Positron annihilation spectroscopy in materials structure studies

The Effect of Curing Light Intensity on Free Volume Size in Some Dental Composites

“…The Ps has two spin states, called para-positronium (p-Ps), and ortho-positronium (o-Ps), which are related to antiparallel or parallel spins of the positron and electron, respectively. In a vacuum, p-Ps decays into 2 gamma quanta with a mean lifetime of 0.125 ns, while o-Ps decays into 3 photons with a lifetime of 142 ns [10]. In molecular solids, the lifetime of o-Ps is reduced to a few nanoseconds, due to a competitive process called pick-off annihilation, in which the positron is annihilated by a foreign electron from surrounding atoms with an opposite spin direction.…”

“…This is the so-called thermalization process of the positron. A thermalized positron may be annihilated by an electron and emit two ~511 keV gamma rays [10]. In polymeric materials that have relatively open structure, there is a chance for a positron to form a bound state with an electron during the thermalization process.…”

“…Positron annihilation lifetime spectroscopy (PALS), a non-destructive technique, measures the time interval between the arrival of a positron into a medium and its annihilation. The positron is a nuclear probe to investigate free volume in polymers [6][7][8][9][10][11][12]. As Mostafa et al noted, "When a positron is injected into polymers, it interacts with molecules by an inelastic collision process leading to their ionization and excitation" [9].…”

“…Experimentally, one expects to have 4 distinct lifetime components in a typical lifetime spectrum: free positron annihilation, p-Ps annihilation lifetime, o-Ps annihilation in polymers, and o-Ps annihilation in free volume holes [10][11][12][13][14][15]. Each lifetime component has an associated intensity, corresponding to the fraction of the annihilations taking place with the respective lifetime [10][11][12][13][14][15]. O-Ps annihilation in free volumes provides information about the size and the density of the free volume holes.…”

Estudio e implementación de algoritmos digitales para la mejora de la resolución temporal en sistemas de tomografía por emisión de positrones

“…Así pues, se tiene que la aniquilación directa ocurre un 64 % de las veces, 63 % de las veces cuando el positrón alcanza el equilibrio térmico con el entorno y 1 % de las veces a altas energías, al vuelo. En el primer caso se producen dos fotones con una desviación de la colinearidad que viene dada por [30]:…”