Large Ionization Chamber Research Articles

The Advantages and Disadvantages of Large Chamber Measuring Apparatus

ANY ionization chamber larger than 2 or 3 cubic centimeters in volume is too large for use in a phantom or for the determination of radiation intensity upon the patient's skin. For the purposes of this paper, any chamber of volume greater than 3 cubic centimeters is considered a “large” chamber. However, the actual volumes of ionized air in the “large” chambers with which we have had experience have been from 20 to 300 cubic centimeters. Large ionization chambers may be (1) of metal construction, suitable for permanent mounting in the filter position; (2) of the all-air type, suitable for direct determination of the r (so-called “primary standard”), or (3) of organic material, or a special material and construction such as will insure the ionization current being constantly related to the “primary standard” readings. As advantages of the “large” chamber may be listed the following: 1. The ionization current can be large enough to allow of use of a relatively rugged “micro-ammeter” for measuring the current. 2. Even when not designed for use with a micro-ammeter, current may be measured with a galvanometer instead of an electroscope. The special advantage of this is the ease with which one can avoid leakages and field distortions, because the galvanometer plate and lead are within a fraction of a volt of the guard ring potential. 3. With suitable construction and precautions, a “large” chamber may be built so that it will measure the x-ray beam directly in r per minute (a so-called “primary standard”). Lack of portability is one of the disadvantages of the “large” chamber and this is not merely because of the large size of the chamber itself but because the charged plate or plates must be supplied with fairly large currents at high voltages, necessitating either large and heavy dry cells or “B-battery eliminator.” The principal disadvantage of the large chamber is the impossibility of using it on the patient's skin or incorporating it in a phantom. Therefore, it can be used only for determination of intensity in the free beam (without scatter-back). In spite of the disadvantages just enumerated, the large ionization chamber may be used success fully and with a higher degree of accuracy than can be obtained with the small ionization chamber alone, provided all readings are taken in the beam without scatter-back and are then carefully modified by proven factors based upon the quality of radiation, skin-target distance, and size of field. Such factors have been supplied to us by the work of Dessauer, Holthusen, Erskine, Failla and Quimby, and others. It is an unfortunate fact that no two of these are in perfect agreement. This lack of agreement is probably incurable because of certain defects inherent in the small ionization chambers which have been used with phantoms and on the patient's skin. We recently became interested in obtaining similar correction factors suitable for use in connection with radiation to the neck.

Further Studies on the Influence of Radiation Quality on the Erythema Dose Measured in Physical Units

IN a paper entitled “Erythema Doses in Absolute Units” (1), presented at the last Annual Meeting of this Society in Cleveland, we called attention to the fact that the roentgen-ray doses, measured in ionization units, which produce similar reactions on the human skin, are to a marked degree dependent upon the radiation quality employed. At that time we also described our method of skin dose estimation as well as the method of physical dose determination; these shall, therefore, not be dealt with again in this paper, since they have remained the same as formerly described. It should be recalled that our erythema reaction is always accompanied by epilation, which fact was not especially stressed in our former communication on the subject though the erythema dose as here calculated is practically 20 per cent over the epilation dose used in tinea capitis. Most of our doses have repeatedly been transferred to other therapy installations (40 within the last year, in addition to the 70 reported at the Cleveland meeting). The reactions produced with the doses on these 110 machines are, as far as we know, very similar to our own. Within the last year we have extended our observations, correlating the ionization current, measured in electrostatic R-units, with the reaction on the human skin to still softer rays than previously reported, thus finding some interesting phenomena. We also have studied the influence of backscattering from the patient (or water phantom) upon the erythema doses, measured in electrostatic R-units. This was done in an attempt to make a more exact comparison of the dosage, as measured with the different ionization units employed in several laboratories in this country as well as abroad. Radiation Quality For our experiments we used nine different radiation qualities, starting with an X-ray beam giving a half value layer with somewhat under ¾ mm. of aluminum, produced with approximately 45 peak kilovolts, unfiltered, and going as high as 215 kilovolts, strongly filtered, resulting in an X-ray beam showing a half value layer of over 14 mm. of aluminum. A detailed description of the conditions under which these nine qualities were produced is presented in Table I. The half value layers in aluminum and copper (given in Columns IV and V, Table I) of the nine radiation qualities were determined by absorption measurements, using a fine X-ray pencil and a large air ionization chamber. The figures obtained correspond closely to those found by a photographic method, recently described by William H. Meyer (2). Correlation of Skin Effects and Physical Dose for Various Radiation Qualities The findings reported last year (1), that the number of electrostatic R-units per erythema decreases with decreasing penetration of the rays (using qualities C to I in Table I) were again confirmed. The same observation has been reported by other investigators (Dessauer 3, Gruhn 4, Dieterich 5, Wintz and Rump 6, Glocker 7, Sievert 8).