Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 6. CHARACTERIZATION 33
Figure 6.6: Screenshot of the Diamond Station data acquisition software.
Finally, the calibration constant is C cal = 66:2 e ADC ,1 , and the noise gure is
= 211 e. With a diamond detector connected, the latter slightly increases due to
additional wiring to the order of = 270 e. VA2 channels which are not connected show
an ENC of approximately = 93 e. This gure comes close to the value stated by the
VA2 manufacturer. The reason for the excess noise of the input channel is the external
wiring. The placement and values of the elements in this circuit are critical and have been
optimized empirically, but they still add thermal and other noise and stray capacitance.
The pedestal mean value is subjected to a mid-term drift due to temperature variations,
while the calibration constant (or, gain) turned out to be quite stable. Therefore,
the pedestal has been taken before and after each measurement series, while the calibration
was done occasionally.
6.3 Fit Model
With a homogeneous detector material, a \perfect" Landau distributed pulse height spectrum
is expected. In practice, a small fraction of particles, due to misalignment and
scattering, traverse the trigger, but not the test detector, thus adding a small pedestal
contribution to the spectrum. The signal and pedestal parts are well separated with silicon
detectors. However, with diamond samples, especially those with low collection distance,
the two contributions cannot easily be distinguished. Fig. 6.8 shows two examples of pulse