Status of the development of the imaging proportional counters for the Stellar X-Ray Polarimeter

Summary The Stellar X-Ray Polarimeter SXRP will be flown at the focus of the SODART X-ray telescope aboard the Russian satellite SPECTRUM-X-Gamma by the end of 1995. Four imaging proportional counters will detect photons diffracted by a graphite crystal (2.6 and 5.2 keV) and scattered from a lithium rod enclosed in a beryllium case (from 5 to 20 KeV). The counters are position-sensitive by the Wedge and Strip (WS) readout method. The gas mixture is 80% xenon, 10% argon and 10% methane. In this contribution we resume the results of the measurements performed on the engineering models of the counters. Position resolution of about 1.5 mm is attained with an energy resolution of about 25%. The pulse shape discrimination offers more than 98%⁶⁰Co events rejection, with at least 90% X-ray acceptance. Some improvements are foreseen for the flight units in terms of sensitivity and uniformity of response.     

SXRP: an X-ray polarimeter for the SPECTRUM-X-Gamma Mission

Summary TheStellar X-ray Polarimeter (SXRP) is a focal plane instrument which will be flown on the SPECTRUM-X-Gamma mission in 1993. The polarimeter is composed of two separate instruments: the first exploits the dependence on the polarization of the Bragg reflection from a graphite crystal, and of the Thomson scattering from a metallic lithium target. The second instrument makes use of the recently discovered polarization dependence of X-ray photoemission from CsI. The SXRP will permit sensitive measurements of several hundreds of known X-ray sources. X-ray polarization measurements will allow us to constrain the physical mechanisms and the geometries of several classes of galactic X-ray sources, such as X-ray pulsars, black-hole candidates and supernova remnants. Moreover, and for the first time, SXRP will be able to perform highly sensitive measurements of the brightest extragalactic sources. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

ELECTROCHEMICAL LUMINESCENCE WITH N(5)-ETHYL-4a-HYDROXY-3-METHYL-4a, 5-DIHYDROLUMIFLAVIN. THE MECHANISM OF BACTERIAL LUCIFERASE

It has been proposed in the literature that the chemiluminescence of the flavoenzyme of bacterial luciferase is caused by a chemically initiated electron-exchange luminescence mechanism which provides an excited 4a-hydroxy-4a,5-dihydroflavin ([4a-FlHOH]*) as product of 1e- reduction of the radical 4a-FlHOH+.. Electrochemical/photon counting experiments were performed to assess the feasibility of this proposal. Potentials for step-wise oxidation of N(5)-ethyl-4a-hydroxy-4a,5-dihydroluminflavin (4a-FlEtOH) have been determined in dry N,N-dimethylformamide (DMF). Photon counting was carried out during the 1e- reduction of 4a-FlEtOH+.in both DMF and acetonitrile by use of an apparatus consisting of a photocell mounted below a Pt ring-disk electrode. By use of the ring-disk electrode a steady state concentration of [4a-FlEtOH]* could be maintained by continuous 1e- oxidation of 4a-FlEtOH----4a-FlEtOH+.and 1e- reduction of 4a-FlEtOH+.----4a-FlEtOH. A maximum of 14% collection (theoretical maximum is 18%) of FlEtOH.+ at the ring electrode was obtained below 5000 rotations per minute. Calibration of the apparatus using 9,10-diphenylanthracene allowed approximation of the quantum yield for 1e- reductive capture of 4a-FlEtOH+.as 10(-6) to 10(-4) in DMF and 10(-7) to 10(-5) in acetonitrile. No fluorescence for 4a-FlEtOH in DMF could be observed; if fluorescent, the efficiency of 4a-FlEtOH can be no greater than approximately 3 x 10(-5). No electrogenerated chemiluminescence is observed on the electrochemical recycling of FlEt+----FlEt2+ and FlEt2+----FlEt+.     

Predicting Properties of Iron(III) TAML Activators of Peroxides from Their III/IV and IV/V Reduction Potentials or a Lost Battle to Peroxidase

A cyclic voltammetry study of a series of iron(III) TAML activators of peroxides of several generations in acetonitrile as solvent reveals reversible or quasireversible Fe^III/IV and Fe^IV/V anodic transitions, the formal reduction potentials (E°′) for which are observed in the ranges 0.4–1.2 and 1.4–1.6 V, respectively, versus Ag/AgCl. The slope of 0.33 for a linear E°′(IV/V) against E°′(III/IV) plot suggests that the TAML ligand system plays a bigger role in the Fe^III/IV transition, whereas the second electron transfer is to a larger extent an iron-centered phenomenon. The reduction potentials appear to be a convenient tool for analysis of various properties of iron TAML activators in terms of linear free energy relationships (LFERs). The values of E°′(III/IV) and E°′(IV V⁻¹) correlate 1) with the pK_a values of the axial aqua ligand of iron(III) TAMLs with slopes of 0.28 and 0.06 V, respectively; 2) with the Stern–Volmer constants K_SV for the quenching of fluorescence of propranolol, a micropollutant of broad concern; 3) with the calculated ionization potentials of Fe^III and Fe^IV TAMLs; and 4) with rate constants k_I and k_II for the oxidation of the resting iron(III) TAML state by H₂O₂ and reactions of the active forms of TAMLs formed with donors of electrons S, respectively. Interestingly, slopes of log k_II versus E°′(III/IV) plots are lower for fast-to-oxidize S than for slow-to-oxidize S. The log k_I versus E°′(III/IV) plot suggests that the manmade TAML catalyst can never be as reactive toward H₂O₂ as a horseradish peroxidase enzyme.