Military roentgenology / War Department.

Date:
[1944]
Catalogue details

Licence: Public Domain Mark

Credit: Military roentgenology / War Department. Source: Wellcome Collection.

    51/470 page 41
    K Series L. Series M series IONIZATION or INTENSITY <+— DECREASING INCREASING ———> Figure 40. Roentgen ray emission spectrum. 55. ABSORPTION SPECTRA. a. True absorption. When roentgen rays are directed upon a portion of the body, different events occur. Some rays are absorbed, some are scattered, and some pass entirely through without bemg changed. The latter are called transmitted “primary rays.” (See fig. 1.) In the event of true absorption, entire quanta of roentgen rays are transformed, on collision with atoms, to different types of energy; that is, heat (very small) and photoelectrons. (See fig. 41.) The photoelectrons, in turn, dissipate their energy by jonization. True absorption is independent of the physical state of the absorber. Thus, one gram of ice, one gram of water, and one gram of steam will absorb equal portions of roentgen rays. Elements of higher atomic weights in the roentgen ray path have greater true absorption. b. Scattering absorption. Scattering of roentgen rays occurs when a quantum of ray energy (fig. 41) hits an electron, divides its energy with the electron and then continues in a different direction as a new ray of a longer wavelength. On the other hand, the quantum may merely change direction upon collision without a loss of energy. The former type of scatter- ing is called “‘modified (Compton effect) scattering”’; the latter is called “unmodified.” The electrons associated with modified scattering are called “recoil electrons.”” They, too, dissipate their energy by ionization. Scattering increases with volume of tissue in the roentgen ray beam. Both photoelectrons and recoil electrons give rise to new radiations called “‘secondary”’ rays. 56. MEASURE OF ROENTGEN RAYS. a. Quanti- tative. (1) General. A heterogeneous beam _ of roentgen rays is characterized by its intensity and penetrating power. Intensity is a quantitative characteristic; penetrating power of wavelength is a qualitative characteristic. The quantitative meas- ure of roentgen rays is, thus, a measure of intensity. (See table VIII, app. V.) Intensity of radiation Is the quantity of radiant energy passing perpen- dicularly through each square centimeter of surface. The physical factors which determine intensity are: tube kilovoltage, tube milliamperage, filter, and distance. There is no true, simple mathematical relationship between intensity and these four factors. Certam factors are related, however, by the follow- ing ratios: TI, = May, I, = D, I, = kv,?, I, = Mae (= st0> the kvo? for kv greater than 70 kilovolts, where I; and /, are intensities of radiations for milliamperages, May and Map, for distances, D,; and Ds, and for kilo- voltages, kv; and kv» respectively. to i] h¥ h¥ ONS. hd TRANSMITTED UNCHANGED hd TRUE ABSORPTION _ PHOTO ELECTRONS —( P ) SECONDARY RAYS —(S) SECONDARY RAYS —(S)
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