γ Spectroscopy in the superdeformed minimum of 240Pu
Introduction
Superdeformed (SD) nuclei have been observed in a broad range of nuclei and a wealth of experimental data on the resulting rotational bands has been accumulated in recent years [1]. Based on an idea originally developed by Strutinsky [2] to explain fission isomerism in the actinides [3], the occurrence of nuclear superdeformation can be explained within the liquid drop model by taking into account the deformation-dependent shell effects, which can cause a second minimum (SMin) in the nuclear potential energy surface at a quadrupole deformation corresponding to an axis ratio of approximately 2:1 [4]. For A>230 nuclei the second well is already deep enough at zero rotational frequency to force excited SD low spin states to decay within the SMin to its ground state, which is metastable against the electromagnetic decay into the first minimum as well as against fission. Actinides are thus ideal candidates to investigate basic properties of SD nuclei under conditions avoiding additional effects caused by the rotation.
The delayed decay of the shape isomer provides an unique experimental tag to identify the SD configuration. In fact, the detection of the isomeric decay was crucial for the discovery of superdeformation [3], for the first observation of a SD rotational band [5], for lifetime measurements of these bands [6], for conversion electron spectroscopy in the SMin [7], and for the first detailed observation of the γ back decay of a SD state into the normal deformed (ND) minimum [8].
Experimental information on the excitation energy and the vibrational or quasi-particle nature of excited SD configurations in the actinides is extremely rare. Therefore, an experiment was performed to measure non-rotational γ transitions between states in the SMin of 240Pu.
Section snippets
Experiments and data reduction
Excited states in 240fPu were populated via the 238U(α,2n) reaction at Eα=25 MeV by using a selfsupporting 238U target of 1.7 mg/cm2. The target thickness was chosen to stop the 240Pu reaction products but not the fission fragments. The pulsed α-beam (pulse frequency 13.5 MHz, pulse width <1 ns) was provided by the Tandem accelerator of the Max-Planck-Institut für Kernphysik in Heidelberg.
The fission fragments were detected by eight segmented position sensitive Parallel Plate Avalanche Counters
Results
From the conversion electron measurement of the ground state band (g-band) of 240fPu by Specht et al. [5], who employed the same reaction and beam energy, an estimate for the spin distribution of the states populated in the SMin can be deduced: the distribution reaches up to spins of 8ℏ and its average spin is 4.5ℏ. Since the rotational in-band E2 transitions are converted for spins <10ℏ, the γ lines observed in Fig. 2 above Eγ≈150 keV have to be assigned to the γ decay of intrinsically excited
Discussion and conclusion
In contrast to common expectations [16], that the low lying intrinsic excitation modes in the SMin can be assigned to collective surface vibrations of the β- and γ- or Kπ=0− octupole type, the strongest non-rotational excitations in 240fPu observed in the present experiment are characterized by Kπ=2− and Kπ=1−(2−) (Fig. 4). No evidence was found for the β and γ vibration, and only a weak signal for a Kπ=0− octupole vibration, even though from investigations of transmission resonances in
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