Radionuclides as radiation sources

The application of radionuclides as radiation sources in X-ray fluorescence analysis is illustrated in Fig.17.5. The X rays or y rays emitted by a radionuclide are absorbed in the sample and the X rays emitted by the sample are measured by means of a semiconductor in combination with a multichannel analyser. Quantitative evaluation of the spectra is possible by use of suitable standards. In comparison with excitation by means of X-ray tubes, the main advantages of radionuclides are... 

The application of compounds labelled with suitable radionuclides as radiation sources makes it possible to measure the incorporation and discharge of these substances in certain organs of the body, thus providing information about the metabolism and the function of organs of interest. In this way, malfunction and disorder can be detected at very early stages. 

The height of filling in technical installations, e.g. in vessels, containers or tubes, can easily be controlled from outside by means of radionuclides as radiation sources. The radionuclides are selected according to the most appropriate y energy for the task, in particular the diameters of the equipment and the thickness of the walls. The phase boundary between two liquid phases dilfering markedly in their properties as neutron moderators can be located from outside by use of a neutron source and a detector for low-energy neutrons. 

The application of radionuclides as radiation sources for analytical applications has been described in sections 17.10 and 17.11. 

In contrast to the methods described in the preceding paragraphs, radiophotovoltaic conversion is a direet method. In a semiconductor the incident radiation generates free charge carriers, that are separated in the n,p-barrier layer of the semiconductor. Suitable radiation sources are radionuclides emitting p particles with energies below the limits of radiation damage in the semiconductors. These limits are about 145 keV for Si and about 350 keV for Ge. Therefore only Pm, Ni and T are suitable as radiation sources. By use of the combination Pm/Si, efficiencies of about 4% are obtained. 

Nuclear radiation absorption methods have many technical applications. These methods are not to be confused with radioisotope tracer methods, although radioisotopes may be used as radiation sources. In the tracer method the chemical properties of the radionuclide are important while in the applications discussed in this section only the type and energy of radiation emitted are irqx>rtant. 

As radiation sources. X-ray tubes or radionuclides are applied. The polychromatic radiation of the tube is scattered by the Compton or Rayleigh effect towards the detector, which results in a high bremsstrahlung background and line interferences caused by the characteristic lines of the anode material and the resulting Compton peak. 

Fig-1 Schematic presentation of the radiopharmaceutical design. The targeting biomolecule serves as the vehicle to carry a radionuclide to the receptor site on tumor cells. The radionuclide is the radiation source. The bifunctional chelator (BFC) is used for radiometal chelation and attachment of the targeting molecule. The linker is often used for modification of pharmacokinetics. Diethylenetriaminepentaacetic acid (DTPA), 1,4,7,10-tetraaza-cyclododecane-N>N, N">Ar,"-tetraacetic acid (DOTA)... 

Application of radionuclides in life sciences is of the greatest importance, and the largest single user of radionuclides is nuclear medicine. Shortly after the discovery of Ra in 1898 by Marie Curie and its subsequent isolation from pitchblende in amounts of 0.1 to 1 g, the finding that this element was useful as a radiation source led to the first application of radionuclides in medicine. In 1921, de Hevesy investigated the metabolism of lead in plants by use of natural radioisotopes of Pb. 

It is an attractive object of research to synthesize labelled compounds that are taking part in specific biochemical processes or able to pass specific barriers in the body, with the aim of detecting malfunctions and of localizing the origin of diseases. Complexes of short-lived no-carrier-added radionuclides and high yields of the syntheses are of special interest. In the case of short-lived radionuclides, such as fC, the synthesis must be fast and as far as possible automated. Labelled organic molecules can also be used to transport radionuclides to special places in the body for therapeutical application, i.e. as specific internal radiation sources. 

For therapeutical purposes, natural radionuclides, mainly Ra and Rn, were the first to be applied as external and internal radiation sources. For example, encapsulated samples of Ra have been attached to the skin or introduced into the body, and Rn has been recommended for the treatment of the respiratory tract by inhalation in radon galleries or it has been encapsulated in small thin-walled gold tubes and introduced into the body for treatment of cancer. 

In computer tomography (CT) with radionuclides, one or several radiation detectors, a computer and a display are used. The detector array is moved in relation to the patient, and the variations in counting rates with the absorbancies of the radiation in the body as a function of the geometry are processed by the software of the computer to give an image on the screen. This procedure is repeated in subsequent sections (slices) of the body, thus providing a three-dimensional picture. The resolution of the scan is of the order of 1 mm. The method is similar to that used in X-ray CT, but in the latter both the radiation source and the detector array can be moved in relation to the patient. 

Absorption or scattering of radioactive radiation is applied in industry for measurement of thickness or for material testing. For example, the production of paper, plastic or metal foils or sheets can be controlled continuously by passing these materials between an encapsulated radionuclide as the radiation source and a detector combined with a ratemeter, as shown in Fig. 20.2. After appropriate calibration, the ratemeter directly indicates the thickness. The radionuclide is chosen in such a way that the radiation emitted is eflFectively absorbed in the materials to be checked. Thus, the thickness of plastic foils is measured by use of f emitters, whereas Cs or other y emitters are used for measuring the thickness of metal sheets. 

Internal radiation sources (medical application of radionuclides such as " Tc) Diagnosis 1-1000 mSv O.l-lOmSv a 0.02mSv/y... 

USE 238Pu as heat source as radioisotope thermoelectric generator in radionuclide batteries for pacemakers with Be as neutron source. in atomic weapons in power reactors. Caution Radiation hazard concentrates in bone. 

If the intended use of the radionuclide is as an external radiation source (A4a - B2 - C2.1 or C3) its chemical matrix is of minor importance. Such sources are used for radiation treatment of cancer (C3b), radiation sterilization of food (C3c), etc. The radiation ects on biological systems are discussed separately in Ch. 18. In this chapter we focus our interest on radionuclides with specific chemical properties, in the order of column C, Table 9.3. 

The radionuclides come in the form of calibration/reference sources, and as inorganic compounds and labeled organic compounds. They are packed in various ways, most commonly in a glass ampoule packed in a small aluminum can. To reduce the surface radiation dose rate (cf. Ch. 18) for high intensity radiation sources (usually > 10 MBq) the can is put in a lead block which is packed in a wooden crate for shipment. Opening of the aluminum can and glass ampoule may require remote control and should be done only by experienced, licensed personnel. 

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