Slow Neutron Background Simulation

Long-lived neutrons created, diffuse around collision hall

They get captured by nuclei, emitting a photon

Compton scattering or photoelectric effect makes MeV electrons, which cause hits in muon chambers

Why is Neutron Background Hard to Simulate?

• Because neutrons can live up to a second before making a signal

• They can’t be treated like ordinary minimum-bias pileup, because millions of collisions in the past can contribute

The Way It Was Done Before:Parametrization

~6 years ago, UC Davis group (Hessian, Fisyak, Breedon)

Based on 2000 simulated min-bias events, simulated down to low energies and long times

Start with “mother” hits with some distribution in energy, position, and direction

Add some number additional hits in same layer

Propagate each hit to next layer and repeat

Disadvantages of Parametrization

• Hard to maintain Many parameters Needs to be done separately for each detector type for

CSC, DT, and RPC Needs to be updated when geometry or shielding changes

• Can we use the original events rather than a parametrization of them?

What I’ve Done: Database of Neutron Hit Patterns

• Start with a sample of simulated min-bias events• Take the events apart. Treat each chamber with hits as

an independent event. Zero out the time.

Database of Chamber SimHit PatternsStore these patterns of neutron hits in a ROOT file, grouped by

chamber type:

ME1/A ME 1/1 ME1/2 ME1/3 ME2/1 ME2/2 ….

….

When I need to add neutron background to the simulation, I just read in some number of these patterns and superimpose them

•Done before electronics simulation, of course, so things pile up correctly.

How Many Patterns to Superimpose?Say we simulate a window of +-10 bunch crossings around the event

That’s ~280 min bias events (at 1034) that may create signals in future crossings.

I think we can assume that the amount of neutron signal in our 21-bx window is the total amount that would come from ~280 minimum bias events in the past.

If ME2/1 chambers have a neutron-induced occupancy of 0.25% per min-bias event per chamber, we should superimpose a Poisson mean of 280*0.25% = 0.7 hit patterns per chamber

Further Studies

The Thai group seems interested

• They have good GEANT3 and GEANT4 skills

Lots to do:

• Comparing fluxes and spectra from different generators

• Creating and maintaining neutron datasets