Hall A Beamline Activation

June 22, 2007 Hall A Collaboration Meeting

Hall A Beamline Activation

John J. LeRose


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.



Thursday March 22Experiment is running smoothly. Two

partial surveys indicate no excessive activation in the Hall.


Friday March 23 – MCC takes the beam away during day shift to work on

Hall B’s 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.

– 16:18 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 γ-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.


Saturday March 24 – Owl shift runs smoothly– 10:06 harps scans indicate beam is OK (small

spread, y~150 μm)– 11:30 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.


Sunday March 25 – More smooth sailing– MCC takes beam for 5 minutes at 13:30 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.

– 18:25 MCC makes an adjustment of the vertical beam spotsize (238 μm→113 μm)


Monday March 26

– 01:31 target camera fails– 04:30 Hall access to reset right NMR.

Remote reset doesn’t 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.

– 09:15 Full survey of the Hall indicates serious activation all along the beamline.


General Comments/Observations

Hall 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 doesn’t 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


General Comments/Observations

MCC• 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 B’s spec’s 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 (σE ~ 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

Target Ion Chambers Mar. 22-26 (R/hr). Trip level is at 3000

EP Ion Chambers Mar. 22-26 (R/hr). Some increased low level activity mid-day Sunday. Spike mid-day Saturday is from harp scans Compton Ion Chambers Mar. 22-26 (R/hr), similar to EP.


Radiation monitor output for RM-29 located in the Hall A tunnel. (γ’s on the left, n’s on the right). Both γ’s and n’s, especially γ’s, show a dramatic increase during the weekend in question.


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.


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 (> 5R/hr γ’s at RM-29) for extended periods of time (>10 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 )

Documents

Hall A Beamline Activation