Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of⁴He and¹H has been measured in coincidence with fission for reactions of 469-MeV⁵⁶Fe+²³⁸U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the⁴He/¹H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for⁴He and¹H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed⁴He/¹H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For⁴He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For¹H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of⁴He/¹H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.     

Fission and emission of H and He in the reactions of 215 MeV16O with181Ta,208Pb and238U

The reactions of 215 MeV¹⁶O with¹²C,¹⁸¹Ta,²⁰⁸Pb and²³⁸U have been studied. Inclusive measurements for⁴He emission are given from each target, and for fission and^1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for⁴He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of¹⁶O with¹²C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.     

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.     

Angular distributions of fission and deep inelastic products of the system Ar+Au at 285 MeV

The angular distribution of the heavy products from the interaction of 285 MeV ions⁴⁰Ar on an Au target has been analysed through a large angular range (4°–165° lab angles). The results exhibit two components: the fission component corresponding to relatively symmetric masses, and a deep inelastic one which peaks in the angular range 100°–140° (c.m angles). The fission component exhibit the usual 1/sinθ pattern, proving that, in spite of a zero fission barrier, the life time of the system exceeds the rotation period. The relative cross sections for the fission and deep inelastic processes are discussed.     

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.     

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     

Studies of4He emission in both fusion-like and inelastic reactions of 340-MeV40Ar+238U

The fission-fragment angular correlation technique has been used in conjunction with a position-sensitive avalanche detector to identify the fusion-like and inelastic collisions of 340-MeV⁴⁰Ar+²³⁸U. For each of these two reaction types, we have characterized the correlated⁴He emission by measuring three-fold coincidences between two fission fragments and the light charged particle. We find an abundance of both evaporation-like and direct⁴He emission, each of which is associated with both fusion-fission and sequential-fission processes. Several comparison tests of both the coincidence and singles data strongly indicate that very little⁴He emission is due to evaporation from accelerated fission fragments in this system. The evaporative⁴He emission appears instead to have a strong contribution originating from the composite system prior to fission. The forward angle⁴He emissions, characterized by prominent forward-peaking and relatively high energies, suggest an origin associated with the very early stages of the collision.     

Particle spectra from reversed kinematics: Powerful probes of hot rotating nuclei

Inclusive¹H/⁴He emission has been studied for three reactions in “reversed kinematics”,⁴⁰Ar/⁵⁶Fe/⁸⁶Kr (8.5MeV/u)+¹²C. The⁴He spectra include the heretofore unobserved region of low c.m. energies in the direction of the heavy reactant. A test of forward-backward symmetry is made for the hot-spot, moving source model; the data do not support the model. Effective emission barriers for⁴He evaporation are found to be within ～10% of the corresponding fusion barriers, while for¹H evaporation the emission barriers are more than 25% lower. It is possible that deformations predicted by the rotating liquid drop model can account for the⁴He evaporation spectra, but substantial change in the standard evaporation treatment is required to explain the spectra for¹H.     

自受激拉曼晶体Nd3+:SrMoO4的光谱性质研究

通过拉曼散射光谱，吸收光谱，荧光发射寿命和808 nm LD激发下的红外荧光光谱的实验测量，系统研究了Nd³⁺:SrMoO₄晶体的自受激拉曼光谱性质.分析指认了拉曼散射光谱中各拉曼峰所对应的晶格振动模式，得出了其SRS活性最强的声子频率约为898 cm^-1，对应于(MoO^2-₄)离子团的完全对称光学伸缩振动A_g模；通过J-O理论对晶体的吸收谱进行了全面的光谱参数计算，得出⁴F_3/2→⁴I_11/2跃迁的积分发射截面达0.57×10^-18 cm²，自发辐射概率为141.06 s^-1；同时，实验测得该跃迁的荧光发射寿命约为0.2 ms.最后，结合808 nm LD激发下的红外波段荧光光谱，论证了SrMoO₄晶体中Nd³⁺离子1068 nm发射通过拉曼频移获得1180 nm一级斯托克斯激光发射的可能性，为Nd³⁺:SrMoO₄晶体的自受激拉曼激光器研究提供了理论依据. 关键词： 3+离子')" href="#">d³⁺离子 4 晶体')" href="#">SrMoO₄ 晶体 自受激拉曼散射     

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.     

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.     

Two mode fission: Existence of two independent paths in the fission of actinide elements

We attempted to confirm the existence of two-mode-fission mechanism in the²³²Th and²³⁸U plus⁴He systems. Angular distributions and differential ranges of the fission fragments were measured for both systems. Two distinct groups, symmetric and asymmetric, were observed in the anisotropy of the fragments obtained from the angular distributions in either systems. Kinetic energies of the fragments were deduced from the measured ranges and the effective distances were calculated from the data. The distribution of the resulting distances was classified into two groups also, indicating the existence of the symmetric and asymmetric modes. This strongly suggests existence of two independent paths in the fission process.Communicated by: P. Armbruster     

Multiplicities for evaporative4He emission in heavy ion reactions: Relationships to spin and lifetime expectancy of the composite nucleus

Multiplicities are compared for⁴He evaporation in reactions of⁴⁰Ar and⁵⁶Fe projectiles (8.5 MeV/u) with with^natAg and²³⁸U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like⁴He emission from the composite nuclei is shown to be large for mean spins ≦100?, and to decrease dramatically with increasing spin. Coincidence measurements between two⁴He particles have also been studied for⁴⁰Ar+^natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the⁴He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.     

Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.     

The 6He emission accompanying the spontaneous fission of 252Cf

The results of a multiparameter investigation of the ⁶He emission accompanying the spontaneous fission of ²⁵²Cf are presented. The energy spectrum and the yield of the ⁶He particles are found to be in accord with previous measurements, but their angular distribution is observed to be narrower at 13° ± 3° (FWHM) than the value of ? 32° deduced in a previous investigation.Comparisons of the experimental with published calculated energy spectra and angular distributions for ³H, ⁴He and ⁶He particles are shown to provide evidence for a compact scission configuration in ternary fission.     

Evidence for nonequilibrium particle emission before symmetric disintegration of a composite system formed in the 16O+40Ca reaction at 230 MeV

Measurement of fragment-fragment correlations in the reactions of 230 MeV ¹⁶O with ⁴⁰Ca and 280 MeV³²S with ²⁴Mg have been used to isolate processes in which symmetric decay follows nonequilibrium emission of one or two alpha particles. At the higher energy per nucleon. in contrast to previous observations for lower velocity projectiles, nonequilibrium emission followed by symmetric decay has approximately the same probability as the symmetric fission following complete fusion.     

Production of high energy deuterons in the reaction:3He+3He→4He+2H+e ++v

Experiments on the reaction:³He+³He→⁴He+²He using a single four detector telescope have shown evidence for the production of deuteron pulses, consistent with the²He→²H+e⁺+v fusion reaction in the final state. New measurements on the same reaction using improved techniques viz.A) a windowless³He gas target andB) α- d correlation detection with 5 ns resolving time and antipileup circuitry have been performed. Calibrations were obtained from the⁷Li+³He→²H+⁴He+⁴He reaction spectra. The correlation angles were 30° and 97° for the alphas and deuterons respectively. Deuteron pulses have been observed, correlated with alpha particles. A cross section of (1.7 ±0.5)nbsr^?1MeV^?1 was obtained consistent with previous measurements using a single telescope.     

Evidence for Coulomb-field induced light-particle emission during scission

Energy and angular correlations of protons and α particles were measured in coincidence with fission fragments in the reactions 200 MeV, 254 MeV ³⁷Cl + ¹²⁴Sn and 318 MeV ²⁸Si + ¹⁴¹Pr. The light-particle yield is dominated by statistical emission from composite nucleus and from fission fragments, but α spectra also display the characteristics of evaporation during scission. We give Coulomb-field induced light-particle emission as a novel and hitherto only consistent explanation of this emission process.