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Executive Summary of the Background Remediation workshop

From the Background Remediation Steering Committee

  1. BABAR sensitivity to PEP-II backgrounds

In this section, we will first derive a figure of merit for the machine improvements that are needed for successful operation of BABAR at high machine currents. We will then describe the problems associated with high intensity scrubbing with BABAR on the beam line and conclude with a recommendation encouraging maximum scrubbing to be performed before BABAR roll-on.

1.1 BABAR sensivity

The measurements performed by the various commissioning detectors and recent simulation/reconstruction work allows us to assess (within a factor 2 or so) the status of the tracking detectors, would BABAR have been put on the machine as it was in December 1998. The first SVT z-layer (relevant for CP measurement) and the first DCH layer would have had occupancies of 89% and 46%, respectively, for 1 A in the LER and 0.35 A in the HER. Since these detectors are likely to operate effectively only up to occupancies of about 10%, significant improvements in the machine are needed. Current plans include the installation of collimators in IR4 to reduce distant LER backgrounds and pressure improvements by scrubbing the LER beam pipe. Currently, the dynamic pressure in the LER is assumed to be 60 nT/A. The table below shows for various configurations the corresponding background and occupancies. The background is given relative to what we expect for 1nT uniform pressure in each ring and 1 A in each beam. Preliminary studies show that this 1nT-1A background level is approximately equivalent to 10 times the nominal estimates contained in the BABAR TDR. The detailed assumptions used for the calculation of the quantities in the Table are given in Appendix A.

LER current

LER dynamic pressure (nT/A)

IR4 collimator

Total background

(relative to 1nT, 1A in each beam)

SVT occupancy

DCH occupancy

1 A




89 %

45 %

1 A




51 %

25 %






14 %

1 A






1 A






1.5 A







This table shows that the requirement to be able to use a 1 A LER beam in BABAR is to get below 15nT/A of LER dynamic pressure and the presence of IR-4 collimators.

A simple figure of merit that can be derived for these conditions is the beam lifetime. The LER beam life will be 1 hour at 1 A current for 15 nT/A of dynamic pressure. In December 1998, the LER beam had a 1 h lifetime at 150 mA. This comparison sets the scale of the desired pressure improvement. Note also that the 1.5 A operation is going to be difficult even with a 7.5 nT/A LER vacuum.

    1. Machine Operation with BaBar on the Beamline

The presence of the BABAR detector on the beam line will add constraints to high current operation. Even with all BABAR components turned off, several items listed below are subject to potential degradation of their performance if submitted to a high radiation dose. The most sensitive elements are the following.

-the CsI electromagnetic calorimeter, where energy resolution in the weakest crystals (the crystal resistance to radiation is known to vary according to the production process) will begin to degrade after ~1 krad exposure.

-the SVT electronic chips and the PIN diode protection system, certified up to 2 Mrad.

-The drift chamber electronics, certified to 20 krad.

The table below shows the doses that could be received by these components during a dedicated week of LER scrubbing and the subsequent 3 month BABAR run.

The assumptions are described in the table below.



LER dynamic pressure

IR-4 collimators

Factor for injection damage


Scrubbing week

7 days at 100% eff.

30 nT/A

50% closed



3 month operation

84 days at 50% eff.

7.5 nT/A

100% closed



The factor for injection damage is arbitrarily set to 2 during the scrubbing week to take into account doses that would be received during the frequent injection required to maintain the high beam current.




DCH electronics

Maximum dose (design)

2 Mrad

10 krad

20 krad

Annual allocation for 10 year operation (design)

0.2 Mrad

1 krad

2 krad


1 Mrad/year/25% occupancy

1 krad/year/TDRx10



Estimated dose in scrubbing

0.25 Mrad

0.6 k rad


Estimated dose in 1999 running

0.42 Mrad

0.7 krad


(*) The radiation exposure per crystal in the calorimeter is unfortunately not currently available and getting more information on this aspect is our first action item. The total sum of radiation in the CsI is deduced from a Monte Carlo prediction where important background sources were not included. The dose in the most exposed crystals in the forward area could therefore be significantly higher than the estimate given here.

Some dose limits given in this table are not precisely known but are to be used in a conservative manner. The BABAR protection system will limit injection rates to try to respect these limits. This will produce a machine scrubbing operational efficiency certainly significantly lower than the one that can be currently achieved without BaBar in place.

From this table, we can conclude that the dose likely to be received by the passive (i.e., turned off) BABAR components during a dedicated scrubbing week could represent a significant fraction of the annual allocated dose for a 10-year lifetime of these devices and a dose comparable to the smoother 3-month BABAR operation in 1999.


The Background Remediation Steering Committee therefore recommends to BABAR and PEP-II management to proceed with the planned installation of the IR-4 collimators and to consider an extension of the February 1999 PEP-II run (and hence a potential delay in BABAR roll-on schedule) if this can avoid significant scrubbing at high initial pressure with BABAR on the beam line. A LER lifetime better than 1 hour at 1 Amp is likely to allow proper BaBar operation.



2. Actions items

The Action Items from the workshop are numerous and have been classified in six areas. Only the most important action items are given below. The complete list of action items will be made available on the Web.

  1. Protection
    1. Re-estimate the radiation damage in the CsI calorimter per year under the June 1999 assumptions. Produce a quantitative damage figure of merit. (e.g. reconstruction efficiency of 200 MeV pi0s) (A .Mc Kemey, 5/2)
    2. Instrument an input to the Beam abort system using an appropriate technology for the background sensor in the forward calorimeter and in the Drift Chamber Electronics areas. (Aaron Roodman 29/1, A. Mckemey, 5/2)
    3. Measure SVT sensitivity to radiation damage without bias voltage. (C. Hast, 29/1)
  2. Online Issues
    1. Implement a systematic check of the SVT FEE registers as frequently as required and at least at the end of each run. Extend this check to the other subsystems as time permits. (M. Huffer, 29/1)
    2. Estimate the BABAR event size as a function of background level and its consequences. (Gregory Dubois-Felsmann, 29/1)
    3. Implement dead time monitoring in the DAQ system (Gregory Dubois-Felsmann, 12/2)
  3. Shielding
    1. Put in place a lead curtain on the forward side (incoming LER) . Define its optimal thickness. (John Seeman, T. Geld, 5/2)
    2. Put in place a concrete wall on the backward side (incoming HER). Define its optimal thickness (John Seeman, T. Geld, 5/2)
    3. Investigate possibilities of shielding in backward Q2/Q4 region (G. Wormser, 5/2)
  4. Communications
    1. Implement monitoring of BABAR sensors in PEP-II control room (5/2)
    2. Investigate feasibility and design with PEP-II a summary display with crucial BABAR/PEP-II operation parameters. (?,12/2)
    3. Put in place and train a BABAR-PEP-II liaison group (G. Wormser, 12/2)
    4. Make available history charts in EPICS (G. Abrams, 12/2)
  5. Operation
    1. Implement a hardware interlock between DCH_HV_ON, SVT_bias_ON and No_Inject. (?, 29/1)
    2. Demonstrate a BABAR-PEPII hand-shake cycle. Measure the time it takes (Mauro Morandin, 29/1)
    3. Improve operation limits on DC and IFR high voltages systems.(A. Roodman, H. Band, 5/2)
  6. Offline
    1. Define recommendations for background frame production Done
    2. Produce the frames. (C. Young, 5/2)



3. Open issues

There are several areas of concern that are currently under active study. The committee will follow closely the developments in these areas.

  1. Online issues
  2. 1.1 Feature extraction in the ROM was reported to be a limiting factor for the EMC, to a 10% average occupancy at 2 kHz, and in the DCH (200 hits total).

    1. Large SVT occupancies may require more ROM backplanes. Is it possible to acquire and install them?
  3. Photon reconstruction in the calorimeter
  4. Will the available CPUs be a limitation for physics extraction. Is a ‘L4’ filter before full reconstruction needed?
  5. Trigger jitter time, neutral and charged: variation wrt to background
  6. Policy for best lifetime usage of EMC , SVT .




Appendix : Assumptions used in the computation of the background and occupancies table.

Machine parameters




Intensity ratio



Ring-averaged base

Pressure (nT)



Ring-averaged dynamic pressure (nT/A)


Varied from 60 to 7.5

Arc background weight factor



IR-12/4 collimator suppression factor


Varied from 1 to 0.5

Straight section pressure (nT)



Straight section weight factor



Occupancy extrapolations


SVT-1st layer

DCH-1st layer

Initial measurement

2.5% occupancy z-layer at 250 mA LER Dec 1998 in the SVT commissioning module

60 MHz: track rate measured in the miniTPC as function of LER current and extrapolated to 1 Amp

B Field effect x Geometry from a scoring cylinder using latest MC



Timing resolution

1 microsecond

1 microsecond

Number of channels



Relative HER/LER sensitivity (from scoring cylinder MC)