X-ray emission in the “water-window” from laser-produced boron-nitride plasma

Summary form only given. Soft x-rays in the water-window (λ=2.3-4.4nm) are an attractive source for cellular microscopy due to high contrast between water and carbon rich cellular structures. Water-window microscopy allows for sub-10
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nm resolution without the lengthy preparation techniques needed for transmission electron microscopy. Currently synchrotron radiation is the most common source of soft x-rays for cellular microscopy, but synchrotron facilities are extremely large and access is limited. In order for water-window microscopy to become a widely spread and commonly used technique, compact laboratory based systems must be developed. A laser-produced plasma (LPP) x-ray source offers the advantages of being compact with high brightness, making it popular for developing compact water-window microscopes. We investigated the effect of excitation wavelength on 'Water-Window' emission from laser-produced boron-nitride (BN) plasma. BN was chosen as the laser target because nitrogen emits strong spectral lines Ly-α λ=2.478 nm and He α λ=2.879 nm with low continuum emission. BN planar target was placed in the center of a vacuum chamber pumped down to 10" Torr, then irradiated by a Q-switched Nd:YAG laser operating in the fundamental harmonic (λ=1064 nm), second harmonic (532 nm), and fourth harmonic (266 nm), with power density ranging from 10π-1013 W/cm2. A diffraction grating spectrometer is used to measure the spectrum in the water-window wavelength region and two photodiodes with direct deposit filters of Al/Mn and Ti/Zr/Al are used to measure the emission intensity Ly-α and He-α λ=2.879 nm spectral lines. Photodiode measurements of x ray emission for irradiation by λ=1064 nm and λ=532 nm laser pulses show that for equal power densities 1064 nm pulses resulted in higher emission intensity than 532 nm pulses, and emission intensity versus power density follows a second order polynomial trend. The nitrogen Ly-α and He-α line intensity and spectral features are carried out using grazing incidence spectrograph and comparisons are made for different conditions.