Nuclear Physics – Physics Target Laboratory

Introduction

For majority of nuclear physics experiments conducted at iThemba LABS, a particle beam from the separated sector cyclotron (SSC) impinged upon a target, i.e. a metallic foil (either self-supporting or on a backing) of an element that has close to 100% natural abundance. There has been increasing demand for thin targets (of the order of 100-1000 mg/cm2) of enriched isotopes, e.g. 6Li, 48Ca, 114Cd, 208Pb. Thin foils of isotopically enriched materials are not commercially available and must be made in a dedicated target laboratory facility available at iThemba LABS since 2005. Isotopically enriched targets are preferred in nuclear research since they provide the nuclides of interest only and these assist researchers during data analysis since there is a minimum interference by other impurities. The targets of single nuclides (isotope) are mostly used in studies for high–resolution nuclear spectroscopy and investigation of nuclear reactions. iThemba LABS is in the process of growing the existing facility to cater also for radioactive targets. For radioactive material, a separate vacuum evaporator is crucial in order to avoid unnecessary contamination to other target materials and also, with regards to radiation protection regulations, handling of such material has to take place in a monitored and controlled environment. At present, the target laboratory facility at iThemba LABS is the only available facility in South Africa that can cater for the requirements of researchers utilizing the particle beam from the SSC. Requested targets are either supplied in a solid metallic form (thin films), or liquid form (ice) maintained under certain conditions or gas form contained in a gas cell. All of these requests are determined by the requirements of a planned experiment and the researchers provide their target conditions to the target marker for manufacturing.

Example of a deuterated polystyrene target mounted on an AFRODITE frame (left picture and lead target mounted on a U-shaped K600 target frame (right picture)

There are various methods that are used to manufacture targets and these depends on various aspects before deciding on a method, i.e. purity or isotopic enrichment, size and the availability of the material. Other aspects that are looked at before making the targets are also: physical and chemical characteristics of the material and not forgetting the safety of the laboratory personnel during the manufacturing process. In physical characteristic, melting and freezing points of the material are considered, malleability, ductility and others when it comes to chemical properties, e.g. some materials oxidises readily when exposed to oxygen or moisture or in both conditions, or they react with certain gases or acids and bases. Under safety aspects there are those materials which are harmful and poisonous to the humans and the environment. Targets are currently manufactured using vacuum evaporation systems with different heating sources which are resistive and electron sources. For mechanical deformation of materials, a rolling mill is used. The target marker at times is required to develop new methods for targets that have not being prepared before and this forms part the research component of the laboratory.

The research is also conducted in collaboration with local and international universities/institutes. The key users of the targets are nuclear physics researchers from iThemba LABS, researchers and students from local universities. Targets are also supplied to international collaborators who uses the facilities of iThemba LABS and/or at their home institutions for research and trainings.

target
On the left side of the picture is a vacuum evaporation system and on right a thickness measurement device