Gas Systems for the SHMS Cherenkov Detectors

Transcription

1 Gas Systems for the SHMS Cherenkov Detectors Brad Sawatzky Jefferson Lab SHMS Workgroup Meeting (Aug 26, 2010)

2 Noble Gas Cherenkov: e/π separation for P>~5 GeV/c 2.5 m long gas radiator in simple enclosure at 1 atm. Four mirrors --- each 75/2cm tall x 80/2cm wide. Four 5 PMTs at top and bottom (Hamamatsu R1584). Rough volume estimate is 2500 liters 2

3 Radiator Gas Choices (NGC) e - /π - separation requires 1/ e,min <n<1/β π,max 1/β e,min π threshold at 1 atm: Argon: ~5.5 GeV/c. Adding Neon: up to 11 GeV/c. Gas Pr π Threshold 6 GeV/c Pr π Threshold 10 GeV/c e - N p.e. relative Neon Argon 0.95 atm 0.28 atm 4.3x Nitrogen Neon Helium SHMS Workgroup Meeting (Aug 26, 2010) Neon can be used over the entire SHMS momentum range, but at the cost of fewer photoelectrons collected at lower momentum, in comparison to Argon. 3

4 Allow for Argon/Neon mix at ~ 1 atm Simplest system is to flush, fill, and seal gas is cheap enough that a slow flow would also be cost effective if contamination is an issue Performance monitored online (# p.e.'s) Issues: Basic Gas System for NGC Prior notes have suggested using pre-mix Ar/Ne bottles Easy, cost/bottle is low (~$200 for 8000 STP), but... How much tweaking of the mix is anticipated? frequency of kinematic changes how finicky will the gas ratio vs. performance characteristics be? getting system to fill to a particular ratio will take time(?) Do we need / want a new mixer system for the NG Cherenkov? $2k/MFC head unit + $5k $8k for System controller = $9k $12k

5 HGC Recapture Option Devil may be in the details though... BTeV RICH was ~1 atm device flushed with Argon. C 4 O was then fed into the system and the mixture was compressed in the tank (how fast?). Argon would maintain the pressure in the cond. tank (and preferentially vent through relief valve). When cond. tank was full of liquid (how do they know?) they would use it like it was a factory bottle by taking gas off the top of the tank. (That left them with a residual ~6% Ar contamination associated with minimum gas volume at top of tank.)

6 Heavy Gas Cherenkov (HGC): π/k separation for P>3.4 GeV/c To maintain good π/k separation, it is necessary to reduce the gas pressure above 7 GeV/c. Gap between the `set and `K curves takes into account the SHMS momentum bite and a possible 0.1 atm error in the setting of the gas pressure regulator.

7 HGC Basic Requirements Pressure Range: 0 ~1 atm Operating Pressures: ~ atm Tank Volume: ~2900 liters Gas: C 4 O Minimal system: pump out, fill, seal-off monitor performance online (# p.e.'s)

8 C 4 O Properties C 4 O is stable, non-toxic, non-ozone depleting C 4 O liquifies at -3 C (STP) Requires heated pump heads etc, must watch expansion zones low vapor pressure: 21 C Cost runs roughly $1/liter at STP gas is actively manufactured (not scarce/ banned like C 4 F 10 ) Transmission and n.p.e. studies have been done here at JLab (Hall A, Hall B) performance looks very good Used in BigBite Gas Cherenkov (JLab, 2009) real performance: 8 9 p.e.'s over 40 cm path estimate using meas. mirror refl: p.e.'s 8

9 Basic operation (HGC) Periodic pump-out and re-fill (my preferred option) Hall C HMS has successfully run a sealed heavy gas Cherenkov system this way for years vacuum/pump-down requirement and simple geometry should make a sufficiently leak-tight system equally achievable (contamination hasn't been a problem with HMS) Issue 1: Will require some consideration to optimize run-plans. Don't want to be pumping and refilling to different pressures on a daily basis. Issue 2: Ex. run high-momentum settings first (low pressure), then walk down in p 0 and bleed gas into the tank to bring up pressure. Cost effectiveness of a recapture system will be determined by how frequently we have to pump out the tank (Issue 1). What is experience with HGC in HMS? 9

10 HGC Fill Procedure How to fill to correct (final) density? Filling evacuated system with gas from bottle will tend to over-fill tank (since gas will be cold, not in thermal equilibrium) Could wait for system to equalize, then bleed off excess pressure. How long does this take? Will it interfere with data taking? What is experience with HGC in HMS? Or, use mass flow controller and just deliver the correct amount of gas right off the bat.

11 Confirmation of baseline gas systems (Aye, or Nay?) Are we comfortable with the simple fill and seal approach for the heavy gas Cherenkov? fill and 'forget' system was successful with HMS Cerenkov Are we comfortable with the simple fill and seal approach for the noble gas Cherenkov? with the provision that we can slow-flush gas if desired/necessary What are are tolerances on Ar/Ne gas mixture? Realtime quality/density monitoring is desired and will be actively developed. 11

12 Gas System Project List Want to do some R&D in this area Frank Wesselmann has applied for NSF grant to support this work Some modest Hall C funding for 2011 Measure refractive index directly (optical system) Note: need to be careful to optically isolate this system from PMTs and still have it sample the bulk gas adapted commercial refractometer? (FRW) Faraday effect (?) Measure real-time density using acoustic sensors Jack Segal (Hall A) has simple test setup, but hasn't had much time to play with it Optical and Acoustic systems are both available off the shelf, can also try rolling our own need to determine whether sensitivity levels are appropriate for our needs Trace Oxygen sensors are commercially available that operate at the ppm level 100ppm is (rough) operational limit use Oxygen level as canary gas to monitor contamination Need more details (rad. hardness, calibration stability, etc) Take a closer look at cost effectiveness of C 4 O recapture BTeV RICH uses a relatively simple system devil's in the details though... All are excellent grad-student projects for 2011, Hall C summer students? If you're interested in helping, contact brads@jlab.org 12

13 Random Backup Slides 13

14 Pressure Stability of Sealed System (HGC) Will periodic pump and refill be sufficient from a pressure regulation perspective? Atmospheric pressure fluctuations are tolerable Max storm-related pressure fluctuations in N.News are < 0.08 atm (1 2 time/year) Daily pressure variation is ~0.01 atm Thermal pressure fluctuations are also tolerable ΔT of 20 C ΔP < 0.08 atm No problems here 14

15 HGC Recapture Option Based on system used for BTeV RICH Simple design, no fine-tuning, no PID loops involving precise pressure monitoring Condensation tank removes impurities Should allow us to recapture a large fraction of the C 4 O

16 HGC Recapture Option Devil may be in the details though... BTeV RICH was ~1 atm device flushed with Argon. C 4 O was then fed into the system and the mixture was compressed in the tank (how fast?). Argon would maintain the pressure in the cond. tank (and preferentially vent through relief valve). When cond. tank was full of liquid (how do they know?) they would use it like it was a factory bottle by taking gas off the top of the tank. (That left them with a residual ~6% Ar contamination associated with minimum gas volume at top of tank.)

17 HGC Recapture Option So, not as simple as it looked. Needs to be adapted to our situation (our gas should remain quite pure). Possible modifications include: Put condensation tank in freezer (< -5C) to speed liquifaction (really back to a distillation system with a storage tank now...) Take liquid off the bottom, or simply flush residual gaseous impurities using C 4 O from bottle when cond. tank is full. Need some pressure feedback loop (and appropriately sized tank) to avoid filling cond. tank too quickly and venting C 4 O Need liquid level monitor.

18 Dynamic Pressure Regulation? System using a temperature-controlled open bucket inside a ballast tank Vapor Pressure == Gas Pressure Regulate temperature to maintain pressure setpoint Expected temperature range: > -50C Or, just use a fixed temperature and vent excess gas to low-pressure tank at low rate. Replenish liquid as needed. Concerns: cost, complexity, stability? significant R&D effort needed 18

19 HGC Performance vs. Momentum 19

20 Radiator Gas Options The traditional heavy gas choice is C 4 F 10. 3M stopped production years ago, but still available from Europe (>$300/kg). C 4 O (OctaFluoroTetraHydroFuran) appears to be a good substitute. Easily available from many commercial suppliers (~$100/kg). Used in semiconductor industry for plasma etching. Never before used in an operating experiment, but extensively studied by BTeV for use in RICH detector, including prototype beam tests. T. Skwarnicki, NIM A 553 (2005) N. Artuso, et al., NIM A 558 (2006) C 4 O Properties: Gas phase ~10 times heavier than air (9.19 g/l at 21 o C). Boiling point: -5 o C. Vapor pressure: o C. Stable, non-toxic, non-explosive, non-reactive (except with alkali halide metals). Unlike C 4 F 10, it does not destroy ozone. (n-1) only ~4% smaller than C 4 F 10. Optical transmission cuts off slightly earlier in UV: vs 160nm (C 4 F 10 ). Frank Wesselmann plans to study this gas in more detail. SHMS Workgroup Meeting (Aug 26, 2010) 20

21 Suggestion for 1 atm operation Run NGC and HGC at 1 2 H 2 O over atmosphere when possible BigBite Cerenkov gas system ran in this mode simple system using float-switch in cylinder attached Pressure set-point is depth of float switch Tracks atmospheric pressure Simple, reliable, low-cost Operation cost for C 4 O in BigBite system: ~$40/day in spent gas (matched predictions) 21