Phy433_Manual_ch5-6_rev09

Phy433_Manual_ch5-6_rev09 - 5 Photomultiplier Tubes and...

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5 Photomultiplier Tubes and Practical Considerations 5.1 High Voltage Photomultiplier Tubes may be run with either positive or negative high voltage power supplies. In either case, of course, the anode must be more positive than the cathode (electrons are negative). The bases with resistive dividers inside are built for one polarity or the other so one must know which it is. Most of the bases are labeled. If you don't know which it is; ask. Typical maximum voltages go up to about 2000 volts. Check the manufacturer's specifications for a particular tube. The current drawn depends on the resistive divider chain whose design depends on the rate of pulses expected. Typical divider chains draw from 1 to 3 mA. Warning : Most high voltage power supplies can deliver large currents: perhaps 10-20 mA or more and are therefore potentially lethal . Check personally that the power supply is off before handling tube. Obviously you should never remove or attach a base or a high voltage cable with power on. 5.1.1 Negative Voltage When a negative supply is used the cathode is at the high negative potential while the anode where the signal is developed is near ground. Usually a resistor (1 k perhaps) is placed between the anode and ground and the anode is connected directly to the output connector. In this configuration the electrostatic shield that surrounds the glass envelope on most tubes is at high voltage. Often this shield is connected to the high voltage through a big, (1 M or more) resistor for safety: but don't count on it. In this configuration since the tube envelope is at high voltage it is a poor idea to place metal in proximity to the tube, in particular the cathode region. Doing so could be dangerous but in any case may result in large noise pulses and breakdown due to leakage currents. Tubes should never be handled when the high voltage is on. 5.1.2 Positive Voltage
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Phy433_Manual_ch5-6_rev09 - 5 Photomultiplier Tubes and...

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