Parameters of targets

Targets used in nuclear research in most cases are made of the enriched isotopes. They have various shapes and dimensions, thicknesses, physical (solid, liquid, gaseous) states and chemical (elemental or compound) forms. They can be self-supporting or on a backing, they can be irradiated inside the accelerator (so called internal targets), or at external set-ups placed on the beam lines.
The majority of the experiments in nuclear physics studies use thin solid state samples and thus most of the attention in the publications is given to the preparation of such targets.

Nuclear physics studies usually require targets with an areal density (i.e. thickness) of ~1 μg/cm2 up to 10-20 mg/cm2. Targets in the thickness range of a hundreds mg/cm2 are used for radioisotopes production with accelerators, mainly for medical application. The values given above are of general character only as final target parameters depend on experimental conditions.

The size of target in general is related to the kind of projectiles. Traditionally for beams of light projectiles that can be focused to a 2-3 mm spot the target diameter can be and often is below 1 cm and targets with diameter of 1 to 2 cm are used in experiments with heavy ion beams. To avoid the projectile interaction with the frame material the larger targets have to be used in studies with neutrons. Sometimes also targets for other than neutron experiments are produced as small spots placed at a distance from the frame (see photo-collage on the About target page) to avoid interaction of the reaction products with the frame material.

As mentioned above the parameters of targets depend on the experiment but there are some requirements which are common. Apart from having the requested thickness the target should in most cases have a relatively uniform thickness, good mechanical strength and stability under the beam, as well as high chemical purity (the isotopic purity mainly depends on the enrichment of the source material).