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A C McEwan

Publications and source records attributed to A C McEwan.

At least 19 recordsLinked to original sources

Some reflections on the role of the Scientific Advisory Panel to the Marshall Islands nationwide radiological study.

As a consequence of the U.S. Atomic Weapons Testing Program in the Trust Territory of the Pacific, now the Republic of the Marshall Islands, numerous scientists have advised the Marshallese on matters of radiation and radioactive contamination. Some of the previous advice has appeared to vary or conflict resulting in consequent uncertainty for the people. In a new initiative in 1989, the RMI Government engaged a five member multi-disciplinary Scientific Advisory Panel to oversee the assessment of, and to advise on, the radiological status of the entire nation. The formation of the Panel was accompanied by the establishment of a Resident Scientist position, and ultimately a small scientific team and laboratory on Majuro. The nationwide radiological study was conducted using ground survey methods over the period 1990-1994. Tasks undertaken by the Panel included formulating reasonable objectives for the study and attempting to establish effective communication and understanding of issues with political leaders and RMI Government agencies and people, as well as advising on and monitoring the scientific integrity of the study itself. The attempt was also made to initiate investigations to address matters of concern that emerged. The problem was faced of providing not only technical guidance on radioactivity and radiation measurements, but also explaining the significance of measured values and concepts, such as risk and probability of health effects to a diverse but nontechnical audience, generally across cultural and language barriers. The experience of the Panel in providing advice and guidance to the Republic of the Marshall Islands, while unique in many ways, parallels the difficulties experienced elsewhere in communicating information about risks from radiation exposure.

Humans↗

Corrections for scattered photons in free-air ionisation chambers.

Although most recent interest and development in radiation dosimetry has been centered on higher energy radiations, calibrations of conventional x-ray machines for radiotherapy are still based on free-air ionisation chamber measurements. Such measurements require a correction for ionisation produced by secondary photons scattered out of the primary beam, and although the correction is relatively small (generally less than 1%) the data for deriving it are neither as complete or satisfactory as might be wished. This paper describes the derivation of theoretical expressions which allow the calculation of corrections, and compares calculated values with experimental results. It is apparent from the study that in determining the photon gain correction, account should be taken not only of the chamber geometry but also of photon scattering from the wall materials and, for higher energy beams in particular, of contributions from diaphragm scatter and possibly stray radiation penetration of the chamber.

Elementary Particles↗

A theoretical study of cavity chamber correction factors for photon beam absorbed dose determination.

Theoretical expressions have been derived for the contributions to ionisation density in cavity chambers from secondary electrons arising from photon absorption in wall, build-up cap and surrounding medium. The method allows the derivation of C lambda and displacement factors, which are evaluated for the Farmer graphite chamber for a range of photon beams. Differences between C lambda values calculated for air-walled and water-walled chambers are found to be consistent with those determined by Nahum and Greening. The currently recommended displacement factor for the Farmer chamber with cap, however, is found to be in error by about 0.8%. On the basis of a comparison between theoretically predicted air doses in cavities and the experimental results of Fregene it is proposed that no further correction to C lambda is required for the chamber 'effective point of measurement'.

Elementary Particles↗

Theoretical determination of scatter factors at depths in walls of cylindrical chambers.

A theoretical evaluation has been made of the contribution from scattered photons to dose at depths in the walls of cylindrical cavity chambers for photon beams in the energy range 2-30 MV. Scatter factors calculated for graphite, Perspex and water walls over a range of depths and thicknesses show satisfactory agreement with the experimental results of other workers.

Elementary Particles↗

Absorbed doses in the marrow during 131I therapy.

Absorbed doses to red marrow and blood from 131I therapy have been re-evaluated using data which has become available more recently. Uncertainties in the calculations are discussed and a comparison is made with previously published dose estimates. The mean dose to red marrow from the treatment of hyperthyroidism is estimated to be 0.59 rad mCi-1, compared with 0.36 rad mCi-1 for the euthyroid state.

Bone Marrow↗

Unsealed radioactive materials in medical practice in New Zealand.

Information has been collected on numbers and types of radionuclide procedures in medical practice in New Zealand over a three-month period in the latter half of 1973. For in vivo procedures data on the age and sex of patients and the activities of radioactive materials administered were also collected. The changes in numbers of such procedures carried out over a seven-year period are discussed. A genetic radiation dose to the population of 0.24 mrad per person is derived for the year 1973, about 0.20 mrad of this dose arising from diagnostic procedures and 64 percent being attributable to brain imaging. Whereas formerly genetic doses have been estimated from data on activities of 131I and 32P used, this is no longer possible.

Adult↗