NICOLE

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The Nicole On-Line Nuclear Orientation facility comprises a large 3He/4He refrigerator with room temperature side access through which an ion beam from ISOLDE can be introduced to impinge upon a suitable metallic foil maintained at temperatures down to below 10 millkelvin. Using ferromagnetic foils, the implanted nuclei experience hyperfine magnetic fields of order 20 – 200 T, sufficient to produce large degrees of nuclear polarization subject to a spin-lattice relaxation time which can vary between hours and milliseconds. The system has been applied to the study of nuclei of many elements with Z > 20, lighter nuclei experiencing rather weaker fields giving small degrees of orientation. Nuclear alignment can also be achieved through interaction of the nuclear electric quadrupole moment with the large electric field gradient existing in certain none-cubic metals.

The angular distribution of the decay of the polarized or alignednuclei, whether by alpha, beta, gamma or direct or beta-delayed particle emission, shows, in general, large anisotropy with respect to the orientation axis of the system. This axis is defined either by a small field to magnetise the ferromagnetic foil or by the axis of the electric field gradient. The anisotropy is measured using either external detectors for emissions which emerge from the cryostat (gamma, high energy beta, neutrons) or internal detectors for less penetrating emissions (alpha, beta and protons). Measurement of the anisotropy as a function of the degree of orientation, e.g. of nuclear temperature (determined by a nuclear orientation thermometer), yields information concerning the level spin sequence and the multipolarities/partial wave amplitudes involved in the decay. The orientation of the sample means that this valuable information is obtained by singles counting, with no coincidence requirement, and samples having decay rates as low as a hundred s-1 can be studied.

Disturbance of the angular distribution caused by exposing the oriented system to an rf field at the nuclear hyperfine splitting frequency is a sensitive detector of NMR in the radioactive parent nuclear ground state and is used to determine this frequency with typical precision of order 0.1%. NMR on Oriented Nuclei (NMR/ON) has been applied to measure the nuclear magnetic dipole moments of a wide range of isotopes from scandium to polonium.

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