RA3. A New Experimental Technique for Decay Spectroscopy: Ranging Out of
Ions from an Accelerated Beam
(C. J. Gross)
Beta-decay studies on nuclei far from stability have traditionally been carried out at isotope separator facilities and at the focal plane of recoil and fragment separators. The isotope separator facilities extract and accelerate beams to a few tens of kilovolts, mass analyze the beam to one part in 1000, and rely on the purity of the resulting beam to study these nuclei. Recoil and fragment separators rely on the reaction kinematics to convey enough energy to spatially separate and/or electronically tag the ions prior to implantation and study of the decay properties. In both cases, very weak components of the beams can be swamped by contaminants. In order to improve upon the isotope separator technique we propose to accelerate the ion beams to approximately 3 MeV per nucleon and use a transmission ionization chamber to detect and enhance the purification of a beam of neutron-rich nuclei.
Fig. RA3-1 - A photograph of the transmission ionization chamber used in the present study. The chamber is approximately 8 cm long with 6-anodes and one common cathode. The windows are mylar and approximately 16 mm in diameter.
A picture of our small ionization chamber is shown in Fig. RA3-1. The energy loss of energetic ions in gas is dependent upon the Z of the ion; at our energies high-Z ions lose more energy than low-Z ions. For neutron-rich RIBs, the more exotic species has a longer range in the gas. Thus, by operating the ion chamber at high enough pressures the high-Z components of the beam will stop in the gas or exit window while the lower-Z components are transmitted to the measuring station.
We have performed tests using a A=120 beam consisting of radioactive Ag and In components (ΔZ=2). In fig. RA3-2 a γ-ray spectrum showing the decay of the 0.3-second isomer in Ag and transitions following In β decay is shown at two gas pressures. A factor of 5 relative reduction in the amount of In has been observed. The overall transmission efficiency is better than 70%.
Fig. RA3-2 - A portion of spectra taken at two different gas pressures corresponding to full transmission of all ions and optimal transmission of 120Ag ions with maximum reduction of 120In ions. The spectra are normalized to the 1461-keV transition from room background.
Improvements to the setup are possible. There was no shielding of the single Ge detector used for this test. In addition, the support wires of the ionization chamber's windows will be changed to minimize bowing of the window. Initial experiments will occur this summer and will involve isotopes with Z~30.
Our collaboration consists of scientists from ORNL, University of Tennessee, Mississippi State, University, Louisiana State University, UNIRIB, Joint Institute for Heavy Ion Research, and Vanderbilt University.