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Polarex for nuclear orientation

Contact : F. Le Blanc


Complementing the current trend of studying nuclear dynamics from reactions with exotic nuclides are the necessary, unambiguous determinations of ground-state properties such as spin, half-life, mass and shape. The measurement of these basic properties over a large range of the nuclear chart will permit to test the effectiveness of the theoretical models. Single-particle properties in nuclei are decisive for such determinations and a particularly powerful tool for their elucidation is the study of exotic nuclei in spin-oriented states.

These polarized nuclei can be used to measure nuclear magnetic moments, ground-state spins, and for far-reaching studies of fundamental interactions involving beta-asymmetry and isospin mixing for example. To reach this physic, one needs an experimental system able to measure gammas, neutrons, alphas, and betas. Coupling this system with nuclear magnetic resonance (NMR), very accurate measurement of the g-factor of the implanted nuclei can be reached. Electric quadrupole moments of beta-emitting nuclei can also be measured. On the other hand, this spin-oriented ensemble can be used for fundamental symmetry test and also for solid-state physics.


Polarex uses the On-Line Nuclear Orientation (OLNO) method to observe the decay of a spin-oriented ensemble of nuclei. This method combines the on-line implantation of radioactive beam of interest with the Low-Temperature Nuclear Orientation (LTNO) technique. The polarization, needed to get a spin-oriented system, is induced by on-line implantation of the exotic nuclei on a ferromagnetic host foil held at a temperature of the order of 10 mK attached to the cold finger of an 3He-4He dilution refrigerator. An applied magnetic field (0.01-0.5 T) magnetises the ferromagnetic foil and the implanted nuclei are oriented through the internal hyperfine field. Combination of the OLNO method with nuclear magnetic resonance (NMR) can provide a very accurate measurement of the g-factor of the implanted nuclei (On-Line Nuclear Magnetic Resonance of Oriented Nuclei). A modulated RF field produced in a resonance coil inside the cryostat with axis normal to the applied magnetic field is used to excite resonance between the hyperfine split substates of the oriented nuclei. As the RF frequency is varied, resonant absorption can be detected by reduction in the observed anisotropy of the decay products. An experimental set-up like Polarex can be used with two different methods, off-line and on-line. With the first method, the exotic nuclei are implanted in a foil, which will then be inserted in the cryostat. As with the second method, the exotic nuclei are directly implanted on the cold foil in the cryostat.

On-line studies will focus on neutron rich nuclei around the N = 50 and N = 82 regions, where many critical data are missing. Some measurements of interest are given in three Letters of Intent that involve the full Polarex collaboration (CSNSM, IPN, LPSC, ILL, University of Tennessee, Niigata University) :

Orientation of 137I and decay of high-level excited states of 137Xe : magnetic dipole moment of 137I, parity admixture in excited states of 137Xe, and beta-delayed neutron emission from 137Xe

Magnetic-Moment Measurements of Sb and I nuclei close to 132Sn : 134Sb, 136I and 137I

Gamma-ray angular distribution measurements from oriented nuclei using Polarex : M1+E2 mixing ratios in 130Te

Right : setup of the cryostat connected to the ALTO RIB line. Left : Already installed cryostat on the Polarex platform in the ALTO experimental area.



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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