Author: popov

Cathode Drift Chambers for the GlueX Experiment at Jefferson Lab
Lubomir Pentchev for the Forward Drift Chamber group and the GlueX collaboration
(pentchev@jlab.org)

The Detector

The GlueX experiment at JLab (Virginia, USA) uses tagged polarized photon beams from the recently upgraded 12GeV Continuous Electron Beam Accelerator Facility (CEBAF) to search for exotic hybrid mesons (quantum numbers not allowed in quark-antiquark configuration) as predicted by lattice QCD. GlueX detector— hermetic coverage (to enable partial wave amplitude analysis) for charge and neutral particles using calorimetry and tracking within 2T-solenoid; high trigger (up to 200 kHz) and data acquisition rate (up to 1 Gby/sec).

12,672-channel drift chamber system consisting of 24 1m-diameter chambers, grouped in 4 packages and inserted in the magnet bore.
Pipelined data from 10,368 flashADC (125MHz) channels and 2,304 TDC channels.

Each drift chamber consists of two cathode planes divided into strips facing the wire plane.
Wire plane—sense (20 mm) and field wires (80 mm), 5mm apart.
Cathodes—copper strips with a pitch of 5mm. Coupling of the 2mm-thin strips to the pre-amp connector challenging.
Light frame made, mostly of Rochacell, to allow detection of low energy photons by outside e.m. calorimeters.

Performance

Same avalanche is seen in three projections (both cathodes and the wire). Such redundancy allows to study uniquely the charge induction process and the strip resolution.
Cathodes used to reconstruct avalanche (and hit) position along the wires.
Each chamber gives a point in space of the track.

Drift time used to reconstruct hit position perpendicular to the wires. Cathodes also can be used to reconstruct avalanche (not hit) position in direction perpendicular to the wires—this is used to study cathode resolution .

Up right—wires (avalanches) reconstructed by the cathode strips in one chamber. The widths of the projections on the x-axis (up left in blue) are used to estimate (left plot) strip resolution after correction for the strip angles. External tracking is used to estimate the strip resolution along the wire (y-axis) (red points). The blue-red discrepancy - due to transverse diffusion or finite avalanche extent as discussed on the right panels.

Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. The U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for U.S. Government purposes.

High Cathode Strip Resolution and 3D Avalanche Reconstruction — allows unique studies of avalanche development around wire using 55Fe source

The two cathode planes “see” the avalanche position in 3D:
Perp. to cathodes (Z) from ratio of the total charges on the cathodes.
Perp. to wire (X) from strip crossing
Along wire (Y) from strip crossing
2D transverse distribution (right plot) of avalanches around wire (XZ) from 55Fe source at bottom side.

Cell 10x10 mm

S10931 - 025P ( 25 mm)

Plotted is the apparent avalanche position as “center-of-gravity” seen by the strips. Corrections to the reconstructed positions as a result of the induced charge distortion due to the proximity of the avalanche to the wire not applied.

Garfield simulations predict cathode charge asymmetry depending on the position of the avalanche around the wire. The weighting field (right plot) used to calculate the charge induced on the top (positive Z) cathode, demonstrates the asymmetry. Garfield prediction for the up/down charge ratio is shown in the plot below (blue lines).

The cathode charge ratio dependence on the total charge (plot above) can’t be reproduce within Garfield only. A possible explanation is the azimuthal extent of the avalanche mediated by UV photons, that increases with the total charge.
Plot on the left shows the azimuthal extent of the avalanche vs teh charge estimated by distributing electrons around the wire (to simulate the UV photons) and then propagating the avalanches with Garfield microscopic simulations. Examples of such simulations shown on the top plots.

Cluster Counting Studies—space charge effects within the same track; longitudinal avalanche extent
One chamber has been modified with 4cm drift gap (left). High single electron efficiency and big drift times allowed to count the number of clusters for each track using fADC (right).
The number of clusters turned out to be strong function of the track angle w.r.t. wire (bottom). For tracks perpendicular to the wire the electrons travel almost the same paths (bottom left) encountering higher space charge at the wire from the previous electrons. This can explain the lower number of cluster for such tracks. From the plot we estimate a lateral extent of each cluster avalanche of the order of a few hundred microns.

Summary and conclusions: The cathode drift chamber tracking system for the GlueX experiment has been successfully operated, giving already physical results. Redundancy in avalanche reconstruction allows operation at low threshold and precise calibration thus reaching resolution of 150 microns with beam tracks at high efficiency. The resolution might be limited by the finite extent of the avalanche. Higher resolutions have been obtained with 55Fe source demonstrating for a FIRST TIME the possibility to reconstruct the position of the avalanche w.r.t. the wire in TWO DIMENSIONS (one dimensional reconstructions have been done before). The dependence of the cathode charge ratio on the total charge can be explained by photon-mediated azimuthal extent of the avalanche - the dependence of the angular extent on the charge has been estimated for Ar/CO2 gas mixture. Cluster counting studies with 4cm-gap modified chamber show sharp decline in the cluster number for tracks perpendicular to the wire. This demonstrates the effect of the space charge along the wire within the same track. The uncertainty of the longitudinal extent of the avalanche may contribute to the discrepancy between the cathode resolutions along and perpendicular to the wire.

Compare time resolutions for the SiPM array (S12045) and 3 mm2 SiPMs with the pixel size of 25 mm (S10931-025P) and 50 mm (S10931-050P)
- measure resolution of the array (S12045) at different temperatures
Estimate time resolution by measuring time difference between two SiPMs
Use Hamamtsu PLP-10 laser with a pulse width of 70 ps and the wavelength of 405 nm.
- good laser light yield stability up to at least 10 MHz rate
Use fast blue LED driven by a pulse generator ( HP 8116 ) to simulate scintillator response. Rise time of the light pulse ~ 3ns (pulse width 8 ns)
Electronics: discriminator CFD Ortex 935, Caen VX 1290 25-ps TDC, 12-bit flash ADC. Use Canberra 2145 time-to-amplitude converter for timing measurements of the SiPM array
Typical SiPM operation voltage 1 V over breakdown
SiPM laser pulse shape
S10931 S12045
Rise time (ns) 5 (1.5 intrinsic amplifier) 6 (4.5)
Pulse width FWHM (ns) 16 60