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Background Simulation
Terry Geld
Background Remediation Workshop
15 Jan 1999

Status of Beampipe Simulation and Background Simulation

  • The PEP-II beamline geometry has been updated and now simulates the beamline out to +-8m. This includes B1, Q1, Q2, Q4, & Q5. Lots of detail: masks, pumps, flanges, etc. This is now the final beamline geometry. Long standing overlaps between IFR/endplugs and SVT/beampipe have been solved. Magnetic fields have been updated: bbsim nominal beam orbit agrees with accelerator nominal to within 100microns at IP after 8m.
  • Pix of new beamline .
  • New TURTLE rays are available for the new geometry. These rays include Brems and Coulomb scatters from around the entire ring. (See talk by T. Fieguth later in session.)

Why do we need such an extended beamline & TURTLE simulation?

  • Backgrounds from CS in the far distant arcs tend to get lost in the Q4/Q5 region. These produce backgrounds comparable to those from sources in the |z|<60m region (the region simulated in TURTLE for the TDR). By not putting in Q4/Q5, we neglect these contributions.
  • The dynamic pressure (beam-induced pressure at 1A) through most of the ring is on the order of 10nTorr, rather than the TDR 1nTorr (PEP-II CDR claims final 5nTorr in arcs).
  • Note: the background commissioning detectors have been used to validate the MC predictions. By including full ring TURTLE, reasonable pressure profiles and beamline simulation out to Q5, we can get agreement between MC and data to within factor of 1.5 - 2.0.

How can the BaBar detector groups use the new simulation?

  • New background frames are in the pipeline (a few preliminary ones are available).
  • For some detectors, may be appropriate to use the TURTLE rays directly to study occupancies and energy depositions (CPU issue). See later talks
  • Need release 7.8.1 or higher.
  • Need flag use_bkgd_bpuse=1 to track TURTLE ray through beamline fields.
  • Need flag mode_bpfld=2 for both beamline and solenoid fields.
  • I have created a useful README html and posted to BkgCom and Simulation HN.

Other considerations

  • Probably need to redo Bhabha scattering studies. These will hit in Q2 septum which has significantly changed. Possible effect on SOB and forward EMC.
  • Need to determine energy cutoff of simulation. Energy spectrum of showers from lost particles is quite soft. Lots of 100keV gammas. The X-ray detector sees strong 30keV peak in DCH volume (prob due to scattering out of support tube). Crystal ring sees 30% effect in energy deposition by changing from a 100keV to 10keV gamma cutoff. Where is sensitivity cutoff of detectors?

Last modified: Thu Feb 4 10:02:05 PST 1999