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 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…

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 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…..

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 lH2 smaller by 0.8

- 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; 2nd-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 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)

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