New Type of Asymmetric Fission in Proton-Rich Nuclei

Abstract

A very exotic process of -delayed fission of 180 Tl is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the fissionfragment mass distribution of the post--decay daughter nucleus 180 Hg (N=Z ¼ 1:25) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two 90 Zr fragments, with magic N ¼ 50 and semimagic Z ¼ 40. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for -delayed fission of 180 Tl is 3:6ð7Þ Â 10 À3 %, approximately 2 orders of magnitude larger than in an earlier study. DOI: 10.1103/PhysRevLett.105.252502 PACS numbers: 24.75.+i, 23.40.Às, 27.70.+q Nuclear fission, discovered more than 70 years ago [1], represents one of the most dramatic examples of a nuclear metamorphosis, whereby the nucleus splits into two fragments releasing a large amount of energy. Initially, the fission process was described within the liquid-drop model [2,3], in which shape-dependent surface and Coulomb energy terms define the potential-energy landscape through which fission occurs. However, this macroscopic approach naturally leads to symmetric fragments and cannot explain observed asymmetric mass splits of actinides. Only by including a microscopic treatment based on shell effects can asymmetric fission be described [4]. Importantly, only in fission below or slightly above the barrier, so-called low-energy fission, can the interplay between the macroscopic liquid-drop contribution and the microscopic single-particle shell corrections be most fully explored.Until recently, such low-energy fission studies were limited to nuclei from around thorium (Th) to fermium (Fm) using spontaneous fission, fission induced by thermal neutrons or -delayed fission. These studies showed the dominance of asymmetric fission over symmetric fission for most isotopes of these elements [5][6][7] and suggested that structure effects due to, specifically, the spherical shell structure of doubly magic 132 Sn dominate the mass split. A decade ago, a new technique, developed at GSI [8]-Coulomb-excited fission of radioactive beamsallowed for a more extensive experimental survey of lowenergy fission in other regions of the nuclidic chart. These studies demonstrated the transition from mostly asymmetric fission in the actinides towards symmetric fission as the dominant mode in the light thorium to astatine region. This is also consistent with earlier studies by Itkis et al. [9], in which fission of stable targets in the mass 185-210 region was induced by bombardment with protons and 3;4 He beams. Itkis et al. found mostly symmetric mass distributions in the region around 208 Pb, with about four systems in the mass A $ 200 region having a slight reduction of PRL 105, 252502 (2010)

Evolution of fission-fragment mass distributions in the neutron-deficient lead region

“…A few relevant cases for the present discussion (see also Fig.1) are 195 Au, 198 Hg and 208,210 Po, studied by means of charged-particle induced reactions [20][21][22] and 204,206,208 Rn studied via electromagnetically (EM)-induced fission [23,24]. In contrast to this, recent β-delayed fission experiments have established a new region of asymmetry around nuclei 178,180 Hg [25][26][27], which in fission divide into neutron-deficient fragments with most probable mass numbers around A L ∼ 80 and A H ∼ 100. The mechanism behind the asymmetric MD is different from that in the uranium region, since strong shell effects in the respective FFs are absent in the neutron-deficient lead region.…”

“…These intriguing cases will be further studied at the RILIS [45] or CRIS [46,47] , with fission-fragment masses on the horizontal and their relative yields on the vertical axis, for even-N neutron-deficient isotopes between gold and radium at excitation energies slightly above the theoretical fission-barrier heights B f,th [33]. The calculated yields are compared with selected experimental MDs (red) from particle-induced [20,21], β-delayed ( [25,26], this work) and EM-induced fission [23,24]. The border of the lightest known isotopes is shown by the thick solid line, β-stable nuclei are shown on a gray background.…”

“…Surface ionization of francium or laser-ionization of astatine [38] in the ion source of ISOLDE are employed for the respective element selection. After extraction, acceleration to 30 keV and mass separation the isotopically-purified beam is transported to the 'Windmill' detection setup, described in detail in [25,27,39]. There, the ion beam is implanted into one of ten 20 µg/cm 2 thick carbon foils, which are mounted on a rotatable wheel.…”

“…A high-purity germanium detector was installed in close vicinity to the implantation point for γ detection (see Fig. 1 from [25]). The experimental campaign consisted of two parts, a summary of acquired statistics is given in Table I 194,196 At and 200 Fr were mainly acquired at the General Purpose Separator (GPS) in 2012, although a limited number of βDF events for these nuclei was observed at the HRS, see Table I.…”

“…A high-purity germanium detector was installed in close vicinity to the implantation point for γ detection (see Fig. 1 from [25]). The experimental campaign consisted of two parts, a summary of acquired statistics is given in Table I.…”