MUID Shielding Status

Vince CiancioloMuon Meeting

Santa Fe, June 16th, 2003

The Problem…• Throughout the run (especially at the

beginning of fills) the MUID suffered from high current draw and high trigger rates.

• I will argue that the primary culprits are particles scraping somewhere along the ring, falling out of orbit, and showering in the beampipe upstream of the MUID.

High Currents

• Current monitor when backgrounds were particularly bad.

• Trend-spotting is not trivial because beam conditions changed by orders of magnitude:

– Intensity– Steering– Collimation

• Also problems on our end which distort patterns:

– Some chains have individual tubes w/ high current draw.

– A relatively small number of chains had trouble @ 4300V, even w/o beam – will be worked on this summer.

• Nevertheless, some general patterns can be seen:

– Upper panels worse than lower panels.

– Currents increase w/ depth.– Horizontal/Vertical tube trends

are somewhat different for upper and lower panels

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Why are High Currents a Problem (A)?• Indicate significantly higher hit-

rate than expected.• In fact, we looked at the hit

rate/current correlation with a run of clock triggers.

– Essentially held a gate open for 500K events * 200 ns = 1/10th of a second and counted hits.

– Rates were 100’s of kHz per channel (!) and correlated well with the current draw.

• Putting the background rates into perspective:

– The average hit MUID hit rate for MB pp events is 1 hit/arm, and this has a significant contribution from background hits.

– Making the worst-case assumption that this rate was entirely due to collisions at the highest luminosity expected (20 MHz MB pp collisions) we would see an average rate of 7 kHz/tube.

– AuAu hit rates at 40 x design luminosity are similar.

• This leads to worries about premature aging of the Iarocci tubes.

• We will study this over the summer and implement a bubbler that will be capable of introducing trace amounts of isopropanol and water, which has been shown to eliminate and even reverse aging in similar detectors with similar operational gas.

Rate (kHz) vs Current (uA)

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Current (uA)

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Expected hit rate from collisions for 40x Blue Book pp luminosities.

Gap-4

Gap-3

Gap-1

Gap-2

Gap-0

Rate (kHz) vs Current (uA)

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0 5 10 15 20 25

Current (uA)

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Expected hit rate from collisions for 40x Blue Book pp luminosities.

Gap-4

Gap-3

Gap-1

Gap-2

Gap-0

Why are High Currents a Problem (B)?• The HV circuit delivers

voltage via 400M current-limiting resistors on each Iarocci tube.

• Therefore 0.5A/tube corresponds to a 200V reduction in the effective voltage and reduces the efficiency in a time-dependent manner.

Why Is That $^&$& Resistor 400 M?• Resistance value was chosen to

allow operation of an HV chain even with four tubes broken on the chain. – This assures us of 95%

efficiency after 10 years of non-serviceable operation assuming 1%/year tube death rate.

• 400 M results from this allowance for four broken tubes and two other pieces of information we had at the time:– 100 A maximum– 5000V operation.

• As it turns out, we can get 200A out of the supplies and the tubes have not had nearly the expected mortality, so that a resistance of 100 M would have been acceptable.– This is not an order of

magnitude.– The resistors can’t be changed.

Probability for a chain to be enabled (assume 1% failure rate per year)

0 1 2 3 41 0.82 0.98 1.00 1.00 1.002 0.67 0.94 0.99 1.00 1.003 0.54 0.88 0.98 1.00 1.004 0.44 0.81 0.96 0.99 1.005 0.36 0.74 0.92 0.98 1.006 0.29 0.66 0.89 0.97 0.997 0.23 0.59 0.84 0.95 0.998 0.19 0.52 0.79 0.93 0.989 0.15 0.45 0.73 0.90 0.97

10 0.12 0.39 0.68 0.87 0.96

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Maximum number of broken wires

High Trigger Rates• The MUID trigger rates greatly exceed the MB trigger

rate.– For pp, we expect the rate to be below the MB rate by

~x500.

• We are forced to require coincidence w/ MB trigger.– Loss in acceptance (if we can get a decent offline vertex

with the MVD).– Loss in systematic check of the MUID trigger efficiency

(looking at unconditioned MUID triggers for BBC-scaled trigger events).

• W/ the MB trigger in coincidence our trigger is dominated with accidental coincidences and we are still forced to scale down and/or lose acceptance with more selective (e.g., Deep-Deep) triggers.

How to Study/Combat the Problem?• Install some test shielding

– Qualitative observation was that the shielding (~2-feet Fe equivalent “heavy” concrete) dsitributed the currents in gap-4 and greatly reduced currents in gap-3.

– Difficult to quantify effects• Rapid, massive changes in beam conditions• Inability to perform systematic studies in which shielding parameters (e.g., positions,

thickness, composition) were changed.

• Provide feedback to MCR– Scintillators, current monitors– We learned when it was safe to turn chambers on and start a “muon-in” run.– MCR learned to tune and collimate to take our needs into account.

• Clock triggers– Study hit rate vs. current– Look for component that penetrates partially, but not enough to fire trigger.

• Imaging source– Determine whether there is a component which we will be unable to shield against.

The Problem* is Beam-Scrape• It’s not collisions:

– The MUID trigger rates are far higher than the BBC rates.– The backgrounds are present with only one beam (the beam

entering from behind the MUID).• It’s not beam-gas:

– The backgrounds change by orders of magnitude when the beams are steered and/or collimated.

– The backgrounds can be minimal prior to bringing the beams into collision.

• We’ve seen that we are very sensitive to beam-scrape byproducts:– The presence of the polarizer targets more than ½-way around

the ring increases trigger rates by orders of magnitude.• Evidence of beams scraping quad triplets:

– BRAHMS dosimeter studies.– MCR expectations.– Activation-component (Fe) seen by scintillators.

* primarily, at least

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Yellow collimators reduce scintillator backgrounds.They can come in farther, and it would help - PHENIX

Blue collimators haven’t helped yet in this store, butthey can come in significantly farther.

Beam essentially at full energy and no scintillator rate…Problem does not seem to have a significant beam-gas component (yet).

Cogging…

Squeezing

Rotators

Polarizers

TransitionCogging…

Squeezing during ramp

Iron activation seen by scintillators

Shielding Studies (Kin Yip)• Tool: MCNPX (newest

version 2.5.c)• Sources: protons (100

GeV) scraping the inner radii of Q2/Q3 magnets

• Only protons/neutrons turned on at the moment

• Major problem (!): MCNPX does NOT have magnetic field.

• Figure shows background flux at MUID according to this simulation (before shielding).

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Horizontal (cm)

Shielding Studies (Kin Yip), cont.• Several shielding configurations, compositions tried.• Conclusions:

– Interaction length matters, even for slow neutrons, so use steel (and lots of it).

• Note, important to use steel, not 56Fe in simulations to see this result (suggested by Y. Efremenko, confirmed by N. Mokhov (FNAL), confirmed by K. Yip.

– Shielding much more effective as it gets closer to MUID backplate, even if source is far upstream.

• Argues for forward-going scrape products rescattering along length of beam-pipe before entering into MUID.

Limiting Fragmentation• BRAHMS data beautifully

illustrates the relevance of the concept of “limiting fragmentation” to high-rapidity particle production.

– From their own data we see that particle production for > 3 is independent of centrality.

– By scaling to ′ = - y (the beam frame) and scaling by Npart/2 (the number of projectile participants) we see agreement with CERN-energy heavy ion collisions (and this holds generally).

– This is understandable because any particle near the beam-frame must have undergone (or been produced by) only “soft” collisions.

– For fixed r = 6.35cm, the distances particles at different travel before striking the beam pipe are shown.

– Note: there will also be many “spectators” at higher which will strike the beam pipe even further downstream.

• At these glancing angles the 2mm beam pipe looks many cm’s thick and so showers will be created, making the entire beam pipe downstream of scraping locations a line source.

• In retrospect this seems obvious since any particles which originate from upstream sources must emerge at rather shallow angles to get into the MUID (and must therefore pass through the beam pipe where they are likely to shower).

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Heavy Ions have Spectator Nucleons Too…

• Measurement above by CERN emulsion-based experiment.

given by pbeam in beam direction and Fermi momentum in transverse direction.

• At 100 GeV/c this has spectators hitting beam pipe 31 meters downstream of initial scraping.

• Heavy ion beam source likely more extended along beam.

Goal – shield MuID from entire beamline line-of-sight by manyI

Beamline coverage• Walls on previous

slide each cover a stretch of the beamline z-extent for a given MUID transverse radius.

• Regions between black lines covered by downstream wall.

• Regions between red lines covered by middle wall.

• Region above blue line covered by upstream wall.

Current Activities

• Document for RHIC shielding task force.• Charlie Pearson thinking about a series of

walls that will block the MUID from beamline line-of-sight by as much steel as possible.– 4-foot goal– Main wall will likely go immediately upstream

of the DX magnet.

• RHIC is also looking into putting an improved, two-stage collimator.