Applications of polarized neutrons V.R. Skoy Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research141980 Dubna, Moscow Region, Russia Currently at Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 790 – 784, Korea
Main fields of Application • Nuclear structure parameters • Angular and polarization correlations in neutron scattering and capture reactions. Scattering lengths and nuclear pseudomagnetism. • Tests of fundamental symmetries • Space parity nonconservation in transmission, fission and capture reactions with unpolarized nuclei. Time reversal invariance test in neutron reactions with polarized or aligned nuclei. • Fundamental properties of neutron • Anomalous neutron dipole moment. • Magnetic properties of matter • Investigations of domen structure of ferromagnetic. Dynamics of the phase transitions.
Definition of Polarization P = 0 P = 6/12 = 0.5
Polarized neutrons production • Transmission through polarized targets • Polarization via Scattering (n,p) reaction • Polarization via Capture 3He(n,p) reaction • Reflection from magnetic single crystals or magnetic mirrors • (cold and thermal neutrons)
For (n,p) scattering cross section below 100 keV Proton Target Polarization via Scattering
RF - Field Dynamical Nuclear Polarization For polarization via scattering LMN single crystal was initially used. Now some sorts of alcohol and polyethylene are used. Dynamical Nuclear Polarization method which requires: T < 1 K H > 1 Tesla RF - pumping This method can be used for polarization of other nuclei. For example, La in LaAlO3
For 3He(n,p) capture cross section and 3He Target Polarization via Capture
Optical polarization of the Noble Gases • The high nuclear polarization of odd isotopes of any noble gases can be built by means of the following by two – stage process • The circularly polarized resonance laser light optically pumps an alkali – metal vapor (usually Rb), aligning the optical electron spins along the quantization axis (direction of laser beam). • In the presence of noble gas nuclei with non – zero spins, the electron polarization of the alkali – metal atoms is transferred to the nuclear polarization of these nuclei via the hyperfineinteraction during the collisions (the spin – exchange process).
Helmholtz Coils (20 – 40 Gauss) • RF – Coils (NMR) • Glass Cell with 3He (1 – 10 atm.) • Pick – Up Coil (NMR) • Laser Beam (795 nm, > 15 Watt) • Photodiode Typical Installation Design
Relaxation (decay) of 3He polarization • The origin of 3He polarization relaxation are: • The impurities of the paramagnetic atoms inside cell wall and bulk. Thus, to achieve of long decay time (> 10 hours), the cell and gases mast be very clean. • Inhomogeneity of the external magnetic fields. • The last characteristics time can be expressed as: If one put a cell inside the cylindrical magnetic shield, then Here, S is transverse field attenuation factor, and S|| is longitudinal one. Usually: S > S||
Neutron Spin Filter Based On Optically Polarized 3He In a Near – Zero Magnetic Field Frank Lab. of Neutron Physics, JINR 2001
Reflection from magnetic single crystals or magnetic mirrors Polarization in outgoing neutron beam after reflection from magnetic single crystal arise due to interference between nuclear and magnetic scattering amplitudes: Polarization in outgoing neutron beam after reflection from magnetic mirror arise due to the difference of the refraction coefficients n for two neutron spin states: Thus, by adjustment of field H, for one of spin states the mirror reflection can be realized. Both above methods may be effectively used for thermal and cold neutrons (E 0.025 eV) because the wavelength should be of order of a distance between the crystal planes or the thickness of mirror surface coating.
Angular and polarization correlations in (n,) reactions Differential cross section of neutron capture by unpolarized nuclei (17 terms): : unit vector along incident neutron momentum : unit vector along outgoing - ray momentum :unit vector of neutron polarization Terms: Indexes “s,p” refer to capture of neutron with orbital momenta 0 (s – wave) and 1 (p – wave) respectively. Some of terms depend from space parity nonconservation weak matrix element wp. Simultaneous investigation of above correlations can give information about the parameters of the s – and p - wave resonances of nuclei and matrix elements of the weak interaction.
Nuclear Pseudomagnetis Let neutron momentum coincides with nuclear polarization. Then, neutron forward scattering amplitudes for parallel (+) and antiparallel (-) directions of neutron and nuclear spins would be: Here, b are the coherent lengths for the correspondent spin states. This distinctions means difference between neutron refraction coefficients: From other hand, it means rotation of transverse neutron polarization around nuclear one like around magnetic field. The frequency of such pseudomagnetic rotation is
Neutron Electrical Dipole Moment (EDM) Neutron spin is a single direction can be associated with dipole moment: If P – invariance holds, then: Thus, 0. In case of PNC, 0 if holds CP – symmetry (which implies T – invariance via CPT – theorem). Indeed: Thus, nonzero neutrons EDM means simultaneous Parity nonconservation and Time Reversal Invariance violation. Present experimental limit on neutron EDM (ILL):
Space parity nonconseravation (PNC) and time reversal invariance (TRI) test in neutron transmission Neutron forward scattering amplitude on polarized nuclei has a form: A: P,T – even strong interaction B: P,T – even strong spin dependent interaction(pseudomagnetism) C and E: P–odd, T-even PNC weak interaction D: P–odd, T-odd TRI violation - weak interaction PNC effects in nuclear reactions are explained using the assumption of mixing of compound - states with opposite parities by the weak nucleon - nucleon interaction. From nowadays experimental data: and expected value of