Radon daughters: the concentration of radon gas and its decay products depends greatly on temperature and weather patterns. When the air is cooler, or when there is an inversion layer "holding down" air near the ground, there can be a higher concentration of radon gas and its decay products -- especially Lead-214 and Bismuth-214, which are strong gamma and beta emitters and are quite radioactive. Other weather conditions can cause large variations as well, especially rain. Rain washes the radon progeny out of the air. So when it rains, there should generally be a short spike in the gamma and beta counts followed by a period of lower counts.
There may be other radionuclides present such as Iodine 131 or Iodine 135 and their progenies. Without spectrographic analysis it is not possible to identify the specific offending radionuclide.
Monitoring during the rain event demonstrated elevated airborne count readings at 1 meter in excess of 100 counts per minute (CPM) as reflected on the Radiation Network dot com.Ground readings were higher than 150CPM.
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