RF background simulations MICE collaboration meeting Fermilab Rikard Sandström

Outline Reminder –How it was done –The results Future plans

RF background At high fields, electrons are emitted from irregularities in the RF cavities. Electrons are accelerated along beam line according to RF phase settings. –Energy loss in beryllium windows, vacuum and absorber windows. Electrons lose energy in form of ionization and bremsstrahlung in absorbers. They all die there. Bremsstrahlung photons leave absorbers and creates hits in trackers, or low energy electrons in windows etc (typically high Z material). –Very rare, but RF electron emission frequent. Dark currents from RF cavities can have a negative impact on the tracker performance. –To a lesser extent also a problem for TOF2. –TOF1 is shielded by the diffuser.

RF electron acceleration Time and energy of RF electron were calculated numerically in Matlab. The RF phases were optimized for a mu+ at 2 00 MeV/c on axis. –Assumed phase difference between neighboring cavities was constant. –Phase diff = rad = ns. –This gave the muon an energy gain of 10.8 MeV per set of four RF-cavities, including energy loss in Be windows. Electrons were assumed to be emitted when the E-field is at the extremes. –Gave starting time and phase for each cavity.

Accelerating e- in the RF, 2 cavs Downstream direction Upstream direction

Accelerating e- in the RF, 4 cavs Downstream direction Upstream direction

Please note: The RF phases are set such that –electrons have higher energy in upstream direction. –some electrons turn around if starting with downstream direction. –hence, both rates and energies higher upstream. If MICE is optimized for mu-, most of the background will be in downstream direction. –Malcolm showed trackers are more sensitive to background downstream, since the electrons travel with the expected direction.

From Matlab to G4MICE Generating the background as calculated in Matlab at red locations. Extracting data at green locations.

Initial state of e- in G4MICE The electrons were distributed evenly over 21 cm in radius just outside the outer beryllium windows. Particles were assumed to be paral l el to beam line initially. –B-field curves trajectories. Matlab gave time of arrival at absorbers, background from different RF periods was achieved by repeating with an integer RF period offset in time. Off crest emission –If the particles were allowed to be emitted somewhat off crest E peak, energies might change.

Implementation of absorbers Absorbers and vacuum windows supports different geometrical shape. –Flat, spherical, torispherical. All optional absorber shapes had the central window thickness and central liquid hydrogen thickness set to most up to date design (fall 2004).

Result Only flip-mode studied. –Non flip could defocus electrons less, and thus more photons in trackers. Particles at upstream tracker (worst direction): –5.8x10 -6 e- per RF electron. –8.3x10 -4 gamma per RF electron. –With existing emission data from MTA, this means 0.15 MHz of electrons and 21.2 MHz of photons. –Energies up to 7.0 MeV achieved, typically much lower. –Only ~20% of photons leaving cooling channel enter trackers.

Future plans New 201 MHz measurements in MTA will give more accurate information on situation for MICE. –Total rates. –Angular & spatial distributions. With improved knowledge on RF emitting phenomena, the simple assumptions previously used could be more elaborate and accurate. –For example emission angle, and off crest emission. Other possible improvements: –Can we implement a more accurate absorber shape in reasonable time? –Should also non-flip field mode be simulated? Or even only non-flip mode?

Geant 4 versions Geant4.5 (basis of old study) –Simple models for multiple scattering, ionization and related processes. The stuff we care about! For example, G4.6 showed 22% more photons in trackers. (small statistics) –Could not simulate an electron decelerating to zero energy, and then reaccelerate. It was assumed dead at the zero crossing. Geant4.7/8 –Implementations of recent papers on multiple scattering models, and other improvements in the tracking of particles. –Not sure if reaccelerating zero energy particles is yet supported. I spoke with them fall 2004 about this. Hence, differences in underlying model suggests the RF background should be resimulated if we take the problem seriously.

A future simulation Simulating the problem in 2004 was very demanding on processing power. –Due to rare nature of RF induced bremsstrahlung events. Now, I have access to a GRID cluster in Geneva. –Would be helpful to get the MICE VO started / fully functional. Still, timescale is weeks, not days or hours of processing time. –Also disk space could prove a problem. Terry Hart and Malcolm Ellis are checking if the code I and Yagmur Torun wrote in 2004 is still doing what it is supposed to. –Nobody has touched it ever since, so nothing should be broken that we know of.

Sub jobs of the simulation 1.Implement updated model, with fresh data from MTA. 2.If possible, cross check that electrons arrive at absorbers at the same energy and time in G4MICE simulation (depends on whether G4 can do it). Matlab calculation 3.Use spectrum at absorber entries to shoot through absorbers, collect particles on the other side. The most time consuming part. 4.Use spectrum of particles which made hits on other side of absorbers to generate background for detectors.

Summary The RF background problem has not been touched for some time. New data from MTA will give useful information. Geant4 model changes suggests a new simulation should be run.