Development of an Intense Positron Beam lifetime spectrometer

Abstract

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.