Coincident Fission Fragments Research Articles

Absolute probabilities of fission induced by stopped antiprotons

Absolute probabilities for the fission induced by stopped antiprotons have been determined for U, Th, Au, Ho by measuring the fission fragments in coincidence. The numbers of antiprotons stopped in the targets were obtained from an activation analysis. The fission probabilities are 77(4)%, 70(4)% and 3.1(3)% for 238U, 232Th and 197Au. For 165Ho an upper limit of 1.1% was determined. The results are compared with intranuclear cascade calculations.

Excitation and fission decay of 232Th by inelastic scattering of 17O at 84 MeV/u

Inelastic scattering of 84 MeV/u 17O projectiles has been used to excite the giant resonances (GR) in 232Th. The deduced parameters (width, energy, EWSR) of the isoscalar giant quadrupole resonance are in agreement with the known systematics. The observed giant monopole resonance strength, 100% of the EWSR, is consistent with other recent experimental results using heavy-ion projectiles. Strength is also observed at high excitation energy. The fission probability of GR in 232Th has been measured by detecting the fission fragments in coincidence with the inelastically scattered particles. A structure is observed in the fission coincidence spectrum around 10 MeV, which can be attributed to the fission decay of giant resonances. The measured fission probability is consistent with statistical decay of the GQR.

Spontaneous fission properties of 103259Lr.

We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission of $^{259}\mathrm{Lr}$. The $^{259}\mathrm{Lr}$ was produced via the $^{248}\mathrm{Cm}$ ${(}^{15}$N,4n) reaction with a production cross section of 100 nb using 81-MeV projectiles. The kinetic energies and times of the alpha particles and coincident fission fragments were measured using our rotating wheel system. From these data the half-life, mass, and kinetic-energy distributions were derived. The total kinetic-energy (TKE) distribution appears to consist of a single component with a most probable pre-neutron-emission TKE of 215\ifmmode\pm\else\textpm\fi{}3 MeV. The mass distribution is predominantly symmetric with a full width at half maximum of about 20 mass numbers. These results are consistent with trends observed for other trans-berkelium spontaneously fissioning isotopes. We determined the half-life to be 6.14\ifmmode\pm\else\textpm\fi{}0.36 s by measuring its alpha decay and the observed spontaneous fission half-life was consistent with that value. An energy of 8.439\ifmmode\pm\else\textpm\fi{}0.010 MeV was measured for the main alpha transition of $^{259}\mathrm{Lr}$. We measured a spontaneous fission to alpha-decay ratio of 0.25\ifmmode\pm\else\textpm\fi{}0.03 which results in a partial half-life for spontaneous fission of 31\ifmmode\pm\else\textpm\fi{}4 s, if there are no other appreciable modes of decay.

Dissipation-fluctuation dynamics for fusion, deep-inelastic collisions, and heavy-ion induced fission with particle evaporation

It is shown on selected examples how heavy-ion collisions (below 15 MeV/u) can be described by Langevin Monte Carlo dynamics. Fusion excitation functions, fusion spin distributions and Wilczynski diagrams of deep-inelastic collision (DIC) are calculated and compared with data. The bifurcation of fusion and DIC down to subbarrier energies is demonstrated. Time scales of DIC are obtained from calculating energy spectra of σ-electrons (atomic clock). By combining a (one-dimensional, overdamped) Langevin dynamics for the fission process with the statistical model (when the Kramers limit is reached after a transient time) we are not only able to calculate neutron multiplicities but also fission probabilities and (H.I.,xn) cross sections. Extending these investigations to a two-dimensional (elongation and neck) Langevin dynamics we obtain also kinetic energy distribution of fission fragments in coincidence with neutron multiplicities. By applying this model to a multi-dimensional decay rate for fission (derived with path integral methods) we calculate quantal corrections to the quasistationary Kramers result, which are not small even at temperatures reached in heavy-ion induced fission.

Light particle emission in fusion-like 14N + 197Au collisions at E/ A = 35 MeV

Single- and two-particle inclusive cross sections for light nuclei (p, d, t, 3,4He, 6–9Li, 7,8,10Be) were measured for 14N induced reactions on 197Au at E/ A = 35 MeV. At forward angles, θ ≈ 20°, quasi-elastic peripheral reactions and more violent fusion-like collisions were discriminated by measuring the folding angle between two coincident fission fragments resulting from the decay of the heavy reaction residue. More pronounced correlation functions, but very similar emission temperatures are observed for fusion-like collisions as compared to peripheral collisions.

Symmetric splitting for the system 32S+238U at energies near and below the barrier.

The total capture cross section for the system /sup 32/S+/sup 238/U has been measured at energies from 0.93 to 1.08 times the s-wave interaction barrier by detecting coincident fission fragments following full momentum transfer reactions. The subbarrier cross section cannot be reproduced by a one-dimensional barrier penetration model. Using a quantum mechanical coupled channels model, good agreement is obtained. The measured angular distributions of fission fragments were compared to the predictions of saddle and scission point transition state theory. Saddle point transition state model calculations fail to reproduce the data, while scission point transition state calculations are in agreement with their qualitative trend. Evidence for nonequilibrium processes is presented.

Linear momentum transfer in 40-150 MeV proton-induced reactions with 238U.

Angular correlations between coincident fission fragments have been measured for 40-, 90-, and 150-MeV proton-induced reactions on a /sup 238/U target. From these data linear momentum transfer distributions have been derived, which are then compared with other results for protons and more complex projectiles. On a per nucleon basis, protons exhibit the largest average linear momentum transfer values of any projectile over the energy range from E/A = 40 to 1000 MeV. This can be understood in terms of fundamental scattering cross sections, and is qualitatively demonstrated by intranuclear cascade calculations. Both cascade and precompound decay calculations reproduce the linear momentum transfer distributions with moderate success.

A low background detection system for fragments and neutrons from spontaneous fission sources

A low background system for the detection and characterization of spontaneously fissioning nuclides has been developed. It allows the simultaneous registration of single and coincident fission fragments and their kinetic energies, and of the number of neutrons emitted per fission event together with the time intervals between fission-fragment and neutron pulses. In addition, α-particle spectra can also be measured. The system consists of six pairs of surface barrier detectors surrounded by 30 3He-filled neutron counters embedded in paraffin. It is placed between two large plastic scintillation detectors in order to identify cosmic-ray induced background events. The detector assembly has been used in the search for superheavy elements in reactions of 238U projectiles with 238U and 248Cm targets and of 48Ca with 248Cm at production rates of a few atoms per experiment.

Angular distributions of light charged particles from 252Cf fission in the ranges 0–46° and 134–180°

Abstract Alpha particles, tritons, deuterons and protons accompanying 252 Cf fission were registered in coincidence with both fission fragments by means of a system containing two-dimensional position-sensitive silicon detectors. Angular distributions, kinetic energy spectra of light charged particles as well as mass distributions of fission fragments in coincidence with light charged particles were measured. The experimental results are compared with some theoretical models.

Fission of heavy nuclei induced by energetic pions.

Fission of /sup 238/U, /sup 209/Bi and /sup 197/Au induced by both positive and negative pion beams at incident energies of 60 to 100 MeV is examined. Energy and mass spectra and angular correlations of coincident fission fragments are compared to theoretical calculations and to other data. Excitation energies of the fissioning systems in excess of 100 MeV are deduced, implying that the fission examined in this study is caused primarily by true absorption of the pion. The masses of the coincident fission fragments are found to have a wider distribution than those from fissioning systems of similar excitation energy induced by nucleon beams. The fission cross sections are close to the reaction cross sections for /sup 238/U and scale with the fissility for the other nuclei studied.

Mass and kinetic energy distributions in near-barrier fission ofW182

Mass and kinetic energy distributions have been measured for the fission of /sup 182/W at an excitation energy above the fission barrier E(-E/sub f/roughly-equal20 MeV. A primary motivation for these studies was to search for evidence of asymmetric mass division and anomalously low total kinetic energy release, as predicted by scission-point model calculations which include deformed (Strutinsky) shell effects. Double kinetic energy measurements of coincident fission fragments were performed, from which fragment mass distributions were derived. From the data it is concluded that at this excitation energy, liquid-drop behavior dominates the fission of /sup 182/W and any shell strength, if present, is less than 20 percent of full strength.

Fission ofEs255,256,Fm255−257, andMd258at moderate excitation energies

The fission of 255,256Es, 255-257Fm, and 258Md has been studied in the excitation energy range from threshold to 25 MeV. A target of 254Es was used in the direct reaction studies; (d,pf), (t,pf), (3He,df), (3He,pf), and in the compound induced fission reactions formed with p, d, t, and α particle projectiles. Coincident fission fragment energies were recorded along with (in the direct reaction studies) the outgoing light charged particle. The mass and kinetic energy distributions were studied as a function of nuclear excitation energy. The observed bulk properties were consistent with established systematics in that they exhibited an asymmetric mass distribution and a phenomenologically consistent total kinetic energy. However, the systems demonstrated a fission decay mode which we ascribe to high energy symmetric fission decay. This component, though somewhat arbitrary in its definition, showed a general decrease in yield as a function of increasing nuclear excitation energy. This observed rapid change in fission properties between normal and high energy symmetric fission probably points to the important observable consequences that can occur from small variations in the potential energy surface.

Fission fragments in coincidence with quasi-projectile in the reactions of 16O on 232Th and 197Au at 31 MeV/nucleon

We studied the linear momentum transfer in the reaction 16O on 232Th and 197Au at 31 MeV/u. For both targets, the most probable transferred linear momentum is 3. GeV/ c corresponding to 80% of the incident momentum. A linear dependence between the θ AB fission fragment correlation angle and the detected ejectile mass is observed. For projectile like fragments, the most probable value of θ AB is associated with beam velocity events. For light particles, we observe the dominance of large momentum transfer components whatever the measured energies.

Limitations on linear momentum transfer in 14N induced reactions on 238U at [formula omitted

Linear momentum transfer distributions have been dermimined for 14N induced reactions on 238U by measuring the folding angle between two coincident fission fragments. The data follow the systematic trends previously established for alpha conjugate projectile nuclei. Over the range of energies investigated, the most probable linear momentum transfer has not reached a limiting value. Extrapolation of the present results indicates that complete fusion should cease to be a relevant reaction mechanism above incident energies of E A =40−45 MeV

Fission of polonium, osmium, and erbium composite systems

Fission cross section excitation functions were measured from near threshold to \ensuremath{\sim}10 MeV/nucleon using $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16,18}\mathrm{O}$, $^{24,26}\mathrm{Mg}$, $^{32}\mathrm{S}$, and $^{64}\mathrm{Ni}$ beams. The systems studied included $^{210}\mathrm{Po}$ formed in $^{12}\mathrm{C}$ and $^{18}\mathrm{O}$ induced reactions; $^{186}\mathrm{Os}$ formed in $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{26}\mathrm{Mg}$ reactions; and $^{158}\mathrm{Er}$ formed in $^{16}\mathrm{O}$, $^{24}\mathrm{Mg}$, $^{32}\mathrm{S}$, and $^{64}\mathrm{Ni}$ reactions. In addition, the composite systems $^{204,206,208}\mathrm{Po}$ formed with $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$ projectiles were studied. In the experiment the velocities and emission angles of two coincident fission fragments were measured using position-sensitive multiwire proportional counter stop detectors and a thin (\ensuremath{\sim}200 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) gas start detector. The measured fission excitation functions along with previous data from $^{4}\mathrm{He}$ and $^{11}\mathrm{B}$ bombardments for the $^{186}\mathrm{Os}$ and $^{210}\mathrm{Po}$ systems and recent data on the $^{200}\mathrm{Pb}$ system are compared to statistical model calculations using recent angular momentum dependent fission barriers calculated by Sierk. Comparisons of calculated and measured fission excitation functions show good overall agreement between data and theoretical predictions. These comparisons give good agreement for both a wide range of mass of the composite system and for a wide range of mass asymmetry in the entrance channel. It is concluded that the fission barriers of Sierk give a good description of both the mass and angular momentum dependence of fission barriers in this region.

Momentum transfer to the target in peripheral collisions of relativistic heavy ions

The angular correlations between coincident fission fragments emitted in the reactions of /sup 197/Au with 5-GeV protons, 1.25-GeV/nucleon /sup 4/He, and 0.25--2.10-GeV/nucleon /sup 20/Ne were measured. The longitudinal momentum transfer to the target calculated from these correlations increased with increasing mass loss from the target and was inversely proportional to the projectile velocity, independent of projectile mass. These observations are shown to be consistent with simple two-body models of peripheral reactions.

Unusually low fragment energies in the symmetric fission ofMd259

The 103-min isotope $^{259}\mathrm{Md}$ has been identified as the daughter of an electron-capture decay branch of $^{259}\mathrm{No}$ produced via the $^{248}\mathrm{Cm}(^{18}\mathrm{O},\ensuremath{\alpha} 3n)$ reaction. Chemical separations were used to confirm the identity of $^{259}\mathrm{Md}$, which decays by spontaneous fission. The kinetic energies of coincident fission fragments were measured, corresponding to a fragment mass which is highly symmetric, similar to those of $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$. However, the total kinetic energy distribution for $^{259}\mathrm{Md}$ is considerably broader (FWHM \ensuremath{\sim}60 MeV) than those of $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$, and peaks at 201 MeV, about 35-40 MeV lower in energy. Furthermore, the maximum total kinetic energy of 215 MeV for mass-symmetric events is about 30 MeV lower than for similar events from the spontaneous fission of $^{258}\mathrm{Fm}$ and $^{259}\mathrm{Fm}$. A hypothesis that this energy difference resulted from the emission of light, hydrogen-like particles at scission in a large fraction of $^{259}\mathrm{Md}$ spontaneous fission decays was shown to be unfounded. From experiments to observe such particles with counter telescopes, an upper limit of 5% was determined for the fraction of fission events accompanied by light-particle emission. The total kinetic energy deficit at mass symmetry must, therefore, be distributed between internal excitation energy and fragment deformation energy at scission. Although the presence of a large amount of fragment deformation energy seems incompatible with symmetric fission into spherical Sn-like fragments, we prefer this explanation because the low total kinetic energy suggests a lowered Coulomb energy resulting from greater separation of the charge centers of deformed fragments at scission.RADIOACTIVITY, FISSION $^{259}\mathrm{Md}$ (SF); measured ${T}_{\frac{1}{2}}$, fragment-fragment coin, deduced mass, TKE distributions. $^{259}\mathrm{No}$; measured EC decay to $^{259}\mathrm{Md}$, upper limit to SF decay.

Angular momentum, statistical equilibrium and sequential fission in very asymmetric systems

The in-and out-of-plane angular distributions for fission fragments in coincidence with projectile-like products from the reaction of 252 MeV20Ne with197Au and238U have been measured. The results are compared to a statistical model which has successfully explainedγ-ray anisotropies from a heavy symmetric system. The agreement is rather good after proper consideration of the direction of the line-of-centers at contact.

The kinetic energy deficit in the symmetric fission of 259Md

Abstract

Light-particle spectra observed in central and peripheral collisions of 16O + 238U at 20 MeV/nucleon

Energetic light particles have been measured in coincidence with two outgoing fission fragments which results from the reaction 16O + 238U at 315 MeV. Central and peripheral collisions can be distinguished by the momentum transferred to the target residue as deduced from the angle between coincident fission fragments. A remarkable similarity is observed in the forward angle light particle spectra observed in peripheral and central collisions.