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Boron (5 B) naturally occurs as isotopes 10 B and 11 B, the latter of which makes up about 80% of natural boron. There are 13 radioisotopes that have been discovered, with mass numbers from 7 to 21, all with short half-lives, the longest being that of 8 B, with a half-life of only 771.9(9) ms and 12 B with a half-life of 20.20(2) ms.
The nuclear industry enriches natural boron to nearly pure 10 B. The less-valuable by-product, depleted boron, is nearly pure 11 B. [149] Enriched boron or 10 B is used in both radiation shielding and is the primary nuclide used in neutron capture therapy of cancer.
Boron-10 atoms strongly absorb neutrons to form a metastable state of boron-11, which undergoes α-decay. By accumulating boron-10 in cancerous cells and subjecting the tumor to neutron radiation, high-energy α particles are selectively delivered only to the target cells. [1]
The development of boron delivery agents for BNCT began in the early 1960s and is an ongoing and difficult task. A number of boron-10 containing delivery agents have been synthesized for potential use in BNCT. [9] [20] [21] The most important requirements for a successful boron delivery agent are:
The only stable nuclides having an odd number of protons and an odd number of neutrons are hydrogen-2, lithium-6, boron-10, nitrogen-14 and (observationally) tantalum-180m. This is because the mass–energy of such atoms is usually higher than that of their neighbors on the same isobaric chain, so most of them are unstable to beta decay.
An example of cosmic ray spallation is a neutron hitting a nitrogen-14 nucleus in the Earth's atmosphere, yielding a proton, an alpha particle, and a beryllium-10 nucleus, which eventually decays to boron-10. Alternatively, a proton can hit oxygen-16, yielding two protons, a neutron, and again an alpha particle and a beryllium-10 nucleus.
Observed cross sections vary enormously: for example, slow neutrons absorbed by the (n, ) reaction show a cross section much higher than 1,000 barns in some cases (boron-10, cadmium-113, and xenon-135), while the cross sections for transmutations by gamma-ray absorption are in the region of 0.001 barn.
Approximately 90% of the tritium in PWR coolants is produced by reactions of boron-10 with neutrons. Since tritium itself is a radioactive isotope of hydrogen, the coolant becomes contaminated with radioactive isotopes and must be kept from leaking into the environment. Additionally, this effect must be taken into account for longer cycles of ...