(a) By referring to a reference on NAA or by using an Internet search engine to find the information, describe in some detail the instrumental and radiochemical differences between prompt gamma-ray neutron activation and delayed gamma-ray neutron activation.
(b) What are the types of elements for which prompt gamma-ray activation analysis is most applicable?
(c) Why is delayed gamma-ray emission most often used in NAA?
(d) Why is NAA considered to be a very selective and sensitive method?
(e) A crystal of potassium fluoride is to be studied via NAA. The following table summarizes the behavior of all naturally occurring isotopes in the crystal.
19F, 20Ne, 39K, 41K, and 42Ca are stable, and we will assume that 40K is also stable because it has a half-life of 1.3 x 109 years. What sort of irradiation and detection sequence would you use if you wanted to base your analysis on fluorine?
(f) What sort of irradiation and detection sequence would you use if you wanted to base the analysis on potassium?
(g) Refer to part
(e) and calculate the activity due to 20F and 42K in a 58-mg (1.0-millimole) sample of pure potassium fluoride that has been irradiated for 60 s. The thermal neutron cross sections for 19F and 41K are 0.0090 X 10-24cm2 and 1.1 X 10-24cm2, respectively. Assume a flux density of 1.0 x 1013 neutrons cm-2 S-1.
(h) Find a method in the literature that describes the use of NAA for determining selenium in freshwater ecosystems. Describe the method in detail. Include the neutron source, the irradiation time, and the calculations used. Give the advantages and disadvantages of the method over other analytical techniques.
