HAPPEX-III and PVDIS targets D.S. Armstrong July 22 ‘08 1)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 - 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 9 Be 4.4 MeV 16 O 2.5 MeV 181 Ta 0.2 MeV Thus H 2 O 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 H 2 O, 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)

Water cell results from HAPPEX-II Determined  to  0.01° in 2005 Note: Compared to 6°,  Q 2 /  is 42% as large at 13° and 24% as large at 20° scattering angles

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

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

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 16 O/hydrogen similar to H-II : 16 O excited states down relative to hydrogen elastic by factors of: 5 (3 1 - ), 8 (1 1 - ) and 20 (2 1 + ) Conclusion: Doable, but fits will have to rely on 16 O elastic and the (6.13 MeV) state entirely (I have not looked into 56 Fe peaks, don’t expect to see them)

Summary: pointing angle measurement 1.Water cell is best choice, if one can tolerate the changeover time (scheduling) Could use BeO, Ta, as less-invasive alternates to water cell… 2.Need to go to 1-pass beam for both HAPPEX-III and PVDIS  measurements. 3.PVDIS  measurement at 20° with water cell more challenging, but precision demands reduced…. However PVDIS proposal goal is a Q 2 contribution to error budget of 0.12% at  =20° which means  0.2 mrad (  0.01°), which matches HAPPEX-II precision…

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

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 ) 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 lH 2 to lD 2 on a loop is a couple of shifts; much better than replacing entire cell block on target ladder…..

Cell length for HAPPEX-III Should we go to 25 cm cell also? Advantages: 1.Swap-compatible with PVDIS cell (spares) 2.Targets group only has to make/certify one design 3.Ratio of Al (windows) to lH 2 smaller by  reduced QE background - perhaps reduced boiling (if film boiling at window dominates) 4.(Maybe) reduced acceptance at detector for Al windows Disadvantages: 1.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) 2.Perhaps a bit harder to manufacture; can same window thicknesses be maintained as for 20 cm cell? 3.Maybe boiling performance worse, if bulk-dominated… -Dave Meekins: cryogen load scales as dE/dx times target length; 2 nd -order effects not well known. Will do some fluid flow optimization after we settle on geometry.

Aluminum window thickness HAPPEX-1 15 cm “beer can” cell: HAPPEX-II 20 cm “racetrack” cell: Entrance window2.8 mil Exit window3.7 mil Side walls7.0 mil Entrance window7.0 mil Exit window2.8 mil Side walls5.4 mil 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