Helen ApSimon founding member and Chairman of the European Association for the Science of Air Pollution. ApSimon is well known for her research into the transport of radioactivity from the Chernobyl disaster. In the immediate aftermath, Russia issued a media blackout and ApSimon was one of few European scientists to detect rising radiation levels. She modelled the nuclear fallout, calculating Chernobyl released 15 - 20 megacuries of Iodine-131 and 1 - 2 megacuries of caesium-137. During the first few weeks after the disaster, ApSimon calculated the radioactive plum covered Scandinavia and the North coast of Europe. She travelled to Hungary, Poland, Czechoslovakia
Isotopes of caesium only long after the nuclear explosion is over. The combined yield of Cs and Cs is given as 6.7896%. The proportion between the two will change with continued neutron irradiation. Cs also captures neutrons with a cross section of 140 barns, becoming long-lived radioactive Cs. Caesium-134 undergoes beta decay (β), producing Ba 134 directly and emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV). Caesium-135 is a mildly radioactive isotope of caesium with a half-life of 2.3 million years, decaying, via emission of a low-energy beta particle, into the stable isotope barium-135. Caesium-135 is one of the 7