RF background, update on analysis Rikard Sandström, Geneva University MICE Analysis phone conference, October 30, 2007

Outline  What is new  Reminder of radiation yield, attenuation.  “New” MTA data.  Angles.  Number of photons per electron.  At 10.5 MV/m, simulated and calculated.  At 8 MV/m.  Rate in MICE Stage 6.  Other effects.  Phasings of cavities.  RF electrons heating absorbers.  Summary. 2R. Sandström

What is new  Distances are now have more exact values.  Al thicker, Cu thinner.  A bug in G4MICE prevented some photons to be registered by the Virtual Planes. It has been circumvented.  Only affected photons at large angles.  As recommended by D. Huang, using PMT#16 data (NaI) instead of PMT#8 data (Sci. paddle).  This makes a difference in rate, but NaI data is more reliable, below saturation point.  When using a series of cavities, maximum rates is downstream.  Calculations from 2004 contained a sign error on the phase. 3R. Sandström

Radiation yield = fraction of kinetic energy lost through radiative processes. Heavy elements (X 0 ) give higher radiation yield. Range of 1 MeV e- in Al = 2 mm. MICE: 0.18 mm Al window. Hence, combination of photon production in Al and LiH2 (+ Be, Cu). MTA: Electron ranges out in copper. R. Sandström4

Attenuation for PMT#16 5R. Sandström

MTA data Raw data scaled to events per RF period (5ns). PMT#8 data taken over full 125  s. PMT#16 data taken over 88.6  s flattop only. PMT#16 is smaller, but closer, than PMT#8. Virtually all photons hitting PMT#16 deposit all their energy in the detector, while only a fraction interact with the paddle, and not all energy is confined in active volume. PMT#16 is saturated at ~10MV/m. R. Sandström6 Areas mark where I have confidence in results

Simulation, angles In order to cross check, MTA was simulated using 3.6 million initial electrons. Solid black line is angular spectrum (after attenuation in metal) if isotropic angular distribution. R. Sandström7 e-e-

Results, number of photons per electron  At 10.5 MV/m, simulation gave  The calculation gave  The results show very good agreement!  For PMT8 even better agreement, within 0.4%. R. Sandström8

At 8 MV/m  This value is energy dependant  rad. yield, photon energy spectrum, attenuation, fraction above threshold  Simulation with E=1.226 MeV corresponding to 8MV/m:  Extrapolation between points of MTA data gave n  =4.03·10 -5 at 8 MV/m.  This implies n e = 705 per RF (half-) period.  Equivalent to 142 GHz per cavity and direction. 9R. Sandström

MICE Stage 8 MV/m, 200 MeV/c   These values implies total RF electron emission in MICE Stage 6 is 2.27 THz.  At the tracker ref plane with worst photon rate (per total number of electrons), based on old G4 simulations:  Hence, 2 GHz of photons in the tracker reference plane.  Electrons in track ref 23 MHz.  But very small statistical sample. 10R. Sandström

Other unaccounted effects  The multipactorlike avalanche effect I mentioned at CM19 would increase number of electrons from linac.  Unknown if rate increase is significant.  B > 0 increases background.  With a factor according to D. Huang, based on 805 MHz cavity data.  This would give a factor 16 at 3 T if applicable to 201 MHz cavity. 11R. Sandström

Old sins not forgotten  An error on the sign of the phasing of cavities was found.  It implies that maximum BG rate is always highest downstream. If running at negative polarity, same rates, but delayed by ½ RF period.  The rates per electron and energy spectrum based on those results unchanged, but upstream should be changed for downstream and vice versa. 12R. Sandström

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Phasing of cavities The energy of most electrons rather insensitive to exact phasing. The “turnaround electrons” however are very sensitive. Left: Phases optimized for 140 MeV/c muon. Note the double reversing electron. R. Sandström14 Downstream Upstream timeposition

On request: Heating of absorbers  142 GHz electrons emitted per cavity and direction.  Electron energy almost fully contained within AFC module.  For due to reversing electrons, upstream absorber sees 2 peaks, downstream absorber 6, and center absorber 8.  Thus, center absorber always has maximum load, no matter if electrons are reversing.  For cavities phased by reference 200 MeV/c muon, average electron energy is 3.80 MeV.  142*8*3.8 = 4.317·10 18 eV Hz = W  Less for other absorbers.  This is within MICE specifications (TRD: 15 W). 15R. Sandström

Summary  MTA PMT16 (NaI) data more reliable for field strengths relevant to MICE.  Calculated rates agree with simulated rates.  Old simulations of MICE + new simulations of MTA + MTA data results in 2 GHz of photons at the downstream tracker reference plane.  One order of magnitude lower rate upstream. 16R. Sandström