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Dosimetry of Beta and Low-Energy Photon Radiations

Objective


The characterisation of beta radiation fields in terms tissue depth and angle of radiation incidence have been performed for different source constructions with radionuclides covering a broad variety of beta energies. The dose rate values were obtained from measurements by extrapolation chambers. The characterised beta radiation fields have been used in this project for a number of studies of energy and angular responses of dose ratemeters and dosemeters. For ruthemium-106/rhodium-106, strontium-90/yttrium-90, thallium-204 and promethium-147 two types of source constructions were studied, the sources produced by the Amersham Company with the radionuclides encapsulated in silver and another type of sources (OMH type) produced at the Hungarian Institute of Isotopes (HII) with backing materials of PMMA and only covered with thin foils of aluminised mylar and therefore expected to produce minimal bremsstrahlung. Studies of the ruthenium-106/rhodium-106 obtained from the Amersham Company showed that the source contained a significant amount of radioactivity from a different radioisotope. The Amersham Company was not able to deliver a replacement source; so for this radionuclide only the HII type source have been available for this project. The carbon-14 source was obtained from the Amersham Company as 0.9 mm thick PMMA sheets with the carbon-14 radionuclide contained in the source as part of the PMMA material. The nickel-63 was also produced by the Amersham Company with the nickel-63 radioactivity electro-deposited on siver and without any further protective cover. Due to the strong absorption in air of beta particles from nickel-63, implying a significant degradation of the spectrum at short distances from the source, a special container, of diameter 50 cm and length 35 cm, enabling irradiation under reduced air-pressure conditions was constructed at Risø for this source. In this way it was possible to achieve a dose rate of 235 mGy/h at a distance of 20 cm from the source. The nickel-63 source is not categorised as an ISO Series 2 type source; however it was found useful to extend the range of the energies to the low energy of nickel-63, in particular for studies of energy and angular responses of thin solid state detectors.

The dose rate homogeneity of the established beta radiation fields were studied using different methods. The results showed that the ISO requirements of variation in dose rates being within ± 5% for the thallium-204 source and sources with higher energies and ± 10% for the promethium-147 source and sources with lower energies can be fulfilled for all the established sources for beam diameters ranging between 5 and 10 cm.

The ISO have set a lower limit for the residual maximum energy, E(res), present at the calibration position to ensure against excessive degradation of the beta spectrum taking place in the source itself and/or the absorber between source and calibration point. E(res) values have been determined for different beta sources. The beta spectra obtained for the HII type sources show E(max) energies for these sources very close to the theoretical E(max) values. The results of the E(res) measurements for other sources showed that the ISO requirements could be fulfilled for all the established ISO Series 2 type sources.

A serious problem of beta calibration fields is the presence of bremsstrahlung. The results of the study show that an important factor for the production of bremsstrahlung is the source material itself, and they emphasise the importance of performing separate analyses of the bremsstrahlung spectrum and dose contribution for each beta source.
THE PROPOSED RESEARCH PROJECT IS A CONTINUATION OF THE CURRENT CEC RTD CON TRACT PROJECT "DOSIMETRY OF BETA AND LOW-ENERGY PHOTON RADIATIONSJ USING EXTRAPOLATION CHAMBER AND THIN SOLID STATE DOSEMETERS". THE PROPOSED PROG RAMME WILL EXTEND AND COMPLIMENT THE ACTIONS INITIATED UNDER THE PRESENT CONTRACT PROJECT. ULTIMATE OBJECTIVES OF THE PROJECT ARE TO DEVELOP STAN DARD CALIBRATION FACILITIES AND TO ESTABLISH STANDARDISED MEASUREMENTS PRO CEDURES FOR DOSIMETRY OF WEAKLY PENETRATING RADIATIONS. THE MAIN RESEARCH AREAS OF THE PROPOSED PROGRAMME COMPRISE : IDENTIFICASTION AND EVALUATION OF INFLUENCING PARAMETERS IMPORTANT FOR THE DOSIMETRIC CHARACTERISTICS OF THE RADIATION FIELD IN DESIGNING STANDARD BETA CALIBRATION BEAMS, E.G. SOUR CE CONSTRUCTION, SURROUNDING SHIELDINGS, BEAM-FLATTENING FILTERS, AND EMER GENT SPECTRA.- CHARACTERISATION OF BETA RADIATION AND THE ACCOMPANYING PHO TON RADIATION (BREMSSTRAHLUNG) AND CHARACTERISTIC X-RAYS) FIELDS IN TERMS OF DOSIMETRIC QUANTITIES AS WELL AS IN TERMS OF ENERGY AND ANGULAR SPECTRA. INVESTIGATION AND DEVELOPMENT OF MEASUREMENT TECHNIQUES AND PROCEDURES FOR CHARACTERISING BETA RADIATIONS (WITH THE ACCOMPANYING PHOTON RADIATIONS) FIELDS AND FOR INDIVIDUAL MONITORING FOR DOSES FROM THESE FIELDS.- ESTA BLISHMENT AND CHARACTERISATION OF LOW-ENERGY PHOTON RADIATION CALIBRATION BEAMS AND CHARACTERISATION OF INDIVIDUAL DOSEMETERS FOR DOSIMETRY OF THESE FIELDS. RESULTS WILL BE OBTAINED THROUGH EXPERIMENTS AND BY COMPUTATIONAL METHODS. COMPUTATIONS USING A MONTO CARLO CODE WILL BE APPLIED FOR ANALY SING THE ANGULAR AND ENERGY SPECTRA AND THE DOSE CHARACTERISTICS OF THE BETA RADIATION FIELDS. A BETA SPECTROMETER WILL BE OPERATED TO OBTAIN EX PERIMENTAL DATA ON ANGULAR AND ENERGY DISTRIBUTIONS OF THE BETA PARTICLES AND MAY VALIDATE THE CALCULATED VALUES. DOSE DISTRIBUTIONS EVALUATED FROM SPECTROMETRIC ANALYSIS WILL BE COMPAIRED WITH DOSE HOMOGENEITY MEASUREMENTS USING PHOTOGRAPHIC FILMS, TLD, TSEE AND OTHERS. THE EXTRAPOLATION-CHAMBER MEASUREMENT METHOD WILL BE FURTHER STUDIED AIMING AT AN OPTIMAL EVALUATION PROCEDURE AND ACCURACY OF MEASUREMENT OF THE METHOD. SOLID STATE DOSEMETERS BASED ON TLD AND TSEE WILL BE FURTHER DEVELOPED AND STUDIED FOR APPLICATION FOR CHARACTERISING FIELDS OF WEAKLY PENETRATING RADIATIONS AND FOR INDIVI DUAL MONITORING FOR DOSES FROM THESE FIELDS. THE PROJECT WILL INVOLVE BETA SOURCES OF DIFFERENT DESIGNS AND OF ENERGIES (E MAX).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

RISOE NATIONAL LABORATORY
Address
399,Frederiksborgvej 399
4000 Roskilde
Denmark

Participants (7)

AEA Technology plc
United Kingdom
Address
Harwell Laboratory
OX11 0RA Didcot
Commissariat à l'Energie Atomique (CEA)
France
Address
Avenue Des Martyrs
38041 Grenoble
Commissariat à l'Energie Atomique (CEA)
France
Address
Centre D'études De Fontenay-aux-roses
92265 Fontenay-aux-roses
JUSTUS-LIEBIG-UNIVERSITY OF GIESSEN
Germany
Address
Heinrich-buff-ring 14-16
35392 Giessen
NATIONAL RADIOLOGICAL PROTECTION BOARD
United Kingdom
Address

OX11 0RQ Didcot,harwell,chilton
Nuclear Electric plc
United Kingdom
Address
Berkeley Nuclear Laboratories
GL13 9PB Berkeley
Université de Montpellier II (Université des Sciences et Techniques du Languedoc)
France
Address
Place Eugène Bataillon
34095 Montpellier