Dr.BHAGURE G.R. Department of Chemistry, Dnyanasadhana College,Thane-400604.

1. Dr.BHAGURE G.R. Department of Chemistry, Dnyanasadhana College,Thane-400604.

2. Introduction : Neutron Activation Analysis (NAA), discovered in 1936, is an important technique for quantitative multi-element analysis of major, minor, trace, and rare elements. The initial step in neutron activation analysis is irradiating a sample with neutrons in a nuclear reactor or sometimes in other neutron sources (Siddappa et al., 1996). The stable nucleus absorbs one neutron and becomes a radioactive nucleus. The concentration of the stable element of interest in the sample can be measured by detecting the decay of these nuclei (Nuclear Science Division and Contemporary Physics Education Project 2000).

3. The radioactive nuclei emit characteristic gamma rays. Detection of the specific gamma rays (of specific energy) indicates presence of a particular element. Suitable semiconductor radiation detectors may be used for quantitative measurement.

4. General Principles: • In typical NAA, stable nuclides (AZ, the target nucleus) sample undergo neutron capture reactions in a flux of (incident) neutrons (Missouri University homepage 2003). The radioactive nuclides (A+1Z, the compound nucleus) produced in this activation process usually decay by emission of a beta particle (ß-) and gamma ray(s) with a unique half-life. A high-resolution gamma-ray spectrometer is used to detect these ‘delayed’ gamma rays in the presence of the artificially induced radioactivity in the sample for both qualitative and quantitative analysis. • The sequence of events that occur during the most common type of nuclear reaction used for activation analysis is shown in Fig. 1.

5. The incident neutron hits the target nucleus, which captures the neutron and is converted into a compound nucleus. The latter immediately emits radiation called prompt gamma radiation and forms the radionuclide, which then kicks out a beta particle and emits the delayed gamma radiation (since it is emitted after some time delay), forming the product nucleus.

6. n1 Fe 59 N A A PATH WAY + Fe 58 gamma rays + B + Beta +

7. Theory of NAA: • Let x be the weight of sample taken for neutron bombardment. X gm the sample contain N atoms of the element under study.If the sample is irradiated with netrons for sufficiently long time ,the activity produced is given , • At = N Ø Ó (1-e-^t ) ………………………(1) • Where N=No.of atom of element under study. • Ø = is the neutron flux in neutrons Cm-2S-1. • Ó= is the neutron capture cross section in Cm‑2 S-1. • t = time of irradiation of the sample • ^ (Lambda) = is the decay constant of the radioactive product in S-1.

8. The decay constant is related to half life of the element ,T1/2 of radioactive element by equation, • ^= 0.693/ T1/2 ………………………( 2) • by introducing the value of Decay constant in equation no.1 we get; • At = N Ø Ó (1-e- 0.693/ T1/2X t ) ………………………(3). • At is the ¥ activity at the end of the irradiation period i.e. at time t. • However some time lapses between the end of irradiation and the commencement of counting which some activity is lost.

9. Hence the activity which is actually measured after the time interval t’ is given by; • At’ = At = e-^t’ ) N Ø Ó (1-e-^t ) ) e-^t’ ………………………(4). • At’ • N= ---------------------------------- …………………………(5) • Ø Ó (1-e-^t ) ) e-^t’ • The weight W gm of the element in x gm of the sample is given by; • N x M M At’ • W= ---------------- = --------------------- x ------------------------- • 6.023x 10 23 6.023x 10 23 Ø Ó (1-e-^t ) ) e-^t’

10. Where M is the atomic weight of element. • In actual practice the standard sample( Ws) and Sample containing unknown amount ( Wg) of element both are irradiated simultaneously. If the corresponding activities are (Ats) and (At) respectively then;

11. W At • ----- = ------- therefore; • Ws (Ats) • At • W= Ws x ------ • Ats

12. Calibration Curve Method= • A series of known samples containing W1, W2, W3………etc. g of the element under study are irradiated along with unknown sample. The activities are then determined using scintillation eight of the element under is under counter. The activities are plotted against the corresponding amounts of the element. From the graph, the weight of the element under study can be found out.

13. Advantages : • NAA is highly selective technique and enables the estimation of several element up to concentration 10-12 g. • It is non destructive technique sample remains as it is. • The technique is suitable for analysis of precious and rare samples like ancient coins, archaeological specimens, Jewels and precious stones.

14. Applications: • Neutron activation analysis is applied in several fields like Material science, environmental monitoring, trace metal analysis in medicine and forensic science. • Estimation of metals like Zinc,Copper, Mn, and Cobalt in plant and biological samples. • Trace element in soils, rocks, ores and alloys can be estimated. Ex. Iridium in soils can be estimated. • Detection of trace impurities in silicon and germanium samples used in transistors. • Detection of trace impurities in moderators and other construction materials used in nuclear reactors. • Oxygen is detected in steel by NAA. • The automobile industry – to test steel quality in the manufacture of cars and to obtain the proper thickness of tin and aluminum

15. The aircraft industry – to check for flaws in jet engines • Construction – to gauge the density of road surfaces and sub-surfaces . • Pipeline companies – to test the strength of welds • Oil, gas, and mining companies – to map the contours of test wells and mine bores, and • Cable manufacturers – to check ski lift cables for cracks. • Some other Applications of NAA

16. Agriculture : beet pulp, lipids, hay, oil, fish • Archeology/Anthropology : Ceramic utensils, obsidian, teeth, bones • Biology : chemicals, sugar, enzymes, solutions, ants • Botany :wheat spores • Chemistry : oxides, salts, pure crystals, and metals • Engineering & Industry :pure metals, chemical compounds, oils, thin film deposits, plastic films, alloys, rocks • Fisheries :fish, shells • Forestry : wood, phloem, tree needles, soil • Geology : rocks, meteorites, moon samples, gems, minerals • Medicine : water, skin, hair, nails, alternative medicine components • Oceanography :fossils, sediments, basalts • Pharmacy :chemicals • Forensic : bullets, paint, glass, metals, gunshot residue swabs

17. Limitations : • NAA cannot used for elements whose radioisotope having very short half lives.Ex. Li, B,Mg, Al,Ne, V, W, and Nb can not detected because these elements having few seconds of half life. • NAA can not useful for analysis of element which have high neutron absorption during irradiation. Eg. Cadmium