Geiger Counter Uses Cockroft-Walton Multiplier


The recent tsunami in
Japan and the on-going calamity with the Fukushima nuclear power plant
has apparently greatly increased sales of radiation meters, not only in
Japan but elsewhere around the world. This device will allow an
estimation of the level of radioactivity, being sensitive enough for
background radiation monitoring or to provide an estimation of the level
of radioactivity from sample objects such as Thorium gas mantles in LPG
lamps. The circuit is compatible with several Geiger Muller tubes and
three types of indication are provided: the good old-fashioned audible
click with each discharge, a flashing LED or an analog meter providing a rough average of radiation levels.

Circuit

Circuit diagram

A normal background count in New Zealand with the smaller GM LND712
tube is around 30 counts per minute, while the larger and more-sensitive
LND7312 pancake tube will count about four times this figure. Both GM
tubes will detect alpha, beta and gamma radiation. Unless the tube is
“filtered”, there is no way of knowing just what type of radiation is
being detected, although a rough guess can be made. Alpha particles will
be stopped by placing a sheet of paper between the tube and the source,
Beta particles (electrons) will be stopped with a few layers of
aluminium foil and the more lively Gamma rays will need a layer of lead.

The circuit provides a regulated 500V supply for the Geiger Muller
tube. This voltage places the tube into its linear operating mode so
that a discharge inside the tube will occur when a particle enters
through the mica window of the tube and causes the gas to ionise. The
very short pulse produced is stretched and used to signal that a
discharge has occurred. The power supply consists of an oscillator and
small transistor driving the 6V secondary of a 240VAC mains transformer.
The stepped up output of the transformer is fed to a Cockroft-Walton
voltage multiplier consisting of diodes D3-D7 and the associated 47nF
630V metallised polyester capacitors.

IC1 is a 40106 Schmitt trigger inverter and IC1a is connected as an
oscillator running at several hundred hertz. This is buffered by IC1b
and fed to the base of NPN transistor Q1 which
then drives the abovementioned transformer. IC1c acts as an error
amplifier to regulate the high voltage fed to the GM tube. A portion of
the DC voltage produced at the junction of diodes D4 & D5 is
monitored by a voltage divider consisting of the 4.7MO and 47kO
resistors, in combination with trimpot VR1. When the voltage from D5 is
below the positive threshold of IC1c, its output will be high and IC1a
will be able to oscillate. Hence, the oscillator will pulse on and off,
to maintain the 500V set by VR1.

Each time there is a discharge in the GM tube, the resultant current triggers the BT149 SCR
which discharges the associated 100nF capacitor and thereby acts as a
pulse stretcher to drive the three remaining inverters in IC1. These in
turn drive a high-brightness red LED (LED1), a
piezo transducer and an analog metering circuit which is based on an
old VU meter movement with a scale graduated in counts/minute. The
current drain of the circuit is 10mA and a small 9V battery should run
the counter for many hours. Warning: do not touch the window of the GM
tube. These are very fragile and made of very thin mica, to allow the
low-energy alpha particles to pass through. With the LND 712, 200 counts per minute is roughly equivalent to 0.3 micro-seiverts.


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