Microtrons

Fig. 7.14 ° Ni Mossbauer spectrum of BaNi02 at 4.2 K. The source, also kept at 4.2 K, of the parent Co was Nio.gsCro.is (97% enriched) activated at Mainz Microtron via the nuclear reaction Ni(y, p) Co. The spectrum shows resolved quadrupole splitting (from [31])...

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). 

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. 

The HS-GC-MIP system consists of a modified HS-6 semi-automated headspace sampler (6) (Perkin Elmer) mounted on a F M-700 gas chromatograph. The outlet of the GC-column is connected to a heated four-way valve for solvent ventilation (Valeo GC-T). From there a heated transfer tube (150°C) is connected to a quartz capillary (2 mm i.d.) that is centered in a 1/4-wave Evenson-type cavity (Electro Medical Supplies, Model 214L). The microwave generator (Electro Medical Supplies, Microtron 200) is operated at 75 W, providing a maximum signal to noise ratio. Emission measurements are carried out with a Perkin-Elmer AAS-403 at 253.7 nm. 

Fig. 7 A bird s eye view of the world s smallest SR machine called AURORA. It has an outer diameter of 3 m. A compact racetrack microtron is used as its injector, which can be seen in the right upper corner of the picture...

The application of microtron photon activation analysis with radiochemical separation in environmental and biological samples was described by Randa et al. (2001), and both flame and plasma emission spectroscopic methods are also widely used. A more recently developed technique is that of laser-excited atomic fluorescence spectrometry (LEAFS) (Cheam et al. 1998). 

Randa Z, Kucera j and Soukal L (2001) Possibilities of simultaneous determination of lead and thallium in environmental and biological samples by microtron photon activation analysis with radiochemical separation. J Radioanal Nucl Chem 248 149-154. 

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. 

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. 

Layout of the microtron (left) and betatron (right). Note that the viewpoints of the sketches are different the beam lies in the plane of the diagram of the microtron, whereas the plane of the beam is perpendicular (horizontal) to that of the cross section of the betatron... 

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... 

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