Positron Escape Research Articles

Harvesting the decay energy of 26Al to drive lightning discharge in protoplanetary discs

Chondrules in primitive meteorites likely formed by recrystallisation of dust aggregates that were flash-heated to nearly complete melting. Chondrules may represent the building blocks of rocky planetesimals and protoplanets in the inner regions of protoplanetary discs, but the source of ubiquitous thermal processing of their dust aggregate precursors remains elusive. Here we demonstrate that escape of positrons released in the decay of the short-lived radionuclide 26Al leads to a large-scale charging of dense pebble structures, resulting in neutralisation by lightning discharge and flash-heating of dust and pebbles. This charging mechanism is similar to a nuclear battery where a radioactive source charges a capacitor. We show that the nuclear battery effect operates in circumplanetesimal pebble discs. The extremely high pebble densities in such discs are consistent with conditions during chondrule heating inferred from the high abundance of sodium within chondrules. The sedimented mid-plane layer of the protoplanetary disc may also be prone to charging by the emission of positrons, if the mass density of small dust there is at least an order of magnitude above the gas density. Our results imply that the decay energy of 26Al can be harvested to drive intense lightning activity in protoplanetary discs. The total energy stored in positron emission is comparable to the energy needed to melt all solids in the protoplanetary disc. The efficiency of transferring the positron energy to the electric field nevertheless depends on the relatively unknown distribution and scale-dependence of pebble density gradients in circumplanetesimal pebble discs and in the protoplanetary disc mid-plane layer.

The late-time light curve of the Type Ia supernova SN 2011fe

We present late-time optical $R$-band imaging data from the Palomar Transient Factory (PTF) for the nearby type Ia supernova SN 2011fe. The stacked PTF light curve provides densely sampled coverage down to $R\simeq22$ mag over 200 to 620 days past explosion. Combining with literature data, we estimate the pseudo-bolometric light curve for this event from 200 to 1600 days after explosion, and constrain the likely near-infrared contribution. This light curve shows a smooth decline consistent with radioactive decay, except over ~450 to ~600 days where the light curve appears to decrease faster than expected based on the radioactive isotopes presumed to be present, before flattening at around 600 days. We model the 200-1600d pseudo-bolometric light curve with the luminosity generated by the radioactive decay chains of $^{56}$Ni, $^{57}$Ni and $^{55}$Co, and find it is not consistent with models that have full positron trapping and no infrared catastrophe (IRC); some additional energy escape other than optical/near-IR photons is required. However, the light curve is consistent with models that allow for positron escape (reaching 75% by day 500) and/or an IRC (with 85% of the flux emerging in non-optical wavelengths by day 600). The presence of the $^{57}$Ni decay chain is robustly detected, but the $^{55}$Co decay chain is not formally required, with an upper mass limit estimated at 0.014 M$_{\odot}$. The measurement of the $^{57}$Ni/$^{56}$Ni mass ratio is subject to significant systematic uncertainties, but all of our fits require a high ratio >0.031 (>1.3 in solar abundances).

34 - Short-lived Positron Emitters in Beam-on PET Imaging During Proton Therapy

Positron emission tomography is so far the only method for in-vivo dose delivery verification in hadron therapy that is in clinical use. PET imaging during irradiation maximizes the number of detected counts and minimizes washout. In such a scenario, also short-lived positron emitters will be observed. As very little is known on the production of these nuclides, we determined which ones are relevant for proton therapy treatment verification. In order to be relevant, nuclides have to be produced close to the distal edge and thus at rather low proton energy. Therefore we measured the integral production of short-lived positron emitters in the stopping of 55 MeV protons in carbon, oxygen, phosphorus and calcium. The experiments were performed at the irradiation facility of the AGOR cyclotron at KVI-Center for Advanced Radiation Technology, University of Groningen. The positron emitters were identified based on their half-life. In order to do this, the proton beam was pulsed, i.e. delivered as a succession of beam-on and beam-off periods, and the time evolution of the 511 keV positron annihilation photons was recorded. A half-life analysis of the beam-off period allowed to determine the production rates of separate nuclides. The 511 keV photons were detected by a germanium clover detector [1]. A correction for the escape of positrons from the target, determined via Monte Carlo simulations, was applied. In the stopping of 55 MeV protons, the most copiously produced short-lived nuclides and their production rates relative to the relevant long-lived nuclides are: 12N (T1/2 = 11 ms) on carbon (9% of the 11C production), 29P (T1/2 = 4.1 s) on phosphorus (20% of the 30P production) and 38mK (T1/2 = 0.92 s) on calcium (113% of the 38gK production). No short-lived nuclides are produced on water (i.e. oxygen). The experimental production rates are used to calculate the production on PMMA and a representative set of 4 tissue materials. [fig. 1] The number of decays per 55 MeV proton stopped in these materials, integrated over an irradiation, is calculated as function of the duration of the irradiation. The most noticeable result is that for an irradiation in (carbon-rich) adipose tissue, 12N will dominate the PET image up to an irradiation duration of 70 s. On bone tissue, 15O dominates over 12N after 8-15 s (depending on the carbon-to-oxygen ratio). Considering nuclides created on phosphorus and calcium, the short-lived ones provide 2.5 times more decays than the long-lived ones during a 70 s irradiation. Bone tissue will thus be better visible in beam-on PET compared to PET imaging after an irradiation. The results warrant detailed investigations into the energy-dependent production of 12N, 29P and 38mK and their effect on PET imaging during proton irradiations.

VERY LATE PHOTOMETRY OF SN 2011fe

The Type Ia supernova SN 2011fe is one of the closest supernovae of the past decades. Due to its proximity and low dust extinction, this object provides a very rare opportunity to study the extremely late time evolution ($>900$ days) of thermonuclear supernovae. In this Letter, we present our photometric data of SN 2011fe taken at an unprecedented late epoch of $\approx 930$ days with GMOS-N mounted on the Gemini North telescope ($g=23.43 \pm 0.28$, $r=24.14 \pm 0.14$, $i=23.91 \pm 0.18$, and $z=23.90 \pm 0.17$) to study the energy production and retention in the ejecta of SN 2011fe. Together with previous measurements by other groups, our result suggests that the optical supernova light curve can still be explained by the full thermalization of the decay positrons of Co56. This is in spite of theoretical predicted effects (e.g. infrared catastrophe, positron escape, and dust) that advocate a substantial energy redistribution and/or loss via various processes that result in a more rapid dimming at these very late epochs.

Monte Carlo modelling of the propagation and annihilation of nucleosynthesis positrons in the Galaxy

We want to estimate whether the positrons produced by the beta plus decay of 26Al, 44Ti and 56Ni synthesised in massive stars and supernovae are sufficient to explain the 511 keV annihilation emission observed in our Galaxy. Such a possibility has often been put forward in the past. We developed a Monte Carlo Galactic propagation code for ~MeV positrons in which the Galactic interstellar medium, the Galactic magnetic field (GMF) and the propagation are finely described. This code allows us to simulate the spatial distribution of the 511 keV annihilation emission. We test several GMFs models and several positron escape fractions from type-Ia supernova for 56Ni positrons. We consider the collisional ballistic transport mode and then compare the simulated 511 keV intensity spatial distributions to the INTEGRAL/SPI data. Whatever the GMF configuration and the escape fraction chosen for 56Ni positrons, the 511 keV intensity distributions are very similar. The main reason is that ~MeV positrons do not propagate very far away from their birth sites in our model. The direct comparison to the data does not allow us to constrain the GMF configuration and the escape fraction for 56Ni positrons. In any case, nucleosynthesis positrons produced in steady state cannot explain the full annihilation emission. The comparison to the data shows that: (a) the annihilation emission from the Galactic disk can be accounted for; (b) the strongly peaked annihilation emission from the inner Galactic bulge can be explained by positrons annihilating in the central molecular zone but this seems to require more positron sources than the population of massive stars and type Ia supernovae usually assumed for this region; (c) the more extended emission from the Galactic bulge cannot be explained. We show that a transient source at the GC, such as a starburst episode, could explain this extended component.

Positron annihilation characteristics in multi-wall carbon nanotubes with different average diameters

Positron lifetime spectroscopy was used to study multi-wall carbon nanotubes. The measurements were performed in vacuum on the samples having different average diameters. The positron lifetime values depend on the nanotube diameter. The results also show an influence of the nanotube diameter on the positron annihilation intensity on the nanotube surface. The change in the annihilation probability is described and interpreted by the modified diffusion model introducing the positron escape rate from the nanotubes to their external surface.

Annihilation emission from young supernova remnants

A promising source of the positrons that contribute through annihilation to the diffuse Galactic 511keV emission is the beta-decay of unstable nuclei like 56Ni and 44Ti synthesised by massive stars and supernovae. Although a large fraction of these positrons annihilate in the ejecta of SNe/SNRs, no point-source of annihilation radiation appears in the INTEGRAL/SPI map of the 511keV emission. We exploit the absence of detectable annihilation emission from young local SNe/SNRs to derive constraints on the transport of MeV positrons inside SN/SNR ejecta and their escape into the CSM/ISM, both aspects being crucial to the understanding of the observed Galactic 511keV emission. We simulated 511keV lightcurves resulting from the annihilation of the decay positrons of 56Ni and 44Ti in SNe/SNRs and their surroundings using a simple model. We computed specific 511keV lightcurves for Cas A, Tycho, Kepler, SN1006, G1.9+0.3 and SN1987A, and compared these to the upper-limits derived from INTEGRAL/SPI observations. The predicted 511keV signals from positrons annihilating in the ejecta are below the sensitivity of the SPI instrument by several orders of magnitude, but the predicted 511keV signals for positrons escaping the ejecta and annihilating in the surrounding medium allowed to derive upper-limits on the positron escape fraction of ~13% for Cas A, ~12% for Tycho, ~30% for Kepler and ~33% for SN1006. The transport of ~MeV positrons inside SNe/SNRs cannot be constrained from current observations of the 511keV emission from these objects, but the limits obtained on their escape fraction are consistent with a nucleosynthesis origin of the positrons that give rise to the diffuse Galactic 511keV emission.

Soft gamma-ray background and light dark matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the ($S$- or $P$-wave) annihilation of light dark matter particles into ${e}^{+}{e}^{\ensuremath{-}}$. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about $4\text{ }\text{ }\mathrm{keV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

Late Light Curves of Type Ia Supernovae

We extend earlier efforts to determine whether the late (t ≥ 60 days) light curves of Type Ia SNe are better explained by the escape of positrons from the ejecta or by the complete deposition of positron kinetic energy in a trapping magnetic field. We refine our selection of Ia SNe, using those that have extensive BVRI photometry 35 days or more after maximum light. Assuming that all SNe within a given Δm15(B) range form a distinct subclass, we fit a combined light curve for all class members with a variety of models. We improve our previous calculations of energy deposition rates by including the transport of the Comptonized electrons. Their nonlocal and time-dependent energy deposition produces a correction of as much as 0.10 mag for Chandrasekhar-mass models and 0.18 mag for sub-Chandrasekhar-mass models.

A spectrometer for the study of angular correlations in polarized-orthopositronium decay

We describe a setup for experimental testing of the CPT-invariance by observing the angular correlations in the decay of polarized orthopositronium atoms. Annihilation events are detected by five NaI(T1) sensors of 150-mm diameter and 100-mm thickness, located at the vertices of a regular pentagon. The sensor signals are detected in the double-coincidence circuits gated by signals with different time delays from the positron escape. The setup has a time resolution of 8.0 ns. In order to reduce the effect of the long-term instability and the instrumental asymmetry on the experimental results, the direction of positron incidence on a SiO2 target was reversed with a program-driven period during data acquisition. The value of the asymmetry, measured with due account of statistical and systematic errors, is 0.0008±0.00091, which is twice better as compared with the previous experiment.

Positron Escape from Type Ia Supernovae

We generate bolometric light curves for a variety of type Ia supernova models at late times, simulating gamma-ray and positron transport for various assumptions about the magnetic field and ionization of the ejecta. These calculated light curve shapes are compared with light curves of specific supernovae for which there have been adequate late observations. %The selection of models is generally not based upon the %ability to fit the late observations, but rather because the %model has been demonstrated by other authors to approximate the spectra %and early light curves of that specific SN. From these comparisons we draw two conclusions: whether a suggested model is an acceptable approximation of a particular event, and, given that it is, the magnetic field characteristics and degree of ionization that are most consistent with the observed light curve shape. For the ten SNe included in this study, five strongly suggest $^{56}$Co positron escape as would be permitted by a weak or radially-combed magnetic field. Of the remaining five SNe, none clearly show the upturned light curve expected for positron trapping in a strong, tangled magnetic field. Chandrasekhar mass models can explain normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar mass models have difficulties with sub- (and potentially normally) luminous SNe. An estimate of the galactic positron production rate from type Ia SNe is compared with gamma-ray observations of Galactic 511 keV annihilation radiation. Additionally, we emphasize the importance of correctly treating the positron transport for calculations of spectra, or any properties, of type Ia SNe at late epochs ($\geq$ 200 d).

Hubble Space TelescopeSpectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas

The Hubble Space Telescope 2000-8000 A spectrum of SN 1987A observed on 1995 January 7 (7.87 yr after the explosion) is dominated by Hα and UV lines, including Mg II-Mg I λ2825 (equal to Hα in luminosity), Fe II UV 2 (two-thirds the intensity of Hα), Fe II UV 3 (one-half the intensity of Hα), and a 3730 A emission feature identified with a blend of [O II] λ3727 and Fe I emission lines. [O I] λ6300 and lines of [Ca II] and Na I, as well as some Fe II optical forbidden and permitted lines are present at visual wavelengths. Also present are a number of weak emission features, which are presumably metal lines produced by photon degradation as a result of reprocessing of UV radiation into metal lines. Modeling the Mg II-Mg I lines provides the velocity of the outer visible radius of the envelope, 9000 ± 500 km s-1 in the Mg II λ2800 line, which is consistent with the earlier direct HST imaging at near-UV wavelengths. The UV/optical emission lines originate from the radioactive luminescence of the cool gas (T ≈ 130-160 K). The metal lines reflect the instantaneous reprocessing of the energy deposited from 44Ti radioactive decays through collisions with fast electrons, while the Hα emission primarily comes from the recombination of previously ionized hydrogen. The overall luminosity of the Fe II emission lines, ~1035 ergs s-1, can be explained if the bulk of the positrons from a mass (1-2) × 10-4 M☉ of 44Ti release their energy in the iron-rich material, which suggests the presence of a magnetic field B > 5 × 10-13 G prohibiting the escape of positrons into oxygen and hydrogen gas. The ionized fraction in the iron-rich material is small (0.2-0.3), and the total UV/optical emission from Fe I should be comparable to that from Fe II. Most of the 1036 ergs s-1 deposited by the 44Ti positrons should be emitted in the Fe II 26 μm line. The observed Hα luminosity decrease, by 5 orders of magnitude between the ages of 1 to 8 yr, is reproduced in a time-dependent model of ionization and cooling with the standard amount of radioactive nuclides. However, an additional source of energy at the present epoch with a deposition rate 30 ergs s-1 g-1 (≈ 1036 ergs s-1 in the whole envelope) is not ruled out. The present average temperature in the hydrogen envelope predicted by the time-dependent model is 130 K, which is lower than the value T ≈ 350 K obtained from the observed Balmer continuum shape. However, the shape is affected by a possible contribution of metal lines to the Balmer continuum. The luminosity of the [O I] λ6300 doublet is consistent with that expected for the deposited energy of γ-rays from (1-2) × 10-4 M☉ of 44Ti for an assumed 1.5-2 M☉ of oxygen. If the oxygen mass does not exceed 2 M☉, 1 × 10-4 M☉ is a lower limit for the amount of 44Ti in SN 1987A. The maximum fraction of the 44Ti positron energy deposited into oxygen-rich material does not exceed 5%, which is consistent with positron trapping in Fe-rich material. The [O I] λ6300 line intensity rules out the presence of a central source of γ-radiation (hν > 100 keV) with a luminosity Lγ > 4 × 1036 ergs s-1.

Positron lifetime in oxide superconductors YBa2(Cu1-xMx)O7-y (M=Fe, Ni, Zn)

The authors present the temperature dependence of positron lifetime parameters measured on a series of specimens of YBa2(Cu1-xMx)3O7-y (M=Fe, Ni, Zn) in the temperature range 20-300 K. In a non-doped specimen, the defect-related component tau 2 showed a step-like increase in the temperature range 40-100 K. Above 180 K, the mean lifetime tau mean decreased with increasing temperature. These variations in tau 2 or tau mean are explained in terms of a thermally activated escape of positrons from the two kinds of trapping centres. The bulk lifetime tau bulk showed a slight decrease in the region 100-300 K. The iron doping moved the shift of the transition region of tau 2 towards a higher temperature and removed the decrease of tau mean out of the temperature range in which experiments were performed. These phenomena are ascribed to the enhancement of the positron-defect binding energies, which is theoretically predicted. On the other hand, nickel and zinc did not much alter the behaviour of tau 2, but affected that of tau bulk. A direct correlation between the superconducting critical temperature (Tc) and the lifetime-temperature curve was not found.

Diffusion heating and cooling of positrons in constrained media

Thermalized positrons in media with spatial constraints can be trapped by sinks, and annihilate with electrons into $\ensuremath{\gamma}$ quanta with characteristics that are discernibly different from those of positrons annihilating in the bulk. The dependence of the cross sections for scattering in the medium on positron velocity determines whether the fastest or the slowest particles in the velocity distribution disappear preferentially from the volume. This causes the cooling or heating of the positrons remaining in the medium. The diffusion equation is solved to derive the annihilation characteristics of positrons, with the result that diffusion heating and cooling always reduce the effective diffusion coefficient pertinent for trapping relative to that in the infinite medium. The effects on the annihilation characteristics are those of a reduced trapping rate and of an apparent initial positron population in sinks, as it is recorded by an instrument with finite time resolution. The results pertain to positronium diffusion in small gas cells, to the escape of positrons or positronium from small solids, and to the trapping of positrons by vacancies or voids in solids. They are compared with experimental results.

Positron escape from annihilation centers in electron-irradiated Si crystals

The mean positron lifetime in irradiated Si diminishes with increasing temperature between 77°K and 300°K. Isochronal high-temperature crystal anneals reduce the effect. Analysis suggests that thermally activated positrons escape after trapping in radiation-induced defects.

Solution of the Dirac Equation for Strong External Fields

The 1s bound state of superheavy atoms and molecules reaches a binding energy of $\ensuremath{-}2m{c}^{2}$ at $Z\ensuremath{\approx}169$. It is shown that the $K$ shell is still localized in $\mathcal{r}$ space even beyond this critical proton number and that it has a width $\ensuremath{\Gamma}$ (several keV large) which is a positron escape width for ionized $K$ shells. The suggestion is made that this effect can be observed in the collision of very heavy ions (superheavy molecules) during the collision.