Background Issues: Real Time Radiation Measurement S.M.Yang EPC.IHEP Mini-Workshop on BEPCII Background Study March 2008 Institute of High Energy Physics,CAS

2 Outline  Detectors introduction  Experiments introduction  Some preliminary results  Next to do

3 Detectors  PIN Diodes  Online dose rate measurement  Installed on vacuum surface near IP  For detector protection  RadFETs  Online integrated dose measurement  About 80 RadFETs mounted on EMC crystals  Monitoring dose of CsI crystals

4 PIN Diode  XRB100s-CB380  Hole-electron pairs generated with income particles and bound electrons interaction  1 cm 2, 380 μm  Sensitivity: 0.25 nA/(mrad/s)  Epoxy window depressing light sensitivity  Guard ring halving dark current  Radiation damage: nAs/krad

5 Dark Current  Dark current increased with bias voltage  Terminal capacitance ↓  Response speed ↑  Charge collection efficiency ↑  Dark current deeply depends on temperature  1.1 times /°C  Temperature compensation needed  Thermistors must be provided for each PIN Diode

6 Current Measurement  Capacitance integration Amplifier  7 internal + 1 external capacitance  Ranges: 50pC /100pC…/350pC…  2 Integrator, no dead time  20 bits ADC  Frequency up to 2KHz

7 RadFET  RadFETs  p-type MOSFETs  Threshold voltage will increase after exposure

8 RadFETs Readout & Calibration  Operation Modes:  Exposure Mode  Readout Mode  Analog switch used  10min cycle  Accuracy: 1mv  Calibration  60Co γ Source 

9  First period of run  Before Aug  SCQ was not installed  Collimator was unavailable  6 PIN Diodes & 8 RadFETs was used Background Measurement

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11 Crotch Pipe Both Beams e- Beam

12 Radial Direction Distribution 75cm from the IP (e - upstream) Horizontal ones suffered much more dose Injections brought much higher background Inner Outer Down Up

13 Z Direction Distribution Crotch pipe position seemed to be the worst Mostly from electron beam Smaller vacuum chamber radius than design value (bottleneck) Crotch pipe (2.2m to IP) 3.3m to IP

14 Int. Dose to the CsI Crystals 30cm 40cm 50cm 60cm

15 Int. Dose to the CsI Crystals 40cm 50cm 60cm

16 Int. Dose at the crotch pipe

17 Preliminary Results The maximum Int. dose at the position of the inner most CsI crystals was about 10rads/day, which was about 10 times that of the design value. Injections brought much more doses. The total Int. dose from the electron beam was higher than that from the positron beam.

18  Second period of run  Oct, 2007 ～ Jan,2008  SCQ was installed  Collimators were installed  Detectors were rearranged Background Measurement

19 Detectors Arrangement (Oct.07) ID Position (Z) Position PD10.26m West from IP (58 # BLM) (e+ upstream)Outside of Outer Ring PD22.1m West from IP (57 # BLM)Outside of Outer Ring PD36m West from IP (56 # BLM)Outside of Outer Ring PD40.26m East from IP (61 # BLM) (e- upstream)Outside of Outer Ring PD52.1m East from IP (62 # BLM)Outside of Outer Ring PD66m East from IP (63 # BLM)Outside of Outer Ring RF01.75m East from IP (e- upstream)50cm Down to Beam Pipe RF11.75m East from IP45cm Down to Beam Pipe RF21.75m East from IP40cm Down to Beam Pipe RF31.75m East from IP60cm Down to Beam Pipe RF51.75m West from IP (e+ upstream)40cm Down to Beam Pipe RF61.75m West from IP50cm Down to Beam Pipe RF71.75m West from IP60cm Down to Beam Pipe

20 Detectors Arrangement (Oct,07)

21 Status of Detectors (2007/12/28)  Dark Currents of PIN Diodes  GT ±6m  For a temperature precision of 0.02 °C ， the poor dose rate accuracy would be only 4mrad/s!!!  GT ±0.26m  Newly Installed: 2.1m (e- upstream) about 100nA -2.1m (e+ upstream) about 10nA  RadFETs  Suffered Int. Dose from 1500 to 2700 Rad  Accuracy was getting worse (±5rad)

22 Early in the second period The position of inner most CsI Crystals upstream e- beam More than 100rads for 10 days The position of inner most CsI Crystals upstream e+ beam More than 100rads for 10 days Accelerator people were doing experiments

23 Early in the second period 0.26m upstream e- beam  Background was still not acceptable  Mostly from injections and beam aborts  Usually, electron beam was much worse electron beam injections or aborts

24 Experiments of Collimators  Storage ring collimators and transport line collimators.  Influences of collimators for stable beam run and injections were studied.

25 Storage Ring Collimators Time Current （ mA,10bu cket ） R4OCH02 Collimator position outsideinside 14: : (open) 14: : (14  ) 14: : : : : : : : : : : : (11.3  )  Sharp signals typically 0.2~0.3mrad/s caused by beam lifetime decrease were detected  No obvious effect was observed by tuning the collimator aperture!

26 Time Current （ mA ） (electron beam Injection) R4OCH02 Collimator position outsideinside 15:42:06- 15:42: (whole open) 15:43:26- 15:43: (14  ) 15:43:39- 15:43: (11.3  ) 15:45:36- 15:45: (11.3  ) 15:47:59- 15:48: (11.3  ) Storage Ring Collimators Injection background was much lower (0.2rad most). Still no obvious effect for beam injection!!! Beam aborts brought much higher doses!! ① ② ③ ④ ⑤

27 Beam Aborts Background More than 9 rad For most serious PIN Diode (2.1m upstream e-), Beam abort could bring several rad of int dose. For detector safe, Dose of one injection and beam abort should be depressed to no more than 1 rad. Beam abort must be well treated.

28 Transport Line collimator Still high beam abort background Much lower injection background The energy dispersion is ±0.3% in e- transport line. The collimator aperture for setting emittance is ±3mm,namly ±1.7 . R4OCH02 is whole open.

29 Positron Beam Background was well controlled!

30 Beam Abort Improved Temporary beam abort system were used to let the beam lose in injection region by using injection kickers and a local bump in that region Beam abort and injection were all well controlled!

31 Electron-Positron Collision 0.26m upstream e- 2.1m upstream e- Single beam background was good enough! But there was some trouble with e+e- collision. e- always be worse!

32 Beam Separated Injection Everything seems OK now!

33 SR Runing (From Feb.2008) Background seemed to be acceptable even for difficult injections

34 The inner most RadFET SR Runing (From Feb.2008) The value was getting smaller with time, It seemed to be annealing

35 Conclusions  Background was being effectively decreased.  For single beam, background from injections and beam aborts is safe for CsI crystals.  Synchrotron radiation is also safe enough.  Injection with collision is not ideal, Beam must be separated by tuning RF phase for injection.

36 Next to do  Study of background source and proportion for radiation protection.  Improve the performance of radiation detectors for steady run measurement.  More experimental study of collimators.  Background study after BESIII finally installed.  Suggestions from you…

37 Thank you!