Atomic Absorption Coefficient Research Articles

Absorption Coefficients for Homogeneous X-Rays

Absorption Coefficients of Various Organic Liquids for X-rays of Wave-length 0.715 \AA{}.---By means of a spectrometer with narrow lead slits and a calcite crystal, the ${\mathrm{K}}_{\ensuremath{\alpha}}$ radiation from the Mo target of a Coolidge tube was reflected into an ionization chamber, and the effect of interposing a cell containing a known thickness of liquid before the first slit, was measured. Thus absorption coefficients were determined for carefully purified samples of pinene (${\mathrm{C}}_{10}$${\mathrm{H}}_{16}$), limonene (${\mathrm{C}}_{10}$${\mathrm{H}}_{16}$), benzene (${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$), toluol (${\mathrm{C}}_{7}$${\mathrm{H}}_{8}$), isopropyl alcohol (${\mathrm{C}}_{3}$${\mathrm{H}}_{8}$O), methyl proprionate (${\mathrm{C}}_{4}$${\mathrm{H}}_{8}$${\mathrm{O}}_{2}$), ethyl acetate (${\mathrm{C}}_{4}$${\mathrm{H}}_{8}$${\mathrm{O}}_{2}$), acetone (${\mathrm{C}}_{3}$${\mathrm{H}}_{6}$O), ethyl formate (${\mathrm{C}}_{3}$${\mathrm{H}}_{6}$${\mathrm{O}}_{2}$) and trimethylene glycol (${\mathrm{C}}_{3}$${\mathrm{H}}_{3}$${\mathrm{O}}_{2}$).Atomic absorption coefficients for C, H, and O for X-rays of wave-length 0.715 \AA{}. were computed from the above data, assuming absorption to be an additive atomic property, and came out from 11.00 to 11.55 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$ for C, 0.45 to 0.50 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$ for H and 29.9 to 31.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$ for 0. Using the mean values, the computed coefficients are within $\frac{1}{3}$ per cent. of the observations, on the average. But the fact that the computed values for acetone and water are 1 per cent. too low suggests that the assumption that absorption is an atomic property may not be accurately true.

The Laws of Absorption of X-Rays.

Measurement of Absorption of X-rays.---The use of a filtered beam of rays can at best give only approximate values of the coefficients, for such a beam is not homogeneous. It is pointed out that logarithmic absorption is no proof of homogeneity. In these experiments homogeneous radiation was secured by using a Bragg spectrometer and the effects of secondary radiation were avoided by placing the absorbing material between the two slits which limited the beam. By taking these and other precautions a precision of 1-2 per cent. was obtained. For wave-lengths below 0.15 \AA{}. it was found better to place the thin analyzing crystal of NaCl nearly at right angles to the beam and to use the radiation reflected from atom planes perpendicular to the ones usually used.X-ray Absorption by Ag, Al, Cu, Mo, Pb and ${H}_{2}$O for Wave-lengths about 0.13 to 0.7 \AA{}.---The linear relation, established by Hull and Rice, between the mass-absorption coefficient and ${\ensuremath{\lambda}}^{3},{\ensuremath{\mu}}_{\ensuremath{\rho}}=F{\ensuremath{\lambda}}^{3}+\frac{\ensuremath{\sigma}}{\ensuremath{\rho}}$, holds accurately for these substances until, as $\ensuremath{\lambda}$ is increased, the characteristic $K$ limit is approached, when the values of $\frac{\ensuremath{\mu}}{\ensuremath{\rho}}$ are increasingly too small. The linear relation also holds above the $K$ limit, $F$ having a much smaller value but $\frac{\ensuremath{\sigma}}{\ensuremath{\rho}}$, the mass-scattering coefficient being about the same. The atomic absorption coefficient for wave-lengths below $K$ is a linear function of ${N}^{4}$, where $N$ is the atomic number, in agreement with Owen's results. The general law of X-ray absorption is therefore as follows: ${\ensuremath{\mu}}_{\ensuremath{\alpha}}=k{N}^{4}{\ensuremath{\lambda}}^{3}+{\ensuremath{\sigma}}_{a}$ where $k$ is a universal constant equal to 2.29 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}27}$ below the $k$ limit and to 0.33 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}27}$ above that limit. The experimental curves give no evidence for the existence of the $"J"$ radiation reported by certain observers for Ag, Al, Cu and ${\mathrm{H}}_{2}$O. The absorption for water was measured down to 0.093 A but shows no indication of an approach to the absorption found for the hard $\ensuremath{\gamma}$-rays of RaC.

The absorption of X-rays

Introduction . —Previous to the discovery of the behaviour of X-rays with regard to crystals, the most homogeneous radiation obtainable was that of the characteristic radiation of an element which is excited when that element is exposed to X-radiation of suitable hardness. These characteristic radiations are now found, however, by the new method of analysis, to be constituted of a number of radiations of different wave-lengths. Moseley, shortly after the discovery of the reflection of X-rays, showed that the characteristic radiations of most of the metals he examined consisted of two prominent wave-lengths; Bragg later found that, in the case of rhodium, palladium and silver, each of these lines could be further resolved into two components. Hence the spectra of the characteristic radiation of the K series of these elements consist of at least four different wave-lengths. The analysis of a beam of X-rays into its constituent radiations by reflection at a crystal face provides a means, therefore, of obtaining radiation of a definite wave length and of such intensity as to enable its absorption coefficient in different materials to be accurately measured. Bragg and Pierce have already measured the absorption coefficients of the two most prominent lines in the spectra of the elements Rh, Pd and Ag, in a number of metals. To make the absorption coefficient more directly comparable with other atomic characteristics, they gave their results in the form of atomic absorption coefficients: the atomic absorption coefficient expresses the proportion of the energy of an X-ray pencil which is absorbed in crossing a surface on which lies one atom to every square centimetre. The ordinary mass absorption coefficient can be calculated from this quantity by dividing it by the mass of the absorbing atom. The experimental results showed that the ratio of two absorption coefficients is independent of the wave-length of the radiation over considerable ranges, a result previously deduced by Barkla from his experiments; also, that the atomic absorption coefficient is proportional to the fourth power of the atomic number of the absorber.