radioactivity measurement and surveying

radioactivity measurement and surveying The radioactivity of a sample is measured by electrical or photographic methods and is expressed as the number disintegrations observed in unit time. The basic unit for the measurement of radioactivity is the curie (Ci), which is defined as the quantity of any radioactive nuclide in which the number of disintegrations in 3.7 × 10¹⁰ per second. For radium, a curie is the amount of radioactive disintegration in 1 g of ²²⁶₈₈Ra. The SI unit of radioactivity is the becquerel (Bq), which is defined as the amount of the radioisotope that gives one disintegration per second. Thus, 1[Bq] = 27 × 10⁻¹² [Ci].

Radioactivity needs to be surveyed because the passage of ionizing radiation through biological tissues causes injury, although the symptoms of injury generally become evident much later. The nature and severity of the threat posed by different kinds of particles and radiations depend on the type of particle or radiation, its energy, the duration of the exposure, the parts of the body that are exposed, and whether the exposure is internal or external. The emission energies of alpha, beta, and gamma (α, β, and γ) particles vary as well as the emission range. Beta particles of 0.5 MeV have an emission range of 1 m in air; whereas ? MeV particles have a range of 10m. When taken up by obstacles, beta particles produce a more penetrative secondary radiation known as bremsstrahlung. The best protective barriers are solids of low atomic number such as aluminium, Perspex, and rubber. For example, 2.5 cm of Perspex will protect against beta emission of energy as high as 4 MeV. A beta particle with an energy of 1 MeV will penetrate only 4 mm of human skin. The body-penetrating power of alpha particles is even less. Gamma rays are more difficult to stop because they are not ionic. Their intensity falls away exponentially. Materials of high atomic energy may exhibit photoelectric absorption, the energy of gamma rays is then transferred to an electron, which is then ejected from the atomic shield. Cobalt-60, ⁶⁰Co, a dangerous product of nuclear fission, emits gamma rays with a mean energy of 1.26 MeV. To reduce their dose by a factor of 10, 4.6 cm of lead is needed; and for each subsequent tenfold reduction a further 4.6 cm of shielding is necessary.

Exposure to radiation is measured in rads. One rad leads to the release of 100 ergs g⁻¹ (10⁻⁵ J g⁻¹) in body tissue. This is also equivalent to exposure to a level of one roentgen. Exposure is also measured in rems. The rem takes account of a ‘quality factor’, which for the more dangerous alpha particles is 10. Exposure to radiation at the level of 1 rad is rated as 10 rems. The gray (Gy) is the SI unit for absorbed dose, 1 J kg⁻¹. The SI unit for dose equivalent is the sievert (Sv), which is measured in grays times a quality factor for the type of radiation and the weighting factor for the tissue irradiated. The sievert is numerically equivalent to the gray for electrons and for X-rays irradiating the whole body.

Concern about human radiation exposure in the natural and built environment is currently focused on radon, the only radioactive gas. It is formed by the decay of uranium and thorium, and is found in the atmosphere. If inhaled, the gas can cause damage to the lungs. ²²²Rn has a half-life of 3.82 days. In the open air, the concentration of radon is generally very low. However, in buildings constructed on or out of rocks containing high quantities of uranium, such as granite or bricks, radon levels can rise well above background levels. In the UK the National Radiological Protection Board has defined areas where radiation of over 200 Bq m⁻³ is measured in more than 1 per cent of houses as ‘action areas’. In these areas, which are primarily centred around the granites of Devon and Cornwall, special measures must be taken in ventilating houses.

K. Vala Ragnarsdottir