RF background simulation:
proposal for baseline simulation
Video conference 22/9 -04
Rikard Sandström
Geneva University
1
Outline
1. Status of the background simulations
1. Reminder of Osaka results
2. What we now know was wrong at Osaka
3. New results
1. Energy dependence
2. “Real” scenario, upstream side
2. Proposal for the baseline background simulations
2
Reminder of Osaka - setup
e-
e-
mu+
• 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…)
3
Reminder of Osaka - rates
• I was assuming 3 GHz of background electrons
reaching each absorber (from MICE proposal).
• That assumption gave these background rates:
Photons
Abs.window TPG
Electrons
Abs.window TPG
Vagabond
79.2 MHz
20.3 MHz
Vagabond
26.3 MHz
24.3 MHz
Immigrant
-
0
Immigrant
-
22.9 MHz
Emigrant
240 kHz
78 kHz
Emigrant
78 kHz
1.2 MHz
Confined
-
18 kHz
Confined
-
64.0 MHz
4
Energy dependence
hard
easy
35
30
25
20
15
10
5
0
TPG, vag e-
Energy [MeV]
11.100
8.325
E=?
10.000
5.550
TPG, vag
gamma
Abs, e-
2.775
Rate [MHz]
Energy dependence
Abs, gamma
(x-axis not linear)
5
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.
6
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.
7
Picture direct from G4MICE (CVS)
Be windows,
flat but thinner
in the middle
Flat vacuum
windows
Flat absorber
windows
Line of no
importance
8
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.)
9
Energy dependence, photons
Bremsstrahlung,
linear dep is expected
10
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.)
11
Photons leaving vacuum windows
Previous asymmetry
resolved
exp(-E) behaviour
Downstream
Upstream
12
Electrons leaving vacuum window
Downstream
Upstream
High E-loss observed
13
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.
14
Photons leaving absorber windows
Downstream
Upstream
15
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).
p
ρ
θ
z
• 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).
16
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.
17
E-dependence, photons
18
E-dependence, electrons
No e- from
lowest E-peak
19
Energy when leaving vacuum win.
20
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!
Photons
Vacuum window
Vagabond
1.6511 % of initial e- (49.5 MHz at 3 GHz initial)
Emigrant
0
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)
21
Extrapolation, rho vs xpol rho
TPG Kapton tube
SciFi active region
TPG active region
22
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.
23
Proposal for baseline BG simulations
Assumptions & requirements
1. 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 absorber
The 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-.
24
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.
2.
3.
4.
Flat absorber windows, no vacuum windows
Flat absorber windows, flat vacuum windows
Curved absorber windows, no vacuum windows
Curved absorber windows, curved vacuum windows
25
Proposal assumptions & req. (cont)
8.
The simulations should give a bank of electrons and photons at
the entrance of the tracker volume.
•
9.
Information in the bank will be (for each entry, lab frame)
1.
2.
3.
4.
5.
6.
positon (3-vector)
momentum (3-vector)
time
particle ID
parent particle ID
kinetic energy of primary generated particle
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.
26