doctoral thesis

20 CHAPTER 1. INTRODUCTION
in which the particle tracks were photographed. Nevertheless, the technology of particle
detectors has with time evolved from these optical methods to electrical methods.
Scintillation counters coupled to photo-multipliers are a successful example. Starting
in the late nineteen-sixties high voltage operated gaseous detectors such as wire or
drift chambers [12] have successfully replaced the scintillation counter in experiments
requiring a high spatial resolution. Conversely the scintillator is still a very commonly
utilized technique to obtain high time resolution in current apparatus for sub nuclear
research.
1.3.1 Time Resolution
The wire based gaseous detectors are indeed not competitive with the scintillator as
far as time resolution is concerned. The reason lies in the fact that the distance of the
closest primary cluster to the wire is exponentially distributed. Due to the 1/r field the
amplification is limited to the region around the wire and all electrons need to drift into
this region before amplification and the signal generation set in. This introduces a time
jitter and limits the time resolution of wire based detectors to a few nanoseconds. A
better time resolution is achievable if a strong uniform electric field is used instead of
that of a charged wire. Here the avalanche amplification sets in instantly for all primary
clusters. The intrinsic detector time resolution is then dominated by the avalanche
statistics.
1.3.2 Spark Counter
The first gas detector taking advantage of the improved time resolution in strong uniform
electric fields was the Keuffel Spark Counter, a gaseous avalanche detector with
parallel plate geometry, that was introduced in 1948 [26, 27]. It indeed offered a time
resolution (around 1 ns) by far better than any of the Geiger-Müller Counters that were
commonly used at that time (around 100 ns) [28]. This development opened the possibility
for the construction of accurate timing systems to measure the velocity of fast
charged particles.
Spark Counters generally consist of two planar metal electrodes with a high voltage
applied to them. The gap between the plates is filled with a gas. The passage of a
charged particle leaves an trail of free charge carriers (primary ionization) in the gas
which triggers avalanches of charge carriers in the electric field. At a certain size of
the avalanches they transform into a streamer. A streamer is defined as a state where
photons contribute to the spread of free charge carriers. At a later stage a conducting
plasma filament connecting the two electrodes is formed. Through this channel the
electrodes are discharged; a spark is created. The rapidly growing anode current is
transformed by a resistor into a fast voltage signal and this signal can be taken as a