Detectors of Recoil Ions

A powerful method of identifying recoil particles and distinguishing them from unwanted 'leaky' beam is to measure their kinetic energy. A good general-purpose detector for low-energy heavy ions is silicon, in the form of a thin reverse-biased semiconducting diode. Much of the energy loss by recoil ions stopping in silicon goes toward creation of electron-hole pairs. The applied bias causes the charges to separate, inducing a signal on front and back electrodes. The induced signal, suitably amplified and shaped, gives a measure of the ion's energy with a resolution of several percent and its time of arrival within 1-2 nanoseconds. If the metal electrodes are segmented into strips, the front strips being at right angles to the back strips, it is possible to deduce where the ion hit the detector to within 1 strip width in each direction.

Gas-filled ionization chambers can provide a good measurement of heavy ion energy, with additional information not available from a silicon counter. As the ion loses energy in the gas (typically isobutane), a fraction of that energy goes to ionization--formation of pairs of electrons and positively charged molecules of the gas. A transverse electric field causes the electrons to drift toward an anode and the ions to a cathode. Suitably amplified and shaped, the resulting electrical signal gives a good measure of particle energy (though the timing information is limited, due to the relatively long drift times). Ions of different atomic number have different profiles of energy-loss vs distance travelled in the gas: segmentation of the anode provides a powerful means of particle identification by comparison of initial rate of energy loss vs total energy of the ion.

Another method of distinguishing masses of recoil ions vs leaky beam is to measure the time-of-flight (t) over a certain distance, combining this with the kinetic energy (E) and calculating ET^2, which is proportional to mass (in the non-relativistic limit). A foil/MCP detector is able to measure times-of-flight with resolution of a few tenths of a nanosecond. When the heavy ion passes through an ultra-thin carbon foil, a few secondary electrons are emitted from the foil surface. By means of an electrostatic mirror, the low-energy electrons are accelerated and directed into a multichannel plate (MCP--an electron multiplier consisting of many fine capillary tubes). Short transit times for the electrons permit good time resolution. It is possible also to deduce where the recoil passed through the foil, from signal division at the MCP anode. A pair of foil/MCP detectors can measure time-of-flight, without undue additional energy straggling in the thin foils, before the heavy ion passes to an energy detector."