Tribute to John Pickering

John left word that he did not want a lot of eulogies, so there is much I could say about him that I do not because I honor his request

Development of a Krypton target for Cyclone-30 at KFSH&RC

Introduction Krypton-81m is a radioactive gas with a half-life of 13 s, and found to be useful in many applications in nuclear medicine, particularly for lung perfusion studies and ventilations. Due to high demands for 81mKr, we have developed an automated Krypton system to be installed in one of the Cyclotron’s beamlines at King Faisal Specialist Hospital and Research Centre (KFSH&RC) and to deliver large activity of the radioactive gas. Material and Methods The effective cross section of producing 81Rb is between 15 and 30 MeV [1]. Therefore, range and stopping power of the effective cross section were calculated with respect to gas density of 0.0185 g/cm3. This value is equivalent to gas density at 5.0 bars at room temperature. SRIM calculations resulted in a range of 589 mm. However, due to limitation in fabricating such long target chamber, the target length is chosen to be 250 mm. Attached to the end of target body is a special water circulating flange ‘back-pool’, its purpose is to absorb the rest of the energy and protons Bragg peak. The target body is made of Aluminum with the inner part being electroplated with nickel. The target body is of conical shape. The target body is electrically isolated from other parts to allow accurate beam current reading. Full access to the target loading/unloading steps is made through touch screen technology (FIG. 2) for user access. Additionally, the target control system is designed to be protected through chain of interlock steps. The production cycle of 81Rb is explained as follow. Target is evacuated to approximately 10−3 mbar before being filled with natKr at pressure of 5 bars. At the end of bombardment, recovery of natKr is done via cryogenic vessel. Finally, the radioactivity is washed with KCl and pushed to Hotcells through the nitrogen gas for chemistry processing. Irradiation time was approximately 30 min. Results and Conclusion Experimental results clearly showed a fairly good activity of 81mKr as shown in TABLE 1. In all experiments, the radionuclidic purity of 81mKr was above 99.59%. 79mKr and 79Kr were also measured with a percentage of, respectively, 0.34 and 0.07 %. Special attention has to be drawn to last experiment where the yield significantly in-creased, due to the period where the KCl left inside the target (10 min) before pushing the solution to the Hotcell

Isotope tracing of submarine groundwater discharge offshore Ubatuba, Brazil : results of the IAEA–UNESCO SGD project

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Journal of Environmental Radioactivity 99 (2008): 1596-1610, doi:10.1016/j.jenvrad.2008.06.010.Results of groundwater and seawater analyses for radioactive (3H, 222Rn, 223Ra, 224Ra, 226Ra, 228Ra) and stable (2H, 18O) isotopes are presented together with in situ spatial mapping and time-series 222Rn measurements in seawater, direct seepage measurements using manual and automated seepage meters, pore water investigations using different tracers and piezometric techniques, and geoelectric surveys probing the coast. This study represents first time that such a new complex arsenal of radioactive and non-radioactive tracer techniques and geophysical methods have been used for simultaneous submarine groundwater discharge (SGD) investigations. Large fluctuations of SGD fluxes were observed at sites situated only a few meters apart (from 0 cm d-1 to 360 cm d-1; the unit represents cm3/cm2/day), as well as during a few hours (from 0 cm d-1 to 110 cm d-1), strongly depending on the tidal fluctuations. The average SGD flux estimated from continuous 222Rn measurements is 17±10 cm d-1. Integrated coastal SGD flux estimated for the Ubatuba coast using radium isotopes is about 7x103 m3 d-1 per km of the coast. The isotopic composition (δ2H and δ18O) of submarine waters was characterised by significant variability and heavy isotope enrichment, indicating that the contribution of groundwater in submarine waters varied from a small percentage to 20%. However, this contribution with increasing offshore distance became negligible. Automated seepage meters and time-series measurements of 222Rn activity concentration showed a negative correlation between the SGD rates and tidal stage. This is likely caused by sea level changes as tidal effects induce variations of hydraulic gradients. The geoelectric probing and piezometric measurements contributed to better understanding of the spatial distribution of different water masses present along the coast. The radium isotope data showed scattered distributions with offshore distance, which imply that seawater in a complex coast with many small bays and islands was influenced by local currents and groundwater/seawater mixing. This has also been confirmed by a relatively short residence time of 1-2 weeks for water within 25 km offshore, as obtained by short-lived radium isotopes. The irregular distribution of SGD seen at Ubatuba is a characteristic of fractured rock aquifers, fed by coastal groundwater and recirculated seawater with small admixtures of groundwater, which is of potential environmental concern and has implications on the management of freshwater resources in the region.This research was supported by IAEA and UNESCO (IOC and IHP) in the framework of the joint SGD project. Science support for some U.S. investigators was provided by grants from the National Science Foundation (OCE03-50514 to WCB and OCE02-33657 to WSM)

Mechanical Strength of 17 134 Model Proteins and Cysteine Slipknots

A new theoretical survey of proteins' resistance to constant speed stretching is performed for a set of 17 134 proteins as described by a structure-based model. The proteins selected have no gaps in their structure determination and consist of no more than 250 amino acids. Our previous studies have dealt with 7510 proteins of no more than 150 amino acids. The proteins are ranked according to the strength of the resistance. Most of the predicted top-strength proteins have not yet been studied experimentally. Architectures and folds which are likely to yield large forces are identified. New types of potent force clamps are discovered. They involve disulphide bridges and, in particular, cysteine slipknots. An effective energy parameter of the model is estimated by comparing the theoretical data on characteristic forces to the corresponding experimental values combined with an extrapolation of the theoretical data to the experimental pulling speeds. These studies provide guidance for future experiments on single molecule manipulation and should lead to selection of proteins for applications. A new class of proteins, involving cystein slipknots, is identified as one that is expected to lead to the strongest force clamps known. This class is characterized through molecular dynamics simulations.Comment: 40 pages, 13 PostScript figure

HIV Status Disclosure and Retention in Care in HIV-Infected Adolescents on Antiretroviral Therapy (ART) in West Africa

We assessed the effect of HIV status disclosure on retention in care from initiation of antiretroviral therapy (ART) among HIV-infected children aged 10 years or more in Cote d'Ivoire, Mali and Sénégal.Multi-centre cohort study within five paediatric clinics participating in the IeDEA West Africa collaboration. HIV-infected patients were included in this study if they met the following inclusion criteria: aged 10-21 years while on ART; having initiated ART ≥ 200 days before the closure date of the clinic database; followed ≥ 15 days from ART initiation in clinics with ≥ 10 adolescents enrolled. Routine follow-up data were merged with those collected through a standardized ad hoc questionnaire on awareness of HIV status. Probability of retention (no death or loss-to-follow-up) was estimated with Kaplan-Meier method. Cox proportional hazard model with date of ART initiation as origin and a delayed entry at date of 10th birthday was used to identify factors associated with death or loss-to-follow-up.650 adolescents were available for this analysis. Characteristics at ART initiation were: median age of 10.4 years; median CD4 count of 224 cells/mm³ (47% with severe immunosuppression), 48% CDC stage C/WHO stage 3/4. The median follow-up on ART after the age of 10 was 23.3 months; 187 adolescents (28.8%) knew their HIV status. The overall probability of retention at 36 months after ART initiation was 74.6% (95% confidence interval [CI]: 70.5-79.0) and was higher for those disclosed compared to those not: adjusted hazard ratio for the risk of being death or loss-to-follow-up = 0.23 (95% CI: 0.13-0.39).About 2/3 of HIV-infected adolescents on ART were not aware of their HIV status in these ART clinics in West Africa but disclosed HIV status improved retention in care. The disclosure process should be thus systematically encouraged and organized in adolescent populations