Objectif
THIS PROPOSAL WILL PROVIDE MORE INFORMATION ON DOSIMETRY FOR THE PROTECTION OF WORKERS AND THE POPULATION. NEW CONCEPTS AND QUANTITIES WILL BE IMPLEMENTED INTO RADIATION PROTECTION PRACTICE.
The investigations lay the basis for further development towards the realisation of operational dose equivalent quantities in mixed neutron and photon fields. This development requires extensive calculational support.
The combination of neutron transport calculations and energy deposition calculations is required for the theoretical simulation of dosemeters such as tissue equivalent low pressure proportional counter (TEPC) because the measured energy deposition spectrum and such derived quantities as absorbed dose or dose equivalent are influenced by the neutron scattering in the detector wall and adjacent material.
The code of Caswell and Coyne installed at PTB enables the calculation of neutron induced secondary charged particles and their energy deposition in spherical cavities. Recently a Monte Carlo code for neutron and photon transport calculations (MCNP) was also installed at PTB. First results were obtained by calculating the spectral neutron fluence in the wall and the gas of a TEPC in various irradiation geometries and by taking these results as input data for the Caswell/Coyne code to calculate the energy deposition spectra.
Together with the above calculations the time of flight (TOF) technique, employed for neutron photon separation at low and high energies, should now enable the study of the combination of TEPC and phantom. This technique can also be utilised to investigate the possibility of measuring dose equivalent at high neutron energies (En > 30 MeV). Initial results have been obtained from measurements performed at the monoenergetic neutron beam facility at PSI Villigen (collaboration with Universitaet des Saarlandes, UCL Louvain-la-Neuve and University Basel); they show considerable low energy neutron contributions which could be corrected for by using the TOF technique.
Routine radiation protection usually encounters doses way below the investigation threshold. Wearing one dosemeter is sufficient and the influence of spectral and angular distribution may be accounted for by using appropriate calibration factors and positions for wearing the dosemeter. For these routine cases a calibration with neutron fields such as thermal or the heavy water moderated californium 252 source and using the simple anterior-posterior irradiation geometry is felt to be sufficiently safe. The results obtained for neutron albedo dosmeters are in compliance with independence of shape approximation for such routine applications. This should influence the intense discussion on calibration practice still underway in literature: it gives room for several basic concepts, as their implementation leads to interpretational but not physical differences. The interpretational differences, however, can be quite important in view of juridical and metrological problems.
The data presented may serve to derive appropriate correction factors for phantom shape and material for both the dosemeter reading and the dose equivalent quantity used if this should be desirable in the context of a specific calibration concept. These calculated factors can be termed reliable as demonstrated by intercomparing some of the calculations with experiments.
In the case of non-routine applications, ie in environments where the investigation threshold might be exceeded, independence of shape approximation may be not sufficient. Here a careful analysis of the distribution of the field in energy and angle is required and additional experimental and theoretical work needed.
The dependence of dosemeter readings on phantom shape and material has also been found for photon radiation experimentally and in calculations. These results are also in compliance with independence of shape approximation. In summary independence of shape approximation is a valid concept for practical calibration and routi ne radiation protection.
The aim of this research was the development and investigation of neutron spectrometers for use in real radiation fields. Various neutron spectrometers designed for application in radiation protection practice were carefully investigated. The experimental calibration of the response of 4 sets of Bonner spheres (BS) (2 x PTB, GSF, NPL) required most of the effort. 3 series of measurements were necessary in order to cover the entire energy range form thermal to approximately 15 MeV neutrons. The experimental data could not be explained by simple ANISN calculations. Preliminary and more realistic Monte Carlo simulations describe these data much better. Systematic calculations are in progress. Properly modified ANISN calculations fitting the experimental data have been used to calculate the response matrix needed for the few channel unfolding procedure. While the spectral neutron fluence derived from few channel BS data exhibits large uncertainties and depends heavily on the a priori information required in some unfolding procedures, the integral data such as fluence or dose equivalent are determined with reasonable uncertainties (<= 10%).
The proton recoil spectrometers cover the energy range most important for dosimetry, as the fluence to dose equivalent conversion factor increases by a factor of 10 - 20 for neutron energies about 10 keV. However, the application of the small spherical proportional counters is limited by their neutron detection efficiency , while its sensitivity to photon radiation may be the drawback of the liquid scintillation spectrometer, even if fast n/gamma discrimination circuits are available.
The spectrometers will be used in combination depending on the environmental conditions and the particular properties of the fields to be investigated.
1. DEVELOPMENT OF NEUTRON SPECTROMETERS FOR RADIATION PROTECTION PRACTICE.
2. REALIZATION OF DOSE EQUIVALENT QUANTITIES FOR PHOTONS AND NEUTRONS USING MICRODOSIMETRIC METHODS.
3. INVESTIGATION OF DOSE EQUIVALENT QUANTITIES FOR INDIVIDUAL DOSIMETRY.
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CORDIS classe les projets avec EuroSciVoc, une taxonomie multilingue des domaines scientifiques, grâce à un processus semi-automatique basé sur des techniques TLN. Voir: Le vocabulaire scientifique européen.
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- sciences naturelles sciences physiques physique théorique physique des particules photons
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