| Research Abstract |
In order to carry out a systematic measurement of electromagnetic moments for unstable nuclei and thereby to drastically extend the experimental reach of nuclear structure studies, we developed experimental methods to produce spin polarization in radioactive nuclei and did β-NMR measurements on some light-mass neutron-rich nuclei.
To achieve high degrees of polarization for a wide variety of nuclei, we took a polarization method that is based on the atomic beam technique. The key to a successful application of this method to radioactive nuclei is the realization of an efficient stopping of high-energy reaction products from the projectile fragmentation reaction in some thin stopping material, since this reaction is known to provide the most powerful production method for radioactive beams. After some test experiments and computer simulation works on the performance of a few different stopping methods, we decided to stop nuclei in a high-pressure, large-volume helium gas, collect them in
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their charged state into a mall volume by using a set of electrodes, neutralize them, and then spout them out from a nozzle in a flow of the carrier gas. A stopping gas chamber, electrodes, and a Laval type nozzle were designed and fabricated. A beam of radioactive nuclei ^<17>N, obtained from the radioactive beam line RIPS at RIKEN, was introduced in the gas chamber and the stopping and spouting of the ^<17>N nuclei were investigated by counting the β-rays emitted from ^<17>N activities. We thus confirmed that an electric field profile with an exponentially growing strength towards the nozzle position allowed the collection of 40 % or higher of the stopped radioactivities at the nozzle position. We also confirmed an extraction of ^<17>N atoms from the nozzle. The Laval type nozzle was designed to implement a sharp forward-directed jet of gas, and a setup with a laser was constructed to investigate the shape of the gas jet and the velocities of the atoms spouting from the nozzle. The setup is now under an improvement, and a spin analyzer sextuple magnet under fabrication. We also carried out measurements of magnetic moments for neutron-rich nuclei ^<17>C and ^<15>C and quadrupole moment for ^<17>B by means of the conventional polarization method through the projectile fragmentation reaction.
Thus, a basis for our method to produce spin-polarized radioactive beams has been established, and a whole setup from the radioactive atom sources upto the spin analyzer will soon be tested. We will then proceed to the development of the remaining rf transition-spin analysis section, and after three years when the completion of a new radioactive beam facility RIBF at RIKEN, we will apply the present method to measurements of nuclear moments for wide regions of nuclei far from stability. Less
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