Accelerators microtron

Almost since the earliest attempts to produce well-defined beams of low energy positrons, various types of accelerator have been used for this purpose, e.g. electron linear accelerators, microtrons and cyclotrons (see e.g. Dahm et al., 1988 Itoh et ai, 1995). Positron beams have also been developed at nuclear reactors (Lynn et ai, 1987). 

This regime is relevant to operation at short wavelengths in the infrared and optical spectra. These experiments typically employ electron beams generated by radiofrequency linear accelerators, microtrons, storage rings, and electrostatic accelerators in which the total current is small. 

Mills Jr., A.P., Shaw, E.D., Chichester, R.J. and Zuckerman, D.M. (1989b). Production of slow positron bunches using a microtron accelerator. Rev. 

Fast positrons are created by bremsstrahlung pair-production in an electron microtron accelerator. These are moderated and bunched into 25 ns packets at 30 Hz, each comprised of 2 x 104 slow positrons. The positrons are guided by a 150-G magnetic field and implanted at 1-2 keV kinetic energy onto an Al(lll) crystal heated to 576 5 K as shown in Figure 7a. About 30% of the incident positrons come off the surface as thermal positronium with a velocity distribution that is a beam Maxwellian. 

Of these accelerators, two types have mostly been used for photon activation analysis, namely the linear accelerator (also called linac) and the microtron. These and other accelerators will not be described in detail here since normally the analyst is engaged in the sample handling, activity measurement, and data processing rather than in the operation of the radiation sources, which is usually done by separate operating personnel. 

Microtrons Cyclic accelerators in which the particles execute circular motion in a uniform magnetic field that carries the beam through an rf accelerating cavity, one in each cycle. 

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. 

He might have been right had not competition from accelerators been growing by leaps and bounds. By 1972 about one hundred and twenty medical accelerators had been sold world-wide, including over a hundred by Varian. Each one represented lost business for CPD. Worst of all, after two years of effort, neither of CPD s development programs was close to producing a marketable machine. The microtron was proving... 

The novelty of the microtron vras also offtet by the fact that Chalk River had developed a radical new accelerator concept of its own. It employed an electromagnet at one end of the standing-wave accelerator cylinder to reverse the direction of accelerated electrons. Once the electrons were turned around, they were subjected ain to the same propellant forces. The result was a device that produced twice... 

Betatrons, microtrons and Van de Graaff accelerators are also used. The latter can generate electron beams of very high intensity (several mA) with energies limited to 10 MeV. 

The microtron is a circular electron accelerator with particularly attractive possibilities but is still rarely used. It is rugged, easy to operate and incurs lower operating costs than a linear accelerator at equivalent performance. This gives it significant advantages over the latter. 

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