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Homebrew Geiger Müller Counter

A different Approach - based on the Arduino™-Board can be found here.

Radiation

Radiation is a process in which a body emits energy that propagates through a medium,
to be reflected or absorbed by other bodies.

Among thermal radiation (e.g. from a heatsink) and others there is ionising radiation.

This ionising radiation may be divided into four categories.

	α-Radiation : Alpha (α) radiation consists of a fast moving helium-4 (4 He) nucleus and may be stopped by a sheet of paper.
	β-Radiation : Beta- (β-) radiation consists of an energetic electron. Beta+ (β+) radiation is the emission of positrons.
	γ-Radiation : Gamma (γ) radiation consists of photons with a frequency greater than 1019 Hz. They can be stopped by a sufficiently thick layer of material with high atomic number, such as lead or depleted uranium.
	Neutron-Radiation : High-energy (high-speed) neutrons have the ability to ionize atoms and are able to deeply penetrate materials. Neutrons are the only type of ionizing radiation that can make other objects, or material, radioactive.

Drawings courtesy of Wikipedia™

Now, as α-radiation is inoffensive, we must detect β and γ radiation.

This may be done with a (Geiger) Counter Tube. We used :

Geiger-Müller Tubes used

	ZP-1300 from DISTRELEC (CHF 150.-)  Datasheet   α, β Supply: 450-600V, Delay < 20µs
	ZP-1320 from DISTRELEC (CHF 114.-)  Datasheet Supply: 450-650V, Delay < 45µs

How does a Geiger (-Müller) Tube work ?

The picture below shows a Geiger-Müller Counter Tube.

Drawing courtesy of Wikipedia™

A Geiger–Müller tube consists of a tube filled with a low-pressure (~0.1 Atm) inert gas. (Helium, Neon, Argon,...). The GM-Tube has a coaxial shape, where the inner conductor (Anode) is biased with a high voltage (~500V) relative to the outer conductor (Cathode). The Tube has a window (left side) made of mica.

When ionizing radiation passes through the window, some of the gas molecules are ionized, creating positively charged ions, and electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles. This results in a short, intense pulse of current which passes from the negative electrode to the positive electrode and is measured or counted.

Concept

This GM-Counter is build on 2 PCB's. One is a standard high Voltage generating circuit, whilst the second is a Counter based on an ATMega16™ which also handles serial Communication with a host (Environmental Control).




The High Voltage generator is based on a 100 Hz Chopper, which is build around a '555' in combination with a standard Transformer and a Cascade to achieve Voltages from 400 to approx 900 V. (adjusteable) The Regulation is just on-off (Burst) which will result in approx 1% Drift. This Circuit consumes about 20 mA at a 9 V (Battery). (more when starting up :-)




This is the second board which is housing the Tube, the Controller and an LCD including a RS-232 interface which is transmitting the measured value. Protocol is 8 N 1. With a jumper it can be selected to count during a second or a minute. Power Consumption is approx. 30 mA
at 9 V.

Definition of Terms

Plateau

This is the (operating) Voltage, where the number of pulses is almost independent of the Supply Voltage.


(Maximum) Plateau Slope

This is the (maximum) change in % pulses per 100 V change of the operation voltage.


Maximum Background (shielded)

This is the number of pulses when the tube is shielded. Due to background radiation this number is not zero. A higher Sensitivity results in a higher background pulse rate.


(Minimum) Dead Time

This is the (minimum) time (in µs) which the tube needs to recover from an entered particle to go back into passive state.

Setting-up Operation

First of all, we have to decide, which tube we want to use. At the moment (April 2011), Availability is the main criteria ...

Let's say we are going to use the ZP-1320. From the Datasheet we know that the recommended operating voltage is 500 V. Apply + 9 V to the High-Voltage Board. Wait a moment. Connect a Voltmeter at the output and adjust to 500 V. Disconnect the Battery and wait till the High Voltage dropped to anything below 30 V.

From the Datasheet we also know, that the recommended Anode Resistor is 3.3 MΩ. Assemble 3.3 MΩ for R1 (Schematics).

to be continued ...

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