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RF background simulation in G4MICE

RF background simulation in G4MICE. Rikard Sandstr öm Université de Genève Video Conference 30 /6 -04. Assumptions. Amount of background MICE proposal says 3GHz of RF induced electrons hit the outer absorbers.

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RF background simulation in G4MICE

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  1. RF background simulationin G4MICE Rikard Sandström Université de Genève Video Conference 30/6 -04

  2. Assumptions • Amount of background • MICE proposal says 3GHz of RF induced electrons hit the outer absorbers. • (I misunderstood the proposal, they were talking of one absorber, I am shooting onto two. Due to symmetry of the experiment my results corresponds to one absorber and one tracker.) • Good muon rate = 600 per ms. • This gives 5000 e- per mu+, or 625 e- per mu+ and cavity. • Z-position of emitters • Using position of where a reference particle hits the beryllium windows these positions are set to z = -1849, -1379.72, -916.45, -394.76, 434.55, 900.8, 1367.3, 1833.55 [mm] • Cuts • 0.5 mm maximum step length was used in absorbers and absorber windows for precision. • Energy cuts are Geant4 defaults, for example 1keV for ionization.

  3. Method A • 625 e- per mu+ was generated at 8 circular disks, corresponding to cavity boundaries. • The electrons were given an initial direction towards closest tracker, and a random initial kinetic energy of 1-3 keV. • The electrons were accelerated in the field using Geant4 until they hit an object and interacted via other processes.

  4. Problems with method A • The spatial position of emitting sites on the cavities is nontrivial. • The time information of the field was unknown, so the electrons did not gain the maximum energy possible. • Hopefully this can be solved very soon. • Simulating 5000 e- per mu+ is very time consuming. • Poor statisitics.

  5. Method B • Two emitting disks where used, positioned inside the last cavity up- & downstream respectively. • At each disk four energy peaks are used for setting the initial kinetic energy of the RF electrons. They correspond to the energy gain of an integer number of traversed cavities, given by the default value of G4MICE parameter. (E = 2.775, 5.55, 8.324, 11.1 [MeV])

  6. Problems with method B • The energies of the RF induced background electrons must be set by hand. • This method does not allow particles to be bent in the field at low energy so that they never reach the absorbers. • Still, 5000 electrons per mu+ is time consuming, but now they almost immediately reach the absorbers where they do some good.

  7. Result with standard MICE physics list 10 mu+, 5000 e- photons accidental cut

  8. Result with standard MICE physics list 10 mu+, 5000 e- electrons accidental cut

  9. Comment to first results • Only one tenth of proper background electrons were used. • Still with the 5000 electrons (corresponding to the background to 1 muon track) 200 electrons are leaving the outer absorber windows heading towards the trackers. • The bremsstrahlung in G4MICE is flawed. • New simulations were performed using different ordering of physical processes applied to electrons:

  10. Result with modified physics list 10 mu+, 50000 e- (proper) photons From downstream From upstream (why not symmetrical?)

  11. Result with modified physics list 10 mu+, 50000 e- (proper) electrons

  12. Comment to second results • With a higher process priority of bremsstrahlung the amount of electrons leaving the absorber windows towards the tracker was reduced to 305 electrons per 50000 electrons generated inside the RF cavities. • The computed efficiencies are 30/5000 = 0.6% for e-, and 3.1% for photons. • The processes inside the absorbers were now (per muon track): • e : ionization = 2762, bremsstrahlung = 235.2 • gamma: compton = 157.2, photoconv = 24.7

  13. Discussion • We need to look closer at exactly what process rates we should expect in the liquid hydrogen. • Some of the electrons leaving the absorber windows are created inside the windows by photons. These are typically on the low energy side of the spectrum. However most of the electrons that are leaving the outer absorber windows have traveled through the entire absorber. • Even with the reordering of processes, 30 electrons per muon track manage to leave the absorbers. However due to the high angles at their departure few will reach the trackers. • This needs to be confirmed. Alain says the electrons will reach the trackers due to the field. • The 150 photons per muon track are much more aligned to the beam line and will hit the trackers.

  14. RF Background & SciFi • With a SciFi gate of 20 ns and the computed electron efficiency there will be 0.36 RF background electrons in the gate for each tracker. • The corresponding number of photons is 1.87 in the gate per SciFi tracker. • These values assume that all particles leaving the absorber window will hit the tracker.

  15. RF Background & TPG • The following slide contains a graphical illustration of a digitized typical event with the background turned on. • Please note that both background and muon trajectory is for one muon at the given rates. In reality the situation will be worse due to overlapping tracks. • The time information is not set, so they will not necessarily enter the tracker at the same time (as in the picture). • Tracks like these should be fed into reconstruction written by Gabriella et al and the simplified reconstruction written by Olena Voloshyn. • With an open gate of 60 microseconds and 3GHz of RF e- emitted, the electron efficiency rescales to 1080 electrons per drift time. • This is a serious problem indeed if all electrons do enter the tracker. • The corresponding number for photons is 5598. • An estimation of how many RF electrons and photons that reach the trackers is desirable. This will take very long computing time due to the rareness of such events (perhaps 1 particle per 50000 generated).

  16. Typical event with background, TPG mu+ mu+ e-

  17. Summary • Method A is desirable compared to method B, but it requires more of G4MICE. • The proper phases must be used to set the time of emission. • We would benefit from having an accurate description of spatial distribution of emitting sites. • More time consuming than method B. • Physics and absorbers in G4MICE • The process rates inside the absorbers need to be calculated by hand to confirm the behavior of G4MICE. • With given rates, 157 photons (effiency = 3.1%) and 30 electrons (efficiency = 0.6%) are leaving the absorber windows towards the trackers per muon track. • Most photons are close to parallel with the beam line, the emitted electrons have more random “emission” angles. • It still looks like the TPG will have problems with the RF induced background. • Higher statistics needed. • This will take time and/or a super computer.

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