In this paper, we report on the influence of nitrogen concentration in metal melts on the growth processes, morphology, and defect-and-impurity structure of diamond crystals. In two series of experiments, the concentration of nitrogen in the growth system was varied by adding Fe 3 N and CaCN 2 to the charge; the other parameters and conditions of the growth were constant: FeNiC system, P = 5.5 GPa, T = 1400 °C, and duration of 65 h. It has been found that, with increasing nitrogen concentration (C N ) in the metal melt from 0.005 to 0.6 atom %, the growth of single crystal diamond is followed by formation of aggregates of block twinned crystals and then by crystallization of metastable graphite. At the stage of single crystal growth, an increase in C N results in an increase in nitrogen impurity concentration in diamond crystals from about 200 ppm to approximately 1100 ppm, an increase in density of dislocations, twin lamellae, and internal strains, and a change in crystal morphology. Further increases in C N result in formation of aggregates of block crystals with nitrogen concentration around 120-300 ppm. At nitrogen concentration in the melt higher than a certain critical value, nucleation and growth of diamond are terminated and graphite crystallizes in the diamond stability field.
A fter the accident at the Chernobyl nuclear reactor in 1986, the concentration of radioactive caesium ( 134 Cs and 137 Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time needed to reduce the average caesium concentration by 50%) was 0.3 to 4.6 years 1,2 . Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout 3,4 . We have measured radiocaesium in nearly 4,000 fish, taking samples 2-4 times every year from spring to autumn. We find that the decline in radiocaesium was initially rapid for 3-4 years and was then much slower. About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8-22 years.The concentration of 137 Cs, the longlived radioactive caesium isotope with a physical half-life of 30.2 years, peaked in 1986. The radioactivity was three times higher in brown trout than in Arctic charr (geometric means: 10,468 and 3,097 Bq kg ǁ1 ). The decline in 137 Cs from its maximum in 1986 to 1998 is modelled by singleand two-component decay functions: Q t ǃQe ǁkt and Q t ǃQ 1 e ǁk 1 t +Q 2 e ǁk 2 t , where Q is the caesium concentration, k is the decay rate and t is time in years after the peak. The ecological half-lives are ln2/k, and are an indication of how long it will take the fish to rid themselves of radioacti-vity. The proportional contribution of the maximum radioactivity with slow decay rates was estimated as Q 2 /(Q 1 +Q 2 ).The decline in 137 Cs was rapid during the first three (brown trout) and four (Arctic charr) years, and was then slower. Based on the initial rapid decline, ecological half-lives were estimated using a single-component decay function at 1.0 and 1.5 years for brown trout and Arctic charr, respectively, as in other post-Chernobyl studies 1,2 , but this underestimates the time that 137 Cs persists in the fish. A two-component decay function
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