NEUTRON ACTIVATION ANALYSIS


Neutron activation analysis is a way of analysing the atomic composition of ordinary non-radioactive materials. A flux of mainly low-energy neutrons is directed at the object to be analysed. The incident neutrons are absorbed by the nuclei (by the neutron capture process) in the object (or target) forming an excited compound nucleus which posseses one (or more) more neutrons than is normal. The compound nucleus formed in this activation process will immediately emit a prompt gamma ray, which will in most cases will leave behind a beta-radioactive nucleus which will be subject to normal beta decay where both an electron and a delayed gamma ray are emitted with a unique halflife. The delayed gamma ray will have a unique energy. A high resolution gamma ray spectrometer is used to detect the delayed gamma rays emitted by the neutron-irradiated object. The energy of these detected gamma rays is used to identify the nucleus emitting them, and hence the element that that nucleus corresponds to.

Actually, there are two methods of Neutron Activation Analysis in use, the most common method being that described above where it is the second (delayed) gamma ray that is measured. The half-lives of the nuclides that are detectable by this method may range from a few seconds to several years. Any longer and the flux of delayed gamma rays is too small to detect. The irradiation period can range from a few minutes to several days, and the integrating measurement period can range from a few seconds to half an hour or more, depending upon the elements/nuclides being measured.

The accuracy of the measurement varies from 1% to 10% of the reported value. With a neutron flux of 1x1013 per square centimetre per second, then the following is the approximate detection limit:

Sensitivity (picograms) E l e m e n t
1 -10 In, Lu, Mn
10 - 100 Au, Ho, Ir, Re, Sm, W
100 - 1000 Ag, Ar, As, Br, Cl, Co, Cs, Cu, Er, Ga, Hf, I, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, V, Yb
1000 - 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

As you can see, the method is most sensitive to indium, lutetium and manganese, and a million times less sensitive to lead and sulphur. Neutron Activation Analysis is used in identifying artifacts such as coins and pottery from the past, or in analysing the elemental composition of tiny flecks of paint from genuine paintings in order to produce a tell-tale 'fingerprint' of artists' paintings in order that forgeries can be later identified. It is also used to inspect luggage in customs and airports in order to 'see' any suspect items within (drugs, guns, etc).

A useful portable source of neutrons is a small sample of californium-252, which is a spontaneously fissionable isotope with a fairly high probability of fission as opposed to alpha decay (branching ratio: 3.1% SF, 96.9% alpha). It has a halflife of 2.65 years, so needs replenishing fairly often. The fission products and alpha particles are blocked by a thin foil.


Show Decay Chain of Cf-252 [POP]

The less well used method is to measure the energies of the prompt gamma rays, those emitted by the compound nucleus. This generally requires an irradiating neutron flux a million times greater than the other method, and thus is a million times less sensitive still.


[University of Missouri]
Neutron activated analysis gamma ray spectrum of a human fingernail


[University of Missouri]
Gamma Ray spectrum of a sample of pottery irradiated for 5 seconds, allowed to decay for 25 mins, then counted for 12 mins

GAMMA RAY DETECTORS

A gamma ray spectrum can be obtained by using a gamma ray detector consisting of a crystal of cadmium zinc telluride, CZT, a semiconductor. When a gamma ray impinges upon the crystal, it knocks a few electrons out of position producing also holes. If the CZT crystal has a voltage applied across two faces, the electrons are accelerated producing a cascade of electrons, an electrical current which is proportional to the energy of the incident gamma ray.