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National Advisory Committee for Aeronautics, Technical Notes - Application of X-Ray Absorption to Measurement of Small Air Density Gradients

An analysis of two X—ray absorption methods for determining
small air—density gradients is presented. One method utilizes a
Geiger—Mueller counter for detection and the other uses photographic
film for detection. The methods are a refinement of a previous
similar application. With the resulting sensitivity in density
measurement, the first method was successfully applied to the evalu-
ation of a 6-percent density gradient of a flat-plate boundary layer
of about 0.024—inch thickness in a two-dimensional-flow channel.

In the other method, the divergence of the X-ray beam passing the
absorber poses a problem. A solution to this problem is postulated.
In addition, example calculations of the expected sensitivity in
boundary-layer density measurements are given for X—ray parameters
selected for optimum operation.

The application of soft X-ray absorption as a technique for the
determination of air densities has a potential advantage over the
optical radiation methods, especially in determining air densities
over a long path length, in that X-ray refraction errors are reduced
by at least a factor of 105. In addition, all radiation methods have
the advantage that no probes have to be inserted into the stream when
evaluating density gradients in a wind tunnel. Radiation methods,
however, can only be employed when two-dimensional flow can be pre-
supposed. In references 1 and 2, the possibilities of the X—ray
absorption method are recognized and are applied to the investigation
of densities in wind tunnels -(reference 1) and shock waves in super-
sonic flow (reference 2). All investigators usingthis method con-
cluded that the full potential of the X-ray absorption method can only
be realized if the probing X-ray beam can be made substantially smaller
than that used in references 1 and 2 by providing an X-ray source of P
higher intensity and an X-ray detector of higher sensitivity.

A stud;r of two different X—ray detecting methods was made at the
NACA Lewis laboratory and the results and the conclusions obtained are
presented herein. The sensitivity of the air—density measurement can
be increased fourfold, even though the probing X—ray beam would be
decreased in size by a factor of 30 with reference tc the instrument
previously described (references 1 and 2) by employing Geiger-Mueller
counters instead of an ionization chamber to provide a detector with
increased sensitivity and by utilizing a tungsten-target X—ray tube
instead of one with a chromium target to increase the X—ray intensity.