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The Orange Spectrometer at the IKP

Conversion electron spectroscopy at the IKP

Copper coil body of the BIG ORANGE
Copper coil body of the BIG ORANGE

Conversion electron spectroscopy is possible at the IKP via two iron-free Orange-spectrometers. The BIG ORANGE spectrometer is installed in a fixed position at a beam line of the FN-tandem accelerator. For coincidence measurements it is possible to couple it with the SMALL ORANGE spectrometer (Double-Orange-spectrometer). Alternatively a small detector array can be installed, on which, for example, (shielded) HPGe-detectors or LaBr3 scintillation detector can be mounted. This flexible and unique setup, in combination with the capabilities of the 10MV FN-tandem accelerator, makes many interesting experiments possible. The instrument is ideal for conversion electron spectroscopy (especially interesting for E0 transitions and highly converted low energy transitions), with an energy resolution of the BIG ORANGE of about 1%. It is also a very efficient and exact instrument for fast timing experiments, allowing for electron-electron and electron-gamma timing with pulsed or continuous beam or with activated targets.
Especially heavy nuclei (rare earth region and heavier) are good candidates to be studied with this spectrometer as conversion becomes more likely with increasing Z.

Working principle

An Orange-spectrometer is a magnetic spectrometer. It makes use of the momentum-dependent dispersion of charged particles, in this case electrons, in a magnetic field. The field is created by an electric current, applied to a cylinder symmetric, toroidal coil. The coil segments are arranged equiangularly, like the segments of an orange; hence the name.

Copper coil body of the BIG ORANGE
Orange electron path

Mono energetic electrons, coming from the target, are refocused to a detector at the other end of the spectrometer. The "focus energy" is defined by the applied current. An energy spectrum is obtained by changing the current in small steps and, for each step, counting the electrons that arrive at the focus. This is done using a plastic scintillation detector.

The energy resolution of the BIG ORANGE is variable between 1-2% via an entrance ring slit at the counting detector. The absolute efficiency of the BIG ORANGE lies between 15-25% respectively.
The electron energies accessible with the BIG ORANGE spectrometer range from about 20 keV to about 2 MeV. The lower limit lies somewhat higher in in-beam experiments with a continuous beam. Delta-electrons emitted due to beam-target interaction, have a continuous energy distribution which increases exponentially towards low energies. This means background at low energies is massive. In in-beam experiments the lower limit typically lies around 50 keV.

Fast timing

The Double-Orange-spectrometer is ideal for fast timing experiments. Due to a lead-shield between target and detector, no Compton events contribute to the background, which is a huge advantage over fast-timing with gamma-rays, especially when studying low intensity transitions at low energies. The delta-electron background is random and disappears in coincidence measurements. Another advantage is the 1% energy resolution, which is superior to that of scintillation detectors like BaF2 and LaBr3:Ce.
Experiments using electron-gamma timing, combining the BIG ORANGE with LaBr3:Ce scintillation detectors, were very successful.

Articles about the IKP Orange-spectrometer

  • Sub-nanosecond lifetime measurements using the Double Orange Spectrometer at the cologne 10 MV Tandem accelerator, Regis et.al., NIM-A 606, 466 DOI: 10.1016/j.nima.2009.04.008
  • Improvement of the intrinsic time resolving power of the Cologne iron-free orange type electron spectrometers, Regis et.al., Rev.Sci.Instrum. 81, 113505 (2010) DOI: 10.1063/1.3499259
  • Lifetime of the first excited 2+ state in 172W and 178W, M. Rudigier et al., Nucl. Phys. A847, 89 (2010) DOI: 10.1016/j.nuclphysa.2010.07.003