Objective
The partner laboratories will perform work to develop different types of dose equivalent meters for radiation protection dosimetry in neutron and photon fields. The methods used include low pressure tissueAequivalent proportional counters and high pressure ionization chambers as area monitors, and a semiconductor type detector as individual dosemeter. The research is aimed at the implementation of these techniques for various tasks in practical radiation protection work. The technical development of these instruments will be combined with experimental and theoretical investigations necessary to improve their performance.
Measurements will be performed in reference photon and neutron fields in order to compare the dose equivalent response of the dosemeters. Neutron fields in the energy range from thermal up to 20 MeV will be produced among the partner laboratories. Neutron energies above 20 MeV will be investigated in collaboration with high energy accelerator facilities (Paul Scherrer Institute, Villigen, Ch; Cyclotron CYCLONE, UCL, LouvainAlaANeuve, Belg.). The determination of basic physical characteristics of the beams will be an important part of the work, in particular the knowledge of photon and neutron fluences will enable the comparison of experimental and theoretical data.
The dosemeters developed will also be tested in radiation environments of practical relevance for radiation protection (nuclear power plants, research reactors, physical and medical accelerators) which will enable the operational characteristics of the instruments in routine conditions to be verified.
Basic physical data of relevance for neutron dosimetry will be obtained (fluenceAtoAkerma conversion factors, average WAvalues, gasAtoAwall dose conversion factors) for neutron energies above 20 MeV for which theoretical data are inaccurate and experimental data are scarce.
Basic characteristics of radiation protection instruments (electrical discharge in gas, cavity theory, semiconductor properties) will be investigated on theoretical and experimental bases. These studies will contribute towards a better knowledge of the detector properties and to extend their range of applicability for ionizing radiation dosimetry and microdosimetry, with practical consequences for the development and optimization of dose equivalent meters. This work will be performed in collaboration with Working Committee 10 of EURADOSACENDOS.
Development of the ambient dose equivalent meter HANDI (Hamburg area neutron dosimeter), based on a low pressure tissue equivalent proportional counter (TEPC), has continued, and an operational version has been produced. Work has been concentrated on improving the performance of the TEPC, in particular its response to neutrons of low and intermediate energies and the gas gain stability as a function of temperature.
Experiments showed that the TEPC dose equivalent response can be improved at low neutron energies by reducing the simulated diameter to 1 um, using thin detector walls, and adding only a few percent of helium-3. Optimum detector characteristics have not yet been identified.
In practice, ambient temperatures might vary from 15 to 40 C. Therefore, the temperature dependence of the gas amplification was investigated by testing the variation of the alpha source calibration peak for TEPCs maintained in a thermostatically controlled chamber. Higher stability with temperature and better gas gain properties were achieved using isobutane rather than propane.
Reference neutron fields have been investigated for the purposes of detector calibration. For neutron in the energy range 20 to 40 MeV, a cyclotron pulsed proton beam and a 2 mm thick beryllium target were used for neutron production and time of flight (TDF) spectrometry for detection. NE213 scintillation detectors and low pressure proportional counters were used to separate neutron and photon dose fractions.
For neutron energies in the range 2 to 20 MeV, the use of scintillation detectors allowed the neutron fluence to be determined within an uncertainty of 3%, by converting the TDF spectra to energy spectra. Such conversion proved unsatisfactory for energies above 25 MeV.
The absolute fluence of neutron fields has been determined to about 8% using a proton recoil telescope, consisting of a hydrogen containing radiator, 2 gas filled proportional counters and 2 semiconductor silicon detectors operating in coincidence. Extensive Monte Carlo simulations of the experimental arrangement have been performed in order to avoid systematic uncertainties in the neutron fluence analysis.
Kerma factors for carbon and A-150 tissue equivalent plastic were determined in almost monoenergetic neutron beams with nominal energies between 14 and 20 MeV, 26 MeV and 38 MeV. The kerma was measured with low pressure proportional counters with walls made of graphite and A-150 plastic. The neutron fluence was measured with a proton recoil telescope and the spectral neutron fluence with an NE213 scintillation detector. The kerma factor ratio for ICRU muscle tissue to A-150 plastic is about 0.93 for neutron energies above 14 MeV.
Neutron transport calculations and charged particle energy deposition spectra in small volumes have been performed in order to assess the dose equivalent response of a small TEPC in a phantom, that is, a sphere of A-150 plastic for use in instrument calibration.
Development of a field instrument for measuring the dose equivalent in mixed neutron and gamma ray fields has concentrated on high pressure ionization chambers (HPICs). Such chambers exhibit increased sensitivity with increased gas pressure, and therefore the pressure dependence and the ion recombination characteristics can both be used to assess the radiation field. However, the increased sensitivity is only available if the leakage current is small. Measurements of leakage current in HPICs showed that it would be necessary to reduce it by a factor between 10 and 100 to obtain the desired sensitivity.
The response of the system under conditions relevant for radiation protection situations was studied. Methane was used to obtain a detector with high neutron sensitivity, and argon for a photon sensitive device. The effect of the composition of the chamber wall axis studied by using tissue equivalent (TE) and A1 HPICs. Gas pressures ranged from 0.1 to 8 MPa and collecting potentials from 50 to 600 V. Results of measurements made at 2 sites similar radiation quality in a generator hall and maze entrance showed agreement for both gases for the TE HPIC. The relative readings of the argon filled A1 chamber were about 10% lower than the argon filled TE chamber.
Development of a semiconductor detector for a coupled semiconductor ionization chamber dosimeter commenced. The detector will be operated as a microdosimetric ionization yield detector. The design is based on a sandwich, where the semiconductor device is covered by a layer of tissue equivalent material. The minimum thickness of this layer in such that secondary charged particle equilibrium is established at the position of the semiconductor to tissue interface. The effect of the interface is not known. The sensitive elements are specially constructed diodes, where depletion layer is almost cubical in shape with a length of side in the range 2 to 4 um.
The neutron photon transport code MCNP was adapted to implement a humanoid phantom. Transport calculations were then performed for a variety of monoenergetic photon and neutron fields a californium-252 neutron source, and mixed neutron photon fields. Target organs included the brain, kidneys and thyroid. Microdosimetric calculations were then performed, using a program developed for simulate proportional counter measurements.
Electron molecule collision cross sections for the organic vapours propane and isobutane have been determined, using information from experimental determination of swarm parameters (including drift velocities, diffusion coefficients and ionization coefficients). All these coefficients depend on the electric field divided by the pressure. The various swarm parameters are defined by the quadrature of the distribution function of electrons and the electron molecule cross sections. It follows that knowledge of the variation versus electric field over pressure of these parameters allows the determination of the energy variation of the cross sections.
The low energy set of cross sections derived for pure propane are presented. The 25 different vibrational processes occurring in propane were assembled in 3 different cross sections. Superelastic collisions in vibration were taken into account and were seen to be important at thermal and subthermal electron energies.
For propane and argon propane mixtures, experimental swarm parameters are compared to those calculated using the cross sections obtained by deconvolution. The results show good agreement, proving the suitability of the unfolding parameter.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- natural sciences chemical sciences inorganic chemistry noble gases
- natural sciences physical sciences theoretical physics particle physics particle accelerator
- natural sciences physical sciences electromagnetism and electronics semiconductivity
- natural sciences chemical sciences organic chemistry aliphatic compounds
- natural sciences physical sciences theoretical physics particle physics photons
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Coordinator
66123 SAARBRUECKEN
Germany
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