There is no sensory response to exposure from ionizing radiation. Like radio waves, ionizing radiation in normal intensities cannot be seen, felt, tasted or smelled. It can only be detected by radiation detectors, such as Geiger-Mueller counters, film badges and liquid scintillation counters (LSCs).

Ionizing radiation can penetrate tissue. Its ability to penetrate depends on the type (e.g. gamma, x-ray, beta, neutron, alpha) and energy of radiation. Each radioactive isotope has its own type and energy:

Gamma radiation, such as that produced by Cs-137 and Co-60, can easily penetrate tissue, glass, wood and even moderate amounts of metal. These radiation sources can pose both an external radiation risk if adequate shielding is not provided, and an internal radiation risk if the source leaks.

Beta radiation is easily shielded. The level of shielding depends on the energy of the beta radiation. The Ni-63 found in ECDs is easily shielded by even a sheet of paper because of its very low energy of emission. Thus this isotope has virtually no external radiation risk, though it certainly can be a dangerous source of internal exposure if the source is leaking.

Higher energy beta radiation, such as that produced by Phosphorus-32, requires thicker shielding, such as 3/8" of plexiglass. P-32 is an example of a beta emitter that is both an external and internal radiation risk.


Quantities of radioactive materials are measured in units called "Activity". Units of activity and common conversion factors are listed here:

  • milliCuries (mCi)
  • microCuries (uCi)
  • Becquerels (Bq)
  • disintegrations/minute (dpm)

1 Curie = 2.22x10+12dpm = 3.7x10+10dps
1 Curie = 3.7x10+10 Bq
1 Bq = 1 disintegration/second
1 uCi = 2.22x10+6dpm


Radioactive material is constantly undergoing the process of radioactive decay, so the quantity of radioactive material (the Activity) is constantly decreasing. The rate at which it decreases is called the Half-life.

Each isotope has its own rate of decay (half-life). The half-life is the time is takes for one half of the material to be lost by radioactive decay. For example, the isotope Ni-63 has a half life of 100 years. So every 100 years 50% of Ni-63 will have decayed by ionizing radiation to the non-radioactive element Copper-63. It will take 7 half-lives for a radioisotope to decay to less than 1% of its current activity. For example, in the case of Ni-63:

100yrs=50% decayed (50% remaining)
200yrs=75% decayed (25% remaining)
300yrs=87.5% decayed (12.5% remaining)
400yrs=93.75% decayed (6.25% remaining)
500yrs=96.875% decayed (3.125% remaining)
600yrs=98.4375% decayed (1.5625% remaining)
700yrs=99.21875% decayed (0.78125% remaining)