Ackermann, D.

Antalic, S.

Block, M.

Heinz, S.

Hessberger, Fritz-Peter

Hofmann, S.

Kalaninova, Z.

Khuyagbaatar, J.

Kindler, B.

Kojouharov, I.

Lommel, B.

Mann, R.

GSI Repository

GSITemplate2007 10 100 1000 10000 1000000,00,10,20,30,4T1/2 = 1.54 ± 0.08 s255Rf- 41T1/2 = 235 +62 ms259Sg  intensity (arb. units)time / ms105 120 135 150 165 180 195 210 225 240024681012141618          sf(259Sg)          (bsf = 6%)E = 180 MeV (uncorr.)ΔE = 11 MeV (FWHM)          sf (255Rf)          (bsf=50%)E = 165 MeV (uncorrected)ΔE = 31 MeV (FWHM)  Events / 5 MeVE / MeV (Stop detector) Spontaneous Fission Properties of  259Sg and 255Rf * F.P. Heßberger1,2, S. Antalic3, D. Ackermann1, M. Block1,2,  S. Heinz1, S. Hofmann1,4,            Z. Kalaninová3, I. Kojouharov1, J. Khuyagbaatar2,1, B. Kindler1, B. Lommel1, R. Mann1 1GSI, Darmstadt, Germany; 2HIM, Mainz, Germany, 3Comenius University, Bratislava, Slovakia,  4Goethe Universität, Frankfurt a. Main, GermanyIn a recent study decay properties of 259Sg, produced in the reaction 206Pb(54Cr,n)259Sg, were investigated at SHIP [1]. Alpha decay from two nuclear states in this isotope was observed: a) the ground state having a half-life of 411 ms and attributed to the Nilsson level 11/2-[725]; b) an isomeric state located at E* ≈ 92 keV with a half-life of 254 ms, assigned as 1/2+[620]. Search for spontaneous fission branches was rendered more difficult by the fact that the α-decay daughter 255Rf has a fission branch of 52% and a half-life of 1.64 s [2]. Thus we searched for spontaneous fission events following the implantation of an evaporation residue (ER) within 10 s and not being preceded by an α-decay neither with full energy release nor with partial energy release in the focal plane detector. A probability pw < 0.05 that a preceding α decay could have been missed and thus an sf event of 255Rf wrongly assigned as an sf event of 259Sg was estimated from the number of correlations α(259Sg) – α(255Rf) – α(251No),  α(259Sg) – α(251No) (without observation of α’s of 255Rf ), and an ≈20% probability to observe only the escaping α-particle as obtained from α – γ - measurements. Altogether 24 sf events not preceded by an α-decay were observed. Their time distribution (Δt(ER-sf)) is shown in fig. 1. A half-life T1/2 = 235 +62/-41 ms was obtained. As the half-life is in-line with that of the α activity attributed to the decay of the 1/2+[620] level, the sf activity is also assigned to it. From the number of α decays and sponta-neous fission events a branching ratio bsf = 0.06±0.015 was obtained, resulting in a partial half-life of Tsf ≈ 3.9 s. Our branching ratio is in-line with bsf≈0.11, obtained from each one α-sf – correlation in decay studies of 263Hs [3,4]. A hindrance of fission in nuclei having odd proton and/ or neutron numbers compared to neigbouring even-even nuclei is well known. The degree of hindrance can be expressed by a hindrance factor HF defined as HF = Tsf/Tsf(unh), with the partial fission half-life Tsf and the hypothetical ‘unhindered’ fission half-life Tsf(unh), usu-ally obtained as the geometric mean of the fission half-lives of the neighbouring even-even nuclei [5]. Using the known values for the neighbours 258Sg (2.6 ms) [6] and 260Sg (7.0 ms) [7] we obtain Tsf(unh)(259Sg)  = 4.3 ms, and thus HF ≈ 907, which is about an order of magnitude lower than the value for the daughter 255Rf. Striking differences have also been observed for the measured ‘fission energies’, which are displayed in fig. 2. It should be noted, that the probability to stop both fission fragments in the focal plane detector is only 30%. So in most of the cases the observed ‘fission energy’ represents the sum of the kinetic energy of one fragment and the energy loss of the other fragment. We observed for 259Sg an about 10% higher mean energy value and a narrower distribution of the fission energies than for 255Rf. This is presently regarded as sign of two components in the mass distribution of the fission fragments of 255Rf, but only one component in that of  259Sg, i.e. a transition from asym-metric fission of  255Rf  to symmetric fission of 259Sg.             Figure 1: Time distributions Δt(ER-sf) of fission events not preceded by an α decay (259Sg) and Δt(α-sf) of fission events preceded by an α decay (255Rf)..         Figure 2: Energy distributions of fission events assigned to 255Rf and 259Sg. Given energy values are not corrected for pulse height defect.   References  [1] F.P. Heßberger et al. GSI Scientific Report 2011 [2] F.P. Heßberger et al. EPJ A 12, 57 (2001) [3] I.Dragojevic et al. Phys.Rev.C79, 011602(R) (2008) [4] D. Kaji et al. J. Phys. Soc. Japan 78, 035003 (2009) [5] D.C. Hoffman Nucl. Phys. A 502, 21c (1989) [6] F.P. Heßberger et al. Z. Phys. A 359, 415 (1997) [7] F.P. Heßberger et al. EPJ A 41, 145 (2009)  * supported by HI Mainz PHN-ENNA-EXP-06 GSI SCIENTIFIC REPORT 2012136

Spontaneous Fission Properties of $^{259}$Sg and $^{255}$Rf

http://repository.gsi.de/record/52039/files/PHN-ENNA-EXP-06.pdf

Spontaneous Fission Properties of $^{259}$ Sg and $^{255}$ Rf

Abstract

Similar works

Full text

Available Versions

GSI Repository

Spontaneous Fission Properties of 259^{259}259Sg and 255^{255}255Rf

Abstract

Similar works

Full text

Available Versions

GSI Repository

Spontaneous Fission Properties of $^{259}$ Sg and $^{255}$ Rf