Single and Multichannel Detector (SCD) / (MCD)
Figure 15: Detector Sweep, Count rate vs. Voltage
The pulse output depends largely on the applied voltage and, in practice the gain is an
increasing function of the applied voltage until the gain reaches about 10
7
: after which
point increasing the voltage further will cause the eventual breakdown of the CEM.
(With a properly configured oscilloscope, i.e. impedance 50 Ohm, the necessary pulse
height can be checked, usually between 2-10mV, see section 9.1.2 , "Amplifier Check"
on page 76.)
For the PHOIBOS analyzer, an input of one electron to the CEM produces an output
pulse that contains at least 107 electrons and lasts for approximately 10 nanoseconds.
3.5.3.1 Extended CEM
The standard detector systems of the PHOIBOS analyzer series are based on the exten-
ded range CEM. This device is a specially formed and treated glass tube which has the
effect of multiplying a single electron at the input to a pulse of around 10
8
electrons at
the output. Because of the low resistance (about 50 ΜΩ) the extended range CEM’s are
suitable for extremely high count rates.
A voltage between 2.5 - 3.5 kV across the cone and the tail-end initiates the electron
multiplication. Electron multiplication is produced by the emissive layer along the inner
PHOIBOS 29
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