Question

A beam of neutrons is used to study molecular structure through a series of diffraction experiments. A beam of neutrons with a wide range of de Broglie wavelengths comes from the core of a nuclear reactor. In a time-offlight technique, used to select neutrons with a small range of de Broglie wavelengths, a pulse of neutrons is allowed to escape from the reactor by opening a shutter very briefly. At a distance of \(16.4 \mathrm{m}\) downstream, a second shutter is opened very briefly 13.0 ms after the first shutter. (a) What is the speed of the neutrons selected? (b) What is the de Broglie wavelength of the neutrons? (c) If each shutter is open for 0.45 ms, estimate the range of de Broglie wavelengths selected.

Short answer

Question: Estimate the range of de Broglie wavelengths selected by the two shutters, given that the distance between them is 16.4 m, the time taken for the neutrons to travel this distance is 13 ms, and each shutter is open for 0.45 ms. Answer: The range of de Broglie wavelengths selected is approximately \(9.27 \times 10^{-12}\,\text{m}\).

Step-by-step solution

Finding the speed of neutrons

Given that the distance between the two shutters is \(16.4\) m, and the time taken for the neutrons to travel this distance is \(13\) ms, we can find the speed of the neutrons by dividing the distance by time. Speed \(v = \frac{Distance}{Time}\) \(v = \frac{16.4 \mathrm{m}}{13.0 \times 10^{-3}\mathrm{s}}\) \(v \approx 1261.5 \mathrm{m/s}\)

Finding the de Broglie wavelength

Now that we know the speed of the neutrons, we can find their de Broglie wavelength using the formula: \(\lambda = \frac{h}{m v}\) Here, \(\lambda\) is the de Broglie wavelength, \(h\) is the Planck's constant (\(6.63 \times 10^{-34}\,\text{Js}\)), \(m\) is the mass of a neutron (\(1.674927 \times 10^{-27}\,\text{kg}\)), and \(v\) is the speed we calculated in Step 1. \(\lambda = \frac{6.63 \times 10^{-34}\,\text{Js}}{(1.674927 \times 10^{-27}\,\text{kg})(1261.5\,\text{m/s})}\) \(\lambda \approx 3.16 \times 10^{-10}\,\text{m}\)

Estimating the range of de Broglie wavelengths

We are given that each shutter is open for \(0.45\) ms. The range of speeds can be found using the formula: \(\Delta v = \frac{\Delta x}{\Delta t}\) Where \(\Delta x\) is the distance between the shutters (\(16.4\,\mathrm{m}\)), and \(\Delta t\) is the time each shutter is open (\(0.45\,\text{ms}\)). \(\Delta v = \frac{16.4\,\mathrm{m}}{0.45 \times 10^{-3}\,\text{s}}\) \(\Delta v \approx 36444.4\,\text{m/s}\) Now, we can estimate the range of de Broglie wavelengths using the formula: \(\Delta \lambda = \frac{h}{m (\Delta v)}\) \(\Delta \lambda = \frac{6.63 \times 10^{-34}\,\text{Js}}{(1.674927 \times 10^{-27}\,\text{kg})(36444.4\,\text{m/s})}\) \(\Delta \lambda \approx 9.27 \times 10^{-12}\,\text{m}\) Therefore, the range of de Broglie wavelengths selected is approximately \(9.27 \times 10^{-12}\,\text{m}\).