Hendrik de Waard

Remanence measurements on individual magnetotactic bacteria using a pulsed magnetic field

We describe pulsed-magnetic-field remanence measurements of individual, killed, undisrupted cells of three different types of magnetotactic bacteria. The measurement technique involved the observation of aligned, individual magnetotactic bacteria with a light microscope as they were subjected to magnetic pulses of increasing amplitude. We show that for MM cells, the hysteresis loop is square, with the coercive field variable from cell to cell. This is consistent with just two magnetization states for the single chain of magnetite particles. For MR and MMP cells, the hysteresis loops are not square, indicating that there are several different magnetization states, and that individual cells can be demagnetized. The coercive fields in the MR and MMP cells are less variable than for the MM cells.

A Mössbauer study of xenon compounds

A large number of xenon compounds have been synthesized and studied by nuclear gamma resonance of the 39.6 keV transition in 129Xe. The quadrupole interaction strengths (Q.S.) of all divalent xenon compounds with linear F–Xe–F groups lie close together, but a small increase of the Q.S. with the acceptor strength of adduct groups is observed. For compounds with F–Xe–O– and –O–Xe–O– groups a reduction of the Q.S. of up to 10% is found. The tetravalent compounds exhibit a Q.S. with absolute magnitude very close to that of the divalent compounds, consistent with a square planar configuration and nearly equal contributions to the Q. S. for each Xe–F bond. Hexavalent xenon in XeF6 also exhibits an appreciable Q.S., indicative of distortion of the octahedral fluorine configuration around the xenon atom in the polymeric solid. An increase of the Q.S. relative to XeF6 is observed in the adducts and in hexavalent compounds with mixed oxygen/fluorine bonding. This is interpreted as due to increased distortion around the central xenon atom.

MULTI COINCIDENCE GONIOMETER FOR ANGULAR CORRELATION MEASUREMENTS

Abstract Four scintillation detectors view the same source. All six coincidence combinations between these are recorded simultaneously. A “coincidence cross delay” speeds up the measurement of angular correlations by another factor of two. It is shown that the instrument is self-monitoring and that the coincidence counting efficieny is an order of magnitude higher than in a conventional arrangement with two detectors. It is concluded that the instrument is suitable for the study of very short lived isotopes. The apparatus was checked with the Co 60 1.17–1.33 MeV cascade and the Bi 207 1.06-0.57 MeV cascade.

Instrument for the measurement of hysteresis loops of magnetotactic bacteria and other systems containing submicron magnetic particles

An electronic control system for the measurement of hysteresis curves of microscopically observed magnetic structures such as chains of magnetosomes in magnetotactic bacteria suspended or swimming in water is described. Using continuous magnetic fields generated by four coils for guidance or orientation of the bacteria or other magnetic structures, and pulsed magnetic fields in two additional coils for changing the degree of magnetization in small steps, hysteresis curves can be traversed. The circuits described can be constructed with readily available components. The guiding- and pulsed-field coils can be fashioned in any standard machine shop. The typical sensitivity of the system is better than 10−12 ergs/G, which makes a wide range of bacteria accessible to quantitative measurement of their magnetosome chains. The electronic circuits as well as the coil systems are described in detail.