2. Recent HRIBF Research - Magnetic Moment Measurements of Short-Lived Excited States in
a Radioactive Environment: 132Te
(N. Benczer-Koller & G. Kumbartzki, spokespersons)
The magnetic moment of a nuclear state is a property that reflects directly the microscopic nature of the state and provides stringent tests of theoretical models. Magnetic moments of short-lived excited states have been measured for many years by a technique which couples Coulomb excitation with the transient field experienced by fast ions traversing ferromagnetic materials.
Recent developments of the experimental procedure involve measurements on beams rather than on targets of the material of interest. This particular approach lends itself well to the use of radioactive beams and opens a large variety of isotopes for investigation. The general details of the method are described in a recent review paper .
The first magnetic moment with a Coulomb-excited radioactive beam utilizing the transient field method (TF) has been measured at Berkeley. The g factor of the 2+1 state of 76Kr was determined . The radioactive beam was produced by the "recyclotron" method. This technique, in a two-step process, involves the production of the isotope of interest by the cyclotron, followed by extraction and re-injection of the isotope into the same cyclotron for acceleration.
However, most of the nuclei that are amenable for study will become available as primary radioactive beams at facilities such as HRIBF and NSCL today, and European, Japanese facilities as well as FRIB in the US in the future.
The first isotope studied at ORNL has been 132Te. The g factor of the 2+1 state has been determined by the Recoil-in-Vacuum (RIV) technique which yields the magnitude of g factor but not its sign. The results have been described in Ref. . The same state has been proposed for TF experiments in order to determine the requirements and limitations of this approach to the radioactive-beam environment.
Figure 2-1: Schematics of target chamber and detector arrangement.
Transient field experiments with radioactive beams necessitate changes to the conventional setup, where the beam typically is stopped in the target and forward-scattered light target nuclei are recorded in a particle detector placed downstream in the beam path. Special care has to be taken so that little radioactive material will be stopped in the target chamber or beam pipes nearby, where its decay radiation can be seen by the detectors. Fig. 2-1 shows the schematic setup for this experiment. The total target thickness was limited to less than 6 mg/cm2 to avoid excessive beam spread. The target was designed to let the beam and Coulomb-scattered projectiles exit from the target and mostly pass through a gap in the particle detector to be stopped down stream in a distant beam dump. A 9.15 mg/cm2 stopper foil of Cu in front of the particle detector let only the forward-scattered light target nuclei reach the detector.
Nonetheless, over time a fair amount of radioactivity accumulates in the target and stopper foil. The stopper foil could be exchanged in principle over the course of an experiment, if necessary several times, depending on the lifetimes of the radioactivities involved. In this experiment the "background" radiation in each Clover detector segment was kept below 1 kHz.
The 132Te beam contains about 10-15% 132Sb (T1/2 = 2.79 min). The Sb decays by β emission into 132Te and populates the 2+1 state of interest. This is a potential problem, since the radioactive decay is much stronger than the Coulomb excitation. But, since the Coulomb-excited projectiles exit from the target and decay in flight, their γ energies are Doppler shifted. The radioactive decay, on the other hand, originates from stopped nuclei and occurs only randomly. As Fig. 2-2 shows, both components are well separated in the gamma spectra and the random-subtraction yields clean coincidence spectra. The removal of the random and stopped components in the peak integration reduces the errors greatly.
The decay-in-flight of the projectiles recoiling into vacuum has a negative side effect. The angular distribution of the decaying &gamma-rays is attenuated due to the hyperfine interaction of the ions recoiling into vacuum. The attenuation leads to a reduced logarithmic slope of up to 30%, compared to that measured for 130Te with a Cu backed target.
Three experiments have been carried out on 132Te at ORNL. The initial run exposed the difficulties of working in a radioactive environment with a beam contaminated by isobars. The target of 10 mg/cm2 Gd proved to be too thick and the accumulated radioactivity near the particle detector and at the exit pipe of the target chamber quickly overwhelmed the γ detectors. A test run with a stable 130Te beam also revealed problems with heat dissipation of the target beam spot. A new, larger chamber was built and a thinner target was prepared. In a second attempt, radioactive 132Te beam was not available, so the new setup was tested with stable 130Te with a beam intensity (3X107 p/s) corresponding to that expected for the radioactive beam to assure thermal stability of the target under beam.
Finally, the radiaoactive beam experiment was carried out, albeit with a 132Te beam weaker than expected. Two different targets were used: 1.3 mg/c2 C on 4.9 mg/cm2 Gd backed by 0.8 mg/cm2 of Cu (added solely to improve thermal conductivity) and 1 mg/cm2 C on 4.4 mg/cm2 Fe. Both targets were cooled to 77 K with liquid nitrogen. The coincidence rate was at best 1 every 18 seconds per Clover detector for a total of about 700 counts per Clover and field direction. The data are still being analyzed and the preliminary result is in agreement with Ref. . An important part of this experiment is the determination of the sign of the g factor, which is as expected positive. The success of this experiment provides a proof of principle and is a guide for future measurements. In a radioactive environment, every isotope carries new challenges and a novel approach may have to be developed for each one.
Figure 2-2: The spectra show from top to bottom a typical γ singles spectrum (random spectrum), a coincidence cut on the prompt particle-γ time and the random-subtracted coincidence spectra for Clover segments at 138° and 58° with respect to the beam. The only peak left is the forward- or backward-Doppler shifted 2+1 -> 0 transition in 132Te. In spite of a small true-to-random ratio, clean coincidence spectra can be obtained.
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