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Americium-241 emits mainly alpha and gamma radiation. It has a half life of 432.2 years, by emitting alpha particles it turns into neptunium-237. The most common source is americium-241 oxide from ionizing household smoke detectors.


Emission spectrum of Americium-241

Type Energy Percentage
Alpha (α) 5.485 MeV 84.5 %
Alpha (α) 5.443 MeV 13.0 %
Beta (β) 52 keV Unknown
Gamma (γ) 59.5 keV 35.9 %
Gamma (γ) 26.3 keV 2.4 %
Gamma (γ) 13.9 keV 42 %

Some beta emission is suspected

Types of radiation

Type Nature Charge Speed
Alpha (α) Helium nucleus +2 < 0.1 c
Beta (β) Fast moving electron -1 < 0.9 c
Gamma (γ) Photon None c

Range and penetrating power of radiation

Type Range in air Thickness of aluminium to halve intensity
Alpha (α) Several centimetres 5 µm
Beta (β) Several metres 0.5 mm
Gamma (γ) Hundreds of metres 8 cm

These values are approximate, the higher the energy the larger the penetrating powwer

Possible experiments

To find out what kind of radiation you are working with you can filter it with aluminium of different thickness and measure the change in intensity.


Fluorescence in Europium-doped yttrium oxide
(30s, f/1.4, ISO 1600)

Alpha particles will excite different types of phosphor. If you cover the sample with phosphor or hold the sample close to it a phosphor of the right type will start to glow weakly.

Types of phosphor to use

  • Europium-doped yttrium oxide (Y2O3:Eu) - Red
  • Silver activated zinc sulphate (ZnS:Ag) - Blue
  • Manganese activated zinc sulphate (ZnS:Mn) - Red/Orange
  • Copper activated zinc sulphate (ZnS:Cu) - Green

How to view the fluorescence

  • Adapt your eyes to the dark by sitting in total darkness for up to 15 minutes. If you then look closely to the side of the light source you can detect flashes of light that contains down to 90 photons and should be able to see a weak glow. If the glow is very weak the eye will not be able to determine the colour of the light and it might just look bluish white.
  • Use a loupe or microscope to see single partcles excite the phosphor, this is called a spinthariscope.
  • Use a digital camera and take long time exposures. A camera makes it simpler to compare different brightnesses and to determine the colour.

Ionization detector

Cloud chamber

CCD chip detector


Americium-241 oxide as found in household smoke dectectors (<35 kBq) is considered safe as long as the sample is not broken up and the dust is inhaled or ingested. Since radiation has a very unpredictable effect on the human body it is safest to treat the sample with respect and store it safely.

Some commercial units can contain 50 times more radioactive material and should be handled with more care. From time to time really dangerous sources can be found and if you don't know exactly what you got you should consider it to be unsafe.

See Radiation safety

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