Radioactive-Series Decay

22.101 Applied Nuclear Physics (Fall 2006) Lecture 13 (10/30/06) Radioactive-Series Decay _______________________________________________________________________ References: R. D. Evans, The Atomic Nucleus (McGraw-Hill, New York, 1955). W. E. Meyerhof, Elements of Nuclear Physics (McGraw-Hill, New York, 1967). ________________________________________________________________________ We begin with an experimental observation that in radioactive decay that the probability of a decay during a small time interval ∆ t, which we will denote as P( ∆ t), is proportional to ∆ t. Given this as a fact one can write P(∆t) = λ∆t (12.1) where λ is the proportionality constant which we will call the decay constant. Notice that this expression is meaningful only when λ ∆ t < 1, a condition which defines what we mean by a small time interval. In other words, ∆ t < 1/ λ , which will turn out to be the mean life time of the radioisotope. Suppose we are interested in the survival probability S(t), the probability that the radioisotope does not decay during an arbitrary time interval t. To calculate S(t) using (12.1) we can take the time interval t and divide it into many small, equal segments, each one of magnitude ∆ t. For a given t the number of such segments will be t / ∆ t = n. To survive the entire time interval t, we need to survive the first segment ( ∆ t)1, then the next segment ( ∆ t)2, …, all the way up to the nth segment ( ∆ t)n. Thus we can write n S(t) = ∏ [1 − P((∆t) i )] i=1 = [1 − λ (t / n)] n → e − λt (12.2) 1 where the arrow indicated the limit of n → ∞ , ∆t → 0 . Unlike (12.1), (12.2) is valid for any t. When λt is sufficiently small compared to unity, it reduces to (12.1) as expected. Stated another way, (12.2) is extension of 1 – P(t) for arbitrary t. One should also notice a close similarity between (12.2) and the probability that a particle will go a distance x without collision, e −Σx , where Σ is the macroscopic collision cross section (recall Lec1). The role of the decay constant λ in the probability of no decay in a time t is the same as the macroscopic cross section Σ in the probability of no collision in a distance x. The exponential attenuation in time or space is quite a general result, which one encounters frequently. There is another way to derive it. Suppose the radioisotope has not decayed up to a time interval of t1, for it to survive the next small segment ∆ t the probability is just 1 - P( ∆ t) = 1 - λ ∆ t. Then we have S(t1 + ∆t) = S(t1 )[1 − λ∆t]] (12.3) S(t + ∆t) − S(t) = −λS(t) ∆t (12.4) which we can rearrange to read Taking the limit of small ∆ t, we get dS(t) = −λ dt (12.5) which we can readily integrate to give (12.2), since the initial condition in this case is S(t=0) = 1. The decay of a single radioisotope is described by S(t) which depends on a single physical constant λ . Instead of λ one can speak of two equivalent quantities, the half life t1/2 and the mean life τ . They are defined as S(t1/ 2 ) = 1/ 2 → t1/ 2 = ln2 / λ = 0.693 / λ (12.6) 2 ∞ τ= and ∫ dt't' S(t') 0 ∞ ∫ dt' S(t') = 1 λ (12.7) 0 Fig. 12.1 shows the relationship between these quantities and S(t). Fig. 12.1. The half life and mean life of a survival probability S(t). Radioactivity is measured in terms of the rate of radioactive decay. The quantity λ N(t), where N is the number of radioisotope atoms at time t, is called activity. A standard unit of radioactivity has been the curie, 1 Ci = 3.7 x 1010 disintegrations/sec, which is roughly the activity of 1 gram of Ra226. Now it is replaced by the becquerel (Bq), 1 Bq = 2.7 x 10-11 Ci. An old unit which is not often used is the rutherford (106 disintegrations/sec). Radioisotope Production by Bombardment There are two general ways of producing radioisotopes, activation by particle or radiation bombardment such as in a nuclear reactor or an accelerator, and the decay of a radioactive series. Both methods can be discussed in terms of a differential equation that governs the number of radioisotopes at time t, N(t). This is a first-order linear differential equation with constant coefficients, to which the solution can be readily obtained. Although there are different situations to which one can apply this equation, the analysis 3 is fundamentally quite straightforward. We will treat the activation problem first. Let Q(t), the rate of production of the radioisotope, have the form shown in the sketch below. This means the production takes place at a constant Qo for a time interval (0, T), after which production ceases. During production, t < T, the equation governing N(t) is dN (t) = Qo − λN (t) dt (12.8) Because we have an external source term, the equation is seen to be inhomogeneous. The solution to (12.8) with the initial condition that there is no radioisotope prior to production, N(t = 0) = 0, is N (t) = Qo λ (1 − e ), − λt t T, the governing equation is (12.8) without the source term. The solution is N (t) = Qo λ (1 − e −λT )e −λ (t −T ) (12.10) A sketch of the solutions (12.9) and (12.10) is shown in Fig. 12.2. One sees a build up of N(t) during production which approaches the asymptotic value of Qo / λ , and after production is stopped N(t) undergoes an exponential decay, so that if λT >>1, 4 N (t) ≈ Qo λ e − λ ( t −T ) (12.11) Fig. 12.2. Time variation of number of radioisotope atoms produced at a constant rate Qo for a time interval of T after which the system is left to decay. Radioisotope Production in Series Decay Radioisotopes also are produced as the product(s) of a series of sequential decays. Consider the case of a three-member chain, λ1 λ2 N 1 → N 2 → N 3 (stable) where λ1 and λ2 are the decay constants of the parent (N1) and the daughter (N2) respectively. The governing equations are dN 1 (t) = −λ1 N 1 (t) dt (12.12) dN 2 (t) = λ1 N 1 (t) − λ 2 N 2 (t) dt (12.13) dN 3 (t) = λ 2 N 2 (t) dt (12.14) 5 For the initial conditions we assume there are N10 nuclides of species 1 and no nuclides of species 2 and 3. The solutions to (12.12) – (12.14) then become N 1 (t) = N 10 e − λ1t N 2 (t) = N 10 N 3 (t) = N 10 λ1 λ 2 − λ1 (12.15) (e − λ1t − e − λ2 t ) (12.16) λ1λ 2 ⎛ 1 − e − λ t 1 − e − λ t ⎜ − λ 2 − λ1 ⎜⎝ λ1 λ2 1 2 ⎞ ⎟⎟ ⎠ (12.17) Eqs.(12.15) through (12.17) are known as the Bateman equations. One can use them to analyze situations when the decay constants λ1 and λ 2 take on different relative values. We consider two such scenarios, the case where the parent is short-lived, λ1 >> λ2 , and the opposite case where the parent is long- lived, λ2 >> λ1 . One should notice from (12.12) – (12.14) that the sum of these three differential equations is zero. This means that N1(t) + N2(t) + N3(t) = constant for any t. We also know from our initial conditions that this constant must be N10. One can use this information to find N3(t) given N1(t) and N2(t), or use this as a check that the solutions given by (12.15) – (12.17) are indeed correct. Series Decay with Short-Lived Parent In this case one expects the parent to decay quickly and the daughter to build up quickly. The daughter then decays more slowly which means that the grand daughter will build up slowly, eventually approaching the initial number of the parent. Fig. 12.3 shows schematically the behavior of the three isotopes. The initial values of N2(t) and N3(t) can be readily deduced from an examination of (12.16) and (12.17). 6 Fig. 12.3. Time variation of a three-member decay chain for the case λ1 >> λ 2 . Series Decay with Long-Lived Parent When λ1 << λ 2 , we expect the parent to decay slowly so the daughter and grand daughter will build up slowly. Since the daughter decays quickly the long-time behavior of the daughter follows that of the parent. Fig. 12.4 shows the general behavior Fig. 12.4. Time variation of a three-member chain with a long-lived parent. (admittedly the N2 behavior is not sketched accurately). In this case we find N 2 (t) ≈ N 10 λ1 −λt e λ2 (12.18) 7 or λ 2 N 2 (t) ≈ λ1 N 1 (t) (12.19) The condition of approximately equal activities is called secular equilibrium. Generalizing this to an arbitrary chain, we can say for the series N 1 → N 2 → N 3 → ... if then λ 2 >> λ1 , λ3 >> λ1 , … λ1 N1 ≈ λ 2 N 2 ≈ λ3 N 3 ≈ ... (12.20) This condition can be used to estimate the half life of a very long-lived radioisotope. An example is U238 whose half life is so long that it is difficult to determine by directly measuring its decay. However, it is known that U238 → Th234 → … → Ra226 → …, and in uranium mineral the ratio of N(U238)/N(Ra226) = 2.8 x 106 has been measured, with t1/2(Ra226) = 1620 yr. Using these data we can write N (U 238 ) N (Ra 226 ) = or t1/ 2 (U 238 ) = 2.8 x 106 x 1620 = 4.5 x 109 yr. t1/ 2 (U 238 ) t1/ 2 (Ra 226 ) In so doing we assume that all the intermediate decay constants are larger than that of U238. It turns out that this is indeed true, and that the above estimate is a good result. For an extensive treatment of radioactive series decay, the student should consult Evans. 8