Polarized ³He targets in medium energy physics at Mainz Microtron
Hyperpolarized 3He can be used for various double polarized experiments at the MAinz MIcrotron (MAMI). Scattering experiments play a crucial role in the exploration of nuclei and their constituents, i.e. protons and neutrons, as well as the forces acting between them. It was found that such interactions are spin dependent. From scattering experiments using spin degrees of freedom one can measure small amplitudes in the presence of strong ones (heterodyne principle), whereas amplitudes which enter quadratically in unpolarized cross-section measurements cannot be measured.
Electric form factor of neutron
Experiments involving the neutron’s spin are hampered by the fact that a target of free, polarized neutrons is not available because they disappear by radioactive decay and by capture in surrounding material. But polarized 3He is a good approximation to a target of polarized neutrons , because it is composed of a neutron and two protons and its nuclear spin of ½ is due to the neutron while the two proton spins are paired off. Since quarks, the constituents of neutrons and protons, are believed to carry fractional electric charges, it is significant for quark models to measure the charge distribution within protons and neutrons, often expressed as a so-called electric form factor. In addition the neutron has a distribution of magnetism resulting in a magnetic form factor which in unpolarized cross-section measurements is about a hundred times larger than the electric one. Using both polarized electrons and a polarized neutron target one becomes sensitive to the interference term between electric and magnetic scattering amplitude (Fig.1).
Fig.1: Double polarized electron scattering to extract the electric form factor Gen via beam helicity asymmetries
At the MAMI the electric form factor Gen was measured using the polarized 3He target in the momentum transfer range Q2 = 0.35-1.5 (GeV/c)2 [2-6].
Validation of the Gerasimov-Drell-Hearn sum rule
The tagged photon facility of the MAMI accelerator has a long tradition in double polarized photoproduction experiments. One of the main physics issues has been the experimental validation of the Gerasimov-Drell-Hearn (GDH) sum rule [7,8] which requires a circularly polarized photon beam and a longitudinally polarized nucleon target in combination with a 4π-detector system. Circularly polarized photons are obtained from bremsstrahlung of longitudinally polarized electrons at an amorphous radiator. The energy information of the photon is provided by the Glasgow Tagger installed at MAMI . For an experimental access to the neutron GDH sum rule the use of nuclear targets such as deuteron or ³He as an effective polarized neutron target is inevitable. So far, only data from polarized deuteron targets exist . Here, polarized 3He (I = 1/2) can provide a complementary approach. The unpolarized protons in 3He only contribute as a constant background to the helicity dependent cross-section. Their contribution therefore (almost) vanishes when doing the difference (NP − NA) between the number of events emitted with parallel (NP) and antiparallel (NA) photon 3He spin configuration. Based on these motivations, for the first time a polarized 3He target suited for a real photon beam experiment has been built and tested. This system comprises cylindrical target cells, and a solenoid inside Crystal Ball (Fig.2). Due to space limitations polarimetry is performed inside large Helmholtz coils on the upstream site of the detector. The target cell can be moved between the two positions via an automatic transport system which allows polarization measurement during data taking without the necessity to enter the experimental hall.
Fig.2: Experimental setup in the photon beam
It has been demonstrated that the system works reliably and that the polarization losses during handling of the polarized gas are under control. Initial polarization values up to 70% and total relaxation times up to 20 hours could be obtained during a first test beam time devoted to the measurement of the double polarized photoabsorption cross section in the Δ (1232) baryon resonance region.
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