Instrumental Neutron Activation Analysis

R. Delmas, Clarisse MARIET, M. Moskura, D. Guiot

The Instrumental Neutron Activation Analysis (INAA) is a method of analysis exclusively elementary. Its implementation consists in irradiating the sample to be analyzed in a flow of adapted particles, neutrons, charged particles… and to identify, then, after irradiation, radioactive isotopes created starting from the elements to quantify. By its principle even, the method is single among the methods of analysis: the excitation relates to the core of the atom and measurements relate to the artificial radioactive isotopes thus created. It results numerous advantages from it: the analysis is not only multielementary and very sensitive (limit of detection near 10^-12 g and less sometimes) but moreover its answer is independent of the chemical shape of the element. The mass of the sample can vary of a few milligrams to several grams, and its preparation is in general very simple. The majority of the risks of error (losses and pollution), which watches for the traditional methods, can be avoided or controlled. After irradiation, a scraping makes it possible to be free from surface pollution and contaminations introduced during possible chemical separations, can neither distort the results nor degrade the detection limits. Radioactivity measurements allow to identify in a very selective way each atom present and to quantify them.

LPS laboratory for the recovery of the samples coming from Orphée or Osiris reactors

For traces quantification, this method is one of rare to usually reach its theoretical limits of detection. Its response times are dependent on the period of decrease of the radioisotopes used and can reach several days. The method applies without major modification to very many materials (metals, semiconductors, archaeological, biological, geological samples…) whose form and mass can vary on the whole. The same applies to the metered quantities which can vary from picogram to gram with a general precision near a few percent, whatever the content, except in the vicinity of the limit of detection. Currently the method is especially used as method of analysis of traces and ultratraces, either as such, or to gauge and check the results obtained by other methods, in general more accessible and more rapids but which always do not have the reliability of the activation analysis. The possibility simultaneously of quantifying, on the same sample, a great number of elements present in very different areas of concentration, makes it a method of choice for sources or mechanisms studies.

Detection limits estimated by INAA following an irradiation in neutrons flow of 1×10¹³ n.cm^-2 s^-1.

Sensibility(picogram)	Elements
1	Dy, Eu
1 – 10	In, Lu, Mn
10 – 100	Au, Ho, Ir, Re, Sm, W
100 – 10³	Ag, Ar, As, Br, Cl, Co, Cs, Cu, Er, Ga, Hf, I, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, V, Yb
10³ – 10⁴	Al, Ba, Cd, Ce, Cr, Hg, Kr, Gd, Ge, Mo, Na, Nd, Ni, Os, Pd, Rb, Rh, Ru, Sr, Te, Zn, Zr
10⁴ – 10⁵	Bi, Ca, K, Mg, P, Pt, Si, Sn, Ti, Tl, Xe, Y
10⁵ – 10⁶	F, Fe, Nb, Ne
10⁷	Pb, S

Contacts:
Clarisse MARIET 0169084960
Robert DELMAS 0169085128

Sample préparation

The geometrical shape of the sample imports little. Indeed, the neutrons, because of their zero load, have a great capacity of penetration in the matter, except for some elements (boron, cadmium…) who have a very strong cross section of neutron capture (several thousands of barns). Theoretically, it is possible to apply the method to the solids, the liquids and gases. In fact, except exception, because of the phenomena of radiolyse and the risks of contamination, the samples are irradiated in solid form, wrapped for example in aluminium paper. This material can be obtained very pure and be led to radioisotopes whose radioactivity decreases quickly after irradiation. The irradiated masses can vary few milligrams with a few grams and even much more if the device planned for the irradiation allows it. In the majority of the cases, the samples with the standards, or the monitors of flow, are placed in a closed shuttle, adapted to the device of transfer. This shuttle can be out of plastic for the short irradiations (less than one hour with 10¹³ n.cm^-2.s^-1), out of graphite or aluminium for the longer irradiations.

Cadmium and plastic shuttles posed on the control board

Sample treatment after irradiation

Sample and standard are extracted from the shuttle of irradiation in a laboratory arranged to this end. The standard, or the flow monitor, either is measured directly, or possibly put in solution to present the same geometry as the sample during radioactivity measurements. In traditional analysis of traces, the sample, if it is solid, is generally pickled to eliminate not easily avoidable surfaces pollution. In activation analysis, the natural pollution introduced after the end of the irradiation cannot distort measurements of radioactivity. In the successful outcome of relatively pure materials activating themselves little or giving radioisotopes of short period (aluminium, magnesium, silicon, their oxides…), it is possible with the modern devices of gamma spectrometry to detect and proportion on the same sample forty elements.