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RF background simulation: proposal for baseline

simulation

Video conference 22/9 -04

Rikard SandströmGeneva University

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Outline

1. Status of the background simulations1. Reminder of Osaka results2. What we now know was wrong at Osaka3. New results

1. Energy dependence2. “Real” scenario, upstream side

2. Proposal for the baseline background simulations

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Reminder of Osaka - setup

• 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], weighted equally.) – This is pessimistic, since the field is synchronized for muons, not

electrons!

• The results presented later correspond to Method B, but only looking at the outermost upstream absorber window, and the upstream tracker. (worst case…)

mu+e- e-

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Photons Abs.window TPG

Vagabond 79.2 MHz 20.3 MHz

Immigrant - 0

Emigrant 240 kHz 78 kHz

Confined - 18 kHz

Electrons Abs.window TPG

Vagabond 26.3 MHz 24.3 MHz

Immigrant - 22.9 MHz

Emigrant 78 kHz 1.2 MHz

Confined - 64.0 MHz

Reminder of Osaka - rates

• I was assuming 3 GHz of background electrons reaching each absorber (from MICE proposal).

• That assumption gave these background rates:

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Energy dependence

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101520253035

2.7

75

5.5

50

8.3

25

10

.00

0

11

.10

0Energy [MeV]

Ra

te [

MH

z]

TPG, vag e-

TPG, vaggamma

Abs, e-

Abs, gamma

Energy dependence

(x-axis not linear)

easy hard

E =

?

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What we now know was wrong• There was a bug in the liquid hydrogen material description.– Fixed.

• There was a major bug in the absorber geometry. Only half was there! :( – This has now been fixed for flat windows.

• Asymmetry observed in June -04 simulations is gone.• Lower electrons rates leaving the cooling channel. (More

optimistic.)

– I am working right now on implementing curved windows as well.

• No news from the Geant4 group regarding the strange behavior of the physics in the absorbers as very dependent on the max allowed step length.– The problem was reproduced using a very simple

example and sent to the Geant4 group.

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New results - setup

• These results are all with flat absorber windows, flat vacuum windows

• Absorbers use the new improved geometrical description.

• The RF electrons are emitted at disks right in front of the inner vacuum windows, and they are distributed uniformly over a radius of 15 cm. Energies are God-given.

• I have made no changes to the physics since Osaka.

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Picture direct from G4MICE (CVS)

Be windows, flat but thinner in the middle

Flat vacuum windows

Flat absorber windows

Line of no importance

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New simulation – energy dependence

• One thing which was requested at Osaka was to investigate the energy dependence of the RF background. This is presented here.– Initial electron kinetic energies goes from 1 MeV

to 20 MeV, with 1 MeV binning. – The BG sample is 5 muons, and the 20 energy

peaks are each 6250 of e- on both outer absorbers.

– Osaka results used 50000 e- per energy peak in one direction only. (4 times larger statistical sample.)

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Energy dependence, photons

Bremsstrahlung, linear dep is expected

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Energy dependence, electrons

e- spiraling on border of active and non-active gas region.

(Each time it enters the active TPG volume is counted as an individual track.)

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Photons leaving vacuum windows

Previous asymmetryresolved

exp(-E) behaviour

UpstreamDownstream

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Electrons leaving vacuum window

High E-loss observed

UpstreamDownstream

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New results – energy dependence

• Very few electrons leaves towards the tracker.– The fact that we see a weak energy dependence

suggests that they are mostly from photon conversion, not energy straggling.

– That 20% of the electrons are created inside the vacuum windows supports this idea. (How many are created by photons in the absorber windows?)

• These results were also the basis for an investigation of emission angle as a function of rho (at flat vacuum window). The results for photons is given on next slide.

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Photons leaving absorber windows

UpstreamDownstream

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Comments on rho-theta dependence

• Many photons are leaving at a large rho, with large angles. – They could still reach the trackers by scattering?

• Theta is not an ideal parameter since it is the angle from the z-axis. It does not say whether a particle will go towards or from the tracker (radially).

• Instead I have been using extrapolation of the photon tracks as they leave the vacuum windows. This gives a rho at vacuum window vs rho extrapolated to tracker entrance. (See later slides).

p

θ

z

ρ

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New simulation – “real” case• In order to investigate what our real background rates

are electrons were assumed to be emitted at peak field 8 MV/m, thus giving four energy peaks at E_kin = {1.3, 2.8, 5.1, 7.8} MeV.

• Only upstream side is simulated– Simulating only the worst direction

• Uniform spatial distribution of RF electrons, radius 15 cm, just in front of the vacuum window.

• Results are based on flat absorber windows and flat vacuum windows.

• Sample is based on a total of 100*(1 mu+ & 5000 e-)• On the first two slides, y-axis is particles leaving

absorber (or arriving at tracker) per initial e- of each E-peak separately. The third slide is per initial e- in total for all four peaks.

• The “tracker” in this case refers to the first plane of the TPG.

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E-dependence, photons

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E-dependence, electrons

No e- from lowest E-peak

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Energy when leaving vacuum win.

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New results• The rates for particles leaving the upstream vacuum

window are given as both (# particles leaving towards tracker)/(# initial e-) and also rescaled to MICE proposal rates.

• Over all lower rates than at Osaka– Photon rate is about 38% lower than Osaka results.– Electron rate is only 0.4 % of the Osaka rates!

Electrons Vacuum window

Vagabond 0.0028 % of initial e- (84 kHz at 3 GHz initial)

Emigrant 0.0012 % of initial e- (36 kHz at 3 GHz initial)

Photons Vacuum window

Vagabond 1.6511 % of initial e- (49.5 MHz at 3 GHz initial)

Emigrant 0

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Extrapolation, rho vs xpol rho

TPG Kapton tube

TPG active region

SciFi active region

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Comment on results• For photons, we observe the expected linear energy

dependence.• 18.5% of photons leaving absorber windows reach the

tracker (here the first plane of the TPG, radius 170 mm). • The extrapolated graph shows a rough linear dependence:

– rho_xpol = 2*rho_window– Extrapolation is to z = -4254 mm (the first SciFi plane) from z =

-2925 mm (the vacuum window).• For electrons, very few electrons leave the absorber system

and those have a weak energy dependence.– Electrons created by the photons in absorber and vacuum

windows.• Of the 19 of 20 e- leaving the vacuum window reach the

tracker (here the first plane of the TPG).• Running the same simulation without background, 20000

mu+, gives a negligible contribution to the background rates from the vacuum windows.

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Proposal for baseline BG simulations

Assumptions & requirements1. Assume we are in Step V, (or that we are in

Step VI, but only a negligible number of e- and gamma traverse the central absorber)• Hence the background source can be treated as

contained in one RF-cell and closest outermost absorber.

2. Assume mu+ beam and upstream absorberThe results for upstream and downstream will be different due to the RF-field. Worst is upstream when RF optimized for mu+, and downstream when optimized for mu-.

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Proposal assumptions & req. (cont)

3. Energy: Assume all electrons emitted at peak RF-field of 8 MV/m. This gives 4 peaks in energy at {1.3, 2.8, 5.1, 7.8} MeV for the worst direction (see point 2) (including E-loss in Be Windows)

4. Assume the four energy peaks to be weighted equally.5. The worst case is probably non-flip B-field. Should

baseline be flip B-field, optimized for beta = 42 cm optics? • Both situations should be simulated before we define the

baseline. 6. Simulate background using

1. Flat absorber windows, no vacuum windows2. Flat absorber windows, flat vacuum windows3. Curved absorber windows, no vacuum windows4. Curved absorber windows, curved vacuum

windows

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Proposal assumptions & req. (cont)

8. The simulations should give a bank of electrons and photons at the entrance of the tracker volume.• Information in the bank will be (for each entry, lab frame)

1. positon (3-vector)2. momentum (3-vector)3. time4. particle ID5. parent particle ID6. kinetic energy of primary generated particle

9. We need statistics for a minimum of 25*106 initial e-.• (e- leaving absorbers)/(e- initial) = 4*10-5, would give 103

electrons in the bank. (this conclusion may change after no-flip mode is simulated)• On my machine this would take 21 days (per run).

10. All code used to create the results should be in CVS repository of G4MICE.

11. G4MICE should be able to use a saved bank of background to generate it right in front of the trackers.