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It is tempting, but technically incorrect, to call this Nuclear
Enterprises RM5/1 a Geiger Counter. It’s not but confusingly, that is what the
one in the photo has become because it is connected to a probe (similar vintage to the RM5/1) that uses a
Geiger Müller tube to detect radioactivity. Don’t worry it’s just another one
of the many quirks and foibles of the nuclear radiation business. The RM5/1 is
actually a Ratemeter and what makes it that, rather than a plain old Geiger
Counter, is down to several useful features. 
The first one is the socket on the front of the case. It’s for the
a probe, which can be one of several types, including the Geiger probe shown –
as well as other types, which we will come to in a moment.
This combination of features makes probe switching a really quick
and simple operation. It greatly increases the instruments flexibility,
especially in the field, which helped to make the RM5/1 a popular choice in
research, industry and medicine throughout the 80s and 90s. Radiation probes
are typically designed to be responsive to one particular or a range of
different types of ionising radiation. The three basic flavours are Alpha, Beta
and Gamma and X-Ray (the last two are closely related) – there are other
varieties but they are beyond the scope of the RM5/1. 
Time for quick and dirty guide to common probe types. On paper
Alpha radiation appears to be quite weak as it doesn’t travel more than a few
centimetres through the air and can be blocked by a thin sheet of paper. This
makes it quite difficult to detect, especially if you are not looking for it,
or unaware that it is there. Although Alpha radiation cannot penetrate skin to
any significant depth, it can do terrible damage to cells and organs if it gets
inside the body, as the Litvinenko Polonium poisoning case in 2006 tragically
proved. The most common type of Alpha sensitive probe is a modified Geiger
Müller tube, sometimes called a ‘pancake’ (they tend to be flat and round),
with a thin ‘window’ at one end made of the mineral Mica. This allows the Alpha
particles to pass through, but prevents gasses inside from leaking out.
General-purpose probes – like the 1257 C in the photo, have Geiger Müller tubes
made of glass or metal. They cannot detect Alpha radiation but they are
sensitive to Beta, Gamma and X-Ray radiation. This particular probe also has a
sliding metal shield that covers the tube. The idea is it blocks Beta radiation
so it can discriminate between the two types. The third class of probe is a
Scintillation detector. Inside the probe there’s a detector ‘crystal’ (made of
organic compounds or a special plastic). It emits weak flashes of light when
exposed to Gamma radiation. The crystal is mounted on the face of a
photomultiplier tube, which amplifies the flashes to produce the pulses that a
ratemeter like the RM5/1 can measures or display on its meter. Incidentally,
Scintillation probes can also be used to identify radioactive isotopes from the
brightness and duration of the flashes, though this requires rather more
sophisticated equipment.
Back now to a roundup of the rest of the RM5/1’s more notable
features, starting with the meter. It is in unusual in that the moving pointer
or needle travels through 270 degrees, which means the scale is significantly longer
than most ordinary panel meters, which typically have 90-degree scales. The
numbers on the scale, showing counts per second (CPS), increase
logarithmically. This eliminates the need for a range switch and it can show
readings, with reasonable accuracy, from just a few CPS right up to a rather
scary sounding 5000 CPS. The upper end of the scale is mainly for the benefit
of pancake and scintillation probes, which can be super-sensitive and might
easily wrap the meter pointer around the end stop on instruments without a
range switch or log-scale. It has a built-in ‘clicker’, heard through a small
loudspeaker mounted on the front of the case. This can be muted from a switch
to the left of the main function knob. It is powered by a pair of 1.5-volt D
cells that should last weeks or months, even with regular use, and these live
under the carry handle, which is held in place by four screws.
I have had this RM5/1 for several years, a swapsie for another
instrument with a fellow rad enthusiast. At the time they were regarded as
quite exotic and rarely seen outside of the usual industrial and scientific
settings. It was in pretty good shape, in good working order and all it needed
to make it presentable for this piece was a circuit check and quick wipe over.
The RM5/1 first appeared in the early 1980s and even back then it was a fairly
basic design. It uses only discrete components, there are no microchips and the
large circuit board has wire-wrapped connections and a super neat, laced wiring
loom. It is the kind of hand-assembled construction that harks back to an
earlier age where money was no object and ruggedness and reliability were the
prime considerations. And that is how it should be, considering that it was
designed to be used in safety critical applications. The lower part of the case
is the RM5/1’s only weakness. The plastic is a bit thin around the corners,
which makes it prone to cracking if it is dropped or roughly handled roughly.