1

B



B



x

 10  3

cm

Development of Polarized

3

He Target Cells for Proton and Neutron Scattering

R.B. Neufeld

a

, T. Katabuchi

a,b

, T. B. Clegg

a,b

, T.V. Daniels

a,b

, S. Kadlecek

c

Triangle Universities Nuclear Laboratory

a,†

, Durham, NC 27708-0308, University of North Carolina

b

, Chapel Hill, NC 27599-3255 Amersham Health

c

, Research Triangle Park, NC 27709

†

Work supported in part by US DOE Office of High Energy and Nuclear Physics

Motivation and Goal*

3

He(p,p) and

3

He(n,n) at low energies

•

Mass-3 System

– A 3-nucleon force is needed to predict 3 H and 3 He binding energies – A

y

data disagree with theoretical predictions ( the “A

y

puzzle”) •

Mass-4 System

– Effort is turning to 4-nucleon-system where p-wave resonant states exist at very low energies.

E

cm

p +

3

He = 1.69 MeV

•

A

y

Puzzle

– Recent theoretical effort shows that the “A

y

Puzzle” persists – Effects are larger in mass-4 system than in the mass-3 system.

•

New data needed

– New target analyzing power and spin-correlation data are sought below

E p

= 6 MeV to resolve theoretical ambiguities.

– A

oy

and A

yy

are the first objective.

*Ph.D. thesis of B.M. Fisher, UNC-Chapel Hill, 2003

3

He Target Cell for Neutrons < 10 MeV

Design Requirements – A successful cell must:

• Withstand the 15 to 20 bar 3 He interior pressures which experiments require; • Provide a timing pulse for scattered neutron time-of-flight measurements; • Be fabricated from materials which minimize loss of 3 He polarization.

Prior Research – Earlier work* had shown that:

• A 1% mixture of Xe with the 3 He provides a light pulse when a neutron scatters;

High-pressure cell with interior MgO coating

• An interior MgO wall coating on the target cell reflects this light efficiently;

Incident neutron beam Light from neutron scattering event Pyrex window with interior DPS coating Photomultiplier tube

• Diphenylstilbene window coating is a good

wave-

length shifter for light emerging to phototube.

New

Developments

– In our research, we:

• Evaporated films of Diphenylstilbene (DPS) and Magnesium Oxide (MgO) onto microscope slides; • Exposed these coated slides to polarized 3 He at ~1 bar pressure to determine the surfaces’ influence on the polarization lifetime.

• Determined the effective 3 He polarization lifetime if covering a 150 cm 3 cell to be 5.7 hrs for diphenylstilbene and 2.2 hrs for magnesium oxide.

Frame for slides

Vacuum evaporator system in which microscope slides were coated with thin films of diphenylstilbene and Kapton foil was coated with 1µm films of copper. The material to be evaporated was heated by passing a current through the tantalum boat. Slides and Kapton foils to be coated were placed on the rectangular frame shown. Evaporations typically were begun after a vacuum of ~10 -5 bar was attained and could then be accomplished in a few minutes.

Tantalum boat * W. Tornow et al., Nuclear Instruments and Methods 133 (1976) 435.

3

He Target Cell for Protons < 6 MeV

Design Requirements – A successful cell must:

• Withstand the ~1 bar 3 He interior pressure needed for experiments; • Have thin foil windows for passage of incident and scattered protons; • Be fabricated from materials which minimize loss of 3 He polarization.

Prior Research – Earlier work* had shown that:

• Pyrex cells can provide the desired 3 He polarization lifetimes (»3 hrs); • Both cylindrical and spherical Pyrex cells are easy to fabricate; • Kapton or 99.99% pure Al or Cu foils are potential window materials; • Kapton foil is stronger per unit thickness than pure Al or Cu; • Al and Cu surfaces enable longer 3 He polarization lifetime than Kapton.

New Developments – In our research, we:

• Simulated cell geometries, 3 He pressures, plus foil window materials and thickness using Monte-Carlo energy-loss code SRIM, to find the combination enabling minimum proton energy loss and straggling; • Determined bursting pressures for test cells of different geometries with Al or Kapton foil windows of various thicknesses glued on using Varian TorrSeal epoxy. • Evaporated ~1µm films of pure Cu onto Kapton seeking a

cell window

to hold 1 bar pressure and provide longer 3 He polarization lifetime. Spherical Pyrex is cell shown with copper-covered Kapton foil epoxied over the window openings. A strip of unused copper-covered foil is shown beneath the cell. Our tests showed that this material combination for window covering did not provide enhanced 3 He polarization lifetime over bare Kapton foil, perhaps because the foil wrinkled and the interior copper film fractured when the cell was evacuated before filling it with polarized

3He

gas.

*T. Katabuchi et al., Proc. of HELION02 http://www.physik.uni-mainz.de/helion02/ and private communication

.

Summary of Results

For 3 He (p,p) Target Cell

 A spherical 50 cm 3 Pyrex cell with 75µm thick Kapton-covered

windows

withstands ~1 bar pressures and provide polarization lifetimes of >4hr;  This cell will facilitate 3 He(p,p) scattering for incident proton energies as low as 2 MeV and for outgoing particle scattering angles up to 120º.

 Cu-covered Kapton does not further enhance 3 He polarization lifetime.

For 3 He (n,n) Target Cell

 Diphenylstilbene is very compatible with polarized 3 He gas; MgO is marginally compatible, so alternative reflective coatings will be sought.

Future Plans

 We will use the Pyrex target cell with Kapton windows for 2 to 6 MeV 3 He(p,p) analyzing power and spin correlation measurements. We will continue development of a target cell for the 3 He(n,n) measurements.