Electron pulse chambers

In liquids with sufficiently high electron mobilities, the ionization electrons produced in the track of an individual particle or quantum can be detected separately. An advantage of this method is the fact that once the ionization electrons have escaped into the bulk of the liquid, no losses due to volume recombination with positive charge carriers occur. A disadvantage is, however, that electron attachment to electronegative impurities influences the electron signal. This is the foimdation of the application of liquids in electron pulse chambers (see Section 9.2). 

Basically, two modes of operation of a liquid ionization chamber can be distinguished (1) ion current measurement and (2) electron pulse detection and counting. Ion current chambers are used in medical physics for dosimetry and radiation field mapping, while electron pulse chambers are employed in the detection of individual elementary particles and quanta in radiation and high energy physics. 

The field of liquid ionization chambers received a fresh impulse because of the interest expressed by Prof. Carlo Rubbia, and Dr. Dieter Schinzel who initiated at CERN, the European High Energy Physics Laboratory at Geneva, a vigorous program on electron pulse chambers to which I had the good fortune of contributing for one year and a half in 1986/87. 

Pulsed electron beam deposition (PED) Pulsed electron beam source emitting 100 ns long electron pulses with 10-20 keV and kA intensity into the deposition chamber, no excimer laser is required, innovative complimentary technique to PLD, further extending the range of materials to be grown as thin films by pulsed energy techniques [128,135]... 

Fig. 18. (a) Ultrafast electron diffraction apparatus consisting of an electron gun chamber, a diffraction chamber, and a detector chamber. Two fs laser pulses are used, one to initiate the chemical change and the second to generate the electron pulse, (b) Detector system incident electrons either directly bombard a small CCD or strike a phosphor-coated fused fiber-optic window. Light emitted from the phosphor is amplified by an image intensifies and brought to a scientific-grade CCD. Both CCDs are thermo-electrically cooled [reproduced with permission from (96), p. 1601. 

The electrons emitted by the photocathode are subsequently accelerated to 50 kV and focused on to a toroid-shaped anode. The anode is made of oxygen-free, high conductivity copper and is maintained at a high positive potential. The electron pulses interact with the copper anode forcing the emission of Cu-Ka x-ray photon pulses, which exit the vacuum chamber through a thin beryllium-foil window. A bend germanium crystal monochromator disperses and focuses the x-rays onto the sample. The duration of the x-ray pulses is measured by a Kentech x-ray streak camera fitted with a low density Csl photocathode. The pulse width of the x-rays at 50 kV anode-cathode potential difference is about 50 ps. This value is an upper limit for the width of the x-ray pulses because the transit time-spread of the streak camera has to be taken into consideration. A gold photocathode (100 A Au on 1000 A peiylene) is used to record the 266-nm excitation laser pulses. The intensity of the x-rays is 6.2 x 10 photons an r (per pulse), and is measured by means of a silicon diode array x-ray detector which has a known quantum efficiency of 0.79 for 8 kV photons. 

The pulsed technique employed by Henchman et uses an ordinary source in which a short electron pulse provides reactant ions. Another short pulse applied to a repeller plate accelerates these to a definite energy (about 1 eV). Before emerging from the exit slit of the chamber, the reactant ions may collide with un-ionized gas to give product ions. The forward velocity of both reactant and product ions is measured by applying a variably delayed gating pulse to a deflection electrode outside of the ionization chamber. A more recent version of the apparatus incorporates separate ion production and reaction chambers, as well as a stopping potential analyzer. ... 

Drift velocity measurements of ions formed by an electron pulse of sec duration convinced Hornbeck that the molecular ions Hc2, Nc2, and Ar2 were not formed by collision of an atom in a metastable state with a ground-state atom, as had been proposed by Arnot and M Ewen In confirmation of this conclusion, Hombeck and Molnar " reported a mass spectrometric study of molecular ion formation in the noble gases. This mass spectrometric technique, which may be called single-chamber, continuous mass spectrometry, was the same as employed by Arnot and M Ewen and by many workers in more recent times. 

The tube of Figure 2-2 can be operated as an ionization chamber, as a proportional counter, or as a Geiger counter. The tube output differs radically from one case to another. Because of these differences, the electronic circuitry associated with the tube must also be different for each case if the pulses from the tube are to be reliably selected and counted. In particular, the circuitry will have to differ in characteristics such as stability, amount of amplification, and time of response. In all cases, linear amplification (amplifier output always proportional to tube output) is desirable. 

Figure 5.6. Diagram of a low-energy, high-angle electron-impact spectrometer. (A) Electron gun (B) monochromator (180° spherical electrostatic energy selector) (C) electron optics (D) scattering chamber (E) analyzer (180° spherical electrostatic energy selector) (F) electron multiplier (G) amplifier and pulse discriminator (H) count-rate meter (I) multichannel scaler (J) X Y recorder (K) digital recorder. (After Kupperman et a/.<42))...

The ion reaction chamber in the present work was at a relatively high pressure (10 torr), so that conditions were similar to those used with our previous pulsed-electron high-pressure sources.8,9 Reactors operating at lower pressures such as 1 torr or less should also be suitable. Thus, ES could probably be easily adapted for use with flow tubes such as FA and SIFT. 

The BF3 proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced. 

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