Narrow Results

Low-lying excitation properties of exotic nuclei in the vicinity of the canonical magic number 8 have been investigated through the level lifetime measurement on the neutron-rich isotope ¹³B with N = 8 and the missing-mass spectroscopy on the proton unbound nucleus ¹²O with Z = 8. The observed trends of the transition probabilities and level schemes have been discussed in comparison with the advanced shell model calculations, which points to the universality as well as the charge independence of the shell quenching phenomena far from stability.

Lifetime measurement of the first excited 2⁺ state in ¹⁸C was performed using an upgraded recoil shadow method to determine the electric quadrupole transition. The measured mean lifetime is 18.9 ± 0.9 (stat) ± 4.4 (syst) ps, which corresponds to a value of 4.3 ± 0.2 ± 1.0 e²fm⁴, or about 1.5 Weisskopf units. The mean lifetime of the first 2⁺ state in ¹⁶C was remeasured to be about 18 ps, about four times shorter than the value reported previously. By incorporating the γ-ray angular distribution measured in this work into the previous measurement, this discrepancy has been explained. The observed transition strengths in ^16,18C are hindered compared to the empirical transition strengths, indicating that the anomalous hindered E2 strength observed in ¹⁶C persists in ¹⁸C.

We have performed the first direct lifetime measurement of the state in ⁷⁴Zn. The neutron-rich ⁷⁴Zn beam was produced by in-flight fragmentation of ⁷⁶Ge at the Grand Accélérateur National d'Ions Lourds and separated with the LISE spectrometer. The lifetime of the state was measured by the recoil-distance Doppler-shift method with the Cologne plunger device combined with the EXOGAM detectors. The lifetime of the state in ⁷⁴Zn was determined to be 27.0(24) ps, which corresponds to a reduced transition probability .

The lifetime knowledge of a nuclear state provides the essential information on transition probabilities for comparison with theoretical nuclear models. A well-known method for picosecond lifetime measurements of excited nuclear states is Recoil Distance Doppler Shift (RDDS) method. Correspondingly a new plunger based on this technique has been developing in CIAE. We use CAD software and Labview to finish the plunger model design and develop a closed control software. A series of methods such as capacitance method, new skills to flatten the foils etc. are applied in building this plunger. Some tests and the latest results with motion and finite element analysis simulations are discussed in this report.