Contrary to PIXE and RBS where forces are respectively electromagnetic
and electrostatic, this kind of microanalysis uses low range nuclear forces.
The basic mecanism is the formation of a compound nucleus in a highly excited
state that desintegrates or desexcites by the emission of gamma ray in a
small time.
The limitation of this method is due to the fact that to have a nuclear
reaction, the repulsive Coulomb barrier has to be surmounted. For light
particles (proton, deuteron, helium 3 and 4) of energy up to 3 MeV, the
only accessible elements are the light elements with Z < 15. Indeed,
the cross sections become rapidly negligible.
The products of the reaction are charged particles, gamma ray or both.
So, we have:
where
The conservation of energy of this kind of reaction is given by
Where Ea, Eb and Ez are kinetic energies, Qi is the Q value of the reaction
(the energy corresponding to the i excited state of the residuel nucleus)
and Egamma is the energy of the emitted gamma ray.
If Qi > 0 then each Ea generates a reaction but for low Ea, the cross
section is very small.
If Qi < 0 then there is a critical Ea for which the reaction becomes
possible.
Contrary to PIXE and RBS methods, PIGE and NRA are not methods with continuous
sensitivity. Each element is a special case depending on the existence or
not of a reaction and of its cross section.
The cross sections of these reactions are quite inferior to those that we
meet in RBS or PIXE but in certain cases, it is possible to use important
resonances.
The charged particle detection is realised by Silicium surface barrior diodes
as in RBS. An interesting aspect of this method is the fact that the energy
spectrum of the emitted particles has usefull information to determine the
concentration profile of the element.
NRA is mostly used to study the corrosion and the vacuum hydrocarbure deposits.
A typical spectrum used to analyse C, N, O on a pure metal surface is shown
below.
In PIGE, the gamma ray emitted is measured by NaI(Ti) or Ge(Li) detectors.
The first one has a good efficiency but a bad resolution while the second
one has a poor efficiency but an excellent resolution. The PIGE method can
not determine profile concentration because the gamma ray beam is attenuated
but its energy is not diminished when it returns to the surface. Nonetheless,
profile information can be obtained by using (p, gamma) reactions with cross
sections that present a strong resonance that can be moved in the sample
by varying the incident proton energy.
PIGE is essentially used to determine light element concentrations in
surface. This is an ideal complementary technique to PIXE as these techniques
can be simultaneously performed. A typical PIGE spectrum is shown below.
This is a matrix composed of C, Na, Al and Li. The peaks at 0.439 and 0.478
MeV used to determine the Li and Na concentrations are well separated due
to the good resolution of the intrinsec Ge detector.
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