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Understanding CPM

Counts Per Minute explained

What CPM Means

CPM (counts per minute) or CPS (counts per second) indicates how many particles (alpha, beta, gamma) the Geiger counter detects, not the total strength of the radiation source itself.

Significance in a Nuclear Event

Following a nuclear explosion, radioactive fallout (radioactive material spread in the air) will create a sharp increase in CPM, often reaching thousands of counts per minute.

Other Potential Causes

High CPM does not exclusively mean a nuclear explosion. It can also be caused by:

Nuclear Reactor Accidents: Such as the Chernobyl incident, which released significant radioactive material.
"Dirty Bombs": Conventional explosives that scatter radioactive materials.
Medical Isotopes: A person who has recently received radiation treatment for cancer might produce high CPM readings.
Industrial/Natural Sources: Radioactive minerals or industrial equipment.

Limitations

CPM varies depending on the detector's sensitivity and type (e.g., a scintillation counter vs. a Geiger-Mueller tube). It does not provide the exact energy level of the radiation, unlike units like Sieverts (Sv) or Roentgen (R).

About µSv/h Values

The microsieverts per hour (µSv/h) values displayed in this app are estimated by applying a conversion factor to the raw CPM reading. This conversion factor is based on known characteristics of common radiation detectors used by reporting stations (such as the SBM-20 Geiger tube), but the actual detector model at each station may differ.

As a result, µSv/h values should be treated as approximate indicators only and may not accurately reflect true dose rates. CPM is the primary, more reliable metric for comparing readings across stations. Where a µSv/h value appears anomalous relative to the CPM reading, it is highlighted in the dashboard station list as a potential data quality issue.