MUID Shielding Status

1
MUID Shielding Status

• Vince Cianciolo
• Muon Meeting
• Santa Fe, June 16th, 2003

2
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.

3
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 _at_
  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

4
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 100s 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.

5
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.

6
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 mA maximum
• 5000V operation.
• As it turns out, we can get 200mA 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 cant be changed.

7
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.

8
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.

9
The Problem is Beam-Scrape

• Its 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).
• Its 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.
• Weve 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
10
Yellow collimators reduce scintillator
backgrounds. They can come in farther, and it
would help - PHENIX
Collimator Positions
Blue collimators havent helped yet in this
store, but they can come in significantly farther.
11
Polarizers
Cogging
Rotators
Squeezing
Beam essentially at full energy and no
scintillator rate Problem does not seem to have
a significant beam-gas component (yet).
12
Squeezing during ramp
Transition
Cogging
13
Iron activation seen by scintillators
14
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).

15
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.

16
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 ? gt 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 cms 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).

17
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.

18
Goal shield MuID from entire beamline
line-of-sight by many LI
19
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.

20
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.