Natural radiation width

Hsieh [64] are forced to the conclusion that the theory is inadequate to explain the observations5, They consider possible reasons for the discrepancy, and point out the likelihood of its originating in the omission from the theory of terms which represent the interaction between the atom and radiation (section 6.10). The magnitude of this effect would be of order a times the fine structure separation, or again, of the order of the natural (radiation) width of the lines, which is roughly the magnitude of the discrepancy. 

We find that the so-called natural radiation width (in frequency units) is given by... 

As a final point in this section, we discuss the cj[uestion of increasing the spectral resolution beyond the natural radiation width Auj = 1 /27rr. Difi er-ent experiments have been performed, in which the observation after pulsed... 

Molecules such as 3,4 and 5 in Figure 2.6, which have a zero velocity component away from the source, behave uniquely in that they absorb radiation of the same frequency Vj-es whether the radiation is travelling towards or away from R, and this may result in saturation (see Section 2.3.4). If saturation occurs for the set of molecules 3, 4 and 5 while the radiation is travelling towards R, no further absorption takes place as it travels back from R. The result is that a dip in the absorbance curve is observed at Vj-es, as indicated in Figure 2.5. This is known as a Lamb dip, an effect which was predicted by Lamb in 1964. The width of the dip is the natural line width, and observation of the dip results in much greater accuracy of measurement of v es. 

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. 

Figure 13.2—Simplified schematic of an atom showing the origin, and the Siegbahn nomenclature, of some fluorescence radiation processes caused by impact of a photon having a high energy. The position of the spectral line is not significantly influenced by the chemical combination in which the atom is found. For example, the Kat line from sulphur is observed at 0.5348 nm for S + and at 0.5350 nm for S°, yielding a shift of 1 eV, which is comparable to the natural line width for X-rays.

These and less important effects which cause line broadening may increase the absorption line to values up to (10 3 nm). Although this is a considerable increase over the natural line width it is still very narrow in practice and very difficult to observe with conventional equipment. Consequently it was not practical to use a continuous radiation source such as a hydrogen lamp. Not only would it be very difficult to isolate the absorption line but the total amount of energy radiated by the light source over such a narrow absorption band would be very small and difficult to measure using conventional detectors. 

As was stated in Section II.A, the energy resolution of the radioactive sources used in conventional Mossbauer spectroscopy is typically 10 eV. This resolution is determined by the natural line width and the maximum energy range obtained by Doppler-shifting techniques. In the case of synchrotron radiation, the energy resolution, which is related to the time period following the excitation of the isotope, is superior to that in conventional Mossbauer spectroscopy. This period can be as short as 2.8 ps, which leads to an energy resolution of about 10 ° eV. However, the... 

The natural line width was first derived from a classical radiation damping model and was later derived on a quantum mechanical basis by Weisskopf and Wigner and Hoyt. It is due to uncertainty in the energy of the excited level having a finite lifetime. The natural line width is much smaller than will be of concern in this paper, of the order of 0.01 cm. ... 

Radiation damping No collisions or or quantum mechanical interactions uncertainty (natural line width)... 

The classical model for natural line widths and resonance radiation is based on several approximations and assumptions which are not valid for condensed systems. 

I do not believe that we can educate a typical member of the public to understand the objective meaning of "a radiation dose rate of 300 /iSv/year" or the "annual deviation width of natural radiation", not to speak of "a risk of 10 /year". 

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