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In addition, shortening the time of exposure and shielding are also effective protective measures to reduce the hazard caused by such external exposure. Internal radiation exposure hazard is caused by radioactive source inside the body. Radioactive material can get inside the body through inhalation, ingestion or by passing through the wounds of your skin. Once radioactive substances enter the human body, it will produce radiation exposure the entire time they are inside the body until the material is no longer radioactive i.

For those radioactive substances with a long half-life or for those materials of which only a small amount could be get rid of by the body through excretion, the radioactive material will stay in the body for a longer time. Furthermore, some radioactive nuclides have affinity for certain human tissues or organs and will accumulate in the organ, causing more damages. For example, iodine beta particle and gamma ray emitter and strontium beta emitter tend to accumulate in the thyroid gland and the bone respectively whereas plutonium alpha emitter mainly accumulates in the bone and the liver.

Print Version. Relative Vulnerability of Alpha Particle 2. Relative Vulnerability of Beta Particle 3. Relative Vulnerability of Gamma Ray 4. Back to content Remarks: External irradiation occurs when all or part of the body is exposed to radiation from an external source. Internal radiation exposure results from radioactive material that gets inside the human body, e. Ionizing radiation consists of particles, including photons, which cause the separation of electrons from atoms and molecules. However, some types of radiation of relatively low energy, such as ultraviolet light, can also cause ionization under certain circumstances.

To distinguish these types of radiation from radiation that always causes ionization, an arbitrary lower energy limit for ionizing radiation usually is set around 10 kiloelectron volts keV. Directly ionizing radiation consists of charged particles.

Radiation - Wikipedia

Such particles include energetic electrons sometimes called negatrons , positrons, protons, alpha particles, charged mesons, muons and heavy ions ionized atoms. This type of ionizing radiation interacts with matter primarily through the Coulomb force, repelling or attracting electrons from atoms and molecules by virtue of their charges. Indirectly ionizing radiation consists of uncharged particles. The most common kinds of indirectly ionizing radiation are photons above 10 keV x rays and gamma rays and all neutrons.

X-ray and gamma-ray photons interact with matter and cause ionization in at least three different ways:. Lower-energy photons interact mostly via the photoelectric effect, in which the photon gives all of its energy to an electron, which then leaves the atom or molecule. The photon disappears. Intermediate-energy photons mostly interact through the Compton effect, in which the photon and an electron essentially collide as particles. Pair production is possible only for photons with energy in excess of 1. However, near 1.

Pair production dominates at higher energies. The photon disappears and an electron-positron pair appears in its place this occurs only in the vicinity of a nucleus because of conservation of momentum and energy considerations. The total kinetic energy of the electron-positron pair is equal to the energy of the photon less the sum of the rest-mass energies of the electron and positron 1. These energetic electrons and positrons then proceed as directly ionizing radiation.

As it loses kinetic energy, a positron will eventually encounter an electron, and the particles will annihilate each other. Two usually 0. For a given photon any of these can occur, except that pair production is possible only for photons with energy greater than 1.

Ionizing radiation, health effects and protective measures

The energy of the photon and the material with which it interacts determine which interaction is the most likely to occur. Figure The most common neutron interactions with matter are inelastic collisions, neutron capture or activation and fission. All of these are interactions with nuclei. A nucleus colliding inelastically with a neutron is left at a higher energy level. It can release this energy in the form of a gamma ray or by emitting a beta particle, or both.

In neutron capture, an affected nucleus may absorb the neutron and eject energy as gamma or x rays or beta particles, or both. The secondary particles then cause ionization as discussed above.

Ionising Radiation and Human Health

In fission, a heavy nucleus absorbs the neutron and splits into two lighter nuclei that are almost always radioactive. The International Commission on Radiation Units and Measurements ICRU develops internationally accepted formal definitions of quantities and units of radiation and radioactivity. The International Commission on Radiological Protection ICRP also sets standards for definition and use of various quantities and units used in radiation safety.

A description of some quantities, units and definitions commonly used in radiation safety follows. Absorbed dose. This is the fundamental dosimetric quantity for ionizing radiation. Basically, it is the energy ionizing radiation imparts to matter per unit mass. The special name for the unit of absorbed dose is the gray Gy.

This quantity represents the number of nuclear transformations from a given nuclear energy state per unit time. It is related to the number of radioactive nuclei N by:. The special name for the unit of activity is the becquerel Bq. This quantity represents the probability per unit time that a nuclear transformation will occur for a given radionuclide. Deterministic biological effect. Cataract induction is an example of a stochastic biological effect.

Effective dose. The effective dose E is the sum of the weighted equivalent doses in all the tissues and organs of the body. It is a radiation safety quantity, so its use is not appropriate for large absorbed doses delivered in a relatively short period of time. It is given by:. The special name for the unit of effective dose is the sievert Sv. Equivalent dose. The equivalent dose H T is the absorbed dose averaged over a tissue or organ rather than at a point and weighted for the radiation quality that is of interest.

The equivalent dose is given by:. The special name for the unit of equivalent dose is the sievert Sv. This quantity is the amount of time required for the activity of a radionuclide sample to reduce by a factor of one-half. Equivalently, it is the amount of time required for a given number of nuclei in a given radioactive state to reduce by a factor of one-half. It has fundamental units of seconds s , but is also commonly expressed in hours, days and years. Linear energy transfer. This quantity is the energy a charged particle imparts to matter per unit length as it traverses the matter.

Mean lifetime. This quantity is the average time a nuclear state will survive before it undergoes a transformation to a lower energy state by emitting ionizing radiation. It has fundamental units of seconds s , but may also be expressed in hours, days or years. It is related to the decay constant by:. Radiation weighting factor. This is a number w R that, for a given type and energy of radiation R, is representative of values of the relative biological effectiveness of that radiation in inducing stochastic effects at low doses.

The values of w R are related to linear energy transfer LET and are given in table Relative biological effectiveness RBE. The RBE of one type of radiation compared with another is the inverse ratio of the absorbed doses producing the same degree of a defined biological end point. Stochastic biological effect. This is a biological effect caused by ionizing radiation whose probability of occurrence increases with increasing absorbed dose, probably with no threshold, but whose severity is independent of absorbed dose. Cancer is an example of a stochastic biological effect.

Tissue weighting factor w T. This represents the contribution of tissue or organ T to the total detriment due to all of the stochastic effects resulting from uniform irradiation of the whole body. It is used because the probability of stochastic effects due to an equivalent dose depends on the tissue or organ irradiated. A uniform equivalent dose over the whole body should give an effective dose numerically equal to the sum of effective doses for all tissues and organs of the body. Therefore, the sum of all tissue weighting factors is normalized to unity. Table The list includes organs that are likely to be selectively irradiated.

Some organs in the list are known to be susceptible to cancer induction. After its discovery by Roentgen in , the x ray was introduced so rapidly into the diagnosis and treatment of disease that injuries from excessive radiation exposure began to be encountered almost immediately in pioneer radiation workers, who had yet to become aware of the dangers Brown The first such injuries were predominantly skin reactions on the hands of those working with the early radiation equipment, but within a decade many other types of injury also had been reported, including the first cancers attributed to radiation Stone Throughout the century since these early findings, study of the biological effects of ionizing radiation has received continuing impetus from the growing uses of radiation in medicine, science and industry, as well as from the peaceful and military applications of atomic energy.

As a result, the biological effects of radiation have been investigated more thoroughly than those of virtually any other environmental agent. The evolving knowledge of radiation effects has been influential in shaping measures for the protection of human health against many other environmental hazards as well as radiation.

Energy deposition. In contrast to other forms of radiation, ionizing radiation is capable of depositing enough localized energy to dislodge electrons from the atoms with which it interacts. Thus, as radiation collides randomly with atoms and molecules in passing through living cells, it gives rise to ions and free radicals which break chemical bonds and cause other molecular changes that injure the affected cells.

The spatial distribution of the ionizing events depends on the radiation weighting factor, w R of the radiation see table Effects on DNA. Any molecule in the cell may be altered by radiation, but DNA is the most critical biological target because of the limited redundancy of the genetic information it contains. Most such lesions are reparable, but those produced by a densely ionizing radiation for example, a proton or an alpha particle are generally less reparable than those produced by a sparsely ionizing radiation for example, an x ray or a gamma ray Goodhead Effects on genes.

The fact that the mutation rate appears to be proportional to the dose is interpreted to signify that traversal of the DNA by a single ionizing particle may, in principle, suffice to cause a mutation NAS In Chernobyl accident victims, the dose-response relationship for glycophorin mutations in bone marrow cells closely resembles that observed in atomic bomb survivors Jensen, Langlois and Bigbee Effects on chromosomes. Radiation damage to the genetic apparatus may also cause changes in chromosome number and structure, the frequency of which has been observed to increase with the dose in radiation workers, atomic bomb survivors, and others exposed to ionizing radiation.

The dose-response relationship for chromosome aberrations in human blood lymphocytes figure Effects on cell survival. Among the earliest reactions to irradiation is the inhibition of cell division, which appears promptly after exposure, varying both in degree and duration with the dose figure Although the inhibition of mitosis is characteristically transitory, radiation damage to genes and chromosomes may be lethal to dividing cells, which are highly radiosensitive as a class ICRP Mature, non-dividing cells are relatively radioresistant, but the dividing cells in a tissue are radiosensitive and may be killed in sufficient numbers by intensive irradiation to cause the tissue to become atrophic figure The rapidity of such atrophy depends on cell population dynamics within the affected tissue; that is, in organs characterized by slow cell turnover, such as the liver and vascular endothelium, the process is typically much slower than in organs characterized by rapid cell turnover, such as the bone marrow, epidermis and intestinal mucosa ICRP It is noteworthy, moreover, that if the volume of tissue irradiated is sufficiently small, or if the dose is accumulated gradually enough, the severity of injury may be greatly reduced by the compensatory proliferation of surviving cells.

Clinical Manifestations of Injury Types of effects. Radiation effects encompass a wide variety of reactions, varying markedly in their dose-response relationships, clinical manifestations, timing and prognosis Mettler and Upton The effects are often subdivided, for convenience, into two broad categories: 1 heritable effects, which are expressed in the descendants of exposed individuals, and 2 somatic effects, which are expressed in exposed individuals themselves.

The latter include acute effects, which occur relatively soon after irradiation, as well as late or chronic effects, such as cancer, which may not appear until months, years or decades later. Acute effects. The acute effects of radiation result predominantly from the depletion of progenitor cells in affected tissues figure For this reason, such effects are viewed as nonstochastic, or deterministic, in nature ICRP and , in contradistinction to the mutagenic and carcinogenic effects of radiation, which are viewed as stochastic phenomena resulting from random molecular alterations in individual cells that increase as linear-nonthreshold functions of the dose NAS ; ICRP Acute injuries of the types that were prevalent in pioneer radiation workers and early radiotherapy patients have been largely eliminated by improvements in safety precautions and treatment methods.

Nevertheless, most patients treated with radiation today still experience some injury of the normal tissue that is irradiated. In addition, serious radiation accidents continue to occur. For example, some nuclear reactor accidents excluding the Chernobyl accident were reported in various countries between and , irradiating more than 1, persons, 33 of them fatally Lushbaugh, Fry and Ricks The Chernobyl accident alone released enough radioactive material to require the evacuation of tens of thousands of people and farm animals from the surrounding area, and it caused radiation sickness and burns in more than emergency personnel and fire-fighters, injuring 31 fatally UNSCEAR The long-term health effects of the radioactive material released cannot be predicted with certainty, but estimates of the resulting risks of carcinogenic effects, based on nonthreshold dose-incidence models discussed below , imply that up to 30, additional cancer deaths may occur in the population of the northern hemisphere during the next 70 years as a result of the accident, although the additional cancers in any given country are likely to be too few to be detectable epidemiologically USDOE Less catastrophic, but far more numerous, than reactor accidents have been accidents involving medical and industrial gamma ray sources, which also have caused injuries and loss of life.

A comprehensive discussion of radiation injuries is beyond the scope of this review, but acute reactions of the more radiosensitive tissues are of widespread interest and are, therefore, described briefly in the following sections. Cells in the germinal layer of the epidermis are highly radiosensitive.

As a result, rapid exposure of the skin to a dose of 6 Sv or more causes erythema reddening in the exposed area, which appears within a day or so, typically lasts a few hours, and is followed two to four weeks later by one or more waves of deeper and more prolonged erythema, as well as by epilation hair loss. If the dose exceeds 10 to 20 Sv, blistering, necrosis and ulceration may ensue within two to four weeks, followed by fibrosis of the underlying dermis and vasculature, which may lead to atrophy and a second wave of ulceration months or years later ICRP Bone marrow and lymphoid tissue.

Lymphocytes also are highly radiosensitive; a dose of 2 to 3 Sv delivered rapidly to the whole body can kill enough of them to depress the peripheral lymphocyte count and impair the immune response within hours UNSCEAR Haemopoietic cells in the bone marrow are similarly radiosensitive and are depleted sufficiently by a comparable dose to cause granulocytopenia and thrombocytopenia to ensue within three to five weeks.

Alpha, beta, and gamma radiation demonstrated

Such reductions in granulocyte and platelet counts may be severe enough after a larger dose to result in haemorrhage or fatal infection table Stem cells in the epithelium lining the small bowel also are extremely radiosensitive, acute exposure to 10 Sv depleting their numbers sufficiently to cause the overlying intestinal villi to become denuded within days ICRP ; UNSCEAR Denudation of a large area of the mucosa can result in a fulminating, rapidly fatal dysentery-like syndrome table Mature spermatozoa can survive large doses Sv , but spermatogonia are so radiosensitive that as little as 0.

Oocytes, likewise, are radiosensitive, a dose of 1. Respiratory tract. The lung is not highly radiosensitive, but rapid exposure to a dose of 6 to 10 Sv can cause acute pneumonitis to develop in the exposed area within one to three months.

Radiation Quantities

If a large volume of lung tissue is affected, the process may result in respiratory failure within weeks, or may lead to pulmonary fibrosis and cor pulmonale months or years later ICRP ; UNSCEAR Lens of the eye. Cells of the anterior epithelium of the lens, which continue to divide throughout life, are relatively radiosensitive.

Other tissues. In comparison with the tissues mentioned above, other tissues of the body are generally appreciably less radiosensitive for example, table Noteworthy also is the fact that the radiosensitivity of every tissue is increased when it is in a rapidly growing state ICRP Whole-body radiation injury. Rapid exposure of a major part of the body to a dose in excess of 1 Gy can cause the acute radiation syndrome. This syndrome includes: 1 an initial prodromal stage, characterized by malaise, anorexia, nausea and vomiting, 2 an ensuing latent period, 3 a second main phase of illness and 4 ultimately, either recovery or death table Localized radiation injury.

The main phase of the illness typically takes one of the following forms, depending on the predominant locus of radiation injury: 1 haematological, 2 gastro-intestinal, 3 cerebral or 4 pulmonary table Effects of radionuclides. Some radionuclides - for example, tritium 3 H , carbon 14 C and cesium Cs - tend to be distributed systemically and to irradiate the body as a whole, whereas other radionuclides are characteristically taken up and concentrated in specific organs, producing injuries that are correspondingly localized.

General features. The carcinogenicity of ionizing radiation, first manifested early in this century by the occurrence of skin cancers and leukaemias in pioneer radiation workers Upton , has since been documented extensively by dose-dependent excesses of many types of neoplasms in radium-dial painters, underground hardrock miners, atomic bomb survivors, radiotherapy patients and experimentally irradiated laboratory animals Upton ; NAS The benign and malignant growths induced by irradiation characteristically take years or decades to appear and exhibit no known features by which they can be distinguished from those produced by other causes.

With few exceptions, moreover, their induction has been detectable only after relatively large dose equivalents 0. The molecular mechanisms of radiation carcinogenesis remain to be elucidated in detail, but in laboratory animals and cultured cells the carcinogenic effects of radiation have been observed to include initiating effects, promoting effects, and effects on the progression of neoplasia, depending on the experimental conditions in question NAS In addition, the carcinogenic effects of radiation resemble those of chemical carcinogens in being similarly modifiable by hormones, nutritional variables and other modifying factors NAS It is noteworthy, moreover, that the effects of radiation may be additive, synergistic or mutually antagonistic with those of chemical carcinogens, depending on the specific chemicals and exposure conditions in question UNSCEAR and Dose-effect relationship.

Existing data do not suffice to describe the dose-incidence relationship unambiguously for any type of neoplasm or to define how long after irradiation the risk of the growth may remain elevated in an exposed population. Any risks attributable to low-level irradiation can, therefore, be estimated only by extrapolation, based on models incorporating assumptions about such parameters NAS Of various dose-effect models that have been used to estimate the risks of low-level irradiation, the one that has been judged to provide the best fit to the available data is of the form:.

Non-threshold models of this type have been applied to epidemiological data from the Japanese atomic-bomb survivors and other irradiated populations to derive estimates of the lifetime risks of different forms of radiation-induced cancer for example, table Such estimates must be interpreted with caution, however, in attempting to predict the risks of cancer attributable to small doses or doses that are accumulated over weeks, months or years, since experiments with laboratory animals have shown the carcinogenic potency of x rays and gamma rays to be reduced by as much as an order of magnitude when the exposure is greatly prolonged.

In fact, as has been emphasized elsewhere NAS , the available data do not exclude the possibility that there may be a threshold in the millisievert mSv dose equivalent range, below which radiation may lack carcinogenicity. It is also noteworthy that the estimates tabulated are based on population averages and are not necessarily applicable to any given individual; that is, susceptibility to certain types of cancer for example, cancers of the thyroid and breast is substantially higher in children than in adults, and susceptibility to certain cancers is also increased in association with some hereditary disorders, such as retinoblastoma and the nevoid basal cell carcinoma syndrome UNSCEAR , ; NAS Such differences in susceptibility notwithstanding, population-based estimates have been proposed for use in compensation cases as a basis for gauging the probability that a cancer arising in a previously irradiated person may have been caused by the exposure in question NIH Low-dose risk assessment.

Epidemiological studies to ascertain whether the risks of cancer from low-level exposure to radiation actually vary with dose in the manner predicted by the above estimates have been inconclusive thus far. Populations residing in areas of elevated natural background radiation levels manifest no definitely attributable increases in cancer rates NAS ; UNSCEAR ; conversely, a few studies have even suggested an inverse relationship between background radiation levels and cancer rates, which has been interpreted by some observers as evidence for the existence of beneficial or hormetic effects of low-level irradiation, in keeping with the adaptive responses of certain cellular systems UNSCEAR The inverse relationship is of questionable significance, however, since it has not persisted after controlling for the effects of confounding variables NAS In summary, therefore, the data available at present are consistent with the estimates tabulated above table High levels of radioactive fallout from a thermonuclear weapons test at Bikini in have been observed to cause a dose-dependent increase in the frequency of thyroid cancer in Marshall Islanders who received large doses to the thyroid gland in childhood Robbins and Adams Similarly, children living in areas of Belarus and the Ukraine contaminated by radionuclides released from the Chernobyl accident have been reported to show an increased incidence of thyroid cancer Prisyazhuik, Pjatak and Buzanov ; Kasakov, Demidchik and Astakhova , but the findings are at variance with those of the International Chernobyl Project, which found no excess of benign or malignant thyroid nodules in children living in the more heavily contaminated areas around Chernobyl Mettler, Williamson and Royal The basis for the discrepancy, and whether the reported excesses may have resulted from heightened surveillance alone, remain to be determined.

In this connection, it is noteworthy that children of south-western Utah and Nevada who were exposed to fallout from nuclear weapons tests in Nevada during the s have shown increase in the frequency of any type of thyroid cancer Kerber et al. The possibility that excesses of leukaemia among children residing in the vicinity of nuclear plants in the United Kingdom may have been caused by radioactivity released from the plants has also been suggested. An ineffective aetiology for the observed clusters of leukaemia is implied by the existence of comparable excesses of childhood leukaemia at sites in the UK that lack nuclear facilities but otherwise resemble nuclear sites in having similarly experienced large influxes of population in recent times Kinlen ; Doll, Evans and Darby Heritable effects of irradiation, although well documented in other organisms, have yet to be observed in humans.

For example, intensive study of more than 76, children of the Japanese atomic-bomb survivors, carried out over four decades, has failed to disclose any heritable effects of radiation in this population, as measured by untoward pregnancy outcomes, neonatal deaths, malignancies, balanced chromosomal rearrangements, sex-chromosome aneuploidy, alterations of serum or erythrocyte protein phenotypes, changes in sex ratio or disturbances in growth and development Neel, Schull and Awa Consequently, estimates of the risks of heritable effects of radiation must rely heavily on extrapolation from findings in the laboratory mouse and other experimental animals NAS ; UNSCEAR From the available experimental and epidemiological data, it is inferred that the dose required to double the rate of heritable mutations in human germ cells must be at least 1.

Arguments against this hypothesis, however, are:. On balance, therefore, the available data fail to support the paternal gonadal irradiation hypothesis Doll, Evans and Darby ; Little, Charles and Wakeford Radiosensitivity is relatively high throughout prenatal life, but the effects of a given dose vary markedly, depending on the developmental stage of the embryo or foetus at the time of exposure UNSCEAR During the pre-implantation period, the embryo is most susceptible to killing by irradiation, while during critical stages in organogenesis it is susceptible to the induction of malformations and other disturbances of development table The latter effects are dramatically exemplified by the dose-dependent increase in the frequency of sever mental retardation figure Source: Brill and Forgotson Susceptibility to the carcinogenic effects of radiation also appears to be relatively high throughout the prenatal period, judging from the association between childhood cancer including leukaemia and prenatal exposure to diagnostic x rays reported in case-control studies NAS Although, paradoxically, no excess of childhood cancer was recorded in A-bomb survivors irradiated prenatally Yoshimoto et al.

The adverse effects of ionizing radiation on human health are widely diverse, ranging from rapidly fatal injuries to cancers, birth defects, and hereditary disorders that appear months, years or decades later. The nature, frequency and severity of effects depend on the quality of the radiation in question as well as on the dose and conditions of exposure. Most such effects require relatively high levels of exposure and are, therefore, encountered only in accident victims, radiotherapy patients, or other heavily irradiated persons.

The genotoxic and carcinogenic effects of ionizing radiation, by contrast, are presumed to increase in frequency as linear non-threshold functions of the dose; hence, although the existence of thresholds for these effects cannot be excluded, their frequency is assumed to increase with any level of exposure. For most effects of radiation, the sensitivity of exposed cells varies with their rate of proliferation and inversely with their degree of differentiation, the embryo and growing child being especially vulnerable to injury. An alpha particle is a tightly bound collection of two protons and two neutrons.

It is identical to a helium-4 4 He nucleus. Indeed, its ultimate fate after it loses most of its kinetic energy is to capture two electrons and become a helium atom. Alpha-emitting radionuclides are generally relatively massive nuclei. Almost all alpha emitters have atomic numbers greater than or equal to that of lead 82 Pb. When a nucleus decays by emitting an alpha particle, both its atomic number number of protons and its number of neutrons are reduced by two and its atomic mass number is reduced by four.

For example, the alpha decay of uranium U to thorium Th is represented by:. The left superscript is the atomic mass number number of protons plus neutrons , the left subscript is the atomic number number of protons , and the right subscript is the number of neutrons. Common alpha emitters emit alpha particles with kinetic energies between about 4 and 5.

Such alpha particles have a range in air of no more than about 5 cm see figure Alpha particles with an energy of at least 7. Alpha emitters generally do not pose an external radiation hazard. They are hazardous only if taken within the body. Beta particles A beta particle is a highly energetic electron or positron.

A positron is the anti-particle of the electron. It has the same mass and most other properties of an electron except for its charge, which is exactly the same magnitude as that of an electron but is positive. Beta-emitting radionuclides can be of high or low atomic weight. Radionuclides that have an excess of protons in comparison with stable nuclides of about the same atomic mass number can decay when a proton in the nucleus converts to a neutron.

When this occurs, the nucleus emits a positron and an extremely light, very non-interacting particle called a neutrino. The neutrino and its anti-particle are of no interest in radiation protection. When it has given up most of its kinetic energy, the positron ultimately collides with an electron and both are annihilated. The annihilation radiation produced is almost always two 0.

A typical positron decay is represented by:. Note that the resulting nuclide has the same atomic mass number as the parent nuclide and an atomic proton number larger by one and a neutron number lesser by one than those of the original nuclide. Electron capture competes with positron decay. In electron capture decay, the nucleus absorbs an orbital electron and emits a neutrino.

A typical electron capture decay is given by:. Electron capture is always possible when the resulting nucleus has a lower total energy than the initial nucleus. However, positron decay requires that the total energy of the initial atom is greater than that of the resulting atom by more than 1. In this case, the nucleus emits a negatron energetic electron and an anti-neutrino.

A typical negatron decay is represented by:. Here the resulting nucleus gains one neutron at the expense of one proton but again does not change its atomic mass number. Alpha decay is a two-body reaction, so alpha particles are emitted with discrete kinetic energies. However, beta decay is a three-body reaction, so beta particles are emitted over a spectrum of energies. The maximum energy in the spectrum depends on the decaying radionuclide.

The average beta energy in the spectrum is approximately one-third of the maximum energy see figure Typical maximum beta energies range from The range of beta particles in air is approximately 3. Beta particles of at least 70 keV energy are required to penetrate the epidermis. Beta particles are low-LET radiation. Gamma radiation is electromagnetic radiation emitted by a nucleus when it undergoes a transition from a higher to a lower energy state. The number of protons and neutrons in the nucleus does not change in such a transition. The nucleus may have been left in the higher energy state following an earlier alpha or beta decay.

That is, gamma rays are often emitted immediately following alpha or beta decays. Gamma rays can also result from neutron capture and inelastic scattering of subatomic particles by nuclei. The most energetic gamma rays have been observed in cosmic rays. It shows a cascade of two gamma rays emitted in nickel 60 Ni with energies of 1. Such an excited nucleus is called an isomer. It illustrates that some radionuclides decay in more than one way.


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While alpha and beta particles have definite ranges in matter, gamma rays are attenuated exponentially ignoring build-up that results from scattering within a material as they pass through matter. When build-up can be ignored the attenuation of gamma rays is given by:. The mass attenuation coefficient depends on gamma-ray energy and on the material with which the gamma rays are interacting. Mass attenuation coefficient values are tabulated in many references. Build-up occurs when a broad gamma-ray beam interacts with matter.

The degree of build-up depends on the geometry of the beam, on the material and on the energy of the gamma rays. Internal conversion competes with gamma emission when a nucleus transforms from a higher energy state to a lower one. In internal conversion, an inner orbital electron is ejected from the atom instead of the nucleus emitting a gamma ray. The ejected electron is directly ionizing. As outer orbital electrons drop to lower electronic energy levels to fill the vacancy left by the ejected electron, the atom emits x rays. Internal conversion probability relative to gamma emission probability increases with increasing atomic number.

X rays are electromagnetic radiation and, as such, are identical to gamma rays. The distinction between x rays and gamma rays is their origin. Whereas gamma rays originate in the atomic nucleus, x rays result from electron interactions. Although x rays often have lower energies than gamma rays, this is not a criterion for differentiating them. It is possible to produce x rays with energies much higher than gamma rays resulting from radioactive decay.

Internal conversion, discussed above, is one method of x ray production. In this case, the resulting x rays have discrete energies equal to the difference in the energy levels between which the orbital electrons transit. Charged particles emit electromagnetic radiation whenever they are accelerated or decelerated. As a result, electrons emit much more x radiation than heavier particles such as protons, all other conditions being equal.

X-ray systems produce x rays by accelerating electrons across a large electric potential difference of many kV or MV. The electrons are then quickly decelerated in a dense, heat-resistant material, such as tungsten W. The x rays emitted from such systems have energies spread over a spectrum ranging from about zero up to the maximum kinetic energy possessed by the electrons before deceleration. Gamma photons are the most energetic photons in the electromagnetic spectrum. Gamma rays are a form of electromagnetic radiation EMR. They are the similar to X-rays, distinguished only by the fact that they are emitted from an excited nucleus.

Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each travelling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount or bundle of energy, and all electromagnetic radiation consists of these photons. Gamma-ray photons have the highest energy in the EMR spectrum and their waves have the shortest wavelength. Scientists measure the energy of photons in electron volts eV.

X-ray photons have energies in the range eV to , eV or keV. Gamma-ray photons generally have energies greater than keV. For comparison, ultraviolet radiation has energy that falls in the range from a few electron volts to about eV and does not have enough energy to be classified as ionising radiation.


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The high energy of gamma rays enables them to pass through many kinds of materials, including human tissue. Very dense materials, such as lead, are commonly used as shielding to slow or stop gamma rays. The key difference between gamma rays and X-rays is how they are produced.