TPG digitization update Phone conference 18/2 -04 Rikard Sandström Geneva University

Purpose Code in CVS assumes all particles are ideal muons when it comes to ionization of the TPG active gas... The present way of calculating electrons in hit uses Riemann zeta function without cutoff. We need to treat delta- and RF-electrons differently than muons since 1.They have lower energy. (Most important reason.) 2.They are emitted parallell to the gas slices, the muons are not. (Only applies to delta electrons and electrons created by photons on the cylinder surface.) 3.They take a longer effective path. (Scattering)

Two main paths In order to come to terms with these problems I have developed a procedure to solve it. Edda has a second idea how to do this. I will put the code in the CVS once we converge, and the code is written and checked.

Path Edda (as I have understood it) For muons, should not create delta electrons explicitly. They are included in the clusters/cm and electrons/cluster from GARFIELD. –They are now given as mean values but should be special input files (histograms). Photo-electrons have low E and hence loose energy too quickly to use the above method for muons:

Path Edda (cont.) “A simple model for the electrons is the following: A) Calculate the average electrons range for the given energy (see BLUM/ROLANDI first pages...). B) Divide the energy by the average ionization energy for the given gas mixture -> this gives the total number of electrons on the track. C) In reality the number of ionization electrons increases towards the end of the track.../ /...(Bragg Peak), but this is not so crucial, I would just distribute the electrons evenly along the track. There is one problem is with electrons that have sufficient energy to leave the gas volume, then only a fraction of their energy is deposited in the gas and we have a situation that is somehow between 1) and 2) For this you must invent somthing clever... between 1) and 2)” This I can do but it requires changes to MICESteppingAction The formula I sent to the mailing list

Path Rikard #1 Basic idea is to take parameters for an ideal muon and scale it down to the energy of the particle. –dE/dx ~ 1/E(kin) –Valid for muons up to p ~ 100MeV/c (This could be a problem but we should be within 50% of correct results at worst.) Min(range) in TPG gas = stripspacing, implies min(E) = 1keV for delta e-. All particles below this threshold are treated with the “GARFIELD-cluster method” and not explicitly simulated. –Electrons higher than threshold ionize with the same procedure as muons (creating clusters). –We could create values for higher thresholds as well if too slow

Path Rikard #2 Hence I want to scale –clusters/cm with 1/E(kin) dependance, using Poisson distribution around the scale mean value. –electrons/cluster with a normalizing factor given by the maximum ionisation as a function of the particle energy. I will use P(n)=N/n 2 with N =1/  i -2, i = 1, 2,......, E/W, where W is mean energy to ionize gas (material constant). (In fact, W is required by both paths.)

Path Rikard #3 These changes will be needed to Sim.out: –Add kinetic energy (for scaling) –Add tracklength inside a gas slice (to allow trajectories not along beam line) –Probably no information of particle type (e/mu etc) is necessary as long as only charged particles are generating hits in the SensitiveDetectors. Potential problem: –If the delta electrons carry off a large fraction of the energy from the cluster energy conservation will be broken, unless these electrons are excluded in the GARFIELD simulation.

GEANT4 GARFIELD A problem both paths have is the interface between G4 and Garfield: Since G4 changes energy and direction of the particle we have to rely on that the energyloss is close to equal with that of GARFIELD, or we will have energy conservation violation on a macrolevel. Should they both give the same energy loss, we would still have energy conservation violation in each step but it would average out in the long run.

So... Which method will be –most accurate? –fastest to run? –fastest to put into code?