Radiation from isotopes

Oeschger, H., Houtermans, J., Loosli, H., Wahlen, M., The constancy of cosmic radiation from isotope studies in meteorites and on the Earth, Nobel Symposium, Vol. 12, p. 471-498, 1970. 

The number of protons identifies an element. Some elements may have atoms that contain different numbers of neutrons. The different atoms of the same element are then called isotopes. Because of the different number of neutrons in one isotope of an element, it can be quite radioactive that is, the nucleus is progressively breaking apart and giving off radiation. The type and amount of radiation determines the degree of risk. The radiation from isotopes is used in controlled situations - for example, cobalt therapy, for treating cancer. Cobalt 60 therapy uses a cobalt isotope with 33 neutrons. Normal cobalt in vitamin B12 or soil fertihzer almost alw s contains only 32 neutrons. Your home smoke detector may contain a 37kBq source of americium 241. Some natural elements, e.g. chlorine, are a mixture of two long-lived isotopes of the element. 

Knowledge about the radiations from each isotope is important because as the uses of the radioisotopes have iacreased, it has become necessary to develop sensitive and accurate detection methods designed to determine both the presence of these materials and the amount present. These measurements determine the amount of radiation exposure of the human body or how much of the isotope is present ia various places ia the environment. For a discussion of detection methods used see References 1 and 2. 

The modem ionization chamber, called a dose caUbrator in this appHcation, is capable of linear measurements of radioactivity having a precision in the range of several percent coefficient of variation over a range of 370 kBq (10 -lCi) to at least 370 GBq (10 Ci). This extraordinary range is the chief advantage of this instmment. It may only be used when the sample is known to have only a single isotope. It has no capacity to distinguish radiation from different isotopes. 

The same type of radiation emitted by different isotopes may differ signifieantly in energy, e.g. y-radiation from potassium-42 has about four times the energy of y-radiation from gold-198. 

Half-lives can be interpreted in terms of the level of radiation of the corresponding isotopes. Uranium has a very long half-life (4.5 X 109 yr), so it gives off radiation very slowly. At the opposite extreme is fermium-258, which decays with a half-life of 3.8 X 10-4 s. You would expect the rate of decay to be quite high. Within a second virtually all the radiation from fermium-258 is gone. Species such as this produce very high radiation during their brief existence. 

Radioactivity. Methods based on the measurement of radioactivity belong to the realm of radiochemistry and may involve measurement of the intensity of the radiation from a naturally radioactive material measurement of induced radioactivity arising from exposure of the sample under investigation to a neutron source (activation analysis) or the application of what is known as the isotope dilution technique. 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

CobaltCII) iodide, uses, 7 240t CobaltCII) ion, 7 229 CobaltCIII) ion, 7 229 Cobalt isotopes, residual radiation from, 17 553-554... 

The effect of ionising radiation is described in Section 4.2. Most often, accelerated tests are carried out using gamma radiation from an isotope source or an electron beam from an accelerator. Radiation from nuclear reactors can also be used but will be a mixed radiation which may or may not be suitable for the simulation. The penetration of an electron beam is inherently limited which means that only relatively thin samples can be treated. Hence, gamma irradiation is the more versatile technique. With thin samples, such that penetration limits are not a problem, there are conversion factors to approximately equate the various radiations and energies to an equivalent gamma dose. 

Enriched or should be used for the reactor irradiation to eliminate unwanted background radiations from the other tellurium isotopes. Radiation damage to the source is unimportant since annealing (13) the ZnTe after irradiation did not change line intensity or width. 

J. C. Overley, Determination of H, C, N, O Content of Bulk Materials from Attenuation Measurements, International Journal of AppHed Radiation and Isotopes, 36 (1985) 185. 

Strontium-90, a radioactive strontium isotope with a half-hfe of 29 years, is a dangerous fallout source of radiation from atmospheric nuclear bombs. If a person is exposed to it, it will rapidly accumulate in bone tissue and interfere with the production of new red blood cells... 

Alkali metals (K, Rb, Cs) behave similarly and sometimes one is accumulated preferentially when another is deficient. A similar case is made for Sr and Ca (Whicker and Schultz 1982a). The most important alkali metal isotope is Cs because of its long physical half-life (30 years) and its abnndance as a fission prodnct in fallout from nuclear weapons and in the inventory of a nuclear reactor or a fuel-reprocessing plant. Cesium behaves much like potassium. It is rapidly absorbed into the bloodstream and distribnted throughout the active tissues of the body, especially muscle. The P and y radiation from the decay of Cs and its daughter, Ba, result in essentially whole-body irradiation that harms bone marrow (Hobbs and McClellan 1986). 

The result of our effort to develop the best possible detector for MES is as follows. Our detector has a resolution of approximately 2 KeV (fwhm) at 15 KeV as shown in Figure 6. There is virtually no deterioration in performance over a period of several months. The overall efficiency of the detector when used for MES with 14-KeV y-radiation is such that a 0.001-inch thick sample of stainless steel type 302 (natural isotopic abundance) gives a spectrum with the peak height some 400% of the base line, Figure 7. (For comparison, when we started we were quite content with 50%.) With our 10-mc Co-57 source, the data acquisition rate in the peak is approximately 500 counts/min. This means that in a matter of a minute or less one obtains a recognizable spectrum. As a bonus, the observance of 6-KeV x-rays yields an effect of approximately 50% of the baseline. To accomplish this, we interpose a plastic filter between the source and the sample to absorb most of the 6-KeV radiation from the source (which does not contribute to the effect but is elastically... 

A variety of radioactive isotopes is available having gamma rays diller-ing in penetrating ability, and with half-lives varying from a few minutes to many years. Radioactive iodine with an 8 day half-life and radioactive bromine with a l -day half-life were used for most tests. Radiation from these isotopes passes easily through the walls of pipe found in the oil field. 

Radiophosphorus is used in the treatment of patients with a number of diseases, lliis element has a half-life of about 14 days and emits beta rays. It is taken up by die body in the greatest quantity by those tissues which manufacture blood cells. In polycythemia vera, a condition in which too many red blood cells arc formed, the radiation from this isotope often brings about a sufficient suppression of the blood cell-making tissues to alleviate some of the symptoms of the disease. Leukemia patients, in whom there is an excessive production of white cells, are offered added comfort and, m some instances, prolongation of life by the use of radiophosphorus. This element also may be used m treatment of metastatic cancer to the bone and, although the treatment is not used in an attempt to eradicate cancer, it can result in significant palliation of pain in some patients. 

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

The emission of nuclear particles or radiation from an isotope during its disintegration is commonly referred to as radioactivity. These emissions are classified as of the three basic types a, 0 and y. a-Particles, which are helium nuclei, have only weak penetrating power and may be stopped by 5-10 cm of air or thin metallic sheets. They are, however, highly energetic, ranging from 4 to 10 MeV. Thus they have a high ionization capacity... 

The selection rules state that the total angular momentum quantum number may change by 1 or 0. Thus an element with several isotopes each with its own nuclear spin will present a line spectrum with a very complex and, under most experimental conditions, unresolved hyperfine structure. Nevertheless, as we shall see later, the overlap between the hyperfine components of a spectrum line is sufficiently incomplete to permit preferential excitation of one isotope in a mixture of isotopes by radiation from a lamp containing that same isotope. 

Some of the radiation from radium is constantly being released into the environment. It is this release of radiation that causes concern about the safety of radium and all other radioactive substances. Each isotope of radium releases radiation at its own rate. One isotope, radium-224 for example, releases half of its radiation in about three and a half days whereas another isotope, radium-226, releases half of its radiation in about 1,600 years. 

Impulseless resonance absorption of y-quants (gamma radiation) from a radioactive isotope, here Cobalt 57Co 57Fe + y (main quant 122 keV quant used for spectroscopy has a different energy)... 

---