HAPPEXIII and PVDIS targets

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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.

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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 (MM) 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)
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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
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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 q1.47 fm-1 H-III E
3.46 GeV ?13.8 Q2 0.625 GeV2 q4.0 fm-1
H-III (2-pass) E 2.32 GeV ?13.8 Q2
0.290 GeV2 q2.7 fm-1

(not feasible) H-III (1-pass) E 1.18
GeV ?13.8 Q2 0.077 GeV2 q1.41 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
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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
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Water cell for PVDIS
PVDIS (1-pass) E 1.2 GeV ?12.9 q1.36
fm-1 (no problem)
?20 q2.11 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, dont expect to see them)
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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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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

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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..

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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.

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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