Nuclei containing more than 100 protons are often refered to as being 'superheavy'. These nuclei can be explored both in terms of their structure and, more interestingly, their mere existence. Until today no nuclei with a proton number in excess of Z=118 have been observed. The lack of experimental knowledge allows different theoretical models to thrive in this heavy mass region since there are no 'right' or 'wrong'. Experimental superheavy element (SHE) research aims to reveal the existence of an anticipated 'Island of Stability' - superheavy nuclei being particularly stable against nuclear decays. In fact, the predictions of the island date back to early days of nuclear structure theory. This is in particular interesting in connection with the development of the deformed shell model - named the Nilsson model in honour of Sven-Gösta Nilsson signing responsible for its development in Lund in the 1950's.
TASCA at GSI
At the European large-scale accelerator facility GSI, located just outside Darmstadt in Germany, a new gas-filled ion separator has been constructed. The separator has been given the name TASCA (transactinide separator and chemistry apparatus). It provides new opportunities to find and study the superheavy elements. In an extended focal plane, TASISpec places pixelized silicon detectors for particles and high-resolution germanium gamma-ray detectors around it in an extremely compact way. Implanted SHE can thus be investigated by means of their subsequent decay in a detection setup with unprecedentedly high efficient detection of gamma-rays and X-rays: Recent (2010-2011) simulations show that this setup exceeds 50% detection efficiency at around 150 keV for the gamma decay of the implanted nuclei. The alpha particles are also detected with a very high efficiency, around 80%.
The new sophisticated set-up, TASISpec (TASCA Small Image mode Spectroscopy), is described in this article. is led by the nuclear structure group in Lund with physicists and chemists involved from mainly GSI and the Universities of Mainz in Germany, Liverpool in the United Kingdom, and UNAL Bogota, Colombia.
At the time being, information on the nuclear structure of SHE nuclei has been derived mainly via studying the alpha-particles emitted as the SHE nuclei decay. This means that there still are question marks remaining regarding their true origin as the proton number has not been measured directly for any of the claimed new superheavy nuclei and, hence, new chemical elements 113-118. For even mass nuclei such decays involve almost exclusively alpha decays from ground state (mother) to ground state (daughter). For odd-mass nuclei on the other hand, studies of the decays should in principle involve also transitions between excited states, thus revealing parts of the nuclear shell structure and give hints on magic numbers at the upper end of the nuclidic chart. However, with very limited statistics and spectroscopic information, it is difficult to draw conclusions from the experimental data.
TASISpec has been granted eight weeks of beamtime for fingerprinting the superheavy nuclei (A=287, Z=115, and daughters) by means of the characteristic X-rays. This means that instead of identifying the superheavy nuclei indirectly via their subsequent decay (alpha particles) the nuclei will now be directly identified via the emitted X-rays. Such an experiment is very challenging and has therefore not been done before for the heaviest nuclei. Naturally, the success of this experiment lies in the hand of three main pre-requisites:
1. High production rates and long decay chains.
2. The population of excited states in the daughter nuclei, which will have to decay with highly converted electromagnetic transitions.
3. High detection efficiency of the K and L X-rays emitted in coincidence with the alpha particles and associated conversion electrons.
The TASISpec setup is hence ideal with the detection efficiency peaking around 150 keV (this is where K-conversion X-rays are expected for these nuclei) and still being transparent enough to detect the L-conversion X rays (at 30 keV). The experiment has been prepared in Lund and at GSI in 2011 and is ready to run at any time in hopefully the near future. Of course, both unique UNILAC beam characteristics, the opportunity for the production and use of actinide target material at Mainz and at GSI, respectively, and the TASCA separator are pre-requisites for such an endeavour!
The complete TASISpec silicon and germanium array has now been constructed virtually using the simulation package Geant4. It is described in this article. Great effort has been put into reproducing all the details of the setup to accurately reproduce reality and thus realistic TASISpec properties. The figure above shows some of the details such as all the detector PCBs, the canisters for the Ge-detectors, the HV fingers in the Ge crystals etc.