Skip to main content

CALCULATION AND MEASUREMENT OF DOSES FROM PARTICULATE RADIOACTIVE SOURCE

Objective

There is an increasing awareness of the importance, particularly in the USA, of the dosimetry of radioactive particles (hot particles, microscopic, high activity radioactive particulates) which commonly arise in reactor operations from neutron activation product debris and fissioned fuel contamination and in hospital and other industrial situations where such sources present a potential skin hazard. Current methods of calculating doses from particulates use semiempirical beta depth dose data such as those of Cross which are based on semiinfinite, tissue geometries and cannot be easily applied to real geometriesEand high atomic number particles. This project will provide improved and validated methods of calculations of dose from radioactive particulates.
A critical evaluation of the use of the extrapolation chamber for the measurement of doses from point sources has been made, and comparison made with radiochronic dye films. A commercial 1 mm diameter strontium yttrium-90 source was used, appropriate for studies of hot particle effects.

The tissue equivalent parallel plate extropolation chamber provides an absolute method for determining dose rates from beta emitting sources. For planar sources and in the vicinity of punctiform sources, an understanding of the effect of dose nonuniformity on the form of the relationship between chamber ionization current ad electrode separation is required. Dose estimates from both linear and nonlinear extrapolation models have been obtained and compared with the provided by radiochronic dye film dosemeters. For a large area planar source, linear extrapolation gave good agreement. Nonlinear extrapolation provided significantly improved agreement in the case of the punctiform source.

The CEC contract gathers results from M W Charles (University of Birmingham, GB) P J Darley (University of Birmingham, GB), J P Patou (University of Toulouse, FR) and Y Herbaut (CEA Grenoble, FR) concerning the beta dose on contact of microscopic radioactive particulates.

The cobalt 60 spherical sample (200 um in diameter, 0.8 Mbq in activity)was provided by Dr Charles. Our contribution was to compare extrapolation chamber measurements (PTW cahmber model 23391, whose main characteristics have been determined, has been used with the 10 mm diameter collecting electrode) and codes . The calculations underestimate by a factor of 2, PTW experiments.

Similar experiments close to a TM 170 radioactive source are in the project. They will be performed with smaller collecting electrode areas.

A computer program has been developed to calculate energy spectra and local absorbed dose distribution around high activity radioactive particulates. Idealized cobalt-60 beta spherical sources were considered.

The reliability of the code was tested by simulating the transport of beta particles in a complex geometrical configuration used for dosimetric measurements. Monte Carlo methods were used for sampling each beta ray emission position and for simulating the decleration. For each step and for each material the pathlength and deflection distributions or calculated (using Landase Theory and Goudsmit and Saunderson Theory, respectively). The probability of inelastic scattering and the energy spectra of secondary electron are derived from the Moller cross section.
The NCRP and ICRP have recently provided recommendations for limiting the occupational exposure of the skin from radioactive particulates. Their deliberations relied upon the interpretation of animal studies which used a range of radiation sources (including particles) for which detailed dosimetry was limited and for which no low energy beta source data were available. This work will provide improved dosimetry and a supply of well characterized Co-60 particle sources for related CEC animal skin studies (CEC BI7-0041).

This project will produce beta emitting radioactive particulates by neutron activation (Radiological Protection Branch, Nuclear Electric plc at Berkeley Nuclear Laboratories, BNL) and measure the radiation doses using TLD, extrapolation chamber, (BNL & Commissariat a l'Energie Atomique, Centre d'Etudes Nucleaires de Grenoble, CENG) and radiochromic dye film (BNL).

A Monte Carlo computer code, suitable for PC use, will be developed to predict doses from the specific designs of source used (Department of Biophysique and Pharmaceutique, Tolouse University, TU) and these will be compared with measurements and the simplified empirical calculation methods currently in wide use. Such a programm will be useful to the scientific and health physics community at large. Experiments using cobalt-60 particles will initially be used because of the important practical relevance of this radionuclide to the nuclear industry, but can be extended to include fission products.

Pure cobalt particles will be produced with diameters of about 100, 500, and 1000 microns and mounted on ultrapure aluminium supports. Neutron activation will be used to produce individual Co-60 particle activities with maximum values of about 0.1EmCiE(5 MBq). Doses will be measured over various areas and at various depths (using tissue type plastic absorbers) relevant to epidermal and dermal target cells in the skin (10 to 700 microns).

The extrapolation chambers will be based on the original Failla extrapolation chamber (BNL) the Far Western (CENG) and Bohm chambers (BNL & CENG).

Target dates:

1990 to 1991: Production of smallest low activity Co-60 sources. Extrapolation chamber and radiochromic dye film measurements. Initial development of a Monte Carlo code and comparison with initial dosimetry data. .SP 1 1991 to 1992: Comparison of measurements for larger, higher activity sources with theory. Refinement of MC code and comparison with semiempirical methods.

1992 onwards: Extension of studies to other neutron activation and fission product particle sources.

The results of this work will enable potential skin doses, and their hazards, from practical radioactive particles to be calculated more reliably. The data will also help to interpret the results of skin radiobiology studies (CEC BI7-0041) and aid their application to practical particle exposure situations. This work is carried out under the auspices of EURADOS committee 2. The related radiobiology studies form part of the current programme of EULEP.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Central Electricity Generating Board (CEGB)
Address

EC1A 7A4 London
United Kingdom

Participants (2)

COMMISSARIAT A L'ENERGIE ATOMIQUE
France
Address
Rue De La Federation 31-33
75015 Paris
UNIVERSITE PAUL SABATIER DE TOULOUSE III
France
Address
Route De Narbonne 118
31062 Toulouse