Radioactive Decay - Printable Version +- HP Forums (https://www.hpmuseum.org/forum) +-- Forum: Not HP Calculators (/forum-7.html) +--- Forum: Not remotely HP Calculators (/forum-9.html) +--- Thread: Radioactive Decay (/thread-13691.html) Radioactive Decay - SlideRule - 09-20-2019 01:57 PM The Journal of Nuclear Medicine {vol 19, num 11} pgs. 1270-1271 Calculation of Radioactive Decay with a Pocket Calculator JAMES S. ROBERTSON Mayo Clinic Rochester, Minnesota "Radioactive decay is customarily expressed by the equation (1) A = A₀ e^(-λt) where t = time; λ = decay constant in t⁻¹ units; A = activity, usually μCi or mCi; A₀ activity at t = 0; and e = 2.718 …, the natural logarithm base. However, the decay parameter most readily available is not λ, but the half-life, T. Therefore, the relationship λT = ln(2) = 0.693 … is invoked and the decay equation becomes (2) A = A₀ e^(-0.693t/T) From this it would appear that the way to calculate A, given A₀, t and T, is first to determine x = -0.693 t/T and then to obtain A/A₀ from e^(x). However, a simplifying feature that is overlooked in this procedure is that e^(-0.693) = …½, and the decay equation may therefore be expressed as (3) A = A₀ ½^(t/T) Thus, if a pocket calculator having a y^x function is used, A/A₀ may be calculated simply by entering 0.5 as y, calculating t/T as x, and calling y^x. This saves several steps when compared with using Eq. 2. For negative values of t, the same procedure works, but alternatively the number 2 may be entered as y and the absolute value of t used. These procedures are similar to the slide-rule method of setting T against 0.5 or 2.0 on a log-log scale and reading A/A₀ against t, a method now in danger of being forgotten with the proliferation of inexpensive pocket calculators." 'Ahh, memories' BEST! SlideRule RE: Radioactive Decay - Albert Chan - 09-20-2019 02:40 PM (09-20-2019 01:57 PM)SlideRule Wrote:  (1) A = A₀ e^(-λt) ... (2) A = A₀ e^(-0.693t/T) ... ... simplifying ... (3) A = A₀ ½^(t/T) Why go thru all these ? (3) is the *definition* of half-life