give a deep red colour. This same effect identifies strontium cations in the flame test. Fireworks consume about 5% of the world's production. Strontium carbonate is used in the manufacturing of hard ferrite magnets. Strontium chloride is sometimes used in toothpastes for sensitive teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight. Small amounts are used in the refining of zinc to remove small amounts of lead impurities. The metal itself has a limited use as a getter, to remove unwanted gases in vacuums by reacting with them, although barium may also be used for this purpose. The ultra-narrow optical transition between the [Kr]5s2 1S0 electronic ground state and the metastable [Kr]5s5p 3P0 excited state of 87Sr is one of the leading candidates for the future re-definition of the second in terms of an optical transition as opposed to the current definition derived from a microwave transition between different hyperfine ground states of 133Cs. Current optical atomic clocks operating on this transition already surpass the precision and accuracy of the current definition of the second. Radioactive strontium 89Sr is the active ingredient in Metastron, a radiopharmaceutical used for bone pain secondary to metastatic bone cancer. The strontium is processed like calcium by the body, preferentially incorporating it into bone at sites of increased osteogenesis. This localization focuses the radiation exposure on the cancerous lesion. 90Sr has been used as a power source for radioisotope thermoelectric generators (RTGs). 90Sr produces approximately 0.93 watts of heat per gram (it is lower for the form of 90Sr used in RTGs, which is strontium fluoride). However, 90Sr has one third the lifetime and a lower density than 238Pu, another RTG fuel. The main advantage of 90Sr is that it is cheaper than 238Pu and is found in nuclear waste. The Soviet Union deployed nearly 1000 of these RTGs on its northern coast as a power source for lighthouses and meteorology stations. Biological role Acantharea, a relatively large group of marine radiolarian protozoa, produce intricate mineral skeletons composed of strontium sulfate. In biological systems, calcium is substituted to a small extent by strontium. In the human body, most of the absorbed strontium is deposited in the bones. The ratio of strontium to calcium in human bones is between 1:1000 and 1:2000, roughly in the same range as in the blood serum. Effect on the human body The human body absorbs strontium as