1. Simulation for Fast Focusing DIRC R&D
Focusing DIRC R&D effort at SLAC:
Jose Benitez #
Gholam Mazaheri #
Larry L. Ruckman +
Gary S. Varner +
David W.G.S. Leith #
Blair N. Ratcliff #
Jochen Schwiening #
Jerry Va’vra #
# SLAC University of Hawai’i
Acknowledgements:
M. McCulloch and B. Reif (prototype construction)
I. Bedajanek and J. Uher (software development)
K. Suzuki (beam test)
Most of the Geant4 simulation work
done by Ivan Bedajanek (Prague) during six month stay at SLAC in 2005/2006.

2. Motivation Goal: Super-B will have 100x higher luminosity
Backgrounds are not yet understood, but
they would scale with the luminosity if they
are driven by the radiative Bhabhas
→ Future DIRC needs to be smaller and faster:
Focusing and smaller pixels can reduce the expansion volume by a factor of 7-10
Faster PMTs reduce sensitivity to background.
Additional benefit of the faster photon detectors:
Timing resolution improvement: s ~1.7ns (BABAR DIRC) → s 150ps (~10x better) which allows measurement of photon color to correct the chromatic error of qc (contributes s ~ 5.4 mrads in BABAR DIRC)
Focusing mirror effect:
Focusing eliminates effect of the bar thickness (contributes s ~ 4 mrads in BABAR DIRC)
However, the spherical mirror introduces an aberration, so its benefit is smaller.

3. For latest status of our R&D see my talk
Beam Test Setup in 2006
SLAC 10 GeV/c electron beam in End Station A
Beam enters bar at 90º angle
Prototype is movable to 7 beam positions along bar
Time start from the LINAC RF signal, but correctable with a local START counter
SLAC-built amplifier and constant fraction discriminator
TDC is Phillips 7186 (25ps/count), CAMAC readout
Beam spot: s < 1mm
Lead glass:
For latest status of our R&D see my talk
at RICH 2007 workshop
Local START time:
s ~36ps

4. Focusing DIRC Prototype Optics
Radiator:
1.7 cm thick, 3.5 cm wide, 3.7 m long fused silica bar (spares from BABAR DIRC).
Optical expansion region:
filled with a mineral oil to match the fused silica refraction index (KamLand oil).
include optical fiber for the electronics calibration (PiLas laser diode).
Focusing optics:
a spherical mirror with 49cm focal length focuses photons onto a detector plane.

5. Focusing DIRC Prototype Photon Detectors
1) Burle MCP-PMT (64 pixels, 6x6mm pad, sTTS ~50-70ps)
snarrow ≈70ps
time (ns)
Timing resolutions were obtained using a fast laser diode (PiLas) in bench tests with single photons on pad center.
2) Hamamatsu H-8500 MaPMT (64 pixels, 6x6mm pad, sTTS ~140ps)
snarrow ≈140ps
time (ns)
3) Hamamatsu H-9500 Flat Panel MaPMT (256 pixels, 3x12mm pad, sTTS ~220ps)
snarrow ≈220ps
time (ns)
Nucl.Inst.&Meth., A 553 (2005) 96

6. Prototype Simulation
Each detector pixel determines many photon parameters assuming an average :
c, cos  cos  cos Photon path length, time-of-propagation, number of photon bounces.
We obtain those photon parameters for each pixel using Geant4 simulation
Detailed Geant4 simulation
complete physics, photon transport and detection
for studying performance, backgrounds, corrections, systematics, etc
future optics design, etc
Fast Geant4 simulation
simple “photon gun” at end of bar (no scattering or inefficiencies)
for calculating for each pad the photon track parameters for average λ
Fast ray tracing toy Monte Carlo and graphical ray tracing software
for initial design and cross-check of Geant results

7. Detailed Geant4 Prototype Simulation
10 GeV/c electron beam at 90 deg angle, beam position adjustable, beam spot size randomized
3.675 m long DIRC fused silica bar, mirror at one end
Detailed photon propagation includes reflection coeff as function of λ and polish as measured at SLAC
Fused silica block at other bar end
Epotek glue between bars and at block
Complex shape of expansion volume
KamLand mineral oil in expansion volume
49 cm focal length spherical mirror
Up to 7 PMTs mounted on quartz window
Different timing resolution for Burle MCP-PMTs and Hamamatsu MaPMTs, incl. non-Gaussian tail
Individual uniformity/efficiency variation for each PMT as measured at SLAC
Each PMT location aligned to match beam test data occupancy

8. Generation of photons blue: electron yellow: outline of radiator bar
magenta: tracks of detected Cherenkov photons

9. Detection of photons
Use SLAC R&D measurement results in 2D scanning setup to simulate:
Hits on PMT registered according to individual PMT and pad efficiencies
Probability for hit to cause charge sharing in neighbor pad(s)

10. Simulated Event
Up to ~1000 Cherenkov photons produced ~300 transported (cuts on photon angles)
Typically detected

11. Simulation Results
Predicted photon hit location on detection plane without pixelization (Full G4 sim)
→ “ring” image significantly wider in outer slots (longer photon paths - plus other effects showing up in the wings?)
y position (cm)
x position (cm)
Note different scale in x and y
Size of single pixel is 6mm x 6mm / 3mm x 12mm

12. Simulation Results
Predicted photon hit occupancy for fast Geant sim (photon gun to all PMT pixels)
low statistics plot, 2005 detector config.
Predicted photon hit location for central PMT without pixelization
(fast Geant sim)
Interesting structures become visible.
y position of hit (cm)
x position of hit (cm)

13. Simulation Results
Predicted photon hit location for production polar angle fixed to 822mrad (fast Geant sim) → observe good focusing in center of detection plane,
large optical aberrations (“fringes”) towards outer slots
result of our current mirror design, need to improve in future design
y position of hit (cm)
x position of hit (cm)

14. Simulation Results
Another way of looking at the optical aberration (ray tracing toy MC)
→ fringe effect grows from 0 mrad at ring center to 9 mrad in outer wings of Cherenkov ring
note different x,y scale compared to previous plot

15. photon propagation time (ns)
Simulation Results
Photon propagation time in bar for beam position closest to detector (Full G4 sim)
Good agreement but simulation has less background than data (also true for G4 simulation in BaBar DIRC)
peak 1
reflections at glue/bar boundaries
red: beam data
blue: G4 simulation
Mirror
peak 2
photon propagation time (ns)

16. Location of the Cherenkov ring (Occupancy)
Aug 2006 data
full Geant4

17. θc resolution θc resolution (mrad) Photon path length (m)
Comparison shown for small pixels (Hamamatsu H-9500), 3mm x 12mm pad size

18. Conclusion
We have a detailed standalone Geant4 simulation of our Focusing DIRC
prototype in End Station A.
Photon propagation in bar simulated in more detail than BaBar DIRC.
The simulated occupancy and resolution agree well with beam test data.
The simulation could be useful as basis for SuperB barrel PID simulation.
Our local Geant expert left in 2006, our ability to modify/upgrade simulation quite limited (change alignment and PMT properties).
Looking towards the near future: try different focusing optics designs, identify best location for photon detectors in new standoff box,
study coupling of existing DIRC bars and bar boxes to new standoff box, etc – this goes beyond our current level of expertise