Isotopes radioactivity calibration

An isotope dose calibrator is shaped as a cylinder and is often built in beneath the working area in the safety cabinet. It measures the radioactivity of a prepared dose in a vial or syringe. Each individual radionuclide can be measured accurately. Other equipment to measure radioactivity are scintillation counters (e.g. the Nal well counter) and semiconductor-based instruments (e.g. the Germanium detector). 

Quantitative Whole-Body Autoradiography (QWBA) is based on the RLG technique and the use of standards obtained from dilution series containing known concentrations of radioactivity. Isotopes used in QWBA are mainly 14C and 3H. These standards were cut together with the whole-body sections to ensure an identical thickness and used for the internal calibration. The information of the calibration curve allows the determination of the concentrations in the organs and tissues of interest which can be derived from the measured area and the section thickness. 

Beginning in 1937, both Kamen and Kurie became associated with E. O. Lawrence s newly organized radiation laboratory. At that time, recoil tracks were used to calibrate cloud chamber experiments, but little was known about the isotope s physical characteristics. It was assumed that it was radioactive with a half-life of a few hours or, at most, a few days based on an analogy with the 0.8-sec half-life of He (J2). In view of its assumed short half-life, no determined eflFort to isolate it was undertaken immediately since the Lawrence laboratory between 1937 and... 

In principle, the applications of ICP-MS resemble those listed for OES. This technique however is required for samples containing sub-part per billion concentrations of elements. Quantitative information of nonmetals such as P, S, I, B, Br can be obtained. Since atomic mass spectra are much simpler and easier to interpret compared to optical emission spectra, ICP-MS affords superior resolution in the determination of rare earth elements. It is widely used for the control of high-purity materials in semiconductor and electronics industries. The applications also cover the analysis of clinical samples, the use of stable isotopes for metabolic studies, and the determination of radioactive and transuranic elements. In addition to outstanding analytical features for one or a few elements, this technique provides quantitative information on more than 70 elements present from low part-per-trillion to part-per-million concentration range in a single run and within less than 3 min (after sample preparation and calibration). Comprehensive reviews on ICP-MS applications in total element determinations are available. " ... 

Following the classification of the analytical methods given by ISO 32 [3] two major type of calibration materials can be certified. For relative methods such as all spectrometric ones, pure substances are necessary. They can be certified for the stoichiometry and degree of purity but also for isotopic composition. The latter case is a prerequisite for measurements of radioactive materials and for stable isotope mass spectrometry (isotope dilution TIMS or ICP-MS). For comparative methods, pure substances and mixtures of substances are necessary, as well as matrix materials for which the element to be determined is perfectly known and also the major compounds that produce a matrix influence on the signal (e.g. alloys, gases). 

Calibration. It is necessary to equate units of radioactivity with units expressing amount absorbed. In general, the chemical composition of a radioactive solution was determined by standard chemical methods an aliquot of the solution was prepared for counting and its radioactivity was compared with the radioactivity of the treated fibers. Since great attention must be paid to details in this calibration process it is convenient to consider the calibration for each isotope separately. 

Isotope dilution begins with the addition of a known amount of calibrated radioactive tracer solution R[ to a sample of mass nii in solution. By measuring the mass recovered mi and the radionuclide amount at the end of the procedure, Ri, one calculates the initial mass. Designating Si and as the specific activity (i.e., the ratio of radionuclide amount to the mass of the same element, at the beginning and end, respectively) and OTri as the mass associated with the radionuclide tracer initially gives the following relationships ... 

Quantitative determination of the monoisotopic element cesium can only be performed by using a radioactive isotope or a different alkali element as an internal standard or by establishing a calibration curve as an external standard. A plot of such a curve is given in Fig. 13 for [Cs]" . For measurements, a number of standard solutions of CsCl in CCI4 are prepared and analyzed. The resultant values are displayed with an error of 22% corresponding to 2a (a = standard deviation). 

An alternate method exists to determine in situ bulk density usually only in very shallow water such as bays and estuaries, without the necessity of using radioactive isotopes. To accomplish the electrical resistivity of the soil and also of the pore water (to correct for variations in salinity) is measured using two probes (Rietsema and Viergever, 1979). This is a convenient measurement, although the equipment may be somewhat complicated to use because of having to use two separate probes to obtain one measurement in addition, the laboratory calibration procedure is not simple. What appears to be a more versatile device has been described by Bennett et al. (1983). 

The principle of isotopic wear studies is to label the material to be worn and monitor the material loss by radiation measurement. A typical example is studying car engine piston rings labeled, e.g., by neutron activation and measuring the accumulation of radioactivity in the lubricant oil. The measured count rates can be converted to mass by the appropriate calibration. Another example from the vehicle industry is to study the wearing of car tyres. 

There are very strict limits on the amount of radioactivity that can be administered for each type of investigation to be performed. These limits are set by the legislative bodies in individual countries. Therefore, all radioactive preparations must be checked for activity before administration. Most radionuclide calibrators used in radiopharmacy are ionization chambers. Commercial calibrators have built in scaling factors for individual radionuclides that take into account the ionizing ability of the isotope and give a readout in the appropriate units (kBq or MBq, or mCi). However, this type of calibrator is not ideal for all radionuclides. Low-energy radiation, such as that produced by iodine may be attenuated before... 

Standard In AMS, standard sample is that where the ratio of the long-lived radioactive isotope to stable isotope is known. The standard samples can be diluted to meet the level desired for AMS analysis. Isotope ratios in the range from 1 to 1 x 10 are usually convenient for system calibration. 

Precision and accuracy. As in all other techniques, precision is a measure of reproducibility while accuracy deals with the closeness of the result with the true value. The precision of an AMS measurement is evaluated from the reproducibility or spread of repeat measurements of the isotopic ratio of a sample. In AMS, stable and radioactive isotopes are measured simultaneously. The precision in AMS is limited by the uncertainty of the counts of the ions arriving in the detector. Precision or uncertainty is also dependent on the sample size. To have a more accurate result all the instruments required in various steps of AMS measurement should be calibrated properly (Hotchkis et al. 2000). 

The sample matrix plus standards for the elements of interest are irradiated for a select period of time in the neutron flux of a research reactor. After irradiation and appropriate radioactive decay, the y energy spectrum is measured by counting the sample with a high resolution (to separate various y-transitions of close-by energies) y-detection system. The NAA technique provides highly resolved analysis of elemental composition by the identification of characteristic y-ray energies associated with different isotopes. Quantitative analysis is provided by element-to-element comparison of the number of y-rays emitted per unit time by the unknown sample to the number of y-rays emitted per unit time by the calibration standards. 

Determination of uranium in soil samples can be carried out by nondestructive analysis (NDA) methods that do not require separation of uranium (needed for alpha spectrometry or TIMS) or even digestion of the soil for analysis by ICPMS, ICPAES, or some other spectroscopic methods. These NDA methods can be divided into passive techniques that utilize the natural radioactive mission (gamma and x-ray) of the uranium and progeny radionuclides or active methods where neutrons or electromagnetic radiation are used to excite the uranium and the resultant emissions (gamma, x-rays, or neutrons) are monitored. In many cases, sample preparation is simpler for these nondestructive methods but the requiranent of a neutron source (from a nuclear reactor in many cases) or a radioactive source (x-ray or gamma) and relatively complex calibration and data interpretation procedures make the use of these techniques competitive only in some applications. In addition, the detection limits are usually inferior to the mass spectrometric techniques and the isotopic composition is not readily obtainable. 

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