Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Alternative radioactive source for the diffusion cloud chamber

The diffusion cloud chambers sold by school science equipment suppliers in the 1960s and 70s used small radium paint sources of nominal activity ~ 1 kBq. These sources are no longer available and suitable new sources in the UK are not easy to obtain. This is a nuisance because diffusion cloud chambers are easy to use and have great educational benefit in allowing students to observe actual radiation tracks. 

To overcome this, a thoriated tungsten electrode can be used as a cloud chamber source. These are designed for TIG (tungsten inert gas) welding fabrication and commonly available from welding supplies shops. The type used in the test experiments was an SWP brand 2% thoriated tungsten electrode, type WT20, rod diameter 3.2 mm and length 150 mm, conforming to ISO 6848. The price of an electrode is about £3. The electrode has an identifying red colour tip. 
 
A new cloud chamber was purchased from Ideas in Education, and modified by drilling a 3.5 mm hole directly opposite the existing 3.5 mm hole designed for inserting sources. The electrode was put through the cloud chamber and corks/bungs placed firmly on each end to keep it in place. The electrode centre was about 7 mm above the chamber floor. 

cloud chamberThe felt was dampened with ethanol, and dry ice was placed in the lower chamber. The photograph shows the appearance of alpha tracks; they appear at a rate of about one or two every second. The alpha tracks also show evidence of collisions because some tracks show a sudden small change in direction. The alpha tracks vary in length, up to a maximum length of around 5 cm, because some alpha emissions come from just within the electrode surface and lose energy before emerging. 
 
The tracks from beta can be observed, but they are much fainter; low-energy beta emissions produce very irregular tracks. Another way to identify beta tracks is to take a number of photographs at 1 second intervals with a digital camera and flash, say 10 or so, and then download the images to a computer. Zoom in on the photographs and with luck you may be able to pick out some images that show beta tracks. The contrast may be better if the pictures are changed to greyscale. 

TrackPhotograph 2. The irregular track of a low energy beta can be seen moving right to left midway down from the electrode. The photograph has been digitally changed to greyscale and the brightness increased. 
 
To store electrodes, they can simply be removed from the cloud chambers and kept with other radioactive sources in a secure store. The electrodes are usually supplied in a handy plastic storage case that holds ten at a time. 

Electrode pack
 
Photograph 3. The electrodes are usually supplied in a handy storage pack. 

Safety

The equivalent dose rate at the surface of the electrode is very low, no more than a few microsieverts per hour, and the equivalent dose received during use as a cloud chamber source will be insignificant. In standard WT20 type TIG electrodes, thorium is evenly dispersed throughout the rod – during manufacture tungsten and thorium oxide powder are sintered into a metal alloy rod and the thorium is firmly bound in the metal. It is almost inconceivable that thorium could become released from the electrode, even if it were roughly handled. (Thorium is released in small quantities when grinding the electrode, or to a lesser extent during welding, neither of which happens when using the electrode as a cloud chamber source!) Purchase electrodes from a reputable supplier and make sure they are type WT20 with the thorium sintered homogeneously in the tungsten. 
 
Where permitted, it would be justifiable for responsible students under 16 to use the cloud chambers with this source fitted because the activity is low and the radiation risk is negligible. 
 
Thoriated tungsten electrodes are conditionally exempt from the Radioactive Substances Act 1993 by the Radioactive Substances (Uranium and Thorium) Exemption Order 1962. Buying and disposing of thoriated electrodes does not require registration or authorization by the regulatory authority for the environment. However, using thoriated electrodes does fall under the Ionising Radiations Regulations 1999, and although the risks are low, most employers will require that you to obtain their permission before you obtain and use the ‘sources’.