1. INFN-Sezione di Pavia, Italy
Plans for the 3He target
Paolo Pedroni
INFN-Sezione di Pavia, Italy
Work by S.Altieri, P.Bartolome-Aguar, M.Martinez, A.Thomas

2. Why is a (polarized) 3He target useful ?
2H: m ' mp + mn ) ­ ­
IGDHneutr.' IGDHdeut.-IGDHprot. -INuclear
3He: m ' mn ) ­ ­ ¯ (S-state with ' 90% prob.)
IGDHneutr.' IGDHHe3-a¢ IGDHprot. -INuclear
(a ' 10%)
Spin Structure of 3He ' Spin structure of the neutron

3. Measurement of the helicity dependent total cross section (sp -sa)
Inclusive method (no partial channel separation)
No nuclear effects (free nucleons inside nuclei)
3He gives (by far) the most accurate evaluation of the GDH sum rule for the neutron
(nuclear corrections » the same for both nuclei)

4. Nuclear corrections (of course can not be avoided…): the possibility of comparing the nuclear models for two different nuclei, will greatly enhance their reliability and decrease the systematic errors of the nuclear corrections.

5. “Experimental” polarized 3He targets
3He gas can be polarized at room temperature and low pressure (»mbar) with a laser using the so-called “optical pumping method”
Polarized gas can then be compressed to » 6-8 bar without a big loss of polarization
At Mainz, A1 Collaboration has used a high pressure glass cell for the neutron form factor measurement, but …
=) electron beam intensity much higher (» 104) than the intensity of the A2 tagged photon beam
=) A1 used thin Cu in-beam windows (not to depolarize the gas); the particles thus emitted were not “seen” by their spectrometers.
=) A1 had a spherical target; the polarization degree was continuously monitored using a Helmoltz coil.

6. A polarized 3He target for the A2 photon beam
Very long target (» 20cm)
Cylindrical shape (how to monitor polarization ?)
Very thin in-beam windows with light materials (do
they they depolarize the gas ?)
It has to be kept inside a very homogeneous magnetic
magnetic field (D B/B » 10-5 ) . Very long solenoid
(50 cm) around the target (feasible).
First basic questions: do we “see” enough events from the gas target ? Are we “drowned” by events from the in-beam windows ?

7. Estimates from our past experience with DAPHNE
(charged particles; on line rejection of most of the e§) H My
3He My
1 bar

8. 3He My
8 bar
50 m
At 8 bar and 50 m

9. First feasibility test : Rate test
Target prototype
shape (ball+ cylinder) is a “relic” from the past. This will not be the final one …

10. mylar not suited: it does not stand the high temperatures (200o) needed to “prepare” the target before the filling of polarized helium
Kapton window (50 mm)
Pressure measurement
Gas valve (in/out)

12. A first test has to be performed before the end of the year (before the end of P.B-Aguar diplomarbeit)
The test can be done wihout moving the hydr.target cell now in place
The glass cell will be placed downstream just after the last (vacuum) window of the (empty) hydr.target
the input window of the glass cell and » half of its length can still be “seen” under a relatively high solid angle by MWPCs.
Comparison of rates and MWPCs “vertexes” between “empty” target and target filled with gas at different pressures
An additional test could eventually be done when the hydr.target will be dismounted.