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Widget Of The Week

Dosimeter Corp. MiniRad II Rad Monitor, 1977

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.

 

This MiniRad was part of a job lot of Geiger Counters bought a while ago on ebay. It’s hard to say how much it cost me but since it was the oldest and least sophisticated part of the package I estimate it accounted for around £10 of the overall price. I hadn’t seen one of these before and it was only when I tried to find out more about it that it became apparent just how old and unusual it was; more on that shortly.

 

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 (mSv/hr) SI units. It wouldn’t be too difficult to re-scale the meter to show true counts per minute or second but it’s always preferable to preserve vintage instruments in their original condition, so the lead foil will eventually be replaced.

 

What Happened To It?   

There’s very little on the web about the Dosimeter Corporation of Cincinnati, Ohio apart from some patents filed in the mid to late 1980s and references to the company’s apparent disappearance at some point in the nineties. By the way, the date of 1977 for this MiniRad II is based on markings on the PC board, and that’s backed up by the use of mainly discrete components on the circuit board, which are typical of the time. During what appears to have been a fairly short period of operation the Dosimeter Corporation made a variety of instruments, including a version of the classic US Civil Defense CDV-700 and passive pen-type dosimeters. It seems likely that the company was merged or taken over in the early to mid 90s, possibly by the Bendix Corporation’s, which had busy military and instrumentation divisions. But this is all guesswork so as usual feel free to put me right on the origins and history of this enigmatic company. Incidentally, the MiniRad name seems to have been appropriated by several companies, none of whom, as far as I can see, have any connection with the Dosimeter Corporation.

 

Geiger Counters and dosimeters are still with us and thanks to digital chippery and solid-state displays, are a good deal smaller, smarter and cheaper these days. I wouldn’t mind betting this one cost several hundred dollars when new but if the number of appearances of this model in Google Images is anything to go by (precisely none…) comparatively few were sold, or survived. Indeed this may be the only one. Clearly that makes it very rare and extremely valuable, at least it should. In the real world relatively few vintage gadgets become sought after collectibles, and it is even more unusual for specialised devices like this to make the cut, at least not ones made as recently the late seventies. Whilst collecting old Geiger counters is never going to make anyone wealthy it is -- at the moment -- a relatively cheap way to get into an interesting hobby. What’s more, those of us who do get involved may have the last laugh come the next nuclear conflict or radioactive incident, assuming of course we keep our collections, and ourselves, in bombproof shelters…     


DUSTY DATA

First Seen:                   1977?

Original Price:             £?

Value Today:               £30.00 (1219)

Features:                     Geiger Müller tube sensor, Gamma & X-Ray sensitivity, Hi/Lo range, moving coil meter display, scale 0 – 5mR/hr & 0 – 500 cpm, independent scale calibration presets, battery check function, built-in sounder, headphone socket, belt and lanyard clip

Power req:                     1 x 9 volt PP3

Dimensions:                   120 x 66 x 28mm

Weight:                          245g

Made (assembled) in:     USA

Hen's Teeth (10 rarest):   9



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