Magic composite pulses

I describe composite pulses during which the average dipolar interactions within a spin ensemble are controlled while realizing a global rotation. The construction method used is based on the average Hamiltonian theory and rely on the geometrical properties of the spin-spin dipolar interaction only. I present several such composite pulses robust against standard experimental defects in NRM: static or radio-frequency field miscalibration, fields inhomogeneities. Numerical simulations show that the magic sandwich pulse sequence, a pulse sequence that reverse the average dipolar field while applied, is plagued by defects originating from its short initial and final \pi/2 radio-frequency pulses. Using the magic composite pulses instead of \pi/2 pulses improves the magic sandwich effect. A numerical test using a classical description of NMR allows to check the validity of the magic composite pulses and estimate their efficiency.Comment: 22 pages, 6 figure

Influence de la température sur l'accumulation par la voie directe du 60Co chez un poisson dulçaquicole

L'influence du facteur thermique sur la contamination de la carpe par le 60Co a été étudiée pour 4 températures : 8 °C, 15 °C, 20 °C et 25 °C. Pour chaque expérience, un lot homogène de 10 individus, d'une masse initiale d'environ 2,3 g et nourris ad libitum, est introduit dans 5 litres d'eau, renouvelée tous les 3 à 4 jours et contaminée à raison de 1,2.105 Bq.l-1.Entre deux renouvellements, la concentration du 60Co dans l'eau décroît sensiblement et l'état ionique du radionucléide, introduit à 100 % sous forme cationique, évolue vers des formes anioniques et neutres. Ces deux faits sont liés au métabolisme des poissons dont la croissance pondérale est corrélée à la température, la distribution de la nourriture étant adaptée aux besoins des individus.Globalement, la quantité totale de radiocobalt fixée par les carpes apparaît très fortement liée au facteur thermique mais, selon le niveau des températures, des situations différentes se dégagent pour des mêmes écarts. Il n'y a pas proportionnalité comptéte entre élévation de température et accumulation du 60Co mais une augmentation discontinue témoignant de l'existence de paliers, dont le plus important se situe à 20 °C.Le facteur de concentration du 60Co est, par contre, peu influencé par le facteur thermique pour les écarts inférieurs à 10 °C ou quand la température maximale est inférieure à 20 °C. L'incidence de la température devient sensible quand les écarts dépassent 10 °C et notamment lorsque sont atteintes des valeurs correspondant à l'optimum thermique des carpes. A l'équilibre, le facteur de concentration moyen du 60Co est de 3,6 à 8 °C, 4,4 à 15 °C, 5,1 à 20 °C et 6 à 25 °C.The problems of simultaneous thermal and radionuclide pollutions have led to a number of laboratory and field studies of the temperature effects on the radioactive contamination of aquatic organisms. AMIARD and KHALANSKI (1981) concluded from their bibliographic study that a temperature rise generally has a positive effect on biological accumulation of radionuclides in freshwater organisms. Due to insufficient data, however, this conclusion does not appear to be substantiated for 60Co uptake by fish. An experimental study was therefore undertaken to assess the effect of temperature on 60Co uptake by Caprinus carpio.The importance of 60Co results from the fact that, apart from tritium, radioactive cobalt isotopes represent the major components of liquid wastes discharged from pressurized water reactors, which are widely operated in France (BARDIN et al., 1982; PICAT et al., 1986). The common carp (Caprinus carpio L.) was selected as the subject of this experimentation because of its wide geographic distribution and the frequency with which it is found in field samples, making it a suitable biological indicator (D'AUBENTON and SPILLMANN, 1978; FOULQUIER and PALLY, 1982; FOULQUIER et al., 1985).The effect of temperature on 60Co contamination of the carp was investigated by conducting the same experiment at four temperatures (8 °C, 15 °C, 20 °C and 25 °C) determined by ecological and physiological criteria. In each experiment, young carps with a mean weight of 2.3 g were placed in a tank containing 5 liters of water contaminated with 1.2 x 105 Bq.l-1 of 60Co. The water was renewed at intervals of 3 - 4 days. Modifications in the ionic form of the radio-active cobalt dissolved in the water were monitored by percolating filtered water samples through ion exchange resin columns. The carps were fed to satiation with an industrial food containing 12 % minerals, 8 % fats and 58 % proteins (dry weight) together with vitamins and trace elements including 0.2 mg.kg-1 of cobalt. Radioactivity levels in the fish were measured, after mild anesthesia, with a single-channel amplitude selector equipped with a thallium-activated flat sodium iodine probe.Between successive water renewals, the 60Co concentration diminished (Table 1) and the ionic form of the radionuclide remaining in solution was significantly modified. The 60Co was introduced in chloride form and was initially found exclusively as a cation; anionic or neutral forms appeared and became dominant after a few days (Figure 1). These two phenomena are probably related to the fish metabolism which is temperature dependent. Thus, food was distributed according to the individual requirements, and an exponential correlation was observed between the temperature and the weight gain in the carps (Table 1). The appearance of anionic and neutral forms may be attributed to excessive amounts of organic matter due to the presence of the fish and food (FUKAI and MURRAY, 1973; KILLEY et al., 1984). This evolution also results from a significant increase in ions such as HCO3-, Cl-, SO4--, PO4-- and NH4+ (FRITSCH and BAUDIN, 1984) which may combine with Co++ ions to form neutral molecules and anionic complexes (TRISHAN et al., 1981). Radioactive decay in the mater is also temperature-dependent : the amount of fecal matter, which strongly adsorbs radioactive cobalt, is proportional to the metabolic intensity in the fish.The overall 60Co uptake by the carps (Table 2) appears to be strongly related to the temperature, but the effect of a given temperature rise varied with the initial temperature (Table 4). The 60Co uptake was not fully proportional to the temperature rise, but was observed to increase in stepwise fashion indicating the existence of various thresholds. The most significant of these was noted at 20 °C, which is the lower limit of the optimum temperature range for carps.The 60Co concentration in th fish (Table 3) was influenced to a much lower extent by the temperature than the total uptake. Small temperature variations (5 - 7 °C) had little or no effect on the concentration factor; this was also true for larger variations, from 8 °C to 20 °C (Table 4). Temperature effects were only significant between 8 °C and 25 °C, and between 15 °C and 25 °C.The 60Co concentration variation in time was similar for all four temperatures, with two successive phases (Figure 2). It can be expressed mathematically by an exponential model based on the existence of several compartments in living organisms characterized by the influx and efflux rates for the element under consideration (GOLSTEIN and ELWOOD, 1971; AOYAMA and INOUE, 1973; PENTREATH, 1975; GONTIER, 1983). The maximum value and the time required to reach equilibrium were determined by extrapolation from the equations for the concentration factor variations. The results were 3.6 and 510 days at 8 °C, 4.4 and 220 days at 15 °C, 5.1 and 125 days at 20 °C, and 6 and 110 days at 25 °C. These values indicate that a temperature rise diminishes the time required to reach equilibrium and slightly increases the concentration factor by an amount that is statistically significant only for major temperature differences (Table 4).Detailed investigations of various freshwater species, notably Caprinus carpio, have demonstrated the decisive effect of temperature on metabolic activity in these animals (LEVESQUE, 1980). The results presented here indicate that the 60Co uptake rate in the carp is not proportional to the temperature rise as is generally the case for physiological processes. From a radiation protection standpoint, this observation suggests that the temperature is not an essential factor in 60Co contamination of freshwater fish

Données sur les transferts du 137Cs et du 60Co dans un écosystème fluvial : le Rhône

L'étude radioécologique du Rhône permet d'évaluer qualitativement et quantitativement les radionucléides présents dans le fleuve. Les études menées in situ posent des questions concernant les modalités de transfert des radionucléides. Dans ce travail des expériences sont mises au point, afin d'analyser les mécanismes de bioconcentration dans les écosystèmes aquatiques. Pour le césium-137 les échanges entre l'eau, le sédiment et divers organismes aquatiques ont permis d'élaborer un modèle mathématique que l'on peut confronter aux valeurs mesurées sur le terrain. En ce qui concerne le Cobalt-60 les auteurs décrivent des expériences permettant l'évaluation de la contribution relative de l'eau et de la nourriture dans l'accumulation du radionucléide par un poisson.The radioecology of the Rhone Basin has been studied for the last 15 years. This has been an opportunity to make a quantitative and qualitative evaluation of radionuclides as a function of their different sources. Special attention is given to 137Cs (present both in fallout and liquid wastes) and 60Co, which characterize the liquid wastes of pressurised water reactors. In order to assess the transfer and bioconcentration of these two radionuclides in freshwater ecosystems, several experimental studies were undertaken.The 137Cs transfer studies were carried out with a 5-component experimental ecosystem and the data were included in a mathematical model. For 60Co, the experimental study concerns the relative contribution of water and food in the accumulation of the radionuclide by Cyprinus carpio.Water, sediment, plants and fishes were taken from 60 sampling stations set up along the river (figure 1). Water was filtrated, then percolated on resin columns. Sediment, plants and fishes were dried and burnt to ashes in an oven at 500° C. Radioactivity was measured by gamma spectrometry and radiochemistry.137Cs experimental transfers were studied between water, sediment, midge larvae, daphnid and carp. These components were taken in pairs in order to estimate the radionuclide transfer from one to the other. Thus ten experiments were carried out (figure 2). In order to study the relative importance of food and water as 60Co sources for the carp, an experiment was carried out simultaneously on three homogeneous groups of ten juvenile fishes. The individuals of the first group were maintained in separate aquaria and offered 45 daily rations of labelled food over a 63-day period. Bach carp of the other two groups was placed in a compartment of a large tank with contaminated water. One group was fed with radioactive food, the other with non-radioactive food (table 1).Natural radioactivity remained steady all along the river. It ranged around 1 Bq.l-1 in water, 2250 Bq.kg-1 DW in sediment, 1700 Bq.kg-1 DW in aquatic plants, 110 Bq.kg-1 WW in fish. The fallout impact was characterized by 137Cs presence. PWR liquid wastes contained mainly, 58Co, 60Co, 134 Cs, 137Cs. The Chernobyl fallout gave an increase of Cs and the presence of 103Ru and 106RU+Rh specially during May and June 1986 which later decreased (tables 2, 3 and 4).137Cs transfer between water and sediment was very fast and important. Less than 2 % of the radionuclide was released from sediment into a non-radioactive water. During the transfer from water to chironomids the larvae radioactivity increased steadily (figure 3). Conversely, the transfer from the sediment to larvae did not seem to depend on the contact time. The transfer from water to carp was regular without any steady state during the 63 days of the experiment (figure 3). Then the fish concentration factor was less than 5. For 42 days, the transfer factor from sediment to carp was 3.6.10-3. The retention factor from food to carp was 0.03 when fishes were fed with daphnids and 0.13 with chironomids. An experiment showed that the various ways of 137Cs transfer could have an added impact. Thus the carp radioactivity was the sum of the separate transfers. Water was responsible for 4 % of the fish 137 Cs concentration, sediment for 45 % and chironomids for 51 %.It is possible to include the different kinetic equations in a mathematical model. If the radioactivity of one of the components is known, the nuclide concentration can be computed in others, as a relation of the contact time, the quantity and quality of ingested food, etc... This model gives a concentration factor for juvenile carp of 1000 in 180 days and 500 for 3-year old fish. Considering the field conditions (e.g. seasonal nutritive cycles) the computed concentration factor in fish was between 200 and 350. For a 1 mBq.l-1 137Cs concentration in water, the model gave a concentration of 0.2 to 0.35 Bq.kg-1 WW in carp, which was the 137Cs radioactivity level measured in the Rhone fish before the Chernobyl accident.During the 60Co accumulation phase, the mean weight of the fish in the three groups increased exponentially and the resultant relative weight gain was 52-59 % after 63 days (table 5).The 60Co accumulation kinetics showed that the steady state should be reached after 165 days for fish exposed to 60Co in food, 92 days for fish exposed to radiocobalt in water and 120 days for fish exposed to 60Co in both sources (figure 4). According to the 60Co concentration in the fish in the three treated groups, the accumulation from water accounted for 75 % of the total radioactivity and the accumulation of the radionuclide from both water and food was in addition.Depuration of 60Co from carp was a relatively intensive process reflecting a high Co turnover. Biological half-lives for loss from the long-lived compartment ranged from 35d in fish previously contaminated by food, to 87d in fish previously contaminated by food, to 87d in fish previously contaminated by water (figure 5).137Cs and 60Co are the most concentrated radionuclides in liquid wastes of the pressurised water reactors, and they are often measured in the aquatic ecosystem components. Though it accounts for the highest fraction of total radioactivity in the liquid wastes, 60Co cobalt is not the most concentrated radionuclide in fish. Experimental studies show that it is primarily transferred from the water so it is logical that its concentration in fish remains at a low level. Conversely the 137Cs has a low concentration in water but as it is transferred simultaneously from water, sediment and food, its concentration in fish is still important. Moreover its 30 years half-life means that the cesium contamination of fish is a long and important process, all the more so as the source terms can add their own effects during time and space

Long-Term Economic Development and Demand for Forest Products

The FAO Outlook Studies were published in October 1986. Projections of the demand and supply of forest products were presented up to year 2880 by country and for aggregates of countries. The Outlook Studies were based on two alternative economic scenarios: the FAO and Chase-Manhattan GDP growth-rate projections. The effect of varying the input to the FAO model system was, however, modest and did not reflect the inherent uncertainty of long-term projections. As an alternative, a pessimistic economic scenario has been developed based on simple time-series extrapolations of GDP growth rates. This subjective/statistical scenario gives generally substantially lower growth rates than the main alternatives of the Outlook Studies. This paper presents the methods of making projections of the FAO model system and demonstrates the effect of varying the input economic growth rates. Furthermore, it stresses the uncertainty aspects of long-term projections of the demand for forest products

The underlying physical meaning of the νmax−νc\nu_{\rm max}-\nu_{\rm c} relation

Asteroseismology of stars that exhibit solar-like oscillations are enjoying a growing interest with the wealth of observational results obtained with the CoRoT and Kepler missions. In this framework, scaling laws between asteroseismic quantities and stellar parameters are becoming essential tools to study a rich variety of stars. However, the physical underlying mechanisms of those scaling laws are still poorly known. Our objective is to provide a theoretical basis for the scaling between the frequency of the maximum in the power spectrum ($\nu_{\rm max}$) of solar-like oscillations and the cut-off frequency ($\nu_{\rm c}$). Using the SoHO GOLF observations together with theoretical considerations, we first confirm that the maximum of the height in oscillation power spectrum is determined by the so-called \emph{plateau} of the damping rates. The physical origin of the plateau can be traced to the destabilizing effect of the Lagrangian perturbation of entropy in the upper-most layers which becomes important when the modal period and the local thermal relaxation time-scale are comparable. Based on this analysis, we then find a linear relation between $\nu_{\rm max}$ and $\nu_{\rm c}$, with a coefficient that depends on the ratio of the Mach number of the exciting turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&