Resolving nuclear reaction products at solenoid focal spot

For studies of astrophysical interest and nuclear reactions with radioactive nuclei, charged particle products emitted from nuclear reactions such as ⁷Li + ⁹Be are routinely produced, separated and focused onto a focal spot by ion focusing separators such as a superconducting solenoid. The separation and focusing of desired ion groups onto the focal plane is however not complete, being also accompanied by unwanted ion groups. The mass- and energy-resolving but also the spatial focusing power of these separators are also limited and often spread out. In order to evaluate and determine and minimize these effects we decided to analyze the spectral and spatial distributions of ion reaction products at a superconducting solenoid. For this purpose we use the position-sensitive quantum-counting pixel detector Timepix which provides energy or time sensitivity per pixel. This device resolves different ion groups and measures the spectral and spatial spread distributions. Single ions are detected and individual ion groups can be identified and distinguished by multi-parameter event-by-event and pattern recognition analysis of the characteristic ion signal response in the pixelated detector. We present the capabilities of the technique on the indicated reaction above which is studied at the radioactive ion beam facility RIBRAS in Brazil. Several ion groups arriving at the focal plane are established such as ^7,8Li, ^4,6He and ³H together with their spatial and spectral distributions. The energy- and spatial-spread of the solenoid resolving power at the focal plane can be thus evaluated with a resolution of tens of keV and few μm, respectively. The detector serves moreover as an online verification and calibration imager for the ion optics separator and beam focusing monitoring. Results are presented for measurements with a 24 MeV ⁷Li beam and a thin ⁹Be target with the solenoid tuned for select- on and focusing of produced radioactive ⁸Li.