Radionuclides average life

The average life t of a given radionuclide corresponds to the inverse of the decay constant—hence, to l/ln2 times the half-life tn2, as obtained by integration of the time of existence of all radionuclides divided by the initial number Nq. 

The number of radioactive atoms present and hence the rate of disintegration decreases to one-half in one half-life, to one-quarter in two half-lives, to one-eighth in three half-lives, and so on. The half-life is characteristic of any particular radioisotope. Another useful quantity is the mean life or the average life of a radionuclide which is the reciprocal of the decay constant, = 1/A,. 

Radionuclide Half-life (d) Gamma emissions (keV) Average beta emissions (keV) Mean tissue penetration range (pm)... 

Radionuclide Half-life (days) Mean range (pm) Average (i energy (keV)... 

The straight solid line in Figure 9.5 represents the average, steady-state abundances ofthe radionuclides at a galactic age of 7.5 Ga, when the solar system formed. For times that are much longer than the mean lifetimes of the short-lived radionuclides, the slope of this line is given by the mean life, x, divided by the galactic age, /. As the mean life approaches /, the steady-state abundance falls below the xlt line and the trend asymptotically approaches the effective production ratio ( = 1 on this plot). The vertical position of this line and the curves derived from it depend on our choice of t. 

Although the half-life of a given radionuclide is a defined value, the actual moment of disintegration for a particular atom can be anywhere from the very beginning of the nuclide s life to infinity. The average or mean life of a population of nuclei can, however, be calculated. The mean life t is naturally related to the decay constant and is, in fact, simply the reciprocal of the decay constant ... 

Step 8. Repeat measurement to obtain duplicate result then repeat at selected intervals. The decrease in the count with time rate at a peak ROI is used to measure the half life with which it decays. Peaks of longer-lived radionuclides are measured as they become visible when gamma rays from shorter-lived radionuclides disappear from the spectrum. Increase the counting time in later measurements to compensate for radioactive decay and lower activity of longer-lived radionuclides. Increase the time interval between counts as the average half life of the sample increases. For example, count the sample daily for the first week, weekly for the next 4 weeks, and then monthly. 

Radioisotopes of some metals are naturally present in soils. The most conmion is potassium-40, with a natural abundance of 0.012% and a half-life of 1.28 x 10 years. Since potassium is an essential nutrient, the average content of the human body is 4000 Bq. Similarly, another alkali earth metal, rubidium, has a natural radioisotope, Rb, with a half-life of 4.75 x 10 years and accounts for about 600 Bq hi the human body. Other natural radionuclides, such as - U and whose distribution and relative abundance may be altered by mining and other human activity, may contribute significantly to radiation dose but are not... 

Radionuclide range Half-life %,max(MeV) Max. / + range (mm) in water Average /3+ range (mm) in water... 

Mean life(r). The period of time a radionuclide exists on the average before disintegration. It is related to the half-life and decay constant by r = 1/A = 1-440/2. 

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