Search of the Decay of Heaviest Isotopes of the Element 112

 

¹ Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia

² Gesellshaft fur Schwerionenforschung, D-64220 Darmstadt, Germany

³ Institute of Physics, SK-84228 Dubravska 9, Bratislava, Slovakia

⁴ Department of Physics, Comenius University, SK-84215, Bratislava, Slovakia

⁵ Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-01, Japan

 

Abstract

 

To produce heavy isotopes of element 112 in the reaction ⁴⁸Ca+²³⁸U two experiments at different beam energies were performed.

In the first experiment the beam energy was 231 MeV which lead to the production of the compound nucleus ²⁸⁶112 with an excitation energy of E_x = 31 MeV.

In a 20-day irradiation of the ²³⁸U target with beam dose of 3.5*10¹⁸ on the focal plane of the recoil separator VASSILISSA two spontaneous fission events were detected. No - particle emission preceding spontaneous fission as well as any - correlation in the energy range from 8 to 13 MeV in the time interval up to 10000 s. have been observed.

The half life of the new spontaneous fissioning nuclide is about 100s. The most probable explanation of the data obtained in this experiment is that the observed spontaneous fission corresponds to the decay of the even-odd isotope (N=171) of element 112 produced in the reaction ²³⁸U(⁴⁸Ca, 3n)²⁸³112 with a cross section ~ 5pb.

In the second experiment the beam energy was 238 MeV which increase the excitation energy of the compound nuclei up to E_x= 39 MeV. The total beam dose in this case was 2.2*10¹⁸. No events due either to spontaneous fission or sequential - decay in the energy range from 8 to 12 MeV and time interval of 1000s. were detected. This data gives an upper limit of 3 pb for the production cross section of the even-even isotope ²⁸²112 in the reaction ²³⁸U(⁴⁸Ca,4n)²⁸²112.

The work was performed in the Flerov Laboratory of Nuclear Reactions JINR.

 

PACS: 21.10 Dr; 23.60.+e; 25.70.-z; 27.90.+b

 

 

*-Corresponding authors; Tel: ++7-09621-62261, Fax: ++7-09621-65083,

E-mail: spm111@yandex.ru and spm111@yandex.ru




Introduction

 

            According to the macro-microscopic theory predictions, the stability of the super heavy nuclei has to increase sharply, while approaching the spherical neutron shell N=184. The synthesis of the super heavy spherical nuclides, even if they are distant from the shell N=184, will take place only with a significant neutron excess in the colliding nuclei. Thus, the isotopes of element 112, which have a spherical shape and which are therefore relatively stable, could be produced in the reaction ⁴⁸Ca+²³⁸U^[1].

 

The compound nucleus ²⁸⁶112 formed in this reaction appears to be weakly excited due to the significant mass excess of the double magic ⁴⁸Ca; the excitation energy at the Coulomb barrier is only E_x = 33 MeV. At such excitation energy, the shell effects are still presented in the heated nucleus. This may increase the survival probability of the evaporation residues (ER). The high mass asymmetry in the entrance channel (A₁/A₂ = 0.2, and Z₁Z₂= 1840) seems to decrease the dynamic limitations for the fusion of the interacting nuclei, earlier observed for the more symmetric cold fusion reactions ^[2].

 



 
 
 
 
 
 
 


Fig 1.

 

The theoretical predictions for partial half-life of the isotopes with Z=112. Open points and open squares connected by solid lines are T and T_sf taken from ^[3] respectively.

Blank point – experimental data; circle - T for the isotope ³⁷⁷122 obtained in the reaction ²⁰⁸Pb (⁷⁰Ni, n) ²⁷⁷122; square - T_sf for the isotope ²⁸⁸112 produced in the reaction ²³⁸U(⁴⁸Cn, 3n)²⁸³112 – present work

 

 

 

            The expected decay properties of the isotopes with Z=112 are quite specific. According to the calculations provided by R. Smolanczuk ^[3], the even-even isotopes ²⁸²112 and ²⁸⁴112 have partial half lives for -decay respectively: T = 0.005 s. and 1.0 s. (Fig.1). Their spontaneous fission half-life (T_s.f.) is slightly higher than T. At the same time the isotope ²⁸⁰110, which is the a-decay daughter of the ²⁸⁴112 nucleus, will mainly decay by spontaneous fission. As for the chain of sequential decays of the other isotope ²⁸²112, all products with Z<106 will have T_s.f << T _, too. For the isotope ²⁸³112 the predictions are less clear, because the odd number of neutrons may lead to significant limitations for both -decay and spontaneous fission. Here, the strong competition between the two modes of decay is also highly expected.

 

It should be noted, that calculations performed by P.Moller and R.Nix ^[4] , predict for the same nuclei the half-life against -decay to be hundreds and thousands times longer than T from ^[3]. However, it seems less significant, since it does not change the main decay properties of the isotopes with Z=112, produced in the reaction ⁴⁸Ca + ²³⁸U. Either they will decay by spontaneous fission, or by the chains of their -decay (one or several) will end by the spontaneous fission anyway. On the other hand, the modern separation techniques allow synthesizing and identifying the recoil nuclei, if their half-life is more than one microsecond. It covers the range of all predictions and thus the experimental results obtained using these techniques can be used as a direct test of the predictive capabilities of the existing theoretical models.

 

Experiment

            The production of the intensive ion beam from the rare and extremely expensive ⁴⁸Ca isotope presents itself as a key problem in our issue on the synthesis of the isotopes of element 112.

 

            The neutral atoms of calcium were injected into the plasma of ECR-4M ion source ^[5] by the process of controlled heating the metallic sample of ⁴⁸Ca(50mg) enriched to 70%. The sample was made from the calcium oxide right before being placed in the oven of the ECR source. The whole preparation procedure of metallic Ca as well as the recuperation of the material from the ion source chamber was controlled by measuring the yield of -rays from the isotope of ⁴⁷Ca (4.5 d), produced in the ⁴⁸Ca(,n)⁴⁷Ca reaction.

 

The new injection system and the modified beam optics of the cyclotron allowed to obtain the internal beam of ⁴⁸Ca⁵⁺ ions with intensity up to 1 pA at material consumption rate of about 0.3 mg/h. The beam extraction from the cyclotron U-400 was provided by the stripping method. The variation of the energy of the extracted beam in the range from 200 MeV to 280 MeV was performed by smooth variation of the magnetic field and by precise positioning of the stripping foil. The mean beam intensity of ⁴⁸Ca on the target was 2.2*10¹²pps.

 

            The evaporation residues were separated in-flight from the beam particles and other reaction products with the electrostatic recoil separator “VASSILISSA”^[6]. Enriched isotope of ²³⁸U (99.999%) with thickness of 0.3 mg/cm² was used as a target material. It was deposited uniformly with a thickness of 1.6 mg/cm² on an aluminum disk 125 mm in diameter. The disk was rotating with frequency =2000 rpm. The same target design with enriched isotopes of Tb, Yb, and Pb was used in the several test experiments.

 

            For registration and identification of the recoils a system of time-of-flight detectors and silicon position sensitive strip detector array was installed in the focal plane of the separator. After registration in the TOF detectors, the recoils were implanted into the 16-strip silicon detector with an active area of 60x60 mm². Each of the strips had longitudinal position sensitivity. Special measurements of the position resolution along the each strip were performed. For sequential - decays it was 0.6 mm, for correlated recoil and -particle - 1.0 mm, and for correlated recoil and spontaneous fission events it was 1.5mm. The energy resolution for -particles within the energy range from 6 to 9 MeV was 20 keV. The time accuracy for the recorded events was about 1sec. The strip detector was surrounded with four other identical silicon detectors and the entire array had a shape of a cube with dimensions 60x60x60mm³. The geometrical efficiency of the silicon array was 85% of 4. In the present series of experiments each of the four neighboring strips of backward detectors were connected galvanicaly and formed 16 energy sensitive segments.

 

Scattered low energy projectiles were the main contribution to the background. An additional bending dipole magnet allowed placing the detectors at the angle of an 8^o with respect to the primary beam direction. After all, the counting rate of the frontal detector was only 25-30Hz. In front of the silicon detectors 3 m degrader foil (mylar) was placed to reduce the number of very low energy projectiles reaching focal plane of the separator.

 

            The separation efficiency of EVRs from the reaction ⁴⁸Ca+²³⁸U was obtained in the test experiments. The cross sections of xn-evaporation channels were measured in the bombardment of targets of ¹⁵⁹Tb, ¹⁷⁴Yb and ^206,208Pb by the ⁴⁸Ca projectile in wide energy range. These measurements showed that about 25% of the recoils ^286-x112, produced on the U-target, would be implanted in the frontal detector. The signals from the time-of-flight detectors were used both for measurements of the velocity of the recoils and for distinguishing decays of the previously implanted nuclei. The high efficiency of the TOF detectors allowed to obtain very clean decay spectra and to widen significantly (up to several hours) the time window for measuring decay chains. The latter was particularly important for identification of the EVRs with a long half-life and the continuous time structure of the beam.

 

Results

            To estimate the yield of EVRs in the reaction ²³⁸U (⁴⁸Ca, xn) ^286-x112 and to choose the optimal beam energy for ⁴⁸Ca, a set of additional experiments was performed. On the recoil separator VASSILISSA, the excitation functions for the reaction ^206,208Pb(⁴⁸Ca,xn) ^254,256-xNo were measured. At the same time, on the time-of-flight fission fragment spectrometer CORSET^[7] data were obtained on the cross-sections and mass distribution of fission fragments in the reactions ⁴⁸Ca ^{+ 206,208}Pb and ⁴⁸Ca + ²³⁸U. For all the reactions these compound system was formed at low excitation energies, close to the Coulomb barrier.

 

These experiments will be described in full in a separate article;^[8] here we shall limit ourselves to the main conclusions drawn from the data. As it follows from the analysis of the data observed above, the highest cross-section for the reaction  ⁴⁸Ca + ²³⁸U is expected for the 3n and 4n-evaporation channels. The excitation functions reached their maximums at E_x = 31 and 39 MeV, which corresponds to the beam energy in the middle of the target E_L = 231 MeV, and 239 MeV, respectively. The absolute value of the cross-section for xn-evaporation channels can be estimated much less definitely. According to different calculations, the cross-section at the maximum of the 3n-evaporation channel varies from one to a few tens pb; although, it still remains by a factor of 3-5 higher than for the 4n evaporation channel.

 

On the basis the data presented above, two long-term irradiation of a ²³⁸U target with ⁴⁸Ca at beam energies of 231 ± 3 MeV and 238 ± 3 MeV were performed. The doze of 3.5*10¹⁸ projectiles was collected in the first experiment in March 1998. Two spontaneous fission events were detected at that time. For both events the fission fragments were registered as two coinciding signals with a high-energy deposition in both front and backward detectors.

 

            The total kinetic energy (TKE) for the events was also measured (Fig.2). The detectors were calibrated by fragments from spontaneous fission of the implanted recoils of ²⁵² No, produced in the reaction ²⁰⁶Pb (⁴⁸Ca, 2n). The determined values of the TKE for the two registered events are 190 and 212 MeV respectively. If the observed spontaneous fission relates to the decay of a heavier nucleus, say with Z=110-112, the above mentioned values of TKE must be increased roughly by 10 MeV.

 

The analysis of all events collected in the experiment has been provided in order to find genetic decay links of the implants. For that purpose, the data of the position and energy resolutions of the correlated events, measured by the frontal detector for the various modes of decay were used. Thus, for recoil-- as well as recoil--…-SF for an -particle in the energy range from 8 to 13 MeV and in the time  interval up to 10000 s. no correlation were found. But at the same time, there were

 



 

 

 

 

 

 

 

Fig. 2.

The spectra of TKE for spontaneous fission of  ²⁵²No. Points – data obtained with VASSILISSA recoil separator in the reaction ²⁰⁶Pb (⁴⁸Ca, 2n)²⁵²No; dashed line–data taken from ^[9].

Black triangles correspond to the events from the reaction ⁴⁸Ca+²³⁸U.

 

 

two correlated signals from the recoil and spontaneous fission found in strips #12 and #15 within the position window ±0.8 mm. In the first case (strip #12), the time interval between the signals “recoil” and “spontaneous fission” was 182.4 s, while in the second case (strip #15), the time interval was 52.0 s. As it became evident from the data of the long-term detection, at the position window ±0.8mm the signals, similar to EVRs, were detected in strip #12 with a mean frequency of 0.001Hz, while in strip #15 the frequency was 0.005 Hz.

            In the second experiment, the doze of 2.2*10¹⁸ projectiles was collected at the beam energy E_L=238 MeV (E_x=39MeV). No spontaneous fission events were detected and no - correlation’s were observed in the whole -particle energy range from 8 to12 MeV in the time interval up to 1000 s. The upper limit for EVRs cross-sections at this excitation energy is 3.0 ± 1.5 pb.

 

It is very important to note that the two events observed in these experiments were collected practically with no background conditions. The fact that no SF- isomers as well as any other known SF-nuclei were observed in the two experiments with the total beam doze of 5.7*10¹⁸ makes evident an extremely high selectivity of the device for evaporation residues detection. That is why we also have to exclude a possibility to explain the effect in terms of a rare and unknown decay mode of known nuclei formed in the reaction ⁴⁸Ca + ²³⁸U.

 

As mentioned above no -particles, preceding the spontaneous fission, were observed for either event despite the high efficiency of the detector array. The most reasonable explanation is that we are observing the spontaneous fission of the heaviest new even-odd isotope ²⁸³112 with the half-life T_1/2=80.7^+195.2_-33.5 s. Despite the fact that only spontaneous fission was observed in the experiment, we can not exclude also the -decay with a branching ratio b 50%. The cross-section for the new isotope is 5 ± 2 pb (the statistical error is presented here; the absolute cross-section accuracy value is within a factor of 2). The absence of any effect at the excitation energy E_x=39 MeV is not surprising. The cross-section of the 3n-evaporation channel decreases with the increase of the excitation energy, while the cross-section of 4n-evaporation channel is several times lower than it is for the 3n-channel, as it was already mentioned above.

 

Conclusion

 

            The present article describes a new attempt to synthesize super heavy spherical nuclei in the reaction ⁴⁸Ca + ²³⁸U. Comparing to all previous experiments with ⁴⁸ Ca ions involved, the present one has more than 100 times higher sensitivity. Two spontaneous fission events were detected at beam energy below the Coulomb barrier. As it follows from the data analysis, the events were most probably triggered by the decay of the even-odd isotope ²⁸³112 formed in the 3n-evaporation channel. The half-life of the new isotope is about 100 sec. It could be that it also undergoes -decay with a branching ratio b 50%.

 

            The half-life of the new isotopes is more than 5 orders of magnitude longer than the already known isotope ²⁷⁷112 (T_1/2~ 0.15 ms), obtained in the cold fusion reaction ²⁰⁸Pb(⁷⁰Zn, n)²⁷⁷112.^[10] The difference could indicate that the stability increases sharply when we enter domain of the spherical shell corrections.

 

            The present experiment is the first one in our long-term research program with the ⁴⁸Ca beam, dedicated to the synthesis and properties of super heavy elements. We have a strong intention to increase the intensity of the beam , which will allow us to try to synthesize other isotopes with Z=110 and 114.

 

 

 

 

Acknowledgments

 

Authors would like to thank sincerely Prof. Ts. Vylov and V.G. Kadyshevsky for their support at all stages of carrying out of this work. We are thankful to Dr. B.I. Pustilnik for calculations of fusion cross sections induced by ⁴⁸Ca ions. We would like to thank as well Drs. R. Smolanczuk and Y. Yano for their help and interesting discussions. We also express gratitude to Drs. S.N. Dmitriev, A.B. Yakushev, and. V .Ya. Lebedev for developing methods for preparation of the metal samples from the enriched ⁴⁸Ca isotope and its recuperation from the chamber source; to A.N. Shamanin and E.N. Voronkov for their help in maintenance of the separator “Vassilissa”; and finally to the personnel of the cyclotron U-400 under the direction of A.V. Tihomirov for obtaining an intensive and stable ⁴⁸Ca beam.

 

References

 

[1] Yu. Ts. Oganessian, in: Proc. Of Int. Conf. Nuclear Physics at the Turn of the Millenium: “Structure of the Vacuum and Elementary Matter”, Wilderness, South Africa, (World Scientific, Singapure), p.11 (1996).

[2] J.P. Blocki, et. al., Nucl. Phys. A 459, 145 (1986).

[3] R. Smolanczuk, Phys. Rev. C 56 812 (1997).

[4] P. Moller, et. al., Atomic Data and Nuclear Data Tables 59 185 (1995) and P. Moller et. al.,
“Nuclear Properties for Astrophysics”, http://t2.lanl.gov/publications/astro/normal.html.

[5] V.B. Kutner, et. al., in: Proc. of 15^th Int. Conf. On Cyclotrons and their Applications, 14-19 June, 1998, Caen, France (submitted).

[6] A.V. Yeremin, et. al., Nucl. Instr. And Meth. B 126 329 (1997).

[7] M.G. Itkis et. al., Il Nuovo Cim. (to be published).

[8] A.V. Yeremin, et. al., JINR Rapid Communuications, (1998) (in press).

[9] E.K. Hulet, Yad. Fiz. 57 1165 (1994).

[10] S. Hofmann, et. al., Z. Phys. A 354 229 (1996).