The Beta-Nuclear Magnetic Resonance Apparatus

The beta-NMR apparatus measures the magnetic properties of radioactive nuclei having very short half-lives. This is accomplished by observing the response of the short-lived nuclei to incoming radio waves of differing frequencies.

Expanded Description

The general methods employed in the beta-NMR apparatus are the same used today to diagnose bodily injury with Magnetic Resonance Imaging (MRI).

The beta-NMR apparatus consists of a small electromagnet with a four-inch gap between pole faces. A foil is place at the center of the pole gap to catch the fast moving radioactive beam. Surrounding the foil is a pair of plastic scintillator telescopes used to detect beta particles emitted from the captured radioactive beam. The telescopes are placed on the north and south pole faces of the electromagnet. Small, multi-turn copper coils placed around the implantation foil are used to introduce radio-frequency waves into the sample.

A beta-NMR spectrum is obtained by determining the ratio of the counting rates in the north and south beta detectors as a function of the incoming frequency of the radio waves. At resonance, a deviation of this north/south counting ratio is observed. The frequency of the radio waves required to reach resonance is directly related to the magnetic strength of the radioactive nucleus.

Typically, large samples are required for NMR and MRI experiments. However, by detecting the emitted beta particles from the radioactive sample, a sensitivity gain of over 14 orders of magnitude is realized by beta-NMR measurements over conventional NMR. Successful beta-NMR measurements at NSCL have been completed with sample sizes smaller than 100 radioactive nuclei implanted per second.

The Importance of the Beta-NMR Apparatus

The magnetic properties of nuclei have been well studied for stable nuclei that exist in nature. The origins of these magnetic properties are also well known. Determination of magnetic properties of short-lived radioactive nuclei provides a simple yet sensitive test for complicated theoretical models. Short-lived nuclei have very different neutron-to-proton ratios as compared with nuclei at stability. This may result in very different magnetic properties, which can be directly measured using beta-NMR.

Since a nucleus is comprised of both protons (positive charge) and neutrons (neutral charge), it may have a net charge. The movement of this net charge due to the spin and orbital motion of the nucleus will result in a distribution of magnetic currents. The magnetic dipole moment is the quantitative measurement of the magnetic properties of a given nucleus.

The magnetic dipole moment can serve as a stringent test of current models that attempt to describe the nature of nuclei. Many of these models make simplifying assumptions on the orientation of proton and neutron spins in the nucleus. Therefore, the magnetic dipole moments can be readily calculated. This holds true even for nuclei that may have neutron-to-proton ratios that differ tremendously from those found at the valley of beta stability. By measuring magnetic dipole moments of short-lived radioisotopes using beta-NMR, the predictive power of these models can be evaluated.

The magnetic properties of nuclei are also used to probe the behavior of bulk condensed matter. The study of the magnetic properties of short-lived radioisotopes may provide new nuclear probes for advanced materials.

Technical Information

The beta nuclear magnetic resonance station (beta-NMR) station at NSCL was developed for the measurement of ground state moments. It has also been used to measure the spin polarization of nuclei produced in fast fragmentation reactions. It consists of two beta telescopes surrounding a thin catcher foil. The telescopes are located at 0° and 180° relative to the direction of the holding field of the magnet, and are each placed 24 mm from the catcher foil. A set of radiofrequency (rf) coils which provide the oscillating magnetic field for the resonance measurement are also located around the catcher foil. The coils are two 30-turn loops of magnet wire arranged in a Helmholtz-like geometry. A vacuum chamber fitting into the pole gap containing mounts for the catcher foil and the rf coils has also been developed. The detector telescopes reside outside the chamber.

Status: Operational

Location: S2 vault, N2 vault

Contact person: Paul Mantica

Funding acknowledgement: The beta-NMR station is supported in part by the National Science Foundation under grant PHY-0110253.

Reference:

P.F. Mantica et al., Nucl. Instrum. Meth. A 422 (1999) 498.
doi: 10.1016/S0168-9002(98)01073-0