Pulsed electron sources, very high

The Dosimetry of Very High Intensity Pulsed Electron Sources Used for Radiation Chemistry II. Dosimetry for Gaseous... 

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

Radiation grafting [83, 84, 85, 86, 87, 88, 89] is a very versatile and widely used technique by which surface properties of almost all polymers can be tailored through the choice of different functional monomers. It covers potential applications of industrial interest and particularly for achieving desired chemical and physical properties of polymeric materials. In this method, the most commonly used radiation sources are high-energy electrons, y-radiation, X-rays, U.V.-Vis radiation and, more recently, pulsed laser [90], infrared [91], microwave [92] and ultrasonic radiation [93]. Grafting is performed either by pre-irradiation or simultaneous irradiation techniques [94, 95]. In the former technique, free radicals are trapped in the inert atmosphere in the polymer matrix and later on the monomer is introduced into... 

Figure 12.8 The two mtegories of detectors used for energy dispersive X-ray fluorescence spectrometry. (a) Proportional counter used in pulse mode (b) Cooled Si/Li diode detector using Peltier effect (XR detector by Amptek Inc.) (c) Functioning principle of a scintillation detector containing a large size reverse polarized semi-conductor crystal. Each incident photon generates a variable number of electron-hole pairs. The very high quantum yield enables the use of low power primary sources of X-rays (a few watts or radio-isotopic sources).

There is first the fundamental of a strong infrared laser. For the sake of illustration, we have chosen the case of a Ti sapphire laser operated at coi = 1.55 eV, i.e. a>i = 0.057 a.u., which is representative of the recently operated "femtosecond" sources. We have considered intensities li around lO l W/cm, which are typical. Note that, although such intensities are very high by laboratory standards, they remain quite moderate when compared to the "atomic" intensity, Iq = 3.5 lOl W/cm, which is associated to the field strength experienced by an electron on the first Bohr orbit in hydrogen, namely Fq = 5.1 10 V/cm. At intensities li =10 W/cm the atom can experience multiphoton ionization and even ATI, as shown in Fig, 1, which displays the simulation of a photoelectron spectrum for a peak intensity 1l = 2. 10 W/crri. Here, the pulse shape is assumed to be trapezoidal with linear tum-on and turn-off durations of one laser period Ti = 110 a.u., i.e. Ti = 2.6 fs), the total duration of the pulse being 8Tl. 

PEPICO experiments work best with a continuous light source that is relatively weak, and with very high collection efficiencies for both electrons and ions. A pulsed source such as a 10 Hz pulsed VUV laser would not work because each laser pulse would produce many electrons and ions within a 10 ns interval. Thus, it is impossible to distinguish which electron is associated with which ion. 

A family of vacuum-tube MMW sources is based on the propagation of an electron beam through a so-called slow-wave or periodic structure. Radiation propagates on the slow-wave structure at the speed of the electron beam, allowing the beam and radiation field to interact. Devices in this category are the traveling-wave tube (TWT), the backward-wave oscillator (BWO) and the extended interaction oscillator (EIO) klystron. TWTs are characterized by wide bandwidths and intermediate power output. These devices operate well at frequencies up to 100 GHz. BWOs, so called because the radiation within the vacuum tube travels in a direction opposite to that of the electron beam, have very wide bandwidths and low output powers. These sources operate at frequencies up to 1.3 THz and are extensively used in THZ spectroscopic applications [10] [11] [12]. The EIO is a high-power, narrow band tube that has an output power of 1 kW at 95 GHz and about 100 W at 230 GHz. It is available in both oscillator and amplifier, CW and pulsed versions. This source has been extensively used in MMW radar applications with some success [13]. 

Photolysis is a selective way to make radicals from a suitable precursor. It is very clean and particularly useful in emission experiments. The energy required to photolyze a precursor is typically about 6 electron volts. This means that photolysis sources will be pulsed lasers or resonance lamps. Lasers such as XeCl (A. = 308 nm) or ArF (A = 193 nm) with pulse widths of about 10 ns and repetition rates of around 100 Hz will thus make transients species in short bursts. It is a difficult task to couple such a system to a Fourier transform spectrometer but it has been done in a dynamics study [23]. Resonance lamps, while providing the appropriate energy, usually do not provide the flux necessary to generate the high concentrations of unstable species needed for a successful spectroscopy experiment [24],... 

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