BABAR
Predicting accurately the background level using dedi- cated simulations is not an easy task, whether the detector plans to run at the intensity or at the energy frontier. Yet, background is a major concern for any HEP experiment as it can severely impact the data taking: first, by slowing down the acquisition system and creating dead time; then, by decreasing the quality of the logged data when signal signatures get lost in a mass of random hits; finally, by degrading or even destroying detector components. There- fore, special care is given to design detectors able to handle background levels corresponding to the predictions (with significant safety margins added), while numerous probes monitor the background during the data taking. When the conditions become unsafe for the detector, automated systems make its HV ramp down to safer levels and can even dump the beams.
Figure 2.2.18 shows an overview of the BABAR back- ground monitoring system: several probes monitor quanti- ties sensitive to background (radiation doses, rates recorded by scaler boards, channel currents, etc.) in real time and compare the measured values with pre-defined alarm lev- els. The status of each variable (in alarm or not) is indi-
Data Taking System RUNSUM MOND ECL_BHA KEKB Monitors BELBMIF Trigger frontend Level-1 Trigger System KLMTRG GDL evdisp DQM1 CDC frontend COPPER TDC
TOF frontend FASTBUS TDC
SVD frontend flashADC
ACC frontend
ECL frontend FASTBUS TDC
KLM frontend
FASTBUS TDC
EFC frontend FASTBUS TDC EFCFBVME
Control System MASTER expertwin localwin local CDC HV local TOF HV local SVD HV local ACC HV local KLM HV TXSEQ runwin logwin (SVD trigger) CDC trigger TOF trigger x8 ECL_BH2 ENVMON COPPER TDC EFC PC DQM2
BOLD font - NSM nodes
Italic font - non NSM components
Thick arrow - data flow Mid arrow - trigger flow Thin arrow - HV control
RFARM2 RFARM1 E1TRK E1VXA E1VXB E1NEU E2NEU E2VTX E2TRK E3 EFARM2 E1TRK E1VXA E1VXB E1NEU E2NEU E2VTX E2TRK E3 EFARM1 x8 x8 x8
Offline Data Processing
HV System HVC SEQ TTD COPPER TDC ECL1VME ECL2VME ECL3VME ACC PC SVD TOF VME CDC PC ECL trigger COPPER TDC TRG PC KLM VME PC00 ... 05 PC06 ... 11
Figure 2.2.17.The configuration of the Belle DAQ system at the end of data taking.
Figure 2.2.18. Snapshot of the globalBABARbackground display (Aubert, 2013) taken at a time when the background was low: all but a couple of probes are green which, in theBABARframework, means ‘safe level’ – alarm states are indicated by yellow (warning level reached) and red (concern) colors. This display was available 24/7 in the control room to help shifters get a real time overview of the background levels around theBABARdetector. The longitudinal and end cross-sections show the locations of the background probes which survey all systems: SVT radiation monitors, current levels in the DCH superlayers, rates in the DIRC, EMC and IFR or neutron rates on both ends of the beampipe.
cated by the color of the display. New alarms produce vi- sual and audio alerts in the control room while automated systems can modify the detector state or even abort the beams if the background becomes worrisome.
There were two main active detector protection sys- tems in BABAR to ensure a safe operation of the sensi-
tive tracking system. First, the SVTRAD which monitored both the instantaneous and the integrated radiation doses received by the SVT. Originally, rates were measured by 12 PIN diodes located on both ends of the SVT in three horizontal planes (one at the beam level, the other two 3 cm above/below it) and on the inside and outside of
Figure 2.2.16.A pipeline TDC module based on COPPER.
the PEP-II rings. As expected, the middle-plane diodes accumulated the highest radiation doses and started to become less reliable due to damage. Therefore, in 2002 two diamond sensors were added to the SVTRAD sys- tem – this was the first time such sensors were used in a HEP experiment – and they worked well until the end of the data taking. Another advantage of these detectors with respect to the PIN diodes is that they are insensitive to temperature fluctuations. The maximum total dose af- ter nine years of operations was measured to be around 4 MRad, i.e.less than the SVT radiation budget, set to 5 MRad. The SVTRAD was also able to abort the beams, either when instantaneous doses were too high or because the integrated dose was consistently above some thresh- old during 10 consecutive minutes. Beam aborts induced by the SVTRAD protection system occurred a few times a day on average. When PEP-II started to deliver beams in trickle injection mode (particles are injected in existing bunches at a few Hz frequency, see Section 3.2.2 for de- tails), the SVTRAD was modified to monitor in addition the dose associated with each injection of particles in the collider rings. This provided a complementary feedback on the trickle injection quality. The second active protection system was based on the monitoring of the DCH currents and was used to prevent damage to the drift chamber wires and the associated front-end electronics; it is described above in Section2.2.2.
The main BABAR background probes were also dis- played in the accelerator control room, providing valuable information about the beam status and helping operators reduce the background levels. For instance, the accelerator crew was notified when the SVTRAD 10-minute counter was enabled; this signal would tell them that the beams were to be tuned and that they also had some time to try and fix the problem before a beam abort would be issued. In addition to the real-time monitoring and protec- tion system, various shieldings around BABARhave been built and improved over the years. The main additions with respect to the original detector design have been a DIRC shielding around the beamline components at the backward end and shielding walls on the forward side of
BABARto protect the outer IFR layers.
2.2.9 Conclusion: main common points, main