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Babar sensitivity to backgrounds and BABAR protection system

May 13, 1999

G. Wormser (wormser@lal.in2p3.fr)

Version 1.2

 

This note summarizes BABAR sensitivity to background and the protection system in place.

1/ BABAR sensitivity to PEP-II background

The BABAR detector suffers in three different ways in presence of high machine backgrounds:

i) The radiation can cause permanent damage to some BABAR subdetectors

ii) Large backgrounds can force BABAR out of its normal operating point (ie reduced voltage,..)

iii) Backgrounds reduce the quality and usability of data on tape.

    1. Permanent damage to BABAR subdetectors

Three subdetectors are subject to damage due to a large accumulated dose:

    • The silicon vertex detector (SVT)
    • The electromagnetic calorimeter (EMC),
    • The central drift chamber (DCH).

For SVT and DCH, the sensitivity is somewhat reduced when the detectors are turned off but their electronics remain exposed. These two detectors always have to be put to this reduced sensitivity point during injection. For the EMC, there is no safe operating condition.

The maximum dose allowed in each detector is given in the table below

Detector Total allowed dose Weekly allowed dose Maximum dose rate
SVT 2 Mrad 6 krad 10 mrad/s
DCH 20 krad 50 rad 0.5 rad/hour
EMC 10 krad 12 rad 0.1 rad/hour

 

While EMC and DCH are only sensitive to accumulated doses, SVT has in addition a sensitivity to large instantaneous dose (1 rad in less than 1 second). It is critical to avoid such incidents. The protection system has a threshold at 100 mrad in 1 msec.

    1. Change in operating conditions
    2. High background can cause reduction of DCH high voltage, change in the trigger, reduction in data acquisition speed due to increased event size. BABAR shift leader in conjunction with the BABAR/PEP-II will determine when this becomes not tolerable and requires corrective action.

    3. Reduced quality of data

The presence of many background hits mixed with the normal BABAR data will result in a loss of data quality. The BABAR shift leader in conjunction with the BABAR/PEP-II liaison shift person will determine when this becomes not tolerable and requires corrective action.

  1. The BABAR protection system

The BABAR protection system consists of the following elements :

    • A set of background sensors input to :
    • Two complementary modules, the analog module and the SVTRAD modules.
    • Hardware injection interlock related to the HV status of BABAR subdetectors (DCH, SVT, IFR)
    • Software hand-shake mechanism for injection, thru EPICS

.

2.1 The BABAR sensors

To implement the above goals, BABAR has equipped all its sub detectors with sensors capable of monitoring machine backgrounds, even if BABAR is not in operation. The complete list of all the BABAR sensors is given in appendix. It is the responsibility of the BABAR-PEP-II liaison person to keep track of all these sensors. The PEP-II operator will only be concerned with the following subset of these:

    • The sensors which are input to the beam abort system
    • The fast sensors signal for fast tuning
    • The global BABAR figure of merit sensor (not available at the beginning)
      1. The sensors input to the beam abort system

The sensors which are input to the beam abort system are given below.

Sensor Beam abort module Number Coming thru
SVT PIN diodes Analog module 2 Hardwired/SCP
EMC Pin diodes Analog module 1 Hardwired/SCP
DCH Pin diodes Analog module 1 Hardwired/SCP
DRC PMT Analog module 1 Hardwired/Video
SVT Pin Diodes SVTRAD module 4 Hardwired/EPICS

2.1.2 The fast sensors

A fast PMT signal coming from BABAR DIRC is directly sent to MCC for fast tuning. It is available on a video screen (channel 2-60), together with a DCH diode output. The scope is presently triggered by the Injection Trigger. A single photon corresponds to a 20 mV signal, 20 ns wide. When background rates are high, more than 1 photon will be seen and the amplitude will get higher. As a example, dumping the beam using B3=0 creates a 2 V signal. The first shot thru BABAR gave a 40 mV signal.

2.1.3 The BABAR figure of merit

The best mixture of all BABAR sensors will form the BABAR figure of merit which will be used to assess the quality of the BABAR data. The global summary page is available thru BABAR EPICS. The BABAR main EPICS page is available thru BABAR account and can be set up by the liaison person on any convenient screen. (by typing the command

source /nfs/bbr-srv01/u1/babar/boot/apps/babar.login/EpicsSetup from a babar account)

 

By clicking on the BKG button one gets the background global summary page:

where most, if not all, EPICS_available BABAR useful information is displayed .Integrated doses can

be found by clicking on the DCH, SVT or EMC buttons (here is an SVT display)

 

The EPICS address of all BABAR channels is given in the appendix

2.1.4 Other BABAR information available thru SCP

Four DIRC scalers are also available in SCP . The information is equivalent to the one present in the global summary EPICS page but it comes thru an alternate channel and will archived in the MCC database. They are accessible thru the BABAR Panel and then BABAR Analog Ch100-131. Note that the name for Strip chart should be PR02:ASTS:CH116 etc.

Counters SCP address Conversion factor Threshold Comments
DIRC_TOP ASTS PR02 DATA CH100 1V = 0.2 MHz 0. 5 V  
DIRC_HER ASTS PR02 DATA CH101 1 V= 0.2 MHz 0.5 V  
DIRC_LER ASTS PR02 DATA CH102 1 V=0.2 MHz 0.5 V  
DIRC_TOP ASTS PR02 DATA CH103 1 V=0.2 MHz 0.5 V  

  The status of all BABAR Interlock can be found in the SCP panel : BABAR Panel then

BABAR Protec Digitl.

 

The four column correspond respectively to LER Injection, LER stored beam, HER sotored beam and HER Injection.

The first line corresponds to the SVTRAD module, the next 4 to the each of the 4 inputs of the analog abort module, (the two SVT, the DCH and EMC pin diodes). The sixth line corresponds to the interlock from the BABAR High Voltage status (SVT, DCH and IFR).

2.2 The protection systems

2.2.1 The SVTRAD module

The SVTRAD module is used for both monitoring the 12 SVT pin diodes thru EPICS and to provide input to the beam abort system. It has two thresholds: one for ‘chronic dose’ (initially set to 10 mrad/s) and one for ‘acute dose’ (initially set to 100 mrad , integrated in ~ 1 ms). If any of its 4 abort inputs exceed the thresholds, stored beams will be dumped and injection inhibited. This module does not separate the two beams, ie it will dump both. The 4 inputs channels can be (and should be) monitored through an EPICS strip chart (technical name given here). In addition to its input signal, the output signal of the SVTRAD module is also available on EPICS, indicating whether some threshold has been exceeded: one for 70% of the dump threshold, one for the dump threshold. The information for the SVTRAD module is given in the following table, the MCC EPICS system as:

SVT RAD info Address Threshold (Unit=mrad/s) 
FE-TOP PB60:SVT:BG:SIG1  10
FE-BTM PB60:SVT:BG:SIG2  10
FW-TOP  PB60:SVT:BG:SIG3  10
FW-BTM  PB60:SVT:BG:SIG4  10
 BE-TOP  PB60:SVT:BG:SIG5  10
 BE-BTM PB60:SVT:BG:SIG6  10 
BW-TOP PB60:SVT:BG:SIG7 10
BW-BTM PB60:SVT:BG:SIG8 10

2.2.2 The analogue module

This module is more flexible than the previous one, in the sense that it offers injection rate limiting rather than completely inhibiting injection. It can also identify which is the bad beam through its injection timing gates. Three modules will be in operation, one looking at two SVT Pin diodes , one looking at four EMC PIN diodes (OR-ed together) and one looking at four DCH pin diodes (OR-ed together). In addition, a fast signal from DIRC PMT will dump the stored beams in case of an incident during an injection gate. These inputs are available in the SCP system from the BABAR Panel and then in BABAR Protec Analog panel , as indicated in the following table: (for strip charts, use the notation PR02:ASTS:SVT1TOTL

Analog input signal Name Threshold
SVT PIN diode 1  ASTS PR02 DATA SVT1TOTL  0.3 V (Stored beam)
  SVT1HINJ 0.3 V (HER Injection)
  SVT1LINJ 0.3 V (LER Injection)
  SVT1NINJ 1 V (non gated fast signal )
  SVT1PEAK 1 V (peak outside injection)
  SVT1TEMP Temperature
SVT PIN diode 2  SVT2TOTL  0.3 V (Stored beam)
  SVT2HINJ 0.3 V (HER injection)
  SVT2LINJ 0.3 V (LER Injection)
  SVT2NINJ  1 V (Non gated fast signal)
  SVT2PEAK 1 V (peak outside injection)
  SVT2TEMP  
DCH Pin Diode  ASTS PR02 DATA DCH_TOTL  0.3 V (Stored beam)
  DCH_HINJ 0.3 V (HER injection)
  DCH_LINJ 0.3 V (LER Injection)
  DCH_NINJ 1 V (Non gated fast signal)
  DCH_PEAK 1 V (Peak outside injection)
  DCH_TEMP  
EMC PIN diode  ASTS PR02 DATA EMC_TOTL 0.1 V  (Stored beam)
  EMC_HINJ 0.1 V (HER Injection)
  EMC_LINJ 0.1 V (LER Injection)
  EMC_NINJ 1 V (non gated fast signal)
  EMC_PEAK 1 V (Peak outside injection)
  EMC_TEMP  

  2.2.3 The High Voltage Interlock

If the High Voltage of the SVT, or DCH is at their nominal values, a hardware interlock (the output of a BABAR SIAM module) is sent to the Beam abort logic and prevents injection. Currently the IFR HV is not in the hardware interlock.

2.2.4 BABAR-PEP-II Injection hand shake

BABAR will adopt the appropriate configuration for PEP-II configuration (injection, tuning, physics running), such as turning down HV to be more robust during tuning or injection. It is the responsibility of the PEP-II operator to make sure that the proper handshake with BABAR has been initialized. Never inject by bypassing the initial EPICS dialog and without letting BABAR know. The detailed instructions for the injection sequence are available in the control room. The general spirit is given below:

    1. Go to the BIC panel
    2. Check BABAR status (Inject_permit or inject_deny)
    3. If in inject_deny, check with the liaison person why it is so and ask BABAR to go to inject_permit
    4. Once in injection_permit, Request Injection. Wait for BABAR to become Safe (State BABAR_safe). The BABAR injectable flag will be set.
    5. Inject !
    6. IF BABAR sends an hardware interlock, the injection will be automatically stopped.
    7. When tuning is requested after injection, no change of mode is presently required.
    8. When stored beams are in reasonable shape, deassert the Request for Injection. BABAR will then normally go to Inject_DENY mode and ramp up its voltages for physics.

 

  1. Administrative and human protection

3.1 Administrative limits

In addition to these protection systems, BABAR will also rely on administrative limits. These will consist of limits on the following quantities, for each beam:

  1. Total current
  2. Number of bunches
  3. Current per bunch
  4. Injection rate

These limits will be fixed during the daily BABAR-PEP-II meeting and can only be changed by common agreement of the BABAR and PEP-II run coordinators.

    1. Tuning during stored beam

When tuning during a stored beam is required, inform the liaison person. (in the future , this could take the form to a transition to a PEP-II_tuning state). This will have the following advantages:

  • BABAR will undergo a transition to a safer state (it should not take more than 30-60 seconds)
  • The proper accounting of radiation damage will be possible (see below)
  • In the future, it is planned that higher thresholds values be made available in this mode so the probability of dumping the beam will be reduced.

3.4 Accounting

It is important to get a detailed accounting of the dose received by BABAR in the various periods of machine operation. Three states have been identified: physics running, PEP-II tuning, injection. In addition, PEP-II can be in a physics period and or in a MD period. It is the responsibility of the PEP-II operator to make sure that the machine status is always correct.

    1. Human help

A BABAR-PEP-II liaison person, fully knowledgeable about background and protection will permanently reside in MCC. S/he will monitor the various background sensors, and warn BABAR and PEP-II in case of problems. S/he will assist the operator to identify any BABAR related problems and inform BABAR of what is going on in PEP-II. S/he is there to help, do not hesitate to ask him/her to do so!

  1. Conclusion

Because of the large currents in PEPII, BABAR will be submitted to a potentially damaging machine background. BABAR radiation budget can be thought of as a normal $$ budget: it is finite, always too small and you have to buy a lot for it, in this case: integrated luminosity for the lifetime of the experiment.

It has therefore to be spent judiciously! There is only one difference with a real budget : overrunning it will be much more difficult than usual. This situation will force us to be very cautious at the beginning and it is likely to unfortunately impact machine performances and development. Patience, strict obedience to the rules and dedicated work will thus be necessary to overcome these problems

 

 

 

 

 

 

 

 

 

Appendix

Background Monitor Signals

Epics Names

SIGNAL

EPICS NAME

PIN diodes -SVTRAD abort PB60:SVT:BG:SIGx (x=1,8)#
SVT Pin Diodes BABAR EPICS SVT:FW:BG:TOP_RINST
SVT Pin Diodes BABAR EPICS SVT:FW:BG:MID_RINST
SVT Pin Diodes BABAR EPICS SVT:FW:BG:BTM_RINST
SVT Pin Diodes BABAR EPICS SVT:FE:BG:TOP_RINST
SVT Pin Diodes BABAR EPICS SVT:FE:BG:MID_RINST
SVT Pin Diodes BABAR EPICS SVT:FE:BG:BTM_RINST
SVT Pin Diodes BABAR EPICS SVT:BW:BG:TOP_RINST
SVT Pin Diodes BABAR EPICS SVT:BW:BG:MID_RINST
SVT Pin Diodes BABAR EPICS SVT:BW:BG:BTM_RINST
SVT Pin Diodes BABAR EPICS SVT:BE:BG:TOP_RINST
SVT Pin Diodes BABAR EPICS SVT:BE:BG:MID_RINST
SVT Pin Diodes BABAR EPICS SVT:BE:BG:BTM_RINST
SVT Occupancies (AToM) Not yet available
DCH RADFETS DCH::ENV:EPL_R:RAD_x (x=0-3 backwards)

DCH::ENV:EPL_F:RAD_x (x=0-1 forwards)

DCH PIN diodes not available yet
DCH Currents DCH::HV:PS_1:I:SL# (superlayers)

DCH::HV:PS_1:I_TOT

DIRC Currents not available yet
DIRC PMT Scaler rates DRC::ENV:VSM:CNTx (x=0-11)
EMC diode leakage currents not available yet
EMC RADFET's EMC:BB:ENV:RMB_x:VOLT_y (x=0,1; y=0-31)

EMC:EC:ENV:RMB_x:VOLT_y (x=0,1; y=0-31)

IFR Currents IFR:sector:HV:PS#:IMON_chan

IFR:sector:HV:PS#:STATUS_chan

IFR RPC's IFR:sector:RT:RPC:bkg#