Positrons

There are other less common types of radioactive decay. Positron emission results in a decrease by one unit in the atomic number K capture involves the incorporation of one of the extranuclear electrons into the nucleus, the atomic number is again decreased by one unit. 

Corpuscular emission (neutrons, protons, electrons, positrons) + + ... 

Duke C B 1994 Interaction of electrons and positrons with solids from bulk to surface in thirty years Surf. Sc/. 299-300 24... 

Natural titanium is reported to become very radioactive after bombardment with deuterons. The emitted radiations are mostly positrons and hard gamma rays. The metal is dimorphic. The hexagonal alpha form changes to the cubic beta form very slowly at about 88O0C. The metal combines with oxygen at red heat, and with chlorine at 550oC. 

Also arising from relativistic quantum mechanics is the fact that there should be both negative and positive energy states. One of these corresponds to electron energies and the other corresponds to the electron antiparticle, the positron. 

Converting a proton to a neutron results in the emission of a positron, i[3. 

K. F. Hubner, J. CoUmann, E. Buonocore, and G. W. Kabalka, Clinical Positron Emission Tomography, Mosby Year Book, St. Louis, Mo., 1991. 

In addition to Compton scattering, y-rays having energies above 1022 keV interact with matter by a process called pair production, in which the photon is converted into a positron and an electron. The y-ray energy in excess of the 1022 keV needed to create the pair is shared between the two new particles as kinetic energy. Each j3 -particle is then slowed down and annihilated by an electron producing two 511-keV photons. 

IT = isomeric transition, EC = electron capture, l3 = positron emission, and j3 = beta decay. 

Alternatively, smaller on-site cyclotrons can provide immediate access to substantial quantities of short half-life positron-emitting isotopes such as... 

Positron Imaging. Creating images of distributions of positron emitters requires a somewhat different type of apparatus. Positron cameras use many of the same technologies as do cameras for other isotopes, but there is a broader array of methods and physical arrangements. AH of these systems take advantage of the physical characteristics of positrons. 

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