where a 1 and a 2 are the activities measured for standard and analyte respectively under identical
conditions, and w 1 and w 2 refer to the respective masses. Neutron activation analysis is an extremely
sensitive, selective and precise technique (Table 10.2).
Table 10.2 Sensitivity of neutron activation analysis in determination of elements by (n,
γ) reactions with a neutron flux of 10^12 n cm–^2 s–^1
Grams Element
10 –^12 Eu, Dy
10 –^11 Mn, Pd, In, Sm, Ho, Re, Ir, Au10 –^10 Na, Sc, Cu, Ga, As, Br, Kr, Y, Sb, Pr, Tb, La, Er, Tu, Yb, Ta, W, Th, U
10 –^9 P, Ar, K, Rb, Co, Ru, Cd, Cs, Ba, Ce, Nd, Gd, Hf, Os, Pt, Hg
10 –^8 Cl, Si, Ni, Zn, Ge, Se, Mo, Ag, Sn, Te, Xe, Tl10 –^7 Ca, Sr, Fe, Zr, Bi
10 –^6 S, Pd10 –^5 Mg, PbThese valuable characteristics derive from a combination of factors. Firstly, extremely sensitive
instrumentation with a facility for spectrometric distinction between radionuclides is available. In
particular, a Ge(Li) or intrinsic solid state detector together with a multichannel analyser can be used to
make activity measurements by γ-ray spectrometry, often with a bare minimum of sample treatment.
Secondly, activation cross sections can be large (up to 10^5 barns) and intense neutron fluxes are
available (10^14 neutrons cm–^2 s–^1 ) whence NYλY becomes large (equation (10.13)). Thirdly, the reagent
blank problem which is so common in trace element analysis is largely eliminated, for contaminants so
introduced after activation will not be active and thus not detected. Finally, when sample processing
prior to measurement is needed, the problem of working with microgram amounts of material can be
simplified by the addition of non-active 'carrier' which does not affect the final activity measurement.
Neutron activation analysis is an attractive method in many trace element problems, or where the total
amount of sample is limited. Many geochemical studies of trace constituents and semi-conductor
developments have used the technique, whilst in recent years pollution investigations have provided a
new focus. In forensic science small flakes of paint, single hairs and a variety of other small samples
have been analysed and identified by activation analysis. In recent years activation analysis has lost
further ground to ICP-MS which provides more comprehensive information and is more readily
operated. Sensitivity is also comparable in many cases.