Hall D Photon Beam Simulation and Rates

1. Hall D Beam Line and Tagger Review Jan. 23-24, 2006, Newport News Hall D Photon BeamSimulation and Rates Richard Jones, University of Connecticut Part 1: photon beam line Part 2: tagger

2. I. Photon Beam Line Simulation • estimate background rates • evaluate options for shielding • has a detailed model of coherent bremsstrahlung • describes the detailed beam line geometry, fields • simulates electromagnetic processes accurately • includes photonuclear interactions at some level To accomplish these goals, we need a Monte Carlo simulation that:

3. Photon Beam Line Simulation • Detailed photon beam line simulation: HDGeant • has built-in coherent bremsstrahlung generator to simulate beam line with a realistic intensity spectrum • beam photons tracked from exit of radiator • assumes beam line vacuum down to a few cm from entry to primary collimator, followed by air • beam enters vacuum again following secondary collimator and continues down to a few cm from the liquid hydrogen target • includes all shielding and sweep magnets in collimator cave • monitors background levels at several positions in cave and hall • The same simulation also includes the complete GlueX target and spectrometer, detector systems, dump etc.

4. Photon Beam Line Simulation • HDGeant:all of the std Geant3 physics models, plus: • muon pair production • modified Geant to turn a fraction of the e- e+ pairs into μ-μ+ • cross section formulas translate simply me mm • rates are down by factor (me/mm)2 -- but not insignificant • photonuclear reactions • large mass of data on these cross sections, not all relevant • final results will dependon hadronic interactionsin any case. • GELHAD package from BaBAR was adopted • gN quasi-elastic scattering, single p+/p0 production, p+p- production (vector dominance model), p+n emission (quasi-deuteron model)

5. Photon Beam Line Simulation cut view of simulation geometry through horizontal plane at beam height Hall D collimator cave Fcal tagger building Cerenkov vacuum pipe spectrometer

6. Photon Beam Line Simulation overhead view of collimator cave cut through horizontal plane at beam height 12 m collimators concrete air vac vac sweep magnets iron blocks lead

7. Photon Beam Line Simulation cut view of simulation geometry through horizontal plane at beam height Hall D collimator cave virtual detector plane x z Fcal tagger building Cerenkov vacuum pipe spectrometer

8. Photon Beam Line Simulation Development • Progress so far: • Simulation has been used to optimize the amount and placement of shielding in the collimator cave. • Study of background rates in the GlueX start counter and trackers are being used to constrain the design of those detectors. • Ongoing development: • The Hall D team is working with the Jlab RadCon group to cross-check our rate results for the Hall D beam line. • Elements from HDGeant (geometry, coherent bremsstrahlung generator, fields) have been shared, and many things checked. • Latest results (P. Degtiarenko) are consistent with conclusions based on previous Hall D studies. • Some errors have been corrected: results follow

9. Beam Energy Spectrum: counts photon energy (GeV)

10. Background Rates: z = -1 m particle flux (/cm2/s)

11. Bethe-Heitler muons m+/m– flux vs. position • At these energies, the primary source of muons is pair production in the collimator. • With appropriate shielding (included in the simulation) the rates at the detector are within an order of magnitude of cosmic rays. flux (muons/cm2/s) radial position (cm)

12. II. Photon Tagger Simulation • estimate background rates in focal plane counters • evaluate options for shielding • detailed field map of the tagger • design for the vacuum box downstream exit region • focal plane hodoscope model Elements needed for tagger simulation (in addition to beam line)

13. Photon Tagger Simulation cut view of simulation geometry through horizontal plane at beam height Tagger Building goniometer Hall D photon beam line quadrupole magnet tagger dipoles microscope fixed array hodoscope vacuum chamber to electron beam dump detailed magnetic field map (from TOSCA) 350 x 30 x 1600 = 17 M points

14. Photon Tagger Simulation • events begin with a CBg,e- pair inside the diamond radiator • beam emittances realistic for the 12 GeV machine • electron tracked in the magnetic field of the quadrupole and dipoles • focal plane scintillators are “sensitive volumes” photon beam exits from tagger inside vacuum, continues 70 m down to collimator cave without leaving vacuum Runge-Kutta method • hodoscope exit window is 1 mm Kapton • other walls of vacuum chamber are 1 cm Al • exposed side of electron exit channel is 5 mm Al

15. Photon Tagger Simulation One issue highlighted in the GlueX Detector Review report, October 2004: A potential concern is the high flux of electrons with energies close to the endpoint interacting with the mechanical structure of the vacuum chamberor the dump pipe. Because of the shallow bend angle of the spectrometer, downstream spray could cause background in the tagging detectors. Recommendation: Perform a Monte Carlo simulation of the tagging system with particular attention to background in the tagging counters caused by high-energy electrons.

16. Photon Tagger Simulated Events concrete wall

17. Photon Tagger Simulation Resolution in focal plane microscope in energy and emission angle

18. Photon Tagger Simulation monitor all particles reaching one of the central fibers of the FP microscope gammas only gammas positrons electrons all hits, single fiber all hits, combine neighboring hits

19. Summary • A complete set of tools for detailed physics simulation has been developed covering the Hall D Photon Beam and Tagger systems. • Backgrounds at the entrance to the detector and also at the tagger focal plane have been examined. • The shielding for the collimator region has been optimized based on the simulation, leading to acceptable background rates in the GlueX detector. • Backgrounds coming from the exit region of the tagger vacuum will be minimized using the simulation by refining the mechanical design and optimizing the shielding in that area.