[en] A small size polyethylene moderator for use on neutron dosemeters based on silicon diodes has been studied using the MCNP-4A code to determine the size and geometry that allow an improvement to the dosimeter's response for neutrons in the energy range 10 keV - 1 MeV. Various dimensions of cylindrical and spherical moderators with or without a central cylindrical opening have been modelled for 1 MeV starting neutrons at normal incidence on the detector area. Since silicon diodes provided with a borate polyethylene converter allow thermal and fast neutron detection, the number of fast and thermalized neutrons reaching the detection area is observed as a function of the detector positioning inside the moderator for each geometry considered and compared with results obtained for Bonner Spheres. The evolution in the number of thermalized neutrons with starting neutron energy is shown for some of the geometries and a moderator with a natural boron absorber layer is proposed. (author)

[en] Large area silicon diodes used in electronic neutron dosemeters have a significant over-response to X- and gamma-rays, highly non-linear at photon energies below 200 keV. This over-response to photons is proportional to the diode's active area and strongly affects the neutron sensitivity of such dosemeters. Since silicon diodes are sensitive to light and electromagnetic fields, most diode detector assemblies are provided with a shielding, sometimes also used as radiation filter. In this paper, the influence of materials covering the diode's active area is investigated using the MCNP-4A code by estimating the photon induced pulses in a typical silicon wafer (300 μm thickness and 1 cm diameter) when provided with a front case cover. There have been simulated small-size diode front covers made of several materials with low neutron interaction cross-sections like aluminium, TEFLON, iron and lead. The estimated number of induced pulses in the silicon wafer is calculated for each type of shielding at normal photon incidence for several photon energies from 9.8 keV up to 1.15 MeV and compared with that in a bare silicon wafer. The simulated pulse height spectra show the origin of the photon-induced pulses in silicon for each material used as protective cover: the photoelectric effect for low Z front case materials at low-energy incident photons (up to about 65 keV) and the Compton and build-up effects for high Z case materials at higher photon energies. A simple means to lower and flatten the photon response of silicon diodes over an extended X- and gamma rays energy range is proposed by designing a composed photon filter. (author)

[en] A moderator type neutron dosemeter is described and its response to 'realistic neutron fields' and fission sources is investigated, for irradiations with and without a poly-methyl methacrylate (PMMA) phantom. Changes in response induced by the use of the moderator are observed throughout the experiments, with respect to the energy distribution of the neutron fluence and the dose equivalent. The experimental dosemeter shows an over-response to thermal neutrons and for on phantom irradiations, and a low sensitivity for fast neutrons. Compared to a classical neutron dosemeter based on silicon diodes (without moderator), the actual design shows, for the same active area of the silicon detector, a sensitivity (counts per mSv) from 10 to 40 times higher for 'realistic neutron fields'. (author)

[en] Monoenergetic neutron sources in the intermediate energy range are scarce and the development of neutron therapy techniques depends on the design of neutron beam producing facilities. The paper deals with a theoretical and computational demonstration of the possibility to select quasi-monoenergetic intermediate neutrons from broad energy spectra by elastic scattering. Although heavy materials scatter nearly isotropically intermediate neutrons, on special geometrical conditions as thin blades, it is possible to get anisotropic response for neutrons of a certain energy and to select them from a broad energy spectrum. Materials useful for such selectors are those presenting at least one dominant elastic scattering cross-section peak in the intermediate neutron energy range, like iron and nickel. A verification of the theoretical demonstration has been performed by numerical simulation using the MCNP-4A code. Computation results for selecting devices made of iron and nickel show that a quasi-monoenergetic neutron field of 28.2 keV can be obtained from iron and respectively 15.6 keV from nickel. Due to the small efficiency of the scattering process, the use of a focusing device is necessary to significantly enhance neutron selection efficiency. The design of such a focusing device is proposed in the paper. (author)

[en] This article describes the medium frequency pulsed dc power operation of a Penning-type (opposed target) magnetron to sputter copper in an argon atmosphere. The current-voltage characteristics of the magnetron are detailed in dc and pulsed dc conditions. The variations in optical emission from the excited species in the plasma are measured as a function of time during the applied pulses. The intensity of the argon emissions closely follows the variations of the target voltage but the different behavior of the copper emissions shows some evidence of self-sputtering of the target by copper ions. The energy and flux of the ions arriving at the substrate are measured as a function of time and it is shown that during pulsed operation, as the pulse off-period is extended, there are large increases in ion energy and ion flux. The initial overshoots in the target voltage cause a rapid expansion of the plasma sheath at the analyzer probe and wide variations in plasma potential leading to an initial pulse of high energy ions during the first 500 ns. The plasma potential then settles to ∼40 eV and there is a second peak in ion flux at this energy with a peak at ∼4000 ns due to the increased ion drift resulting from electron extraction from the target

[en] A moderator type neutron dosemeter is described and its response to realistic neutron fields from C. E. N. Cadarache-France is investigated, as well as its sensitivity to the gamma rays generated along with the realistic neutron fields. Changes in response induced by the use of a moderator are observed throughout the experiments, with respect to the energy distribution of the neutron fluence and the dose equivalent. The silicon detector sensitivity to the gamma ray component of these fields is also discussed. Compared to an A moderator type neutron dosemeter is described and its response to realistic neutron fields from C. E. N. Cadarache-France is investigated, as well as its sensitivity to the gamma rays generated along with the realistic neutron fields. Changes in response induced by the use of a moderator are observed throughout the experiments, with respect to the energy distribution of the neutron fluence and the dose equivalent. The silicon detector sensitivity to the gamma ray component of these fields is also discussed. Compared to an electronical neutron dosemeter without a moderator, for the same active area of the silicon detector, the actual design shows a response 10 to 40 times higher to realistic neutron fields and a sensitivity to gamma rays up to 10 times lower. (author)

[en] Large area Silicon diodes used in electronic neutron dosemeters have a significant over-response to X and gamma rays, highly non-linear at photon energies below 200 keV. This over-response to photons is proportional to the diodes active area and strongly affects the neutron sensitivity of such dosemeters. Since Silicon diodes are sensitive to light and electromagnetic fields, most diode detector assemblies are provided with a shielding, sometimes also used as radiation filter. In this paper, the influence of materials covering the diode's active area is investigated using the MCNP-4A code by estimating the photon induced pulses in a typical silicon wafer (300 μm thickness and 1 cm diameter) when provided with a front case cover. There have been simulated small-size diode front covers made of several materials with low neutron interaction cross-sections like aluminium, TEFLON, iron and lead. The estimated number of induced pulses in the silicon wafer is calculated for each type of shielding at normal photon incidence for several photon energies from 9.8 keV up to 1.15 MeV and compared with that in a bare silicon wafer. The simulated pulse height spectra show the origin of the photon induced pulses in silicon for each material used as protective cover: the photoelectric effect for low Z front case materials at low energy incident photons (up to about 65 keV) and the Compton and build-up effects for high Z case materials at higher photon energies. A simple means to lower and flatten the photon response of silicon diodes over an extended X and gamma rays energy range is proposed by designing a composed photon filter. (author)

[en] A small-size polyethylene moderator for use on neutron dosemeters based on Silicon diodes has been studied using the MCNP-4A code to determine the size and geometry that allow an improvement to the dosemeter response for neutrons in the energy range: 10 keV - 1 MeV. Various dimensions of cylindrical and spherical moderators with or without a central cylindrical opening have been modelled for 1 MeV starting neutrons at normal incidence on detector area. Since Silicon diodes provided with a borate polyethylene converter allow thermal and fast neutron detection, the number of fast and thermalized neutrons reaching detection area is observed function of detector positioning inside the moderator for each considered geometry and compared to results obtained for Bonner Spheres. The evolution in the number of thermalized neutrons with starting neutron energy is shown for some of the considered geometries and a moderator with a natural Boron absorber layer is proposed. (author)

[en] The assessment of H and H₂ densities by mass spectrometry methods in silane-hydrogen (SiH₄-H₂) plasma-assisted CVD processes requires estimation of the depletion factor for the parent molecules (SiH₄ and H₂). Such an estimation is non-trivial as H₂ is both depleted and generated in SiH₄-H₂ plasmas, and the usual depletion factor becomes a 'depletion-generation' factor. A method for calculating the depletion-generation factor in SiH₄-H₂ plasmas is presented and applied to de-convolute energy distributions of H and H₂ from energy-resolved residual gas analysis (ERRGA) performed using a HIDEN EQP300 mass-energy analyser. The ERRGA method is described and the energy distribution of the H radicals and H₂ molecules is obtained for an LEPECVD plasma during the transition from a-Si to nc-Si deposition at constant discharge operation parameters for gas input flow rates of 50 sccm Ar and 10 sccm SiH₄, with the addition of H₂ (0-50 sccm) at reactor pressures of 1-3 Pa. The H and H₂ energy distributions show two main groups, one below 0.7 eV and another at 1.2-1.5 eV, with a strong variation in total density with increase in H₂ input. Above 30 sccm H₂ input, a step increase in the lower energy component is observed. Energy distributions are discussed in relation to H and H₂ generation reactions and gas heating in PECVD processes.

[en] The control and management of neutron doses, received by workers in nuclear power or research facilities, requires a knowledge of cumulated dose equivalent or dose equivalent rate in real time. Individual dosemeters so far developed for this purpose are scarce and not very satisfactory. Passive dosemeters such as TLD systems based on the albedo effect, nuclear emulsions or solid track detectors, do not give sufficiently accurate measurements. Furthermore, the increase in the quality factor and the more restrictive new ICRP recommendations diminish the maximum admissible threshold making currently used systems obsolete. Other than bubble dosemeter systems, based on thermodynamic effects of a superheated gel, no simple electronic device is available at the present time. The development of diode based dosimetric gamma badges, having a size similar to that of credit cards, has stimulated us to design and develop a personal neutron dosemeter based on a double diode system. The results obtained are very encouraging and practical models should become available in the near future. (author)