Study of 240Cf and new isotope 236Cm

Decay properties of neutron-deficient isotopes of actinide elements provide valuable information on the nuclear mass surface close to the proton drip line, on nuclear structure of deformed heavy nuclei and on fission properties far from stability. However, detailed experimental data on decay properties of neutron-deficient nuclei in the region of Z>96 are still scarce due to low production rates. Recently, attempts were carried out to investigate the decay properties of neutron-deficient isotopes of fermium [1]. Due to this renewed interest on decay properties of neutron-deficient isotopes we aimed to study the decay of the isotope 240 Cf at SHIP by measuring simultaneously α decay and spontaneous fission. The isotope 240 Cf was first identified in [2] by α decay. Spontaneous fission of this isotope was measured in [3] and a fission branching of ≈0.02 was deduced. However, in [3] only fission events were measured. Therefore, the estimated fission branching and deduced partial spontaneous fission half-life was based on a calculated total production cross-section in the fusion-evaporation reaction 208 Pb( 34 S,2n) 240 Cf [3]. Consequently, this important decay property was relatively uncertain. In addition, an α decay branching of 240 Cf being close to one would open a possibility to search for the so far unknown isotope 236 Cm using the α-α and α-SF correlations. The reactions 36 S( 207 Pb,3n) and 36 S( 206 Pb,2n) were used for production of 240 Cf. Beam energies were at Elab=170.3 and 163.3 MeV, respectively. Corresponding α decay spectra measured in the energy region (6500–7700) keV are shown in figs. 1a and b. The strongest line, observed at an energy of (7585±20) keV was attributed to 240 Cf according to the literature data [2]. In the reactions 36 S( 206 Pb,2n) 240 Cf and 36 S( 207 Pb,3n) 240 Cf, a total number of ≈70 and ≈770 α decay events were observed, respectively, having full energy release in the focal plane detector. A so far unknown α line at E=(6954±20) keV was observed in both irradiations during the beam-off periods. To assign this line we performed a position and time correlation analysis (see fig. 1c). The scatter plot marks events which are in delayed coincidence within a position window of ±0.3 mm and within a time window of 25 min. The scatter plot clearly reveals a cluster of events at parent-daughter energies of 7585 and 6954 keV, respectively. In addition to these events we observe correlations to known α decays of 232 Pu, 228 U, 224 Th and 220 Ra. Therefore, we unambiguously assign the group of events at (7585±20) and (6954±20) keV to 240 Cf and to the new isotope 236 Cm, respectively. Fig 1: Alpha spectra taken during the beam-off periods in irradiations of 206 Pb with 36 S at Elab=163.6 MeV (a) and of 207 Pb with 36 S at Elab=170.3 MeV (b). Two dimensional α-α correlation plot (c). Dashed lines mark the energies of α decays of 240 Cf and its decay products. See text for details.