Chemical identification of dubnium as a decay product of element 115 in the rection 48CA+243AM



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CHEMICAL IDENTIFICATION OF DUBNIUM AS A DECAY PRODUCT OF ELEMENT 115 PRODUCED IN THE REACTION 48Ca+ 243Am
S.N. Dmitriev, Yu.Ts. Oganessyan, V.K. Utyonkov, S.V. Shishkin, A.V. Eremin, Yu.V. Lobanov, Yu.S. Tsyganov, V.I. Chepygin,.E. A. Sokol, G.K. Vostokin, N.V. Aksenov, M. Hussonnois,
M.G. Itkis, H.W. Gäggeler1, D. Schumann1, H. Bruchertseifer1, R. Eichler1, D.A. Shaughnessy2, P.A. Wilk2, J.M. Kenneally2, M.A. Stoyer2, and J.F. Wild2
1 Paul Scherrer Institute, Villigen CH-5232, Switzerland

2 Lawrence Livermore National Laboratory, L-236 Livermore, CA 9455, USA




In experiments1-6 performed during the past five years at the Flerov Laboratory of Nuclear Reactions, JINR, 17 isotopes of new chemical elements of the Periodic Table with atomic numbers 112-118 have been synthesized. The complete fusion reactions resulting from accelerated 48Ca ions bombarding targets of 238U, 242,244Pu, 243Am, 245,248Cm, and 249Cf were used for production of the above-mentioned superheavy elements (SHE). The nuclei of new elements appeared to mainly undergo -decays (one or several) until the decay chain ends with a spontaneous fission (SF). The half-lives of the new nuclides range from 0.5 ms to 0.5 min, depending on the proton and neutron number of the synthesized nuclei. These results point to a considerable increase in the nuclear stability of isotopes of SHE with increasing neutron number. Essentially, they provide the first experimental confirmation of the theoretical predictions of the existence of an “Island of Stability” in the domain of hypothetical superheavy elements.

Chemical identification of isotopes in the observed decay chains could give us the identity of the atomic numbers of nuclei in the decay chain and provide independent evidence for the discovery of a new element (elements). Such an experiment was proposed7 and performed8 for element 115.

An isotope of element 115 with mass number 288 was synthesized6 in the reaction 48Ca+243Am288115+3n. It undergoes five sequential -decays (115  113  111   109 107  105 ) ending with the spontaneous fission of 268Db. The total time of the first five -transitions is about 20 seconds. The half-life of the spontaneously fissioning final nucleus 268Db was estimated from the three observed events to be T1/2= hours.

For chemical identification, the element should be separated according to its group properties. For this purpose, we developed a method of sorption extraction for the group 5 elements as anionic fluoride complexes. Bearing in mind that the Z=105 isotope of interest undergoes SF, we paid special attention to separating the group 5 elements from the actinides and, most importantly, from spontaneously fissioning isotopes of californium, 252Cf (T1/2=2.65 y, SF – 3.1%) and 254Cf (T1/2=60.5 d, SF – 99.7%).

In this report the results of the chemical identification of Db as the terminal isotope of the decay of element 115 produced via the 243Am(48Ca, 3n288115 reaction are presented. The experiment was performed on the U-400 cyclotron of FLNR, JINR. The 243Am target was bombarded with a beam dose of 3.41018 48Ca projectiles at energy of 247 MeV in the center of the target. The reaction products were collected in the surface of a copper catcher block, which was removed with a lathe and then dissolved in concentrated HNO3. The group 5 elements were separated by sorption onto Dowex 50x8 cation-exchange resin with subsequent desorption using 1 M HF, which forms anionic fluoride complexes of group 5 elements. The eluant was evaporated onto a 0.4 m thick polyethylene foils that were placed between a pair of semiconductor detectors surrounded by 3Не neutron counters for measurement of -particles, fission fragments and neutrons. Over the course of the experiment, we observed 15 spontaneous fission events (Table 1) with T1/2=32 h which we



Figure 1. Scheme of the experiment.
attribute to 268Db. The production cross section for the 243Am + 48Ca reaction was 4.2 pb. These results agree with the original element 115 synthesis experiment where 268Db was first observed as the terminal isotope following the five consecutive -decays from the 288115 parent nucleus in the Dubna gas-filled separator (Table 2).

The data from the present experiment give independent evidence for the synthesis of element 115 as well as element 113 via the 243Am+48Ca reaction.


Table 1. Conditions and results of the experiment


No


Time of irradi-ation

/hours


48Ca- ion beam dose


Energies of fission fragments E1/E2 (MeV)

Neutron multiplicity registered in every event of spontaneous fission

Time of regis-tration

/hours


1

20

2,51017

120/126

2

20

2

22

3,71017

 /86

1

74

3

22

3,41017

131/124

116/122


1

2


15

72


4

22

2,91017

104/120

97/125


100/128

1

1

1



22

29

51



5

38

6,71017

117/118

108/107


110/104

-- /76


2

3

0



2

6

9

15



68

6

23

3,91017

120/114

2

39

7

22

3,61017







8

45

7,41017

119/110

118/105


65/58

2

2

3



5

93

174




Table 2. Results of physical and chemical experiments





Physical experiment [6]

Chemical

experiment



Separation method

Kinematic separator

Radiochemical separation

Separation efficiency

35%

80%

Detection method

Decay chains of nuclei with Z=115

SF of nuclei with Z=105

Energy of the 48 Ca-ion beam at the center of target layer

246 MeV

247 MeV

Total 48Ca beam dose

4.5·1018

3.4·1018

Thickness of the 243Am target

0.36 mg/cm2

1.2 mg/cm2

Number of detected spontaneous fission events

3

15

Formation cross section for the nuclei with Z= 115

2.7 pb

4.2 pb

Half-life

16hours

32hours

Total kinetic energy of spontaneous fission fragments

~ 225 MeV

~ 235 MeV

Average neutron multiplicity per fission act

--

4.2

Identification method of SF–decaying nuclei in the48Ca+243Am

Method of excitation functions (Z=115)

Isolation of Group V elements
(Z=105)


References

1. Yu.Ts. Oganessian et al., Phys. Rev. Lett., 1999, 83, 3154.

2.  Yu.Ts. Oganessian et al., Phys. Rev., 1999, C 62, 041604(R).

3.  Yu.Ts. Oganessian et al., Phys. Rev., 2001, C 63, 011301(R).

4.  Yu.Ts. Oganessian et al., Communication of the Joint Institute for Nuclear Research, Dubna, 2002, D7-2002-287.

5.  Yu.Ts. Oganessian et al., Phys. Rev., 2004, C 69, 054607.

6.  Yu.Ts. Oganessian et al., Phys. Rev., 2004, C 69, 021601(R).

7.  S.N. Dmitriev et al., Extended Abstracts of the Second International Conference on The Chemistry and Physics of the Transactinide Elements (TAN 03), Napa, California, USA, 2003, 204.



8.  S.N. Dmitriev et al., Report on International Symposium on Exotic Nuclei, Peterhof, Russia, July 5-12, 2004, JINR E12-2004-157.


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