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Trap-assisted conversion-electron spectroscopy


Conversion-electron measurements are an excellent method of determining the multipolarity of low-energy transitions of exotic nuclei. Comparisons of K-to-L electron, electron-to-gamma or electron-to-X-ray ratios allow conversion coefficients to be determined. As conversion coefficients generally vary by around on order of magnitude between one multipolarity and the next measurements containing low statistics can be used. Determining the multipolarity of a transition is essential when building reliable level schemes, which can then be compared to model predictions.

The LSPC Grenoble possess a Si(Li) conversion-electron detector, which is liquid-nitrogen cooled. This detector is segmented in two, to allow electron-electron coincidences, has an area of 2x6 cm2 and is windowless, allowing electrons below 10 keV in energy to be measured. The detector is also only 4 mm thick and so is relatively transparent to gamma rays, has an excellent energy resolution ( 2.5 keV) and as it contains no electromagnetic focusing, it can work in areas containing a stray magnetic field.

Due to the multiple peaks detected in a conversion-electron measurement it is difficult to perform beta-conversion-electron measurements with standard mass-separated beams. By combining the LPSC conversion-electron detector with a trap-purified beam and a tape system, to remove longer-lived activities, such measurements will be possible. The head of the detector is also thin enough to allow Ge detectors to be placed in close proximity to the beam, allowing beta-gamma or beta-gamma-electron measurements to be performed in the same experiment. Such a setup will allow conversion coefficients for transitions from very exotic nuclei to be measured down to low energy with low-intensity beams.

Figure: Conversion-electron setup to be used at DESIR

Contact: Gary Simpson, LPSC Grenoble

 

 

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