Backgrounds and Remediation at BaBar

[ Motivation | Links | Methodology | Thoughts so far ]

Backgrounds Group

"We are the first, last, and only, defense of the BaBar Experiment against the background scum of the Universe"

The structure of the Backgrounds group is currently in a state of flux. Part of the work has been discharged through the successful formation and discharge of duties of the High Luminosity Backgrounds Task Force, (see their report). Nominally, the Backgrounds group has collected together BaBar and PEP-II physicists in an effort to understand and mitigate the machine backgrounds at BaBar. The group was formed under Guy Wormser with a charge.

In the past, the Backgrounds group has also been responsible for managing, training, and supervising the BaBar/PEP-II Liaison shift role in the daily operations of the experiment. The current (September 2002) Liaison Coordinator is Stan Hertzbach.

Connecting to our Work

Here is a collection of useful links that is enjoyable and satisfying.

Methodology

This part is under serious construction.

The high currents needed in the two PEP-II rings to achieve the design luminosity (0.7 and 2 A in the High Energy Ring (HER) and Low Energy Ring (LER)) can engender significant beam induced backgrounds in BABAR. The dominant production mechanism is electromagnetic showers due to in teraction of off-momentum or off-axis electrons, positons or photons with materials in the detector vicinity. Since Beam-gas interactions (Coulomb or Bremsstrahlung scattering) are the processes responsible for creating these off-acceptance particles, the background will be most quadratic with beam current.

The potential damage of these particles is three-fold: They can permanently damage the detector after some dose has been integrated, they can hamper detector operation by exceeding limits on drawn currents, event sizes or trigger rates and they can deteriorate data quality because of the superimposed noise.

Babar has taken these problems very seriously from the very beginning and attacked them on all fronts:

      background simulation, backgrounds experiments during machine 
      commissioning, built-in safety margin in detector design and 
      construction , sophisticated detector protection systems, 
      careful strategic planning

Observations

This text is selected from previously written documents circa August 2000.

Operationally, the acceptable level of backgrounds at an unprecedented machine like PEP-II is determined primarily by the radiation hardness of the sub-detectors (SVT, EMC) and by the tolerable drift chamber current. The Level-1 (L1) trigger rate and the occupancy in other detectors (DIRC) also constitute occasional limitations. Careful measurement, analysis and simulation of the background sources and of their impact, has led to a detailed understanding and an effective remediation of these effects.

The primary causes of backgrounds in PEP-II are, in order of increasing importance:

• Synchrotron radiation (SR) generated in the bending magnets and final focussing quadrupoles in the incoming high- and low-energy ring (HER and LER) beam lines. Careful layout of the interaction-region area and a conservative SR masking scheme have proven very effective.
• Two-beam backgrounds from three sources: enhanced beam-gas interactions in the HER, due to low-energy ring IP synchrotron radiation impinging onto the incoming HER beam pipe; photons and low-energy e+- from radiative-Bhabha scattering hitting nearby vacuum components; and tails generated by the beam-beam interaction and/or by the electron-cloud-induced blowup of the low-energy beam.
• The interaction of beam particles with residual gas around the rings (beam-gas), which constitutes the primary source of radiation damage, and that with the largest impact on operational efficiency.

While instantaneous background conditions do vary because of IP orbit drifts, and of the sensitivity of beam tails to small tune adjustments, reproducible patterns have emerged.

The HER beam-gas contribution is typically dominant: the combination of a 40m long straight section, almost devoid of magnetic bending upstream of the final doublet, and of the magnetic beam-separation scheme, conspire to direct abundant bremsstrahlung-induced electromagnetic debris into the IP vacuum pipe. The same process occurs in the LER, but to a lesser extent because of a shorter drift section and lower primary energy. Most BaBar subdetectors, therefore, exhibit occupancy peaks at phi=0 and 180 degrees, reflecting the fact that the separation dipoles bend energy-degraded particles in opposite directions. Such local beam-gas interactions dominate the SVT instantaneous dose rates, the total drift chamber current, and the L1 trigger rate. Limited vertexing of L1 pass-through events identifies the beampipe wall and several aperture restrictions within 100cm of the IP as the primary impact points of lost particles. Maintaining a low pressure in the region from 4m to 60m upstream of the IP in the incoming HER beam line, is vital to minimize this background. Scrubbing, which has reduced the HER-averaged dynamic pressure below 50% of its design value, also proved effective.

Both the DIRC and the drift chamber appear sensitive to beam-gas Coulomb scatters around the entire LER. In addition, the DIRC proved particularly vulnerable to tails generated by beam-beam or electron-cloud induced blowup of the low-energy beam. Even though partially eliminated by a set of betatron collimators in the last arc, such tails tend to scrape near the highest-beta point of the final LER doublet, located inside the SOB. This results in photomultiplier counting rates sometimes exceeding 200 kHz; the problem has been alleviated by local lead shielding, and additional collimation will be installed during the fall shutdown.

Whereas trigger-rate and occupancy considerations define acceptable dynamic running conditions, it is the total integrated radiation dose that determines the lifetime of the sub-detectors. Despite a significant investment in radiation-hard technology, the innermost layers of the SVT silicon and front-end electronics remain the most susceptible to radiation damage. The accumulated dose has been maintained below budget, through a strict program of hardware interlocks, administrative controls, and real-time monitoring. To date, the worst spot of the SVT has been exposed to approximately 1.3 Mrad, about 40% of which is contributed by injection periods.