Overview
Research proposes and simulates a compact, laser-driven source designed to generate intense $\gamma$-rays, positrons, and neutrons. This system integrates laser plasma wakefield acceleration (LWFA) with oriented crystalline targets to augment the conversion of relativistic electron beams into high-energy photons and secondary particles. The objective is to make capabilities typically associated with large accelerator facilities accessible to smaller, university-scale laboratories.
Research Context
Conventional production of intense $\gamma$-rays, positrons, and neutrons often necessitates large accelerator facilities. The development of compact sources offers a pathway to decentralize and broaden access to these radiation and particle-beam capabilities. The proposed system leverages the interaction of relativistic electron beams with specific materials to achieve this compaction and enhancement.
Approach
The core approach involves the use of LWFA to generate relativistic electron beams, which are then directed into oriented crystalline targets. The specific interaction mechanism relies on the alignment of electrons with major crystallographic directions within the target material. This alignment facilitates coherent interactions, including channeling radiation and coherent bremsstrahlung. These coherent processes are posited to significantly enhance photon emission compared with interactions in amorphous targets.
The research employed Geant4 simulations to investigate these interactions. The simulations were conducted using electron energies of 300 MeV, 1 GeV, and 3 GeV, interacting with an oriented tungsten crystal. Further investigation into tunable quasi-monochromatic radiation utilized a thin diamond crystal to demonstrate coherent bremsstrahlung.
Findings
- **Enhanced Photon Emission:** Electrons aligned with major crystallographic directions within oriented crystalline targets undergo coherent interactions (channeling radiation and coherent bremsstrahlung), resulting in substantially enhanced photon emission compared to amorphous targets.
- **Angular-Spectral Correlation:** The emitted radiation exhibits a strong angular-spectral correlation, enabling the generation of collimated $\gamma$-ray beams with reduced spectral bandwidth.
- **Increased Particle Production Efficiency:** Crystal-enhanced emission is indicated to increase particle-production efficiency. This includes positrons, generated through $\gamma$ conversion into electron-positron pairs, and neutrons, produced via photonuclear reactions.
- **Yield Enhancements:** Geant4 simulations showed that crystal orientation increases the yields of $\gamma$-rays, positrons, and neutrons by up to a factor of approximately two.
- **Production Rates:** For a 200 pC electron bunch with a duration of a few femtoseconds, the simulations predicted production rates of approximately $10^{24}\gamma/s$, approximately $10^{23}e^+/s$, and approximately $10^{21}neutrons/s$.
- **Collimated $\gamma$-ray Brightness:** The collimated $\gamma$-ray brightness reached approximately $10^{24}\gamma/s/mm^2/mrad^2/0.1\%BW$. This represents an enhancement factor of 6-8 compared with random crystal alignment.
- **Tunable Quasi-Monochromatic Radiation:** Coherent bremsstrahlung in a thin diamond crystal demonstrated the generation of tunable quasi-monochromatic radiation, achieving a brilliance of approximately $3.5\cdot10^{20}\gamma/s/mm^2/mrad^2/0.1\%BW$.
Why This Matters
The development of compact sources for intense $\gamma$-rays, positrons, and neutrons has the potential to bring radiation and particle-beam capabilities to smaller university-scale laboratories. This contrasts with the current reliance on large accelerator facilities for such capabilities, suggesting a democratization of access to advanced particle and radiation sources.
Potential Applications
The research discusses potential applications of the technique proposed.