The Importance of the Time Scale in Radiation Detection Exemplified by Comparing Conventional and Avalanche Semiconductor Detectors

• Published in: Physica scripta 1976-02, Vol.13 (2), p.83-92
• Main Authors: Tove, P A, Cho, Z H, Huth, G C
• Format: Article
• Language: English
• ISSN: 1402-4896; 0031-8949; 1402-4896
• DOI: 10.1088/0031-8949/13/2/003

The profound importance of the time scale of a radiation detection process is discussed in an analysis of limitations in energy resolution and timing, with emphasis on semiconductor detectors used for x-ray detection. The "basic event detection time" involves stopping of the particle and creating a distribution of free electrons and holes containing all desired information (energy, time position) about the particle or quantum, in a time ≃ 10 -12 s. The process of extracting this information usually involves a much longer time because the signal is generated in the relatively slow process of charge collection, and further prolongation may be caused by signal processing required to depress noise for improving energy resolution. This is a common situation for conventional semiconductor detectors with external amplifiers where time constants of 10 -5 - 10 -4 s may be optimum, primarily because of amplifier noise. A different situation applies to the avalanche detector where internal amplification helps in suppressing noise without expanding the time scale of detection, resulting in an optimum time of 10 -9 - 10 -8 s. These two cases are illustrated by plotting energy resolution vs. time constant, for different magnitudes of the parallel and series type noise sources. The effects of the inherent energy spread (due to statistics) and spatial inhomogeneities are also discussed to illustrate the potential of these two approaches for energy and time determination. Two constructional approaches for avalanche detectors are briefly compared, viz. the deep Ga diffused and the Read-diode type design and their respective merits are pointed out. The similar use of internal gain in other types of detectors (gas ionization, scintillation counters) is compared. Although the noise analysis is done for nuclear radiation events, it applies also to the detection of bursts of optical photons such as are encountered in pulsed laser, radar, or high-speed digital optical communications.