X-ray emission from clusters of galaxies and cosmological parameters

Clusters of galaxies are excellent probes of cosmic structure and evolution. X-ray studies of clusters provide some of their key parameters, viz., temperature of the hot intra-cluster gas, its metallicity, X-ray luminosity and surface brightness giving mass distribution and mass-flow rate in the case of cooling flows. X-ray measurements for a large sample of clusters have lead to estimates of the total gravitating mass in them, which can be compared to the virial masses derived from dynamical considerations and gravitational lensing in some of them. X-ray derived total masses are consistent with masses obtained from the other methods after the effects due to the presence of cooling flows are taken into account in the analyses. Estimated virial masses, lack of evolution in X-ray properties, and detection of several very hot clusters at high redshifts indicate a Universe with a low value (≤ 0.3) for the Ω parameter.     

Design of broadband RF pulses with polynomial-phase response

The achievable bandwidth of common linear-phase RF pulses is limited by the maximum feasible B1 amplitude of the MR system. It has been shown previously, that this limitation can be circumvented by overlaying a quadratic phase in the frequency domain, which spreads the power across the pulse duration. Quadratic-phase RF pulses are near optimal in terms of achieving minimal B1max. In this work, it is demonstrated that further B1max reduction can be achieved by combining quadratic with higher-order polynomial-phase functions. RF pulses with a phase response up to tenth order were designed using the Shinnar-Le Roux transformation, yielding considerable increases in bandwidth and selectivity as compared to pure quadratic-phase pulses. These benefits are studied for a range of pulse specifications and demonstrated experimentally. For B1max = 20 microT and a pulse duration of 2.1 ms, it was possible to increase the bandwidth from 3.1 kHz for linear and 3.8 kHz for a quadratic to 9.9 kHz for a polynomial-phase pulse.     

A multi-sample 94 GHz dissolution dynamic-nuclear-polarization system

We describe the design and initial performance results of a multi-sample dissolution dynamic-nuclear-polarization (DNP) polarizer based on a Helium-temperature NMR cryostat for use in a wide-bore NMR magnet with a room-temperature bore. The system is designed to accommodate up to six samples in a revolver-style sample changer that allows changing samples at liquid-Helium temperature and at pressures ranging from ambient pressure down to 1 mbar. The multi-sample setup is motivated by the desire to do repetitive in vivo measurements and to characterize the DNP process by investigating samples of different chemical composition. The system can be loaded with up to six samples simultaneously to reduce sample loading and unloading. Therefore, series of experiments can be carried out faster and more reliably. The DNP probe contains an oversized microwave cavity and includes EPR and NMR capabilities for monitoring the DNP process. In the solid state, DNP enhancements corresponding to ～45% polarization for [1-(13)C]pyruvic acid with a trityl radical have been measured. In the initial liquid-state acquisition experiments described here, the polarization was found to be ～13%, corresponding to an enhancement factor exceeding 16,000 relative to thermal polarization at 9.4 T and ambient temperature.     

Exotic decay in cerium isotopes

Half life for the emission of exotic clusters like ⁸Be, ¹²C, ¹⁶O, ²⁰Ne, ²⁴Mg and ²⁸Si are computed taking Coulomb and proximity potentials as interacting barrier and many of these are found well within the present upper limit of measurement. These results lie very close to those values reported by Shanmugam et al using their cubic plus Yukawa plus exponential model (CYEM). It is found that ¹²C and ¹⁶O emissions from ¹¹⁶Ce and ¹⁶O from ¹¹⁸Ce are most favorable for measurement (T _1/2<10¹⁰ s). Lowest half life time for ¹⁶O emission from ¹¹⁶Ce stress the role of doubly magic ¹⁰⁰Sn daughter in exotic decay. Geiger-Nuttall plots were studied for different clusters and are found to be linear. Inclusion of proximity potential will not produce much deviation to linear nature of Geiger-Nuttall plots. It is observed that neutron excess in the parent nuclei slow down the exotic decay process. These findings support the earlier observations of Gupta and collaborators using their preformed cluster model (PCM).     

Equi-ripple design of quadratic-phase RF pulses

An improved strategy for the design of quadratic-phase RF pulses with high selectivity and broad bandwidths using the Shinnar-Le Roux (SLR) transformation is proposed. Unlike previous implementations, the required quadratic-phase finite impulse response (FIR) filters are generated using the complex Remez exchange algorithm, which ensures an equi-ripple deviation from the ideal response function. It is argued analytically that quadratic-phase pulses are near-optimal in terms of minimising the B1-amplitude for a given bandwidth and flip angle. Furthermore, several parameter relations are derived, providing practical design guidelines. The effectiveness of the proposed design method is demonstrated by examples of excitation and saturation pulses applied in vitro and in vivo.     

Preparation and characterization of MgB2 superconductor

The MgB₂ superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB₂ is sensitive to atmospheric degradation.     

RF pulse concatenation for spatially selective inversion

It is shown that spatially selective inversion and saturation can be achieved by concatenation of RF pulses with lower flip angles. A concatenation rule which enables global doubling of the flip angle of any given excitation pulse applied to initial z magnetization is proposed. In this fashion, the selectivity of the single pulse is preserved, making the high selectivity achievable in the low flip-angle regime available for inversion and large flip-angle saturation purposes. The profile quality achievable with exemplary concatenated pulses is investigated in comparison with adiabatic inversion. It is verified that by using concatenated inversion in the transfer insensitive labeling technique (TILT), the MT artifact is suppressed. Copyright 2000 Academic Press.     

Noise figure characterization of preamplifiers at NMR frequencies

A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy. At high radiofrequencies, such impedance variation is typically achieved with transmission-line tuners, which however are not available for the relatively low range of typical NMR frequencies. To solve this issue, this work describes an alternative approach that relies on lumped-element circuits for impedance manipulation. It is shown that, using a fixed-impedance noise source and suitable ENR correction, this approach permits noise figure characterization for NMR and MRI purposes. The method is demonstrated for two preamplifiers, a generic BF998 MOSFET module and an MRI-dedicated, integrated preamplifier, which were both studied at 128MHz, i.e., at the Larmor frequency of protons at 3 Tesla. Variations in noise figure of 0.01dB or less over repeated measurements reflect high precision even for small noise figures in the order of 0.4dB. For validation, large sets of measured noise figure values are shown to be consistent with the general noise-parameter model of linear two-ports. Finally, the measured noise characteristics of the superior preamplifier are illustrated by SNR measurements in MRI data.