Spontaneous Fission Activity Research Articles

New studies and a short review of heavy neutron-rich transfer products

We present new results on multi-nucleon transfer reactions in low-energy collisions of ^{48}hbox {Ca}+{}^{238}hbox {U} measured at the velocity filter SHIP of GSI Helmholtz Centre, where we observed around 90 different nuclides from Tl to Am (Z= 81–95). We followed the idea to use uranium targets for the synthesis of neutron-rich MNT products, particularly in the region below lead, which was triggered by model calculations. The gamma , alpha and spontaneous fission activities of the populated nuclides have been analyzed for their identification. The cross-sections of the observed isotopes for elements Z = 81–93 as a function of their mass number have been investigated. Excitation energy, total kinetic energy and the influence of nuclear shell effects on the production cross-sections of the observed transfer products have been studied. Also we present a compact review and comparative analysis of various multi-nucleon transfer and fragmentation reactions which are aimed at the synthesis of neutron-rich nuclides along the N=126 shell closure in heavy nuclei.

Production ofDb262in theCm248(F19,5n)Db262reaction and decay properties ofDb262andLr258

The nuclide $^{262}\mathrm{Db}$ was produced in the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,5$n$)$^{262}\mathrm{Db}$ reaction at beam energies of 103.1 and 97.4 MeV. Decay properties of $^{262}\mathrm{Db}$ were investigated with a rotating wheel apparatus for \ensuremath{\alpha} and spontaneous fission (SF) spectrometry under low background conditions attained by a gas-jet transport system coupled to the RIKEN gas-filled recoil ion separator. Based on genetically correlated \ensuremath{\alpha}-\ensuremath{\alpha} and \ensuremath{\alpha}-SF decay chains, a total of 76 chains were assigned to $^{262}\mathrm{Db}$ and its \ensuremath{\alpha}-decay daughter nuclide $^{258}\mathrm{Lr}$. No correlated decay chain was observed for Db produced in the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,4$n$)Db reaction and its daughter Lr. The half-lives of $^{262}\mathrm{Db}$ and $^{258}\mathrm{Lr}$ were measured to be ${T}_{1/\phantom{\rule{0.0pt}{0ex}}2}=33.{8}_{\ensuremath{-}3.5}^{+4.4}$ and $3.{54}_{\ensuremath{-}0.36}^{+0.46}\mathrm{s}$, respectively, and they confirmed the literature data. The \ensuremath{\alpha}-particle energy $({E}_{\ensuremath{\alpha}})$ and intensity (${I}_{\ensuremath{\alpha}}$) of $^{262}\mathrm{Db}$ were revised to ${E}_{\ensuremath{\alpha}}$ = 8.46 \ifmmode\pm\else\textpm\fi{} 0.04 MeV (${I}_{\ensuremath{\alpha}}$ = 70 \ifmmode\pm\else\textpm\fi{} 5%) and ${E}_{\ensuremath{\alpha}}$ = 8.68 \ifmmode\pm\else\textpm\fi{} 0.03 MeV (${I}_{\ensuremath{\alpha}}$ = 30 \ifmmode\pm\else\textpm\fi{} 5%). The SF activity with ${T}_{1/2}$ = 30.2 \ifmmode\pm\else\textpm\fi{} 6.1 s was assigned to $^{262}\mathrm{Db}$, and the SF branch of $^{262}\mathrm{Db}$ was determined to be ${b}_{\mathrm{SF}}$ = 52 \ifmmode\pm\else\textpm\fi{} 4%. From an observation of two SF decays which correlated to the \ensuremath{\alpha} decay of $^{262}\mathrm{Db}$, an electron capture (EC) decay with ${b}_{\mathrm{EC}}$ = 2.6 \ifmmode\pm\else\textpm\fi{} 1.8% was suggested in $^{258}\mathrm{Lr}$. The production cross sections for the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,5$n$)$^{262}\mathrm{Db}$ reaction were determined to be 2.1 \ifmmode\pm\else\textpm\fi{} 0.7 nb at 103.1 MeV and $0.{23}_{\ensuremath{-}0.11}^{+0.18}\mathrm{nb}$ at 97.4 MeV, whereas, those for the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,4$n$)Db reaction were the upper limits of \ensuremath{\le}0.064 nb at 103.1 MeV and \ensuremath{\le}0.13 nb at 97.4 MeV. These cross sections are discussed by comparing with the literature data as well as the theoretical prediction.

Long-termα- and spontaneous fission measurement of aRf/Dbsample chemically prepared in aCa48onAm243experiment

Results from long-term measurements of a chemically separated Db/Rf sample prepared from the products of a $^{48}\mathrm{Ca}$ on $^{243}\mathrm{Am}$ irradiation are presented. The sample with the highest spontaneous fission activity out of eight samples produced in the course of chemical experiments performed in 2004 was selected for these measurements. We conclude that there is no evidence for SF-decay originating from heavy actinide isotopes in this sample. Hence, it is appropriate to assign the SF-events observed in this experiment to decay products of $^{288}115$.

Spontaneous fission of neutron-deficient fermium isotopes and the new nucleus 241Fm

Spontaneous fission of neutron-deficient fermium isotopes (N ≤ 144) was investigated. The nuclei were produced in fusion-evaporation reactions of 40Ar beams with 204,206–208Pb targets. Excitation functions of the two-and three-neutron evaporation channels were measured. The new isotope 241Fm was identified, and its decay properties were determined. A previously made assignment of a 0.8 ms spontaneous-fission activity to the isotope 242Fm could not be confirmed. Isomeric states were not observed in any of the investigated isotopes. The experiments were performed at the velocity filter SHIP of GSI in Darmstadt.

Impact of nuclear data uncertainties on the incineration of 237Np and 241Am targets

In this work, we have explored the impact of nuclear data uncertainties on the incineration of 237Np and 241Am targets and on the evolution of correlated safety quantities such as alpha and spontaneous fission activities and radiotoxicity. These studies were performed in three representative neutron flux spectra: EFR (fast reactor), GT-MHR (moderated reactor) and HTFR (thermal reactor). In order to get a common scale of comparison between these scenarios, the simulations were based on given incineration rate rather than irradiation time. Afterwards, the dominant contributors to alpha and spontaneous fission activities and radiotoxicity were identified in each case and the impact of nuclear data uncertainties on the formation of these isotopes was assessed. Finally, data which have the highest contributions were highlighted and their multi-group errors were detailed as a function of energy, to show on which energetic domains future measurements have to be focused on.

Attempts to chemically investigate element 112

Summary Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48Ca with 238U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283112 for the 238U(48Ca,3n) 283112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of element 112 with half-lives (t 1/2) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of 283112. Therefore, these events could not be unambiguously attributed to the decay of 283112. In contradiction with earlier publications newer reports from FLNR Dubna claim that 283112 decays by α-particle emission (E α = 9.5 MeV) with t 1/2 = 4 s followed by a SF-decay of 279Ds (t 1/2 = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of 283112. However, during this experiment neither short α-SF correlations nor SF coincidences were detected. The conclusion is that 283112 was not unambiguously detected, neither in Experiment I nor in Experiment II.

Synthesis of elements 115 and 113 in the reactionAm243+Ca48

The results of two experiments designed to synthesize element 115 isotopes in the {sup 243}Am+{sup 48}Ca reaction are presented. Two new elements with atomic numbers 113 and 115 were observed for the first time. With 248-MeV {sup 48}Ca projectiles, we observed three similar decay chains consisting of five consecutive {alpha} decays, all detected in a total time interval of 30 s. Each chain was terminated by a spontaneous fission (SF) with a high-energy release and a lifetime of about a day. With 253-MeV {sup 48}Ca projectiles, we registered a different decay chain of consecutive {alpha} decays detected in a time interval of 0.5 s, also terminated by spontaneous fission, but after 1.8 h. The decay properties of the eleven new {alpha}- and SF-decaying nuclei are consistent with expectations for consecutive {alpha} decays originating from the parent isotopes {sup 288}115 and {sup 287}115, produced in the 3n- and 4n-evaporation channels, respectively. Support for the assignment of the atomic numbers of all of the nuclei in the {sup 288}115 decay chain was obtained in an independent experiment in which a long-lived spontaneous fission activity, {sup 268}Db (15 events), was found to be chemically consistent with the fifth group of the periodic table.more » The odd-odd isotope {sup 288}115 was observed with largest cross section of about 4 pb. In the SF decay of {sup 268}Db, a total kinetic energy of 230 MeV and a neutron multiplicity per fission of 4.2 were measured. The decay properties of the 11 new isotopes with Z=105-115 and the production cross sections are in agreement with modern concepts of the role of nuclear shells in the stability of superheavy nuclei. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research.« less

On search and identification of short-lived super heavy cosmic-ray nuclei ( Z ≥ 110) by fossil track study of the extraterrestrial crystals: Results and perspectives [II

The existence of relatively stable super heavy elements (SHE) in Nature was predicted theoretically at the midst of the sixties (Nilsson, Nix, Sobichevsky, see Ref. [1]). Basing on nuclear shell model it was estimated, that double magic nuclei with atomic number 110 ≤ Z ≤ 114 and neutron number N = 184, namely, the double “magic” closed shells of nuclei can possess the life time at ≥10 3 up to 10 9 years. Thus, these elements, similarly to Th and U, can survive in the Earth and meteorites since formation of Solar system ∼ 4.6 billion years ago. The present project work was drastically stimulated by recent synthesis and discovery of very stable isotopes of SHE in Flerov Laboratory of Nuclear Reactions During 1999–2000 Oganessian and his colleagues succeed in obtaining of a number of rather neutron-rich isotopes of elements 112, 114 and 116 in the reactions of 48Ca with monoisotopic targets of 238U, 244Pu and 248Cm, respectively [2]. The most stable isotope obtained is odd-even nuclear 285112, which possess the life time in between 10–30 min. Still this isotope has only 173 neutrons which is 11 fewer as compared with the magic number N = 184. For the region of known actinide nuclei (Z = 89 – 98) such a neutron difference for the most stable isotopes provides the stabilization factor of 10 10 – 10 13 in the life time. The discovery of very stable isotope of the new element 112 provides firstly the final unambiguous proof on the existence of new island of very stable SHE nuclei. Now we pointed out that there is no way to get the neutron number N = 184 using present accelerators and target nuclei. The only one way to find out double magic SHE nuclei now is the search for these nuclei in natural samples. The experimental attempts to discover such long-lived SHE nuclei with the life time ≥2×10 8 y in natural samples undertaken during the late sixties up to end of seventies provided some evidence of their existence in a number of both terrestrial samples and meteorites. These experiments were done by the investigation of alpha radioactivity and spontaneous fission activity, which exceeds significantly the effect due to the spontaneous fission of 238U nuclide. Still no decisive information on the existence of SHE in the nature was obtained.

Spontaneous-fission decay properties and production cross-sections for the neutron-deficient nobelium isotopes formed in the 44, 48Ca + 204, 206, 208Pb reactions

Heavy-ion fusion reactions 48Ca + 204Pb and 44Ca + 208Pb leading to the same compound nucleus 252No* were run in attempts to produce new neutron-deficient spontaneous-fission isotopes of 249,250No using the electrostatic separator VASSILISSA. Production cross-sections for the spontaneous-fission activities with the half-lives 5.6 and 54 μs observed in these reactions are compared with the measured ones for the well-known isotopes of 251 - 255No formed in the heavy-ion fusion reactions 48Ca + 206Pb and 48Ca + 208Pb. The obtained excitation functions for the reaction products formed after the evaporation of 1-4 neutrons from the corresponding compound No nuclei have been compared with similar data obtained earlier and results of statistical model calculations.

Decay properties of neutron-deficient isotopes 256, 257Db, 255Rf, 252, 253Lr

Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 were produced by the reaction 209Bi(50Ti,xn) 259-xDb (x = 1, 2, 3) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction 208Pb(50Ti,1n)257Rf. The measured α-decay data of 257Db and its daughter products resulted in the identification of α-decaying isomeric states in 257Db and 253Lr. Two new isotopes, 256Db and 252Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by delayed α-α coincidences. The measured half-lives are (1.6+0.5 -0.3) s for 256Db and (0.36+0.11 -0.07) s for 252Lr. Besides α-decay, a spontaneous fission activity of T 1/2 = (2.3+1.1 -0.6) s was observed and attributed to an electron-capture branch of 256Db, which feeds the fissioning nucleus 256Rf. A branching ratio of 0.36±0.12 was obtained. The isotope 255Rf was produced by the reaction 207Pb(50Ti,2n)255Rf. Improved decay data have been obtained by means of α- and α-γ spectroscopy.

Spontaneous fission and alpha-decay properties of neutron deficient isotopes 257−253104 and 258106

Spontaneous fission and α-decay of253−257104 and 258106 were investigated in irradiations of 204,206,208Pb with 50Ti and 209Bi with 51V, respectively. New spontaneous fission activities were identified and assigned to 253104, 254104, and 258106. The half-lives were measured as T1/2 = (48−10+14) μs for 253104, T1/2 = (23 ± 3) μs for 254104, and T1/2 = (2.9−0.7+1.3) ms for 258106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for 253104, bα ≤ 0.015 for 254104, and bα ≤0.2 for 258106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of 257104 and 255104 have been improved. An isomeric state decaying by α-emission was identified in 257104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus 256104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to 264Hs (Z=108), the heaviest even — even nucleus identified so far.

Identification of new mendelevium and einsteinium isotopes in bombardments of209Bi with40Ar

The four new neutron deficient isotopes246Md (T1/2=(1.0±0.4) s),245Md (T1/2=(0.35 −0.16 +0.23 ) s),242Es (T1/2=(16 −4 +6 ) s) and241Es(T1/2=(8 −4 +6 ) s) were produced in fusion reactions of40Ar ions with209Bi targets at bombarding energies of E p =4.78, 4.93 and 5.12 A*MeV. The assignment was based on evaporation residue-α-α time and position correlations. A spontaneous fission activity of T1/2=(0.90±0.25) ms was observed at a projectile energy of E p =5.12 A*MeV and assigned to245Md.

Determination of the half-life of the spontaneous fission of240PU

Two metallic240Pu sources were certified as reference standards of spontaneous-fission activity and neutron flux. The certification results were used to determine the ratio TαTs&f of the alpha-decay and spontaneousfission half-lives of240Pu. The ratio was found to be (5.68+-0.11)·10−8, which corresponds to Ts&f=(1.16+0.02)·1011 yr for an assumed value Tα=6563+7yr.

Possible synthesis of new and super-heavy elements via cluster decay

Cluster decays of some heavy and super-heavy, formed in heavy-ion 'cold fusion' reactions, are studied within a preformed cluster decay model. The cold super-asymmetric fission of these nuclei is also investigated. Some of the calculated exotic cluster decays from the ground to ground or excited to excited states are found to have half-liver T12/<1025 s, which are measurable with presently available experimental techniques. If such decays of the compound systems are observed as the long lived, super asymmetric, spontaneous fission activity, or simply as the cold fission process, it could give rise to some other new, heavy and super-heavy nuclei. Shell stabilization effects due to both the spherical and deformed shell closures are found to be responsible for selecting both the favourable cold fusion reactions as well as the most probable cluster decays.

A cryosystem for the detection of alpha and spontaneous-fission activities in volatile species

Abstract A system for the on-line detection of very volatile α and spontaneous-fission activities produced in nuclear reactions has been developed. The reaction products are continuously transported by a gas-jet from the target area to the detector unit. Volatile species are condensed on a solar cell kept at low temperature ( ⋍ 40 K ) by a cryogenic pump. An annular surface barrier detector is placed in front of the solar cell. Fission fragments and their energies are recorded by coincidence signals from both detectors, whereas α-particle spectra are measured with the surface barrier detector. The background is less than one coincident event per day, thus allowing measurements at low production rates. A half-life region down to a few seconds can be covered as is shown with short-lived radon isotopes. The system was used in searches for very volatile superheavy elements in the reactions 48Ca+248Cm and 238U+238U.

Actinide production in 136Xe bombardments of 249Cf.

The production cross sections for the actinides from $^{136}\mathrm{Xe}$ bombardments of $^{249}\mathrm{Cf}$ at energies 1.02, 1.09, and 1.16 times the Coulomb barrier were determined. Chemical fractions of the individual actinide elements were prepared and the gamma, alpha, and spontaneous fission activities of the actinide isotopes were measured. The cross section distributions are presented and compared with those from other heavy ion bombardments of actinide targets. Conclusions are drawn as to the utility of these heavy ion bombardments for producing unknown isotopes in the actinide region. It is shown that for new neutron-rich actinide isotopes with atomic numbers smaller than that of the target these very heavy ion bombardments are perhaps the best method of production, and when using the most neutron-rich targets available new neutron-rich isotopes should be produced in quantities sufficient for their identification and study.

The discovery of 260Md and the decay properties of 258Fm, 258m,gMd and 259Md

We have discovered a new neutron-rich isotope, 260Md, from 18O and 22Ne bombardments of 254Es. We observed a spontaneous-fission (SF) activity with a half-life of 32 days in electromagnetically separated fractions with mass number 260 from these bombardments and we measured the mass and kinetic energy distributions of this SF activity. The mass distribution was symmetric with the principal energy peak at a total kinetic energy (TKE) of 234 MeV, similar to previous observations for heavy fermium isotopes. Surprisingly, we also observed a smaller symmetric component with a TKE of 195 MeV. We interpret these two peaks in the TKE distribution as arising from two types of fission in the same nucleus, or bimodal fission. The observed fission activity may be either from the SF decay of 260Md or from 260Fm which would arise from electron-capture (EC) decay of 260Md. We have eliminated the possible β − decay of 260Md by measuring β −-SF time correlations for the decay of 260Md and we plan to determine whether 260Md decays by EC by measuring time correlations between fermium X-rays and SF events. We also measured various properties of the heavy fermium and mendelevium isotopes and obtained 1. (1) more accurate cross-sections for the neutron-rich mendelevium isotopes which we use to predict the production rates of yet undiscovered nuclides, 2. (2) improved half-Me measurements for 258m,gMd and 259Md, 3. (3) confirmation of the EC decay of 258mMd by measurement of the fermium X-rays preceding the SF decay of 258Fm and 4. (4) very substantially improved mass and TKE distributions for the SF decay of 258Fm and 259Md.

Search for superheavy elements in damped collisions between 238U and 248Cm.

Negative results for the production of superheavy elements in damped collisions between $^{238}\mathrm{U}$ projectiles and $^{248}\mathrm{Cm}$ targets are reported. This reaction was believed to permit a closer and more widespread approach to the predicted island of stability near Z=114 and N=184 than any practical fusion reaction. Aqueous and gas phase chemistry techniques were used to isolate superheavy element fractions. The fractions were counted for spontaneous fission activity, fragment kinetic energies, and neutron multiplicities. Cross-section limits for half-lives from hours to several years are g4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{2}$.

Spontaneous fission of rutherfordium isotopes.

We have found several new spontaneous fission (SF) activities and suggested assignments for some of them to rutherfordium (element 104) isotopes. Their half-lives and production cross sections have been measured by collecting recoils on a moving nickel or steel tape and transporting them past stationary mica track detectors at known velocities. The following tentative assignments are based on several cross bombardments and comparisons between experimental and calculated production cross sections: $^{256}\mathrm{Rf}$(9\ifmmode\pm\else\textpm\fi{}2 ms), $^{257}\mathrm{Rf}$(3.8\ifmmode\pm\else\textpm\fi{}0.8 s, 14\ifmmode\pm\else\textpm\fi{}9% SF), $^{258}\mathrm{Rf}$(13\ifmmode\pm\else\textpm\fi{}3 ms), $^{259}\mathrm{Rf}$(3.4\ifmmode\pm\else\textpm\fi{}1.7 s, 9\ifmmode\pm\else\textpm\fi{}3% SF), $^{260}\mathrm{Rf}$(21\ifmmode\pm\else\textpm\fi{}1 ms), and $^{262}\mathrm{Rf}$(47\ifmmode\pm\else\textpm\fi{}5 ms). Presently we are unable to assign several other new spontaneous fission activities with half-lives of \ensuremath{\sim}15 ms, \ensuremath{\sim}22 ms, \ensuremath{\sim}100 ms (Zl104), \ensuremath{\sim}1.6 s, \ensuremath{\sim}5 s, \ensuremath{\sim}5 s, \ensuremath{\sim}30 s, and \ensuremath{\sim}47 s. Contrary to other observations, we have not found any indication for an 80-ms spontaneous fission activity, earlier credited to the spontaneous fission decay of $^{260}\mathrm{Rf}$, in reactions in which we expected to produce this isotope. Our measurements support a shift in the spontaneous fission half-life systematics at element 104, first proposed by Flerov and Oganessian et al. and later predicted by Randrup et al. and Baran et al. and attributed to the disappearance of the second hump of the double-humped fission barrier and a weakening of the 152-neutron subshell.

Further attempts to isolate superheavy elements in the meteorite Allende

The search for superheavy elements /SHE/ in the carbonaceous chondrite Allende was continued. This time we carried out a thermochromatographic separation at 1050 °C in O2-, resp., H2-gas flow. The volatile fraction /mostly Pb/ was collected on a Pt-foil or trapped in KOH-solution. The heavy element probes were investigated for spontaneous fission activities. We observed zero spontaneous fission decays during 421 days. This yielded an upper limit of ≤2.9 fission x kg−1 x year−1 /95% confidence limit/. We were unable to confirm the existence of a spontaneous fission activity in the meteorite Allende.