First observation of a resonance ionization signal on242mAm fission isomers

The feasibility of a hyperfine spectroscopy on^242mAm fission isomers has been demonstrated at the low target production rate of 10/s. The experimental method employed is based on resonance ionization spectroscopy in a buffer gas cell with detection of the ionization process by means of the fission decay of the isomers. The resonance ionization has been performed in two steps, utilizing an excimer dye laser combination with a repetition rate of 300 Hz. The first resonant step proceeds through theJ=7/2 term at 21440.35 cm⁻¹, which has been excited with the tuncable dye laser beam of a wavelength of 466.28 nm, the second non-resonant step is achieved with the 351 nm radiation of the excimer laser itself, running with XeF. The frequency scan of the tuneable dye laser exhibits a broad resonance ionization signal, the width of which is most likely explained by the magnetic hyperfine interaction.     

Investigation of the dynamics of dissipative U+Au collisions with δ-electron spectroscopy

δ-electron emission in elastic and dissipative collisions of U+Au at E/A = 8.65 MeV has been investigated. The velocity vectors of the reaction products were measured in coincidence with electrons of energies up to 3.2 MeV. The δ-electron yield measured for elastic collisions is in good agreement with coupled-channels calculations. The δ-electron spectra of dissipative reactions show a clear dependence on the violence of the collision, i.e. the total kinetic energy loss (TKEL). The shape of the spectra are analysed with an atomic model by a fitting procedure using phenomenological trajectories. The results indicate an increasing contact time of the united system with increasing total kinetic energy loss reaching 1.16(4) × 10^?21 s at < TKEL > = 375 MeV.     

Excited states in neutron deficient even-even thorium isotopes (218≦A≦222)

The nuclei^{218, 220, 222}Th were investigated by conversion electron andγ-ray spectroscopy after compound nucleus reactions of¹⁴N with²⁰⁹Bi and^{16, 18}O with²⁰⁸Pb. The intenseγ-background from fission was suppressed by spectroscopy of conversion electrons andγ-rays in coincidence with the evaporation residues or theirα-decay. Level schemes were determined for²¹⁸Th up toI ^π=10⁺ and for²²⁰Th and²²²Th up toI ^π=15^?. The observed structure of²¹⁸Th may be explained in the spherical shell model with residual interaction by two neutron excitation. ForN≧130 completely different level schemes are observed showing very intense, collective electric dipole transitions (B(E1)?10^?2 W.u.) with energies of typically 200 keV. Interpretation in terms of stable octupole deformations of the ground state as well as in anα-cluster picture are discussed.     

Perspectives for laser spectroscopy of the element nobelium

Strong efforts are undertaken at GSI in Darmstadt preparing for laser spectroscopy of the synthetic element nobelium. Several excimer- and dye lasers will be used in the forthcoming search for the $^1$P$_1$ -level in ²⁵⁴No. Based on the highly-efficient Radiation Detected Resonance Ionization Spectroscopy (RADRIS) technique, the identification of this excited state predicted around 3.8 eV becomes possible within a relatively short period of beam-time. This will form the basis for future studies of the atomic structure of the heaviest elements.     

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ∼100 kA in ∼10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics. PACS 41.60.Cr; 52.38.Kd     

Towards optical spectroscopy of the element nobelium ({\sf Z }= \mathsf{102}) in a buffer gas cell

For the investigation of the atomic level structure of heavy elements which can only be produced at on-line facilities such as GSI, a novel experimental procedure has been developed. It is based on Radiation Detected Resonance Ionization Spectroscopy (RADRIS) and can be applied to elements like nobelium produced at rates of a few ions per second. Fusion reaction products are separated from the primary beam by the velocity filter SHIP at GSI, stopped in a buffer gas cell, collected on a tantalum filament and then re-evaporated as atoms. The ions produced by resonance ionization with tunable laser beams are detected via their characteristic α decay. First on-line experiments on α-active ¹⁵⁵Yb, which is supposed to have an atomic level structure similar to nobelium, were performed. These test experiments focused on the optimization of the collection and re-evaporation process of the radioactive ions, the laser ionization efficiency and the detection via α decay. An overall efficiency for RADRIS of 0.8% with respect to the target production rate was measured. While further improvements of this efficiency are in progress it should already be sufficient for the search for atomic levels in nobelium.     

Chemical Perturbation of Oncogenic Protein Folding: from the Prediction of Locally Unstable Structures to the Design of Disruptors of Hsp90–Client Interactions

Protein folding quality control in cells requires the activity of a class of proteins known as molecular chaperones. Heat shock protein-90 (Hsp90), a multidomain ATP driven molecular machine, is a prime representative of this family of proteins. Interactions between Hsp90, its co-chaperones, and client proteins have been shown to be important in facilitating the correct folding and activation of clients. Hsp90 levels and functions are elevated in tumor cells. Here, we computationally predict the regions on the native structures of clients c-Abl, c-Src, Cdk4, B-Raf and Glucocorticoid Receptor, that have the highest probability of undergoing local unfolding, despite being ordered in their native structures. Such regions represent potential ideal interaction points with the Hsp90-system. We synthesize mimics spanning these regions and confirm their interaction with partners of the Hsp90 complex (Hsp90, Cdc37 and Aha1) by Nuclear Magnetic Resonance (NMR). Designed mimics selectively disrupt the association of their respective clients with the Hsp90 machinery, leaving unrelated clients unperturbed and causing apoptosis in cancer cells. Overall, selective targeting of Hsp90 protein–protein interactions is achieved without causing indiscriminate degradation of all clients, setting the stage for the development of therapeutics based on specific chaperone:client perturbation.