RF background, analysis of MTA data & implications for MICE Rikard Sandström, Geneva University MICE Collaboration Meeting – Analysis session, October 8, 2007

Objective  Understand origin of observed photon spectrum.  Deduce initial electron emission from MTA data.  Apply lessons learned to MICE conditions. R. Sandström2

Outline  Hypothesis & model  Calculations of effective radiation yield  Bremsstrahlung  Cross section  Radiation yield  Attenuation, energy distribution  Angular dependence  Simulation  Angular dependence  Energy spectrum, compared with calculations and data  Scaling results to MICE condition  Implications  Another possible annoyance  How to deal with it? 3R. Sandström

Hypothesis & model  My hypothesis:  electrons are emitted at high E-fields  almost all photons are created from bremsstrahlung  only photons survive to the detector  My model:  electrons emitted only when field is maximal  monochromatic electron beam, function of gradient  electrons hit the surface parallel to the surface normal  Seltzer-Berger tabulated values used for bremsstrahlung cross section R. Sandström4

Bremsstrahlung cross section (#1) 1. Seltzer-Berger parameterization 2. Simplest approximation Good agreement. R. Sandström5 Fraction of kinetic energy given to photon Infrared divergence!

Bremsstrahlung cross section (#2) Multiplying differential cross section by photon energy: Infrared divergence disappears. Simplest approximation performs poorly at large x. R. Sandström6 Markers: Seltzer-Berger tabulated values Red: 1-x+0.75x 2 Black: 3 rd degree polynomial fit

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. Sandström7

Attenuation At 1 MeV, Compton scattering dominates. At lower energies, PE dominates -> Much larger cross section! Hence above 1 MeV only moderate attenuation (transmission≈0.4), no photons at keV scale. MICE: less material than MTA  less attenuation. R. Sandström8 MTA PMT#8

Attenuated bremsstrahlung spectrum R. Sandström9 Assume radiation yield p y. Then the average photon energy is and since the differential cross section can be normalized despite infrared divergence which together with attenuation gives the spectrum on the right.

Photons detected per initial electron  Assumptions:  E=10.5 MV/m  T = MeV  Energy spectrum as previously illustrated.  Photons emitted isotropically in a half sphere.  12 m to a 1dm 2 detector.  Interaction probability in detector = 5.6% (Beer’s law for 1 cm polystyrene).  Detector threshold 420keV (from MTA measurement).  Energy distribution of Compton scattering flat up to Compton edge. R. Sandström10

Simulation, angles In order to cross check, MTA was simulated using 3.6 million initial electrons. The photon rate is strongly suppressed at larger angles, since they are more attenuated. If homogeneous distribution, and this is the only effect, expect blue line. (Red line is a Gaussian fit.) Comparing with calculations, this effect suppresses two orders of magnitude of total number of photons leaving metals. R. Sandström11 e-e-

Comparison of energy spectra Simulated Calculated R. Sandström12 OK agreement, but calculations give too many high energy photons. No energy loss accounted for. Additional path length at large angles. The parameterization does not work well at high x.

Measured energy spectrum R. Sandström13

Results, number of photons per electron  Using the same assumptions regarding detector efficiency etc as for the calculated rates, the fraction of initial RF emitted electrons which give a hit above threshold in the detector is  The calculations gave 31% lower gain. Hence, fair agreement.  We can obtain reasonable results without tedious simulations! (81 photons out of 3.6 million initial electrons hit an area 0.8 by 0.8 m) R. Sandström14

MTA measured rates (J. Norem et al.) R. Sandström15

Results, number of electrons emitted  MTA measures rates per pulse.  Pulse length = 125·10 -6 s.  RF period = 1/201.25·10 6 = 4.97·10 -9 s.  Extrapolation from data shows ~15 photons per pulse at 10.5 MV/m, hence 6.6·10 -4 photons per RF period, equivalent to ~60’000 electrons emitted from the cavity per period.  At 8 MV/m, there are approximately 0.23 photons per pulse.  Simple scaling from 10.5 MV/m photon yields, results in approximately 900 electrons emitted per period, or slightly less if using simulated photons yield. R. Sandström16

Implications for MICE  Studies by Sandström & Torun in 2004 assumed 8 electrons emitted from the cavity  per period and direction.  New results show 80 or 110 times (depending on calculation or simulation) higher background rates.  Results from 2004 are still reliable, but total rates must be scaled up by two orders of magnitude.  M. Ellis presented results for exactly this at CM7: R. Sandström17 Efficiency in %Efficiency out %Purity in %Purity out %  4D bias %  4D resolution % No RF99.99(85)99.81(17)99.15(12)99.17(66) With RF99.99(85)99.83(43)99.13(14)99.17(57) x RF100.(00)99.(73)95.(28)96.(47) N/A

Another possible annoyance Multipactoring: When an electron hit the Be window of the other side of the cavity, secondaries are accelerated in opposite direction if field is nearly maximal. Resonance effect! Similar to multipactoring, when an electron hit the Be window of a neighboring cavity, the field is nearly maximal. Not previously considered. Would give even higher rates. By how much? Accelerated through less cavities, lower energy photons. R. Sandström18

How do we deal with the background?  By energy:  Photons never have more energy than its initial electron.  8 MV/m: 1.12 MeV for one cavity, 7 MeV maximum for a linac.  Can we remove photons in TOF by threshold?  Integrated over one TOF “open gate” (~500 ns), the previous 23 MHz upstream is now ~2GHz. If 10% of the photons cause hits in the detector, each hit 0.5 MeV, the integrated energy deposit is 500 MeV…  Not all photons that hit the trackers will hit the TOFs though.  By timing:  Spectrum follows repetitive pattern in time…  Conclusion: Xomeone needs to think how to deal with this. R. Sandström19

Summary  Bremsstrahlung is assumed to be primary source of photons.  Effective photon to electron ratio at MTA PMT 8 is approximately 1.6·10 -8, for 10.5 MV/m.  New measurements imply RF background rates simulated in 2004 should be increased by two orders of magnitude. 20R. Sandström