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The partial stopping power of a target material for an ion or an atom is the energy lost per target particle per cm 2 in all types of collisions in which the ion or the atom is the projectile, with the exception of those collisions in which the projectile leaves with its charge changed. Partial atomic stopping powers of target gases for atoms are measured by imposing a strong transverse magnetic field on the stopping cell. For partial ionic stopping-power measurements, the ion beam is held in a circular orbit, closely defined by vanes and slits, as it traverses the stopping cell. The partial stopping powers e 0 (for He 0 traversing H 2 gas), ei (for He + ), c 2 (for He ++ ) have been measured in the kinetic energy range 40-460 keV, and new measurements of the ordinary, or total stopping power of H 2 gas for helium from 40 to 220 keV have been made. Using the known charge composition of He beams in H 2 , it is possible to deduce the fraction of the stopping power losses due to charge changing collisions, and this varies from 37% at 140 keV to 27% at 400 keV. At 120 keV, the total energy loss in completing the charge changing cycle He + -> He 0 -* He + is 95±9 eV, rising to 117=b 10 at 160 keV. . 5 In a simple capture and loss cycle involving He 0 and He + in a beam of kinetic energy E electron volts, the number of cycles completed in an absorber is easily shown to be

A fully automatic method for the determination of organic matter has been implemented using a robotic station. The overall process involves weighing, dilution, oxidation in very harsh working conditions (K2Cr2O7 + concentrated H2SO4) dilution, centrifugation and photometric monitoring of the Cr(III) formed. Batches of six samples are manipulated by the robot, which also calibrates and delivers the results of the sample determination, both in soil and fertilizers, as percentages of organic matter which require slightly different sample treatment. The precision and validation of the method have been established using both types of samples and the results obtained compare well with those of the method used for routine analysis in official agricultural laboratories.

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