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Blair Ratcliff. Status Update: the Focusing DIRC Prototype at SLAC. Representing: I. Bedajanek, J Benitez, J. Coleman, C. Field, D.W.G.S. Leith, G. Mazaheri, M. McCulloch, B. Ratcliff, R. Reif, J. Schwiening, K. Suzuki, S Kononov, J. Uher. Focusing DIRC Prototype Goals.

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status update the focusing dirc prototype at slac
Blair RatcliffStatus Update: the Focusing DIRC Prototype at SLAC

Representing: I. Bedajanek, J Benitez, J. Coleman, C. Field, D.W.G.S. Leith, G. Mazaheri, M. McCulloch, B. Ratcliff, R. Reif, J. Schwiening, K. Suzuki, S Kononov, J. Uher.


Focusing DIRC Prototype Goals

  • Work with manufacturers to develop and characterize one or more fast, pixelated photon detectors including;
    • basic issues such as cross talk, tube lifetime, and absolute efficiency
    • operation in 15 KG field
  • Measure single photon Cherenkov angular resolution in a test beam
    • use a prototype with a small expansion region and mirror focusing, instrumented with a
    • a number of candidate pixelated photon detectors and fast (25 ps) timing electronics.
    • demonstrate performance parameters
    • demonstrate correction of chromatic production term via precise timing
    • measure N0 and timing performance of candidate detectors.

Prototype Optics

  • Radiator
    • 3.7m-long bar made from three spare high-quality BABAR-DIRC bars
  • Expansion region
    • coupled to radiator bar with small fused silica block
    • filled with mineral oil (KamLand experiment) to match fused silica refractive index
    • include optical fiber for electronics calibration
    • would ultimately like to used solid fused silica block
  • Focusing optics
    • spherical mirror from SLD-CRID detector (focal length 49.2cm)
  • Photon detector
    • placed in fixed slots allowing easy replacement.
    • typically using 2 Hamamatsu flat panel PMTs and 3 Burle MCP-PMTs in focal plane
    • readout to CAMAC/VME electronics with 25 ps resolution.
    • Limited number of channels available.

Typical Scanning System results

(Burle 85011-501)

  • Burle 85011-501 MCP-PMT
    • bialkali photocathode
    • 25μm pore MCP
    • gain ~5×105
    • timing resolution ~70ps
    • 64 pixels (8×8), 6.5mm pitch

Typical Scanning System Results

(Hamamatsu H-8500)

  • Hamamatsu H-8500 Flat Panel Multianode PMT
    • bialkali photocathode
    • 12 stage metal channel dynode
    • gain ~106
    • timing resolution ~140ps
    • 64 pixels (8×8), 6.1mm pitch

Beam Test Setup

  • 10 GeV/c e- beam in End Station A at SLAC.
  • Beam enters bar at 90º angle.
  • 10 Hz pulse rate, approx. 0.1 particle per pulse
  • Bar contained in aluminum support structure
  • Beam enters through thin aluminum foil windows
  • Bar can be moved along long bar axis to measure photon propagation time for various track positions
  • Trigger signal provided by accelerator
  • Fiber hodoscope (16+16 channels, 2mm pitch) measures 2D beam position and track multiplicity
  • Cherenkov counter and scintillator measure event time
  • Lead glass calorimeter selects single electrons
  • All beam detectors read out via CAMAC (LeCroy ADCs and TDCs, Philips TDC, 57 channels in total)



e– beam




Start counters, lead glass

Mirror and oil-filled detector box:

Movable bar support and hodoscope

Radiator bar in support structure


Prototype Readout

  • For 2005 beam test read out two Hamamatsu Flat Panel PMTs and three Burle MCP-PMTs (total of 320 pads).
  • Elantec 2075EL amplifier (130x) on detector backplane
  • SLAC-built constant fraction discriminator
  • Eight Philips 7186 TDCs (25ps/count) for 128 channels
  • Four SLAC-built TDC boards: TAC & 12 bit ADC (~31ps/count) for 128 channels
  • Connect only pads close to expected hit pattern of Cherenkov photons
  • Calibration with PiLas laser diode (~35ps FWHM) to determine TDCs/ADCs channel delays and PMT uniformity

PMT with amplifiers

Photodetector backplane

Simulated eventsin GEANT 4

Photodetector coverage in focal plane


Beam Test Data

Expansion region


  • In July, August, and November 2005 we took beam data during five periods, lasting from few hours to several days.
  • Total of 4.1M triggers recorded, 10 GeV/c e–
  • Reconstructed 201k good single-track events
  • Beam entered the radiator bar in 7 different locations.
  • Recorded between 100k and 700k triggers in each beam location.
  • Photon path length range: 0.75m–11m.

Occupancy for accepted events in single run, 400k triggers, 28k events


Timing versus Beam Position

Expansion region


Hit time distribution for single PMT pixel in three positions.

Position 1




Position 1

Position 4

Position 4

Position 6

Position 6

hit time (ns)


Chromatic Broadening

Example: chromatic growth for one selected detector pixel in position 1

  • First peak ~75cm photon path length
  • Second peak ~870cm photon path length
  • Important: careful calibration of all TDC channels to translate counts into ps
  • Use accelerator trigger signal as event time
  • Calculate the time of propagation assuming average <λ>≈410nm
  • Plot ΔTOP: measured minus expected time of propagation
  • Fit to double-Gaussian
  • Observe clear broadening of timing peak for mirror-reflected photons

75cm path


ΔTOP (ns)



calculate from reco

ΔTOP (ns)

hit time (ns)

burle mcp pmt with 10 micron holes sensitivity to magnetic field angular rotation wrt z axis b 15kg
Burle MCP-PMT with 10 micron holes: sensitivity to magnetic field angular rotation wrt z axis ( B = 15kG)
Photon detector performance continues to be improved by manufacturers, and is approaching the required level for timing resolution, and single photon efficiency. Burle MCP-PMT detectors with 10 micron holes have acceptable gain and timing resolution in magnetic fields up to 15 KG.

Single photon Cherenkov angular resolution performance of DIRC prototype in timing mode looks fine, and meets MC expectations.

A fast DIRC is operationally challenging. Calibration is and will be a major issue.

We hope that many of the basic performance issues will be addressed during the next year with the prototype.

Many photon detector questions remain:

Geometry, aging, rate capability, cross talk, sensitivity to magnetic field, quantum efficiency, reliability, electronics, number of channels, and cost.


Data Set

run 1

position 4

5,590 tracks

run 2

position 4

4,650 tracks

run 3

position 1

9,651 tracks

run 5

position 7

4,126 tracks

run 4

position 7

8 tracks

run 6

position 6

22,911 tracks

run 8

position 2

6.232 tracks

run 7

position 1

31,561 tracks

run 9

position 3

5,058 tracks

run 12

position 1

31,914 tracks

run 11

position 4

20,414 tracks

run 10

position 5

5,107 tracks

Photon Pathlength in bar [cm]

Most of the data taken in positions 1, 3, 4, 5, 6

run 14

position 5

17,475 tracks

run 13

position 3

36,880 tracks


Beam Detectors

e –

Energy (ADC counts)



Lead glass: single track ADC distribution

Hodoscope: single track hit map

x coordinate (cm)

Cherenkov counter: corrected event time

z coordinate (cm)


Corrected time (ns)


Cherenkov Angle Resolution

Position 1, mirror-reflected photons (longest photon path)

θc from time of propagation


θc from time of pixels



Hamamatsu H-9500

  • Hamamatsu H-9500 Flat Panel Multianode PMT
    • bialkali photocathode
    • 12 stage metal channel dynode
    • gain ~106
    • typical timing resolution ~220ps
    • 256 pixels (16×16), 3 mm pitch
    • custom readout board – read out as 4×16 channels

Efficiency relative to Photonis PMT, 440nm, H-9500 at -1000V

σnarrow ≈220ps


BABAR-DIRC Resolution Limits

Photon yield: 18-60 photoelectrons per track (depending on track polar angle)

Typical PMT hit rates: 200kHz/PMT (few-MeV photons from accelerator interacting in water)

Timing resolution: 1.7nsper photon (dominated by transit time spread of ETL 9125 PMT)Cherenkov angle resolution:9.6mrad per photon → 2.4mrad per track

Focusing DIRC prototype designed to achieve • 4-5mrad qc resolution per photon,

• 3σπ/K separation up to ~ 5GeV/c


Chromatic Effects

Chromatic effect at Cherenkov photon production cos qc = 1/n(λ) bn(λ) refractive (phase) index of fused silica n=1.49…1.46 for photons observed in BABAR-DIRC (300…650nm)→ qcγ= 835…815mradLargerCherenkov angle at production results in shorter photon path length → 10-20cm path effect for BABAR-DIRC(UV photons shorter path)

Chromatic time dispersion during photon propagation in radiator barPhotons propagate in dispersive medium with group index ng for fused silica: n / ng = 0.95…0.99 Chromatic variation of ng results in time-of-propagation (ΔTOP) variation

ΔTOP= | –L l dl / c0 · d2n/dl2 |(L: photon path, dl: wavelength bandwidth)→ 1-3ns ΔTOP effect for BABAR-DIRC(net effect: UV photons arrive later)



Precisely measured detector pixel coordinates and beam parameters.→ Pixel with hit (xdet, ydet, thit) defines 3D propagation vector in bar and Cherenkov photon properties (assuming average )x, y, cos cos cos Lpath, nbounces,c, fc , tpropagation