◆ The Geiger Counter
As noted in the section on plasmas, gases conduct electricity only
when some of their atoms are ionized, i.e. are split up into free electrons and ions.
Fast electrons and ions emitted by radioactive materials do ionize atoms
with which they collide, and Hans Geiger, an associate of Rutherford (ions, history)
used this property to invent a sensitive detector for such particles.
◆ A Geiger Counter
A "Geiger counter" usually contains a metal tube with a thin metal wire along its middle,
the space in between them sealed off and filled with a suitable gas,
and with the wire at about +1000 volts relative to the tube.
Here is a 700 volt power supply for powering a Geiger tube or charging a capacitor in a flash unit.
The circuit uses a 1:1 telephone isolation transformer typically used in modems.
The voltage is determined by the string of devices including the neon bulbs.
Select a combination of neon lamps, varistors or zeners to achieve the desired voltage.
Zeners are available at high voltages but neon lamps are probably easier to obtain.
The circuit shows the power supply in a typical Geiger counter circuit.
Geiger tubes draw about 100uA when they pulse but the pulses are very short and relatively far apart
so the supply need only source a few micro-amps to the output capacitor.
Geiger tubes are available in a variety of sizes and styles including units
with thin mylar windows in the end for detecting larger particles besides x-rays.
This circuit is nice because it cuts back current consumption to less than 1 ma when the load current is low.
It works well from either 9 or 12 volts but it will supply more current with higher supply voltages.
◆ High Voltage Generator for Geiger Tubes
Here is a 700 volt or less power supply for powering a small Geiger tube or other very low current device.
The circuit is very efficient when no current is being consumed which is typically the case in a Geiger counter.
Geiger tubes draw about 100 uA when they pulse but the pulses are very short and relatively far apart (hopefully).
The current drain from the 9 volt battery is less than 1 mA with no load.
The circuit will supply a bit more current when operated from 12 volts.
The output voltage is set by the string of devices that includes the neon bulbs.
Select a combination of neon lamps, varistors, zeners, etc to achieve the desired voltage.
Good results may be had by selecting one or two ordinary small-signal silicon diodes
with the desired breakdown voltage.
The current in the diode will be quite low and no damage will result.
The circuit shows the power supply in a typical Geiger counter circuit
but it may be desirable to use a larger resistor
from the high voltage to the tube - see the manufacturer's recommendations.
A 10 megohm is a safe value for most tubes.
This generator cannot supply much current.
An ordinary 10 megohm voltmeter will pull the output voltage quite low;
a very high impedance voltmeter is needed to directly measure the output voltage.
A 1000 megohm resistor in series with a 10 megohm voltmeter will give a 100 to 1
voltage reduction and will not overload the circuit.
Large Geiger tubes may also draw too much current in moderate x-ray fields
so a more robust generator may be needed. (See the 3V version below.)
참고 : 사용한 GMT는 하마마츠 옵토닉스의 D3372 입니다.
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