The influence of precipitation
In order to detect a radiological incident, Dr. Alegra took the dose rate of gamma radiation, i.e. the amount of energy we receive per unit of mass over a specified unit of time. Her prime goal was to establish values for normal radiation situations, their origin being in natural sources of radiation: cosmic radiation from the Sun and interstellar spaces; and terrestrial radiation from radionucleids (originating in uranium or potassium, amongst others) present in the earth. Ms Alegra has chronologically ordered recorded dose rate values and shown that their evolution is constant (being represented on a horizontal line), although they do have an irregular component due to changes in meteorological values such as rainfall and lightning. That is, recorded values increase during periods of precipitation. The fact is that rainfall makes radioactivity present in the air fall to the earth's surface (or to the rooftop where the probe or detector is located), and this causes the recorded dose rate to increase, without the cause being a radiological incident.
Effectively, the meteorological variables make it impossible for the evolution of radioactivity to be graphically represented using histograms. This is why Dr. Alegra decided to separate the behaviour of the dose rates over different periods, depending on the level of precipitation. Thus, she defined a dry period (0 litres per square metre), a wet period (whenever rainfall is greater than 0 litres per square metre) and a transition period, that encompasses the period from the end of the precipitations to the point where the gamma radiation dose rates return to thos
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