Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 6. CHARACTERIZATION 32
90 -
Collimated Sr β Source
Diamond
+HV
8.2MΩ
Si Trigger Diode
+45V
Collimator
3.3pF
10nF
100MΩ
VA2
Repeater
To
CAMAC
Module
100kΩ
100nF
50Ω line
50Ω
line
39Ω
10kΩ
50Ω
Calibration
Pulse
11Ω
Trigger
Amplifier
To
CAMAC
Module
Figure 6.5: Schematics of the Vienna characterization station.
6.2 Calibration and Noise Measurements
With a pedestal measurement, the overall noise performance of the characterization setup
can be determined. However, this is only given in ADC counts, as long as there is no
absolute calibration, which can be done through the injection of a known step pulse into
the VA2 input as described above.
To obtain an accurate calibration, it is essential to know the involved parameters,
in particular the exact capacitance value (C) and the voltage step size at the capacitor.
The capacitor has been measured with a Hewlett Packard 4285A precision LCR meter,
while the small voltage step cannot be measured directly with required precision. Thus,
the attenuation of the voltage dividers (r) has been measured with DC voltages much
higher than used in the calibration. The step pulse output of the CAMAC module can
be switched alternatively to both DC levels to precisely measure the voltage dierence
before attenuation (V ). The number of electrons injected into the VA2 is given by
N = Q e = CV : (6.4)
re
The rise time of the step pulse is indierent, as long as it is substantially shorter than the
integration time of the VA2, which is also the minimum length of the pulse.
Fig. 6.7 shows a measurement of both pedestal and calibration peaks in the pulse
height spectrum, tted with Gaussian distributions. In this case, the parameters were
C =3:37 pF, V = 227:6mV (terminated) and r = 1022, yielding an injected charge of
4691 e. From the histogram t parameters, a pedestal RMS of 3:192 ADC counts and a
peak location dierence of 70:9 ADC counts are obtained.