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Detectors and detection limits in INAA

I. General theoretical relationships between detector specifications and detection limits

Journal of Radioanalytical and Nuclear Chemistry
Author: P. Bode

Abstract  

Expressions have been derived which link detection limits in INAA with the Ge-detector specifications, provided by the vendors: relative efficiency, peak-to-Compton ratio and resolution. Situations have been distinguished in which a peak has to be detected on a Compton background, or on a natural background. The expressions allow for a direct indication of the improvement in detection limits, or the shortening of tumaround-time to attain equal detection limits, when a given detector is replaced by a detector with better specifications.

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Detectors and detection limits in INAA

II. Improvements in detection limits with large Ge-detectors, well-type Ge-detectors and anti-Compton spectrometers

Journal of Radioanalytical and Nuclear Chemistry
Author: P. Bode

Abstract  

Detection limit improvement factors have been calculated to predict the improvement in detection limits when replacing a traditional 20% coaxial detector by either a 100% coaxial detector, a well-type detector or by equipping the detector with a Compton suppression shield. It is show that, when measuring sources on the end-cap of the detector, a system with a Compton suppression shield and a 100% detector have almost equal potentials. But when e.g. geometrical consideration larger source-to-end-cap distances are required with coaxial detectors, the well-type detector offers the best outlook for attaining better detection limits. The calculated predictions have been compared with experimental observations and the results agree well.

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Abstract  

The improvement of detection limits for trace elements in geological samples by epithermal neutron activation analysis is examined. The relative merits of cadmium, boron and composite cadmium+boron filters are compared for trace elements Ni, As, Pd, Cd, Sb, W, Ir, Pt and Au, and interfering elements Na, K, Sc, Cr, Fe, Co and Cu. A boron filter gives optimum sensitivity for the trace elements based on interference from46Sc, but the detection limits are only improved 2–5 times. Ma imum possible improvement, which is shown by Ni, gives sensitivities 5 times better under cadmium and 15 times under boron.

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Abstract  

This paper describes some highlights from the author’s efforts to improve neutron activation analysis (NAA) detection limits through development and optimization of radiochemical separations, as well as to improve the overall accuracy of NAA measurements by identifying, quantifying and reducing measurement biases and uncertainties. Efforts to demonstrate the metrological basis of NAA, and to establish it as a “Primary Method of Measurement” will be discussed.

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Abstract  

The widespread introduction of rapid pneumatic sample transfer systems has enabled instrumental neutron activation analysis to be based on an increasing number of very short-lived activities. Furthermore, these transfer systems have been interfaced to computer-based MCA's so that the experimenter has complete control over irradiation, decay and counting times, as well as being able to arrange the automatic transfer of numbers of samples between the various stations. Thus the analyst now has a series of options available to him to make the best use of time and facilities. Based on the requirements of detection limits and precisions, he will choose between various irradiation and counting régimes (a) single i.e. conventional (b) cyclic and (c) repeated; or he may choose to replicate the sample a number of times. This paper examines how detection limits and precisions are affected by the above options. By considering a specific isotope, being detected in backgrounds of different half-lives, it is possible to calculate signal-to-noise ratios in each of these cases, and hence compare these régimes from this aspect. Based on calculations for the isotope77mSe (17.5 s), which is now being widely accepted as the basis for selenium analysis, it is shown that, if a low detection limit is the prime consideration, then replicating samples is the procedure of choice; however, if commercial considerations of sample throughput are important then a pseudocyclic régime would provide the best compromise.

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Abstract  

Method detection limits have often been misunderstood and misused. The basic definitions developed by Lloyd Currie and others have been combined with assumptions that are inappropriate for many types of radiochemical analyses. A practical way for determining detection limits based on Currie's basic definition is presented that removes the reliance on assumptions and that accounts for the total measurement uncertainty. Examples of proper and improper use of detection limits are also presented, including detection limits reported by commercial software for gamma spectroscopy and neutron activation analyses.

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Abstract  

The (pX, X) technique is a very reliable tool for trace element analysis when large amounts of a neighboring element are present. This paper describes the experimentally determined detection limits of some available sources of GaAs, Ge and Zr, to evaluate the utility of this technique. The theoretical detection limits calculated with a computer program are also described.

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Abstract  

The advantages of an underground laboratory for the measurement of activated pure materials are demonstrated. Background spectra and experimentally determined detection limits for the INAA of semiconductor silicon are compared with literature data and interference-free detection limits are calculated.

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Abstract  

This paper extends previous work for decision levels to detection limits. After transforming the net count to an integer, the probability density function for the transformed net count can be readily determined when the transformed net count is greater than zero. The right tail of the distribution is summed and the detection limit is determined to four decimal places. The code under discussion works well when the product of the ratio of the blank count time to the sample count time with the expected blank count in the sample count time is not greater than 100.0.

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Abstract  

The cause of the low detection efficiencies of Cherenkov radiation by the liquid scintillation counter was investigated. The detection limit of the liquid scintillation counter was assumed and the increase of counting rates for the increase of refractive index was calculated from Cherenkov photo yields and -distribution.

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