RBS Instrumentation: Spectroscopy
Alpha particles come from the accelerator and strike the sample surface. Energies must be measured for those few particles that recoil back into the detector. Surface barrier silicon detectors are used in RBS. Since these devices are essentially diodes, they are often called semiconductor diode detectors. The high energy charged particles produce electron-hole pairs in the semiconducting material. The detector is operated with an electrical potential (typically 4 kV) between the front and back surfaces. In the resulting electric field, the electron-hole pairs produce a current proportional to the energy of the charged particle.
The average energy expended by He++ to produce one electron-hole pair is approximately 3.7 eV. This is sometimes called the ionization energy of the detector. Each 1 MeV particle produces about 2700 electron-hole pairs. The fluctuation or variance in the number of charge carriers affects the spectroscopic resolution. The theoretical minimum variance (which follows Poisson statistics) is equal to the number of charge carriers. The standard deviation equals the square root of the variance. The Fano factor is the ratio of the observed to this theoretical minimum variance. The Fano factor implicates other sources of peak broadening, typically incomplete charge collection and variations in dead layer loss.
Incomplete charge collection is minimized by high purity semiconductors which provide relatively few sites for electron-hole pair recombination. The energy lost before the charged particle reaches the active volume of the detector (dead layer loss) is minimal because this layer is thin (about 100 nanometer) in surface barrier detectors. Since this thickness corresponds to only about 0.4% energy loss for 1 MeV He++, small variations in the energy loss are insignificant for typical RBS experiments. High quality silicon surface barrier detectors are thus nearly ideal for alpha particle spectroscopy.
Particle arrival times at the detector are randomly spaced in time, leading to the possibility of interference between measurements when particles arrive at nearly the same time. This phenomenon, called pulse pile-up, becomes a serious problem at high particle arrival rates. There are two types of pile-up. Tail pile-up involves the superposition of pulses on the long duration tail or undershoot from a preceding pulse, leading to reduced spectral resolution. High quality electronic circuits minimize tail pile-up. Two pulses sufficiently close together to be treated as a single pulse undergo peak pile-up, the second type. Detector dead time is the minimum time between successive ion arrivals if they are to be measured separately. Peak pile-up ultimately limits the rate at which RBS data collection can occur.