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Almost everyone coming into contact with
radioactivity on a day-to-day basis can usually spot a radiation detection or
monitoring instrument at 50 paces. Typically it’s the distinctive shape, size,
colour, or the fact that it’s dotted with radioactivity symbols, ticks or
beeps, that gives the game away. But every so often one comes along that could
be almost anything. At first glance the MiniRad II Radiation Monitor looks like
a simple electrical test meter, maybe a guitar tuner or even some sort of
photographic gadget. The name should be a clue but even that is quite discrete,
and you would have to be very close to read what the meter is measuring, which
for the record is counts per minute and mR/h (millirems per hour). 
This is a compact instrument, designed to be
clipped to the user’s belt or worn around the neck on a lanyard, and whilst
‘dosimeter’ is printed on the front panel (actually the US manufacturer’s logo
– Dosimeter Corporation) it’s basically a simple Geiger counter. The meter
needle moves and it ticks in the presence of radioactivity, giving the user an
approximate indication of the strength of the radiation field they are being
exposed to. More up to date dosimeters generally record radiation dose. In
other words they keep a record of how much radioactivity the wearer has been,
or is being exposed to, over a preset period – 12 or 24 hours say. More
advanced models may also sound an alarm when what is deemed to be a safe dose
is at or close to being exceeded.
Inside the MiniRad’s metal case is a tiny DCA
5310 Geiger Müller (GM) tube, situated just behind the meter. This is the
widget that detects radioactivity and white crosses on the outside of the case
helpfully indicate its position and orientation, for making close-up
measurements of radioactive sources. This GM tube is configured to be sensitive
only to Gamma and X-Ray radiation, which, in sufficiently strong doses, are
amongst the most penetrative and hazardous types of radioactivity, able to pass
through most materials, including flesh and bones, and on the way through
crashing into living cells, potentially doing serious damage. They are only
stopped or impeded by really solid, dense and heavy stuff like metal, rock and
concrete. All types of radioactivity have problems with lead and the tube in
this instrument is wrapped in a thin layer of lead foil. This technique, known
in the trade as energy compensation, is designed to shield the tube from other,
lower energy types of radioactivity, which can skew readings as dose
measurement is normally mostly concerned with heavy-hitting Gamma and X-Ray
radiation.
So much for the theory. Thankfully operating of
the MiniRad II is very straightforward. There are two rocker switches. The one
on the left is for selecting high or low range, and if you see the needle move
on the high range, you need to step away from the source, quickly! The right
hand switch is for battery test and switching the unit on. On the top panel
there’s a hole for the sounder or ‘clicker’ and a socket for a set of
headphones (which mutes the sounder). On the right side of the case there are
two presets for calibrating the dual meter scales. The lower part of the case
is the battery compartment. This houses a 9-volt PP3 type battery that with
normal use will last for several days. Around the back there’s a spring metal
belt clip and a loop for a lanyard. The all-metal case is around the size of a
pack of 20 cigarettes and really tough, suggesting it’s was designed to be used
in harsh environments, such as mines, building sites and so on.
It was sold as working and outwardly the
condition was surprisingly good with only a few very minor scuff marks on the
case. Inside was no different and it did indeed work, but sensitivity was
quite poor. The reason for that turned out to be the energy-compensating
shielding around the tube and as an experiment I carefully removed it. The
increase in sensitivity was dramatic, up by around 50 to 60 percent, making it
a useable little instrument, responsive to normal background radiation –
clicking once or twice a second -- and genuinely handy for basic detection,
prospecting and monitoring. Unfortunately removing the shield means the meter
scales are all wrong. Not that it matters much for everyday use, but in any case
mR/hr readings are pretty much obsolete these days, having been replaced by
more meaningful microsieverts per hour (