Table of contents

Cavity ionization theory is applied to the dosimetry of bones incorporating beta - and gamma -emitting radioactive isotopes. The assumptions inherent in different theoretical approaches to bone dosimetry are reviewed and compared with those adopted in cavity theory. In presenting the theory attention is given to the effects of electron and photon attenuation and to the absence of electronic equilibrium which may occur in bone. The mean marrow dose is calculated for a range of cavity size and gamma -ray energies and for a beta -ray emitter. The mean marrow dose and the mean endosteal dose are calculated for a human lumbar vertebra and some comparisons made with the results of other workers.

Mean dose factors have been calculated for those tissues in bone that are relevant to the induction of late irradiation effects; namely, red bone marrow and endosteal tissues lining trabecular bone surfaces. The calculations are based on a Monte Carlo computer method for beta-emitting radionuclides which are distributed uniformly throughout the volume of mineralized bone. Results are given for the radionuclides ¹⁴C, ⁴⁵Ca, ²²Na, ¹⁸F, ³²P, ⁹⁰Y and ⁹⁰Sr+⁹⁰Y (all considered for this purpose as volume-seekers) for seven bones from the adult human skeleton and calculations are also presented of average skeletal dose factors. Where possible, results are compared with those derived from other methods based on simple geometrical models.

A TLD system based on lithium borate which has been used both in powder form and made up into solid discs by incorporation within PTFE is described. The predominant advantages of this material over other more widely used TLD materials are its closer approximation to soft tissue, its simple glow curve which eliminates the need for complicated annealing procedures and its low cost. The luminescence from the lithium borate is measured with a specially designed reader which uses radiofrequency heating. At present, using lithium borate powder, the system is capable of measuring doses down to 10 mrad and using lithium borate PTFE discs it can measure down to 50 mrad with adequate precision. Preliminary measurements on solid glassy discs of lithium borate suggest that this form of the material may in some respects be superior for patient dosimetry.

The increasing use of fast neutron sources makes dosimetry intercomparisons (intercalibrations) by mail desirable. After comparing the relative advantages and disadvantages of the various available integrating fast neutron detectors, fission fragment track etching was chosen because such detectors can be made sufficiently small, rugged, fading resistant, inexpensive and accurate. Using several combinations of ²³²Th or ²³⁷Np as fissile materials, and organic and inorganic track detectors, it was established that both automatic spark counting and visual track counting techniques can be developed to cover the desirable dose range ( approximately 50-500 rad) with sufficient accuracy ( sigma <or=5%).

A systematic investigation of the effects of cooling rates in the range 10^-1 to 2*10⁵ degrees C min^-1 applied to TLD-700, LiF thermoluminescence dosemeters has shown that the 'transfer sensitivity' effect observed by Booth, Johnson and Attix (1972) is only of importance for cooling rates >10³ degrees C min^-1. Although it is concluded that for practical dosimetry purposes the effect may be ignored it is not clear why such large changes were observed and until this discrepancy is explained it is recommended that a low temperature pre-irradiation anneal should be used.

A simple method for measuring ultraviolet exposure using the thermo-luminescent properties of CaSO₄ : Dy is presented in this paper. In general, three peaks at 145, 215 and 365 °C were observed by ultraviolet irradiation of virgin phosphor. The peak at 365 °C was found to be very small and that at 145 °C fades away within a week; no appreciable fading was observed for the 215 °C peak. Further, the peak at 215 °C was found to coincide with the gamma dosimetric peak and this peak was chosen for ultraviolet exposure measurements. A standard quartz pen-ray lamp was used for calibrating the phosphor both by peak height measurement and by integration. The dosimetric peak has been found to be linear from the detection threshold of 400 erg mm^-2 to 4 x 10⁴ erg mm^-2. Beyond this exposure the phosphor behaved supralinearly to ultraviolet exposure up to the studied range of 5 x 10⁵ erg mm^-2. The phosphor exhibited strong dependence on photon energy. The TL sensitivity increases sharply by a factor of 300 as the wavelength is decreased from 250 to 200 nm. The response is negligibly small above 350nm. Other important characteristics such as particle size dependence, fading characteristics, thickness dependence and re-usability are also presented. The ultraviolet sensitivity increased by a factor of 3 when the grain size range was changed from 105-210 μm to 0-37 μm. Self-shielding by a factor of 2 was observed when the sample thickness was increased from 10 to 72 mg cm^-2.

It is generally believed that after an intravenous injection of Hippuran its concentration in the plasma can be described as the sum of two exponentials. However, by collecting samples during the first minute after injection of the tracer, a third exponential term was found with a half-life of less than one minute. To explain its presence it was assumed that the plasma shares its Hippuran with two peripheral compartments, the rate of transfer from one compartment to another being proportional to the amount in the first of them. The proportionality factors have been determined for 20 adults and averaged. Substituting these averages in the differential equations for the distribution process, the only variable remaining is the renal excretion rate (the fraction of plasma cleared per minute by the kidneys).

Calculations indicate that an assembly consisting of an antimony-beryllium source at the centre of a 4 cm radius water sphere surrounded by a 1 mm thick shell of boron-10 will emit neutrons with a broad spectrum at intermediate energies. A device based on this design was constructed using a water-filled, boron-carbide loaded, plastic shell with an antimony-beryllium source located at the centre. The output spectrum was calculated by Monte Carlo program and the computed total yield agreed well with measurements made with a manganese bath system. The main peak has an effective energy of 0.5 keV and the total yield is 18% of the antimony-beryllium source strength. Experience with this source suggests some possible avenues for future development.

Californium-252 promises to be an effective radium substitute in brachy-therapy. In certain situations, such as cancer of the uterus, the dose rate near a linear source may be of interest. Paterson and Parker (see Meredith, 'Radium Dosage-the Manchester System', p.23, 1967), have tabulated the number of milligram hours for various active lengths of radium sources to give 1000 rad at different distances from the centre of the source. Similar results for Cf-252 (microgram hours to give 1000 rad neutron dose) and the results of experimental measurements of neutron depth dose distributions for both single line sources and arrays are reported. Kodak NTA films and a phantom were used. The experimental results are found to agree well with the theoretically predicted results.

The reasons for the reduction in response of a multidetector radiation counting system when operated at high count-rates were examined and a convenient method of correction which is accurate to within +or-1% up to 0.6*10⁵ counts per second was established. In its application the method uses the observed count-rates from each detector at the time of measurement and previously determined values of critical time intervals of the electronic system. The detected signals establish these intervals in which further signals occurring within them are lost from the response. These further signals can also cause the loss of the signals establishing the intervals. The correction is of particular value when high-sensitivity whole-body counters are used to measure the activity of administered isotopes in metabolic studies.

The literature on the phase relationships between frequency components of a Fourier analysis is reviewed, with examples and theories from acoustics and neurophysiology. Given n sinusoids of different frequencies and phase angles, it is shown that for n>or=2 the set of initial phase angles allowing the n sinusoids to be in phase at some time t₀ consists of one or more planes of constant dimension 2 and that for n=2 such a time t₀ always exists. The conditions under which the common phase of n sinusoids at one time t₀ will be the same as the common phase at another time t₀ are also investigated. The importance of incommensurately related frequency components is emphasized by proofs which do not depend on harmonic relationships. Proofs are formulated in a linear algebra format to demonstrate the versatility of the method for analysing long sequences of frequencies and phases.

The infrasonic part of the spectrum of the carotid artery wall vibration in the neck was obtained. Differences between the spectral content and vibrational amplitude in normal and occluded carotids were found. The application of this technique in clinical practice could be useful in the detection of the carotid insufficiency syndrome.

Calculational results are presented which indicate that, for an equivalent boron concentration and neutron flux, the effects of geometrical differences between a blood-filled capillary and an array of tumour cells lead to a much smaller dose delivered to the endothelial cells than to either the blood or the tumour cells.

In a limited investigation using a 24 cm thick phantom irradiated with neutrons produced by a cyclotron and by an isotopic source, it was found that the uniformity of the thermal flux distribution can be considerably improved by the introduction of a thin thermal neutron absorber into the premoderating material.

A simple time sharing technique which will permit the investigation of dynamic studies at two energies is described. This conversion has been carried out on an updated Nuclear Enterprises Mark III camera. A second isotope energy switch, having three wafers with associated resistors, has been added to the scintisignal generator. Miniature reed relays direct the outputs of the X, Y and Z channel amplifiers first to one of the switch networks then to the other at a preselected changeover rate. The relays are operated from a timing circuit constructed from TTL dividers. A computer system is used for data collection.

Calculations of tumour dose in rotation therapy are generally based on the determination of an average tissue-air ratio (TAR) or tissue-maximum ratio (TMR) A nomogram, obtained by computer regression analysis of data from 56 cases, which relates TAR to three parameters, is given. The parameters are: the AP and lateral dimensions of the patient and the field size of the radiation beam.