HAPPEXIII, PVDIS, PREx targets

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
• Cant run H2O cell when cryotarget cold
  (duuh)
• Run optics/pointing measurements first with
  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 (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)
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 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
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
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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)
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