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

Abstract

Low-energy β-delayed fission of 194,196 At and 200,202 Fr was studied in detail at the mass separator ISOLDE at CERN. The fission-fragment mass distributions of daughter nuclei 194,196 Po and 202 Rn indicate a triple-humped structure, marking the transition between asymmetric fission of 178,180 Hg and symmetric fission in the light Ra-Rn nuclei. Comparison with the macroscopic-microscopic finite-range liquid-drop model and the self-consistent approach employing the Gogny D1S energy density functional yields discrepancies. This demonstrates once more the need of dynamical fission calculations, as for both models the potential-energy surfaces lack pronounced structures, in contrast to the actinide region.Nuclear fission, the division of a heavy atomic nucleus into predominantly two parts, continues to provide new and unexpected features in spite of a long history of intensive theoretical and experimental studies [1][2][3][4][5][6][7]. The fission process is not only important for several applications, such as energy production and radiopharmacology, but also has a direct impact on the understanding of the fission recycling process in r -process nucleosynthesis [8,9]. Therefore, a description of the fission process with reliable predictive power is needed, in particular for low-energy fission where the fission-fragment (FF) mass distributions are strongly * lars.ghys@fys.kuleuven.be sensitive to microscopic effects [4]. Mass distributions (MDs) are usually predominantly symmetric or asymmetric with the yields exhibiting a single peak or two distinct peaks, respectively. However, in several cases a mixture of two modes was observed [5]. Experimental observables characterizing various fission modes are the width of the MD peak(s), the position of these peaks in asymmetric mass division and total kinetic energy (TKE) of the FFs. The dominance of asymmetric fission in most of the actinide region beyond A = 226 up to about 256 Fm was attributed to strong microscopic effects of the heavier FF, near the doubly-magic 132 Sn [4,10,11]. However, nuclei such as 258 Fm and 259,260 Md exhibit complex MDs, each with a narrow and a broad symmetric component with a higher and lower TKE, respectively. 2This phenomenon is called bimodal fission [12][13][14][15]. Competition between symmetric and asymmetric fission, corresponding to respectively lower and higher TKE and resulting in a triple-humped MD has been reported around 226 Th [16][17][18]. These observations strongly support the hypothesis that nuclei may fission through several independent fission modes corresponding to different pre-scission shapes and fission paths in a multidimensional potential-energy landscape, referred to in literature as multimodal or multichannel fission [4,5,11,[16][17][18][19]. In the pre-actinide region, predominantly symmetric FF mass distributions were measured. 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,...

Calculated fission-fragment yield systematics in the region74≤Z≤94and90≤N≤150

“…[10] with respect to 70 charge yields measured at GSI [12], and in Ref. [13] with respect to new and older data in the neutron-deficient Pb region.…”

Alpha Decay and Beta-Delayed Fission: Tools for Nuclear Physics Studies

“…The use of two silicon detectors in a compact geometry allowed both singles α/fission decays and double-fold fission-fragment coincidences to be efficiently measured. The same method was later used for βDF studies of 178 Tl [120], 194,196 At [131,116] and of 200,202 Fr [131]. A detailed discussion of the results will be given in Section 3.5.3.…”

“…The recent novel experiments at the ISOLDE mass separator at CERN (Geneva) [48] used the same method to identify βDF of 178,180 Tl [129,120,130], 194,196 At [119] and 200,202 Fr [131], they will be discussed in details below.…”

“…204,208 Rn, 210 Ra (which are known to fission symmetrically), the βDF experiments at ISOLDE aimed at studies of transitional βDF isotopes of 194,196 At and 202 Fr [131,116]. The low-energy fission data for daughter (after β decay) isotopes and 202 Rn, respectively, were collected.…”

In-Source Laser Spectroscopy with the Laser Ion Source and Trap: First Direct Study of the Ground-State Properties ofPo217,219

“…The introduction of the resonance laser-ionization technique has increased the element selectivity of the ionization process [8], which has enabled a large research program to be undertaken, among others, in the lead region; see, e.g., Refs. [9][10][11][12][13][14][15][16][17]. Alternative production routes are found in the combination of in-flight fusion-evaporation or fragmentation facilities with a gas catcher [18][19][20].…”