Spontaneous Fission Half-lives Research Articles

Influence of spontaneous fission rates on the yields of superheavy elements in the r-process

The formation of heavy elements in the neutron star merger scenario is considered. In such a scenario, the duration of the r-process is long and when the nucleosynthesis wave passes through the region of actinides, beta-delayed, neutron-induced, and spontaneous fission are added to the main r-process reaction channels. The dependence of the formation of superheavy elements on spontaneous fission model is investigated numerically. The formation of nuclei lighter than the cadmium-peak elements and cosmochronometer nuclei is shown to depend on strongly on the spontaneous fission model used in nucleosynthesis calculations. The regions of nuclei with short spontaneous fission half-lives prevent the formation of superheavy elements in the r-process, but the prediction of their yields is so far inaccurate because of an insufficient accuracy of calculating a number of transactinide parameters. The relative contributions from neutron-induced, beta-delayed, and spontaneous fission have been determined for various spontaneous fission models in the nucleosynthesis scenario considered.

Towards high accurate neutron-induced fission cross sections of240,242Pu: Spontaneous fission half-lives

Fast spectrum neutron-induced fission cross sections of transuranic isotopes are being of special demand in order to provide accurate data for the new GEN-IV nuclear power plants. To minimize the uncertainties on these measurements accurate data on spontaneous fission half-lives and detector efficiencies are a key point. High -active actinides need special attention since the misinterpretation of detector signals can lead to low efficiency values or underestimation in fission fragment detection. In that context, 240,242 Pu isotopes have been studied by means of a Twin Frisch-Grid Ionization Chamber (TFGIC) for measurements of their neutron-induced fission cross section. Gases with different drift velocities have been used, namely P10 and CH4. The detector efficiencies for both samples have been determined and improved spontaneous fission half-life values were obtained.

Measurement of the 240,242Pu Neutron-induced Fission Cross Sections

Abstract The neutron-induced fission cross section of 240,242 Pu has been measured at the Van de Graaff facility of the Institute for Reference Materials and Measurements (JRC-IRMM). A Twin-Frisch Grid Ionization Chamber (TFGIC) has been used in a back-to-back geometry with the secondary standards 237 Np and 238 U to normalize the cross section. The energy range measured is from 0.2 keV up to 3 MeV. Preliminary results show some discrepancies around 1 MeV for the 242 Pu with the ENDF/B-VII.1 evaluation. The spontaneous fission half-life has been measured for both isotopes, too. Preliminary results show reasonable agreement with the recommended values.

Possible α decay chains from isotopes of superheavy element 120

We investigate the a decay half-lives of even-Z superheavy nuclei up to Z = 120 by the modified two-potential approach. Based on the density-dependent cluster model, the a daughter potential is obtained from the double folding integral, taking into account the effect of nuclear deformation. A reasonable formula related with the fissility parameter and the fission barrier height, is applied to evaluate the spontaneous fission half-lives of the studied nuclei. After the performance of the competition between a decay and spontaneous fission, possible a decay chains are proposed in pursuit of being useful for the future experiment.

Feasibility of observing alpha decay chains from 270–301117 super heavy nuclei

The alpha decay of 270–301117 super heavy nuclei is investigated within the Coulomb and proximity potential model for deformed nuclei. We have performed a systematic search for those isotopes of Z = 117 which may be detectable through experiments via alpha decay. We have predicted 3α chains for 293117 and 6α chains for 294117 by the comparison of alpha half-lives and spontaneous fission half-lives and it can be seen that our predictions go hand in hand with the experimental observations. Our study reveals that those isotopes of Z = 117 with A ≥ 299 and with A ≤ 271 do not survive fission and thus the alpha decay is restricted within the range 272 ≤ A ≤ 298. Through our study, we have predicted 1α chain from 272,273,296–298117, 2α chains from 274,275,295117, 3α chains from 276,277,292117 and 5α chains from 288–291117. As our study predicts 5α chains and 3α chains consistently from 288–291117 and 292117, respectively, we hope these findings will provide a new guide for future experiments.

The reaction 48Ca + 248Cm → 296116* studied at the GSI-SHIP

The synthesis of element 116 in fusion-evaporation reactions of a 48Ca beam with radioactive 248Cm targets was studied at the velocity filter SHIP of GSI in Darmstadt. At excitation energies of the compound nuclei of 40.9MeV, four decay chains were measured, which were assigned to the isotope 292116, and one chain, which was assigned to 293116. Measured cross-sections of (3.4−1.6+2.7) pb and (0.9−0.7+2.1), respectively, and decay data of the chains agree with data measured previously at the Flerov Laboratory of Nuclear Reactions in Dubna. As a new result, one α-decay chain was measured, which terminates after four α decays by spontaneous fission. The α energies of the second-to-fourth decay are considerably higher than those measured for the α decays of 289114, 285Cn, and 281Ds and the spontaneous fission half-life is significantly longer than that of 277Hs measured in previous experiments. A possible assignment is discussed in the frame of excited quasiparticle states of nuclei populated in the decay chain from 293116. Also other possible assignments were considered and are discussed. At an excitation energy of 45.0MeV no events were observed resulting in a one-event cross-section limit of 1.6 pb. The technical aspects related with the use of radioactive target material at SHIP are described in detail. The experience gained in this experiment will serve as a basis for future experiments aiming to study still heavier elements at the velocity filter SHIP.

Contribution of fission to heavy-element nucleosynthesis in an astrophysical r-process

During the formation of heavy elements in the neutron star merger (NSM) scenario with a fairly long duration of the r-process, most of the seed nuclei rapidly burn out at the initial stage. The nucleosynthesis wave rapidly reaches the region of actinoids, where beta-delayed, neutron-induced, and spontaneous fission are the main reaction channels. The fission products of transuranium elements are again drawn into the r-process as new seed nuclei to form the yields of elements with mass numbers A > 100. The contribution from the various types of fission to the formation of heavy and superheavy nuclei is investigated. The proposed r-process model applied to the NSM scenario describes well the observed abundances of chemical elements, which confirms the formation of the main r-process component in the NSM scenario. Simple extrapolations of the spontaneous fission half-lives are shown to be inapplicable for the region of nuclei with N ∼ 184, because the formulas do not reflect the increase in half-life when the shell structure changes as the number of neutrons approaches 184. The formation of superheavy elements in the r-process is possible, but their survival depends to a large extent on how reliable the predictions of nuclear parameters, including the half-lives of the forming nuclei from the island of long-lived isotopes, are. The contributions from various types of fission—neutron-induced, beta-delayed, and spontaneous one—to the formation of heavy elements in the main r-process have been determined.

Fission properties of the Barcelona–Catania–Paris energy density functional

Fission properties of the Barcelona–Catania–Paris (BCP) energy density functional are explored by performing constrained mean field Hartree-Fock-Bogoliubov (HFB) calculations along the fission path. These calculations provide us with the quantities required to estimate the spontaneous fission half lives and fragment mass distribution. The results obtained are compared to experimental data and other calculations.

αdecay chains in271−294115 superheavy nuclei

{alpha} decay of {sup 271-294}115 superheavy nuclei is studied using the Coulomb and proximity potential model for deformed nuclei (CPPMDN). The predicted {alpha} half-lives of {sup 287}115 and {sup 288}115 nuclei and their decay products are in good agreement with experimental values. Comparison of {alpha} and spontaneous fission half-lives predicts four-{alpha} chains and three-{alpha} chains, respectively, from {sup 287}115 and {sup 288}115 nuclei and are in agreement with experimental observation. Our study predicts two-{alpha} chains from {sup 273,274,289}115, three-{alpha} chains from {sup 275}115, and four-{alpha} chains consistently from {sup 284,285,286}115 nuclei. These observations will be useful for further experimental investigation in this region.

FISSION HALF LIVES OF FERMIUM ISOTOPES WITHIN SKYRME HARTREE-FOCK-BOGOLIUBOV THEORY

Nuclear fission barriers, mass parameters and spontaneous fission half lives of fermium isotopes calculated in a framework of the Skyrme Hartree-Fock-Bogoliubov model with the SkM* force are discussed. Zero-point energy corrections in the ground state are determined for each nucleus using the Gaussian overlap approximation of the generator coordinate method and in the cranking formalism. Results of spontaneous fission half lives are compared to experimental data.

SPONTANEOUS FISSION HALF-LIVES IN VARIOUS MACROSCOPIC-MICROSCOPIC MODELS

The problem of describing spontaneous fission for the entire region of superheavy nuclei is continually present in theoretical investigations. This study outlines the results of the calculations of fission barriers and spontaneous fission half-lives for superheavy nuclei in the region of Z =100-122 using a mean field model. We examine four distinct macroscopic models and two types of pairing interactions. Our approach is based on the deformed Woods-Saxon potential. Spontaneous fission half-lives are calculated using a multi-dimensional dynamical-programming method, where the action integral is minimized within a three dimensional deformation space (β2, β4, β6).

Systematic study of properties of Hs nuclei

The ground-state properties of Hs nuclei are studied in the framework of the relativistic mean-field theory. We find that the more relatively stable isotopes are located on the proton abundant side of the isotopic chain. The last stable nucleus near the proton drip line is probably the 255Hs nucleus. The \( \alpha\) -decay half-lives of Hs nuclei are predicted, and together with the evaluation of the spontaneous-fission half-lives it is shown that the nuclei, which are possibly stable against spontaneous fission are 263-274Hs . This is in coincidence with the larger binding energies per nucleon. If 271-274Hs can be synthesized and identified, only those nuclei from the upper Z = 118 isotopic chain, which are lighter than the nucleus 294118 , and those nuclei in the corresponding \( \alpha\) -decay chain lead to Hs nuclei. The most stable unknown Hs nucleus is 268Hs . The density-dependent delta interaction pairing is used to improve the BCS pairing correction, which results in more reasonable single-particle energy level distributions and nucleon occupation probabilities. It is shown that the properties of nuclei in the superheavy region can be described with this interaction.

Semi-empirical formula for spontaneous fission half life time

A new semi-empirical formula is proposed for determining the spontaneous fission half lives, which works well for the mass region from 232Th to 286114. The computed spontaneous fission half life times are also compared with other semi-empirical formula predictions. The alpha decay half lives are systematically computed for heavy and super heavy region with proton numbers varying from 90 ⩽ Z ⩽ 122 using Coulomb and Proximity Potential Model. The comparison between computed alpha decay half lives and the present spontaneous fission semi-empirical formula predictions of even–even isotopes with Z = 90 – 122 are studied. It is found that in super heavy region the isotopes 270–274Ds, 272–278112, 272–282114, 274–292116, 276–298118, 276–308120 and 278–314122 will survive fission and can be synthesized and identified via alpha decay.

The systematic study of spontaneous fission versus alpha decay of superheavy nuclei

The competition between the alpha decay and spontaneous fission half-lives of even–even elements with Z = 100–122 is studied using the Coulomb and proximity potential model and the phenomenological formula for spontaneous fission. The isotopes situated within the spontaneous fission half-life parabola will have alpha decay as the dominant mode of decay. We have studied the Seaborg plots connecting the spontaneous fission half-lives with the fissionability parameter (Z2/A) in the superheavy region for isotopes with Z = 100–138. The role of relative neutron excess in spontaneous fission half-lives is also studied. The isotopes in the first quadrant of the plot for the ratio of spontaneous fission to alpha decay half-lives against relative neutron excess will survive fission and can be synthesized and detected through alpha decay. Our computed alpha decay half-lives for all isotopes agree with the values calculated by the Viola–Seaborg empirical formula and with the experimental data.

FISSION PROPERTIES OF ODD-A NUCLEI IN A MEAN FIELD FRAMEWORK

Theoretical tools at the level of the mean field approximation are used to explore the spontaneous fission properties of odd-A nuclei. The tools rely on the equal (or uniform) filling approximation to deal with the unpaired nucleon in a time-reversal preserving manner. Realistic calculations have been carried out with the finite range Gogny force D1S, which was tailored to reasonably reproduce fission properties in the actinides. The preliminary results obtained for the nucleus 235 U are analyzed and the physical origin for the hindrance factor for the spontaneous fission half life is discussed.

Competition betweenαdecay and spontaneous fission for heavy and superheavy nuclei

We systematically investigate the {alpha}-decay and spontaneous fission half-lives for heavy and superheavy nuclei with proton number Z{>=}90. The {alpha}-decay half-lives are obtained by the deformed version of the density-dependent cluster model (DDCM). In the DDCM, the microscopic potential between the {alpha} particle and the daughter nucleus is evaluated numerically from the double-folding model with the M3Y interaction. The influence of the core deformation on the double-folding potential is also properly taken into account by the multipole expansion method. The spontaneous fission half-lives of nuclei from {sup 232}Th to {sup 286}114 are calculated with the parabolic potential approximation by taking nuclear structure effects into account. The agreement between theoretical results and the newly observed data is satisfactory for both {alpha} emitters and spontaneous fission nuclei. The competition between {alpha} decay and spontaneous fission is analyzed in detail and the branching ratios of these two decay modes are predicted for the unknown cases.

Search for long lived heaviest nuclei beyond the valley of stability

The existence of long lived superheavy nuclei (SHN) is controlled mainly by spontaneous fission and \ensuremath{\alpha}-decay processes. According to microscopic nuclear theory, spherical shell effects at $Z=114$, 120, 126 and $N=184$ provide the extra stability to such SHN to have long enough lifetime to be observed. To investigate whether the so-called ``stability island'' could really exist around the above $Z$, $N$ values, the \ensuremath{\alpha}-decay half-lives along with the spontaneous fission and \ensuremath{\beta}-decay half-lives of such nuclei are studied. The \ensuremath{\alpha}-decay half-lives of SHN with $Z=102$--120 are calculated in a quantum tunneling model with DDM3Y effective nuclear interaction using ${Q}_{\ensuremath{\alpha}}$ values from three different mass formulas prescribed by Koura-Uno-Tachibana-Yamada (KUTY), Myers-Swiatecki (MS), and Muntian-Hofmann-Patyk-Sobiczewski (MMM). Calculation of spontaneous fission (SF) half-lives for the same SHN are carried out using a phenomenological formula and compared with SF half-lives predicted by Smolanczuk et al. A possible source of discrepancy between the calculated \ensuremath{\alpha}-decay half-lives of some nuclei and the experimental data of GSI, JINR-FLNR, RIKEN, is discussed. In the region of $Z=106$--108 with $N~$ 160--164, the \ensuremath{\beta}-stable SHN ${}_{106}^{268}{\text{Sg}}_{162}$ is predicted to have highest \ensuremath{\alpha}-decay half-life (${T}_{\ensuremath{\alpha}}~3.2$ h) using ${Q}_{\ensuremath{\alpha}}$ value from MMM. Interestingly, it is much greater than the recently measured ${T}_{\ensuremath{\alpha}}$ ($~22$ s) of deformed doubly magic ${}_{108}^{270}{\text{Hs}}_{162}$ nucleus. A few fission-survived long-lived SHN which are either \ensuremath{\beta}-stable or having large \ensuremath{\beta}-decay half-lives are predicted to exist near ${}^{294}{110}_{184}$, ${}^{293}{110}_{183}$, ${}^{296}{112}_{184}$, and ${}^{298}{114}_{184}$. These nuclei might decay predominantly through \ensuremath{\alpha}-particle emission.

Synthesis of rutherfordium isotopes in theU238(Mg26,xn)264−xRfreaction and study of their decay properties

Isotopes of rutherfordium (258-261Rf) were produced in irradiations of 238U targets with 26Mg beams. Excitation functions were measured for the 4n, 5n and 6n exit channels. Production of 261Rf in the 3n exit channel with a cross section of 28+92-26 pb was observed. Alpha decay of 258Rf was observed for the first time with an alpha-particle energy of 9.05+-0.03 MeV and an alpha/total decay branching ratio of 0.31+-0.11. In 259Rf, the electron capture/total decay branching ratio was measured to be 0.15+-0.04. The measured half-lives for 258Rf, 259Rf and 260Rf were 14.7+1.2-1.0 ms, 2.5+0.4-0.3 s and 22.2+3.0-2.4 ms, respectively, in agreement with literature data. The systematics of the alpha decay Q values and of the partial spontaneous fission half-lives were evaluated for even-even nuclides in the region of the N = 152, Z = 100 deformed shell. The influence of the N = 152 shell on the alpha decay Q values for rutherfordium was observed to be similar to that of the lighter elements (96<_ Z<_ 102). However, the N = 152 shell does not stabilize the rutherfordium isotopes against spontaneous fission, as it does in the lighter elements (96<_ Z<_102).

SPONTANEOUS FISSION HALF LIVES OF Z = 112 ISOTOPES

Barriers and spontaneous fission half lives of superheavy isotopes of Z = 112 (N = 150–190) nucleus are calculated using state dependent δ-pairing and both BCS or Lipkin-Nogami (LN) approach. Four different models of macroscopic part of Strutinsky energy are probed. Results for different macroscopic models combined with δ+ LN approach give the half lives well compared to experimental data. The calculations are performed in three dimensional space of deformation parameters β = {β2, β4, β6}. The probability of spontaneous fission has been obtained by dynamical action minimization method.

ROLE OF THE ZERO-POINT CORRECTIONS IN FISSION DYNAMICS

The way of evaluating spontaneous fission half-lives of nuclei in a multidimensional deformation space is discussed. The cranking as well as the generator coordinate method were used to obtain the collective inertia tensor and the 'zero-point' corrections to the fission barriers. The fission probability was evaluated within the WKB approximation along the least-action trajectory to fission. The influence on the fission life-times of the dynamics in the pairing degrees of freedom as well as the effect of the higher even-multipolarity shape parameters and the role of the reflection asymmetry is examined.