X-rays in MICE
RAL 27/10 -04
Rikard Sandström
Geneva University
1
Outline
• Presentation of the newest RF background model
– Introduction
– Improvements
• Absorber design
• Phase and time information
– Reusing the background
• New results
– Particles leaving vacuum windows
– Particles reaching the trackers
• Summary
2
Introduction
• The RF background problem in MICE is still
present.
– But narrowing it down!
• Together with Yagmur I created a way for the user
to generate background, store it and reuse it
independent of the tracker simulated.
– We now have a standard background defined and used.
• Many improvements were made along the way.
3
Progress made past weeks
• Too much progress to report! This is from our homepage:
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check rf background output Done (October 18)Rikard
generate up-to-date rfBGMultipleSpectraFile for background production Done (October 15)Rikard
commit standard rfBGMultipleSpectraFile Done (October 14)Rikard
fix track visualization in interactive mode Done (October 15)Rikard
add support for generating only background with no beam Done (October 15)Rikard
add time delay when using bg bank file Done (October 14)Rikard
change input bank to SpecialVirtual planes in BGPlane Done (October 12)Rikard
move generatorBank output to EndOfEventAction Done (October 12)Rikard
clean up SteppingAction Done (October 12)Rikard
add individual time offsets for histogram peaks in BGPlane Done (October 11)Rikard
implement looping over input background hits in BGPlane and PrimaryGeneratorAction Done (October 8)Rikard
replace generatorBank output with Interface function in BeginOfEventAction Done (October 7)Rikard
update MICEPrimaryGeneratorAction and MICEBGPlane to handle reading rf background from Sim.out Done (October 7)Rikard
update MICEPrimaryGeneratorAction to handle multiple background planes Done (September 30)Rikard
add class for background planesDone (September 30)Rikardcommit PhysicsList update Done (September 13)Rikard
add torispherical window to AbsorberVessel Done (October 6)Rikard
clean up AbsorberVesse lDone (September 30)Rikard
implement spherical shell window in AbsorberVessel Done (September 23)Rikard
add flat vacuum windows with no body Done (September 13)Rikard
fix absorber vessel geometry Done (September 13)Rikard
fix hydrogen composition Done (August 26)Rikard
add rfBG parameters to handle reading from Sim.out Done (October 7)Rikard
remove integer cast for rfBGParameters.fBGDirection Done (October 7)Rikard
add circle radius offset for torispherical window to AbsorberParameters Done (October 6)Rikard
update RFBackgroundParameters Done (September 30)Rikard
add get/set methods to VirtualHitBank Done (October 7)Rikard
add static WriteSim to generatorBank Done (October 7)Rikard
fix energy output in generatorBank Done (October 7)Rikard
add rfBG parameters to handle reading from Sim.out Done (October 7)Rikard
add torispherical absorber parameter to dataCards Done (October 6)Rikard
• …and much much more done by Yagmur, Malcolm, Chris and
others.
4
Levels of description & understanding
OK in z,t
Not considered x,y
Requires more studies
All good
OK
Improved in Geant4.6?
5
Improved absorber representation
• Absorbers and vacuum
windows supports different
geometrical shape.
– Default is spherical.
• All optional absorber
shapes have the central
window thickness and
central liquid hydrogen
thickness set to the latest
design.
• Flange sizes might need an
update.
6
RF phase calculations in Matlab
• G4MICE now supports fixed E-field.
– Working and tested.
• G4MICE also support for time dependent field.
– Needs more testing to confirm working.
In the meantime, calculate with Matlab (faster):
• The phases for the background electrons assumes phases
optimized for a mu+ at 200 MeV/c on axis.
– This causes a symmetry breaking in z!
– Assumes phase difference between neighboring cavities is
constant.
– Phase diff = 2.0498 rad = 1.621 ns.
– This gives the muon an energy gain of 10.8 MeV per set of four
RF-cavities, including energy loss in Be windows.
– Energy loss is calculated using STAR data for ionization and
bremsstrahlung.
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RF phase calculation, mu+
8
Accelerating e- in the RF, intro
•
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The electrons are assumed to have zero kinetic
energy when emitted from beryllium windows.
The electrons emitted at the peak values of the Efield only (+ and - respectively).
They are accelerated using the same Matlab model
as used for the muon.
This results in
1. energy when leaving the RF-system
2. travel time for leaving the RF-system
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Accelerating e- in the RF, 2 cavs
Downstream
direction
Upstream
direction
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Accelerating e- in the RF, 4 cavs
Downstream
direction
Upstream
direction
11
Comments on Matlab results
• The RF phases are set such that
– electrons have higher energy in upstream direction.
– some electrons turn around if starting with downstream
direction.
• The situation is better with 466 mm cavities than with 430
mm cavities
– In the first case the “turn around” electrons are stopped in the
Be windows as they turn -> Only 2/3 of BG upstream!
• Worth considering:
– If we optimize for a slightly different pµ, can we reduce the
background by the change in RF phases?
– If we allow e- to be emitted off crest, how much worst could it
get?
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Generating the background
• Generating the background as calculated in Matlab at red
locations.
• Extracting data at green locations.
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Feeding Matlab into G4MICE
1. The time of emission from each Be window is calculated using
the phases.
2. The time of arrival is the travel time + the time of emission.
3. The downstream RF system is offset in time by looking at a
reference muon in G4MICE.
4. Background from different RF periods is achieved by repeating
with an integer RF period offset in time.
–
Number of periods is chosen with respect to the flight time of the muon.
5. The electrons are distributed evenly over 21 cm in radius just
outside the outer beryllium windows.
–
r = R*sqrt(rand[0,1])
6. Particles are assumed to be parallel to beam line initially.
7. Emission rate is given by the measured 40 kHz/cm2 at 8 MV/m
measured at Lab-G.
8. Only flip B-field simulated so far.
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Uncertainties
• Rate of e- emission
– Largest uncertainty is area considered for emission. (Bessel
functions…)
– We use worse case here, tool for scaling already exists.
– Will be studied experimentally at Fermilab this winter.
• Off crest emission
– If the particles were allowed to be emitted somewhat off crest
E peak, energies might change.
15
Generated RF e-, E_kinetic, upstream
MeV
16
Generated RF e-, E_kinetic, downstream
MeV
17
Generated RF e-, time
ns
mu enters RF system
mu leaves RF system
18
Background event, few ns
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Reusing the background
Background bank generated, works, supports looping
and scaling:
• Due to the small fraction of particles making it to the trackers, hits
at the tracker entry is saved and can be reused.
– Excellent optimization.
• The user is responsible for setting the expected time when the
muon is arriving at the cooling channel.
– Different input beams -> different t0 and z0.
• Should a simulation run out of such background events, looping
over the background bank is supported.
– Random event from BG bank might be implemented.
• Background can be scaled down from nominal value, should later
be possible to scale up too.
• Malcolm has produced thousands of BG events, Yagmur has
merged and filtered to one file.
– Available on our homepage.
20
Particles leaving vacuum windows
• Using mice2root instead of my old readout code.
– Very nice but not problem free.
• Due to a bug momentum and energy is not saved correctly
to output for photons in vacuum windows.
– Simulation itself handles them fine though. Propagation to
trackers OK.
• No problem with e-, but…
– Not part of the standard output Malcolm is generating -> Must
run locally -> Low statistics.
– Low statistics data shows expected 1-1 correspondence
between what is leaving the vacuum window and what is
reaching the tracker. -> OK to use track ref planes (high stat)
to investigate e- from absorbers.
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e- reaching the upstream tracker
Still not the 1000 requested
MeV
22
e- reaching the downstream tracker
Very few
MeV
23
Photons reaching the upstream tracker
Follows Landau distribution
Reaches full input energy
MeV
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Photons reaching the downstream tracker
MeV
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Results, rates
• Assumed emission rate:
– Given the 40 kHz/cm2 of emitted electrons off a cavity at 8 MV/m,
201.25 MHz frequency, and a conservative assumption of area to be
considered, we have 7.92 e- hitting the absorber per energy peak (and
period).
– 8 energy peaks * 2 linacs * 7.92 = 126.72 e- per period in total.
• Very few particles generate a hit at the tracker entrance but some
do.
• Data for particles reaching trackers given as per total generated,
and scaled to frequency:
BG,
Upstream,
tracker
per
entrance generated
Upstream,
[MHz]
Downstream,
per generated
Downstream,
[MHz]
e-
5.76e-6
0.147
1.72e-7
0.00438
Photon
8.315e-4
21.21
6.934e-5
1.768
26
Future plans
• Close to go to latest release of Geant4.
– This will hopefully solve some issues we have had with
physical processes, step length dependence etc.
• Renormalize BG rate using new data which shall be
taken experimentally at Fermilab.
• Run a few events with non-flip B-field.
• Regenerate the background bank file if necessary.
• Solve a long list of bugs assigned to me by Chris et
al…
27
Summary
• Standard background defined, generated and used
by all.
• Rates in upstream tracker (worst) are 21.21 MHz
for photons, 147 kHz for electrons.
• Future experiment might change assumption of
emission -> Rescale rate!
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