1. A linac-based Mu2e calibration system George Gollin Department of Physics University of Illinois at Urbana-Champaign Urbana, IL 61801 USA g-gollin@uiuc.edu September 27, 2008

2. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 2 Introduction A challenge: determine and monitor the Mu2e spectrometer’s resolution and energy scale near the electron spectrum endpoint. An ideal calibration technique would gauge the precision, absolute energy scale, and level of systematic uncertainties associated with the spectrometer during time that the experiment is live, recording physics data. Do this with a linac, built from an upgraded AØ photoinjector, or a spare Project-X cryomodule? Perhaps do it this way: reconfigure the calorimeter, install a linac, and inject electrons into the downstream end of the Mu2e detector.

3. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 3 Current Mu2e spectrometer Spectrometer is charge symmetric: the geometric acceptance for a positive particle leaving a target foil is about the same as for a negative particle of the same momentum. But the signal of interest will consist entirely of negative electrons, and it is not clear that a charge-symmetric detector affords Mu2e any advantages in sensitivity or precision.

4. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 4 Electrons move along helical paths Consider reconfiguring calorimeter vanes to wrap around the spectrometer axis, like screw threads around the barrel of a screw. Inject calibration electrons from the downstream end.

5. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 5 Inject calibration electrons from downstream ALL electron orbits spiral clockwise when seen from upstream. Downstream-movers will strike calorimeter vanes, while upstream- movers usually will not. Big question: what are the neutron and photon calorimeter noise issues?

6. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 6 Tracks will pass through the tracker twice Measure them coming and going, so as to speak.

7. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 7 Tracks are reflected by the upstream field pinch near the stopping targets

8. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 8 Calibration linac block diagram

9. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 9 Linac: upgraded AØ photoinjector Project-X cryomodule Both would be built around 1.3 GHz superconducting assemblies. Particle energies leaving the cryomodule can be as high as 200 MeV. Install it above ground. ~10 9 e - per pulse is easy; reduce flux and momentum width through collimation and momentum selection I asked Shekhar Mishra if the lab would find itself with spare cryomodules at some point. He said there’d be at least one by 2010. He said it would be reasonable for Mu2e to make use of it. I spoke to Bob Kephart about this. He was enthusiastic, and suggested we also consider requesting employment of the (energy-upgraded) AØ photoinjector for this.

10. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 10 Beam transport Conventional system: shielding, magnets, and vacuum system to transport beam from Linac to Mu2e hall. Emittance preservation is not an issue.

11. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 11 Collimation and momentum selection Conventional system: shielding, magnets, and vacuum system to transport beam from Linac to Mu2e hall. Emittance preservation is not an issue. Beam intensity reduced to ~0.1 e - per shot.

12. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 12 Painting the appropriate region of phase space with calibration electrons Accelerator physics! great stuff!

13. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 13 Want to sample the phase space Sample, rather than attempting to fill the volume completely.

14. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 14 Injection port Interesting problem: shield calibration electrons from solenoid field until injection on-orbit. Need to be able to spray electrons into spectrometer over a wide range of angles. View from downstream…

15. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 15 Injection port Shielded injection port to allow on-orbit calibration e - injection My gut feeling is that there’s a better/more clever way to inject.

16. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 16 What about scattering to kick an electron on-orbit? Clever idea, suggested by Yuri last summer. The problem: even with a large nucleus (I did calculations using gold), there isn’t enough elastic scattering “power” to Moller-scatter a 105 MeV electron through a ~60  angle often enough to be useful. See “Thoughts concerning on-orbit injection of calibration electrons through thin-target elastic scattering inside the Mu2e solenoid” in DocDB for the details.

17. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 17 Some of the advantages, and a worry In situ calibration, recorded during actual running. Compare upstream-moving and downstream-moving track momenta: lots of systematic error studies are possible. My worry: calorimeter exposure to downstream-moving fluff. Magnetic trickery and shielding to solve this, if necessary??

18. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 18 Another calibration technique Inject  +, stop them, look at the positron from See Jim Miller’s discussion in DocDB: “Calibrations for the Mu2e Experiment,” for example. All sorts of differences: different momentum, different charge, perhaps different field, different phase space, etc. etc. Even so: perhaps we want to preserve this as an option.

19. George Gollin, University of Illinois, Mu2e group meeting, January 24, 2009 19 Thoughts at UIUC Modeling of a precision collimation and momentum selection system. Initial studies of selection magnet geometry and field stability Studies of phase space painting techniques and injection port geometry Study of synchrotron radiation energy loss Simulation of the spectrometer’s response to calibration electrons Have Vice Chancellor support for four (undergraduate) research assistants, NSF support for one REU student. I already have all five students signed up! DOE isn’t helping with this [nasty mumblings].