|
Andrzej Wieloch Identification of the super-heavy nuclei produced in nuclear reactions is done by two main detection systems: - the velocity of heavy residue is obtained with time of light (ToF) detector
- while silicon (Si) implantation detector gives the particle kinetic energy together with its localization X-Y.
The heavy ion is implanted in Si detector on the depth ranging from 10-30 µm.
Next, the implantation detector registers subsequent spontaneous decays of the implanted heavy ion: alpha particles or fission fragments. For each particle from the chain the energy, position and time from the instant of the implantation is measured. Such a chain is the signature for the newly created element. One assumes unambiguous identification of the element if some of the alpha particles from the chain come from already known alpha radioactive isotopes. Unfortunately this is not the case for the last discovered elements: Z=112, 114, 116. Another important factor which influence proper chain identification is an experimental background. One assumes that the detected particle is an alpha if the signal from the position Si detector is in anti-coincidence with ToF detector. Such definition for alpha particle is proper when the efficiency of the ToF system is 100% (in that case only the decay of an implanted ion can be the source of signal in the Si detector). For lower efficiencies, this is usually the case, background emerges. This background is created by light particles which implants in the same position as the heavy ion. There are also transfer reactions producing nuclei lighter then the fusion nucleus. Such nuclei can be alpha radioactive giving thus, after implantation, additional decay chains which can be mislead with an appropriate ones. By installing in front of the implantation detector additional detector which can identify charge of the ion and have high efficiency for light particles detection will reduce the background significantly [Wie04].
|
