Education

Structure determination of oil/water emulsions by small-angle neutron scattering

The section NPM2 focuses on the innovative and complementary use of neutrons and positrons in a broad area of fields relevant to health and energy. Neutron scattering accesses the microscopic and mesoscopic length. The development of new experimental methods for neutrons and positrons is one of the traditional fields of excellence of NPM2. The section has a long-standing experience in Larmor labelling, which led to the development of SESANS.

Oil in water emulsions are crucial in many industrial processes, as in oil and food industry. In this project relevant samples are made, and process conditions will be simulated to measure the effect of surfactants, salt, temperature or shear on the microstructure.

Contact person and project supervisor: dr. Wim Bouwman; email: W.G.Bouwman@tudelft.nl 

Positron Annihilation Lifetime Spectroscopy (PALS) is a unique method for identifying defects and their concentrations in materials. These defects, ranging from atomic vacancies to nanovoids, have a significant influence on the properties of many materials, but are usually too small to be made visible (e.g. with electron microscopy).  Positrons are highly sensitive probes for  open-volume defects because the  lacking of repulsive coulomb interaction due to the missing atom(s) creates an attractive potential  for the positively charged positron.  The ultimate fate of  a positron in a material is  annihilation with one of the abundant electrons. During the annihilation process both the electron and the positron “disappear” and their total energy (mass and kinetic) is converted into two photons with a characteristic energy of 511 keV.  Detection of such an annihilation photon thus marks the end of a positron’s existence. If the moment when the  positron enters the material is registered too, its lifetime can be determined.  This lifetime depends on the annihilation rate (i.e. the inverse of the positron lifetime) which is lower for trapped positrons because of the locally reduced electron density. By measuring the lifetimes of many (typically >10⁶)  positrons a so-called positron lifetime spectrum is obtained from which defect concentrations and defect sizes can be derived.  When using  ²²Na isotope  as positron source the moment of positron injection is obtained  through the detection of the simultaneously emitted  1.27 MeV photon. Unfortunately, ²²Na shows a broad  positron kinetic energy distribution,  and therefore the depth at which a positron has become  trapped and annihilated is unknown. This lack of depth resolution can be solved by a process called moderation through which the  broad energy distribution is converted in to nearly single energy. Since this process has a low efficiency a strong primary source of positrons is required.  At the 2MW research reactor facility at the Reactor Institute Delft we have met this requirement by producing an intense beam of positrons with well-defined energy through a process known as pair-production utilizing the high energy gammas near the core of the reactor. Since this method does not a supply suitable “start” gamma, as is the case when using ²²Na, a new method for determining the arrival of a positron at the sample surface is being  developed. This method is based on the detection of secondary electrons that are released when a positron hits and penetrates the surface of a material. The present project status is that a beam energy tunable positrons is efficiently transported and focused onto a sample (in the form of a micro channel plate) after having passed a thin carbon foil used for generation of secondary electrons. The next step (which suits very well for a  Masters End Project) is to set-up and test the detection system for these electrons and to correlate the obtained time signals with the  accompanying annihilation photons.

Contact person and project supervisor: dr. Henk Schut; email: h.schut@tudelft.nl