Yu.Ts.Oganessian et al

EXPERIMENT ON THE SYNTHESIS OF ELEMENT 115

IN THE REACTION

²⁴³Am(⁴⁸Ca, xn)^291-x115

I. INTRODUCTION

Our previous experiments were designed to synthesize even-Z superheavy elements (114-118) in the ⁴⁸Ca-induced reactions with actinide targets ²⁴⁴Pu [1], ²⁴⁸Cm [2] and ²⁴⁹Cf [3]. The observed fusion-evaporation reaction products underwent two or three consecutive α-decays terminated by spontaneous fission (SF).

For the neighboring odd-Z elements, especially their odd-odd isotopes, the probability of α-decay with respect to SF should increase due to hindrance for SF. For such odd-Z nuclei one might expect longer consecutive α-decay chains terminated by the SF of relatively light descendant nuclides (Z<105).

The decay pattern of these superheavy nuclei is of interest for nuclear theory. In the course of α-decays, the increased stability of nuclei caused by the predicted spherical neutron shell N=184 (or perhaps N=172) should gradually become weaker for descendant isotopes. However, the stability of these nuclei at the end of the decay chains should increase again due to the influence of the deformed shell at N=\62.

The observation of nuclei passing from spherical to deformed shapes in the course of their consecutive α-decays could provide valuable information about the influence of significant nuclear structure changes on the decay properties of these nuclei. For these investigations, we chose the fusion-evaporation reaction ²⁴³Am+⁴⁸Ca, leading to isotopes of element 115. According to calculations based on the results of experiments on the synthesis of even-Z nuclei [1-4], the 3/7- and 4/7-evaporation channels leading to isotopes ²⁸⁸115 (N=\73) and ²⁸⁷115 (N=172) should be observed with the highest yields.

II. EXPERIMENT

The experiments were performed between July J4 and August 10, 2003, at the U400 cyclotron with the Dubna gas-filled recoil-separator (DGFRS). The average incident beam intensity was 1.3 pμA). Over this period, equal beam doses of 4.3∙l0^{18 48}Ca projectiles were delivered to the target at two bombarding energies. In this experiment, we chose the laboratory energies for the ^4SCa ions of 248 MeV and 253 MeV in the middle of the target. The systematic uncertainty in the beam energy is ~1 MeV. With the beam energy resolution, small variation of the beam energy during irradiation and energy losses in the target (-3.3 MeV). we expected the resulting compound nuclei ²⁹¹115 to have excitation energies between 38.0-42.3 MeV and 42.4-46.5 MeV, respectively [5].

The 32-cm² rotating target consisted of the enriched isotope ²⁴³Am (99.9%) in the form of AmO₂. The target material was deposited onto 1.5-μm Ti foils to a thickness corresponding to ≈0.36 mg cm^-2 of ²⁴³Am.

The evaporation residues (EVRs) recoiling from the target were separated in flight from the ⁴⁸Ca beam ions, scattered particles and transfer-reaction products by the DGFRS [6]. The transmission efficiency of the separator for Z=115 nuclei was estimated to be about 35% [6].

The detection system consisted of a multiwire proportional counter to measure time-of-flight (TOF) and a 4xl2-cm² semiconductor focal-plane detector array with 12 vertical position-sensitive strips to measure the decay of the implanted recoils. This detector, in turn, was surrounded by eight 4x4-cm² side detectors without position sensitivity, forming a box of detectors open from the front side. The detection efficiency for α-decays of implanted nuclei was 87% of 4π. The detection system was tested by registering the recoil nuclei and α- and SF-decays of the known isotopes of Jl and Th, as well as their descendants, produced in the reactions ²⁰⁶Pb(⁴⁸Ca, 2n) and ^natYb(⁴⁸Ca, 3-5n). The energy resolution for cc-particles absorbed in the focal-plane detector was 60-100 keV, depending on the strip and the position in the strip. For α's escaping the focal-plane detector at different angles and registered by side detectors, the energy resolution of the summed signals was 140-200 keV. The FWHM position resolutions of correlated decays of nuclei implanted in the focal-plane detector were 0.9-1.9 mm for EVR-α correlations and 0.5-0.9 mm for EVR-SF correlations. If both the focal-plane and a side detector detected an α-particle, the position resolution depended on the amplitude of the signal in the focal-plane detector (see, e.g., Fig. 4 in [3]), but was generally inferior to that obtained for the full-energy signal.

Fission fragments from ²⁵²Jl implants produced in the ²⁰⁶Pb+⁴⁸Ca reaction were used for a fission-energy calibration. The measured fragment energies were not corrected for the pulse-height defect of the detectors, or for the energy loss in the detectors' entrance windows, dead layers, and low-pressure pentane gas filling the detection system. The mean sum energy loss of fission fragments from the SF-decay of ²⁵²Jl was estimated to be about 20 MeV.

To greatly reduce the background, we switched off the beam after a recoil signal was detected with preset parameters of implantation energy and TOF expected for Z=115 evaporation residues, followed by an α-like signal with an energy of 9.6 MeV< E_a <11.0 MeV in the same strip, within a position window of 1.4-1.9 mm and time interval of up to 8 s. The beam-off interval was initially set at 2 minutes. If, in this time interval, an α-particle with E_a>8.6 MeV was registered, the beam-off interval was extended to between 12 minutes and 2.5 hours. Thus, all the expected sequential decays of the daughter nuclides with Z<113 could be observed under very low background conditions.

III. EXPERIMENTAL RESULTS AND DISCUSSION

The three similar decay chains observed at 248 MeV are shown in Fig. la. The implantations of recoils in strips 2, 3 and 4 of the focal-plane detector were followed by α-particles with E_a=10.46±0.06 MeV. These sequences switched the ion beam off, and four more α-decays were detected in time intervals of 29 s, 15 s, and 20 s, respectively, in the absence of a beam-associated background. The last α-decay in the first chain and second and fourth α-decays in the second chain were registered by the side detectors only. The energies deposited by these α-particles in the focal-plane detector were not registered. However, with the actual α-counting rates, the probability that these α-particles appeared in the detector (Δt~30 s) as random events can be estimated to be approximately 1.5% [7], so we assign them to the decay of the same implanted nuclei. During the remainder of the 2.5 h beam-off period following the last position-correlated α-particle, no α-particles with E_a>1.6 MeV were registered by the focal-plane detectors. The SF-decay of the final nuclei in these chains was detected 28.7 h, 23.5 h and 16.8 h, respectively, after the last α-decay. A search was performed to identify α-decays correlated closely in time (<60 s) and position to each of the three SF-events in Fig. la. No correlations were found.

In the first two decay chains, the focal-plane and side detectors simultaneously detected fission fragments with sum energies of 205 MeV and 200 MeV. In the third chain, only the focal-plane detector registered a fission fragment. All three SF-events were registered in the corresponding strips and positions where the three EVR-α₁-α₅ decay chains were observed (see Fig. la). Thus, these SF-events should be assigned to the spontaneous fission of the descendant nuclei in these observed chains.

In the course of this experiment at 248 MeV, we observed only seven spontaneous-fission events. In addition to the three previously mentioned events, three other SF-events, with measured energies of 147 MeV, 168 MeV, and 154 MeV, were detected 0.51 ms, 4.1 ms and 2.07 ms after the implantation of corresponding recoil nuclei in strips 12, 10 and 11, respectively. For the second and third event, both the focal-plane and side detectors registered fission fragments. Based on the apparent lifetime, we assign these events to the spontaneous fission of the 0.9-ms ^244mlAm isomer, a product of transfer reactions with the ^24jAm target. The DGFRS suppresses the yield of such products by a factor of 10⁵. An additional long-lived SF event with E_sum=146 MeV was observed in strip 2. This SF event corresponds to the background level of long-lived SF Cf isotopes produced in incomplete fusion reactions in a previous experiment with a ²⁴⁹Cf target [3] where the same set of detectors was used.

At 253 MeV, the aforementioned EVR-α₁-α₅ -SF decay chains were not observed. However, a different decay chain, consisting of lour α-decays and a spontaneous fission, was registered (see Fig. lb). The beam was switched off after the detection of an EVR signal followed in 46.6 ms by an α-particle with E_a=10.50MeV in the same position in strip 7. Three other a decays were detected in a time interval of about 0.4 s in the absence of beam-associated background. After 106 minutes, the terminal SF-event was detected in-beam with a sum energy of 206 MeV in the same position in strip 7 (see Fig. lb). In addition to this SF event with E_sl,_m=206 MeV, the decays of three other long-lived background SF nuclei, with measured fission-fragment energies of 168 MeV, 154 MeV, and 151 MeV, were also observed in strips 4, 3 and 1, respectively.

The radioactive properties of nuclei in this decay chain differ from those of the nuclei observed at the lower bombarding energy. The total decay time of this chain is about 10 times shorter and the α-decays are distinguished by higher α-particle energies and shorter lifetimes. Its production also required increasing the beam energy by about 5 MeV, so this decay chain must originate from another parent nucleus.

It is most reasonable that the different decay chains originate from neighboring parent isotopes of element 115, produced in the complete fusion reaction ²⁴³Arn+⁴⁸Ca followed by evaporation of three and four neutrons from the compound nucleus ²⁹¹115. Indeed, at the excitation energy E =40 MeV, close to the expected maximum for the 3/7-evaporation channel, we observed longer decay chains of the odd-odd isotope ²⁸⁸115.

Increasing the beam energy by 5 MeV results in reducing the ²⁸⁸115-isotope yield and, at the same time, increasing the yield of the 4“-evaporation channel leading to the even-odd isotope ²⁸⁷115. Corresponding cross sections for the 3n- and 4n-evaporation channels at the two projectile energies were measured to be 2.7 pb for 3n-channel and 0.9 pb for 4n-channel. Note that these values are in agreement with results of recent experiments [4] where excitation functions for the reaction ²⁴⁴Pu(⁴⁸Ca,xn) have been measured.

The decay properties of the product nuclei are presented in Table 1. In the decay chains shown in Fig. la, we assigned SF-events to the isotope ²⁶⁸Db following five consecutive a-decays. It is also possible that this isotope undergoes a-decay or electron capture (EC). In the case of the a-decay of ²⁶⁸Ns with T_a>2.5 h (after the beam-off period), one would more reasonably expect SF or EC of ²⁶⁴Rf, because the a-decay of ²⁶⁴Rf seems improbable due to the low expected a-decay energy (g_a=6.84 MeV [8], r_a>100 d). The electron capture of ²⁶⁸Ns or ²⁶⁴Rf leads to the even-even isotopes ²⁶⁸Ku or ²⁶⁴Jl, for which rapid spontaneous fission can be expected (e.g., T_1/2=1.4 s is predicted for ²⁶⁸Ku [9]). Since both fission fragments of these terminal nuclei can have the Z=50 and N=82 closed shell configurations, one would expect their SF-decays to result in narrowly symmetric mass distributions with rather high total kinetic energies, TKE=230-240 MeV (see, e.g., Ref. [10]). Indeed, the sum fission fragment energies observed for the terminal nuclei are close to this value. Note, the likelihood that EC-decay occurs earlier in the observed decay chains is small because of the short T_a for the observed a-decays of elements 115, 113, Rg, Mt, and 107.

In the single decay chain originating from ²⁸⁷115 (see Fig. lb. and Table 1), we propose that we have missed the a-decay of ²⁷¹Bh. This assumption follows from the a-decay properties of nuclei located around N=\62 as predicted by macroscopic-microscopic theory [8], The expected T_a value for ²⁷¹107 should be -10 s (g_o=9.07 MeV [8], T_a~5 s for an allowed transition), which is much shorter than the interval between the last observed a-particle and the terminating SF-event, but is much longer than the intervals between the observed correlated a-particles. A search was performed to identify a-particles with E_a>7.0 MeV correlated in position to the observed a-decays. No correlated a-particles were observed within an hour after the last a-decay. In these experiments, we registered 19 a-particles using a detector with 87% efficiency, so the loss of one a-particle seems rather probable. The experimental decay scheme for ²⁸⁷115 is also supported by the agreement of the observed decay properties of the other nuclides in the decay chain with the expectations of theory (see Fig. 2). This means that the SF occurs directly in the decay of Ns since the calculated a-decay and EC-energies for this isotope are rather low (g_a=7.41 MeV [8], E>_EC=1 MeV [5]) and their expected partial half-lives significantly exceed the observed time interval of 106 min.

For the measured α-decay energies of the newly-produced isotopes, one can estimate half-lives for allowed transitions and compare them with experimental values under the Geiger-Nuttall treatment using the formula by Viola and Seaborg. Parameters are obtained from fits to the T_a vs. Q_a values of 65 known even-even nuclei with Z>82 and N>126. The ratios between experimental r_exp and calculated r_calc half-lives define the hindrance factors caused by odd numbers of protons and/or neutrons in the newly synthesized nuclei. The measured T_exp values closely reproduce the calculated ones for the first two nuclei of these chains; thus the element 115 and element 113 isotopes have rather low hindrance factors, if any, for α-decay. For the isotopes of elements Rg, Mt and 107, the difference between measured and calculated T_a values results in hindrance factors of 3-10. These match the hindrance factors that can be extracted for the deformed odd-odd nuclei ²⁷²Rg and descendants produced in experiments at GSI [11] and RIKEN [12]. One can suppose that in this region of nuclei, a noticeable transition from spherical to deformed shapes occurs at Z= 109-111, resulting in the complication of the level structures of these nuclei and in an increased probability of α-transitions going through excited states. Another sign of such a shape transition might be the significant increase in the difference of α-decay energies of the neighboring isotopes observed as the decay chains reach Z=lll, see Fig. 2. This assumption is in agreement with macroscopic-microscopic calculations [8]. The deformation parameter β₂ was calculated to be 0.072 and 0.138 for ²⁸⁸115 and ²⁸⁴113, respectively. As the decay chain recedes from the shell closure at Z= 114, the deformation parameter β₂ increases to 0.200, 0.211 and 0.247 for ²⁸⁰l 11,²⁷⁶Mt and ²⁷²107, respectively.

The α-decay energies of these synthesized isotopes are plotted in Fig. 2, in which the α-decay energies of known odd-Z isotopes with Z>103 together with theoretical Q_a values [8] for nuclides with Z=103-115 are also shown. The α-decay energies of newly produced isotopes of Mt and 107 coincide well with theoretical values [8]. For the isotopes ^279,280 Rg and ^283,284113 the difference between theoretical and experimental Q_a values amount to 0.6-0.9 MeV. Some part of this deficient energy can be explained by possible γ-ray emission from excited levels populated during α-decay. Thus, low-lying level structure calculations for these nuclei are desirable for a more quantitative comparison with theory.

As a whole, the results of the present work demonstrate that in ⁴⁸Cα-induced reactions one can produce and study new nuclei over a wide range of Z and N. These investigations should result in significant new information on the influence of nuclear structure on the decay properties of superheavy nuclei.

Due to the hindrance to spontaneous fission caused by the odd proton, these odd-Z nuclei can undergo consecutive α-decays leading to the lighter transactinide elements that have a large neutron excess. The number of neutrons in such nuclides can be varied downward by using other target nuclei, e.g., ²⁴¹Am, ²³⁷Np or ²³¹Pa. Therefore, the investigation of the region of nuclei located near deformed shell closures Z=108 and N=162 that has been explored from the N<162 side in the cold fusion reactions [11,12] can be approached from the high-N side using ⁴⁸Ca-induced reactions. Note also that the rather long lifetimes of Ns isotopes permit us to investigate, in more detail, the chemical properties of this element, previously studied only in on-line experiments with the short-lived isotopes ^261,262,263Ns (T_α=1.8 s, 34 s and 27 s) (see, e.g., [16] and Refs. therein).

ACKNOWLEDGMENTS

We are grateful to the JINR Directorate, in particular to Profs. V.G. Kadyshevsky, Ts. Vylov and A.N. Sissakian for the help and support we received during all stages of the experiment. We would like to express our gratitude to the personnel of the U400 cyclotron and the associates of the ion-source group for obtaining an intense ⁴⁸Ca beam. This work has been performed with the support of the Russian Ministry of Atomic Energy and grants of RFBR No. 01-02-16486 and 03-02-06236. The ²⁴³Am target material was provided by RIAR, Dimitrovgrad, and by the U.S. DOE through ORNL. Much of the support for the LLNL authors was provided through the U.S. DOE under Contract No. W-7405-Eng-48. These studies were performed in the framework of the Russian Federation/U.S. Joint Coordinating Committee for Research on Fundamental Properties of Matter.

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