Bremsstrahlung interference

Part-per-billion levels of molybdenum can be measured by neutron activation. After two or more hours irradiation at a neutron flux of 1012 n cm"2 sec 1, the principal gamma at 141 KeV [98Mo(n,y,p) 99mTc] is readily measured. In crude oils, however, some Bremsstrahlung interference may be encountered as a result of activated sulfur or chlorine. 

From equation (12.10), one finds that interference between the direct bremsstrahlung process and polarisation radiation results in an asymmetry of the giant resonance profile observed in fluorescence with electron excitation even when the corresponding photoabsorption profile is symmetrical. This is exactly analogous to the Fano resonances (chapter 6) bremsstrahlung plays the role of the continuum, while the resonant chan-... 

Proton bombardment in heavy elements produces both bremsstrahlung and a strong emission of A -radiation which can interfere seriously with the measurement of the y-rays. The yield of this radiation is in fair agreement with theoretical predictions . In part it may be removed by filtration. Recently the use of helium... 

Photoneutrons are produced at considerable flux densities in the heavy metal of the bremsstrahlung converter. For example, in the linear accelerator used by the authors, a thermal neutron flux density of some 10 °cm s has been produced in the tantalum converter during normal operation (30MeV, 150 pA). This appears a relatively poor flux density compared to that of a standard nuclear research reactor. Nevertheless, in advantageous cases trace analyses can be performed. For example, routine analyses of several elements in air-dust samples were carried out by activation with photoneutrons (see below). Normally, however, the photoneutron flux is analytically negligible, whether as an analytical tool or as a source of interference. 

Time-gated detectors that allow the optical emission from the laser plasma to be recorded at some time delay after the laser pulse are required to accurately capture the emission spectra. For the first few microseconds after the ignition of the laser spark, the plasma emits a strong white light continuum (also called bremsstrahlung), which decays as the plasma cools the characteristic atomic and ionic emission lines only appear as the plasma cools. A detector delay on the order of several microseconds after the laser pulse is used to eliminate interference from the continuum radiation. The principle is demonstrated in Figure 7.52. 

As radiation sources. X-ray tubes or radionuclides are applied. The polychromatic radiation of the tube is scattered by the Compton or Rayleigh effect towards the detector, which results in a high bremsstrahlung background and line interferences caused by the characteristic lines of the anode material and the resulting Compton peak. 

Recent applications of photon activation to the analysis of heavy elements include the determination of Te, Sr, and Pb. Campbell and Steele measured Te in the presence of U by Ge(Li) spectrometry of the activated Te isotope [ Te(y,n) Te] with a half-life of 17 d. The method is useful because it avoids the complications arising in n.a.a. caused by fission of the uranium giving rise to direct and spectral interferences from fission products. The measurement of "Sr induced by Sr(), n) and Sr(y,y ) reactions using 30 MeV Bremsstrahlung has been applied to the analysis of Sr in sea-water at the 8 p.p.m. level this involved a radiochemical separation of the 2—8 h Sr. The (y,n) reaction of Pb has been used for the determination of Pb in milk powder.Measurement of the Pb isotope (/ 52 h) after a non-specific sulphate precipitation of the Pb is sufficient to attain a limit of detection of 0.5 //g. The activity was measured with a NaI(TI) detector after "Sr, which was also produced, had decayed. 

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