Beams magnetic transport

Figure 3.12 Magnetic-transport positron beam system. A-grounded shield B-standoff insulators C-coils for magnetic field D-source/moderator E-ExB plates F-lead shielding G-accelerator H-bellows I = aperture J-guiding coils K = turbopump L = sample manipulator M = sample chamber N = CEMA/CCD camera.

Whereas most magnetic-transport beams are a few m long in order to shield the annihilation gamma detector from the source, this length is notrequired if we are interested in particle spectroscopies, especially if electrostatic transport and focusing is incorporated into the design. Canter [45] has published several papers on positron optics to which the reader is referred. Figure 3.13 shows an example from the author s laboratory. 

Fig. 7.9. The positronium source and laser interaction region used by Fee and coworkers. A magnetic field of 100 G (0.01 T) guides the incident beam onto the target and also transports the positrons liberated from the photoionized 2S positronium to the detector. Reprinted from Physical Review Letters 70, Fee et al, Measurement of the positronium 13Si-23Si interval by continuous wave two-photon excitation, 1397-1400, copyright 1993 by the American Physical Society.

The ion-source of the separator is fed by a capillary transporting the recoils therma— lized in the He-jet chamber. The separator beam is extracted at right angle to the axis of the beam line and then travels in a 120° magnet of index n=1/2. From the focal plane the mass-separated beam is then transported by means of a 6m long Einzel lens to a well-shielded collection chamber. A programmable tape-trans port device carries the activity to the counting station where X-rays, y-rays and particles are detected. 

Fig. 2 Layout of isotope separator in beam line C. The numbers indicate (1) He-jet recoil chamber, (2) beam-stdpper, (3) ion-source, (4) analysing magnet, (3) Einzel lens for transport of mass-separated beam, (6) tape-transport and counting station, (7) power/control rack of tape-transport, (8) power/control of mass-separa tor.

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

The inhibition of the fast electron transport in foams can be explained by the following scenario. The current of the fast electron beam produced in the experiments is greater than the Alfven limit, the maximum current that can be propagated by an electron beam in vacuum [41]. For currents exceeding the limit, the intense self-consistent magnetic field associated with the beam will... 

Because tenfold-lower emittance beams can be generated from photocathodes compared with those from conventional thermionic guns, photocathode systems have significant advantages in efficient transportation of the charge from the gun to the radiolysis target and in magnetic pulse compression elfectiveness. 

Sender E, Martinelli JR, Zulu RA, Weeks RA (1987) The use of ion beam analysis for measrrring ion transport in oxides. Cryst Latt Def Amorph Mater 15 277-282 Staschewski D (1969) Kinetik des mit Bicarbonat katalysierten Sauerstuff-Austausches zwischen Kohlendioxid und Wasser. Chemie-Ing-Techn 41 1111-1118 Stanton TR (1990) High pressure isotopic studies of the water diffusion mecharusm in sihcate melts and glasses. PhD Dissertation, Arizona State University, Tempe, Arizona Stebbins J (1995) Dynamics and stmcture of silicate and oxide melts Nuclear magnetic resonance studies. In Stebbins J, McMillan P, Dingwell D (eds) Stiucture Dynamics and Properties of Sihcate Melts, Rev Mineral 32... 

Fig. 9.30 Schematic experimental setup for optical cooling in the MOT atom transport is achieved in the magnetic trap by switching off laser beam 1...

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