Backgrounds and Remediation at BaBar
[ Motivation |
Thoughts so far ]
"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
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
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
- 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
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.
This page was updated by
Guglielmo De Nardo;
Last modified: Mon Sep 22 06:40:07 PDT 2003