Solid targets

Nebulizers can be divided into several main types. The pneumatic forms work on the principle of breaking up a stream of liquid into droplets by mechanical means the liquid stream is forced through a fine nozzle and breaks up into droplets. There may be a concentric stream of gas to aid the formation of small droplets. The liquid stream can be directed from a fine nozzle at a solid target so that, on impact, the narrow diameter stream of liquid is broken into many tiny droplets. There are variants on this approach, described in the chapter devoted to nebulizers (Chapter 19). 

X-rays High-energy electromagnetic radiation usually produced by the action of high-energy electrons hitting a solid target. 

Investigation of interaction of electrons of different energies with a solid material in plasma processes may be even more intriguing and important, especially in the case of an adsorbed layer of materials contained in the reaction vessel. Provided thin semiconductor films deposited on the walls of the reaction vessel are used as solid targets, these films can be simultaneously used as targets and semiconductor sensors. This is also the case when such films are deposited on the specially manufactured quartz plates with electrodes accessible from the outside of the vessel. These sensors can be placed in any point of the vessel. 

High sensitivity is featured by the electrical methods used to detect EEPs. These are based on measuring small currents that occur in the course of selective ionization of EEPs, or currents of secondary emission of electrons or ions knocked by EEPs out of the surface of solid targets (if such emission is taking place). 

An alternative way to preform a channeled plasma consists in exploiting the nanosecond precursor that usually precedes a short femtosecond pulse in the output of a multi-terawatt laser system. In fact, the amplified spontaneous emission (ASE) pedestal has typically an intensity 106-1010 times lower than the main pulse, which, however, can be sufficient to ionize a gas-jet or a solid target. This drawback can be turned into a benefit assuming that this long precursor can prepare the plasma channel for the short pulse propagation. 

In some experiments, the same solid target acted for both accelerator and radiator. 

The most direct and easy way consists in focusing the laser pulse onto a solid target and to collect the radiation emitted by the produced plasma. The wide emitted spectrum extends from infrared to X-rays and it is produced by different physical mechanisms Bremsstrahlung, recombination, resonant lines, K-shell emission from neutral (or partially ionized) atoms. In particular, this latter mechanism has been recognized, since a decade, as a way of producing ultrashort monochromatic radiation pulses at energy up to several keV. 

Because of their inertia, the effect on the ions of the high frequency electric field of the electron plasma waves dealt with so far averages out to zero. However, surprisingly, light ions and/or protons have been observed in ultrahigh intensity laser pulse interaction with solid targets since the late 1990s [33-36]. Later on, these particles were found to come from contaminant layers on... 

The DPM was revealed to be a very robust device and is now routinely used for most of the experiments carried out on the UHI 10 TW laser chain involving ion acceleration as well as high order harmonics generation from solid targets [62],... 

Fig. 8. Proton shopping cross section in solid targets (a) silicon, (b) nickel and (c) gallium as a function of projectile velocity. Lines and symbols have the same meaning as in Fig. 7.

In a laboratory generator, electrons are accelerated by a potential around 30 kV towards a solid target, where they are stopped by impact. The output contains the line spectmm superimposed upon a continuous spectrum. The line, or characteristic spectrum is characteristic of the element and is used in X-ray fluorescent analysis to identity the type and amount of an element present in a sample. The continuous radiation is also called the Bremsstrahlung, from the... 

The use of radioactivated discs of aluminum and steel to estimate fragment erosion in solid targets at very high velocities was found to be feasible. Activation of discs was accomplished by slow neutron irradiation in a nuclear reactor at a flux of 8 x 1012 neutrons per cm2 per sec for 3 days for the aluminum discs (4g, 2.5cm diam x 0.3cm thick), and for 4 hrs for the steel discs (5g, 2.5cm diam x 0.15cm thick). Gamma-ray spectrometry indicated the presence of 59Fe (half-life 46 days) and 51Cr (half-life 28 days) in ratios 0.5 for aluminum and 1.3 for steel. The radioactivities in the aluminum arose solely from impurities, whereas in the steel they were contributed by the major component, iron, and only supplemented by the chromium impurity. The radioactivity was found by successive acid soln determinations to be distributed evenly in both metals... 

Sputtering A third frequently used method is sputtering (Fig. 8.3), a technique in which metal atoms are removed from a solid target by means of ion bombardment... 

Teubner U, Uschmann I, Gibbon P, Altenbernd D, Forster E, Feurer T, Theobald W, Sauerbrey R, Hirst G, Key MH, Lister J, Neely D (1996) Absorption and hot electron production by high intensity femtosecond uv-laser pulses in solid targets. Phys Rev E 54 4167-4177... 

Using laser-induced X-ray pulses from solid targets, ultrafast time-resolved X-ray diffraction experiments have been performed extensively and new phenomena such as ultrafast melting have been demonstrated [26,27]. In the present study, in order to demonstrate the practical capabilities of the X-rays thus produced, the pulse X-ray diffraction obtained from a Si crystal with this source is examined [28]. 

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