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HAPPEX-III, PVDIS, PREx targets

HAPPEX-III, PVDIS, PREx targets. D.S. Armstrong April 17 ‘09. Cryotargets – what we requested: new 25 cm racetrack cells – status 2) Solid Targets for HAPPEX-III/PVDIS – plan 3) Solid Targets for PREx (from ROM). Cryotargets for HAPPEX-III/PVDIS.

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HAPPEX-III, PVDIS, PREx targets

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  1. HAPPEX-III, PVDIS, PREx targets D.S. Armstrong April 17 ‘09 • Cryotargets • –what we requested: new 25 cm racetrack cells • – status • 2) Solid Targets for HAPPEX-III/PVDIS • – plan • 3) Solid Targets for PREx (from ROM)

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

  3. New Cell Block Design from Dave Meekins (this is for 10 cm cell)

  4. Cell length for HAPPEX-III • 25 cm cell also? • Advantages: • Swap-compatible with PVDIS cell (mutual spares) • Ratio of Al (windows) to lH2 smaller by 0.8 • - reduced QE background • - perhaps reduced boiling (if film boiling at window dominates) • somewhat reduced acceptance (10-15%)at detector for Al windows • Luminosity (assuming cryogen load can be delivered) • Disadvantages: • Increase radiative tail losses: 15-20% 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 • Maybe boiling performance worse, if bulk-dominated… We know 20 cm was good.

  5. Cryotargets – request • Requested identical 25 cm cells 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….. • Can have three cells on cryotarget ladder now (was not the case for HAPPEx-II) • Have one working 20 cm cell (hydrogen: thin-walled) from HAPPEx-II • Requested: • For new (25 cm) cells: • - Side walls thickness: aim for 7 mils. • - Entrance and exit windows: aim for  5 mils. • - Important to have good measurement of window thicknesses, particularly • in 1cm x 1cm region around the nominal central ray • (allowing for 4 mm raster and a 5 mm offset of the beam axis) • ASME issues seemed to have calmed down…

  6. 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) Machining and measurement tolerances; chatter of bit … Need integrating mode data with variable density gas (target warming) to scale “xt” factor

  7. Cryotargets – status • ASME issues seemed to have calmed down (non-trivial!) • Mike Seely resigned from Targets group (December 2008) • I was late getting all specs to Targets group • & getting info to Kees to justify $$ (mea culpa) • Dave Meekins has been busy with other targets (SANE, Qweak…) • Cells not yet made, but still have adequate time: • a few weeks needed for machining & a few weeks for testing • Dave M.: time is “not yet a problem”

  8. Solid Targets – HAPPEx-III & PVDIS • - H2O cell (pointing angle measurement) • - BeO viewer • - Al dummy foils, located 25 cm apart (at the windows of the long • racetrack cells) with 1.00 mm thickness • - Standard C multi-foil optics target as we used in 2005 • - Carbon single foil target • - Ta foil target (pointing angle measurement) • - empty position • Can’t run H2O cell when cryotarget cold (duuh…) • Run optics/pointing measurements firstwith H2O cell installed, then break vacuum (all running at low current to this point), remove H2O cell, cool target. Estimate 2-day turnaround. Note: point angle measurement done at 1-pass beam energy, so there is also parallel overhead to restore 3- pass beam.

  9. Solid Targets – PREx Top of stack (closest to cooling); one cryoloop only • Start with warm target: water cell & truncated target ladder (BeO, Ta, empty?) • Break vacuum, remove water cell (no high current on Pb yet) • Run with cold target and full solid target ladder.

  10. Backup Slides (point angle details)

  11. 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)

  12. 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

  13. 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

  14. 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

  15. 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)

  16. 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…

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