โ Scribed by S.S. Nargolwalla; M.R. Crambes; J.E. Suddueth
No coin nor oath required. For personal study only.
x4-MeV neutron activation analJ3is of thick samI~les, higll precision and accuracy can lx obtained provided appropriate measures are taken to ensure homogeneity of tile incident neutron flus, and corrections are made for self-absorptic)n in tile sample and standard.
An earlier study' shows the necessity for tlicse proccclures in tile precise and accurate analysis for osygcn in samples of widely varying composition. Our recent work 2*:3 describes tccliniques whicll permit tlic evaluation of systematic errors resulting from absorption of fast neutrons and y-rays by tllc sample during the irradiation and measurcment of induced radioactivity processes, respectively.
The successful conclusion of these studies prompted further invcstigation into the possibility of applying the developed techniques to tile determination of photon attenuation correction factors for the genera1 case.
The significance of obtaining accurate e&irical relationsllips which would predict attenuation correction factors in the general case cannot be over-emphasized. Sucll information would permit the use of any standard for the determination of an element in any matri.x. Also, the use of such correction factors would improve the accuracy of general analyses with r.4-MeV neutrons.
In most cases, If-spectrometry associatecl with activation analysis involves measurement of photopeaks resulting from direct nuclear level transitions or annihilation of positrons. From the previous study", it was concluded that the attenuated sample thickness, as given by the slope of the photon correction factor line as a function of the linear absorption coefficient, would be independent of the primary y-energy and dependent only upon the efficiency of the detector system for degraded y-ray contribution within the region of interest. Therefore, it was surmised that the slope for photopeak counting would be greater than that measured for the integral counting of y-rays between 4.S and 8.o MeV from 1"N decay in oxygen analysis, and would approximate a value equivalent to one half the geometrical sample diameter. It was also surmised that annihilation photons would exhibit a small change in the slope because of bremsstrahlung contributions under the annihilation photopeak. The neutron attenuation behavior, however, would remain unchanged if the removal cross-section theory was valid, since this theory assumes that neutrons whose energy have been degraded do not take part in subsequent nuclear reactions within the sample. Confirmation of this could easily be obtained if the nuclear reactions * Present acldrcss: Activation Analysis Group, CEN, 131' iGg. ~5Grcnoblc.
A,tld. ChitiZ.
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