HAPPEX-III, PVDIS, PREx targets

D.S. Armstrong April 17 ‘09

1) 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

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

New Cell Block Design

from Dave Meekins (this is for 10 cm cell)

Cell length for HAPPEX-III

25 cm cell also?

Advantages:

1. Swap-compatible with PVDIS cell (mutual spares)

2. Ratio of Al (windows) to lH2 smaller by 0.8

- reduced QE background

- perhaps reduced boiling (if film boiling at window dominates)

3. somewhat reduced acceptance (10-15%) at detector for Al windows

4. Luminosity (assuming cryogen load can be delivered)

Disadvantages:

1. 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)

2. Perhaps a bit harder to manufacture

3. Maybe boiling performance worse, if bulk-dominated… We know 20 cm was good.

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…

Loop 1 25 cm "race-track" style (hydrogen)

Loop 2 25 cm "race-track" style (deuterium)

Loop 3 20 cm "race-track" style (existing HAPPEX-II/H cell)

Aluminum window thickness

HAPPEX-1 15 cm “beer can” cell:

HAPPEX-II 20 cm “racetrack” cell:

Entrance window 2.8 mil

Exit window 3.7 mil

Side walls 7.0 mil

Entrance window 7.0 mil

Exit window 2.8 mil

Side walls 5.4 mil

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

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”

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 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.

Solid Targets – PREx

1. Start with warm target: water cell & truncated target ladder (BeO, Ta, empty?)

2. Break vacuum, remove water cell (no high current on Pb yet)

3. Run with cold target and full solid target ladder.

Top of stack (closest to cooling); one cryoloop only

Backup Slides (point angle details)

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)

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

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)McCarthy and Sick, Nucl. Phys. A 150(1970)63

1-pass

2-pass

Water cell – excited 16O 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 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)

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 Q2

contribution to error budget of 0.12% at =20° which means 0.2 mrad (0.01°), which matches HAPPEX-II precision…