Cloud Chambers and Cool Dry Ice
A workshop overseen by Gary Williams at the IOP
In this workshop we had a go at making a large cloud chamber and looked at safe radioactive sources.
We were given information on obtaining and using dry ice.
About dry ice
Dry ice is solid carbon dioxide which sublimes at -78.4ºC (changes directly from a solid to a gas)
Dry ice must always be stored in an area which is well ventilated (it’s an asphyxiant) and preferably not below ground (because it’s denser than air).
It needs to be out of direct sunlight and sources of heat (so it lasts longer)
It must be kept in a secure place to prevent unauthorised access.
IT MUST NOT BE handled with bare hands as it can cause severe cold burns and frostbite.
IT MUST NOT BE PLACED IN A WORKING REFRIGERATOR OR FREEZER
REMEMBER – A little bit of dry ice will sublime to a large volume of CO2 gas.
See CLEAPSS General Handbook Section 11.2
Make a Risk Assessment
Managing Ionising Radiations and Radioactive Substances in Schools, etc L93 http://www.cleapss.org.uk/download/L93.pdf
Radiation Protection in School Science: Guidance for Employers Information for local authorities (PS46A)
When asked who discovered radioactivity most people (including me initially, Helen Hare) would answer Henri Becquerel, but this not in fact completely true.
Radioactivity was in fact discovered in the year 1857 by Claude Felix Abel Niepce de Saint Victor (26 July 1805, Saint-Cyr, Saône-et-Loire – 7 April 1870) who was a French photographic inventor.
In 1804 the German chemist Adolph Ferdinand Gehlen (1775-1815) had noticed that when a solution of uranium chloride in ether was exposed to sunlight, it quickly changed colour (from bright yellow to green) and precipitated. In the 1850s, Niépce de Saint-Victor was trying to develop colour photography, using light-sensitive metal salts. Beginning in 1857, long before Henri Becquerel’s famous serendipitous discovery of radioactivity in 1896, Niépce de Saint-Victor observed that, even in complete darkness, certain salts could expose photographic emulsions. He soon realized that uranium salts were responsible for this anomalous phenomenon. Niépce recognized that the “light” that was exposing his photographic plates was neither conventional phosphorescence nor fluorescence: the salts could expose photographic plates long after the salts had last been exposed to sunlight. Niépce’s superior, Michel Eugène Chevreul, recognized the phenomenon as a fundamental discovery (“une découverte capitale”), pointing out that uranium salts retained their power to expose photographic plates even after six months in the dark (“encore actif six mois après son insolation”). By 1861, Niépce stated frankly that uranium salts emitted some sort of radiation that was invisible to the human eye.
On March 1, 1896 Henri Becquerel (Re-) discovers Radioactivity
Antoine Henri Becquerel (15 December 1852 – 25 August 1908) was a French physicist, Nobel laureate, and the discoverer of radioactivity along with Marie Skłodowska-Curie and Pierre Curie,
Describing them to the French Academy of Sciences on February 24, 1896, Becquerel said:
“One wraps a Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative … One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts”
But further experiments led him to doubt and then abandon this hypothesis:
“I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts [of potassium uranyl sulphate], arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images. Here is how I was led to make this observation: among the preceding experiments, some had been prepared on Wednesday the 26th and Thursday the 27th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt. Since the sun did not come out in the following days, I developed the photographic plates on the 1st of March, expecting to find the images very weak. Instead the silhouettes appeared with great intensity … One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies. However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion. I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena.
The Nobel Prize in Physics 1927 was divided equally between Arthur Holly Compton “for his discovery of the effect named after him” and Charles Thomson Rees Wilson “for his method of making the paths of electrically charged particles visible by condensation of vapour”.
Charles Thomson Rees Wilson, CH, FRS (14 February 1869 – 15 November 1959) was a Scottish physicist and meteorologist.
Wilson is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre while working on the summit of Ben Nevis in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centres for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911. In Wilson’s original chamber the air inside the sealed device was saturated with water vapour, then a diaphragm is used to expand the air inside the chamber (adiabatic expansion). This cools the air and water vapour starts to condense. When an ionizing particle passes through the chamber, water vapour condenses on the resulting ions and the trail of the particle is visible in the vapour cloud. Wilson, along with Arthur Compton, received the Nobel Prize in Physics in 1927 for his work on the cloud chamber. This kind of chamber is also called a Pulsed Chamber, because the conditions for operation are not continuously maintained.
More Nobel Prizes
The Nobel Prize in Physics 1936 was divided equally between Victor Franz Hess and Carl David Anderson “for his discovery of the positron” which he photographed in a cloud chamber.
Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist.
Cloud chamber photograph of the first positron ever observed
The Nobel Prize in Physics 1948 was awarded to Patrick M.S. Blackett “for his development of the Wilson cloud chamber method, and his discoveries therewith in the fields of nuclear physics and cosmic radiation”.
Patrick Maynard Stuart Blackett, Baron Blackett OM CH FRS (18 November 1897 – 13 July 1974) was an English experimental physicist known for his work on cloud chambers, cosmic rays, and paleomagnetism.
In 1932, working with Giuseppe Occhialini, Blackett devised a system of geiger counters which only took photographs when a cosmic ray particle traversed the chamber. They found 500 tracks of high energy cosmic ray particles in 700 automatic exposures. In 1933, Blackett discovered fourteen tracks which confirmed the existence of the positron and revealed the now instantly recognisable opposing spiral traces of positron/electron pair production. This work and that on annihilation radiation made him one of the first and leading experts on anti-matter.
The following link gives you instructions for using a ready-made diffusion cloud chamber.
Making a Cloud Chamber
You will need:
A fish tank; A baking tray; Scissors; Saw or Stanley knife; Snips; Tape; Metre stick; Access to roll of felt with adhesive backing; A marker pen
1. Cut a 2 cm wide length of felt about 1m long. Then cut the strip so that you have pieces that fit the ends and sides of the tank.
2. Stick to the “bottom” of the plastic fish tank.
3. Cut the lid of the fish tank.
You’re going to cut out the central part of the “lid” that is inside the lip.
4. Put tape along the slots left in the lid.
5. Tape the baking tray to the lid.
6. Put it all together (Remembering that you ought to have removed the label beforehand)
In the top right picture you can see the alpha source (a welding rod) blu-tacked to the baking tray.
Using the Cloud Chamber
1. Soak the felt in the chamber with Isopropyl alcohol.
2. Get blu tac and a source and put them at one end of the cloud chamber. Use the blu tac to have the source about 5mm above the base.
Welding Electrode sources:
3. Put dry ice in a tray. (With gloves).
4. Put the cloud chamber on the dry ice. Believe it or not I did see tracks from the alpha source but they didn’t photograph well.
5. Arrange the lighting so trails will be lit from the side.
The above image shows the tracks moving from the alpha source.
The tracks from a beta source 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.
Where to get dry ice
Online or by phone:
http://www.dryiceuk.com 0800 0842 040
http://www.chillistick.co.uk/ 08433 192 919
http://www.boconline.co.uk 0800 111 333
Around £60 for 20kg. They don’t deliver on Mondays.
Or ask at pharmaceutical suppliers, airport food companies, universities.
What to do with the left over dry ice
1) Floating bubbles
Place some dry ice in a bowl, and let it sublime. The bowl will fill with a dense layer of carbon dioxide.
Now blow a soap bubble, gently, and try to get it to rest, floating, on the layer of CO2
2) Singing Spoons
Press a spoon against a piece of dry ice – the heat of the spoon will make the dry ice sublime, squealing as it rushes past.
3) Make your own Comet!
Apparatus and materials:
Large polythene sheet to protect floor
Bin-liner bag, to line bowl and draw comet together
Mallet, to crush some dry ice to powder
Substantial plastic bag, in which to crush dry ice
Gardening gloves (heavy duty type)
Balance, to find mass of comet (and hence calculate a hypothetical kinetic energy)
For two comets
Dry ice pellets, 10 kg
Garden sand, 1 kg
Water, 2 litres
Soil, 1 handful (organic constituent)
Worcestershire sauce (organic constituent)
Smelling salts (organic constituent)
Dry ice sublimes at -78°C and will cause serious skin burns on contact, but momentary contact is unlikely to be a problem.
Do not confine in a sealed container as it will explode.
10 kg of dry ice will produce 5 m3 of gas, raising the level of CO2 from 0.035% (natural) to safe-limit (USA) of 0.5% in a room 3 m high by 19 m on a side.
Make sure there is adequate ventilation, although if the dry ice is transported in a substantial expanded polystyrene box, little will sublime.
CO2 is heavier than air therefore pools at ground level.
In theory trapped gas could fracture the comet or cause it to split, but this has never been recorded.
The essential ingredients are dry ice, sand and water. The other items represent the organic molecules thought to be present in a comet.
If it feels as if the comet will not bind into a ‘snowball’, it is because you have not used enough water. There is a natural tendency not to want to use too much water for fear of evaporating all the dry ice.
Do this in a well-ventilated area. Wear safety spectacles and gardening gloves.
a) Line mixing bowl with bin liner.
b) Pour in half a litre of water and several handfuls of sand.
c) Stir and add crushed dry ice.
d) Stir and add Worcestershire sauce, soil and smelling salts.
e) Add more water. Make sure that there is a fairly violent release of CO2, which indicates that you are cooling the mixture.
f) Draw the mixture together with the bin liner and squeeze between your gloved hands. You will feel the comet is binding into a solid mass. If it feels loose you require more water and may require more crushed dry ice. Uncrushed pellets on their own will not cool the water fast enough to form a solid mass.
For help in teaching the radioactivity topic