3. Recent HRIBF Research - Decay Studies of 76-79Cu and 83,84Ga
(J. A. Winger, Spokesperson)
In recent years there has been increased interest in the study of nuclear structure near the doubly magic nuclide 78Ni. This has come about as new techniques have allowed the production and separation of these nuclides. At the HRIBF, we have developed a technique using re-accelerated isobarically separated beams to obtain pure beams or beams with enhanced purity. In this technique, a beam from the high voltage platform is isobarically purified before being accelerated through the tandem. The energy of the beam is high enough to allow it to pass through an ion chamber before the beam is deposited onto a moving tape collector (MTC). Using the ion chamber to identify the components of the beam, the isobaric separator can be fine tuned to provide optimal beam purity. Decay spectroscopy of the purified beams is performed using a CARDS array fitted with four Clover detectors and two plastic β detectors.
Two modes of operation are available. At pressures near 200 torr, the higher-Z components are stopped in the gas or exit window of the ion chamber thereby enhancing the purity of the lowest-Z component of the beam. In the case of Cu ions, the removal of Zn from the beam in the charge exchange cell allowed for pure beams. The ions are stopped on the MTC at a position just past the exit of the ion chamber and then moved into the CARDS array. This method is effective for studying long-lived (>1s) nuclides or the daughters (i.e., Zn). At lower pressures (about 100 torr), the deposition point is moved back to the center of the CARDS array allowing the study of short-lived nuclides. In our experiment, the ion chamber was used to identify all ions entering the system, allowing continuous knowledge of the beam composition. In the low pressure mode this allowed the tagging of the ions which could be correlated with the decays in a short time window following implantation. With this method, it was possible to observe decays at rates down to 0.1 ion per second. Although it was not attempted in the current experiment, it is possible to run the ion chamber in a passive mode with the voltage turned off in order to purify beams with much higher beam rates.
For 76,77Cu, both techniques were used to determine the β-delayed neutron branching ratios by comparison of the primary γ rays from the Zn daughters of the two branches. Spectra for these two measurements are shown in Fig. 3-1. This measurement was greatly simplified by the lack of Zn isotopes in the beam. For 77Cu a number of γ rays were assigned to the decay and will be used to produce a decay scheme. In 78Cu decay, we observed the 907→891→730 keV decay sequence from the yrast band, but found no evidence for the 145-keV transition out of the 8+ isomer. This suggests a 5- or 6- ground state for 76,77Cu, but more analysis needs to be performed to confirm this result. For 79Cu we were able to observe for the first time the 730-keV transition in 78Zn fed by the β-delayed neutron branch but found no γ rays that could be assigned to the β branch. This implies a very large β-delayed neutron branch for this r-process nuclide.
Figure 3-1: Fitted spectra showing (a) the relative peak areas of the 199-keV peak from 76Zn and the 228-keV peak from 75Zn used to deduce the β-delayed neutron branch for 76Cu, and (b) the 189-keV peak from 77Zn and the 199-keV peak from 76Zn used to deduce the β-delayed neutron branch for 77Cu.
The decays of 83,84Ga are dominated by γ rays fed through the β-delayed neutron branch. The intensity of these γ rays will be used to determine the β-delayed neutron branching ratios, but a more interesting result comes in looking into the states fed by the β decay branch. With 83Ga we were able to observe a γ ray at 248 keV (with an approximate 0.5% branching ratio) which we assign as the transition between the νs1/2 first excited state and the νd5/2 ground state. This assignment is supported by observation of the same γ ray in the decay of 84Ga as shown in Fig. 3-2. In 84Ga decay we also observe a γ ray at 623 keV which we tentatively assign as being the 21+→01+ transition. Both γ rays are at lower energies than were expected from systematics of the N=51 and 52 isotones. In fact, the 623-keV γ ray indicates that E(21+) continues to drop below mid-shell which might indicate a weakening of the 78Ni core for N>50.
Figure 3-2: Comparison of the spectra from 83Ga (black line) and 84Ga (red line) showing a γ ray at 248 keV present in both decays. Notice that nature was unkind in the case of 83Ga decay with the 248-keV line being in a close doublet with a line from 82Ge fed in the β-delayed neutron branch.