Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 7. RADIATION HARDNESS 42
d c
Summary @ 1V/µm
Mean Charge per β - [e - ]
6000
5000
4000
A1
A2
B1
B2
C1
C2
D1
D2
E1
180
160
140
120
Collection Distance [µm]
100
3000
80
2000
60
40
1000
20
0
0 20 40 60 80 100 120 140 160 180
0
Total Fluence [E13 π + /cm 2 ]
Figure 7.4: The charge collection distance of various samples vs. pion uence.
By simple calculation, we can obtain the charge observed at the electrodes for a single
traversing pion if we know the beam induced current (I ind ), the pion ux ( ) and the
active area of the sample, which is bigger than the contact pad due to the fringe eld
(this will be discussed in detail in section 8.1). For the beam induced charge calculation,
we will refer to this equivalent area (A e ), obtaining the equation
Q c = I ind
A e
: (7.1)
Using eq. 7.1, we can correlate the measured current of each sample with the number
of electrons generated by a single traversing pion. It is very interesting to compare the
beam induced charge with the collection distance measured with the 90 Sr source at the
same bias voltage of 300 V. These two values should be identical for all uences, but
in fact they aren't. It turns out that the pion induced charge (pic) always exceeds the
electron induced charge (eic).
We dene the excess factor as the ratio pic=eic. Considering all samples, we observed
excess factor curves within the shaded area of g. 7.6. There are two components in
the development of the excess factor vs. uence. Easily seen at low uences, there is