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Cloud Chamber



Use forceps to transfer the radioactive source. 

Do not touch dry ice, use tongs.

Chemicals and Solutions: 

Radiation Source (pitchblende, Po, Pb, etc.)

Dry ice

DRY 100% methanol or isopropanol


Cloud Chamber

Hammer for breaking dry ice

plastic pipet


Make sure the high voltage switch is in the off position before plugging in the cloud chamber.

Turn on the fluorescent light by depressing the switch on the front panel.

Fill the reservoir below the chamber with crushed dry ice.  (The dry ice can be crushed by wrapping it in a heavy towel and smashing it with a hammer.)  Slowly and carefully pour in alcohol.  Initially the mixture will bubble furiously.  The reservoir should be filled to the bottom of the bronze dented spring near the top of the reservoir.  The mixture should be thick and slushy.

Place the glass chamber in the reservoir such that the long center rod enters the dry ice alcohol mixture.  Grasp the top of the glass chamber and slowly rotate it until you feel the small rod at the bottom of the chamber slip into the dent of the bronze spring.  With the high voltage switch in the on position, check to see that the voltage is applied to the base plate by touching the metal tip of the blue ground plug to the black base plate.  A small spark should be produced.

With a pipet, drop alcohol onto the base plate.  Place the radioactive source in the center of the base plate. 

Pour alcohol on the foam in the chamber lid.  The foam should be saturated and excess alcohol should be poured off.

Place the lid on the chamber and plug the ground plug into the blue jack mounted on the lid.  Tracks should appear in ten to fifteen minutes.  Alpha particles appear as long straight tracks.  Beta particles are shorter and are not straight.



The radioactive source (pitchblende) emits ionizing radiation. The ionizing radiation produces ions.  We can see the track of the positive ions produced by the ionizing radiation in the cloud chamber.  The cloud chamber contains an atmosphere supersaturated in alcohol vapor.  The positive ions produced attract alcohol vapor and serve as nuclei for the condensation of liquid droplets. 

The purpose of the 940 volt DC field between the metal plate in the lid and the black base plate of the chamber is to prevent the recombination of electrons and positive ions formed by the passage of ionizing particles.  In this field the negatively charged electrons will move rapidly toward the positively charged base plate and will separate from the positive ions.  The positive ions thus have a chance to attract alcohol vapor and serve as vapor condensation nuclei.  Condensation forms almost immediately on the positive ions; they become visible and heavy and then slowly drift under the force of gravity to the bottom of the chamber, where they disappear.



To schedule a demonstration, please send an email to the demonstration lab.



Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606


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