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High-energy gamma-ray radiography has exceptional penetration ability and has become an indispensable nondestructive testing (NDT) tool in various fields. For high-energy photons, point projection radiography is almost the only feasible imaging method, and its spatial resolution is primarily constrained by the size of the gamma-ray source. In conventional industrial applications, gamma-ray sources are commonly based on electron beams driven by accelerators, utilizing the process of bremsstrahlung radiation. The size of the gamma-ray source is dependent on the dimensional characteristics of the electron beam. Extensive research has been conducted on various advanced accelerator technologies that have the potential to greatly improve spatial resolution in NDT. In our investigation of laser-driven gamma-ray sources, a spatial resolution of about 90 µm is achieved when the areal density of the penetrated object is 120 g/cm2. A virtual source approach is proposed to optimize the size of the gamma-ray source used for imaging, with the aim of maximizing spatial resolution. In this virtual source approach, the gamma ray can be considered as being emitted from a virtual source within the convertor, where the equivalent gamma-ray source size in imaging is much smaller than the actual emission area. On the basis of Monte Carlo simulations, we derive a set of evaluation formulas for virtual source scale and gamma-ray emission angle. Under optimal conditions, the virtual source size can be as small as 15 µm, which can significantly improve the spatial resolution of high-penetration imaging to less than 50 µm. [ABSTRACT FROM AUTHOR]

Single shot gamma-induced positron annihilation spectroscopy (GiPS) based on a laser wakefield accelerator is proposed. In this spectroscopy, a large number of gamma rays generated by laser-wakefield accelerated electrons are injected into a sample in a very short time (<2 ps), and the positron lifetime is detected by measuring the time profile of the annihilation radiation (511 keV gamma ray) from the sample. The processes including gamma-ray production, shielding, annihilation generation, and annihilation detection were simulated by using the Monte Carlo method. The results show that up to 1010 gamma rays with a duration of 1.4 ps can be produced per shot with a 45 TW laser system. A Cherenkov radiator PbF2 was used to detect the annihilation radiation, and its optimal thickness of 15 mm was obtained. A whole process simulation with optimized parameters was conducted. A low statistical fluctuation lifetime spectrum can be produced, which confirms the feasibility of single shot GiPS with a typical 45 TW laser system. [ABSTRACT FROM AUTHOR]

Ultraintense laser driven fast electrons play an increasingly important role in many applications. To predict and optimize the fast electron transport efficiency, we introduce a one-dimensional analytical model including resistive effects to estimate the transport efficiency as a function of transport distance and a key parameter named the penetration path. Based on the model, the transport efficiency of fast electrons with the same penetration coefficient can be calculated for different characteristic parameters including the target thickness and laser intensity. A double-layer Kα fluorescence measurement of fast electron transport efficiency is proposed to eliminate the influence of in-target electrons refluxing from the relative Kα photon yield of the rear and front sides of the target. By fixing the transport distance, we have experimentally measured the penetration path and the efficiency of planar Al2O3 targets, in good accordance with Monte Carlo simulations. The results show that the beam energy can be reduced to 25% in a penetration path of tens of microns. This measurement method provides a feasible route to characterize and compare the fast electron transport in various targets and laser conditions, making it possible to modulate and optimize the transport efficiency in actual research studies, which is of great significance in fast ignition, X-ray emission, positron–electron pair production, and many other applications. [ABSTRACT FROM AUTHOR]

Positron generation by an ultra-short ultra-intense laser in an indirect manner has been studied with two-dimensional particle-in-cell (PIC) and Monte Carlo (MC) simulations. In this generation scheme, positrons are produced with energetic electrons accelerated by an ultra-shot laser pulse propagating through an underdense plasma. The dependence of the positron beam properties on the plasma length and secondary target (converter) thickness was investigated in detail. The simulation results reveal that the positron yield is strongly correlated with the total energy of laser-accelerated electrons; both the temperature and divergence of the positron beam are sensitive to the plasma length; and the positron beam has a pulse duration comparable to the incident electron beam. In addition, it is indicated that even with the optimal converter thickness, only a small fraction (11.4%) of positrons can escape out and most of the detected positrons originate from the back edge of the converter. [ABSTRACT FROM AUTHOR]

The Monte Carlo transport code Geant4 has been used to study positron production in the transport of laser-produced hot electrons in solid targets. The dependence of the positron yield on target parameters and the hot-electron temperature has been investigated in thick targets (mm-scale), where only the Bethe-Heitler process is considered. The results show that Au is the best target material, and an optimal target thickness exists for generating abundant positrons at a given hot-electron temperature. The positron angular distributions and energy spectra for different hot electron temperatures were studied without considering the sheath field on the back of the target. The effect of the target rear sheath field for positron acceleration was studied by numerical simulation while including an electrostatic field in the Monte Carlo model. It shows that the positron energy can be enhanced and quasi-monoenergetic positrons are observed owing to the effect of the sheath field. [ABSTRACT FROM AUTHOR]

Positron production by ultra-intense laser-accelerated electrons has been studied with two-dimensional particle-in-cell and Monte Carlo simulations. The dependence of the positron yield on plasma density, plasma length, and converter thickness was investigated in detail with fixed parameters of a typical 100 TW laser system. The results show that with the optimal plasma and converter parameters a positron beam containing up to 1.9 × 1010 positrons can be generated, which has a small divergence angle (10°), a high temperature (67.2 MeV), and a short pulse duration (1.7 ps). [ABSTRACT FROM AUTHOR]

An extensive comparison of the properties of positron beams produced by an ultra-intense femtosecond laser in direct and indirect schemes has been performed with two-dimensional particle-in-cell and Monte Carlo simulations. It is shown that the positron beam generated in the indirect scheme has a higher yield (1010), a higher temperature (28.8 MeV), a shorter pulse duration (5 ps), and a smaller divergence (8°) than in the direct case (109 yield, 4.4 MeV temperature, 40 ps pulse duration, and 60° divergence). In addition, it was found that the positron/gamma ratio in the indirect scheme is one order of magnitude higher than that in the direct one, which represents a higher signal/noise ratio in positron detection. Nevertheless, the direct generation method still has its own unique advantage, the so-called target normal sheath acceleration, which can result in quasi-monoenergetic positron beams that may serve in some specialized applications. [ABSTRACT FROM AUTHOR]