Hall A Beamline Activation

1
Hall A Beamline Activation

• John J. LeRose

2
What happened?

• March 26, 2007 High Radiation levels are found
  in Hall A after what was thought to be a
  relatively quiet weekend of running.

3
(No Transcript)
4
Thursday March 22

• Experiment is running smoothly. Two partial
  surveys indicate no excessive activation in the
  Hall.

5
Friday March 23

• MCC takes the beam away during day shift to work
  on Hall Bs beam quality. Attempts to fix things
  solely in the fifth pass fail and adjustments are
  made as far back as 2nd pass as well.
• After cleaning up the Hall B beam MCC has trouble
  finding a match for Hall A.
• 1618 40 µA CW beam restored to the Hall.
  Radiation monitor in the tunnel just upstream of
  the Hall, RM-29, immediately shows a 3 order of
  magnitude increase in ?-radiation compared to
  Thursday. Neutron radiation at RM-29 goes up
  markedly as well. This condition remains for the
  rest of the weekend. Activity in the target ion
  chambers is up but not significantly . Compton
  and ep ion chambers are quiet.
• Experiment runs smoothly through the night with
  minor concerns re possibly partially hitting the
  target frame with the rastered beam.

6
Saturday March 24

• Owl shift runs smoothly
• 1006 harps scans indicate beam is OK (small
  spread, y150 µm)
• 1130 MCC takes beam to work on energy spread (30
  minutes)
• Afternoon has repeated attempts to work on the
  energy spread. 1C12 OTR indicates a tail on the
  beam. Harp scans indicate small energy spread but
  SLI disagrees. Ion chambers are still relatively
  quiet. RM-29 continues to scream.
• Swing shift We have good shift. Not any
  problem. Log is full of discussion of raster and
  target frame, but no one is really concerned.

7
Sunday March 25

• More smooth sailing
• MCC takes beam for 5 minutes at 1330 to
  investigate an orbit problem.
• Compton and ep ion chambers show a small peak in
  activity mid-day, but well below any alarm
  thresholds.
• Swing shift is concerned about rastered beam
  hitting the target frame but continues to take
  data.
• 1825 MCC makes an adjustment of the vertical
  beam spotsize (238 µm?113 µm)

8
Monday March 26

• 0131 target camera fails
• 0430 Hall access to reset right NMR. Remote
  reset doesnt work anymore. Partial survey
  indicates some elevated radiation levels. Only
  whole body doses were recorded on the path to the
  right NMR. In keeping with ALARA no one lingered
  to make a full survey.
• 0915 Full survey of the Hall indicates serious
  activation all along the beamline.

9
General Comments/ObservationsHall A

• Shiftworkers were unaware that there was any
  potentially activation causing problem with the
  beam.
• Concern re rastered beam striking target frame
  existed the entire run and doesnt explain
  activation upstream
• The spot became increasingly wide over the
  weekend (200 ? 700 µm).
• Singles rates never varied by more than 10.
• The target OTR was not working
• Requested by experimenters in advance
• Might have been useful

10
General Comments/ObservationsMCC

• On the Accelerator side there was a similar lack
  of realization that there was any serious
  problem.
• Work done on Friday aimed at meeting Hall Bs
  specs involved making adjustments all the way
  back to 2nd pass. After that there was difficulty
  finding a match for Hall A.
• That was the case for the whole running period.
• Knew the beam was not as good as it could be but
  problem indicators like the EP, Compton, and
  Target Ion Chambers showed some extra activity
  but nothing alarming.
• Detailed harp scans performed on Saturday at
  5 µA in fact show pretty good beam with very
  small energy spread (sE  2x10-5).
• However, there was difficulty getting the SLI at
  1C12 to show the same small energy spread.
• Arne Freyberger has indicated that he thinks
  this is because of a beam loading problem that
  sets in above 10 µA
• Beam loss monitors also indicated no significant
  problems

11
Radiation monitor output for RM-29 located in the
Hall A tunnel. (?s on the left, ns on the
right). Both ?s and ns, especially ?s, show a
dramatic increase during the weekend in question.
12
Conclusions

• Indicators on both the accelerator and
  experimental sides, with the notable exception of
  RM-29, indicated that the beam was acceptable.
• All of our alarms and interlocks are geared
  toward sudden dramatic failures in beam
  transport.
• Radiation monitor readings suggest that the beam,
  probably a low energy tail on the beam, scraped
  against something in the Hall A tunnel just past
  the end of the arc (near or at the green wall)
  producing a cascade that tumbled down the
  beamline activating things along the way.
• First suggested by Pavel Degtiarenko
• In the survey of the morning of March 26 one sees
  that radiation levels on the right side of the
  beamline are uniformly slightly higher than on
  the left, supporting the low energy tail
  hypothesis.

13
Recommendations

• Shift workers should observe the radiation
  monitor levels, RM-29 in particular, and alert
  MCC, the run coordinator, and experimental
  spokespersons when high radiation levels are
  observed (gt 5R/hr ?s at RM-29) for extended
  periods of time (gt10 minutes). This is a very
  good indicator that beam is not coming cleanly
  into the Hall
• Machine Ops should setup something similar. If
  they have a strong philosophical objection to
  using RADCON instruments in the machine
  protection scheme, then at least some sort of
  integrated ion chamber signal separate from the
  peak level interlocks could be used. Although, it
  appears the ion chambers are less sensitive to
  this kind of lower level radiation (Ion chambers
  report in increments of 10 R/hr )

14
(No Transcript)
15
(No Transcript)
16
(No Transcript)