Objective
The aim of this contract is to develop a macroelectronic dosemeter which will be compared with optimized HECE, ECE and CE track etch dosemeters. LEPOFI (Limoges) and SIDR (Fontenay-aux-Roses) work on the electronic system; ENEA (Roma), NPD (Tessaloniki) and SFR (Barcelona) are concerned with dosemeters based on track etching.
Experiments were performed on an electronic dosemeter to test and calibrate the device. The characteristics of the 2 diodes have been measured with different values of bias voltage and the adjustment of the electronic system of treatment has been realized. Differential method is used in the configuration of the electronic system. One diode is covered by polyethylene converter implanted with boron and the other is free of converter. Each contribution to the sensor response may be modified by bias voltage, geometry of the sensor and other materials, and thickness and implantation of converter. Experiments and calculations are being carried out to measure each component of the total response. 2 diodes were compared through measurements of the reverse current and measurements of junction (capacitance as a function of bias voltage). Adjustments and linearity tests were done which lead to optimization of the 2 paths. The polyethylene thickness and boron implantation characteristics have been well defined. The device is now suitable for the next joint irradiations.
Some work on the calculation of neutron and gamma contribution began through definition of a theoretical model which allows the study of parameters modifying the response. A Monte Carlo computer code is in progress.
A possible solution to the complex problem of personal fast neutron dosimetry may be provided by the thin film electrochemical etching of neutron induced recoil tracks in large areas of polycarbonate and cellulose nitrate followed by spark counter registration. Polycarbonate thin films have now been investigated. A 2 cell apparatus has been set up similar to that required for conventional electrochemical etching. In the new electrochemical etching (NECE), however, one cell is filled with the etchant electrolyte and the other with a dielectric fluid. In these investigations diesel oil has been used. The plastic detectors used are aluminized on one surface. Direct current (DC) and/or alternating current (AC) voltage is applied between the thin aluminium electrode and the electrolyte etchant. The best nonshorting characteristics are obtained with the aluminium electrode at the positive polarity.
Using a californium-252 source and a 10 um polycarbonate foil electrochemically etched at 25 C with 30% potassium hypdroxide in water, it appears that the number of aluminium spots, produced by normally incident fission fragments, reaches a plateau after only 60 minutes of NECE. The aluminium spot formation is very rapid since the electrolyte foil breakthrough starts at submicroscopic track induced channels. Under these NECE conditions it is possible to see the aluminium spots by the naked eye while it is difficult to see the fission tracks even under high microscope magnification. Furthermore the magnitude of the etched removal layer is less than 1 um.
At present efforts are being made to electrochemically etch alpha particle tracks in 25 um thick cellulose nitrate films.
In the development of a fast neutron detecting system, CR-39 of 500 um was used as a track detector. On 1 surface lithium fluoride is evaporated to obtain a thin film of about 1 um. Polyethylene radiators of various thicknesses were used. The samples were irradiated with 2.5 MeV neutrons at neutron fluxes corresponding to equivalent doses of about 5 mSv. Chemical development of CR-39 samples was applied in 20% sodium hydroxide solution at 70 C. For comparison of the results to that of proton recoils, CR-39 samples were irradiated in the same field of neutrons.
The maximum number of tracks is obtained for 0.5 cm to 1 cm polyethylene thickness. The decreasing number of tracks for thicker radiators could be explained by attenuation of the neutron beam. This means that protonic equilibrium is destroyed although alpha particle number is relatively constant with different radiator thickness. Also with increasing radiator thickness an important number of neutrons is scattered to various angles relative to the beam thus escaping the detection system. The diffusion of the beam inside the radiator also changes the proton recoil spectrum.
Presenting the number of tracks as a function of the removed thickness layer from the detector surface (using 0.5 cm radiator thickness), a stabilization in track number is reached at about 12 um indicating that protons are developed. However, the greater part of alpha particles is completely etched before protons. Counting both protons and alpha particles, the response of the system is higher than that with only proton recoils.
The detecting system can be combined with an electronic real time neutron dose meter as a prototype converter.
The fast neutron detection principle employed is one in which neutrons are detected indirectly through an elastic (n,p) interaction with a hydrogenous material (eg polyethythene or polypropylene). The protons emitted by the converter are detected by a semiconducting diode. The differences between the responses of 2 diodes in the system is mainly due to (n,p) interactions in the converter. Thermal neutrons are detected through (n, alpha) reactions on boron-10 implanted on the converter face in contact with the diode. The experimental setup consists of the detecting unit, 2 diodes and a 2 way multichannel analyzer. A microcomputer control is employed.
Some of the essential characteristics necessary to obtain a significant result when subtracting the background noise from the converter signal are: maintenance of a single depth over the depleted zone in order to assure a minimum energy detection threshold; some mean noise for the 2 diodes; and minimization of detection power by minimizing the depth of the depleted zone. The thickness of the depleted zone is determined by measuring the capacitance of the junction using a polarization type indepence bridge. A good correspondence between the thickness of the depleted zone and the polarizing voltage is observed when a polarizing potential exceeding 5 V is applied. In this case the junction is blocked. The 2 diodes exhibit a sensibly equivalent mean noise.
Some special tests have been conducted in specific radiation fields to distinguish between the spectra of different incident particles.
A Monte Carlo method has been used to simulate the passage of a normally incident fast neutron beam through a hydrogenous material. As a first approximation it has been considered that only elastic (n,p) scattering is relevant in the studied energy range. It has also been considered that a given neutron cannot interact more than once in the radiator, as neutron man free paths for the energies involved are for larger than radiator thicknesses.
Experimental studies have been carried out using CR-39 plates, 250 um thick and 32 hours curing time. Samples have been irradiated with normally incident neutrons from americium lithium and plutonium beryllium sources in free in air geometry and to various doses. A 1 mm polyethylene radiator has been placed beside the CR-39 plates so that neutrons fall in its surface. Electrochemical etching has been carried out with a 6.5 molar potassium hydroxide solution at 60 C. Track density, measured in a centred region of 1.2 cm in diameter, is analyzed by means of an optical microscope. The value of CR-39 layer removed has a very important influence on the response curve for low energies of neutrons. The value which best fitted the experimental data was 3.25 um. There is a good agreement between the calculated and measured values, within experimental errors, so that it can be concluded that the Monte Carlo method can be applied to reproduce the response of a neutron dosemeter if the correct experimental conditions are taken into account.
In the first year of the contract, each group will work with its own dosemeter in order to optimize its system. Calculations and experiments with monoenergetic neutron beams (normal incidence) will be done and gamma contributions on diodes will be calculated. LEPOFI and SIDR will work in permanent collaboration for design of the electronic system. At the end of this phase joint irradiations will be done on the accelerator of SIDR-CEA (energy 2.3 MeV) and results of the five groups will be intercompared.
In the second year, the following problems will be treated by theoretical calculations:
study of angular and energy response;
neutron gamama discrimination;
backgroun;
low doses and low dose rates.
Further intercomparisons will be performed in reference neutron fields and well known practical fields, which the coordinator will identify.
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 alkali metals
- natural sciences physical sciences optics microscopy
- natural sciences chemical sciences inorganic chemistry post-transition metals
- natural sciences chemical sciences inorganic chemistry metalloids
- natural sciences mathematics applied mathematics mathematical model
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Coordinator
87065 Limoges
France
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