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Simulation of RF background in MICE

Rikard SandströmUniversity of Geneva

NuFact’04Osaka

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Introduction

Tracker

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Assumptions• Amount of background

– MICE proposal says 3 GHz (3 per ns) of RF induced electrons hit one of the outer absorbers.

– Good muon rate = 600 kHz (600 per ms) – (For technical reasons one event = 1 muon + 5000 e-)

• Position of emitters– The z-positions of the emitters are set to be at the beryllium

windowsz = -1849, -1379.72, -916.45, -394.76, 434.55, 900.8, 1367.3, 1833.55 [mm]

– Transverse distribution is uniform over the beryllium windows.• 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 discrete ionization.

• Simulation environment– G4MICE was used, which is based on GEANT4.5.2.p02 (October -

03 release)

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Two different methods

• Two different methods (A,B) have been used to simulate the RF background problem.

• Method A tries to simulate emission from the surface and acceleration in E&B fields. Close to physical reality, but some problems.

• Method B generates a spectrum of e- at the exit of cavities. More assumptions, but works.

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Method A

• 1250 e- per mu+ were 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 fields using Geant4 until they hit an object and interacted via other processes.

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Method A

• Interesting issues:• The spatial position of emitting sites on the

cavities is nontrivial.• The MICE proposal gives 3GHz of electrons

reaching the absorber, (projected from 805 MHz measurements in lab G) but it does not say how many electrons are emitted from the surfaces.

» Hence more knowledge of the physics of the emission is required.

Trivial technical problem: the phase of the RF field in different cavities is hard to set correctly, so the electrons did not gain the maximum energy possible.– Hopefully this can be solved very soon.

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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], 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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Results - Introduction

• The resulting particles were categorized and colour coded according to their position of creation and destruction.

”Confined”: The particle was created inside the region of interest and was destroyed there as well.

”Emigrant”: The particle was created inside the region of interest and left the volume.

”Immigrant”: The particle was created outside the region of interest and was destroyed inside the region.

”Vagabond”: The particle was created outside the region of interest and left the volume.

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Electrons leaving absorber

0.9% of200 000 e-(40 mu+)

Min E-loss for the 11.1 MeV peak

~ 8.7 MeV

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Electrons in upstream TPG

High energy electrons coming from RF

Some are later scattered back into the tracker again

High-E e- coming fromthe RF

e- from conversions

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Comment on results, electrons• Running 20000 muons without RF background the following results were found: Abs.window TPG

Vagabond 4.92 kHz 120 kHz

Immigrant - 630 kHz

Emigrant 0.12 kHz 24 kHz

Confined - 546 kHz

Abs.window TPG

Vagabond 26.3 MHz 24.3 MHz

Immigrant - 22.9 MHz

Emigrant 78 kHz 1.2 MHz

Confined - 64.0 MHz

• This should be compared with the RF background turned on:

102 kHz entered from target side

Small contribution

Dramatic change!~20 MHz

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Photons leaving the absorber

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Photons in upstream TPG

Photons entering with angle are mirrored against surrounding kapton

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Comment on results, photons

• Without RF background: Abs.window TPG

Vagabond 0.48 kHz 18 kHz

Immigrant - 0.6 kHz

Emigrant 0.12 kHz 0.18 kHz

Confined - 0.0000

Abs.window TPG

Vagabond 79.2 MHz 20.3 MHz

Immigrant - 0

Emigrant 240 kHz 78 kHz

Confined - 18 kHz

• With RF background:

Almost zero

Dramatic change (again)!<19 MHz

8.0 MHz go through all volume side to side

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Physics inside absorbers

• The computed efficiencies are 0.88% for e-, and 2.65% for photons.– This is for particles leaving the absorber window towards

the tracker.

• The processes inside the absorbers were (per muon track, with background):– e-: ionization = 2931, bremsstrahlung = 267,

multiple scattering = 0.1– gamma: compton = 120.0, photoconv = 30.0– mu+: ionization = 8.6– This is inside the liquid hydrogen, not the windows.

• Behavior of Geant4 has been erratic and I can not say I fully trust electromagnetic processes in Geant4...

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RF Background & TPG• With an open gate of 60 microseconds and 3GHz of RF e- emitted,

the electron efficiency rescales to 1458 high energy electrons per drift time, or a total of 6745 e- with energy higher than 1 keV. 1350 electrons traverses the entire active volume.– This is a serious problem.

• The corresponding number for photons is 1222.• If the plans regarding shortening the TPG to 25 cm go through,

these values should be scaled down to 1/4.

• 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.

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Typical event with background, TPG

Upstream, with BG Downstream, without BG

BAD!

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RF Background & SciFi

– Assuming that the number of high energy (red) electrons and photons are identical for the upstream TPG and upstream SciFi:

• With a SciFi gate of 20 ns there will be 0.5 high energy electrons in the gate for each tracker.

• The corresponding number of photons is 0.4 in the gate per SciFi tracker and therefore will produce much fewer hits than the direct electrons.

The dominant source of background are electrons directly produced from the absorber (and windows)

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Energy dependence

• dE/dx due to ionization decreases with energy.– Hence the number of electrons that leaves the

absorbers increases with energy.

• dE/dx due to bremsstrahlung increases with energy linearly.– This makes the number of photons leaving the

absorbers increase linearly with energy.

• MICE will be very sensitive to how much energy the RF induced electrons gain in the cavities.

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Energy dependence

05

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 (plot)

(x-axis not linear)

easy hard

E =

?

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How realistic are the assumptions?

• Hand calculated phases (optimized for muons) gives lower energies to the RF electrons, if emitted at peak phase:– 1.3, 2.8, 5.1, 7.8

[MeV]– Then we only need to

deal with photons!• But, electrons can still

gain the energies I have simulated if emitted a bit off-peak.

• Bypassed some windows in this simulation.

• More realistic model of emission necessary for further study.

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

8.0000

9.0000

0 2E-09 4E-09 6E-09 8E-09

z

Ekin

p

beta

Thanks Alain!

Deceleration due to field mismatch

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Open questions

• Can we improve these results by changing the design of absorbers?– Thickness– Different material (Z)

• To what level should we trust Geant4?– It is a high energy tool, with low-E extensions.

• RF radiation measurements with prototype 201 MHz cavity.

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Summary• Method A is preferable 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. (=> Lower rates?) – More time consuming than method B.

• Physics and absorbers in G4MICE– Found discrepancies in how Geant4 performs electromagnetic

interaction at low energy.• Particle rates, upstream absorber and tracker:

• It still looks like the TPG will have problems with the RF induced background. SciFi?

• The problem is strongly depending on energy.

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

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Extra slides

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Electrons from 11.1 MeV peak, abs.

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Photons from 11.1 MeV peak, abs.

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Range of electrons in TPG (up)

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Range of photons in TPG (up)