Temporal Narrowing of Neutrons Produced by High-Intensity Short-Pulse Lasers

The production of neutron beams having short temporal duration is studied using ultraintense laser pulses. Laser-accelerated protons are spectrally filtered using a laser-triggered microlens to produce a short duration neutron pulse via nuclear reactions induced in a converter material (LiF). This produces a $\sim 3\mathrm{ns}$ duration neutron pulse with ${10}^4\mathrm{n}/\mathrm{MeV}/\mathrm{sr}/\mathrm{shot}$ at 0.56 m from the laser-irradiated proton source. The large spatial separation between the neutron production and the proton source allows for shielding from the copious and undesirable radiation resulting from the laser-plasma interaction. This neutron pulse compares favorably to the duration of conventional accelerator sources and should scale up with, present and future, higher energy laser facilities to produce brighter and shorter neutron beams for ultrafast probing of dense materials.

Figure 1

Diagrams of the different setups used in this study: (a) $S1$: the broadband, unfocused proton beam, (b) $S2$: proton focusing using the microlens, irradiated by the secondary beam, and (c) $S3$: focused protons at longer distances, where the higher divergence of the lower-energy protons decreases their contribution to the measured spectrum.

Figure 2

Images of RCF for (a) unfocused protons in the $S1$ (30 mm) position, (b) focused protons in the $S2$ (30 mm) position, and (c) focused protons in the $S3$ (0.56 m) position. The energies marked on the films correspond to the average proton energy incident on the layer. All films are plotted with the same spatial scale, but they have different gray scales.

Figure 3

Measured single-shot energy spectra of the proton fluence from $S1$ (the circles), $S2$ (the squares), and $S3$ (the diamonds). Error bars for $S1$ represent variation across multiple shots, whereas $S2$ and $S3$ are single shots with 10% film batch variability uncertainty. (a) The average fluence, which is determined by dividing the total signal on the film by the solid angle subtended by the signal. (b) The total number of protons hitting the film stack, where lines show exponential fits to the spectra, $dN/d\mathcal{E}=(N/T)e^{-\mathcal{E}/T}$. Where $N$ is $1.9\times {10}^{11}$, $2.5\times {10}^{10}$, and $5.5\times {10}^9$, and $T$ is 1.72, 1.96, and 5.32 MeV for the $S1$, $S2$, and $S3$ cases, respectively.

Figure 4

Plot of the ntof traces from the experimental data without neutron production (the light lines), with neutron production (the dark lines), and the simulated neutron signal (the dashed lines). Time zero corresponds to the arrival of the laser. Panels (a) and (b) refer to setups $S1$ and $S3$, respectively.