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hw3part2 - 20.309 Biological Instrumentation and...

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1 20.309: Biological Instrumentation and Measurement Laboratory Fall 2006 Homework Set 3.5 Sensitive optoelectronic detectors: seeing single photons Due by 12:00 noon (in class) on Tuesday, Nov. 7, 2006. This is another hybrid lab/homework; please see Section 3.4 for what you need to turn in. Contents 1 Objectives 1 2 Background 2 2.1 Photomultiplier Tube (PMT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 Noise Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.1 Photon Shot Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.2 Electron Shot Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2.3 Johnson Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 Experimental Procedures 4 3.1 Hardware set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3 Experiment Roadmap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.4 Data Analysis and “Deliverables” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Objectives 1. Understand the principles of operation of photomultiplier tubes (PMTs). 2. Build signal conditioning electronics to capture and detect the optical signal generated by a photomultiplier tube. 3. Observe single-photon events with the detector. 4. Understand some of the noise characteristics of the PMT-circuit system as affected by light level and gain. 1
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20.309: Biological Instrumentation and Measurement Laboratory Fall 2006 2 Background 2.1 Photomultiplier Tube (PMT) For low light-level detection and measurement, you can’t beat the photomultiplier. This clever device allows a photon to eject an electron from a light-sensitive alkali metal photocath- ode. The photomultiplier then amplifies this feeble photocurrent by using a high voltage to accelerate the electron onto successive surfaces (dynodes), from which a cascade of additional electrons is easily generated (Figure 1). This use of “electron multiplication” yields extremely low-noise amplification of the initial photocurrent signal. The final current is col- lected by the anode, usually run near ground potential. Figure 1: Principle of PMT operation: a high neg- ative voltage applied to the photocathode acceler- A PMT is a linear device in the sense that the current output is proportional to the ates electrons down the dynode chain. light power incident on the photocathode. The PMT we use in this lab is a Hamamatsu R7400P, which has a photocathode sensitivity of 60 mA/W (recall that a photon’s energy depends on its wavelength our LED’s light is approx. 565nm). The gain of the dynode chain depends on the applied accelerating voltage. The overall anode sensitivity is the product of the photocathode sensitivity and dynode chain gain (Table 1 provides a summary, and the PMT data sheet has more detail). PMT voltage (V) dynode chain gain total sensitivity (A/W) 500 3 × 10 4 1 . 8 × 10 4 800 10 6 6 × 10 4 Table 1: PMT gains at two operating voltages. Due to its high sensitivity, a PMT can be used to observe individual photoelectron events. At low light levels, this is typically done by following the PMT with charge-integrating pulse amplifiers, discriminators and counters. In this lab, we will simply observe them visually on an oscilloscope.
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