Time resolving ionization monitor for microsecond radiation impulses.

The current induced in the outer circuit of a fast response ionization chamber exposed to pulsed radiation consists of two components, a fast one induced by free electrons and a slow one induced by ions. The fast electron component may be used for the representation of the shape of the ionizing pulse. In order to avoid interference with the slow ion current, the latter has to be removed from the signal. This is achieved by deriving a voltage course from the chamber signal which fits the shape of the ion component and subtracting this from the entire signal. The function of the electronic circuit used for this purpose is described. Some considerations about the time resolution of the chamber gas are to be found in the appendix.

The effect of free-electron collection on the recombination correction to ionization measurements of pulsed radiation.

Three models of the charge collection process in small dosimetric ionization chambers exposed to pulsed radiation are discussed. All three models allow for the presence of a free-electron component in the charge transfer, incorporating this into the model in slightly different ways, and the resulting collection efficiency formulae are compared over the range of variables normally met within clinical dosimetry. Measurements of the free-electron fraction for plane-parallel ionization chambers and for a Baldwin-Farmer 0.6 cm3 chamber are presented. The proportion of free electrons at the normal operating voltage is often high in small chambers but it is obvious that this can only lead to an increase in collection efficiency if the f-value calculated for purely ionic conduction allows for some improvement. Thus, a 50% free-electron fraction in a chamber which collects ions with efficiency f = 0.9950 at low pulse doses will increase this efficiency to only 0.9982. The same chamber, at the same operating voltage, and therefore the same free-electron fraction, if exposed to larger pulse doses, yielding an efficiency of 0.9531 as calculated for ions alone, would have a true efficiency of 0.9830-a large change.

[The reduction of the dose to the lung in whole-body irradiation with cobalt-60 gamma rays (author's transl)]. / Herabsetzung der Lungendosis bei Co-60-Ganzkörperbestrahlung.

In order to reduce the exposure of the lung and oesophagus during whole-body irradiation with cobalt-60 gamma rays the dose rate delivered to the lungs and the neck is diminished by means of a filter. Because of its special shape, sternum and columna vertebralis obtain the unreduced dose. The filter is constructed for irradiation in antero-posterior direction. A fixed position of the patient is not necessary.