Letters to the Editor
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Dear Editor ,
The Mini 6-80 , which was depicted on the cover photograph of the recent edition of Radiation Protection Today , is an excellent instrument , although now an old design . NRPB used theirs as part of the response to Chornobyl in 1986 , and they were then bought in significant numbers by organisations such as local authorities who wanted the capacity to make measurements in their own area . This popularity was deserved and also gave those of us involved in emergency response a comforting sense that , in the event of an incident , we would be supplied with country-wide monitoring results from the same instrument type , making interpretation simple .
The instrument has two vulnerabilities however . The centre of gravity of the probe and tripod is rather high at the recommended 1 m ground-to-detector centre height . If it blows over , which does not require a particularly strong gust of wind , the resulting clonk when the probe hits the ground can crack the glass-to-metal anode tube seal in the GM detector , causing it to fail . This can either happen there and then or , more insidiously , it can fade away over weeks . With the original tripod , one way to pull the centre of gravity down and improve the gust resistance is to screw the hanger , supplied with the kit , onto the bottom of the tripod centre column and hang the scaler / timer ( in the air ) on this hanger , using the strap provided . The tripod pictured on the front of the magazine is newer , which doesn ' t allow use of the clip in this way , but the same principle applies . For these , the strap should be threaded through the radius arms and then the scaler / timer hung on it . The photograph on the cover of the magazine does not show this . The recommended height is 1 metre . At that height , with the original tripod , all three tripod leg sections can be extended , keeping the base wide and , again , reducing the chance of it blowing over when compared to extending only two sections and extending the centre column to get the same height .
The other weakness is the cable , which is a standard nuclear industry PET to PET type . If the scaler / timer is connected and left in the transport case and the lid blows shut , there ' s a significant chance the cable will be damaged . So the scaler / timer should be out of the case when the instrument is assembled for use . Hanging it on the tripod will achieve this . The cable used in the cover picture is also rather twisted . The PET connector is excellent for use in the laboratory – very good RF shielding – and it is also close to waterproof , but generations of instrument users ( and maintainers ) will testify that they would ( very much ) prefer a connector which is a lot more rugged . So it ' s important to keep any stress down to a minimum .
We have to remember that damage to the probe will stop the monitoring programme , which would be a really major problem in the event of an incident , and is also expensive . The picture shows the recommended way to use it with an original tripod .
Pete Burgess
This picture shows the correct use of the Mini 6-80 .
6 Radiation Protection Today www . srp-rpt . uk