Positron drift

Fig. 6.17. Schematic illustration of the positron-drift apparatus used by Paul and coworkers.

Fig. 6.18. Results from the positron-drift experiments of Paul and coworkers. The full curve is v+/(Zefj), obtained from a fit to all the data see text for details. Electron drift velocities (+) are shown for comparison on the left-hand scale. The points to the right of the vertical broken line were taken from runs at molecular hydrogen pressures of 50 torr ( ), 25 torr (a) and 10 torr (o).

Bose, N., Paul, D.A.L. and Tsai, J.-S. (1981). Positron drift in molecular hydrogen. J. Phys. B At. Mol. Phys. 14 L227-L232. 

Charlton, M. and Laricchia, G. (1986). Positronium formation and positron drift experiments. In Positron(Electron)-Gas Scattering, eds. W.E. Kauppila, T.S. Stein and J.M. Wadhera (World Scientific) pp. 73-84. 

It is interesting to compare the properties of positive electrons, positrons, with the properties of electrons in nonpolar liquids. Values of the mobility of positrons, )x +, are now available for a few liquids. Early measurements for in -hexane ranged from 8.5 to 100 cm /Vs [181,182]. In a recent study, the Doppler shift in energy of the 511-keV annihilation gamma ray in an electric field was utilized to measure the drift velocity. This method led to fi+ = 53 cm /Vs in -hexane and 69 cm /Vs in 2,2,4-trimethylpentane [183]. Interestingly, these values are comparable to the mobilities of quasi-free electrons in nonpolar liquids. 

In this section we review the results from positron annihilation experiments, predominantly those performed using the lifetime and positron trap techniques described in section 6.2. Comparisons are made with theory where possible. The discussion includes positron thermalization phenomena and equilibrium annihilation rates, and the associated values of (Zeff), over a wide range of gas densities and temperatures. Some studies of positron behaviour in gases under the influence of applied electric fields are also summarized, though the extraction of drift parameters (e.g. mobilities) is treated separately in section 6.4. Positronium formation fractions in dense media were described in section 4.8. 

Several studies have been made of the behaviour of low energy positrons in gases under the influence of a static electric field e. The broad aim of this work has been to study the diffusion and drift of positrons in order to understand better the behaviour of the momentum transfer and annihilation cross sections at very low energies. The theoretical background has been given in section 6.1, and the diffusion equation with an... 

Paul and Tsai (1979) showed that the fraction Fd of positrons which drift into the foil after stopping in the gas is given by... 

As shown in Figure 6.18, electron drift velocities below e/p = 1 Td (= 1017 V cm2) are at least four times larger than those for positrons. Bose, Paul and Tsai (1981) attributed this difference to higher momentum transfer cross sections for positrons than for electrons at very low (i.e. 

When an electric field was applied across the chamber some positrons annihilated prematurely, following field-induced drift to one of the electrodes. In this case the free-positron component of the lifetime spectrum was field dependent the maximum drift time, rmd, was given by the end-point of the lifetime spectrum and was due to thermalized positrons which had traversed the entire drift length l. The drift speed was then v+ = 1/rmd and the mobility could be found from... 

Charged particles were detected by the telescopes Ti and T2. The track coordinates were measured by drift chambers. The time interval between detector hits in Ti and T2 was measured by scintillation hodoscopes. Electrons and positrons were rejected by gas Cherenkov counters, and muons by scintillation... 

The formation will be done by pushing antiprotons through a rotating positron plasma. The rotation is an unavoidable result of the E x B drift of the positrons in the magnetic field. The rotation frequency depends on the spatial density of the plasma [34]. 

Moderation efficiency could be greatly enhanced by drifting a larger fraction of thermalised positrons to the exit surface. Attempts to realise field-assisted moderation have to date largely foundered because of the interactions of positrons with the interfaces between the material across which an electric field is maintained and the conductive coatings to which the potentials are applied. One reported observation of the enhancement of positron emission by an electric field has been that from a solid gas moderator whose surface was charged by electron bombardment [44]. 

The large area, 2m x 2m, of AGATE has lead to the selection of drift chambers for the tracking detector, rather than the spark chambers used in EGRET. Drift chambers have fewer wires and much less deadtime per event. The power per wire is low enough to use many layers in order to reduce the multiple scattering of the electron and positron before the gamma-ray direction can be measured. 

This device is in principle a classical double-gap buncher with a travel time of 10 ns for the positrons through the drift tube. When compared to a A/4 coaxial resonator, the double-gap resonator is preferable because of the shorter length and the lower rf power consumption resulting from two bunching gaps. The rf power is fed in by a coupling loop and a pick-up loop is used for control and regulation pur-... 

Detecting a tag in two-photon reactions does not require the full power of the detector only the calorimeter and some ability to reject backgrounds. The calorimeter should be able to distinguish the high energy (1-3 GeV) scattered positron from hadron backgrounds, while photon rejection could be provided from the corner region of the drift... 

---