University Physics V · topic UPV-15.01
The decay law, the width Gamma, and where it fails
Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.
Further practice
Work through a fresh case before tackling the extension.
The decay law, Γ, and where it fails: predict a finite change
The decay law says an unstable state's survival probability falls as e^(−λt) with a rate λ = Γ/ħ fixed by the full width of its energy line, so the expected number of survivors in an ensemble is N₀e^(−λt). Use the supplied topic model Y_out = exp(-0.0015193*gam*t), where Y_out is SURVIVAL S(t) (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; exp(x) means eˣ. t is Elapsed time t, measured in ns. Keep gam (Line width Γ (FWHM)) = 4.7 neV fixed. For this exercise use only 74.375 ns ≤ t ≤ 625.625 ns on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. Calculate Y_out at t = 195.65 ns and 272.825 ns. Find the signed output change ΔY_out and the average sensitivity ΔY_out/Δt across this interval. State whether the response increases or decreases; distinguish this interval average from a derivative at one point.
SURVIVAL S(t) against t
Predictions from the supplied The decay law, Γ, and where it fails model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): SURVIVAL S(t)
Horizontal axis (x): t: Elapsed time t / ns
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| t: Elapsed time t / ns | SURVIVAL S(t) |
|---|---|
| 74.375 | 0.5879635597313115 |
| 212.1875 | 0.21977095180813358 |
| 350 | 0.08214670868501589 |
| 487.8125 | 0.0307050667627455 |
| 625.625 | 0.011477040772500615 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Substitute each value of t separately while keeping every other input fixed, then subtract in the same order.
Reveal the worked solution
- At t = 195.65 ns, substitute into Y_out = exp(-0.0015193*gam*t): Y₀ = 0.247318.
- At t = 272.825 ns, the same fixed-parameter model gives Y₁ = 0.142535.
- Signed change: ΔY_out = Y₁ − Y₀ = -0.104783; the input change is Δt = 77.175 ns.
- Average sensitivity is ΔY_out/Δt = -0.00135773 1 per ns. Its negative sign means that this response decreases over the specified interval.
- This is a secant slope between two settings. It equals a point derivative only for a locally linear relation or in a justified small-interval limit.
Answer Y₀ = 0.247318, Y₁ = 0.142535; ΔY_out = -0.104783; average sensitivity = -0.00135773 1 per ns.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Extra challenging questions
Work through the linked stages and explain what each result means.
The decay law, Γ, and where it fails: infer an input with uncertainty
The decay law says an unstable state's survival probability falls as e^(−λt) with a rate λ = Γ/ħ fixed by the full width of its energy line, so the expected number of survivors in an ensemble is N₀e^(−λt). Use the supplied topic model Y_out = exp(-0.0015193*gam*t), where Y_out is SURVIVAL S(t) (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; exp(x) means eˣ. t is Elapsed time t, measured in ns. Keep gam (Line width Γ (FWHM)) = 4.7 neV fixed. For this exercise use only 74.375 ns ≤ t ≤ 625.625 ns on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.053277387, with a bounded reading uncertainty ±0.0062701. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown t by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.055125 ns, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.
SURVIVAL S(t) against t
Predictions from the supplied The decay law, Γ, and where it fails model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): SURVIVAL S(t)
Horizontal axis (x): t: Elapsed time t / ns
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| t: Elapsed time t / ns | SURVIVAL S(t) |
|---|---|
| 74.375 | 0.5879635597313115 |
| 212.1875 | 0.21977095180813358 |
| 350 | 0.08214670868501589 |
| 487.8125 | 0.0307050667627455 |
| 625.625 | 0.011477040772500615 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
The model's endpoint predictions bracket the reading. Solve Y_out(t) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.
Reveal the worked solution
- Endpoint predictions are Y_out(74.375) = 0.587964 and Y_out(625.625) = 0.011477. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives t_line = 74.375 + (0.0532774 − 0.587964)(625.625 − 74.375)/(0.011477 − 0.587964) = 585.655 ns. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
- Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives t = 410.637 ns; substituting back gives Y_out = 0.053277387.
- The interpolation error in the input is |t_line − t| = 175.017 ns. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.047007287 and 0.059547487. Sort the roots even if the response decreases: 395.056 ns ≤ t ≤ 428.172 ns.
- The lower and upper input deviations are 15.5814 ns and 17.5346 ns. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.
Answer t ≈ 410.637 ns; compatible interval [395.056, 428.172] ns. Endpoint interpolation gives 585.655 ns.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.