A new intense positron beam lifetime spectrometer is being developed at the Reactor Institute Delft that should be able to perform positron lifetime measurements with depth profiling. The positrons are generated by pair formation using high energy gamma-photons from the Hoger Onderwijs Reactor. This new positron lifetime spectrometer has a unique way to determine the point in time when a positron is injected into the sample material. Before the positrons are injected, they travel through a carbon foil which will then release secondary electrons. These secondary electrons are detected by a microchannel plate detector which starts a timer. Once the positron annihilates within the sample it releases two 511 keV photons. If one of these photons is detected, the timer is stopped, and the elapsed time is stored. Many measurements of the elapsed time are needed to create a positron lifetime spectrum.

The main research goal is to further develop this new positron lifetime spectrometer such that it is able to measure a positron lifetime spectrum. Multiple adaptations and tests were performed in order to achieve the research goal.

At the start of the project, the positron lifetime spectrometer could not deliver a thermalized positron beam from its 50 nm thick tungsten moderator. This problem was solved by reannealing this moderator. Additionally, a new moderator was prepared and annealed. The Variable Energy Positron (VEP) facility was used to perform characterization measurements on the moderators. This facility was modified to allow the transmission of positrons through the moderator and measure emitted thermalized positrons by the moderator. At the same time, a model based on the positron transport equation was developed to simulate and verify the characterization experiments. Both the old and new moderator have shown that they can emit thermalized positrons. The old moderator has a measured efficiency of 0.073 ± 0.002 when the positron implantation energy equals 3.7 ± 0.1 keV. The new moderator has a measured efficiency of 0.123 ± 0.002 when the positron implantation energy equals 3.2 ± 0.1 keV.

During the project, the microchannel plate detector which measures secondary electrons had to be tested, because no detections were observed in previous experiments. Additionally, the scintillation detector which is used to measure annihilation photons was tested. Eventually, both detectors have showed that they work correctly.

Finally, the positron beam of the lifetime spectrometer was aligned such that the positrons travel through the carbon foil onto the target. Secondary electrons generated by positrons have been measured by the microchannel plate detector. Afterwards, the first positron lifetime spectrum was measured using this new instrument. This lifetime spectrum is still rather crude as it shows multiple peaks and a time resolution function with a full width half maximum of 0.57 ns. The research goal of this project has been achieved, but further development is needed in order to accurately measure positron lifetimes.

The behaviour of ODS 12Cr steel under thermal treatment is studied in this report at micro and macro level. ODS 12Cr steel is always in the ferrite phase and has a melting point at 1500 ºC. Before studying,
the retrieved samples were metallographically prepared in four steps: mounting, grinding, mechanically polishing and electrolytically polishing. A measurement was done to analyse the effect of electrolytically polishing on bulk S and W parameters. This showed that mechanically polishing with 0.04 μm alumina particles has the same effect as electrolytically polishing. Next, the alloy was studied with three different measurement techniques: positron annihilation Doppler broadening spectroscopy, Vickers hardness test and positron lifetime measurements. Before these measurements the samples were annealed at selected temperatures for 10 minutes and naturally cooled.

The Doppler broadening spectroscopy was done with the Variable Energy Positron beam (VEP) at the Reactor Institute Delft (RID). With this set-up the S and W parameter were measured at different im-
plantation energies and converging bulk values of them were determined. The results were fitted with VEPFIT. This measurement was done in the as received state and after annealing from 200 ºC up to
1300 ºC in steps of 100 ºC. The Vickers hardness test was done with a load of 0.3 kgf and in the same temperature range as the VEP measurements but with steps of 200 ºC. A lifetime spectrum was measured in the as received state with two sample sandwiching a 22Na source packed with kapton. The average lifetime was determined by fitting the spectrum with LT 9.2. This measurement was repeated
after annealing for 10 minutes from 200 ºC up to 700 ºC in steps of 100 ºC.

The three different measurement techniques show corresponding results. Up to annealing temperatures of 1000 ºC the results stay constant. The bulk S and W parameters are respectively 0.475 and 0.078.
The Vickers hardness stays at 395 HV and is comparable to literature values. The average lifetime is approximately 208 ps, which corresponds with defects with sizes comparable to or at least larger than
that of divacancies. At this temperatures most thermal vacancies are trapped by the nano-oxide particles and are not able to liberate themselves which causes the constant results. At higher temperatures (larger than 1100 ºC) the thermal vacancy concentration increases and the vacancies gain enough energy to be liberated. This causes, in combination with oxide particles clustering, more defects in the material and explains the increasing bulk S value for temperatures up from 1100 ºC. The high bulk S values, compared with an iron alloy with less chromium, can also originate from clustering of chromium atoms. The surface S parameters are also increased in this temperature domain due to the formation of an oxide layer which can be seen with the naked-eye. Finally, the hardness value decreases to 374 HV after annealing at 1200 ºC which is a significant decrease and indicates that the pinning of the nano-particles decreases, resulting in grains growing easier.