Light Charged Particle Emission Research Articles

Excitation energies in statistical emission of light charged particles in heavy-ion reactions.

Light charged particle emission has been investigated as a function of excitation energy in exclusive experiments on the decay of $^{16}\mathrm{O}$, $^{24}\mathrm{Mg}$, and $^{35}\mathrm{Cl}$ projectiles between 25A and 70A MeV. The systematics of excitation energy removed by Z=1 and Z=2 particles were deduced. The results are similar to a previous study of proton and \ensuremath{\alpha}-particle evaporation in compound nucleus reactions at beam energies below 20A MeV, supporting the idea of a common statistical process.

Nuclear surface, mean field and isospin effects in Boltzmann master equation theory of pre-equilibrium reactions

Pre-equilibrium emission of nucleons and light charged particles is investigated in central collisions leading to the complete fusion of two heavy ions, within the Boltzmann master equation approach. Realistic nucleon momentum density distributions for both projectile and target nuclei are used and mean field effects are schematically introduced in the early stage of the reaction process. The isospin dependence of the calculation parameters is also discussed. Theoretical predictions of the spectra of the particles emitted both in symmetric and asymmetric interactions at incident energies of a few ten MeV/nucleon are in satisfactory agreement with the experimental findings.

Emission of alpha particles during the spontaneous fission ofPu244

We have measured the light charged particle (LCP) emission probabililty for the spontaneous fission of $^{244}\mathrm{Pu}$. For the LCP emission to binary fission probability ratio a value of (2.96\ifmmode\pm\else\textpm\fi{}0.31)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ was found. This result is in agreement with the previously observed trends concerning the LCP emission probability.

Light charged particle and intermediate mass fragment emission in the reaction 640 MeV 86Kr+63Cu.

Light charged particles from the reaction 640 MeV {sup 86}Kr+{sup 63}Cu have been measured in singles and in coincidence with intermediate mass fragments, fissionlike fragments, and other light charged particles. Multiplicities for {sup 1}H and {sup 4}He in association with the evaporation residues, fragments, and intermediate mass fragments have been determined. Composite nuclei, most of which decay to evaporation residues, are the major sources of evaporative light-charged particle emission. Average multiplicities for prescission {sup 1}H and {sup 4}He can be associated with the composite nucleus {ital en} {ital route} to scission; they are relatively large and thus suggest a time scale for fission longer than that for evaporation. The multiplicities for {sup 1}H and {sup 4}He in association with the intermediate mass fragments indicate that these fragments are usually born with a significant quantity of excitation energy. These multiplicities are used to estimate the primary masses and kinetic energies of the intermediate mass fragments.

Emission of light charged particles in photon induced fission.

The emission of light charged particles in the photofission of $^{230,232}\mathrm{Th}$, $^{233,234,235,238}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{242}\mathrm{Pu}$ has been studied with bremsstrahlung with end-point energy of 12, 15, and 20 MeV. The light charged particles are measured using a setup consisting of eight \ensuremath{\Delta}E-E particle identification detector telescopes. The kinetic energy distributions and the emission probabilities of tritons and long-range alpha particles were determined. The presence of photoprotons obscures the identification of ternary protons while $^{6}\mathrm{He}$ particles were observed clearly, however, with insufficient statistics to allow meaningful conclusions concerning their energy distribution or emission probability. The average kinetic energy of the ternary tritons and \ensuremath{\alpha} particles and the width of the triton kinetic energy distributions seems to be practically independent of the compound nucleus excitation energy or ${\mathit{Z}}^{2}$/A. Only the width of the long-range alpha particle kinetic energy distribution shows a slight increase with increasing ${\mathit{Z}}^{2}$/A of the compound nucleus. Our results point toward a slight increase of the ternary alpha particle yield with increasing excitation energy of the compound nucleus. As already observed by Wild et al. for spontaneously fissioning nuclei, this yield shows a clear correlation with 〈Q〉-〈${\mathit{E}}_{\mathrm{tot}}^{\mathrm{*}}$〉, where ${\mathit{E}}_{\mathrm{tot}}^{\mathrm{*}}$ is the total kinetic energy, suggesting that the light charged particle emission probability is determined by the deformation of the system at scission, and that, for the compound nucleus excitation energy studied in our experiments, the excitation energy transferred to the compound nucleus remains in the system as internal heat, and is not transformed into deformation energy.The triton emission to binary fission ratio, t/B, for $^{237}\mathrm{Np}$ is not substantially higher than for the neighboring nuclei, indicating that the unpaired proton in $^{237}\mathrm{Np}$ does not influence in an important way the triton emission probability.

16O breakup in quasielastic reactions near the Fermi energy.

The quasielastic breakup of an $^{16}\mathrm{O}$ nucleus at 32.5 MeV/nucleon on a $^{197}\mathrm{Au}$ target has been investigated by measuring the projectilelike fragments in coincidence with the emitted light charged particles. It is found that the breakup proceeds via sequential, two-step processes in which inelastic scattering and one-neutron pickup are the most significant components in the formation of the primary fragment. This is followed by statistical decay of the highly excited primary fragment. A Monte Carlo simulation was used to identify these processes and to deduce differential and total cross sections for the breakup of $^{16}\mathrm{O}$ into the \ensuremath{\alpha}${+}^{12,13}$C channels.

Nuclear density of states for moving fused compound systems.

The single-particle distribution function of moving fused compounds, with velocities of the order of a few tenths of the velocity of light, are calculated using the discrete spectrum of single-particle states of deformed nuclei. The polarization of single-particle momenta in the direction of flight is clearly brought out. Results are presented for [sup 40]Ca, [sup 64]Ni, [sup 71]As, and [sup 90]Zr. For small temperatures when the internal excitations are small, the nuclear level density varies as [ital E]([ital E][sup 2][minus][ital M][sup 2])[sup 1/2] where [ital E] is the total energy of the moving nuclei, and, as excitation increases, the internal degrees of freedom dominate and the level density [rho][similar to][ital e][sup [ital S]], where [ital S] is the entropy of the system, varies exponentially. The level densities thus calculated are used to evaluate light charged-particle emission from hot fused compounds in motion.

Dynamical effects on the de-excitation of hot nuclei with A

Light charged-particle emission and neutron emission have been measured for the fusion-evaporation and fusion-fission channels in the $^{60}\mathrm{Ni}$${+}^{100}$Mo reaction at 550 and 655 MeV bombarding energies. Temperatures, emission barriers, and multiplicities for the particles detected in coincidence with evaporation residues and fission fragments have been determined. For the evaporation residue data, the ``first-chance'' spectra of particles emitted from the compound nucleus have been isolated and the same initial temperature for the different evaporated particles has been extracted. The inverse level density parameter K=A/a reaches a value of 13.8\ifmmode\pm\else\textpm\fi{}0.7 MeV at ${\mathit{E}}_{\mathrm{th}}$\ensuremath{\simeq}236 MeV. A decrease of the apparent emission barriers for the charged-particle emission at high excitation energy suggests dynamical effects on the de-excitation process. Analysis of the fissionlike events shows a saturation of light particle emission from the fission fragments. Using the average energies and multiplicities of the emitted neutrons, and charged particles, it has been found that for both initial excitation energies, 251 and 293 MeV, scission occurs at an excitation energy \ensuremath{\simeq}140 MeV. For both fusion evaporation and fusion fission, the light charged particles are preferentially emitted during the early part of the de-excitation cascade. Statistical models and dynamical calculations have been used in an attempt to interpret the experimental data.

Light-charged particle emission in fission

The emission of light charged particles in the fission process is of interest as they are believed to emerge from the neck region of the deformed fissioning nucleus at a time close to the scission point and may thus prove a useful probe to investigate the last stages of fission close to the rupture point. Experimental results on light charged particle emission and the efforts made to obtain information about the scission point parameters therefrom are reviewed.

Preequilibrium emission in central collisions of the mass-symmetric system 40Ar+ 40Ca at 20 MeV/u

In the mass symmetric system 40Ar+ 40Ca the emission of neutrons and light charged particles produced in central collisions was studied at an incident energy of 20 MeV/u. Heavy reaction products and light particles were measured inclusively as well as exclusively. The heavy reaction product data allow a separation of central and peripheral collisions. The mass difference between the observed heavy fragments and the compound system can be explained by the emission of light particles with Z ⩽ 2. For the neutrons clear evidence for preequilibrium effects is found from the shape of the energy spectra and angular distributions. The preequilibrium and equilibrium multiplicities and nuclear temperatures are extracted from the data. The experimental results are compared to various models. From a quantal phase-space model an average equilibration time is derived.

Polar light charged particles accompanying spontaneous fission

Simple model of emission of light charged polar particles accompanying fission of heavy nuclei is presented. We assume the emission from the whole surface of fragments after scission, taking into account the influence of the accompanying fragment. The initial energy and time of emission are treated phenomenologically. The motion of light particles and fission fragments are subsequently calculated in the framework of classical dynamics. We get a good agreement with experimental data if we assume a prompt emission. The possibility of the connection with giant resonances is discussed.

Decay of deformed 59Cu nuclei.

The light charged particle emission following the fusion of $^{32}\mathrm{S}$ with $^{27}\mathrm{Al}$ at 100--150 MeV bombarding energies in 10 MeV steps has been studied. Energy spectra and angular distributions of p, d, t, and alpha particles as well as light-particle--light-particle coincidences have been measured. The light charged particle data and the published evaporation residue distributions are compared with statistical model calculations. The comparison shows that the statistical model describes well the experimental observables if account is taken of changes in the level density and barrier apparently due to the onset of deformations at high spins. The spectra of \ensuremath{\alpha} particles emitted from narrow windows of angular momentum with 28\ensuremath{\Elzxh}\ensuremath{\le}${J}_{\mathrm{ave}\mathrm{\ensuremath{\le}}34\mathrm{\ensuremath{\Elzxh}}}$ have been obtained by means of a subtraction procedure and analyzed to extract emission barriers and level density enhancement factors. The deduced barrier lowering with respect to the spherical barrier is somewhat greater than that predicted by assuming rotating liquid drop model (RLDM) shapes. The level density enhancement factor is comparable with that determined in the past for statistically deformed nuclei.

Mechanisms for light charged particle emission in the reactions 247 and 337 MeV 40Ar+natAg.

Light charged particles from the reactions 247 and 337 MeV $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag have been measured in singles and in coincidence with fission-like products. Most of the $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in coincidence with fusion-fission and quasifission can be accounted for by evaporation from the composite nucleus or by the fragments. Multiplicities are determined for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in association with evaporation residues, fusion-fission, and quasifission. The probability for composite nucleus emission decreases with increasing spin zone of the emitter and gives a view of the role of emitter lifetime. The ratio of composite-nucleus multiplicities for quasifission/fusion-fission is about 1/2 ((1/4) for 337 MeV $^{40}\mathrm{Ar}$ (247 MeV $^{40}\mathrm{Ar}$) which implies a similar ratio for the lifetimes of the respective composite nuclei between thermalization and scission. Energy spectra and angular distributions of particles (in coincidence with fusion-fission and quasifission) are consistent with emission from a strongly distorted nuclear system en route toward scission. Prethermalization emission of $^{4}\mathrm{He}$ is observed at 10\ifmmode^\circ\else\textdegree\fi{} in comparable amounts for peripheral collisions (quasifission and fusion-fission) and for central collisions.

Decay properties of highly excited nuclei produced in fast dissipative collisions

At bombarding energies of 10–20 MeV/u, fast dissipative collisions are found to represent a smooth extension of deep inelastic reactions, towards the intermediate energy regime.Although the main trends of the low energy systematics are confirmed new features emerge. In collisions among ( A ≈-100)-nuclei the large amount of kinetic energy deposited is used as a tool to produce nuclei of known excitation energy, up to values of ≈-=1 GeV. The evolution of the decay properties, with the emission of light charged particles as well as complex fragments and fission products, can be investigated continuosly as a function of excitation energy exploiting the full range of kinetic energy losses.The time scale of the emission can be investigated from proximity effects.

Light charged particle emission in40Ar induced reactions on68Zn at 14.6, 19.6 and 35 MeV/nucleon

The light charged particles emitted in the40Ar+68Zn reaction performed at 14.6, 19.6 and 35 MeV/nucleon have been studied inclusively. The energy spectra have been analysed in terms of preequilibrium emission, moving source and coalescence models. A complete coherence between the present data and those obtained separately from heavy fragment studies is achieved. These data are consistent with the onset of projectile fragmentation between 20 and 35 MeV/nucleon correlated with the formation of a highly excited region of the nuclear system inducing light particle emission. The other sources of light particles can be interpreted as statistical and sequential decay of the quasi projectile.

Light charged particle emission in 485 MeV 56Fe + 197Au reactions: Correlations with heavy fragments and relationships to spin and lifetime

Emission of 4He and 1H has been studied in reactions of 485 MeV 56Fe + 197Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the 4He/ 1H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed 4He/ 1H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for 4He and 1H are deduced for each source of emission, and are compared with results from similar systems. The experimental 4He/ 1H multiplicity ratios indicate roughly equal probability for 4He and 1H evaporation from a given excited source. The observation of pre-scission 4He/ 1H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable 4He/ 1H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

Studies of light-charged particle emission in fission at Trombay

Studies of prompt radiations emitted in fission were started at Trombay in the late 1950’s by Dr R Ramanna and over the years extensive investigations on the emission of prompt neutrons, gamma ray and K x-rays in fission were carried out with neutron beams fromapsara andcirus reactors. In the early 1960’s studies on the emission of light-charged particles in fission, which is a rare mode of fission, were also started. This paper reviews some of the recent studies on the emission of light-charged particles (lcp) in fission which were carried out with a view to investigate the mechanism oflcp emission, the scission configuration and the dynamics of the last stages of the fission process.

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive4He and4H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u238U with238U and−197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For238U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For238+238U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In238U+197Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.

Emission of light charged particles and associated heavy partners in reactions induced on Ni by C projectiles at 30, 60 and 84 MeV/u

Measurements of light charged particle emission and of associated heavy nuclei have been used to explore reaction mechanisms in the interaction of 12C projectiles at 30, 60 and 84 MeV/u with a Ni target. In this intermediate-energy range it is shown that the main characteristics for both the low-energy light particles and the target-like nuclei evolve very gently. There is no indication in the present data of a rapid change in the reaction mechanisms when exploring interactions with beam velocities larger than the speed of sound in nuclear matter or larger than the Fermi velocity.

Emission of high-energy charged particles at 0° in Ne-induced reactions

Abstract Inclusive energy spectra have been measured for light charged particles emitted in the bombardment of 232Th, 181Ta, natTi and 12C targets by 22Ne ions at 178 MeV and natTi by 20Ne ions at 196 MeV. The reaction products were analysed and detected by means of a ΔE − E telescope placed in the focal plane of a magnetic spectrometer located at an angle of 0° with respect to the beam direction. In all the reactions studied light charged particles with an energy close to the respective calculated kinematic limit for a two-body exit channel are produced with relatively great probability. The results obtained make it possible to draw some conclusions about the reaction mechanism involving the emission of light charged particles.