Nuclear interferences

If radioisotope B, by which element A is determined, is generated from an element C the analysis is inevitably wrong. This element C that gives rise to radioisotope B by a competing nuclear reaction C(r,7)B may be another impurity or one of the main constituents of the material analyzed. 

Element determined Competing nuclear reactions Approximate threshold energy (MeV) 

Furthermore, by comparing the threshold values given in Tables II-IO and II-ll, it is clear that it is in all cases possible to eliminate totally the effect of competing nuclear reactions, by the appropriate choice of the maximum energy of the gamma photon beam. 

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Proper choice of the charged particle energy (see Nuclear Interference). 

Detection limits down to the pg per kg level have been attained for the most favorable instrumental analyses (e.g., carbon and nitrogen in molybdenum and tungsten) and for radiochemical analyses (e.g., cadmium and thallium in zinc) at least if no nuclear interferences occur. This is below the concentration levels at which these impurities influence the material characteristics and below the detection limit attainable by more common methods of analysis. A precision (reproducibility) of a few percent is possible at the mg per kg concentration level in the most favorable cases. However, at higher concentration levels the precision will not improve significantly. Many systematic errors can be checked experimentally (e.g., interferences, yield of a radiochemical separation) others can be avoided experimentally (e.g., surface contamination). Systematic errors due to reagent blanks do not arise. 

Table IV-1 gives nuclear reactions that can be used for the determination of boron, along with the nuclear interferences. For some reactions the sensitivity is also given. The half lives of the radionuclides produced are 9.97 min ( N), 20.4 min ( C) and 53.3 d ( Be). N and C are positron emitters, Be emits 478 keV 7-rays. For the B(d,n) C and B(o,n) N reactions, energies below the thresholds of the interfering reactions (resp. 5.9 and 13.6 MeV) can be used, so that no nuclear interferences occur.

Table IV-1 Nuclear reactions for the determination of boron and nuclear interferences (threshold energies in MeV)...

Proton activation analysis via the reaction B(p,a) Be is one of the most interesting methods as it is precise and accurate at + 5 %, provided Be is separated radiochemically before counting and the lithium concentration in the sample is very low, which is generally the case. The use of protons of lower energy (e.g. 2.75 MeV) theoretically allows the analysis to be carried out in an instrumental way, but experience proved that it is difficult to obtain a good precision. Deuteron activation analysis via the B(d,n) C reaction is an alternative. This method is free from nuclear interferences but requires a radiochemical separation of... 

Table V-4 gives nuclear reactions that can be used for the determination of carbon along with the nuclear interferences. The radionuclides produced, N and C are -emitters with half-lives of 9.97 and 20.4 min.

No nuclear interferences occur for deuteron energies below 4.9 MeV. From... 

Deuteron activation analysis is free from nuclear interferences for deuteron energies below 4.9 MeV. As shown in Table V-8 the results are comparable to those obtained with photon activation analysis. At the 0.2 to 0.3 Mg/g level both... 

For reaction 1 nuclear interferences do not occur for proton energies below 21.7 MeV. In addition, spectral interferences are not very probable, if the 7-ray at 2313 keV is measured. A disadvantage is the high threshold energy of the N(p,n) 0 reaction, so that for low Z matrices it is not possible to use an incident energy below the coulomb barrier of the matrix to avoid activation of the matrix. 

For reactions 5 and 6 nuclear interferences resp. from boron and carbon and from carbon and oxygen cannot be avoided. 

For the first three methods (activation with protons, helium-3 and a-particles), nuclear interferences cannot be avoided by an appropriate choice of the incident energy. He-activation is the most selective method. 

As shown in Table II-IO nuclear interferences are possible, especially from aluminium itself. By irradiating metal fluorides with photons with a maximum energy of 40 MeV, it was confirmed that the interference of fluorine is... 

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