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A mini-TPC for PEP-II IR Commissioning

PEP-II Interaction Region (IR) Commissioning

Several types of detectors have been installed in PEP-II Region 2 for commissioning the beam interaction region. Both tracking and energy deposition detectors were chosen to measure the charged and neutral components of the machine backgrounds.

The mini-TPC Detector

Tracking with a 3D detector allows good pattern recognition in what could be a hostile background environment. The mini-TPC was designed to provide multiple high-resolution hits on tracks passing through the central interaction region. Ionization amplitude measurements (dE/dx) along the track allow the separation of highly ionizing hadrons from the predominant minimum ionizing electrons and positrons. In addition, low energy Compton scatters can be identified as isolated hits in the chamber drift volume. Collaborators , and Institutions

Mini-TPC Hardware
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The mini-TPC chamber, including the high-voltage field cages and proportional wire planes, was fabricated at LAL , Orsay. The gating grid and shielding wire planes were built by U.Cincinnati. Some pictures of the MiniTPC :

(click icons for full size pictures)

The gating grid electronics and support structure were designed and tested at LBL, Berkeley. Several mechanical drawings including one of the full assembly are available.

Pictures of the MiniTPC and Silicon mock detector in installation tests at LBNL:
TPC mounted on model B1/Q1 3/4 view
TPC mounted on model B1/Q1 full view

Trapezoidal cathode pads are being used to increase the charge sharing. A detailed electrostatic study has been performed.

Mini-TPC Electronics
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The preamplifiers and FastBus readout electronics were fabricated as original prototype systems for the DELPHI TPC detector at CERN. The gating grid pulsing circuit was a prototype for the STAR TPC detector at RHIC. The VMS-based data acquisition system was assembled at Orsay for the micro-TPC proposal to BaBar at SLAC.

Mini-TPC 3D Event Display
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A 3D event display program for scanning data taken with the mini-TPC has been developed. The display framework, called WIRED for Web Interactive Remote Event Display, was developed in Sun's Java Language Environment by the Information and Programming Technology Group at CERN.

The data were taken in the January 1998, PEP-II High Energy Ring (HER) commissioning at different beam currents. You can see from the first low intensity run (#2008) that the tracking is wonderful. The next two runs #2085 and 2102, at beam currents of 200 and 425 mA, demonstrate the capabilities in visualizing the 3D mini-TPC data and the level of HER Interaction Region (IR) background. Event displays for run #2102, event #5 as a 2D XY projection and rotated 3D in both portrait and landscape views are available for printing.

Mini-TPC Simulations
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A full GEANT description of the mini-TPC has been written. A cut-away drawing of the detector is available .

In HER commissioning, the mini-TPC would not fit around the 5 cm radius beam pipe. It was mounted vertically off the beam axis 2.3 m downstream as shown . In an expanded transverse view . one can see showers originating in the Copper mask that produce charged tracks in the mini-TPC. Tracks will also be generated in showers that develop in the beam pipe bellows that is upstream of the mini-TPC as shown in the following events: event #1 , event #2 and event #3 .

We've had some trouble defining the beam pipe geometry. Using the unofficial version of the gnbpip package produces the correct geometrical description for: TREE , COMI and COMD . The release version of gnbpip gives the following geometries which are wrong: TREE , COMI and COMD . The BFAC mother volume specification is also given . The symtom is that GEANT tracks through overlapping volumes without recognizing them. In particular, the mini-TPC doesn't record hits when it overlaps with the COMD volume.

Several simulated beam background events due to interactions of "lost particles" in HER commissioning are available: event #1 , event #2 and event #3 . For these runs the mini-TPC was located in a downstream position within 1 cm of the beam pipe.

Background tracks being swept out by the Q4 quadrupole magnet or originating from electron and photon showers in the Copper mask will suffer secondary scattering in the downstream beam line elements. In particular, during HER commissioning, there was a beam pipe flange in a direct line of sight between the mini-TPC and the mask. The following plot of extrapolated origin displays the spread in Z for higher momentum (>100 MeV) tracks that would be found in the mini-TPC for a source located at Z=0. To test the ability of the mini-TPC to separate different sources of background in the Monte Carlo, the chamber was moved forward by 50 cm. This lost-particle event illustrates tracks from different sources. The following plot of the extrapolated Z origin demonstrates that the mini-TPC in this upstream position could point tracks back to showers in the Copper mask (note enhancement at Z~-1m which should improve in the next higher statistics run). Here a cut on the mini-TPC hit position was loosened to allow more secondary scattering tracks in the Z origin distribution. After scattering or showering in the flange, these tracks appear to be originating from Z=0.

Simulation of the expected backgrounds measured by the mini-TPC detector in HER commissioning is being done at two levels. One approach is a simple analysis based on track vectors saved in ntuples while the other simulates the actual response of the mini-TPC to various background sources. Both efforts are being done for a complete mini-TPC geometry description within the BaBar Monte Carlo package BBSIM. For the ntuple analysis, we present:
1. Two dimensional plots of x vs. z and y vs. z , and projections in the y-z plane display the expected outline and hit distribution in the chamber.
2. Simple histograms show the charge, energy, step size in the mini-TPC and number of hits / 100 micro-sec readout.
The same analysis on recent Lost Particle files for the hcom-latr of 98Mar27 yields similar distributions with higher statistics: x vs. z , y-z plane and charge, etc. histograms . The expected mini-TPC track rate for the Mar '98 hcom lattice simulation is 3/100 micro-sec interval or 30 kHz at the design current of 1A and the expected pressure of 1 nTorr. This result is for tracks with a measurable track length of more than 2 cm in the mini-TPC. (Note: This rate is lower than recent estimates which double counted tracks passing through different elements of the mini-TPC GEANT geometrical description.) Latest results include:
1. Hit distibutions in local coordinates - z vs. y & y vs. x .
2. Charged particle energy distributions in semi-log and linear plots for different energy ranges.
3. Extrapolated z origin of the background charged particles cut on phi direction, the chamber z and r hit location (basically, front outer quarter section) both for low energy (<100 MeV) and high energy tracks.
4. Low energy particle contribution (<10 MeV) is less than 30% of the total MC track rate and is expected to have a small impact on the observable track rate in the mini-TPC.
5. The measurable track length is determined from the total step size in the chamber shown with the total number of steps for both low and high energy tracks.

The expected performance of the mini-TPC detector in measuring beam backgrounds for PEP-II colliding beam commissioning has been simulated within the BaBar Monte Carlo package BBSIM:
1. Measured dE/dx vs. log(p)
2. Measured electron dE/dx vs. log(p)
3. Phi angular distributions at different radii cut on energy.
4. Phi angular distributions at different radii cut on dE/dx.
A simulation of nominal e+/e- beam backgrounds over the full 100 microsec readout of the mini-TPC is available .

Results of the simulation were presented at a workshop on B-Factory backgrounds in Mar '97. Anne-Marie's Hawaii talk


Related Web pages: SLAC, PEP-II machine, Interaction Region, BaBar detector,
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Mike Ronan