Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 5. DETECTOR MATERIAL COMPARISON 23
[mm]
Local dc
Distribution of CVD Diamond
- PRELIMINARY -
[mm]
[ADC counts]
Figure 5.2: Spatial distribution of the local charge collection distance. The histogram at the
bottom gives the distribution of the overall collected charge.
The velocities saturate with higher electric eld. As the target is to achieve the highest
possible charge collection eciency, one aims to apply an electric eld close to saturation.
On the other hand, high voltages are dicult to handle and there is the danger of electric
break-through. Thus, the usual eld strength for CVD diamond characterization has
become 1 V m ,1 , resulting in an applied voltage of several hundred Volts, depending on
the sample thickness.
In g. 5.4, the dependence of the charge collection distance on the applied electric
eld for a high-quality diamond is shown. Measurements with reverse polarity of the
electric eld show that there is no signicant asymmetry, thus there is no sign of longterm
polarization eects.
5.2 Si Detectors
Most solid state tracking detectors presently used are made of silicon, a material that
is easily available from the semiconductor industry and well understood. However, silicon
for detector application must be of higher quality and purity than the material for
semiconducting devices.
The intrinsic carrier density of silicon is too high to operate a silicon detector as-is.
This should be illustrated by a comparison [25] for a commonly used detector thickness of
300 m and an area of 1 cm 2 . The numberofintrinsic carrier pairs inside the bulk volume
is 4:35 10 8 , while one MIP traversing the detector generates a mean signal of only 32400