Spectrometers

 The example of LISE

The primary goal of LISE is to produce and select radioactive ions.

How radioactive ions are produced

Radioactive nuclei are produced from stable nuclei accelerated by the GANIL accelerator, which are fired onto a stationary target with a thickness of the order of one millimeter, made for example from carbon.

Nuclear reactions then occur, including fragmentation of the stable ion beam, leading to the generation of radioactive nuclei. Depending on the energy of the incident ions and the target's nature, the resulting radioactive fragments either still have a few electrons, or are totally stripped of their electrons.
 

This is carried out by different means:

• Using the property of magnetic fields to deviate charged particles. The greater the magnetic field and the particle's electric charge, or the smaller the particle's mass and velocity, the more strongly it is deviated. By selecting a given deviation, it is possible to select the ion to be ultimately studied. With LISE, two magnetic dipoles are used to create such a magnetic field.
• A piece of matter (the moderator) is interposed on the trajectory of the nuclei generated by the target. The nuclei pass through a given thickness of matter and are thus slowed down, or release some of their energy. The trick is to select the thickness of the piece of material, and then to adjust the magnetic fields such that, depending on each particle's trajectory and type, only those nuclei that release the correct amount of energy are selected.
• Finally, a device called a Wien filter, using both magnetic and electric fields, allows the ions to be separated according to their velocity.
   

View of a Wien filter in the LISE spectrometer: the device uses electric and magnetic fields to separate the generated ions according to their velocity.