A Brief Study of Ionising Radiation and the Mineral Collector – Part II
Written by John Cooke, Mineral Enthusiast and Part-Time Curator
Those people who read the previous article (Part I, published Sept 8th) may recall that my last action was to press the “Buy” button on an online platform. Well, a few days later my miniature Geiger counter arrived (A Radex 1503+ which was purchased based on the good reviews). Batteries installed and ready to go. Impressive sensitivity and easy to use and read with a display showing the values in microsieverts per hour (μsv/h). Anyway, keen to prove to my wife the necessity and wisdom of having a GM counter in the domestic environment, I set about devising experiments to demonstrate the value of the instrument (as against the definition of TOY that my wife is employing).
Firstly, I wanted to prove the inverse square law which states that doubling the distance resulted in a quarter of the radiation exposure. So, to this end I arranged the source (a torbenite from Shaba, Zaire) at a height of 15 cms above a wooden surface (i.e. a cardboard box on the dining table) and took readings at distances of 10, 20 and 30 cms away, with the results as follows:
|Distance||Reading in μsv/h|
So, doubling the distance should reduce the count by a factor of 4 and trebling by a factor of 9. All-in-all not too bad a result using my “kitchen sink” science.
The second experiment involved taking the same radioactive source, and still using the detector at a distance of 10 cms, introducing between these some household items to determine any reduction in detected activity. Results were as follows:
|Item introduced||Activity μsv/h|
|Piece of thin paper||6.53|
|Piece of kitchen towel||6.44|
|Piece of thin cardboard||4.72|
|4mm plastic breadboard||0.77|
|4mm glass fruit bowl||0.34|
There is a definite trend in reduction of activity but what does this all mean? The chosen items are merely to represent measures that might be taken to mitigate the effect of radioactive minerals within a collection. The paper and kitchen towel are “nesting” materials that may be used to support a mineral and the cardboard represents the cardboard tray in which the mineral might sit. The plastic may mimic the boxes often used for mineral display and the glass is a representation of the glass doors of a mineral display cabinet.
The resting background activity in this domestic environment is 0.15 μsv/h. So, by taking some simple precautions the amount of radioactivity exposure can be adequately managed.
We can combine the previous experiments so that the detector is placed outside the glass cabinet and activity recorded when increasing the distance within (i.e putting the specimen at the back).
|Distance from detector||Activity μsv/h||Behind glass μsv/h|
Taking 30 cms as the optimum distance which equates to placing the specimen at the back of the display cabinet it can be demonstrated that the activity is greatly reduced and that in this particular case, it is only marginally raised above background radiation.
What is the safe level of radioactivity within the mineral collecting community? There is no hard-and-fast rule and here I use the advice given by Price et al (2013). In their paper they use the best practice from various universities, of which Glasgow and Oxford are employed here. The approach used by Glasgow is to classify a mineral as radioactive if the surface measurement is between 1 and 7.5 μsv/h. Specimens recording activity above 7.5 μsv/h are classified as significantly radioactive. Oxford university measures radioactivity at a distance of 30 cms. Those registering an activity of between background and 2.5 μsv/h are designated as “low level radioactive” and those above 2.5 μsv/h are regarded as “high level radioactive”.
So, what about the specimens? After checking the activity of various torbenites and autunites (both Cornish and foreign) the specimens were moved to sets of drawers and particularly towards the back and within plastic boxes wherever possible. It was surprising to find several long-forgotten specimens lying in drawers, some of which were purchased in the 1960s, such as boltwoodite, carnotite and zeunerite from locations outside of the UK.
Nevertheless, there were exceptions. Whilst doing an external sweep of activity of my Victorian boxed mineral collections – two showed activity. The first is a collection of Cornish minerals from the mid-1850s which on closer internal inspection showed activity in two specimens. These were uranium ores and were then suitably entombed in lead (roof flashing) and this mitigated the activity. The second box, which is of German origin, was opened to reveal a specimen of pitchblende which gave a reading of greater than 10 μsv/h. A coffin of lead was constructed with a covering lid, only to find that the activity reading was now 4.22 μsv/h at 10 cms distance. A much larger reduction in activity was expected, so it was concluded to be emitting a degree of gamma radiation. The specimen and lead overcoat were consigned to the shed. Actually, in lead, then glass and in a drawer. The activity is still detectable but now within tolerable levels (1.41 μsv/h at about 20 cms). Its future is now the subject of debate! On closer inspection of the accompanying label it stated that it was from Joachimsthal, Bohemia (now Jáchymov, Czech Republic). A quick check on Wikipedia stated that this was the mine where Marie and Pierre Curie sourced their original material from which they isolated radium and polonium, hence gamma ray activity!
So, what has been learned? A Geiger counter is not a toy…Q.E.D. That the inverse square law is valid. That simple measures can mitigate against the presence of radioactivity. That the mineral collecting fraternity should be aware of the risks present within their home. There is still a place for kitchen-sink science. AND that a garden shed is particularly useful and not just a “man cave” as my wife suggests.
Price, M., Horak, J. and Faithfull, J. (2013). Identifying and managing radioactive geological specimens. Journal of Natural Science Collections, Volume 1, 27-33