ionised inside the EBIS. Neon used as cooling gas inside the REXTRAP.
| Homepage and schedule | Motivation | Charge breeding system | Linear accelerator | Target and Miniball | Collaboration |
As the intensity of radioactive ions out of the EBIS is much smaller than the intensity of residual gas ions, a mass separator is required. Due to the potential depression of the electron beam inside the EBIS the extracted have a large energy spread, which limits the q/A-resolution of an ordinary magnetic separator system. Instead a mass separator of so-called Nier-spectrometer type is used, consisting of an electrostatic 90° cylinder deflector and a 90° magnetic bender arranged in a vertical S-shape. With the electrostatic deflector the ions are separated according to their energies irrespective of their masses. In the energy focal plane the energetic width of the ion beam can be restricted. The correct charge-to-mass ratio is selected in the focal plane of the bending magnet. With the given design the energy dispersion in the magnet is compensated by the electrostatic bender.
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| Figure 1. Peaks of charge-bred
potassium separated from residual gases ionised inside the EBIS. Neon used as cooling gas inside the REXTRAP. |
The calculated q/A-resolution of the separator for an extraction emittance of the EBIS of ~3 π mm mrad is about 760. Higher emittances will decrease the resolution because of larger entrance angles of the ions and higher order optic effects of the electrostatic deflector. A q/A-resolution of ~150 has been achieved.
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| Figure 2. The principle of a Nier spectrometer with the definition for mass resolution |
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| Figure 3. The achromatic mass separator of REX-ISOLDE with its different elements |
Design and construction of the
separator was done by the LMU of Munich.
Residual gas isotopes in the charge breeder having similar or the same A/q values as the ion of interest cannot be eliminated by the REX mass separator and thus have to be considered further. A mass scan at the exit of REXEBIS shows that the principal contaminants are:
14N, 15N, 16O, 18O, 20Ne, 22Ne, 12C, 13C, 36Ar, 40Ar
Using the Java-Applet below this text, the user can enter the mass of the isotope of interest and obtain a qualitative graph comparing the possible A/q ratios in a given range (default: 3.0-5.0) to the ones of the principal contaminants. It is possible to specify a different range by entering the A/q values in the boxes on the lower left (and hitting enter), or by zooming in using the mouse. In order to decrease contamination of the beam with the separator it is recommended not to use A/q values falling on top of a contaminant and to avoid A/q values falling in a bin just next to a contaminant in the graph. It is possible to add new contaminants in the histogram by just entering their A values and hitting the "Contaminant" Button.
Using the applet below, the user can determine the A values that could be present in a mass spectrum by giving the A/q value measured in the experiment plus the absolute precision on the measurement. The applet is an application that multiplies the given A/q value by the possible integer charge states q and checks whether the obtained A values are in the range of the given precision. It then provides the list of all possible A values and their corresponding charge states.