Measurement radioactivity

Handbook of Radioactivity Measurements Procedures, NCRP Report No. 58, 2nd ed.. National Council on Radiation Protection and Measurement, Bethesda, Md., 1985. 

The results of the theory of quantum mechanics require that nuclear states have discrete energies. This is in contrast to classical mechanical systems, which can have any of a continuous range of energies. This difference is a critical fact in the appHcations of radioactivity measurements, where the specific energies of radiations are generally used to identify the origin of the radiation. Quantum mechanics also shows that other quantities have only specific discrete values, and the whole understanding of atomic and nuclear systems depends on these discrete quantities. 

To determine the ftq, value of Hg a solid sample is used, in which some of the iodine is present as radioactive 1-131. The count rate of the sample is 5.0 X 1011 counts per minute per mole of L An excess amount of Hg2I2(s) is placed in some water, and the solid is allowed to come to equilibrium with its respective ions. A 150.0-mL sample of the saturated solution is withdrawn and the radioactivity measured at 33 counts per minute. From this information, calculate the ft, value for Hg2l2. 

Radioactivity detectors, which count the number of nuclei that decay, provide an example of a conversion from number of particles to moles. If a radioactivity measurement on a blood sample from a patient receiving radiation... 

Quality assurance of radiopharmaceutical preparation and use is obviously a very important topic because of its direct impact on patient diagnosis, treatment and health (see, e.g. Abreu 1996). Reference materials play only a small - but nevertheless important -role in this process, mainly in the area of calibration of radioactivity-measuring instruments. The materials of interest are all pure chemical containing calibrated activities of selected radionuclides used commonly in nuclear medicine (e.g. Co, Ga, I,... 

Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with great precision immediately opened up the entire field of uranium and thorium decay chain studies. This area of study was formerly the poaching ground for radioactive measurements alone but now became part of the wonderful world of mass spectrometric measurements. (The same transformation took place for radiocarbon from the various radioactive counting schemes to accelerator mass spectrometry.)... 

Radioactivity measurement The radioactivities of lead isotopes and their decay products were measured with TRICARB 3380 liquid scintillation counter (Packard Inc.). The radioisotopes concerned, and their decay charateristies are shown in Figure 2. In the case of the direct method, the absolute radioactivity can be obtained by the integral... 

It was also demonstrated that the indirect method was superior for the determination of Rn-220 in a sample in which the ratio Rn-220 / Rn-222 was small. In the direct method, the decay analysis of Equation 1. where the build-up of Po-218 (T1/2 = 3.05 m), Pb-214 (T1/2 = 26.8 m), and Bi-214 (T1/2 = 19.7 m) is taken into account brings about a large uncertainty in the evaluation of the Rn-220 concentration even when the activity itself is not small whereas the decay analysis of Equation 5 gives an unambiguous result since the half lives of Pb-214 and Bi-214 are quite different from that of Pb-212. The lower limit of the Rn-220 concentration that can be determined by the indirect method is calculated to be 0.4 nCi/l for the condition that one-1 iter of gas is sampled and that the radioactivity measurement of Pb-212 is con-... 

Radioactivity measurements in the above case were similar to the measurements indicated for the test facility, except that only one sample per source could be investigated at a time. In order to facilitate the study, four sources were used, two Ra-226 sources and two Th-228 sources. The strengths of the two Ra-226 sources were different. The same applies to the two Th-228 sources. 

Fisenne, I.M. and N.H. Harley, Lung Dose Estimates from Natural Radioactivity Measured in Urban Air, USAEC Report HASL-TM-74-7 New York, NY (1974). 

Finite resolution and partial volume effects. Although this can occur in other areas of imaging such as MRS, it is particularly an issue for SPECT and PET because of the finite resolution of the imaging instruments. Resolution is typically imaged as the response of the detector crystal and associated electron to the point or line source. These peak in the center and fall off from a point source, for example, in shapes that simulate Gaussian curves. These are measures of the ability to resolve two points, e.g. two structures in a brain. Because brain structures, in particular, are often smaller than the FWHM for PET or SPECT, the radioactivity measured in these areas is underestimated both by its small size (known as the partial volume effect), but also spillover from adjacent radioactivity... 

Radiotracers are uniquely well suited to such studies. The sensitivity of detection means that only very small amounts of tracer need be added to follow the chemical pathway of the relevant species. Furthermore, it matters little what the physical or chemical state of the tracer is, for measurements may be made on liquids, solids or gases. Chromatography, solvent extraction and precipitation are amongst separation methods widely studied by means of radiotracers. In the individual separation steps the distribution of the species may be studied by simple radioactivity measurements, and subsequently the tracer will serve as a yield indicator for the overall procedure. 

Immunoradiometric assays (IRMAs) are like RIAs in that a radiolabeled substance is used in an antibody-antigen reaction, except that the radioactive label is attached to the antibody instead of the hormone. Furthermore, excess of antibody, rather than limited quantity, is present in the assay. All the unknown antigen becomes bound in an IRMA rather than just a portion, as in a RIA IRMAs are more sensitive. In the one-site assay, the excess antibody that is not bound to the sample is removed by addition of a precipitating binder. In a two-site assay, a molecule with at least two antibody-binding sites is adsorbed onto a solid phase, to which one of the antibodies is attached. After binding to this antibody is completed, a second antibody labeled with 125I is added to the assay. This antibody reacts with the second antibody-binding site to form a sandwich, composed of antibody-hormone-labeled antibody. The amount of hormone present is proportional to the amount of radioactivity measured in the assay. 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

Secular Equilibrium A condition that occurs when a chain of radionuclides has reached a steady state condition, in which the rate of decay of daughter nuclides is balanced by their rate of formation by decay of each parent. In this condition, the radioactivity (measured in disintigrations per minute) of each radionuclide in a chain is the same. 

At equilibrium, the concentration of free ligand, [L], is the same on both sides of the dialysis membrane. Thus, the difference in the amount of radioactivity measured in aliquots taken from both sides of the membrane can be used to estimate [RL] (see Protocol 4.1)... 

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