Activity minimum detectable

The minimum detectable activity (MDA) is the smallest net count that can be reported with a certain degree of confidence that represents a true activity from a sample and is not a statistical variation of the background. The term MDA is not universally acceptable. In the general case, in measurements not necessarily involving radioactivity, other terms such as lowest detection limit have been used. Here, the notation and applications will be presented with the measurement of a radioactive sample in mind. 

Obviously, MDA is related to low count rates. In such cases of low count rates, the person who performs the experiment faces two possible errors. 

TYPE I error To state that the true activity is greater than zero when, in fact, it is zero. If this is a suspected contaminated item, the person doing the measurement will report that the item is indeed contaminated when, in fact, it is not. This error is called false positive. 

The outcomes of radiation measurements follow Poisson statistics, which become, essentially, Gaussian when the average is greater than about 20 (see Sec. 2.10.2). For this reason, the rest of this discussion will assume that the results of individual measurements follow a normal distribution and the confidence limits set will be interpreted with that distribution in mind. Following the 

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Using the Chart of the Nuclides as a guide, estimate the sensitivity (minimum quantity that can be detected) of neutron activation analysis for europium using a thermal neutron flux of 3 x 1012 n/cm2-s. Assume no irradiation may last more than 1 h and the minimum detectable activity is 10 dpm. 

Taking into consideration the method of °Sr analysis, the activity equilibrium state between °Sr and its decay product °Y is very important. This state is attained 12 days after the separation of radiostrontium [62]. The reliability of the received results of °Sr determination depends on the minimum detectable activity (MDA) [5]. The MDA should be calculated for each analysis sample. Generally, the separation of °Sr with the use of fuming HNO3, and subsequent co-precipitation of radium, lead, and barium as chromates, is used for the analysis of flora, soil, ash filters, and water samples. The fusion products (e.g., Cs) are removed by co-precipitation of the hydroxides, then transformed into yttrium oxalate, and the activity of °Y measured in a low-level proportional counter. The yield is controlled by measuring the activity of Sr (gamma emitter) added to each sample before analysis as an internal tracer [1, 46]. The accuracy of the analytical results obtained should be verified in a validation process with the use of certified reference materials (CRMs). 

The minimum detectable activity (MDA), which is analogous to the minimum detectable concentration, is given by... 

Other methods currently in use for Ca determination in concretes are based entirely on precipitations for purification of the Ca. Suarez et al (2000) utilise a chromate precipitation for removal of Ba and Ra from the solution. This is a pH dependant precipitation, and the chromate solution resulting from this requires a separate waste stream. The method has a total of 9 steps after sample dissolution, and has decontamination factors of >10" for Ba and Co, >10 for Sr and >10 for Eu. The minimum detectable activity by liquid scintillation counting is 0.29 Bq/g for Ca (0.034 Bq/g Ca). 

High-purity detection systems having a very low background are suitable tools for the direct measurement of low-level radioactivity in environmental samples. The background features of the detection system are of considerable importance because they have to be known for one to obtain an estimate of the detection limit and of the minimum detectable activity (Curie, 1968). The natural radioactivity background originates from the uranium and the thorium series from K and from cosmic rays. Natural radioactivity is found in most materials, and it is necessary to shield the... 

The minimum amount of radioactivity that can be detected by a flow-through radiochemical detector is a subject of continuing debate. It is generally accepted that a fairly sharp peak that contains counts that are at least twice background can be detected. One formula for calculating the minimum detectable activity (MDA) is given by... 

Figure 2.12 The meaning of the critical detection limit (CDL) and minimum detectable activity (MDA) in terms of the confidence limits defined by a and /3.

As in the case of minimum detectable activity (Sec. 2.20), two types of errors are encountered when one tries to identify peaks in a complex energy spectrum. Type I arises when background fluctuations are falsely identified as true peaks. Type II arises when fluctuations in the background obscure true peaks. Criteria are set in the form of confidence limits (see Sec. 2.20 and Ref. 38) that can be used to avoid both types of errors. 

Sensitivity of the activation analysis method for a particular element refers to the minimum mass of that element that can be reliably detected. The minimum detectable mass is determined from Eq. 15.4 by assuming the most favorable conditions for the measurement and by setting an upper limit for the acceptable error of the result. The process is similar to the determination of the minimum detectable activity discussed in Sec. 2.20. 

Estimation of the detection limit of the measurement instrument in the time period available for counting, linked with the required detectable concentration and the expected concentration of the radionuclide to be determined, guides selection of sample size for the analysis. Calculation of the minimum detectable activity is discussed in Section 10.4.2. Additional documents of interest are Altshuler and Pasternack (1963), Pasternack and Harley (1971), and Currie (1968). The terms minimum detectable concentration and lower limit of detection also have widespread use. A document that addresses these and other topics pertinent to radiation monitoring and measurement was developed by a committee of the Health Physics Society (EPA 1980a). 

In the future, mass spectrometry (see Chapter 17) may supersede radiochemical analysis for long-lived radionuclides and require a different set of chemical separations. This trend is opposed to a certain extent by chemical separation processes introduced to achieve ever lower minimum detectable activity requirements and by the continued interest in identifying newly created radioelements. 

The laboratory may often be required to make detection decisions about samples, but when the analyte activity is low enough, the relative uncertainty in the result may make it difficult to distinguish between a small positive activity and zero. The performance characteristic of the measurement process that describes its detection capability is called the minimum detectable value, minimum detectable activity, minimum detectable concentration (MDC), or lower limit of detection (LTD). These terms have been used to denote the theoretical concept of the smallest true value of the analyte in a sample that gives a specified high probability of detection. 

For low level measurements in conditions of practices and chronic (prolonged) exposure, the minimum detectable activity of the equipment and the method apphed should be such as to enable the measurement of radionuchde levels that are substantially lower, by one to two orders of magnitude, than estabhshed limits or action levels for radionuclides in the appropriate media. If the established limits are lower than the background levels, however, then a minimum detectable activity that enables the measurement of radiation levels or activity concentrations lower than background levels is sufficient. 

When monitoring data are to be used to assess the annual doses for a critical group and to verify comphance with the dose constraints in the case of practices or to check against the intervention level, the minimum detectable activity of the equipment concerned should be selected so as to enable measurements to be made at levels that are substantially lower than the established reference dose levels, with account taken of multiple pathways of human exposure. For every pathway that has to be checked, a certain fraction of the reference dose should be allocated the minimum detectable activities should be designed to guarantee the detection of these possible contributions to doses. 

In the conduct of practices, rates of release of radionuclides are generally low and the possibilities for a detailed analysis of exposure might be limited ik for example, the external dose rate attributed to releases is of the same order as the fluctuations in the dose rate due to background radiation. In this case, the dose can be assessed as a value less than the dose estimated with the minimum detectable activity for the measurement used as input data. This dose assessment can be assigned an estimated uncertainty that takes into account the uncertainties in the parameters of the dosimetric models. 

Beads packed in microcolumns is the most reported in literature, since users can customize the quantity of resin according to the capacity of it, to the volume of sample to be loaded, and to the minimum detectable activity (MDA) of the detector used. In general, packing is manually replaced in flow systems based on flow injection analysis G A), sequential injection analysis (SIA), multisyringe flow injection analysis (MSFIA) and multipumping flow systems (MPFS). By the contrary, lab on valve (LOV) allows the manipulation of heterogeneous solutions, i.e. bead injection, achieving the automated replacement of the resin. In Chapter 3 are described in detail the parts of the microcolumns and the way to fill them. Table 8.1 summarizes the variety of available resins from TrisKem International [4]. 

In this chapter, I will examine the statistical nature of radioactivity counting. Statistics is unavoidably mathematical in nature and many equations will emerge from the discussion. However, only as much general statistical mathematics wiU be introduced as is necessary to understand the relevant matters. I wiU go on to discuss the statistical aspects of peak area measurement, background subtraction, choosing optimum counting parameters and the often superficially understood critical limits and minimum detectable activity. I end with an examination of some special counting situations. 

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