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 collection efficiency of an ionization chamber in a pulsed and magnetically swept electron beam: limits of validity of the two-voltage technique.

The two-voltage method of determining the collection efficiency of an ionization chamber exposed to a pulsed and magnetically swept electron beam depends upon the form of the saturation curve agreeing closely with the formula based on ionic conduction and negligible space charge. The possible disturbing effects of charge transport by free electrons and of overlapping pulses are discussed and the conditions to be satisfied for the reliable use of the method are defined and demonstrated experimentally.

Dosimetry in a magnetically swept electron beam.

In a magnetically swept electron beam the recombination correction applicable to conventional ionization chambers can become very large. The paper presents the general saturation curve which obtains under these conditions and describes the two-voltage technique for evaluating the true value of the dose. A chart is provided for solving the relevant equation.

The recombination correction for an ionisation chamber exposed to pulsed radiation in a 'swept beam' technique. I. Theory.

The established theory for the collection efficiency of an ionisation chamber irradiated by a series of uniform short pulses is extended to the more complex case of irradiation in a pulsed and electromagnetically swept electron beam. The radial variation of exposure rate is assumed to be Gaussian with scale constant equal to alpha, and the pulses are assumed to be uniformly distributed over the area swept out. A formula for the collection efficiency is derived and the influence of varying some of the parameters is studied. The collection efficiency, phi 1, at voltage V1 can be calculated if the two dosemeter readings, Q1 and Q2, at two widely different chamber voltages, V1 and V2, are known. phi 1 depends primarily upon the ratios Q1/Q2 and V1/V2 and to a much lesser extent upon the geometry of the chamber or the ratio of Gaussian scale constant to a typical linear dimension of the field scanned. Charts for calculating phi 1 are provided.

Current collection and ionic recombination in small cylindrical ionization chambers exposed to pulsed radiation.

The recombination correction for a small cylindrical ionization chamber exposed to pulsed radiation is reexamined in the light of new experimental evidence. A simple two-voltage testing technique is described and demonstrated. In general the existing theory for an air-filled chamber (Boag 1950) is found to be accurate over the range of charge densities examined (0.013 p/pulse to 3.8 p/pulse) where p=3.33 X 10(-4)C. m-3 (1 esu cm-3).

Cell survival at low oxygen tension and dose build-up in argon.

Mammalian cells were exposed to 250 kVp X-irradiation in air, argon and nitrogen to determine whether cells irradiated when severely hypoxic have survival curves with lower extrapolation numbers (n) than their aerobic counterparts. Cells irradiated suspended in liquid showed no significant differences between values of 'n' irrespective of the gas used, neither was the sensitivity of cells irradiated in argon any greater than that of cells irradiated in nitrogen. In contrast, cells attached to glass dishes irradiated with the medium withdrawn were apparently much more sensitive in argon than in nitrogen. It has been demonstrated that the lower survival of cells irradiated in argon could have been caused by the greater photoelectric absorption in argon compared with nitrogen. When the dosimetric discrepancy was removed either by absorption of photoelectrons in liquid or by use of high energy radiations, there was no evidence that severe hypoxia during irradiation could lead to reduced values of 'n'.

Prototype ionographic imaging chamber. Construction and clinical use.

The design and construction of an experimental ionography chamber are described and the principles of this new electrostatic imaging technique are discussed. Examples of medical ionographic images taken with this simple prototype chamber are presented.

Looking both ways. Presidential address delivered at British Institute of Radiology Annual Congress-9th April 1976.

The address looks backward over the progress made in the techniques of radiology--both diagnosis and therapy--over the past 30 years and forward to the possibilities which lie ahead. It draws attention to the continuing need for basic research as the source from which practical advances spring, illustrating this from the development of radiosensitizing drugs. Finally, it emphasized the importance of the Institute as an interdisciplinary forum in the present era of rapid technological advance.

Saturation curve in gases of high atomic number at pressures up to 8 atm. Part II: Freon 13-B1, and mixtures of Freon with xenon and krypton.

The saturation curve has been studied in Freon 13-B1 (CF3Br) and in mixtures of Freon with xenon and krypton up to a pressure of 8 atm. The enhanced initial recombination due to the electron affinity of Freon has been evaluated and an empirical formula constructed that describes the fraction of current which escapes initial recombination over a wide range of voltages, pressures, and electrode spacings. After correction for this initial recombination, the general saturation curve for Freon and for mixtures of this gas with krypton and xenon has been derived and, again, convenient empirical formulae established which allow the current collection efficiency to be calculated for any given parameters within the range investigated. These formulae are of practical value in the design of image-forming ionization chambers.

Radiation exposure to the patient in xeroradiography.

The paper presents measurements of the radiation exposure required in xeroradiography of the breast and of the extremities. The nature of the radiation hazard and the most appropriate quantity by which to assess it are discussed, and it is shown that for a number of radiological procedures xeroradiographs can be taken for about the same radiation exposure to the patient as that required by conventional film techniques.

A test of several parametic statistical models for estimating success rate in the treatment of carcinoma cervix uteri.

The parametric statistical models discussed include all those which have previously been described in the literature (Boag, 1948-lognormal; Berkson and Gage, 1952-negative exponential; Haybittle, 1959-extrapolated actuarial) and the basic data used to test the models comprised some 3000 case histories of patients treated between 1945 and 1962. The histories were followed up during the period treated between 1945 and 1962. The histories were followed up during the period 1969-71 and thus provided adequate information to validate long-term survival fractions predicted using short-term follow-up data. The results with the log-normal model showed that for series of staged carcinoma cervix patients treated during a 5-year period, satisfactory estimates of long-term survival fractions could be predicted after a minimum waiting period of 3 years for stages I and II, and 2 years for stage III. The model should be used with a value assumed for the lognormal paramater S in the range S = 0.35 to S = 0.40. Although alternative models often gave adequate predictions, the lognormal proved to be the most consistent model. This model may therefore now be used with more confidence for prospective studies on carcinoma cervix series and can provide good estimates of long-term survival fractions several years earlier than would otherwise be possible.

Saturation curve in gases of high atomic number at pressures up to 8 atm. I. Krypton and xenon.

The saturation curve has been studied in xenon and in krypton up to a pressure of 8 atm. An empirical formula has been found that describes the fraction of current collected over a wide range of voltages, pressures, ionization intensities, and electrode spacings. This is of practical value in the design of ionography chambers. For krypton the collection fraction fKr = (1 + 0.25eta-1.74)-1, and for xenon fXe = (1 + 0.16eta-1.88)-1, where eta = Fp-0.7Vd-2q-1/2 with F = 3.61 X 10(-7) and 2.50 X 10(-7) for krypton and xenon, respectively. The ranges of the variables covered in the experiments were p = 1-8 atm, V = 5-25000 V, d = 0.3-1.3 cm, and q = 4 X 10(-9)-6 X 10(-8) A/cm3.