Bright Laser-Driven Neutron Source Based on the Relativistic Transparency of Solids

Neutrons are unique particles to probe samples in many fields of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers sufficient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to produce a forward peaked neutron flux with a record yield, on the order of ${10}^{10}\mathrm{n}/\mathrm{sr}$. We present results comparing the two acceleration mechanisms and the first short pulse laser generated neutron radiograph.

Figure 1

Experimental setup. Left: The target consists of ${\mathrm{CH}}_2$ or ${\mathrm{CD}}_2$ foils and a Be converter. The proton or deuteron beam is converted into a spherical ($4\pi$) and a directed neutron component. Right: BTI bubble detectors were placed on the outside of the TRIDENT target chamber to cover different neutron emission angles. Three $n\mathrm{TOF}$ detectors were used to measure the energy spectrum.

Figure 2

Ion spectra from a 300 nm, 90% deuterized plastic target. The bulk ion distribution is clearly represented in the spectrum showing the acceleration of the foil volume rather than surface acceleration.

Figure 3

Neutron Data: Polar plot of an experiment with $3.2\mu \mathrm{m}$ target (left) compared to a 400 nm target (center). In addition to the isotropic neutron yield in the latter case a strong forward directed yield is visible. The neutron spectrum in the forward direction (upper right plot) also shows higher neutron energies.

Figure 4

Ion beam: Autoradiography of the copper shield in front of the beryllium converter at 5 cm distance from the target (left) using CH targets. The interesting feature is the distinct two-lobe formation of the most energetic and intense part of the proton beam (center) for a 300 nm target, a characteristic feature of the BOA acceleration scheme. This is different from TNSA ($3.2\mu \mathrm{m}$ CH target), where usually homogeneous, circular beams are observed (right). The edge features are caused by mounting structures in the ion beam path.

Figure 5

Neutron image of three tungsten blocks. The left side shows a sketch of the arrangement in front of the scintillating fiber array of the neutron imager. A photograph of the fiber array consisting of a $500\mu \mathrm{m}$ diameter fiber configuration is shown in the center. A laser pulse with an energy of 75 J within 800 fs generated a deuteron beam from a 410 nm ${\mathrm{CD}}_2$ target. The right side of the figure shows the superposition of two different shots as in Fig. 6.

Figure 6

Comparison between the measured transmissions for different tungsten objects to MCNPX calculations using the measured neutron energy distribution detected by the $n\mathrm{TOF}$ detector. The data were averaged over two shots to improve the signal-to-noise ratio.