Ultra-Small Focal Spot X-Ray Sources for High Resolution Digital Mammography

Abstract

The laser-based x-ray source utilizes a high-power and coherent light beam for x-rays production. In this device laser beam impinge on a solid target. Initially, a thin layer of solid density cold plasma (with electron temperature Te∼0.5 keV is being created on the surface of the target [1]. In the next stage, energy is transferred from the laser light to free electrons in the solid plasma. As a result, a low density hot plasma is being created above the target with hot electron temperature, Te≈25 keV). Up to 40% of laser light energy can be transferred to hot (suprathermal) electrons. A significant fraction of hot electrons returns to the positively charged space-charge region on the target surface previously created due to emission of suprathermal electrons. Consequently, the hot electrons penetrate the target producing a burst of incoherent x-rays, composed of continuous bremsstrahlung emission and discrete characteristic x-ray emission lines [2], The effective size of the Laser Produced Plasma (LPP) x-ray source and the duration of the x-ray pulse always exceeds the size and the duration of laser light pulse. Nevertheless, it is extremely small (tens of microns) and short (picoseconds). The conversion efficiency of the LPP x-ray source (F x ) defined as the ratio of the energy emitted in the x-ray burst generated by LPP to the energy in the laser light pulse, is approximately proportional to the square root of the laser beam intensity, i.e. to the peak electric field of the laser light, and to the atomic number Z of the target, F x ∝ E peak · Z [3]. It is analogous to the x-ray tube conversion efficiency law, F x ∝ V·Z and similar conversion efficiency can be expected (i.e. below 1%) in both cases.