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Radioactive Materials



Geiger Counter

alpha source = Polonium 210 (stopped by paper)

beta source = Strontium 90 (stopped by glass)

gamma source = Cobalt 60 (stopped by lead brick)

Fiestaware (contains uranium)

Colman mantle (contains Thorium)

Smoke detector (contains americium)

Cardboard, glass, and lead shields


Turn on the geiger counter (with sound on!) and bring its detector up to the Polonium 210 source.  The gieger counter clicks rapidly if ionizing radiation is present.  Polonium 210  emits alpha radiation:

210Po  =>  4He +  206Pb

84              2          82

Demonstrate that alpha radiation can be stopped by  cardboard by slipping a piece between the source and the detector (the geiger counter will stop clicking).

Next  bring the detector up to the Strontium  90 source. Strontium 90 emits beta radiation:

90Sr =>   0e  +  90Y

38          -1       39

Demonstrate that beta emission can not be stopped by a piece of paper, but that it can be stopped by glass.

Bring the detector up to the Cobalt 60 source.  Cobalt 60 emits gamma rays.  Show that gamma rays are not stopped by cardboard or glass or a thin piece of lead.  A lead block is required to stop gamma rays.

Use the geiger counter and shields to determine the type of radiation emitted by a smoke detector (contains americium), a Colman mantle (contain Thorium) and a piece of "fiestaware" pottery (contains uranium).



A Geiger counter consists of a gas filled cylinder connected to a high voltage battery and to a sounding device and a meter.  A gap between the wall of the cylinder and an electrode passing into its center serves to break the electrical circuit connecting the battery to the rest of the counter.  In the absence of ionizing radiation this gap keeps the battery from acivating the sounding device or the meter.  Any ionizing  radiation that passes  through the cylinder produces a stream of ions in the gas, which temporarily completes the circuit.

A smoke detector uses americium 241.  Americium is an alpha emitter.  Because of ionization by alpha particles, air within the detector becomes electrically conductive and relays a current provided by a small battery.  Smoke particles cut down the flow of electrical current in the air, and the resulting decrease in the current sets off the alarm. 



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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