Pulsed positron beam

B. Ghaffari, R. S. Conti, and D. W. Gidley Mat. Sci. Forum 255, 248 (1997) B. Ghaffari A Pulsed Positron Beam to Measure the 23Pi —> 23 Pj Energy Intervals in Positronium Discovery of Chaotic Transport. Ph. D. Thesis, University of Michigan, Ann Arbor (1997)... 

If sufficient positrons can be confined, studies of particle transport within the plasma, etc., similar to those conducted with electrons can be carried out. It may be possible to use the enhanced detection possibilities afforded since positron-electron annihilations can be detected. An ultra-cold source of positrons would also have a variety of other applications.24 For example, it has been proposed to eject trapped positrons into a plasma as a diagnostic.25 Also, positrons initially in thermal equilibrium at 4.2K within a trap would form a pulsed positron beam of high brightness when accelerated out of the trap. 

In the simplest measurement, data collected for two energies can provide a measure of the escape depth, the porosity and the open porosity fraction. The values are extracted by comparing the measurements to a set of calibration curves. With a positron beam flux of 0.2 pA, 100 locations on a wafer could be checked in 30 minutes. Mean lifetime measurements can be carried out with a pulsed positron beam. An intermediate time resolution of about 1 ns will be sufficient. A single measurement can be accomplished in about 1 minute. 

Positronium formation was also found in porous Si obtained by anodization of crystalline silicon in HF acid solutions. Itoh, Murakmi and Kinoshita [11] found a long-lived (>10 ns) component in the positron lifetime spectrum measured by conventional PALS. The authors investigated Ps behavior in porous Si at various temperatures by means of PALS with a monoenergetic pulsed positron beam [12],[13]. 

Suzuki, R., Kobayashi, Y., Mikado, T., Matsuda, A. et al. (1991) Characterization of hydrogenated amorphous silicon films by a pulsed positron beam , Jpn. J. Appl. Phys. 30,2438. 

Suzuki, R., Ohdaira, T. and Mikado, T. (1998) Low-energy pulsed positron beam at the ETL linac facility Proc. Int. Workshop on Advanced Techniques of Positron Beam Generation and Control, RIKEN, Wako, Japan (Committee of Crossover Research Program for Basic Nuclear Science). 

Using the previous equations we can derive the microscopic structure (/. e., the kinds of defects and their concentrations) from the experimentally determined lifetimes and intensities if the problem is homogeneous. This premise, however, does not hold for RPV steels completely. In inhomogeneous problems, the diffusion of positrons from the various implantation sites to the trapping centres must also be considered [125,130]. However, the mathematical difficulties associated with the corresponding diffusion-trapping model (DTM) [73] have so far prevented exact solutions from being obtained for all but the simplest problems [116,117], Thus, it is impossible to qualitatively analyse the very detailed experimental results obtained with a pulsed positron beam. 

Fig. 8.6. Schematic of a proposed configuration for the production of conditions for Bose-Einstein condensation of positronium using a pulsed, brightness-enhanced positron beam (see text for details). Reprinted from Physical Review B 49, Platzman and Mills, Possibilities for Bose condensation of positronium, 454-458, copyright 1994 by the American Physical Society.

The facility-based system is still the main hope for intense positron beam generation. In the case of LINACs the beam is pulsed this property can be exploited in applications where timing is an advantage. 

A simple mass spectrometric experiment with a well-defined positron beam would give us much useful information. Much more information would be obtained by the application of recoil ion momentum spectroscopy (RIMS) [23, 24] to annihilation from positron-molecule resonances. This would provide the energies and masses of all the ionic fragments. One possible configuration of a RIMS spectrometer involves crossed beams of a supersonic molecular beam of target molecules and a pulsed beam of positrons. This experiment is possible with existing technology [25]. 

Suzuki, R., Ohdaira, T., Uedono, A. and Kobayashi, Y. (in press) Positron annihilation in Si02-Si studied by a pulsed slow positron beam , Appl. Surf. Sci. 

Hamada, E., Oshima, N., Katoh, K., Suzuki, T., Kobayashi, H., Kondo, K., Kanazawa, I., Ito, Y. (2001) Application of a pulsed slow-positron beam to low-density polyethylene film . Acta physicapolonicaA. 99,373. 

During the last decade, a number of facilities have been built which use this so-called electroproduction of positrons. A review of the field has been given recently by Dahm et al. [3.14]. The large majority of the electron accelerators used for this purpose are linear machines (LINAC s), but it is also possible to use a microtron (Mills et al. [3.15]). All of the accelerators deliver pulsed electron beams, and their time structure is transferred to the resulting primary slow positron beam. Typical repetition rates are in the order of 100 s, while the pulse duration varies from a few ns to some (is. 

List of facilities where slow positron beams are electro-produced. Maximum electron energy f , repetition rate and beam pulse duration, as well as the largest observed slow positron intensity. (Numbers in parentheses are "expected intensities.) The electron accelerators are UNACs except... 

Although the time structure of the primary positron beams may be of advantage to some experiments (see e g. Howell et al. [3.18]), the saturation and pile-up effects inherently connected with high-intensity bunched beams often cause problems in other experiments. Therefore, a number of present electro-producing positron facilities have been equipped with storage and pulse-stretching devices (see, e.g., Ebel et al. [3.17], Akahane and Chiba [3.19], Ito et al. [3.20], and Hulett et al. [3.21]). 

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. 

A small fraction of the orthopositronium atoms produced pass through the cw-excitation beam, where they are promoted to the 23Si level and then through a multi-pass doubled-YAG beam at 532 nm, where they are photo-ionized. The photo-ionized positron is electro-statically accelerated and magnetically-guided into a channel-electron multiplier array (CEMA) where it is detected. The time-of-Hight between the incident positron pulse and the photo-ionization pulse determines the range of positronium velocities detected. 

Work is currently underway to re-measure the 1S-2S energy level splitting. An electron accelerator beam dump at AT T Bell Laboratories has been shown to produce 4 x 104 slow positrons/pulse at 30 Hz, and further improvements are expected.[ll] We anticipate a 10 increase in the number of available thermal Ps. In collaboration with M. Fee and K. Danzmann at Stanford, we are also working... 

Direct detection of fast positrons is possible with scintillator detectors as the processes involved are the same as for gamma rays without the initial Compton or photoelectric event. In positron experiments the detection of fast positrons by thin plastic scintillators has been put to best use in time-of-flight and lifetime studies [21, 22], Here almost 100% of the positrons from a radioactive source or MeV beam which pass through a thin scintillator disc deposit enough energy (> 20keV) to create photons which produce a useable time-zero tagging pulse in a PM. (See secs. 3.5). 

Figure 10 Thermal positronium-laser beam interaction region. Positronium is formed by a bunch of positrons that is stopped by a clean A1 surface in ultrahigh vacuum. Positronium atoms thermally desorbed from the surface are ionized by the laser and the e fragments are collected by a single particle detector. The laser pulse is narrowed in frequency by the Fabry-Perot interferometer.

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