SSPALS: A tool for studying positronium

Abstract

Single-shot positron annihilation lifetime spectroscopy (SSPALS) is an extremely useful tool for experiments involving the positronium atom (Ps). I examine some of the methods that are typically employed to analyze lifetime spectra, and use a Monte-Carlo simulation to explore the advantages and limitations these have in laser spectroscopy experiments, such as resonance-enhanced multiphoton ionization (REMPI) or the production of Rydberg Ps.Positronium (Ps) [1] is the bound state of an electron and a positron. In vacuum, the components of the particle-antiparticle pair will ultimately annihilate with each other. The mean lifetime against self-annihilation is 125 ps for the n = 1 singlet spin state (1 1 S 0 , para-Ps) [2,3] or 142 ns for the n = 1 triplet spin states (1 3 S 1 , ortho-Ps) [4,5]. Annihilation of p-Ps usually results in two 511 keV gamma-ray photons, whereas o-Ps predominately decays into three with a combined energy of 1.022 MeV [6]. A scintillator coupled to a photomultiplier tube (PMT) can be used to efficiently detect gamma rays with subns timing resolution [7]. This facilitates precision positron annihilation lifetime spectroscopy (PALS) [8, 9] -a simple but powerful technique that was instrumental in the discovery of positronium by Deutsch in 1951 [10].The gross atomic structure of Ps [11] can be described by the Bohr model for hydrogen but with a reduced mass of µ Ps = m e /2; the corresponding energy levels are then given by E n = −6.8/n 2 (eV). Optical excitation from the ground state can be achieved using a pulsed laser synchronized to a time-bunched Ps source (∆t 10 ns) [12][13][14][15][16][17][18]. The annihilation and fluorescence decay rates of the excited states range widely [19,20] and laser excitation to these can have a marked effect on the overall lifetime. But to measure a PALS spectrum each annihilation event must be resolvable in the time domain, which is generally not possible with ns Ps sources. In this case, single-shot positron annihilation lifetime spectroscopy (SSPALS) [21] can be implemented instead. Here, the output of a fast gamma-ray detector constitutes the lifetime spectrum. This is a valid approximation if the Ps formation time and the decay time of the detector are both sufficiently short. PbWO 4 has a scintillation decay time of κ ∼ 10 ns, which is well suited to resolving o-Ps decay (τ = 142 ns). The Cerenkov radiator PbF 2 can be used to improve timing resolution [22] but Email address: a.deller@ucl.ac.uk (Adam Deller) it has a lower light output. For some applications, a slower material with a higher light output, such as LYSO (κ ∼ 40 ns), might be chosen to improve detection efficiency and the signalto-noise ratio [23].Pulsed Ps sources, with time widths of a few ns, can be obtained by implanting time-focused positron beams [24,25] into Ps-converter materials, such as mesoporous SiO 2 [26]. The interconnected network of pores provides a path to vacuum along which Ps atoms cool via inelastic collisions [14,27]. The overall efficiency for emission of o-Ps from...

Effect of Detector Material and Size on Pile-Up Elimination in Positron Burst Measurement

“…Usually, positron annihilation lifetime spectroscopy (PALS) is performed by collecting positron annihilation events one by one, and the time taken to obtain a spectrum can be anywhere from tens of minutes to days, depending on the source intensity and the apparatus structure. Based on the positron trapping technique, a single-shot positron annihilation lifetime spectroscopy (SSPALS) method has been developed for completing the positron lifetime spectrum using a single positron shot [1][2][3][4]. In order to avoid saturation of the detector, a fast Cherenkov radiator, coupled to a fast response photomultiplier, is used to detect the pulse of positrons in this approach [1][2][3][4].…”

“…Based on the positron trapping technique, a single-shot positron annihilation lifetime spectroscopy (SSPALS) method has been developed for completing the positron lifetime spectrum using a single positron shot [1][2][3][4]. In order to avoid saturation of the detector, a fast Cherenkov radiator, coupled to a fast response photomultiplier, is used to detect the pulse of positrons in this approach [1][2][3][4]. Recently, higher intensity positron bursts have been produced with the developments of femtosecond laser induced positron technique [5], and huge number of gamma photons are released in several picoseconds after positrons annihilation.…”

Cronni Plasma o Bositronau

“…Cynhyrfir positroniwm i gyflyrau Rydberg gan ddefnyddio laser a chyflawnir sbectrosgopeg i feintioli'r atom egsotig ac i ddeall ei strwythur (Cassidy, 2018). Gwneir hyn gan ddefnyddio sbectrosgopeg saethiad sengl hyd oes difodiant positronau (Single-shot Positron Annihilation Lifetime Spectroscopy -SSPALS) (Deller, 2019), sy'n defnyddio pwls o bositronau yn hytrach na phositronau unigol fel PALS. Hyd yn hyn, mae'r dechneg yn ddefnyddiol er mwyn astudio positroniwm, ond rhagwelir y gellid gwella nodweddiad solidau yn y dyfodol.…”

Monte-Carlo simulation of positronium laser excitation and anti-hydrogen formation via charge exchange

“…Single-Shot Positron Annihilation Lifetime Spectroscopy, a method for detecting and analysing delayed annihilations due to Ps succeeding the prompt annihilation burst produced by positrons, in order to gather information about the fraction of Ps and its properties. Both kind of annihilations are observed through gamma radiation detected by a fast scintillator, coupled with a rapid photomultiplier and whose signal is recorded by a fast oscilloscope (see[15,16] for the conception and precise definition of SSPALS, as well as[17] for a recent review on the capability to disentangle also Ps laser excitation effects).…”