Radioisotopes (RI) such as 3H, 14C, 32P, and 45Ca are excellent tools in biological research. Most RI are used as tracers in studies of primary and secondary metabolism, drug metabolism, transcription, translation, post-translational modifications such as protein phosphorylation, association of proteins with metals, and transport of metals across biomembranes. Furthermore, some experiments have used neutrons for mutagenesis of microorganisms, animals, and plants.
Biochemical experiments with RI provide information on the fates of metabolites, nutrients, and inorganic ions at each periodic stage of living organisms or cells. In the early era of molecular biology, 32P was used as an essential tool in a large number of laboratories to determine DNA sequences and to identify target DNAs or mRNAs. Phosphorylation of serine and/or tyrosine residues is a key covalent modification of proteins.
The technique of comparing the abundance ratio of a radioactive isotope to a reference isotope to determine the age of a material is called radioactive dating. Many isotopes have been studied, probing a wide range of time scales.
The isotope 14C, a radioactive form of carbon, is produced in the upper atmosphere by neutrons striking 14N nuclei. The neutron is captured by the 14N nucleus and knocks out a proton. Thus, we have a different element, 14C. The isotope, 14C, is transported as 14CO2, absorbed by plants, and eaten by animals. If we were to measure the ratio of 14C to 12C today, we would find a value of about one 14C atom for each one-trillion 12C atoms. This ratio is the same for all living things–the same for humans as for trees or algae.
Once living things die, they no longer can exchange carbon with the environment. The isotope 14C is radioactive, and beta-decays with a half-life of 5,730 years. This means that in 5,730 years, only half of the 14C will remain, and after 11,460 years, only one quarter of the 14C remains. Thus, the ratio of 14C to 12C will change from one in one-trillion at the time of death to one in two trillion 5,730 years later and one in four-trillion 11,460 years later. Very accurate measurements of the amount of 14C remaining, either by observing the beta decay of 14C or by accelerator mass spectroscopy (using a particle accelerator to separate 12C from 14C and counting the amount of each) allows one to date the death of the once-living things.
Other methods of dating are used for non-living things. 40K decays with a half-life of 1.3 109 years to 40Ar which can be trapped in rocks. A potassium-argon method of dating, developed in 1966, measures the amount of 40Ar arising from the 40K decay and is compared to the amount of 40K remaining in the rock. From the ratio, the time since the formation of the rock can be calculated.
The age of our galaxy and earth also can be estimated using radioactive dating. Using the decays of uranium and thorium, our galaxy has been found to be between 10 and 20 billion years old and the earth has been found to be 4.6 billion years old. The Universe must be older than our galaxy. Within experimental error, this estimate agrees with the 15 billion year estimate of the age of the Universe.
Removal of landmines
There are two principal techniques to detect land mines through nuclear reactions. Both rely on the use of neutrons. The first such technique relies on the fact that the vast majority of explosives used in land mines are very nitrogen rich when compared with other materials.
In practice a detection system using this reaction works by subjecting the mine to thermal neutrons while searching for the characteristic gamma ray emitted from the excited state in nitrogen-15; these photons will only be observed when an object containing nitrogen is being subjected to the neutron irradiation.
One possible neutron source is californium-252 which undergoes spontaneous fission. A better neutron source is to use a sealed tube electrostatic D-T neutron generation tube, this has the advantage that the tritium is much less radiotoxic than the californium so in the event of an accident such as an explosion, the nuclear mine detection equipment would pose a smaller threat to humans. This type of explosive detection has been proposed for use in airport security and for the detection of explosives in trucks coming into military bases.
Nuclear powered ships
Nuclear power is particularly suitable for vessels which need to be at sea for long periods without refuelling, or for powerful submarine propulsion. Over 140 ships are powered by more than 180 small nuclear reactors and more than 12,000 reactor years of marine operation has been accumulated. Most are submarines, but they range from icebreakers to aircraft carriers.
In future, constraints on fossil fuel use in transport may bring marine nuclear propulsion into more widespread use. So far, exaggerated fears about safety have caused political restriction on port access. Work on nuclear marine propulsion started in the 1940s, and the first test reactor started up in USA in 1953. The first nuclear-powered submarine, USS Nautilus, was put to sea in 1955. This marked the transition of submarines from slow underwater vessels to warships capable of sustaining 20-25 knots submerged for weeks on end. The submarine had come into its own.
By 1962 the US Navy had 26 nuclear submarines operational and 30 under construction. Nuclear power had revolutionised the Navy. The technology was shared with Britain, while French, Russian and Chinese developments proceeded separately.
The safety record of the US nuclear navy is excellent, this being attributed to a high level of standardisation in naval power plants and their maintenance, and the high quality of the Navy’s training program. However, early Soviet endeavours resulted in a number of serious accidents – five where the reactor was irreparably damaged, and more resulting in radiation leaks. There were more than 20 radiation fatalities. Nevertheless, by Russia’s third generation of marine PWRs in the late 1970s safety and reliability had become a high priority. (Apart from reactor accidents, fires and accidents have resulted in the loss of two US and about 4 Soviet submarines, another four of which had fires resulting in loss of life.)