Transmission resonance spectroscopy in the third minimum of232Pa

Abstract

The fission probability of 232 Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the (d, pf ) transfer reaction on a radioactive 231 Pa target. The experiment was performed at the Tandem accelerator of the MaierLeibnitz Laboratory (MLL) at Garching using the 231 Pa(d, pf ) reaction at a bombarding energy of E d =12 MeV and with an energy resolution of ∆E=5.5 keV. Two groups of transmission resonances have been observed at excitation energies of E * =5.7 and 5.9 MeV. The fine structure of the resonance group at E * =5.7 MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape (h 2 /2Θ=2.10±0.15 keV). The fission barrier parameters of 232 Pa have been determined by fitting TALYS 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the J=4 states, the excitation energy of the ground state of the 3 rd minimum has been deduced to be EIII=5.05MeV.PACS numbers: 21.10. Re; 24.30.Gd; 25.85.Ge; 27.90.+b The observation of discrete γ transitions between hyperdeformed (HD) nuclear states represents one of the last frontiers of high-spin physics. Although a large community with 4π γ arrays was searching for HD states in very long experiments, no discrete HD γ transition was found in the mass region of A ≈100-130 [1-5]. On the other hand, the existence of low-spin hyperdeformation in the third minimum of the fission barrier is established both experimentally and theoretically in the actinide region [6,7]. Observing transmission resonances as a function of the excitation energy caused by resonant tunneling through excited states in the third minimum of the potential barrier can specify the excitation energies of the HD states. Moreover, the observed states could be ordered into rotational bands and the moments of inertia of these bands can characterize the underlying nuclear shape, proving that these states have indeed a HD configuration.Regarding hyperdeformation, the double-odd nucleus 232 Pa is of great interest. Even though low-spin hyperdeformation has already proved to be a general feature of uranium [8-10] and thorium isotopes [11], no HD state has been found in protactinium isotopes so far. In particular, the level scheme of the odd-odd 232 Pa is completely unknown in the 1 st minimum of the potential barrier, only the ground-state properties are known at present (I π gs =2 − ) [12]. The fine structure of the fission resonances of 232 Pa has been studied so far only via the (n, f ) reaction [13] with high resolution, but the results showed no convincing evidence on the existence of HD states. A possible reason was the rather limited momentum transfer of the (n, f ) reaction at that low neutron energy (E n ≈100 keV), which did not allow for the population of rotational bands. In contrast, the (d, p) reaction can transfer considerable angular momentum, thus full sequences of rotational states with higher sp...

Fission barriers at the end of the chart of the nuclides

“…For example Blons and collaborators [40] consistently find that the so called "third minima" of some light actinide nuclei are very shallow and no more than 0.5 MeV or so below the surrounding second and third barrier peaks, which are about 6 MeV above the ground-state minimum, in good agreement with results obtained in our current model [41]. In contrast, Csige and collaborators [42][43][44] find third minima for nuclei in this region that are up to 3 MeV below the surrounding peaks. We find it difficult to reconcile with potential-energy calculations the substantial difference in barrier structure they find between 232 92 U 140 [42] and 232 91 Pa 141 [43] in these studies.…”

“…In contrast, Csige and collaborators [42][43][44] find third minima for nuclei in this region that are up to 3 MeV below the surrounding peaks. We find it difficult to reconcile with potential-energy calculations the substantial difference in barrier structure they find between 232 92 U 140 [42] and 232 91 Pa 141 [43] in these studies. A change by just one proton and one neutron cannot, in potential-energy calculations, result in such large differences in barrier structure.…”

Perspectives for photofission studies with highly brilliant, monochromaticγ–ray beams

“…axis ratio 3:1) third potential minimum remained unclear. In a series of measurements, employing light-particle induced direct reactions, in the isotopic chain of even-even uranium nuclei [13][14][15] (and recently in 232 Pa for the first time also in an odd-A isotope [16]), we succeeded to establish the existence of a deep third potential minimum, in some cases comparably deep as the second minimum. Since for hyperdeformed configurations the rather thin outer barrier results in fission lifetimes much shorter than typical lifetimes of collectively excited states in the excita- Energy (MeV) 00 00 00 11 11 11 00 00 00 11 11 11 000 000 000 111 111 111 00 00 00 00 11 11 11 11 00 00 00 00 11 11 11 11 000 000 000 000 111 111 111 111 00 00 00 00 11 11 11 11 00 00 00 00 11 11 11 11 000 000 000 000 111 111 111 111 000 000 000 111 111 111 000 000 000 111 111 111 000 000 000 111 111 111 00 00 00 11 11 11 00 00 00 11 11 11 000 000 000 000 111 111 111 111 00 00 00 00 11 11 11 11 000 000 000 000 111 111 111 111 000 000 000 111 111 111 00 00 00 00 11 11 11 11 00 00 00 11 11 11 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 000 000 000 111 111 111 000 000 000 000 111 111 111 111 00 00 00 11 11 11 00 00 00 11 11 11 00 00 00 00 11 11 11 11 000 000 000 000 111 111 111 111 00 00 00 11 11 11 00000 00000 11111 11111 00000 00000 00000 11111 11111 11111 00000 00000 11111 11111 Fig.…”

Transmission Resonance Spectroscopy of the Doubly Odd $^{238}$Np in ($d$,pf) Reaction

“…Even though low-spin hyperdeformation has already proven to be a general feature of the light even-even actinides [3,4], no such systematics has been established for the odd-odd actinides. However, in a recent experiment [5], we found conclusive evidences on the existence of HD bands in an odd-odd nucleus ( 232 Pa) [5], which suggested to start a systematic investigation of the odd-odd actinides. 238 Np is a very interesting isotope regarding hyperdeformation: it is an isobar of 238 U, where a hyperdeformed third minimum has already been settled [6].…”