Resonance, absorption Doppler broadening

The half width of elemental lines is of the order of 0.002 nm when observed by emission spectroscopy with flame or electrothermal atomisation. A number of reasons can cause broadening of the linewidth, of which the most important and best understood are natural, pressure, resonance, and Doppler broadening. If a stable and sensitive detection is to be achieved, the linewidth of the excitation radiation must be narrower than the full width at half maximum (FWHM) of the analyte line. Under these conditions, the entire radiant energy produced by the excitation source will be available for absorption by the analyte. The typical line sources used for atomic absorption are element specific excitation sources such as the hollow cathode lamp or the electrodeless discharge lamp. But even continuum sources can be used with appropriate instrumental designs. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

The arguments seen in section 2.3 suggest that resonant y-absorption should decrease at very low temperatures because the Doppler broadening of the y-lines decreases and may even drop below the value of the recoil energy. In his experiments with solid sources and absorbers, however, R.L. Mossbauer ([1] in Chap. 1) observed on the... 

The absorption coefficient, of mercury vapor for a Doppler-broadened resonance line is given by the expression ... 

Doppler broadening, whereby the absorption of neutrons in the resonance region is increased with an increase in temperature. The increase in nonfissioning absorption by uranium-238 means that this effect reduces the reactivity of thermal reactors at higher temperatures ... 

The powerful technique of two-photon absorption, which permits limitations of Doppler broadening to be overcome (see Chapter 1), has been used in a study on sodium atoms in which the Stark effect in the 5s 2Si and 4d D and D levels was observed.188 The radiative lifetimes of the S and D Rydberg levels of Na,189 the use of laser-induced resonance fluorescence for the measurement of small concentrations of Na vapour,170 the quenching of Na(32P) and K(42P) by N2, 02, H2, and HaO,171 the chemi-ionization reactions of photoexcited atoms,172 and excitation of the K(42P ) level in collisions with rare-gas atoms173 have been the subjects of recent reports. 

The reaction rate of a moderated uranium pile depends on temperature. For example, in a reactor in which the neutron spectrum is approximately Maxwellian, the average velocity will increase with temperature, thus decreasing the absorption cross sections for the low-energy neutrons which vary as 1/v. Resonance levels will be broadened by the Doppler effect, and if lumping of the uranium has been made use of to decrease the total resonance absorption as proposed by Szilard, and later found experimentally to be effective, increased temperature will decrease the advantage thus gained. 

G. M. Roe, The absorption of neutrons in Doppler broadened resonances. Knolls Atomic Power Laboratory Report KAPL-1241 (1954). 

For the 3/2 — 1/2 transition of Fe one has D(Ty) = 1-0 x 10 eV at 300 K. Since D Ey) is commensurable with Tr, one can see that Doppler broadening can cause some overlapping between the shifted emission and absorption lines (O Fig. 25.3), and thus a small yield of nuclear resonance absorption is to be expected (and, indeed, achieved see Malmfors 1953). However, if the overlap occurs due to such thermal motion, the essence of using resonant nuclear absorption to develop a spectroscopic method with an extremely high-energy resolution is lost. 

Autonomous reactor operation requires intrinsic self-regulation of reactor power. This behaviour can be obtained by negative reactivity feedback such as that provided promptly by Doppler broadening of neutron absorption resonances in resulting from increased fuel temperature, and quickly by efrects associated with coolant temperature increases and density decreases (i.e., negative void coefficient), and fuel thermal expansion. However, adequate safety consideration would also have to be given to possible reactivity insertion transients initiated by overcooling events. 

G. M. Rise, ( The Absorption of Neutrons in Doppler Broadened Resonances, KAPL-1241 (1954). 

The inherent resolution of collinear-beam spectroscopy is still limited by the residual Doppler broadening. In beams with a broad velocity distribution the labeling of one velocity class by optical pumping, probed in a second Doppler-tuning zone, was exploited already before narrow Doppler widths were achieved. The complete elimination of the first-order Doppler effect in resonant two-photon absorption on Ne I has been discussed in Section 3.3, in connection with a precision measurement of the relativistic Doppler effect. A similar experiment was performed on In I, where the 29p Rydberg state was excited from 5p Pi/2 via 6s Si/2 and detected by field ionization. The linewidth caused by the laser jitter can be reduced to the transit-time limit of a few hundred kilohertz. 

The importance of Doppler broadening of resonance lines in the calculation of resonance neutron absorption was first pointed out by J. A. Wheeler. 

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