Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 5. DETECTOR MATERIAL COMPARISON 22
Mean Signal [e -
]
10000
5000
Sample B
Pion beam
90
Sr source
Sample A
0
0
500 1000 1500 2000
Thickness after lapping [µm]
Figure 5.1: Charge collection distance vs. thickness remaining after lapping for two dierent
diamond samples. The solid line shows the prediction from a calculation including border limits.
that this is not true with CVD diamond samples. Signicant lateral variations of the
collection distance have been encountered on the scale of a few ten micrometers. On some
samples, clusters with higher or lower local d c than the average have been observed in
the sub-millimeter range, which may correspond to the grains. These eects are currently
under investigation. Fig. 5.2 shows a preliminary distribution plot of the charge collection
distance in 100 100 m 2 bins. Each bin contains approximately 120 hit entries and the
shade represents its mean collection distance. The white column to the right corresponds
to a dead readout channel. The histogram at the bottom shows the distribution of the
overall collected charge, which is not exactly Landau-shaped due to the inhomogeneity.
It is intended to achieve smaller binning and higher statistics in the future.
Whenever a charge collection distance value is stated within this work, it is meant
to be the average over a comparatively large volume of the diamond lm. The radiation
hardness studies in particular have been made on diamond samples with pad electrodes
covering an area of 2:5mm 2 and more.
Natural diamond has a charge collection distance of about 30 m. Starting in the early
1990s, the d c of CVD diamond was far below this value. From that time, the collection
distance was permanently improved by rening the manufacturer's growth process as
shown in g. 5.3. By the end of 1997, diamond detectors with a charge collection distance
of up to 250 m (corresponding to a mean signal of 9000 e) were available. Although those
detectors were rather thick (almost 1 mm), a recent sample shows d c = 230 m while it is
only 432 m thick, resulting in a charge collection eciency of 53%.
5.1.2 Collection Distance vs. Electric Field
As in all solid state detectors, the charge collection speed depends on the strength of the
electric drift eld. This behavior origins in the carrier drift velocities, which are a function
of the electric eld, approximated in the linear region by
v = E : (5.6)