Diamond Detectors for Ionizing Radiation - HEPHY

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CHAPTER 6. CHARACTERIZATION 28 The preamplier, which is physically connected to the detector, is the rst stage of the amplier chain. There are two principal congurations [10, 29], the feedback preamplier (or charge-sensitive amplier), which is slow but accurate, and the grounded base amplier, which is fast but contributes more noise. The performance of the preamplier is primarily determined by the noise gure, which is usually stated in terms of equivalent noise charge (ENC). As the electronic noise is approximately Gaussian distributed, the ENC states the RMS value of a noise charge source at the input of the (ctive) noise-free amplier. The ENC depends on the electronic parameters of the input stage and approximately linearly increases with load (detector) capacitance. Within the characterization station, both types of preampliers are used. A lownoise, slow charge-sensitive amplier is connected to the detector under test, while a fast grounded base amplier, connected to a silicon diode, is used as a trigger detector. 6.1.2.1 Charge-Sensitive Amplier The feedback preamplier basically relies on the current integrating capability of a capacitor, given by CV = Z Idt = Q : (6.1) There are various realisations of charge-sensitive ampliers in both discrete and integrated circuits. It is easily seen that the latter have a much better noise gure. As an example, I want to give some details of the VA2 chip, which was used for the characterization station at the HEPHY. The VA2 chip, produced by the IDE AS company [30], is a lower noise redesign of the original Viking chip [31, 32] for silicon strip detector readout. It has 128 equal input channels, one of which is connected to the detector under test. The schematics of the input stage of one channel is shown in g. 6.2, composed of the preamplier, which actually converts charge to voltage, and the shaper. The preamplier Figure 6.2: Schematics of one VA2 input stage, consisting of preamplier and shaper. integrates the input current, while resistor R 1 slowly discharges the integrating capacitor C 1 to avoid pile-up eects. The output of the preamplier is connected to a CR-RC shaper, which lters the preamplier output in order to minimize the noise. Both preamplier and shaper make use of operational transconductance ampliers (OTAs). The resistors
CHAPTER 6. CHARACTERIZATION 29 and amplier bias currents are adjustable to optimize the output signal with respect to the detector parameters. When a particle traverses the detector, a current pulse is injected into the detector with a duration of approximately one (diamond) or a few (silicon) nanoseconds. With respect to the system's time constants, this input current can be simplied in both cases without loss of accuracy to a Dirac delta pulse. The preamplier integrates this current pulse, resulting in a step pulse, while the discharging eect of resistor R 1 can be neglected comparing the time constants. This step pulse is now shaped by the CR-RC stage, which has the (Laplace domain) transfer function H(s) = V out V po sT p = A : (6.2) (1 + sT p ) 2 T p is the peaking time of the output signal, i.e., the time from the charge injection to the maximum of the output voltage and thus the point to sample. According to the bias settings, it can be adjusted in the range of 0:5 :::3s. The internal logic of the VA2 provides a sample/hold circuit and an output multiplexer, shifting out all 128 sampled values in an analog queue, which are digitized externally. Basically two reasons do not allow an on-chip ADC: the eects on the system noise and the additional power consumption (note that several thousands of such chips are utilized in a vertex detector in a comparatively compact volume at an operating temperature of slightly below 0 C). Due to the long integration time, the noise gure of this amplier is excellent. For the original Viking chip a value of approximately ENC 135 e + 12:3epF ,1 , slightly depending on the peaking time, is stated, while the noise of the VA2 redesign, which is optimized for lower load capacitance, could be reduced to ENC 82 e + 14 e pF ,1 . 6.1.2.2 Grounded Base Amplier A second particle-sensitive detector is necessary in order to trigger a readout cycle of the amplier connected to the detector under test. Often these are one or more scintillators connected to photomultiplier tubes. In our setup we decided to use a standard silicon detector connected to a very fast, discrete amplier described below. The major advantages of this trigger compared to a scintillator-photomultiplier combination are its compact size and the lack of high voltage, which would be essential for a photomultiplier. Apart from that, as both the trigger and the test detector are solid state detectors, they sense the same set of particles, i.e., only charged particles. Scintillators, however, are also sensitive to neutral particles. The trigger amplier utilized in the Vienna characterization station is a very fast, nonintegrating grounded base amplier [33]. This circuit, shown in g. 6.3, directly converts the input current to an output voltage, allowing to monitor the charge collection duration in various detector types. The preamplier makes use of low cost HF transistors (2SC4995), which have a transit frequency of f t = 11 GHz, a DC gain of h fe = 120 and a noise gure of a F = 1:1dB at f = 900 MHz. A monolithic amplier (INA-02186) giving a gain of 30 dB and a pass-band at to 1 GHz, is implemented after the preamplier, capable of driving a 50 line. In
Page 1 and 2: Diploma Thesis Diamond Detectors fo
Page 3 and 4: CONTENTS 3 7 Radiation Hardness 37
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Page 57 and 58: Acknowledgements First of all, I am
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Page 63 and 64: Bibliography [1] Austrian Academy o
Page 65: BIBLIOGRAPHY 65 [29] A. Hrisoho, Fr

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