F. Labohm

Physical properties of nitrogenated amorphous carbon films produced by ion-beam-assisted deposition

Abstract Carbon films with up to 32 at.% N (a-C:N) have been prepared using an ion-beam-assisted magnetron, with an N2+ beam at energies between 50 and 300 eV. The composition and density of the films vary strongly with the deposition parameters. Electron energy loss spectroscopy shows that these a-C:N films are mostly graphitic with up to 20% C sp3 bonding. Rutherford backscattering spectroscopy and neutron depth profiling show that the density goes through a maximum as the average deposited energy per unit depth increases. X-ray photoelectron spectroscopy shows that nitrogen is mostly combined with carbon in triple (C≡N) and double (C=N) bonds. Positron annihilation spectroscopy shows that the void concentration in the films goes through a minimum with deposited energy. These results are consistent with a densification induced by the collisions at low deposited energy, and damage-induced graphitization at high deposited energy values.

Rectangular nanovoids in helium-implanted and thermally annealed MgO(100)

Cleaved MgO(100) single crystals were implanted with 30 keV 3He ions with doses varying from 1×1019 to 1×1020 m−2 and subsequently thermally annealed from 100 to 1100 °C. Transmission electron microscopy observations revealed the existence of sharply rectangular nanosize voids at a depth slightly shallower than the helium-implantation range. Monitoring of the defect depth profile and the retained amount of helium was performed by positron-beam analysis and neutron depth profiling, respectively.

Helium desorption from cavities induced by high energy 3He and 4He implantation in silicon

A detailed study has been made of helium release from silicon wafers implanted with MeV helium ions at fluences of 5 × 10 16 cm -2 and 10 17 cm -2 . Thermal desorption spectrometry (TDS), neutron depth profiling (NDP), non-Rutherford elastic backscattering (NREBS) and nuclear reaction analysis (NRA) have been employed to measure the helium content and release rate during isothermal annealing at annealing temperatures of 800 and 1000°C. TDS has also been used for isochronal annealing. Transmission electron microscopy (TEM) is used to monitor changes in morphology in the formed bubble layer. The helium release results can be modeled rather well when it is assumed that the helium initially is present in overpressurized bubbles. The present study reveals a single activation energy for helium release of 1.83 (0.05) eV.

Helium damage and helium effusion in fully stabilised zirconia

Abstract Fully stabilised zirconia (FSZ) samples have been implanted with helium-ions of different energies (200 keV and 1 MeV) and with different fluences (1.4×10 13 –1.4×10 16 He + /cm 2 ). Neutron depth profiling (NDP) for different annealing temperatures and effusion experiments in two different experimental systems with different thermal annealings have been performed on these samples. The samples were analysed by electron microscopy during the various annealing stages. For the low-fluence samples, the diffusion of helium is probably caused by vacancy assisted interstitial diffusion with an activation energy of 1.6 eV. In the highest fluence samples probably high pressure bubbles are formed during thermal annealing.

Lithium depth profiling in thin electrochromic WO3 films

Li profiles in sol-gel prepared electrochromic WO3 thin films have been characterized using three different techniques: neutron depth profiling (NDP), secondary ion mass spectrometry (SIMS), and elastic recoil detection (ERD). The results have been evaluated with respect to their concentration sensitivity and depth resolution. NDP and SIMS led to comparable results in terms of the shape of the profiles. The NDP results were quantified by using a calibration sample. NDP has the advantage that it is a quantitative nondestructive technique while SIMS has a very high sensitivity and depth resolution. Another advantage of NDP is that there is very little risk of neutron-induced Li migration during the measurement because of the very low energy of the incoming thermal neutrons. ERD was found to have a limited depth resolution, attributable to deterioration of the sample as a result of ion bombardment. However, the absolute amount of Li can be very accurately quantified with this technique.

Testing of a nuclear-reactor-based positron beam

Abstract This paper describes the testing of a positron beam which is primarily based on copper activation near the core of a nuclear reactor and extraction of the positrons through a beam guide tube. An out-of-core test with a 22 Na source and an in-core test with the reactor at reduced power have been performed. Both tests indicated a high reflectivity of moderated positrons at the tungsten surfaces of the moderation discs which enhanced the expected yield. Secondary electrons generated in the source materials during the in-core test caused electrical field distortions in the electrode system of the system by charging of the insulators. At 100 kW reactor power during one hour, positrons were observed with an intensity of 4.4 × 10 4 e + s −1 of which 90% was due to positrons created by pair formation and 10% by copper activation.

Annealing effects of helium implanted single crystals and polycrystalline magnesium aluminate spinel

Abstract The effects of transmutation produced α-particles and the thermal behaviour of the incorporated helium is simulated by 900 keV 3 He ion implantations at 2 μm depth in (100) surface oriented single crystals and in polycrystalline samples. Thermal Helium Desorption Spectrometry (THDS) and Neutron Depth Profiling (NDP) have been applied to study the release rate of helium and changes in the depth profile, respectively, during isothermal annealing. Release of the helium took place in at least two stages in a wide temperature interval ranging from 600 to 1600 K. For polycrystalline spinel an additional high temperature release stage at T > 1300 K was ascribed to trapping in pores. For the high dose implanted single crystals mainly helium release in bursts was observed due to thermally induced flaking.

Anormal growth of cavities in MeV He implanted Si covered with a thin Al foil

Abstract Cavities were created by MeV He implantation in silicon through a 1.5 μm Al foil. After a 800 °C-annealing, unexpected growth mechanism was found: a non-uniform layer of cavities with a distribution where bigger cavities are surrounded by smaller ones. Neither Oswald ripening nor migration-coalescence mechanisms can be applied to describe the growth of these cavities. The role of dislocations seem to be important in that distribution. Indeed some cavities seem to be trapped by dislocations. However the motion of the helium does not appear to be the origin of that anormal growth mechanism.

Evidence of very low-energy positron reflection off tungsten surfaces

Two different methods of measuring the reflection coefficient of positrons impinging on tungsten foils with incident energies below the positron workfunction for tungsten have been employed to study reflection. One involves using a 1 keV incident positron energy and monitoring the build-up in annihilation signal as a function of time after a grid in front of the sample is switched to a positive potential to return re-emitted positrons to the sample. The other method involves using very low-energy incident positrons (20 eV) and observing the dying out of the annihilation signal over time after the grid is switched to a potential in excess of +20 V to reflect re-emitted positrons and switch off the primary beam. A reflection coefficient of approximately 0.55 is found from the first experiment and a value of 0.58 is derived from the second experiment.

A new method to determine in situ the transmission of a neutron-guide system at a reactor source

In this paper, a description of a new method to determine the transmission of neutron guides after they are installed in a beam-tube at a reactor source is given. The method is based on activation measurements of gold foils at the entrance of the beam-tube and at the exit of the neutron guides compared to Monte-Carlo calculations. In this method, a quality factor is defined as the ratio between the actual transmission and the theoretical maximum attainable transmission. This method is used to determine the quality of an optimised neutron-guide system developed for beam-tube R2 of the HOR. The HOR is a pool-type nuclear research reactor at the Interfaculty Reactor Institute of the Delft University of Technology. It is shown that the quality factors of the newly installed neutron guides are between 0.49 and 0.63.