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Since the late 1980s many of the portable instruments designed to
detect, measure and warn against ionising radiation have morphed into small and
usually rather dull-looking little boxes. Arguably that’s no bad thing, but in
the olden days of the Cold War, when the threat of nuclear annihilation was
part of daily life, Geiger Counters, survey meters, dosimeters and the like
were typically chunky, hairy-arsed pieces of kit. They looked like they meant
business, with lots of knobs and switches and like cockroaches, probably able
to survive a nearby detonation, even if the humans using them had been blown to
smithereens... 
The Nuclear Enterprises PDR3 featured here appeared in the early
80s. It’s a no-nonsense brick-sized, canary-yellow coloured lump. There are no
fills and its sole purpose is to display radiation dose rate, a measure of how
much radioactivity the user is being exposed to over time, in milliRads per
hour or as an integrated spot measurement in Rads per hour. Either way, the
PDR3 is designed to respond to relatively high, and therefore quite dangerous
levels of Gamma (and X-Ray) radiation. If the needle ever moves you know it’s
time to be somewhere else. In other words it’s not one your everyday radiation
detectors; it’s the kind of thing that would have been a familiar sight in
places and situations where exposure to high levels of radiation was a real
possibility, in nuclear power stations, laboratories, industry, hospitals and
so on.
A small device called a Geiger Müller tube handles the job of
detecting gamma and X-Ray radiation. It’s deceptively simple, a sealed metal
cylinder, filled with a mixture of exotic gasses at low pressure. Metal
shielding around the GM tube shields or ‘compensates’ for normal background
radiation that can skew readings. An electrode inside the tube is held at a
high voltage (around 450 volts in this case) and when a radioactive particle
penetrates the shielding and tube, it ionises gas molecules resulting in a
small but measurable electrical discharge. The unit’s internal electronic
circuitry converts the pulses into a voltage, displayed by a moving pointer the
meter set into the top panel. 
If the radiation level exceeds pre-set values (0.5, 2 and 4 rads)
it triggers an audible alarm and the LED above the meter starts to flash. The
latter also flashes when the battery (2 x 1.5 volt D cells) are about to expire
(battery life is around 100 hours). There‘s also a battery check function on the
3-way on/off toggle switch on the left side of the top panel. Incidentally,
this switch has a clever locking action and it has to be pulled upwards before
it can be moved. It’s a bit like the reverse gear interlock on cars with manual
gearboxes. The red tipped Dose switch on the right side has both locking and
momentary (spring loaded) actions, ensuring that once a reading has been taken
it returns to continuous Dose Rate measurement mode. If all this sounds a bit
of a palaver, bear in mind that if the PDR3 was being used in anger, possibly
in a noisy or smoke filled environment, the operator will almost certainly be
wearing protective clothing, with a mask and breathing apparatus and thick,
heavy gloves, so the instrument needs clear audible and visual indicators and
an On switch that can’t be accidentally switched to the Off position.
The PDR2 is one of several Nuclear Enterprises instruments in my
collection and it was a swapsie with a fellow enthusiast in exchange for a Cold
War era Geiger counter. I can’t be sure who the original owner was, but it had
been very well looked after with almost no signs of use. It is really well made
too with, proper wiring looms, a tough metal chassis and what looks like
bulletproof circuitry. It also appears to be in good working order. I was able
to check the ratemeter and alarm circuitry using a pulse generator, to simulate
the output from a GM tube. The tube itself is also fairly easy to test by
hooking it up to a highly sensitive instrument and seeing if it reacts to a
low-level check source, which it does. To really put it through its paces,
though, enough to get the needle twitching and the alarm sounding would require
exposing it to a very spicy radioactive source, and that’s not something I hope
will happen anything soon.