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Plans for the 3 He target

Plans for the 3 He target. Paolo Pedroni INFN-Sezione di Pavia, Italy. Work by S.Altieri, P.Bartolome-Aguar, M.Martinez, A.Thomas. Why is a (polarized) 3 He target useful ?. 2 H: m ' m p + m n ) ­ ­ I GDH neutr. ' I GDH deut. - I GDH prot. - I Nuclear

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Plans for the 3 He target

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

  8. 3He 8 bar My My 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)

  11. 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.

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