Nuclear astrophysics experiments: Reactions and elastic scattering

Experimental methods in nuclear astrophysics experiments with radioactive beams are described, and evaluated. The importance of performing the (p, p) elastic scattering in parallel to a (p, α) or a (p, γ) reaction is emphasized. Received: 1 May 2001 / Accepted: 4 December 2001     

Explosive processes in nucleosynthesis

There are many explosive processes in nucleosynthesis: big bang nucleosynthesis, the rp-process, the γ-process, the ν-process, and the r-process. However, I will discuss just the rp-process and the r-process in detail, primarily because both seem to have been very active research areas of late, and because they have great potential for studies with radioactive nuclear beams. I will also discuss briefly the γ-process because of its inevitability in conjunction with the rp-process. Received: 1 May 2001 / Accepted: 4 December 2001     

Radioactive beam facilities: European perspectives

During the last ten years, the cooperation in Nuclear Physics between the different European countries has significantly increased, in particular through extensive discussions initiated by NuPECC, the Nuclear Physics European Collaboration Committee. In this context, perspectives in physics with radioactive beams have been reviewed and a high priority has been given to the developments of performing facilities in Europe including both in-flight and ISOL facilities. In this presentation, the main challenges are recalled and the various existing national and transnational projects are reviewed as well as the accompanying research and development programs. A special attention is given to the EURISOL project, a European program aiming at a preliminary design study of a second-generation European ISOL radioactive beam facility. Received: 1 May 2001 / Accepted: 4 December 2001     

A new test for random events of an exponential distribution

A new statistical test procedure is described to evaluate whether a set of radioactive-decay data is compatible with the assumption that these data originate from the decay of a single radioactive species. Criteria to detect contributions from other radioactive species and from different event sources are given. The test is applicable to samples of exponential distributions with two or more events. Received: 24 March 2000 / Accepted: 13 April 2000     

Tuneable micro- and nano-periodic structures in a free-standing flexible urethane/urea elastomer film

We have studied the control and manipulation of tuneable equilibrium structures in a free-standing urethane/urea elastomer film by means of atomic force microscopy, small-angle light scattering and polarising optical microscopy. The urethane/urea elastomer was prepared by reacting a poly(propyleneoxide)-based triisocyanate-terminated prepolymer (PU) with poly(butadienediol) (PBDO), with a weight ratio of 60% PU/40% PBDO. An elastomer film was shear-cast onto a glass plate and allowed to cure, first in an oven, then in air. Latent micro- and nano-periodic patterns are induced by ultra-violet (UV) irradiation of the film and can be “developed” by applying a plane uniaxial stress or by immersing the elastomer in an appropriate solvent and then drying it. For this elastomer we describe six pattern states, how they are related and how they can be manipulated. The morphological features of the UV-exposed film surface can be tuned, reproducibly and reversibly, by switching the direction of the applied mechanical field. Elastomers extracted in toluene exhibit different surface patterns depending upon the state in which they were developed. Stress-strain data collected for the films before and after UV irradiation reveal anisotropy induced by the shear-casting conditions and enhanced by the mechanical field. We have interpreted our results by assuming the film to consist of a thin, stiff surface layer (“skin”) lying atop a thicker, softer substrate (“bulk”). The skin's higher stiffness is hypothesised to be due to the more extensive cross-linking of chains located near the surface by the UV radiation. Patterns would thus arise as a competition between the effects of bending the skin and stretching/compressing the bulk, as in the work of Cerda and Mahadevan (Phys. Rev. Lett. 90, 074302 (2003)). We present some preliminary results of a simulation of this model using the Finite Element package ABAQUS.     

Atomic structure and stability of elliptical carbon onion

Elliptical carbon onions were produced from amorphous carbon by electron beam irradiation. Atomic structure and structural stability of the onion were investigated by high-resolution electron microscopy, molecular mechanics calculation and molecular orbital calculation, and a structure model of C₂₄@ C₈₄ was proposed at the center of the elliptical onion. Received 30 November 2000     

Future developments of INFN-LNL nuclear beam facilities

The accelerator group at INFN-LNL has been mostly engaged, recently, in completing and commissioning the higher current injector of the linac booster ALPI (named PIAVE) and in constructing and assembling the front-end part of a high current driver linac for the RNB facility SPES. PIAVE, designed to accelerate ions with A/Q = < 8.5 up to 1.2 MeV/u, is now completed. The injector has been commissioned with O, Ar, Ne and Xe beams. Neon and argon beams have been delivered to experiments for a total of about 400 hours. A consolidation program of PIAVE and ALPI is planned, so as to deliver a larger variety of beams with a current range  pnA and with an energy exceeding the Coulomb barrier in relevant nuclear reaction cases. The RNB facility SPES, allowing a frontier program in RNB physics, is being designed and prototyped: beams of neutron rich medium-to-heavy mass nuclei will be produced inducing ²³⁸U fission with a 40 MeV 200μA proton beam impinging onto a multi-slice direct target. A further development of ALPI will make it best suitable for the re-acceleration of radioactive nuclear species, after charge breeding and isotope selection.     

Electrical transport characteristic of carbon nanotube after mass-separated ultra-low-energy oxygen ion beams irradiation

Mass-separated ultra-low-energy oxygen ion beams were irradiated to the single-walled carbon nanotubes (SWCNTs) under an ultra-high-vacuum pressure of 10⁻⁷ Pa for the purpose of achieving n-type conduction of nanotubes. The ion beam energy was 25 eV, which was close to the displacement energy of graphite. The incident angle of the ion beam was normal to the target nanotube. The ion dose ranged from 3.3 × 10¹¹ to 3.8 × 10¹² ions/cm². The structure of SWCNTs after the ion irradiation was investigated. The CNTs still have a clear single-walled structure after the ion irradiation. The graphite structure is distorted and some defects are induced in the nanotube by the oxygen irradiation. The oxygen ions with the ion energy of 25 eV are irradiated to the field effect transistor (FET) device with the nanotube channel. The n-type characteristic appears upon the oxygen ion irradiation, and the device exhibits ambipolar behavior. The defects induced by the ion irradiation may act as the n-type dopants.     

Different alloying behaviors of Co-Mo multilayered films upon ion irradiation and thermal annealing

The Co-Mo system is characterized by a small negative heat of formation being −7 kJ/mol, which provides a small driving force for alloying. In the case of the initial multilayered films with closest-packed semi-coherent interfaces, solid-state interfacial reaction is frustrated upon thermal annealing even at a temperature up to 600 °C. However, amorphization through interfacial reaction was achieved upon ion irradiation as the intensive ion irradiation can elevate the initial energetic state by the irradiation energy as well as destroy the semi-coherent interfaces and thus drive atomic mixing in the films eventually becoming disordered. Received: 7 June 1999 / Accepted: 1 November 1999 / Published online: 8 March 2000     

Mössbauer study of the vibrational anisotropy and of the light-induced population of metastable states in single-crystalline guanidinium nitroprusside

M?ssbauer studies were performed on single crystals of guanidinium nitroprusside with different orientations of their principal crystallographic axes (a, b, c) with respect to the incident radiation. The markedly anisotropic Lamb-M?ssbauer factor f _LM , i.e. f _LM ^(a) = 0.118(8), f _LM ^(b) = 0.174(8), f _LM ^(c) = 0.202(8) is in contrast to that of nitroprussides with inorganic anions. The observed anisotropy is ascribed to the anisotropic vibrational mean-square displacement of the nitroprusside anions as a whole which is due to the specific packing of both, anions and cations, as well as the very weak chemical bonding between the ions, typical only for guanidinium nitroprusside. The vibrational anisotropy of iron atoms in barium nitroprusside that has been observed by X-ray structural investigations has a different origin and therefore does not result in an anisotropic Lamb-M?ssbauer factor. We have also investigated metastable states in guanidinium nitroprusside that have been populated by means of incoherent irradiation from light-emitting diodes. With a specific orientation of the guanidinium nitroprusside single crystal a population of the metastable states up to 26% could be achieved. Populations of comparable size on lithium, sodium and potassium nitroprussides have only been reached using coherent laser irradiation. Received 15 December 1998 and Received in final form 3 March 1999     

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by ⁴⁸Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas. Received: 1 May 2001 / Accepted: 4 December 2001     

In situ observation of the ultrafast lattice dynamics of graphite under ion irradiation

We develop a pump-probe experiment system, in which vibrational dynamics of a solid sample under ion irradiation can be measured in real time. In situ observation enables us to monitor small changes induced by ion irradiation, without being influenced by the irreproducibility of the sample quality or the experimental configuration. We apply the experimental system to investigate the femtosecond dynamics of the coherent E_2g1 phonon of graphite under 5 keV He⁺ irradiation. A slight decrease in the dephasing rate of the phonon at the initial stage, as well as a downshift followed by an upshift of the phonon frequency, are clearly demonstrated, all of which were ambiguous in the ex situ experiment due to the poor reproducibility of the surface quality. This technique could also be applied to study femtosecond vibrational dynamics in real time during thermal annealing, film deposition with e.g. ablation and sputter, and molecular adsorption on substrates.     

Early stages of laser mixing process in Sb/Ge multilayer thin films

The onset of mixing at the interfaces between Sb and Ge in thin multilayered films containing two or four layers has been studied. The films were irradiated with nanosecond laser pulses in order to trigger mixing, and in situ reflectivity measurements were used to follow the transformation in real-time. Cross sectional transmission electron microscopy analysis was used to study both the structure and the composition profile before and after irradiation.A threshold irradiation energy exists for the onset of mixing, below which roughening of the interface between the layers is observed, together with recrystallization of the surface Sb layer following melting. The results are consistent with a melting/diffusion process which is inhomogeneously nucleated at the interface between the top Sb and Ge layers. Once mixing is initiated an amorphous Sb-Ge layer of constant thickness is formed, corresponding to mixing along a well defined planar melt front. Voids are observed at the former Sb/Ge interface, which may be related to interfacial stress in the as-grown configuration.     

Effect of radiation dose on the size of bismuth particles produced by radiolysis

Colloidal bismuth is formed in -irradiated solutions , containing BiO⁺, 0.2 M propanol-2 and 2×10^-2 M of polyacrylic acid. Radiolytic reduction of these solutions produces long-lived bismuth clusters which are stabilized on the polymer chains. The clusters have a distinct absorption band at 350 nm with a tentatively assigned to or structures. With continued irradiation, the clusters coalesce into colloidal particles of 5 nm diameter. The absorption spectra of Bi oligomers and colloidal particles are reported as well as the optical changes accompanying their formation.     

Variation of Kβ/ Kα X-ray intensity ratio and lineshape with the effects of external magnetic field and chemical combination

In this work, the effects caused by different chemical combination and external magnetic field in several copper and zinc compounds (Cu, CuBr, Cu₂O, CuI, CuCl, Cu₂Te, Cu₅Si, CuSO₄, CuSeO₄.5H₂O, CuCl₂, Cu(NO₃)₂, CuS, CuSe, CuF₂, CuF2.3H₂O, CuBr₂, Cu(ClO₄)₂.6H₂O, Zn, ZnSO₄.5H2O, Zn(C₂H₃O₂)₂, ZnF₂, Zn(NO₃)₂.6H₂O, ZnO, ZnS, ZnSe, ZnTe and ZnF₂.4H₂O) were studied using a Si(Li) detector. The samples were excited by 22.69 keV X-rays from ¹⁰⁹Cd point radioactive source of strength 10 mCi in the external magnetic field of intensities 0.6 T and 1.2 T. The shift, asymmetry, FWHM and Kβ/Kα intensity ratio values were determined. For B = 0, the present experimental results were compared with the experimental and theoretical data in literature. The results have shown that the atomic parameters such as energy shifts, asymmetry indices, FWHM and Kβ/Kα intensity ratios can change when irradiation is conducted in a magnetic field.     

Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by different methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of films deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel films with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed films, because of the interaction between clusters. Using an effective medium model, it is demonstrated that the random dispersion of clusters in implantation films causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed films induces a screening effect between the clusters of largest size. Received 10 January 2002 / Received in final form 26 April 2002 Published online 19 July 2002     

Target ion source and charge exchange cell development for the EXCYT facility

The EXCYT facility at the INFN-LNS is based on a K-800 superconducting cyclotron delivering stable ion beams on a Target Ion Source (TIS) assembly to produce the required nuclear species, and on a 15 MV Tandem for post-accelerating the radioactive beams. For some ion beams such as for Li, the extraction efficiency from the TIS is higher when obtained by positive ionisation, while the injection into the Tandem is possible only after a charge exchange to obtain negative ions. In this work we present the procedures together with the results of the production of ^6,7,8,9Li beams extracted at EXCYT during the last year. The production of the radioactive elements was performed by sending a ¹³C⁴⁺ primary beam of 45 MeV/u on a graphite target. The ionisation of the production species was achieved by a tungsten positive surface ioniser. The Li⁺ has been extracted from TIS at different energies to cross-check the transmission and the charge exchange efficiency. To perform the conversion from positive to negative ions we employed a Charge Exchange Cell (CEC) containing Cs vapours. The Li beam interacts with the latter in a two-step reaction, thus converting its charge from +1 to –1. The CEC was already characterised during off-line tests; the results obtained at EXCYT confirmed both the isotopic shift effect and the efficiency values at several given extraction energies. Future improvements of the TIS and the CEC are discussed.     

Interfaces under ion irradiation: growth and taming of nanostructures

We have investigated the synthesis of nanostructures, as well as the control of their size and location by means of ion beams. The phase separation and interface kinetics under ion irradiation give new possibilities for controlling the growth of nanostructures. Additionally, the chemical decomposition of the host matrix by collisional mixing can contribute to the self-organization of nanostructures, especially at interfaces. It is shown how collisional mixing during ion implantation affects nanocrystal (NC) synthesis and how ion irradiation through NCs modifies their size and size distribution. An analytical expression for solute concentration around an ion-irradiated NC was found, which may be written like the well-known Gibbs–Thomson relation. However, parameters have modified meanings, which has a significant impact on the evolution of NC ensembles. “Inverse Ostwald ripening” of NCs, resulting in an unimodal NC size distribution, is predicted, which has been confirmed experimentally for Au NCs in SiO₂ and by kinetic lattice Monte Carlo simulations. At interfaces, the same ion-irradiation-induced mechanism may result in self-organization of NCs into a thin δ-layer. Collisional decomposition of SiO₂ may enhance the NC δ-layer formation in SiO₂ at the Si/SiO₂ interface. The distance of the self-organized NC δ-layer from the SiO₂/Si interface renders the structure interesting for non-volatile memory applications. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-351-260-3285, E-mail: K.-H.Heinig@fz-rossendorf.de     

New measurement of exotic decay of 225Ac by 14C emission

The branching ratio of ²²⁵Ac decay by emission of ¹⁴C was remeasured under improved experimental conditions by using a radioactive source produced at the ISOLDE mass-separator at CERN and a nuclear track detector technique. The result, B = λ_14C/λ_α = (4.5±1.4)10^-12, is consistent with the anomalously high value obtained in the 1993 experiment, thus confirming the importance of nuclear-structure effects in this exotic decay. Received: 20 July 2001 / Accepted: 28 November 2001     

Conversion electron spectroscopy of isobarically purified trapped radioactive ions

The feasibility of the JYFLTRAP for in-trap spectroscopy has been studied. Several internally converted transitions have been measured for isomers of fission products with good accuracy. High-resolution spectroscopic data free of source effects have been obtained proving that trapped radioactive ions can provide excellent conversion electron sources. The shortest-lived isomer studied in this work was ^117m Pd with a half-life of 19.1ms, for which a superior peak-to-total ratio and an excellent line shape at the 9.9keV conversion electron line have been observed. Detection efficiencies and related phenomena of the present setup are analyzed.