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HAPPEX-III and PVDIS targets

HAPPEX-III and PVDIS targets. D.S. Armstrong July 22 ‘08. Pointing angle measurements – water cell target some old-style nuclear physics… 2) Cryotargets 20 vs. 25 cm racetrack cells – issues. Pointing Angle Measurement. Important for Q 2 measurement

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HAPPEX-III and PVDIS targets

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  1. HAPPEX-III and PVDIS targets D.S. Armstrong July 22 ‘08 • Pointing angle measurements – water cell target • some old-style nuclear physics… • 2) Cryotargets • 20 vs. 25 cm racetrack cells – issues.

  2. Pointing Angle Measurement • Important for Q2 measurement • Use nuclear recoil technique as was done for HAPPEX-II/HAPPEX-He: • Need targets with different recoil (i.e. different mass) to maximize precision • Can use elastic scattering (M*=M) or inelastic to nuclear excited states (M*M) • At 1.18 GeV, 13.8°: • E – E’ • ¹H 41.3 MeV • 9Be 4.4 MeV • 16O 2.5 MeV • 181Ta 0.2 MeV Thus H2O target gives close to optimal performance; BeO possible, much less lever arm in E – E’, TiH has engineering problems. Dave Meekins designed water cell: 5 mm H2O, 2 1-mil thick steel windows. Note: cannot run with cryotargets; need to de-install water cell to install cryotargets (roughly 2-3 day turnaround)

  3. Water cell results from HAPPEX-II Determined  to 0.01° in 2005 Note: Compared to 6°, Q2/ is 42% as large at 13° and 24% as large at 20° scattering angles

  4. Water cell for HAPPEX-III Will H2O target work at HAPPEX-III, PVDIS kinematics? H-II: E = 2.76 GeV =6.1° Q2 =0.085 GeV2 q=1.47 fm-1 H-III: E =3.46 GeV =13.8° Q2 =0.625 GeV2 q=4.0 fm-1 H-III (2-pass) E =2.32 GeV =13.8° Q2 =0.290 GeV2 q=2.7 fm-1 (not feasible) H-III (1-pass) E =1.18 GeV =13.8° Q2 =0.077 GeV2 q=1.41 fm-1  go to 1-pass beam for HAPPEX-III pointing measurement (cross sections about 30% of HAPPEX-II values) 1-pass 2-pass McCarthy and Sick, Nucl. Phys. A 150(1970)63

  5. Water cell – excited 16O states test 1-pass 2-pass T.N. Buti, PhD thesis (MIT, 1984); T.N. Buti et al. Phys. Rev. C 33(1986)755

  6. Water cell for PVDIS PVDIS (1-pass) E =1.2 GeV =12.9° q=1.36 fm-1 (no problem) “ =20° q=2.11 fm-1 (more of a challenge) at 20°: hydrogen elastic cross section down by factor 30 vs. H-II : ratio of elastic 16O/hydrogen similar to H-II : 16O excited states down relative to hydrogen elastic by factors of: 5 (31-), 8 (11-) and 20 (21+) Conclusion: Doable, but fits will have to rely on 16O elastic and the 31- (6.13 MeV) state entirely (I have not looked into 56Fe peaks, don’t expect to see them)

  7. Summary: pointing angle measurement • Water cell is best choice, if one can tolerate the changeover time (scheduling) • Could use BeO, Ta, as less-invasive alternates to water cell… • Need to go to 1-pass beam for both HAPPEX-III and PVDIS  measurements. • PVDIS  measurement at 20° with water cell more challenging, but precision demands reduced…. However PVDIS proposal goal is a Q2 contribution to error budget of 0.12% at =20° which means 0.2 mrad (0.01°), which matches HAPPEX-II precision…

  8. Cryotargets for HAPPEX-III/PVDIS • HAPPEX-II used 20 cm “racetrack” cell (design: Dimitri Margaziotis, Cal State LA) • Excellent boiling performance • Geometry – no problem using • at HAPPEX-III/PVDIS angles with • up to 4 mm raster • (vertical acceptance is issue) • - PVDIS asks for 25 cm version

  9. Cryotargets – issues • Xiaochao and I met with Dave Meekins (July 3): • Need to build to ASME code (CFR 851: DOE Worker Safety and Health Programs, new as/or Feb 9 2006) • Code for pressure vessels: 1/16” walls (62.5 mils!! – reminder, HAPPEX-II windows 3-7 mils) • need exemption: paperwork, reviews…. Dave needs choice on cell geometry by Labour day to meet schedule. • Could have identical cells (if we want) for PVDIS, HAPPEX-III: • act as mutual spares in case of leaks; changeover of lH2 to lD2 on a loop is a couple of shifts; much better than replacing entire cell block on target ladder…..

  10. Cell length for HAPPEX-III • Should we go to 25 cm cell also? • Advantages: • Swap-compatible with PVDIS cell (spares) • Targets group only has to make/certify one design • Ratio of Al (windows) to lH2 smaller by 0.8 • - reduced QE background • - perhaps reduced boiling (if film boiling at window dominates) • (Maybe) reduced acceptance at detector for Al windows • Disadvantages: • Increase radiative tail losses: 20% increase in radiative effects, taking into account Al windows; and, they are the “worst” kind (before scattering vertex, reduces asymmetry) • Perhaps a bit harder to manufacture; can same window thicknesses be maintained as for 20 cm cell? • Maybe boiling performance worse, if bulk-dominated… • Dave Meekins: cryogen load scales as dE/dx times target length; 2nd-order effects not well known. Will do some fluid flow optimization after we settle on geometry.

  11. Aluminum window thickness • HAPPEX-1 15 cm “beer can” cell: • HAPPEX-II 20 cm “racetrack” cell: Al background: (1.4  0.1) % Al background: (0.91  0.12) % (2004) (0.76  0.25) % (2005) Propose asking for 5 mil entrance/exit/side walls Machining and measurement tolerances … Need integrating mode data with variable density gas (target warming) to scale “xt” factor

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