Radiation is a form of energy propagating through space and material as an electromagnetic wave or particles.
Radioactivity refers to the property of atomic nuclei to transform into other nuclei without extraneous cause, thereby emitting high-energy radiation. The nucleus produced as a result of this transformation ("daughter nuclide") may in turn be radioactive, too, and may be subject to further radioactive decay. Final products of this transformation process are stable atoms that are not radioactive any more.
The process of nuclear transformation is termed "radioactive decay", and the radiation emitted is called "ionising radiation", due to its property to induce ionisation processes on atoms and molecules when penetrating substances.
The term "radioactive" radiation, which is frequently used in common speech, therefore is scientifically not correct. "Radioactive" refers to the atoms which are generally termed "radionuclides".
During radioactive decay the following types of ionizing radiation may be emitted:
Alpha radiation:
Alpha radiation is particulate radiation consisting of nuclei of the element helium (alpha particles). Alpha particles have a very short range (a few centimetres in air, less than one millimetre in water). They may, therefore, be shielded by a sheet of paper, and external irradiation represents no hazard, since the outer skin layers are not permeated.
Radionuclides entering the body (incorporation) may cause considerable radiation exposure. Incorporated with food or breathable air, radionuclides reach the blood and tissues via the intestine or lung, leading to irradiation of cells (see radiation effect) As a result of a high energy transfer over short distance, alpha particles cause particularly serious tissue damage (high biological effectiveness). Uptake of radon and its daughter products with air or food is a typical example of incorporation of alpha particles.
Beta radiation:
Beta radiation is particulate radiation consisting of electrons (beta particles) or - less frequently - of positrons. The penetrating power of beta particles in air is in a range of several centimetres to metres, in soft tissue or synthetics a few millimetres to centimetres.
Radionuclides entering the body (incorporation) may cause considerable radiation exposure. Incorporated with food or breathable air, radionuclides reach the blood and tissues via the intestine or lung, leading to irradiation of cells (see radiation effect).
Gamma radiation:
Gamma radiation is electromagnetic radiation. Its physical nature is like that of visible light, but it is of considerably higher energy and high penetrating power in material. Shielding of gamma radiation therefore requires heavy materials such as lead and concrete. Apart from its origin, gamma radiation is comparable to X-radiation.
Radionuclides entering the body (incorporation) may cause considerable radiation exposure. Incorporated with food or breathable air, radionuclides reach the blood and tissues via the intestine or lung, leading to irradiation of cells (see radiation effect).
Neutron radiation:
Neutron radiation is composed of uncharged particles (neutrons). Neutron radiation is hardly impaired by air at all. Materials used for shielding of neutron radiation have as high a hydrogen proportion as possible (e.g. paraffin, polyethylene, water) in order to slow down the neutrons. Subsequently, the neutrons must be captured by an absorber (e.g. boron or cadmium). The gamma radiation emerging in this process is shielded using lead.
Neutrons are released during nuclear fission, i.e. a special form of nuclear transformation. Nuclear fission is characteristic of heavy atomic nuclei only, e.g. those of the element uranium.
Activity:
The number of nuclear disintegrations occurring per unit time in a substance is termed "activity". The unit of activity of a radioactive substance is Becquerel (Bq;
1 Bq = 1 nuclear disintegration per second).
For use in practice, activity is related to another quantity , for example to an area ("areal activity" in Becquerel per square centimetres), a volume (Becquerel per cubic metres), or a mass (Becquerel per gram). If the radionuclide's activity is related to its mass, the so-called "specific activity" is obtained. This is an important and invariable parameter of any radionuclide: Specific activities vary over a very wide range of values. For example, the specific activity for the radionuclide Caesium-137 amounts to as much as about 3.2 Terabecquerel (Tbq) per gram (1TBq = 1,000,000,000,000 Bq), whereas that for the long-lived uranium isotope U-238 is only 12,400 Bq/g.
Half-life:
The decay of radioactive atoms finally produces stable atoms. The number of radioactive atoms in a defined quantity of material decreases with time. The time elapsing until only half the original radioactive nuclei exist is called "half-life". This is also a measure of the time during which the intensity of the ionising radiation emitted from the radioactive substance in question decreases to half the initial value. Correspondingly, the activity decreases to one fourth after two half-lives and to one eighth after three half-lives (corresponding to 12.5 per cent) of the initial value. After ten half-lives, the activity of the substance is about one thousandth of the initial value.
The half-life of a radionuclide is a characteristic, that is, fixed and exactly known property. The half-lives of the different radionuclides range from split seconds up to some billions of years.
X-radiation:
X-radiation is part of ionising radiation and its physical nature is not different from that of gamma radiation. X-radiation is technically produced when high-energy electrons are decelerated at the anode of an X-ray tube. The very short-waved radiation is the more penetrating the higher the tube voltage applied to accelerate the electrons.
In contrast to nuclear radiation, the existence of which is bound to radionuclides and which is emitted until disintegration of the final radionuclide, the production of X-radiation is brought to an end as soon as the X-ray equipment is deactivated.
Radiation effect:
When ionising radiation hits the human body, radiation exposure occurs. This means that tissues absorb radiation to a varying extent, giving rise to interaction on a molecular level. The “amount” of radiation absorbed in the body is termed dose. The different types of radiation, however, produce strongly differing biological effects related to the same absorbed dose.
When a tissue is exposed to a defined amount of absorbed dose produced by alpha radiation or beta radiation, the biological effect of alpha radiation is approximately 20 times that of beta radiation. Absorbed dose alone, therefore, is not sufficient to describe the biological effect of radiation in the human body.
Ionising radiation, regardless whether of natural or artificial origin, has a directly damaging effect on the cell as the smallest biological unit. Radiation is able to change or destruct cellular constituents and here especially the cellular genetic material (DNA). Cell damage or changes in cells are, however, not automatically the same as development of health detriment.
The organism is able to compensate for cell damage and to identify damaged cells and reconstitute the normal state by repair mechanisms, cell destruction and immune defence.
However, the defence or repair mechanisms my fail or be overstrained. In such cases, decisive factors are, among others, the amount of dose and the type of radiation. The time during which the dose is obtained and the spatial distribution of radiation-induced cell damages are of substantial influence on the repair systems’ "efficiency".
When a given dose is obtained within a short period, thus inducing an increased number of damages within one cell at the same time, the repair system may reach its limits. When the same dose is distributed over a long period - thus producing only relatively few damage at the same time - , the chance for complete repair is higher.
Biological effects of ionising radiation on man occur in the following two ways:
Deterministic radiation effects are radiation effects occurring when high dose levels are exceeded. They may be directly traced back to a certain radiation exposure. They occur immediately or within a few weeks after exposure. They express themselves only when a certain amount of killed or damaged cells is exceeded. This type of damage, therefore, occurs only when a minimum dose – the threshold dose – is exceeded. There are different threshold doses for varying deterministic radiation effects such as anaemia, loss of hair, etc.. The lowest threshold doses for acute whole-body exposure are at 0.1 – 0.5 Sievert (Sv). This dose level may already result in short-term changes of the blood picture only detectably by a doctor. The higher the radiation dose, the more serious the disease.
The organs and tissues of outstanding radiosensitivity are mainly the haematopoietic organs, the mucosae of the gastrointestinal tract and of the respiratory passages, as well as the gonads and embryonal tissue. For teratogenic damage, i.e. congenital malformations following maternal irradiation, a threshold of about 100 Millisievert (mSv) must be assumed.
Acute whole-body exposure to about four Gray (Gy) may lead to death in man.
Stochastic radiation effects are based on random, i.e. stochastic events. When a cell’s information content is changed due to radiation exposure of the nucleus and is not sufficiently repaired by the organism subsequently, this change may be passed on to succeeding cell generations, if such a changed cell remains viable and capable of division.
Stochastic radiation effects occur as a function of dose with a certain probability. The latency, that is, the period between radiation exposure and appearance of the disease, may be years to decades. The outcome may be either a mutation (that is, genetic change/change in genetic traits) with potential health consequences for succeeding generations, or malignancies such as cancer and leukaemia in the radiation exposed individual, depending upon whether a germ cell or a somatic cell is involved. Currently under debate is whether cardio-vascular disease and cataracts of the eye may be stochastic effects.
The probability of a stochastic radiation effect is expressed by the term “detriment”. The risk is estimated on the basis of observed disease incidence in exposed population groups. The largest population group observed in this context are the atomic bomb survivors in Hiroshima and Nagasaki.
On the basis of laboratory experimental findings and biomedical models for cancer induction, this risk is extrapolated to low-dose levels relevant in the environment and working world. Thereby, it is basically assumed - on grounds of radiobiological findings - that even the lowest radiation exposure is associated with a correspondingly low radiation risk.
Radiation-induced cancers can only be identified using epidemiological-statistical methods applied to relatively large groups of individuals based on increased disease rates. They cannot be detected in individuals based on the disease pattern. Disease patterns of radiation-induced diseases do not differ from those of so-called spontaneous diseases.
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