Positron moderators

Choice of moderator depends on the application of the positron beam. For implantation defect spectroscopy the priority is to maximise moderator efficiency, whereas for electrostatic systems a well-collimated parallel beam requires a planar low-(f + surface cooled to minimise thermal smearing. 

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]. 

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Because the positron moderator and the Ps converter require well characterized metal surfaces, the entire positron apparatus is in an ultra high vacuum chamber ( 2x10 torr). 

Fig. 2.10 Positron emission tomography (PET) perfusion images showing a zone at risk. There is a moderate resting perfusion defect indicating a small non-transmural scar in the distribution of the LAD. After dipyridamole stress, the...

Fig. 1.7. Comparison of the energy spectrum of / + particles from a radioactive source with that for moderated positrons.

In solids the free positron lifetime r lies in the approximate range 100-500 ps and is dependent upon the electron density. Following implantation, the positrons are able to diffuse in the solid by an average distance L+ = (D+t)1//2, where D+ is the diffusion coefficient. This quantity is usually expressed in cm2 s-1 and is of order unity for defect-free metallic moderators at 300 K (Schultz and Lynn, 1988). The requirement of very low defect concentration arises because the value of D+ is otherwise dramatically reduced owing to positron trapping at such sites. 

Inserting values for D+ and r into the expression for L+, a value of approximately 1000 A is obtained, and this is typical for metals. One can then find an estimate of the efficiency e of the moderator by multiplying the implantation profile P(x), equation (1.9), by the probability that a positron reaches the surface from a depth x, exp(—x/L+), and integrating over all values of x. Then, since pimpL+ -C 1, the efficiency may be written as... 

It was found that the boron target itself acted as a moderator with a low efficiency of 10-7, but the emitted positrons had a low energy, and therefore a narrow energy width, of approximately 0.1 eV. 

The atomic beam was formed by a multichannel capillary array, placed perpendicular to the positron beam, with a 2.5 mm2 effusing area and a length-to-diameter ratio of 25 1. The head pressure behind the array was kept at 9 torr (ss 103 Pa) in the initial measurements. An annealed tungsten moderator was used to provide a beam of more than 105 positrons per second at 200 eV. A much more intense beam of electrons could also be obtained by reversing the electrostatic potentials on the various elements which made up the transport system. Channel electron multipliers (CEM1 and CEM2 respectively) were used to monitor the incident and scattered beams. In later versions of the apparatus, a third... 

The absolute scale of the cross sections was obtained by making measurements with the secondary electron beam produced by (3+ bombardment of the moderator. Comparing the ion count rates measured at detector 2 obtained from both electron and positron impact gave the ratio [[Pg.179]

Moriarty, P. and Fung, S. (1987). A field-assisted moderator for low-energy positron beams. Appl. Phys. A 42 111-116. 

Greaves, R.G. and Surko, C.M. (1996). Solid neon moderator for positron trapping experiments. Can. J. Phys. 74 445-448. 

Mills Jr., A.P. and Gullikson, E.M. (1986). Solid neon moderator for producing slow positrons. Appl. Phys. Lett. 49 1121-1123. ... 

To monitor tumor response to capecitabine therapy noninvasively, Zheng and co-workers, from the Indiana University School of Medicine, developed the synthesis of the fluorine- 18-labeled capecitabine as a potential radiotracer for positron emission tomography (PET) imaging of tumors.28 Cytosine (20) was nitrated at the C-5 position with nitric acid in concentrated sulfuric acid at 85°C, followed by neutralization to provide 5-nitrocytosine (27) in moderate yield. This nitro pyrimidine was then carried through the glycosylation and carbamate formation steps, as shown in the Scheme below, to provide the 6/s-protected 5-nitro cytidine 28 in 47% for the three-step process. Precursor 28 was then labeled by nucleophilic substitution with a complex of 18F-labeled potassium fluoride with cryptand Kryptofix 222 in DMSO at 150 °C to provide the fluorine-18-labe led adduct. This intermediate was not isolated, but semi-purified and deprotected with aqueous NaOH in methanol to provide [l8F]-capecitabine in 20-30% radiochemical yield for the 3-mg-scale process. The synthesis time for fluorine-18 labeled capecitabine (including HPLC purification) from end of bombardment to produce KI8F to the final formulation of [18F]-1 for in vivo studies was 60-70 min. 

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