Diamond Detectors for Ionizing Radiation - HEPHY

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Chapter 6 Characterization An important issue for judging the quality of a detector is the measurement of its charge collection distance, therefore called characterization. In a laboratory environment, the detector under test is exposed to a source and the pulse height spectrum is recorded. Principally, it is the same measurement that is also made in test beams or detectors in experiments, although in the laboratory more emphasis is laid upon precise measurements, well-dened parameters, reproducibility and a clean analysis. 6.1 Characterization Setup The main elements of a characterization setup are the detector itself, a particle source and a front-end amplier as well as trigger and readout electronics. A few institutes participating in the RD42 collaboration have built such characterization stations. As an example, the setup at the HEPHY is shown schematically in g. 6.1. When a particle from Collimated Source 90 (e.g. Sr) Charge Sensitive Amplifier Detector Under Test Si Trigger Detector ADC Trigger Bias Voltages Fast Trigger Amplifier Figure 6.1: Schematics of the characterization station. the source traverses the detector under test and the trigger detector, a readout cycle is initiated, the amplier output is converted to digital numbers and stored in the computer. Apart from the detector itself, the involved parts will be discussed in the following sections. 26
CHAPTER 6. CHARACTERIZATION 27 6.1.1 Particle Source Penetrating particles are necessary for tracking detector measurements, either coming from an accelerator, a radioactive source or from cosmic radiation. The characterization of tracking detectors usually refers to minumum ionizing particles (MIPs), which transfer the least amount of energy possible (see section 4.1). The muons of cosmic radiation are not suitable for detector tests, since the sensitive area is usually very small (< 1cm 2 ), which results in a very low cosmic rate. Exact reference measurements require a well dened particle beam, which is only available from high energy accelerators such as the SPS (Super Proton Synchrotron) at CERN. However, for practical reasons, it is much easier to use a radioactive source such as 90 Sr, which delivers only beta electrons. In the setup shown in g. 6.1, the detector under test is located between the source and the trigger detector. Electrons with low energies thus stop in the test detector without triggering. This method requires a source with rather low activity, otherwise such a stopping electron and a subsequent (or previous) penetrating electron could overlap within the time constant of the amplier, leading to false signal pulse heights and pile-up eects. Assuming a 500 m thick diamond detector, electrons with a kinetic energy below roughly 0:5 MeV are absorbed. The penetrating particles are a good approximation to MIPs; in 500 m thick diamond they deposit 108% of the MIP energy [8]. Strontium decays in , mode to 90 Y, emitting electrons with a maximum energy of 0:55 MeV [13, 27], most of which do not penetrate the detector under test. The half-life of this rst decay is 28:8years. The 90 Y isotope again decays in the , mode with a half-life of 64:1 hours to the stable 90 Zr isotope; the maximum energy of the electrons is then 2:28 MeV. The practical result of this decay chain is in fact a 90 Y decay with a half-life of 28:8years rather than 64:1 hours [28]. The advantages of 90 Sr compared to other isotopes are the lack of decays, the relatively narrow energy spread and the long half-life, which results in almost constant activity over years. For most applications it is neccesary to collimate the source, as the electrons are emitted in all directions. Furthermore, the collimator has the task of protecting the person handling the source. Due to the low energy, the range of such electrons in solid matter is only a few millimeters, depending on the kind and amount of material in the particle track. For measurements where further penetration of the particles is essential, a high energy particle beam from an accelerator is essential. This may be the case when particle tracks are monitored with a telescope (see section 8.2) or the response of the detector to specied particles and energies is under investigation. However, these studies are usually described as test beam measurements and not as characterization. 6.1.2 Detector Ampliers After a particle has traversed the detector, a certain charge is induced in the electrodes. In the case of MIPs, this charge is of the order of several thousand electrons (and holes), which have to be amplied to a reasonable voltage (or current) level.
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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