Abstract. The neutron deficient isotopes 180-185Tl which are located at the proton drip line have been identified at IRIS - facility in a spectroscopic study using the spallation reaction induced by high energy protons on Th(U)Cx targets.

The beta-delayed fission fenomenon discovered in a light mass region of neutron deficient isotopes and the identification of 180Tl as a precursor nuclide [1] exite an interest to the study of decay properties of other unknown Tl isotopes.

There are some other reasons for interest to extremely neutron deficient nuclides of Tl :

• i) These isotopes belong to the region of very long alpha-decay chains. It allowes to obtain the mass values for nuclides far off stability. For Tl isotopes the proton binding energies can be obtained using atomic mass data of [2]. One can see that nuclides with mass number A<185 could be proton unstable.
• ii) The shape coexistence in nuclei just below Z=82 shell closure was studied for Hg isotopes. The similar picture will possibly arrise for light thallium isotopes, where isomeric states are well known.

2. Experimental set-up

Experiments have been performed at IRIS facility on line with 1 GeV proton beam of the synchrocyclotron [3]. The ThCx and UCx target material was prepared by special method using the pyrolysis of dyphtalocyanines of metals [4]. The target-ion source unit and properties of the target material have been described in detailes in [5].

The target temperature was kept at the level of 2300-2400 C to release the short lived Tl isotopes with the subsiquent ionization on W-Re surface. A mass separated ion beam was implanted into the moving tape and the radioactive sample moved to solid state detectors to detect alpha-,beta-particles and KX-, gamma-rays.

The analysis of spectra was considerably simplified due to high selectivity of the ionisation. Only Tl isotopes were released from target-ion source unit at the mass numbers A=180-185 because of very high ionization potential for neighbouring elements. These values for Hg and Au are V(Hg)=10.4 eV and V(Au)=9.2 eV, whereas for Tl element only V(Tl)=6.1 eV.

3. Results

185Tl.

Only isomeric state with the half-life T1/2=1.8 s was known for 185Tl isotope from heavy ion experiments [6]. The characteristic radiation K[alpha](Tl) with T1/2=2.5 s was observed in our spectra. In alpha-spectra well known line with the energy E[alpha]=5.97 MeV was idetified as the decay of this isomeric state with half-life T1/2=2.0[+-]0.1 s.

In our spectra we can see also the characteristic K[alpha](Hg) radiation which we attribute to the decay of 185Tl ground state with the half-life T1/2=19.5[+-]0.5 s. This radiation could not arise after the isomeric state deexitation in the daughter isotope 185mHg because of very low transition energy which is equal to 65 keV.

In addition to KX-radiation we observed the well known alpha-line with the energy E[alpha]=5.65 MeV which belongs to the decay of 185Hg ground state. It has the very specific time behaviour which reflects the accumulation of this state after the parent isotope decay. It is possible to extract the values of half-lifes which are responsible for this situation. One of them is T1/2=49[+-]2 s. This value is in a good agreement with the known half-life of 185Hg ground state. Another half-life is T1/2=18[+-]2 s which is close to the value obtained from analisys of KX-rays. It leads to the conclusion that we observed the decay of 185Tl ground state with T1/2=19.5[+-]0.5 s.

183Tl.

The characteristic radiation K[alpha](Hg) with the T1/2=6.9[+-] 0.7 s was observed at the mass number A=183.
Alpha particles with the energy E[alpha]=5.9 MeV of known daughter isotope 183Hg are in our alpha-spectrum without any other lines. This alpha-line has a very specific time behaviour. The analisys shows that it consists of two parts. One belongs to the decay of 183Hg ground state with T1/2=10.7[+-]0.8 s. Another one is the new half-life T1/2=6.9[+-]1.4 s which is responsible for accumulation of the daughter 183Hg isotope noticed by mentioned alpha-line. This new half-life is in good agreement with the value obtained from KX-rays and could be interpreted as the decay of 183Tl ground state.

182Tl.

Two alpha-lines were observed at the mass number A=182. The well known line of 182Hg isotope with E[alpha]=5.88 MeV has an accumulation part in time behaviour. The analisys of new alpha-line with E[alpha]= 6.05 MeV gives the value T1/2=2.8[+-]0.6 s for the half-life. It could be attributed to the decay of the new isotope 182Tl. The high energy part of the beta-spectrum in the range 3-5 MeV gives the half-life value T1/2= 2.0[+-]0.3 s. This part of beta-spectrum belongs to the decay of 182Tl because the beta end point energy for 182Hg is less than 3.5 MeV.

Using obtained experimental data we deduced the limit for the alpha-decay branching ratio br[alpha]=[lamb][alpha]/[lamb] for 182Tl. This value is br[alpha]<0.04. No fission fragmets were observed at mass number A=182. The deduced limit for the beta-delayed fission branching ratio is br(df)<4 10-4.

Our results on the half-life and the alpha branching ratio published in [7] are in good agreement with the data of [8] obtained from the gamma-spectra investigation.

181Tl.

There are two alpha-lines in the spectrum at the mass number A=181. The alpha-transition with the energy E[alpha]=6.00 MeV represents the decay of 181Hg (3.6 s) and has an accumulation part in time behaviour curve. Another alpha-line E[alpha]=6.18 MeV gives the value T1/2=3.4[+-] 0.6 s and can be interpreted as the decay of the new 181Tl isotope. The observed value E[alpa]=6.18 MeV is less than predicted one ( from 6.2 to 7.0 MeV ) by different mass formulae [9]. Probably here we have the transition to the exited state of 177Au nuclide.

180Tl.

This nuclide has been identified with the half-life T1/2=0.92 [+-]0.12 s by authors of [1] as a precursor of delayed-fission fragments. In our experiments no alpha-particles which could be interpreted as the decay of 180Tl were observed at the mass number A=180. The high energy part of beta-spectrum ( 4.5 < E-beta < 6.5 MeV ) at this mass number can be attributed to the decay of 180Tl because the beta end point energy for decay of the daughter isotope 180Hg is less than 4.5 MeV. The half-life value for 180Tl deduced from beta-spectra is equal to 1.9[+-]0.9 s which is close to the result of [1].

3. Conclusion

The data obtained in this work are summarized in Table 1. The comparison of experimental half-lifes with theoretical beta decay half-life values from [10] showes a good agreement for all studied nuclides. It means that the values of alpha decay branching ratios are small. It was indeed in the case of 182Tl with br[alpha]<0.04.

The experimental limit for the delayed fission branching ratio of 182Tl ( < 0.04 % ) is at least two orders of magnitude higher than an estimated value.

The lack of experimental infarmation on alpha-decay schemes of studied isotopes does not allow to deduce the exact mass values of these nuclides. Only estimated values of proton binding energies can be analised. The result is that 180Tl and 181Tl are located beyond the proton drip line. The isomeric states of 183mTl and probably of 185mTl are also proton unbound.

The authors have the pleasure to express the gratitude to Academician Yu. Ts. Oganessian for stimulation of this work.

References:

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