Detailed study of Rf and No isotopes radioactive decay properties

The experiments of detailed study of No and Rf isotopes radioactive decay properties in complete fusion reactions 50 Ti + 208 Pb and 48 Ca + 208,206,204 Pb with subsequent neutron evaporation from the excited compound nucleus at the kinematic separator SHELS were performed in FLNR JINR. The data of the 256 Rf decay properties and preliminary data of 250 No decay properties are presented.


Introduction
Improvement in the experimental methods and carrying out experimental investigation with the latest developments in the detection system area are one of the main objectives in modern nuclear physics of superheavy and heavy elements synthesis. The probability of formation is much higher for heavy isotopes than for superheavy (SHE, Z>106) that is why this region of transfermium elements (100 ≤ Z ≤ 106) is more available for studying with the recent investigation methods. Moreover, the transfermium region (neutron-rich isotopes from No to Sg) is the most interesting for spectroscopic researching on its own due to the presents transition from neutron subshell N =152 to N =162, and the cross sections of these isotopes are sufficiently high. We obtain preliminary information about what we should expect in the SHE region by means of studying heavy isotopes region.
The experiments of detailed study of isotopes radioactive decay properties of transfermium elements ( α , β , γ -spectroscopy) are performed in the Laboratory of Nuclear Reactions, JINR at the recoil kinematic separator SHELS (modernized VASSILISSA) [1,2].

Description of the experiments
In 2018-2019 years at U-400 cyclotron at FLNR, JINR experiments on detailed study radioactive decay properties of Rf and No isotopes were performed. These isotopes were produced in the complete fusion reactions of accelerated heavy ion beam of 50 Ti or 48  The targets in the form of segments are mounted on a rotating disk in the target block of the SHELS kinematic separator in order to reduce heat load [1,2]. The transmission eefficiency of recoils, produced in the complete fusion reactions with ions of 48 Ca and 50 Ti from the target to the focal plane of the separator, is equal to 30 -40% depending on the settings of the separator ion-optical system.
Sophisticated detection system GABRIELA is located in the separator focal plane, allows to detect α -particles, γ -quants, β -particles and spontaneous fission fragments (SF), emitted from nuclei under the question [3,4]. After separation from the background products our recoils, flying through the time of flight system, which consists of 2 (start and stop) detectors,are implanted into the focal double side silicon strip detector DSSD (128 × 128 strips, size 100 × 100 mm 2 , thickness 0.5 mm). Additional 8 strip detectors (16 × 16 strips, size 50 × 60 mm 2 , thickness 0.7 mm) are mounted on the side of the focal detector, forming «well» with the depth 6 cm, are used for increasing of the detection efficiency of αβ -particles, and SF fragments, emitted from focal DSSD-detector. The energy resolution for α -particles in the range 6 -10 MeV, which was measured in the previous experiments on the modernized separator [3], is about 15 -20 keV. Four single crystals of germanium detectors are arranged around the «well». Clover four-crystal germanium detector is located maximum close to the focal DSSD detector [4]. The germanium detectors surrounded by BGO protection in order to reduce background influence from γ -quants.

Results
In the experiments of 256 Rf the SF properties studying at the SHELS separator with using neutron detector consists of 543 He-counters, could not be observe a single event of α -decay, corresponding to 256 Rf. The number of SF events related to this isotope equals to 1500 [5,6]. In earlier experiments [7,8], which were performed at GSI, Darmstadt, α -decay events assigned to decay of 256 Rf were observed. The α -decay probabilities were b α = 0.022 +0.073 −0.018 [7] and b α = 0.0032 [8]. In this experiment, decay properties of 256 Rf were refined in investigating the complete fusion reaction 50 Ti+ 208 Pb → 256 Rf * . During the experiment, in the separator focal plate it was detected about 6270 SF and nine α -decay events, which can be assigned to decay of 256 Rf. α -decay events were obtained as a result of recoilαα correlation analysis (Table 2). Table 2. Recoilαα correlation analysis for isotope decay of 256 Rf, E R -recoil energy, ∆ T(Rα 1 ) -time difference between detected mother nucleus and recoil, E α 1energy of mother nucleus, ∆ T( α 1α 2 ) -time difference between mother and daughter nuclei, E α 2 -energy of daughter nucleus. The measured half-lifetimes were equal to (6.9 ± 0.23) m sec for SF and (5.7 ± 1.2) m sec for α -decay, decay probabilities were b SF =99.71% and b α =0.29% respectively, which are in a good agreement with the published data [6][7][8] . The table 3 shows the radioactive decay properties of 256 Rf according to obtained and existing data. Table 3. The existing published data according to isotope decay of 256 Rf and the results of current experiment. N α / N SF -the numbers of detected α -particles/SF fragments, E α -energy of α -particles, b α / b SFα -decay/SF probabilities.

No isotopes. Preliminary results
After two neutrons evaporation from compound nucleus the complete fusion reactions 48 Ca+ 208,206,204 Pb → 256,254,252 No * produces the isotopes 254,252,250 No. These isotopes mainly experience α -decay and SF with half-life times from few microseconds up to few seconds (Table 4). During irradiation of 208 PbS target with accelerated ions beam of 48 Ca we have detected about 600 recoil -SF fragment correlation events in the focal plane. We observed two activities, which could be assigned to SF of 254 No and 252 No. It connects with a high fission probability of 252 No isotope, which is produced on an impurity of 206 Pb in the main target. More over, the SF probability of 254 No is 0.17%, while for 252 No this value is 29.3%. These two isotopes well separate via half-life times due to the large difference between their life times. As a result, about 310 SF events were assigned to decay of 254 No.
At the experiment time, the gained statistic was comparable with previous experiment at GSI Darmstadt [10]. Two SF events with short life times, which could be preliminary assigned to decay of 254 No isomeric state, were observed. There were γ -quants ( E γ =159 keV), are emitted by 254 No nucleus via the transition from 6+ to 4+ level.
During 206 PbS target irradiation with accelerated heavy ion beam of 48 Ca in the focal plane about 2200 SF events were detected, assigned to decay of 252 No.
The obtained statistic was enough for detectors calibration via total kinetic energy (TKE). We have also observed γ -quants ( E γ =167 keV), which are emitted from 252 No nucleus via the transition from 6+ to 4+ level. For these No isotopes the transition from 6+ level to 4+ occurs from 159 keV in 254 No to 167 keV in 252 No, the transition from 4+ to 2+ occurs with photons emission with energy 101 and 107 keV and from 2+ to 0+ with 44 and 46 keV respectively [9]. Based on these data we can imagine decay scheme of 250 No, is shown at Figure 2c. Decay from 6+ isomeric state is accompanied by transitions 914 keV with multipolarity M1 in 6+ and 1090 keV with multipolarity with E2 in 4+ of ground state. The transition from 6+ to 4+ is 176 keV, from 4+ to 2+ is 115 keV and from 2+ to 0+ ≈ 49 keV.

Conclusion
The preliminary results of 250 No isomer state studying were presented and some decay properties of 256 Rf were refined in this paper with α , β , γspectroscopy technique successfully implemented at the kinematic separator SHELS. In spring 2019, the first launch of SHE factory was held at JINR. The data analysis presented in this work based on the used investigation methods, will allow us to study in detail the structure of transfermium elements. We expect be amintensity on the new cyclotron DC-280 approximately 10 times more than we have now at the working U-400 cyclotron. It is planned top roduce heavy ion beams with intensity up to 10 p µA at the SHE factory, FLNR, JINR [11]. The using of such high-intensity beams in combination with effective methods and experimental facilities should open up access to the study of nuclei closer to the center of the "Island of stability".