THIS PROPOSAL WILL PROVIDE MORE INFORMATION ON DOSIMETRY FOR THE PROTECTION OF WORKERS AND THE POPULATION. COLLECTION AND EVALUATION OF DOSIMETRIC DATA. AREA MONITORING WILL BE IMPROVED.
Research is being carried out to determine the basic physical data for the radiation protection dosimetry of neutrons and photons using low pressure proportional counters, and to investigate the practical aspects of employing microdosimetric counters as dose equivalent meters.
Significant progress was achieved in the method of applying low pressure proportional counters (PC) for the determination of basic physical data required in neutron and photon radiation dosimetry. The overall experimental uncertainty was reduced considerably by the introduction of new, and the combination of several, calibration procedures and by improvements in the evaluation procedure by combining measurements using PC's with walls made of different materials. The timing resolution of PC's was optimised with regard to the counting gas mixtures. In collaboration with the Physikalisch Technische Bundesanstalt (PTB) time of flight techniques were applied with these optimised PC's to separate events of different neutron energies. The range of application of tissue equivalent proportional counters (TEPC) was enlarged by applying counting gas mixtures containing small amounts of helium-3 gas combined with the variation of the counter wall thickness. This method was employed to study the attenuation and thermalisation of low energy neutrons in hydrogenous material and the influence of these processes on absorbed dose and dose equivalent.
The principal suitability of PC's to measure absorbed dose and kerma for low neutron energies was investigated in measurements with monoenergetic neutrons in the range from thermal neutrons to 144 keV neutrons (at the PTB). Absorbed dose measurements of high accuracy for various irradiation geometries were also performed at the bare and deuterated water moderated californium-252 reference sources of the PTB.
A main aim was to determine kerma factors and fractional kerma for various secondary particles in different materials for monoenergetic fast neutrons in the energy range 14-20 MeV (at the PTB) and 20-70 MeV (at the Paul Scherrer Institute, PSI, Villigen, Switzerland). In order to achieve the desired accuracy in the evaluation of the measurements using cavity chamber principles, new approaches to evaluate effective W-values and gas to wall absorbed dose conversion factors were developed. This work was of particular importance for the nonhomogeneous counters, ie, the counters with walls made of carbon and other nonhydrogenous materials.
The applicability of cavity chambers of different materials to determine absorbed dose for photons was investigated with PC's in the radiation fields of nearly monoenergetic gamma ray nuclide sources (in the energy range 140 keV-1.2 MeV).
This research concentrated on the construction and implementation of a portable area monitor based on tissue equivalent proportional counters (TEPC) suitable for routine application in operational radiation protection. The suitability of the newly developed system (HANDI) was tested in reference neutron and photon fields and in radiation environments of practical importance by comparison with standard laboratory equipment for experimental microdosimetry (BIO), and with different TEPC instruments developed by other laboratories. This collaboration was coordinated by EURADOS Committee 1. In particular, this laboratory was actively involved in the intercomparison of TEPC dose equivalent meters organised by EURADOS and Physikalisch Technische Bundesanstalt (PTB) and its evaluation. In addition separate and more detailed experiments were performed in the same reference fields with the aim to provide a basis for further improvement of the dose equivalent response of TEPC's at low neutron energies.
A prototype of the area monitor HANDI became operational at approximately the middle of the contractual period. A second version using improved analog and digital electronics developed by computer assisted design (CAD) techniques was finished towards the end of the research and consisted of a portable battery powered instrument suitable for semi-industrial production. The relatively simple mode of operation enabled the instrument to be used for routine operational health physics work. The 2 produced versions showed the anticipated technical and physical performance. In particular, no significant decrease in accuracy or loss in diagnostic capacity was observed if compared to the laboratory equipment.
Measurements with the HANDI system, and partly with BIO, were performed at medical irradiation facilities, high energy accelerators, a nuclear power reactor and a nuclear fuel processing industrial plant. In some cases comparisons could be made with other neutron dose equivalen t meters. These measurements documented the principal suitability of the HANDI system for operational applications and provided evidence for the usefulness of the diagnostic information only available from TEPC area monitors in particular instruments equipped with spectra storage capabilities such as HANDI.
Detailed and comprehensive measurements with TEPC's in reference photon and neutron fields at PTB and at The National Institute for Science and Technology (NIST, formerly NBS, Washington DC, USA) provided useful data on radiation transport and interaction processes in the counter wall and gas. The resulting quantitative understanding of the detector response showed that the too low dose equivalent response at low neutron energies can be substantially improved and was used to investigate the feasibility of a TEPC to be used as a reference or transfer instrument. The operational version of the portable area monitor provides the basis for further improvement in order to construct a truly hand held system.
1. BASIC PHYSICAL DATA FOR THE DOSIMETRY AND RADIATION PROTECTION DOSIMETRY OF NEUTRONS AND PHOTONS WITH LOW PRESSURE PROPORTIONAL COUNTERS.
2. INVESTIGATION OF PRACTICAL ASPECTS OF EMPLOYING MICRODOSIMETRIC COUNTERS AS DOSE EQUIVALENT METERS.