Radiation Safety


Since ionizing radiation cannot normally be detected through the senses, instruments must be relied on and utilized for the detection and measurement of radioactive contamination and radiation fields. As a wide range of radiation-monitoring instruments are available, care must be taken to select an instrument both appropriate and efficient for the application.

Principal investigators (PI) working with beta- (e.g., C-14, S-35, P-33, P-32) or gamma-emitting radioisotopes (e.g., I-125, Cr-51) should possess or have access to a portable radiation detector with the appropriate probe. Certain labs with only sealed sources presenting a minimal radiation exposure hazard are not required to possess a radiation detector. Labs working only with H-3 are not required to possess a portable radiation detector. However, they must have access to a liquid scintillation counter for all post-experiment and monthly surveys.

All radiation detection instruments used for quantitative and qualitative measurements must be calibrated or response-checked every 12 months. Calibrations shall be performed either by EH&S, the instrument manufacturer, or a licensed calibration service provider.

If the calibration label affixed on the side of instruments indicate that the last calibration date has been exceeded by one year, do not use the detector and notify EH&S immediately.

Detector Type

Radioisotopes: C-14, S-35, P-33, P-32


  • Can be used to find gross surface contamination
  • Does not detect H-3 or any very low energy beta
  • Inefficient for detection of photons (x-rays, gamma rays)
  • Typically measures in counts per minute (cpm)

Radioisotopes: Gamma or x-ray fields


  • Measures exposure rate due to gamma or xrays
  • Measures exposure rate due to Bremsstrahlung radiation from beta particles
  • Typically measured in μroentgen(R)/hr or mR/hr

Radioisotopes: I-125, Cr-51


  • Much more efficient at detecting gamma radiation than GM meter
  • Fragile and very hygroscopic
  • Measured in cpm

Radioisotopes: H-3, C-14, S-35, P-33, P-32, I-125, Cr-51


  • Able to quantify H-3
  • Most sensitive detection method for beta particles, although measurement is applicable to all types of emissions
  • Measured in cpm or disintegrations per minute (dpm)

Radioisotopes: I-125, Cr-51 and other gamma-emitting isotopes


  • Efficient detection method for gammaemitting isotopes
  • Measured in cpm or dpm