Neutron activation analysis using thermal or superthermal neutrons with the capture reaction (n, γ) is widely used to determine trace quantities of the elements in most biomedical materials, but it does not permit determining the content of the main elements (C, H, N, and O). An advantage of activation analysis using fast (14 MeV) neutrons lies in the possibility of determining nitrogen and oxygen according to the nuclear reactions (n, p) and (n, α) [1]. One of the most important applications of this method is determining protein in biomedical materials [2][3][4][5][6].A variant of using fast neutrons for determining the nitrogen content and through the nitrogen the protein content in certain kinds of grain and pulse crops has been developed [7]. To prevent errors associated with the instability of the fast-neutron flux and the irradiation geometry for samples and standard materials, the flux was monitored with an activation detector.Nitrogen in grain crops was determined with the reaction 14 N(n, 2n) 13 N, which product is a positron emitter with T 1/2 = 9.98 min, accompanied by annihilation γ-rays with energy E = 511 keV (100.62%).The mass m of nitrogen in the irradiated material was calculated from the induced activity A = 6.023·10 23 kmθM -1 σΦ[1 -exp (-λt irr )]exp (-λt cd ),where k is the detected radiation fraction; θ is the abundance of 14 N (99.635%); M is the molecular mass of nitrogen; σ is the reaction cross-section (0.0058 b); Φ is the neutron flux; λ is the decay constant; and t irr and t cd are the durations of irradiation and cool-down of the irradiated material, respectively. The activity with respect to the number of recorded counts is usually found from the relationwhere I is the number of recorded counts, ω is the detection efficiency for γ rays, and ε is a geometric factor. For the same irradiation and cool-down time for the sample and standard,where the indices refer to the sample (s) and the standard (st). For monitors (m) which are irradiated together with a sample and standard, I m.s /I m.st = (m m.s Φ m.s )/(m m.st Φ m.st ).
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