1

HBD UpdateSetback via Gas Accident

T.K. Hemmick for the HBD Crew

2

Gas System Failure on HBD• Friday August 22 two changes were made to the

system– New scrubbers were installed– a fresh bottle of Nitrogen was installed.

• The system was run for three hours (standard) under watch and the water/oxygen levels dropped (new scrubbers).  All pressures & flows seemed stable.

• Monday August 22 the system was not flowing.– The cause was traced to the top-most valve on the fresh

bottle of nitrogen.  This valve was not fully opened and some time over the weekend (as the bottle pressure dropped), the valve stopped allowing fresh gas to enter the system.

– The status of the system when flow was restored was that the oxygen had risen to several hundred ppm and that the water was off-scale (> 1000 ppm) presumably due to known out-gassing characteristics of the detector.

– The water level is likely damaging to the HBD cathodes.

3

Immediate Actions

• Opened the errant valve.• Increased refresh rate of gas to purge out

the stale gas more rapidly.• Checked for good detector seal by doing a

rate of pressure fall tests.– Quick Test (racing time to reestablish flow)

• No leaks were detected as the detector's pressure changed by less than the meter's sensitivity (0.01 torr) in 5 minutes.

• However, O2 and H2O levels did restore.

4

Gas System sketch

• System operates by having analyzer flow less than supply flow.• Pressure controller exhausts whatever is necessary to maintain

detector pressure compared to outside world.• Loss of gas supply with continued analyzer flow results in under

pressure at detector.

Flow

Gas

Flow

Flow

Pump

Analysis

Pressure Controller Exhaust

5

Pressure interlock mistake.

• A pressure interlock system added to LEGs early this year used the wrong model Magnahelic and only protected against overpressure, not underpressure.

• The right model is now in our hands.

6

False leads in leak chasing

• Switching the HBD in&out of the system to localize the leak lead to confusing and apparently contradictory results.

• The trouble was chased down to a leak in the body of the recirculation pump (stainless bellows has crack).

• The result was that the O2 contamination was a STRONG function of the return line pressure and changed by orders of magnitude when this line went from 1 to 2 torr of overpressure.

• Initial results/conclusions about detector leaks were overwhelmed by these small changes.

• There is a 2 week lead time on the pump.

7

HBD Leak-down Tests difficult.

• No leaks found with Argon or Helium sniffing.• We’ll change to flammable gas sniffing to improve

sensitivity.• Leak-down tests are inconclusive since the rate is

below a fraction of a Torr per hour.– We do compare the HBD to a fixed volume reference.– However, external pressure changes can mimic trouble at

this level by simply squeezing the detector body.

• The real test is contamination level under flow.– If the gas is clear we are OK.– If the gas is not, window would be the first suspect.

8

Plan of action

• GEM photo-sensitivity tests (scintillation) cannot be done in bad gas and we cannot afford to wait for the recirculation pump.

• We are assuming that the GEMs are bad and need fresh CsI.

• HBD East is 6 GEMs short of completion– We have removed the HBD West Au GEMs.– We will wash and re-evaporate 6 for the east.– The other 4 will be kept in vacuum storage for

possible later tests on their condition.

• HBD West will be completed with the final GEMs from the next shipment.

9

Schedule

• HBD East will finish in two weeks.– This will coincide with the availability of the gas

system and allow cosmic tests.

• Final GEM shipment left WIS yesterday.• HBD West will finish ~4 weeks after it is

begun.• This is a 6 week schedule for full availability

and leaves no contingency with installation of the full system by Halloween.– Two weeks of contingency from Don would be

very nice to have.