Intermediate coulomb excitation of N=Z nucleus at GANIL
Intermediate coulomb excitation of N=Z nucleus at GANIL

Intermediate energy coulomb excitation setup at LISE

The aim of this experiment was to extend our study of shape coexistence in light krypton isotopes to the N=Z nucleus 72Kr. Coulomb excitation of 74Kr and 76Kr at energies below the barrier had been performed earlier using radioactive beams from SPIRAL and the EXOGAM spectrometer. From these experiments both transitional and diagonal quadrupole moments were extracted for yrast and non-yrast states using the code GOSIA. The results confirm the shape coexistence scenario in the light krypton isotopes near the N=Z line with prolate ground-state bands and oblate excited rotational bands. In the N=Z nucleus 72Kr the situation is expected to be inverse: an oblate deformation in the ground state and a prolate excited band. Unfortunately, the intensity of a 72Kr beam from SPIRAL is presently not sufficient for coulomb excitation at low energy. At intermediate energy (~40 MeV/A) a thicker target can be used to increase the excitation probability. Such experiments can be performed at GANIL using a fast fragmentation beam from SISSI and the LISE spectrometer. At such high energies nuclear reactions have to be minimized by selecting small scattering angles (between 1.5 and 5 degrees in this case). The interaction time is too short to populate higher-lying states in a multi-step process. Consequently, only the B(E2) value of the first 2+ state can be extracted, whereas low-energy coulomb excitation allows to measure also static quadrupole moments.


 

Intermediate coulomb excitation of N=Z nucleus at GANIL

Intermediate energy coulomb excitation setup at LISE

Intermediate coulomb excitation of N=Z nucleus at GANIL

Prompt doppler-corrected gamma-ray spectra in coincidence with 72Ge (top) and 78Kr (bottom) fragments.

The experiment was performed in July 2004 at the LISE spectrometer at GANIL. An incident 78Kr beam of 73 Mev/A with high intensity (I= 4.5 1011pps)was fragmented on a primary nickel target in the SISSI device. The fragment were selected in the LISE spectrometer and the nuclei were identified event by event by their energy loss and time of flight. The incident beam was tracked by two position-sensitive micro-channel plate detectors. The nuclei were coulomb excited in a thick secondary lead target (220 mg/cm2) which was surrounded by four EXOGAM clover detectors in a close geometry (see pictures). The scattered particles were detected in a telescope of two annular silicon detectors (147 mu and 300 mu thickness) which were segmented into 16 rings and 16 sectors. The unscattered particles were counted and identified in two plastic scintillators (300 mu thickness). The second scintillator, in which the ions were implanted, was tilted 45 degrees with respect to the beam axis. Two alcohol-cooled silicon detectors (50*50 mm2) were used to detect delayed conversion electrons from a low-lying excited 0+ state. Such a state is known to be the first excited state in 72Kr and similar states are expected in neighboring nuclei. The segmentation of the EXOGAM clover and the annular silicon detectors were necessary to correct the doppler effect.


The main goal of the experiment was to identify the unknown 2+2 state in 72Kr and to measure the B(E2) value of the first excited 2+ state. Unfortunately, the yield of 20 pps for this nucleus was not sufficient to achieve these goals. However, new results in several neighboring nuclei such as 66Ge, 64Ga, 62Zn and 68Se can expected to be found. The nuclei 78Kr, 72Ge and 64Zn were produced in seperate runs to test and calibrate the setup; their B(E2) values are well known and can be used as a reference. Preliminary data for 78Kr and 72Ge are shown below. 

The above spectra show the 2+ to 0+ transitions in 72Ge and 78Kr following coulomb excitation. The first excited state in 72Ge is known to be a 0+ state as is the case in 72Kr. Since the ions are fully stripped of their atomic electrons while passing through the spectrometer, nuclei produced in the excited 0+2 can only decay after implantation by regaining electrons in the plastic scintillator. The E0 decay between the two 0+ states is observed in the silicon detectors facing the inclined second scintillator. A spectrum of the E0 decay in 72Ge is shown below.  

Intermediate coulomb excitation of N=Z nucleus at GANIL

Conversion-electron spectrum showing the E0 transition between the two 0+ states in 72Ge

#581 - Màj : 14/02/2005

 

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