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.     

Destruction of26Al via the26Al(n,p)26Mg-reaction

An angular analogue to the Giant Dipole (the Giant Angle Dipole) is described. Such collective vibrational motion is specifically investigated for the case of a spin saturated large particle number system contained in a deformed harmonic oscillator well. The model predicts the ratio of the Angle Dipole to Giant Dipole excitation as equal to the deformation.     

Radiative fusion from very symmetric reactions: the giant dipole resonance in the Au nucleus

The spectra of high-energy γ rays emitted by the Giant Dipole Resonance (GDR) built on moderately excited states associated with the evaporation of 0, 1 and 2 nucleons were measured in the ⁹⁰Zr + ⁸⁹Y symmetric fusion reaction. The radiative fusion data suggest statistical emission from the compound nucleus. In addition, the analysis of the high-energy γ-ray spectra associated with the different evaporation channels at the present temperature of 0.7 MeV and spin range 15–20 show a fairly narrow width of 5.0±0.35 MeV. This value is smaller than what would be expected in a nucleus where shell effects do not play a role.     

Dynamical decay process of <Superscript>219, 220</Superscript>Ra<Superscript>*</Superscript> formed in <Superscript>10, 11</Superscript>B + <Superscript>209</Superscript>Bi reactions

The excitation functions for both the evaporation residue and fission have been calculated for ¹⁰B + ²⁰⁹Bi and ¹¹B + ²⁰⁹Bi reactions forming compound systems ^{219, 220}Ra^* , using the dynamical cluster-decay model (DCM) with effects of deformations and orientations of the nuclei included in it. In addition to this, the excitation functions for complete fusion (CF) are obtained by summing the fission cross-sections, neutron evaporation and charged particle evaporation residue cross-sections produced through the axn\ensuremath \alpha xn and pxn\ensuremath pxn (x = 2, 3, 4) emission channels for the ²¹⁹Ra system at various incident centre-of-mass energies. Experimentally the CF cross-sections are suppressed and the observed suppression is attributed to the low binding energy of ^{10, 11}B which breaks up into charged fragments. The reported complete fusion (CF) and incomplete fusion (ICF) excitation functions for the ²¹⁹Ra system are found to be nicely fitted by the calculations performed in the framework of DCM, without invoking a significant contribution from quasi-fission. Although DCM has been applied for a number of compound nucleus decay studies in the recent past, the same is being used here in reference to ICF and subsequent decay processes along with the CF process. Interestingly the main contribution to complete fusion cross-section comes from the fission cross-section at higher incident energies, which in DCM is found to consist of an asymmetric fission window, shown to arise due to the deformation and orientation effects of formation and decay fragments.     

Hot nuclei — Effective shapes and entrance channel effect

The γ-decay of the Giant Dipole Resonance in hot rotating compound nuclei has provided valuable information on the nuclear structure at finite temperature. A number of experimental results showing that the nuclear shapes change with temperature and angular momentum are here reviewed. In particular we concentrate on the temperature interval from 1 to 2 MeV, rotational frequencies from 0.2 to 1.5 MeV and on nuclei in the mass regionsA≈160–170 andA≈110, characterized by the prolate-oblate and spherical-oblate phase transitions, respectively. The possibility to study the shapes involved in the compound nucleus formation is also discussed. For this purpose long formation times are required and the nucleus¹⁷⁰W formed with the reaction⁶⁰Ni+¹¹⁰Pd, here studied, seems to be a good candidate. The gamma and particle decays were compared to those of the reaction⁴⁸Ti+¹²²Te. The comparison shows that in average the energy of the α-particles is larger for the more symmetric reaction, consistent with longer formation times and larger deformations in the pre-equilibrium stage.     

The dependence of fission time-scales on fragment mass asymmetry in the 20Ne+165Ho reaction at 50A MeV

The pre-scission and post-scission multiplicities of alpha particles were determined in coincidence with fission fragments for four windows of exit channel mass asymmetry in the reaction ²⁰Ne+¹⁶⁵Ho at 50A MeV. The technique used employed a kinematic analysis of the energy spectra of alpha particles as a function of the emission angle with respect to the fission axis. Using a procedure of fitting the experimental energy spectra with those resulting from a Monte Carlo simulation program, the total multiplicity of alpha particles was separated into pre-scission and post-scission components. Using the pre-scission and post-scission multiplicities, neutron multiplicity measurements and other empirical observations, both the initial excitation energy and the excitation energy at scission of the compound nucleus were determined. The pre-scission times in the de-excitation of a highly excited ¹⁷⁸W compound nucleus with initial excition energy of 570 MeV were evaluated using statistical model calculations. Only a small decrease in scission time was derived for the most asymmetric events studied. The possible contribution of very damped deep inelastic processes is discussed.     

The width of the giant dipole resonance built on excited states of Cu compound nuclei

Continuumγ- ray spectra from the decay of⁵⁹Cu formed at an excitation energy of 100 MeV and angular momenta up to 43? by means of the reaction 190 MeV³²S +²⁷Al have been measured and analyzed. The parameters of the Giant Dipole Resonance (GDR) have been extracted using the statistical model. The derived GDR width confirms the sizeable broadening of this resonance in⁵⁹Cu already reported in our earlier investigation at 77 MeV excitation energy (J_crit=38?). Estimates of the GDR width have been performed in the adiabatic approximation. Predicted values account qualitatively for the experimental data of⁵⁹Cu as well as of the heavier isotope⁶³Cu, in which the broadening was not seen up to 77MeV excitation (J_crit=35?). The present analysis demonstrates the strong sensitivity of the GDR to spin effects in this mass region.     

Origin of complex fragments from32S+natAg reaction at 37.5 A·MeV

Fragment emission from collisions of³²S with^natAg at 37.5 A·MeV has been studied with the 4π multi-detector AMPHORA. Production of intermediate mass and heavy fragments as well as of light charged particles has been measured. The total charged particle multiplicity and polar angular distributions have been used to select various classes of collisions. Analysis of angular and energy distributions of fragments and light particles in central collisions indicates the formation of a hot source (excitation energy of≈4.4 A·MeV) with an additional contribution from a preequilibrium process at more forward angles. Azimuthal angle correlations of He-Li, Li-Li, B-B, and C-C pairs have been used as a tool to study the origin of complex fragments. Data at backward angles are well described by considering a thermalized emitter with an angular momentum around 70? and a fragment emission time of the order of 200 fm/c. A microscopic approach of BNV type confirms these emission times and angular momenta indicating the persistence of an incomplete fusion process responsible for the emission of complex fragments at backward angles.     

Characterization of Micro‐ and Nanoscale LuPO4:Pr3+,Nd3+ with Strong UV‐C Emission to Reduce X‐Ray Doses in Radiation Therapy

UV‐C emitting nanoscale scintillators can be used to sensitize cancer cells selectively against X‐rays during radiation therapy, due to the lethal DNA lesions caused by UV‐C photons. Unfortunately, nanoscale particles (NPs) show decreased UV‐C emission intensity. In this paper, the influence of different Nd³⁺ concentrations on the UV‐C emission of micro‐ and nanoscale LuPO₄:Pr³⁺ is investigated upon X‐ray irradiation and vacuum UV excitation (160 nm). Co‐doped LuPO₄ results in increased UV‐C emission independent of excitation source due to energy transfer from Nd³⁺ to Pr³⁺. The highest UV‐C emission intensity is observed for LuPO₄:Pr³⁺,Nd³⁺(1%,2.5%) upon X‐ray irradiation. Finally, LuPO₄ NPs co‐doped with different dopant concentrations are synthesized, and the biological efficacy of the combined approach (X‐rays and UV‐C) is assessed using the colony formation assay. Cell culture experiments confirm increased cell death compared to X‐rays alone due to the formation of UV‐specific DNA damages, supporting the feasibility of this approach.     

Nuclear photoexcitation and delbrück scattering studied in the energy range 2–8 MeV

Elastic scattering by nuclei in the range of mass numbers between 64 and 238 has been studied with monochromatic photons in the energy range between 2 and 8 MeV. These photons were provided either by a Ti(n,γ) source installed in the tangential through channel of the Grenoble high flux reactor, or by²⁴Na and⁵⁶Co sources produced by deuteron bombardment of Al or Fe at the Göttingen cyclotron. The photoexcitation of 23 nuclear levels has been observed and the decay properties and groundstate widths of the majority of these levels have been determined. For the lead scattering target the coherent elastic differential cross section has been studied in detail. There is evidence that below the photo-neutron threshold the elastic scattering via virtual photoexcitation of the nucleus can be approximated by extrapolating the real part of the Giant Dipole Resonance amplitude along a Lorentzian curve. Coulomb corrections to Delbrück scattering seem to play a small role at 6.5 MeV.     

Particle Size of X-ray Pumped UVC-Emitting Nanoparticles Defines Intracellular Localization and Biological Activity Against Cancer Cells

Effectiveness of radiation treatment for cancer is limited in hypoxic tumors. Previous data shows that UVC-emitting nanoparticles enhance cytotoxicity of X-ray irradiation in hypoxic tumor cells. This study examines the impact on cell killing, particle size, uptake into cells, incubation time, and UV emission intensity of LuPO₄:Pr³⁺,Nd³⁺. A549 cells are treated with LuPO₄:Pr³⁺,Nd³⁺ and X-rays. The surviving fraction is evaluated using the colony formation assay after treatment of cells with different particle sizes (D₅₀ = 0.16 and 5.05 µm) and after different incubation times before X-ray irradiation. Nanoparticle uptake into cells is verified by transmission electron microscopy and quantified by inductively coupled plasma mass spectrometry. The microparticles exhibit a five times higher emission intensity compared to nanoparticles. Both particle sizes show an increased cytotoxic effect after X-ray excitation with prolonged incubation times. Surprisingly, the smaller nanoparticles show a significantly higher biological effect compared to the larger particles, despite their significantly lower UVC emission. Nanoparticles accumulate more quickly and closer to the nucleus than the microparticles, resulting in higher localized UVC emission and greater lethality. The results suggest that the number of intracellular particles and their proximity to the cell DNA is more important than the emission intensity of the particles.     

Decay of the compound nucleus179Au formed in the cold fusion reaction90Zr+89Y

For the compound nucleus¹⁷⁹Au formed at an excitation energy of 26 MeV in the fusion reaction⁹⁰Zr+⁸⁹Y, the energy spectra of promptly emitted protons,α particles andγ rays were measured in concidence with the evaporation residues. On the basis of the measured total decay energy, the 1p and 1α decay channels were separated from all other evaporation-residue channels. The energy spectra and absolute cross sections, together with previously measured excitation functions for various decay channels, are successfully described by statisticalmodel calculations with the Monte Carlo code CODEX.     

Fission modes in charged-particle induced fission

The population of the three fission modes predicted by Brosa's multi-channel fission model for the uranium region was studied in different fissioning systems. They were produced bombarding²³²Th and²³⁸U targets by light charged particles with energies slightly above the Coulomb barrier. Though the maximum excitation energy of the compound nucleus amounted to about 22 MeV, the influences of various spherical and deformed nuclear shells on the mass and total kinetic energy distributions of fission fragments are still pronounced. The larger variances of the total kinetic energy distributions compared to those of thermal neutron induced fission were explained by temperature dependent fluctuations of the amount and velocity of alteration of the scission point elongation of the fissioning system. From the ratio of these variances the portion of the potential energy dissipated among intrinsic degrees of freedom before scission was deduced for the different fission channels. It was found that the excitation remaining after pre-scission neutron emission is mainly transferred into intrinsic heat and less into pre-scission kinetic energy.     

Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

Evolution of the giant dipole resonance properties with excitation energy

The studies of the evolution of the hot Giant Dipole Resonance (GDR) properties as a function of excitation energy are reviewed. The discussion will mainly focus on the A ∼ 100-120 mass region where a large amount of data concerning the width and the strength evolution with excitation energy are available. Models proposed to interpret the main features and trends of the experimental results will be presented and compared to the available data in order to extract a coherent scenario on the limits of the development of the collective motion in nuclei at high excitation energy. Experimental results on the GDR built in hot nuclei in the mass region A ∼ 60-70 will be also shown, allowing to investigate the mass dependence of the main GDR features. The comparison between limiting excitation energies for the collective motion and critical excitation energies extracted from caloric curve studies will suggest a possible link between the disappearance of collective motion and the liquid-gas phase transition.     

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.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.     

MeV particles from laser-initiated processes in ultra-dense deuterium D(−1)

Fast particles from laser-induced processes in ultra-dense deuterium D(−1) are studied. The time of flight shows very fast particles, with energy above MeV. Such particles can be delayed or prevented from reaching the detector by inserting thin or thick metal foils in the beam to the detector. This distinguishes them from energetic photons which pass through the foils without delays. Due to the ultra-high density in D(−1) of 10²⁹cm⁻³, the range for 3 MeV protons in this material is only 700 pm. The fast particles ejected and detected are thus mainly deuterons and protons from the surface of the material. MeV particles are expected to signify fusion processes D+D in the material. The number of fast particles released is determined using the known gain of the photomultiplier. The total number of fast particles formed, assuming isotropic emission, is less than 10⁹ per laser pulse at < 200 mJ pulse energy and intensity 10¹²W cm⁻². A fast shockwave with 30keV u⁻¹ kinetic energy is observed.     

Evaporation residue excitation functions from fusion of 63Cu with 65Cu

Mass distributions of evaporation residues from the fusion of ⁶³Cu + ⁶⁵Cu have been measured at seven excitation energies from 55 to 105 MeV in a single irradiation experiment. They are interpreted as a mixture of residues produced by single nucleon evaporation cascades and cascades including α-particle evaporation. Compound nuclei with an average excitation energy of 55 MeV ($\text{51.5 ≦ E}{}^1\text{≦ 59}\text{MeV}$) are still found to have a probability as high as 0.3 % for decaying by emission of a single nucleon. The low-energy behaviour of the excitation function can be interpreted as a fusion barrier effect. The parameters of this barrier are determined. The evaporation residue cross section at higher energies is shown to be limited by the fission of the compound nucleus.