This experiment was first explained by Austen and Brouwer (1997). It can be used to show how the air has naturally low levels of radioactive material in it, and that this decays over time. It is also a demonstration that our environment has low levels of radioactivity, mainly due to the gas Radon, to which we are exposed continuously.
Inflate a rubber balloon, clamp the neck with a clip and suspend it somewhere in the room, better in the cellar or in the garage. It does not have to be high up, but you tend to get better results if the balloon is hung in a place away from draughts. Rub the balloon vigorously for a few moments with woollen gloves until the balloon is friction charged. Leave the balloon for about 30 minutes, as in the picture aside.
Set up a counter with the GM tube. We used the LND 712 (α,β,γ) with a Theremino Geiger Adapter and Theremino Geiger Counter software. We put the GM tube inside a lead-shielded well in order to decrease the background level.
Take care as the GM tube end-window is fragile and easily broken by carelessness. Take the background count for about 1 minute. Put on a pair of disposable gloves and take down the balloon. Deflate it by removing the clip, and put it inside the lead shielded well. Avoid the balloon sagging and touching the GM tube.
Take a count for 1 minute to find the average count rate over the minute. The result can be quite astonishing: from less than 0.1 μSv/h, the activity ramp up to about 10 μSv/h, one hundred time the background level ! If time allows, take 1 minute counts continuously and plot the average count rate against time on a graph. In the picture below we show the result after about 24 hours count.
The cause of the radioactive contamination of the balloon
The radioactivity arises from the decay products of radon. Radon comes from the decay chains of naturally occurring uranium and thorium in the environment. Measurements by Austen and Brouwer revealed that most of the radioactivity was due to Pb214 and Bi214, both Rn222 (radon) progeny, and from Pb212 and Bi212, from the progeny of Rn220 (Thoron). Pb214 has a half-life of 26.8 minutes, Bi214 19.7 minutes and Pb212 10.6 hours while Bi212 61 minutes.
If you record the count over 1 minute at intervals without disturbing the balloon and GM tube equipment, you should obtain a decreasing count rate. The initial decay is roughly exponential. The initial decay curve, mainly from the decay of Pb214 and Bi214, gives an average ‘half-life’ of about 50 minutes. If there is sufficient activity on the balloon such that there is a measurable count rate from it after 24 hours, the decay of the Rn220 progeny Pb212 and Bi212 predominate to give an average half-life of about 11 hours, as shown in the graphs below.
When you have finished, discard the balloon in the waste bin, wipe down the measurement well and then wash your hands.
The software generates a log file with all the activity data. These data can be uploaded in a spreadsheet software like excel which allows the production of the following graphs :
Using Excel’s ‘Add Trendline’ facility and choosing the exponential trendline option, the exponential equation of best fit can be displayed on the graph (Figures below). The half-life can be determined from the exponent of base e. The half-life is ln(2)/−0.015, which is 46.2 minutes in first part of the decay curve (Pb214 + Bi214 decay) and ln(2)/0.0009, which is 12.8 hours in the curve tail (Pb212 + Bi212 decay). These numbers are in good agreement with the true decay constants.
Gamma Spectroscopy of the balloon …
If you have a gamma spectrometer you can make a spectrometric analysis of the “contaminated” balloon in order to verify which are the isotopes present on it surface. After having deflated the balloon, this is inserted in a plastic bag in order to avoid contamination of the sensor. We have used the DIY gamma spectrometer already described in the following post : DIY Gamma Spectrometry with the software Theremino MCA.
The results are presented in the graphs shown below. The first graph has been obtained without compensation for the lines widening, while in the second the compensation algorithm has been applied to obtain a higher resolution and to facilitate identification of the peaks.
From the charts it is evident the presence of the isotopes Pb214 e Bi214, furthermore the strong peak at 240 KeV is a prof of the presence of both isotope Pb214 and isotope Pb212, if it was present only the isotope Pb214 the peak would be much lower.
The use of the rubber balloon allows, as described above, the making of numerous experimental demonstrations with high educational value, such as :
- Radon detecting;
- Evidence of nuclear decay;
- Half-life measurement;
- Decay chain isotopes identification;