<P> The isotopes of nobelium are mostly produced by bombarding actinide targets (uranium, plutonium, curium, californium, or einsteinium), with the exception of nobelium - 262, which is produced as the daughter of lawrencium - 262 . The most commonly used isotope, No, can be produced from bombarding curium - 248 or californium - 249 with carbon - 12: the latter method is more common . Irradiating a 350 μg cm target of californium - 249 with three trillion (3 × 10) 73 MeV carbon - 12 ions per second for ten minutes can produce around 1200 nobelium - 255 atoms . </P> <P> Once the nobelium - 255 is produced, it can be separated out in a similar way as used to purify the neighboring actinide mendelevium . The recoil momentum of the produced nobelium - 255 atoms is used to bring them physically far away from the target from which they are produced, bringing them onto a thin foil of metal (usually beryllium, aluminium, platinum, or gold) just behind the target in a vacuum: this is usually combined by trapping the nobelium atoms in a gas atmosphere (frequently helium), and carrying them along with a gas jet from a small opening in the reaction chamber . Using a long capillary tube, and including potassium chloride aerosols in the helium gas, the nobelium atoms can be transported over tens of meters . The thin layer of nobelium collected on the foil can then be removed with dilute acid without completely dissolving the foil . The nobelium can then be isolated by exploiting its tendency to form the divalent state, unlike the other trivalent actinides: under typically used elution conditions (bis - (2 - ethylhexyl) phosphoric acid (HDEHP) as stationary organic phase and 0.05 M hydrochloric acid as mobile aqueous phase, or using 3 M hydrochloric acid as an eluant from cation - exchange resin columns), nobelium will pass through the column and elute while the other trivalent actinides remain on the column . However, if a direct "catcher" gold foil is used, the process is complicated by the need to separate out the gold using anion - exchange chromatography before isolating the nobelium by elution from chromatographic extraction columns using HDEHP . </P>

The symbol 'co' refers to which chemical element