations of spin precession (QM and classical), geometric phase studies, magnetic field maps D. Beck, S. Clayton, Illinois: 3He simulations I. Logashenko, BU: GEANT-based central cell simulation