Fission accompanied by alpha particle emission in the12C(85MeV)+232Th reaction

Inclusive and coincident spectra of alpha particles and fission fragments were measured for the²³²Th+¹²C (85 MeV) reaction to study the influence of the excitation energy and the angular momentum on the fission of the compound nucleus and to separate different alpha particle emission mechanisms. At backward angles α emission can be accounted for by the evaporation processes. At forward angles the most important contribution is given by the break-up fusion process. Mass distributions for compound nuclei²⁴⁴Cm (E^*=58 MeV,ff coincidences), and²⁴⁰Pu (E^*=37 MeV,ff α coincidences) were obtained. In the case of²⁴⁰Pu mass distribution has a shape different from those obtained in light ion reactions at the same excitation energy, indicating the strong influence of the entrance channel. The dependence of the mass distribution shape on the α particle energy is also examined.     

Influence of shell effects on the angular distributions of fission fragments

A dynamical-statistical model is used to analyze the experimental angular distributions of fission fragments in the reactions α + ²³⁸U, ²³⁷Np at E _α = 20–100 MeV, as well as to determine the Am isotope fission probabilities and the shape isomer yields in the reactions d + ^242,240Pu at E _d = 20–30 MeV. Manifestations of shell effects are found in the fission barrier structure up to the excitation energies of 50–60 MeV.     

Linear momentum transfer in the reaction 232Th(14N,light particles) AT 208 MeV

The longitudinal momentum transferred to a target nucleus (ΔP) has been studied for light-particle emission in the ¹⁴N-induced reaction on ²³²Th at 208 MeV. The transferred momentum was deduced by measuring folding angles between two fission fragments resulting from the sequential decay of the target-like nucleus. It was found that the fraction $\text{?}{}_2\text{=}\text{(〈ΔP〉 + P}{}_{\mathrm{<mtext>out</mtext>}}{}^{\mathrm{\parallel }}\text{)}\text{P}{}_{\mathrm{<mtext>beam</mtext>}}$ averaged over light-particle energy was about 0.68, almost independent of light-particle species and detection angle, where P_beam is the initial momentum of the beam particle and P_out^∥, the momentum component parallel to the beam carried away by the observed light particle. A possible mechanism for describing the phenomena is discussed.     

Light-particle emission in the reaction 32S + 27A1 AT 135 and 190 MeV

The properties oflight particles emitted by the ³²S + ²⁷Al reaction at 135and 190 MeV bombarding energies were studied by means of a coincidence spectrometer. The spectrometer consisted of two large-area ionization chambers which measured the energy, momentum, mass and nuclear charge of the heavy reaction products. By requiring the conservation of those quantities the energy, momentum, mass and charge deficits were determined which are representative of the unobserved light particles. The analysis of the momentum deficit in the event plane did not yield an indication for a fast, direct process of light-particle emission. The alternative analysis in the fragments rest system confirmed the statistical nature of the emission process. The out-of-plane angular correlations were used to determine the spins of the particle-emitting fragments.     

Light-particle emission in the reaction of 93Nb + 14N at 132, 159 and 208 MeV

Nonequilibrium light-particle emissions have been investigated in the reaction ⁹³Nb + ¹⁴N at 132, 159 and 208 MeV by measuring inclusive differential cross sections of p, d, t, ³He and α. The experimental data were analyzed in terms of three models: (i) an extended exciton model, (ii) a coalescence model, and (iii) a moving thermal source model. The angle-integrated energy spectra of the protons were well described by the extended exciton model in which projectile nucleons were assumed to be transferred to the target one by one, but those of composite particles were not. On the other hand, the composite particle spectra (except for α at forward angles) were successfully described by the coalescence model using spectra consistent with those for the protons. Extracted coalescence radii P₀ were about 140 MeV/c for d and t, and about 220 MeV/c for α. The light-particle spectra were also fitted by the moving-source model assuming isotropic emission from a source moving with approximately half of the beam velocity and with much higher temperature than that of the compound nucleus. Extracted temperatures followed the systematics of a recent compilation for the various reactions. A discussion of these analyses is given.     

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

The results of coincidence measurements of⁴He emission with fission fragments in reactions of¹²C(108 MeV) ions with a¹⁹⁷Au target and of¹⁶O(144 MeV) ions with a²³²Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of⁴ He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of⁴He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions.     

Angular momentum dependence of 200Pb fission studied by comparison of 19F on 181Ta with 16O on 184W

The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with ¹⁹F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10⁻¹⁷s. The total fission cross section and angular distribution of fission fragments were also measured for ¹⁹F bombardment of ¹⁸¹Ta in the energy range 84.3–114.7 MeV and for ¹⁶O bombardment of ¹⁸⁴W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The ¹⁹F + ¹⁸¹Ta measurements, when compared with the present ¹⁶O + ¹⁸⁴W cross section and angular distribution measurements and earlier lifetime measurements for ¹⁶O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.     

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997     

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.     

Search for dynamical effects in the fission decay in the 240 MeV 32S + 100Mo reaction

Light particles in coincidence with evaporation residues and heavy fragments have been measured by a 4π charged-particle detector at the INFN Laboratori Nazionali di Legnaro (Padua) for the 240 MeV ³²S + ¹⁰⁰Mo reaction leading to the ¹³²Ce composite system at 152 MeV of excitation energy. Energy spectra of the alpha-particles in coincidence with fission fragments were extracted for many correlation angles both in plane and out of plane. A prominent out-of-plane emission was observed, consistent with the pattern for the near-scission emission. From the fit to the spectra, the pre-scission alpha-particle multiplicity of 0.040±0.006 was obtained. This value is reproduced by the code PACE2 without the inclusion of a delay time for fission. The presence of fast fission, which could be responsible for this result, is discussed. Received: 6 August 2002 / Accepted: 9 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: moro@na.infn.it Communicated by C. Signorini     

态布居测热核的核温度及其与碰撞参数的关系

测量了２５ＭｅＶ／ｕ^４０Ａｒ＋¹⁹⁷Ａｕ反应中两裂片符合下的小角α-α关联．由α-α关联函数提取了⁸Ｂｅ核的相对态布居和热核的核温度．实验测得发射温度随碰撞参数而稍有变化，从中心碰撞的４ＭｅＶ变化至周边碰撞的３.８ＭｅＶ．在周边碰撞中，观察到发射温度随着粒子能量的增加而升高     

Angular momentum effect in prescission particle multiplicities for a light system by diffusion model

The effect of angular momentum on the competition between fission and particle emission during light system fission process was studied via fission diffusion model. The prescission particle multiplicities were found to increase with decreasing angular momentum. The experimental prescission proton and α particle multiplicities can be fitted for 10.6 MeV/nucleon ⁸⁴Kr(²⁷Al,binary fission) reaction with this model. Entrance channel effect found in [1] is proved to be angular momentum effect.     

Kinetic energy and angular distributions of instantaneous fission fragments in a classical model

The process of instantaneous fission in deep inelastic collisions is investigated in a classical model. Kinetic energies and angular distributions of the fragments are calculated for the proposed reaction Pb+U atE _cm ^inc =750 MeV; an experimental setup for the separation of the fragments originating from instantaneous fission from the fragments of thermal fission is explained. We also discuss fusion following instantaneous fission as a mechanism for the production of superheavy elements and arrive at rather promising estimates.     

Temperatures and Excitation Energies of Hot Nuclei in Reactions of 40Ar+natAg,209Bi at MeV/u

Coincidence measurements of fission fragment and light charged particle have been perfotrmed for the reactions of ⁴⁰Ar+^natAg,²⁰⁹Bi at E/A=25MeV using 4 PPAC and 11 sets of ΔE-E telescopes.Angular correlations of fission fragment were ploted as a function of the folding angle between the two detected fission fragments.The linear momentum transfer distributions were derived by measuring angular correlations.The backward spectra of light particles detected in coincidence with fission fragments having different average〈LMT〉are analyzed with Maxwell distribution.After some corrections the initial temperatures of the hot nuclei are determined from the energy spectra.The excitation energies corresponding to the different average〈LMT〉are obtained considering the reaction Q values and pre-equilibrum emission.In the central collision of the ⁴⁰Ar+^natAg,²⁰⁹Bi reactions,excitation energies are measured to be about 4.2MeV/u,2.4MeV/u and temperatures about 6.1MeV,5.5MeV,respecdvely.In semi-central collision,excitaionen energies are measured to be about 3.5MeV/u,1.9MeV/u and temperatures about 5.8MeV,4.8MeV respectively.     

Dynamical approach to calculating angular distributions of fission fragments

A dynamical approach is proposed for calculating the angular distributions of fission fragments. The relaxation time for the degree of freedom associated with the projection of the total angular momentum of the nuclear system onto the symmetry axis and the coefficient of damping of the fission mode are the basic parameters of this approach. Experimental data on the anisotropy of the angular distributions of fission fragments and on the multiplicities of prescission neutrons are analyzed within the proposed model for ¹⁶O+²⁰⁸Pb (E _lab=110–148 MeV), ¹⁶O+²³²Th (120–160 MeV), ¹⁶O+²⁴⁸Cm (110–148 MeV), and ¹⁶O+²³⁸U (96–148 MeV). The relaxation time and the damping coefficient are estimated at τ_K=(5–6)×10^?21 s and β=4×10²¹ s^?1, respectively.     

Correlations between fission fragments and light charged-particles in the reaction20Ne+197Au at 13 MeV/nucleon

rgy of light charged-particles has been measured in coincidence with one or two fission fragments in the reaction²⁰Ne+¹⁹⁷Au at a bombarding energy of 13 MeV/u. The fission cross section was found equal to 1,340±260 mb. Assuming that it represents the totality of the fusion cross section, the critical valuel _c is deduced equal to 93±9?, higher thanl _Bf=0=74 in the rotating liquid drop model. The main emission source for protons and alpha particles seems to be a thermally equilibrated composite system. The competition between fission and charged particle emission is unexpected in the frame of the statistical treatment. A high energy component is observed in the forward direction. These results correspond to the first step of a study programme on the evolution of nucleus-nucleus collisions between 10 and 100 MeV per nucleon.     

Neutron emission and fragment yield in high-energy fission

The KRIS special library of spectra and emission probabilities in the decays of 1500 nuclei excited up to energies between 150 and 250 MeV was developed for correctly taking into account the decay of highly excited nuclei appearing as fission fragments. The emission of neutrons, protons, and photons was taken into account. Neutron emission fromprimary fragments was found to have a substantial effect on the formation of yields of postneutron nuclei. The library was tested by comparing the calculated and measured yields of products originating from the fission of nuclei that was induced by high-energy protons. The method for calculating these yields was tested on the basis of experimental data on the thermal-neutroninduced fission of ²³⁵U nuclei.     

30MeV/u40Ar+159Tb反应中中等质量碎片的发射时标与发射机制

报道30MeV/u⁴⁰Ar+¹⁵⁹Tb反应中碎片-碎片关联函数的实验结果．利用三体弹道模型从关联函数提取了中等质量碎片发射时间．中等质量碎片的平均发射时间随碎片能量而变化，从低能时的约500fm/c下降至高能时的约100fm/c．中等质量碎片发射时间随束流能量的升高而下降，表明随着束流能量的升高中等质量碎片发射机制逐渐从相继两体衰变向多重碎裂发射过渡．对于⁴⁰Ar+¹⁵⁹Tb反应，此过渡能区在35—45MeV/u之间 关键词：     

The effect of nuclear structure in the emission of reaction products in heavy-ion reactions

Study of intermediate mass fragments (IMFs) and light charged particles (LCPs) emission has been carried out for a few reactions involving α-cluster and non- α-cluster systems to see how the emission processes are affected by nuclear clustering. Li, Be, B and α-particles have been studied from α-clustered system ¹⁶O + ¹²C for 117, 125, 145 and 160 MeV bombarding energies respectively. The enhanced yields of near-entrance channel fragment B and large quadrupole deformation of the produced composite ²⁸Si^? extracted from LCP spectra indicate the survival of orbiting-like process in ¹⁶O + ¹²C system at these energies. The same IMFs emitted from the α-cluster system ¹²C (77 MeV) + ²⁸Si and nearby non- α cluster ¹¹B (64 MeV) + ²⁸Si and ¹²C (73 MeV) + ²⁷Al (all having the same excitation energy of ～67 MeV) have also been studied. The fully energy damped (fusion–fission) and the partially energy damped (deep inelastic) components of the fragment energy spectra have been extracted. It has been found that the yields of the fully energy damped fragments for all the above reactions are in conformity with the respective statistical model predictions. The time-scales of various deep inelastic fragment emissions have been extracted from the angular distribution data. The angular momentum dissipation in deep inelastic collisions has been estimated from the data and it has been found to be close to the corresponding sticking limit value.