Gamma radiation as well as neutron radiation can cause RPMs to trigger an alarm procedure. Alarms caused by statistical fluctuations of detection rates are referred to as false alarms. Alarms caused by benign radioactive sources are referred to as nuisance alarms. Causes of nuisance alarms can be broken up into several large categories:
*Naturally occurring radioactive materials (NORM) and technically enhanced NORM (TENORM)
**Ceramic, tiles, porcelain, pottery, granite, clay, and other rock and clay based products contain elevated levels of naturally occurring potassium-40 and to a smaller degree thorium-232 and other isotopes.
**Propane gas tankers, full or empty, contain elevated levels of radium-226.
**Many fertilizers and potash contain elevated levels of potassium-40.
**Cat litter contains elevated levels of thorium-232.
*Medical isotope alarms constitute the majority of alarms in privately owned vehicle lanes at land borders and are usually due to medical treatment of the driver or passengers, mostly due to technetium-99m and thallium-201 and iodine-131.