TY - JOUR
AU - Seemann, Herman E.
AB - A Proposed Diaphragm for Standard Ionization Chambers 1 Herman E. Seemann , Ph.D. From the Kodak Research Laboratories, Rochester, N. Y. Excerpt It is a well-known fact that the results of measurements made with a standard ionization chamber cannot be known with a greater degree of accuracy than the cross-sectional area of the limiting diaphragm. If a diaphragm were constructed with straight sides instead of the conventional round hole, the straight sides could probably be more easily and accurately fine-ground than the round hole. The following design for a diaphragm is proposed with the idea that probably somewhat greater accuracy may be obtained by its use and simple construction be retained. The diaphragm described is only apparently more complicated than a circular hole cut in a solid plate. Two lead-bismuth alloy 2 plates A and A ′ (Fig. 1) are fastened together at their ends with simple metal yokes. Their edges E and E ′ are accurately machined for straightness and are as nearly parallel as possible. If a second pair of plates is similarly arranged and laid over the first pair at right-angles to it, the device in Figure 1 (B) results. The two pairs of plates thus mounted together comprise an easily constructed diaphragm. The size of the opening is most accurately determined by measuring the distance between the plates of each pair at equal intervals along their entire length. Perhaps a slight taper (lack of parallelism) of the edges will be detected, but with reasonably good machine work on the edges the uniformity of the taper should be very good. The mean of these measurements is then the effective width of the diaphragm. It is to be noted that, when measurements are made near the ends of the plates, a more accurate determination of the effective width of the diaphragm is obtained than with measurements on the diaphragm only, assuming accurately straight edges. Great accuracy in crossing the plates at right-angles is not essential since, for example, the area of the parallelogram formed by the crossing of such plates at 89 degrees is only about 0.015 per cent greater than if crossed at 90 degrees. Owing to the necessity for thick plates to absorb the X-rays, a correction must be made for the fact that the two pairs of plates are at different distances from the source. No more data than are normally taken or known in standardization work are necessary to make this correction. Referring to the schematic diagram (Fig. 2), we see that slit “A” is defining the beam in the vertical direction at distance X 1 , and “B” is defining it in the horizontal direction at X 2 It is assumed that the distance X 1 is the most convenient to measure directly. The effective width of “B,” that is, the width it would need to have at X 1 in order to limit the beam to the same extent, is , where “b” is the measured width of “B.” Thus, the effective area of the diaphragm at distance X 1 is . If desired, this type of diaphragm could be made adjustable by equipping the pairs of plates with micrometer screws. The utility of the crossed slit diaphragm does not depend upon the shape of the edge chosen.
TI - A Proposed Diaphragm for Standard Ionization Chambers
JF - Radiology
DO - 10.1148/19.1.55
DA - 1932-07-01
UR - https://www.deepdyve.com/lp/radiological-society-of-north-america-inc/a-proposed-diaphragm-for-standard-ionization-chambers-GnQko0TCA6
SP - 55
VL - 19
IS - 1
DP - DeepDyve
ER -