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Radiation and Health - Chapter 3

Chương này chứa các thông tin cần thiết cho những người quan tâm trong việc ước tính liều lượng và thực hiện tính toán rủi ro trong kết nối với bụi phóng xạ Thuật ngữ nửa cuộc sống đã được đề cập trước đó. Cho đến nay, điều này đã được gọi để chỉ một nửa đời sống vật chất. Khi xem xét sức khỏe và môi trường, một nửa cuộc sống-sinh học cũng được sử dụng. Cuộc sống một nửa được định nghĩa là thời gian trôi qua khi cường độ của bức xạ giảm xuống một nửa giá trị ban. | Chapter 3 Radioactive Decay Laws Half-life This chapter contains the necessary information for those interested in estimating doses and carrying out risk calculations in connection with radioactive fallout The term half-life was mentioned earlier. So far this has referred to only the physical half-life. When considering health and the environment biological half-life is also used. The half-life is defined as the time elapsed when the intensity of the radiation is reduced to one half of its original value. The Physical Half-life The radiation from a radioactive source will gradually be reduced. The rate of this decay is given by the half-life. It is usually denoted as 11 2 but sometimes denoted as tp for the physical half-life. In an experiment in which the intensity of the radiation is measured versus time a curve like that shown in Figure 3.1 is observed. The activity of the radiation is given along the vertical axis 100 when the experiment is started and the time in half-lives is given along the horizontal axis. 2003 Taylor Francis 22 Radiation and Health Figure 3.1. The radiation from a radioactive source decreases with time as shown. The curve can be described by an exponential formula. The figure demonstrates the meaning of the half-life. After one half-life the intensity of the radiation has decreased to 50 . After two half-lives only 25 remains and so on. Each half-life reduces the remaining amount by one half. The Earth still contains large amounts of naturally occurring radioactive isotopes such as U-238. For this to occur the half-lives must be very long. We saw in Figure 2.3 that U-238 has a half-life of 4.47 billion years. The Laws of Radioactive Decay The activity of a radioactive source A i.e. the number of disintegrations per second becquerel is given in the following way 3.1 X is the disintegration constant and it varies from one isotope to another. N is the number of atoms that in time will disintegrate and dN is the change in N during the time .