The production of targets applied in the heavy-ion beam is a typical task of the scientific-technical infrastructure. Depending on the requirements of the experiment standard targets on standard frames are produced as well as new concepts are supported and developed in close collaboration with the other departments of the infrastructure and with the scientific staff from the first vague idea on.
The tasks are divided mainly into three different fields: the procurement of the materials, the production of the targets, and the characterization and the measurement of targets, degraders, windows, or the like.
1.Procurement of the materials:
We purchase the enriched isotopes for targets or as ion source material as well as special materials like for example Beryllium or very pure metal foils for windows and backings. Even in the preliminary stage of the experiments we are open for discussion and we pick up information for you concerning availability, price, delivery period, workability or typical impurities of the wanted material.
Every beginning of the year together with the purchasing department we call for tender of all the isotopes probably needed during the year. Then all year through the materials can be ordered from the best offer, when required. Therefore we kindly ask all experiments to inform us about the kind of isotopes definitely needed, together with the enrichment and quantity required for the beam time wanted. Additionally, we would like to know which isotopes are mentioned in proposals or are discussed in concrete plans for experiments. Each isotope that has to be ordered later separately takes longer to get and means a lot more work for us.
2. Production of the targets:
Thin films are produced by the deposition of material via thermal evaporation, via electron-gun evaporation, or via sputtering. The production of thick films is done by means of cold rolling, that of bulk targets by cutting, lapping, and polishing respectively.
We prepare layers over a wide range of thickness from several hundreds of nm up to several cm. We distinguish the targets from their different production processes.
- Bulk targets:
Thick films from thinner than a mm up to several cm are manufactured by separation, lapping and polishing. In this case, a high value is set on the plane parallelism of the faces and their polish quality. For a lot of materials a surface roughness better than 0,1 µm can be obtained. Typical examples are the targets at the fragment seperator (FRS) and the pion production targets at the HADES-experiment. Material in powder form as for example from most of the oxides can be pressed to pills with varying thickness and diameter.
- Rolled targets:
Thin layers produced by cold rolling are in the order of several mm up to approximately half a µm in thickness. These targets are successively rolled with a standard cold rolling mill in a sandwich process between two hardened stainless steel plates until the desired thickness is reached. There is also a cold rolling mill inside a glove box to work on very sensitive materials like Calcium, Uranium, or rare earth metals in an inert gas atmospher.
- Thin films:
Thin films, meaning layers thinner as about a µm are produced by evaporation or sputtering. If the targets are needed that thin that they cannot be worked on mechanically from the desired materials one deposits them on a soluble interlayer on glass plates or directly on metal foils. On dissolving the interlayer in a liquid solvent or etching the metal foil the film floats on the surface of the solvent or etchant and can be fetched with a frame.
Depending on the thickness, the material wanted, and the active area which is needed, it is sometimes impossible to produce the targets free-standing. In such a case, the material is evaporated on a suitable backing material as for example a thin film of carbon or aluminium, or the target foil is fetched on a supporting grid for stabilization.
3. Characterization and measurement of the targets:
Each target produced in the target laboratory is characterized with regard to its areal weight or its thickness, respectively. Thin films are characterized by weighing or indirectly by measuring the absorption of the layer with a UV-Vis-Spectrometer using a calibration curve, which is developed measuring the areal weight of a large number of samples and the corresponding absorption values for each material separately.
Thick films or bulk targets are also weighed. Additionally their thickness can be measured via mechanical sensors. For very accurate measurements, especially in cases where large areas have to be measured, an automatic Target Scanner is available.
If you have any further questions or remarks, please send an email to B.Kindler.