positron emission is a type of nuclear decay that effectively turns a proton into a neutron.
this type of nuclear decay effectively turns a proton into a neutron?
positron emission
usually unstable nuclides are clearly either "neutron rich" or "proton rich", with the former undergoing beta decay and the latter undergoing electron capture (or more rarely, due to the higher energy requirements, positron decay). however, in a few cases of odd-proton, odd-neutron radionuclides, it may be energetically favorable for the radionuclide to decay to an even-proton, even-neutron isobar either by undergoing beta-positive or beta-negative decay. an often-cited example is the single isotope (29 protons, 35 neutrons), which illustrates three types of beta decay in competition. copper-64 has a half-life of about 12.7 hours. this isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. this particular nuclide (though not all nuclides in this situation) is almost equally likely to decay through proton decay by positron emission (18%) or electron capture (43%) to , as it is through neutron decay by electron emission (39%) to .
beta decay
for the naa procedure to be successful, the specimen or sample must be selected carefully. in many cases small objects can be irradiated and analysed intact without the need of sampling. but, more commonly, a small sample is taken, usually by drilling in an inconspicuous place. about 50 mg (one-twentieth of a gram) is a sufficient sample, so damage to the object is minimised. it is often good practice to remove two samples using two different drill bits made of different materials. this will reveal any contamination of the sample from the drill bit material itself. the sample is then encapsulated in a vial made of either high purity linear polyethylene or quartz. these sample vials come in many shapes and sizes to accommodate many specimen types. the sample and a standard are then packaged and irradiated in a suitable reactor at a constant, known neutron flux. a typical reactor used for activation uses uranium fission, providing a high neutron flux and the highest available sensitivities for most elements. the neutron flux from such a reactor is in the order of 10 neutrons cm s. the type of neutrons generated are of relatively low kinetic energy (ke), typically less than 0.5 ev. these neutrons are termed thermal neutrons. upon irradiation, a thermal neutron interacts with the target nucleus via a non-elastic collision, causing neutron capture. this collision forms a compound nucleus which is in an excited state. the excitation energy within the compound nucleus is formed from the binding energy of the thermal neutron with the target nucleus. this excited state is unfavourable and the compound nucleus will almost instantaneously de-excite (transmutate) into a more stable configuration through the emission of a prompt particle and one or more characteristic prompt gamma photons. in most cases, this more stable configuration yields a radioactive nucleus. the newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons. this decay process is at a much slower rate than the initial de-excitation and is dependent on the unique half-life of the radioactive nucleus. these unique half-lives are dependent upon the particular radioactive species and can range from fractions of a second to several years. once irradiated, the sample is left for a specific decay period, then placed into a detector, which will measure the nuclear decay according to either the emitted particles, or more commonly, the emitted gamma rays.
neutron activation analysis