Ray Hardness

RAY HARDNESS, a term used in radiology to characterize the degree of penetrability of X-rays. Ray hardness depends on the wavelength of the X-rays emitted by the tube: the hardness is greater the shorter their wavelength. The wavelength of X-rays, according to the Duane-Hunt law, is inversely proportional to the voltage applied to the tube's poles, and therefore the rays are harder the higher the voltage, and vice versa. Since the tube is usually powered not by constant but by high alternating voltage, it emits rays of different qualities - more and less hard, corresponding to the change in voltage of the alternating current curve. It has been agreed that in determining ray hardness, the voltage corresponding to the highest peak of the alternating current curve is used; the wavelength corresponding to this peak is called the limiting wavelength and characterizes the ray hardness. It can be determined from the Planck-Einstein equation: λ = 12,398/V, where λ is the wavelength in Angstrom units (Å), and V is the voltage in thousands of volts applied to the tube's poles. In practical work, to characterize ray hardness, the wavelength of the rays is usually not given, but rather the voltage at which these rays are obtained is indicated. In this case, maximum and effective voltage are distinguished; the first corresponds to the maximum peak of the alternating current curve at the poles of the transformer's secondary winding, the second to the entire alternating current curve at the tube's poles: it is always less than the maximum voltage. Christen proposed to indicate ray hardness by the thickness (in centimeters) of a layer of soft human tissues that absorbs half of the rays emitted by the tube, calling it the 'half-absorbing layer' (Halbwertschicht); the thicker this layer, the harder the rays, and vice versa. It is generally accepted to indicate the 'half-absorbing layer' by the thickness of a layer of distilled water; Holtmsen proposes to indicate it in millimeters of copper sheet. Christen's principle forms the basis of the absolute hardness meter proposed by this author. The biological action changes depending on the greater or lesser ray hardness, with which various indications for their use in therapy are also associated.

M. Manikov.