Positron annihilation lifetime spectroscopy using fast scintillators and digital electronics

Abstract

Positron Annihilation Lifetime Spectroscopy (PALS) is a non-destructive radiological technique widely used in material science studies. PALS typically relies on an analog coincidence measurement setup and allows the estimate of the positron lifetime in a material sample under investigation. The positronium trapping at vacancies in the material results in an increased lifetime. In this work, we have developed and optimized a PALS experimental setup using organic scintillators, fast digitizers, and advanced pulse processing algorithms.We tested three pairs of different organic scintillation detectors: EJ-309 liquid, EJ-276 newly developed plastic, and BC-418 plastic, and optimized the data processing parameters for each pair separately. Our high-throughput data analysis method is based on single-pulse interpolation and a constant fraction discrimination (CFD) algorithm. The setup based on the BC-418 detector achieved the best time resolution of 198.3 ± 0.8 ps. We used such optimized setup to analyze two single-crystal quartz samples and found lifetimes of 156 ± 9 ps and 366 ± 22 ps, in good agreement with the characteristic time constants of this material.The proposed experimental set up achieve an excellent time resolution, which makes it possible to accurately characterize material vacancies by discriminating between the lifetimes of either the spin singlet or triplet states of positronium.

Characterization of stilbene-d12 for neutron spectroscopy without time of flight

“…This distance was chosen to obtain We selected gamma-ray pulses in the start detector and neutron pulses in the stop detector through PSD. The timing of each detected event was determined through a digital constant fraction discrimination (CFD) algorithm, with an attenuation factor of 50% fraction and a delay of 6 ns [14]. CFD yields a bipolar pulse, whose time stamp is determined through linear interpolation of the zero-crossing region, between the sample before and after the zero crossing.…”

Positron Annihilation Lifetime Spectroscopy as a Special Technique for the Solid-State Characterization of Pharmaceutical Excipients, Drug Delivery Systems, and Medical Devices—A Systematic Review

“…PALS is a non-destructive radiological technique widely used in material science studies [ 1 ] since it is exceptionally sensitive to the free volume of the material [ 2 ]. The method is based on the lifetime and intensity of ortho-positronium (o-Ps) atoms in free volumes of given structures; positronium (Ps) trapping in vacancies has a characteristic lifetime and, therefore, allows one to measure the size of a vacancy cluster, providing information about the structural characteristics of solids, liquids, and semi-solid materials [ 1 , 3 , 4 ]. PALS is utilized in several sectors for the examination of energy carriers [ 5 , 6 , 7 , 8 ], catalysts [ 9 , 10 , 11 ], and packaging materials [ 12 , 13 ] and for interrogating defects and pores in metals [ 14 , 15 ], ceramics [ 16 , 17 ], and polymers [ 3 ].…”

“…In the matter, o-Ps accumulates in regions of low electron density and thus occupies the core of the voids of the material (the wall of the core is formed by the electrons from neighbouring molecules). It is annihilated via collisions with electrons from the surrounding media by so-called “pick-off” annihilation [ 3 ], and two 0.511 MeV annihilation photons are created [ 1 ], conforming to the matter–energy equivalence principle. The parallel spinning positron is “picked off” by annihilation with an anti-parallel spinning electron from the surrounding wall, and thus the positron is annihilated not by the electron to which it is bound, but by an electron in its neighboring environment [ 4 ].…”

“…The o-Ps lifetime is a direct measure of the proximity of the Ps to the local environment, as the bound electron of the Ps is repelled by the electrons in the medium [ 3 ]. The elapsed time between the initial production of the positron (1.28 MeV gamma ray) and the detection of one of the two annihilation photons (0.511 MeV gamma ray) is therefore a measurement of the positronium lifetime in the material under investigation [ 1 , 4 , 19 ]. Thus, for the detection of the lifetime decay curve, two gamma ray detectors (BaF 2 or plastic scintillators) are tuned to detect the two energy levels (1.28 MeV for the start and 0.511 MeV for the stop) and are connected to a time counter capable of resolving events occurring nanoseconds apart [ 4 , 19 ].…”

“…The PALS method typically relies on an analog coincidence measurement setup and allows one to estimate the positron lifetime in the material sample under investigation [ 1 ]. The spectrum of the positron lifetime is obtained as a histogram of counts [N(t)] (number of measured events) versus time; the resultant time spectrum is a sum of exponentials of lifetimes in a medium [ 19 ] ( Figure 1 ).…”