Positron Annihilation Γ-rays Research Articles

Component characterization and commissioning of a gamma-PET prototype detector system

Hybrid imaging systems, comprising PET and Compton camera modules, have recently gained in interest, due to their capability to simultaneously detect positron annihilation photons and γ-rays from single-photon emitting sources as also used in SPECT. A unique feature of such systems, however, is the capability to also be operated in a so called γ-PET mode. Here, specific β+- emitting radioisotopes (such as 44Sc, 1°C or 14O) are used to detect triple-coincidences between two annihilation γ-rays (in PET imaging) and a third, prompt photon (in Compton imaging), that is emitted by the deexcitation of the decay’s daughter nucleus. Consequently, an intersection between the line-of-response (LOR) and the Compton cone can be determined, which (in principle) allows to localize the photons’ emission vertices on a single decay basis. In practice, however, a few tens of events are required to localize a point source, which still results in a considerable sensitivity improvement compared to conventional PET imaging.For a proof-of-principle study, we used a pixelated GAGG crystal array (16 × 16 crystals; 1.45 × 1.45 × 6 mm3 crystal volume; 25 μm SPAD SiPMs as readout) as Compton camera scatterer and PET detectors, and a three-layered LYSO crystal array (1.2 × 1.2 × 6.66 mm3 crystal volume; 50 μm SPAD SiPMs as readout) as Compton camera absorber. We characterized the individual detector components with regard to their energy resolution and the capability to identify the various scintillator array’s individual crystals. Our first γ-PET prototype was tested in PET-only and Compton-only imaging mode, in which spatial resolutions of 3.2–3.5 mm FWHM (PET-only mode) and 14.4–19.3 mm FWHM (Compton-only mode at 1,274 keV) were achieved, respectively, using a22Na point source and 10 iterations of an ML-EM reconstruction algorithm. By using triple-coincidences in a γ-PET mode (event-wise intersection of the LOR and the Compton cone), we could demonstrate the capability of the prototype to perform a full 3D point source reconstruction using only 77 events.

Open Porosity and Pore Size Distribution of Mesoporous Silica Films Investigated by Positron Annihilation Lifetime Spectroscopy and Ellipsometric Porosimetry.

Tunable mesoporous silica films were prepared though a sol-gel process directed by the self-assembly of various triblock copolymers. Positron annihilation γ-ray energy spectroscopy and positron annihilation lifetime spectroscopy (PALS) based on intense pulsed slow positron beams as well as ellipsometric porosimetry (EP) combined with heptane adsorption were utilized to characterize the open porosity/interconnectivity and pore size distribution for the prepared films. The consistency between the open porosities was examined by the variations of orthopositronium (o-Ps) 3γ annihilation fractions and the total adsorbed volumes of heptane. The average pore sizes deduced by PALS from the longest-lived o-Ps lifetimes are in good agreement with those by EP on the basis of the Barrett–Joyner–Halenda model, as indicated by a well fitted line of slope k = 1. The results indicate that the EP combined with heptane adsorption is a useful method with high sensitivity for calibrating the mesopore size in highly interconnected mesoporous films, whereas PALS is a novel, complementary tool for characterizing both closed and open pores in them.

Steady-state nucleosynthesis throughout the Galaxy

The measurement and astrophysical interpretation of characteristic γ-ray lines from nucleosynthesis was one of the prominent science goals of the INTEGRAL mission and in particular its spectrometer SPI. Emission from 26Al and from 60Fe decay lines, due to their My decay times, originates from accumulated ejecta of nucleosynthesis sources, and appears diffuse in nature. 26Al and 60Fe are believed to originate mostly from massive star clusters. The radioactive decay γ-ray observations open an interesting window to trace the fate and flow of nucleosynthesis ejecta, after they have left the immediate sources and their birth sites, and on their path to mix with ambient interstellar gas. The 26Al emission image obtained with INTEGRAL confirms earlier findings of clumpiness and an extent along the entire plane of the Galaxy, supporting its origin from massive-star groups. INTEGRAL spectroscopy resolved the line and found Doppler broadenings and systematic shifts with longitude, originating from large-scale galactic rotation. But an excess velocity of 200 km s−1 suggests that 26Al decays preferentially within large superbubbles that extend in forward directions between spiral arms. The detection of 26Al line emission from the nearby Orion clusters in the Eridanus superbubble supports this interpretation. Positrons from β+ decays of 26Al and other nucleosynthesis ejecta have been found to not explain the morphology of positron annihilation γ-rays at 511 keV that have been measured by INTEGRAL. The 60Fe signal measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an origin from point-like/concentrated sources is excluded. The 26Al /60Fe ratio is constrained to a range 0.2–0.4. Beyond improving precision of these results, diffuse nucleosynthesis contributions from novae (through 22Na radioactivity) and from past neutron star mergers in our Galaxy (from r-process radioactivity) are exciting new prospects for the remaining mission extensions.

Variable-energy Positron Study of Nanopore Structure in Hydrocarbon–Siliconoxide Hybrid PECVD Films

Abstract Nanopore formation upon heat treatment of hydrocarbon-siliconoxide hybrid films was investigated by means of the variable-energy positron annihilation γ-ray and lifetime techniques. The films were prepared using plasma enhanced chemicalvapor deposition (PECVD),and nanoscopic poresinthe films weredeveloped throughthe decom positionofasacrificialhydrocarbonous porogenbyannealingthe filmsat temperaturesupto600 ° C.Asaresultofthe positron annihilation γ-ray measurements, the line-shape S parameter increases with increasing annealing temperature from 150 ° C to 400 ° C, while it reduces with further increasing temperature. This suggests that more positronium annihilate with carbonous elements remaining on the wall of pores formed at temperatures up to 400 ° C. The lifetime measurements revealed the nanopore sizes in the range from 0.2 nm 3 to 2.0 nm 3 . With increasing the pore size the film refractive index reduced from 1.44 to 1.29. The variable-energy positron techniques were demonstrated to be useful to examine the nanoporosity evolution for porous PECVD films.

Correlation Study between Free-Volume Holes and Molecular Separations of Composite Membranes for Reverse Osmosis Processes by Means of Variable-Energy Positron Annihilation Techniques

The nanoscopic structure of three kinds of commercially available composite membranes (LF10, NTR729HF, and NTR7250) for water purification has been studied by energy-variable positron annihilation. The membranes consist of a functional polymer film and porous polysulfone substrate supported on a nonwoven polyester fabric. Positron annihilation γ-ray and positron lifetime techniques were employed to probe the layer structure of the composite membranes and subnanometer-sized free-volume holes in the functional films, respectively. It was found that the functional film on the porous substrate in LF10 contains two different layers on the substrate, while that in NTR729HF and NTR7250 consists of a single top layer. Comparison of the rejections of different uncharged solutes, determined by a pressure-driven experiment, with the hole size of the functional film, obtained from the positron lifetime data, revealed that the holes probed by positrons can well explain the hindering effect of molecular transport. More...

Positron annihilation study of the disordered phase of hydrogen adsorbed on the Ni (111) surface

The adsorption of hydrogen on transition-metal surface has been studied by both experimental and theoretical methods. Hydrogen adsorbed Ni (111) surface had measured by using slow positron beam apparatus. The positron annihilation γ-ray energy spectra emitted from hydrogen-adsorbed Ni (111) surfaces are analyzed. We have discussed the stability of the disorder phase of adsorbed hydrogens on Ni (111) surfaces. Positron annihilation results showed that the disordered phase is unstable until about 300K.

Vacancy evolution in Ni during irradiation at high temperatures studied by in situ positron annihilation spectroscopy

We present experimental results for in situ study of atomic-vacancy production and its evolution occurring during irradiation by using a slow-positron beam. Thermal stability of the vacancy produced during irradiation is investigated under elevated temperature conditions. An annealed Ni specimen was irradiated with 400 keV He ions at three different temperatures of 296 (RT), 368 and 713 K. Doppler broadenings of positron-annihilation γ-rays were measured and variation of the line-shape parameter S was observed under beam-on (during irradiation) and beam-off (non-irradiation) conditions. Results indicate that variation of the S depends on the specimen temperature, showing that vacancy cluster consisting of about 15 vacancies is formed predominantly via thermal evolution of atomic-sized vacancies under irradiation at high temperatures. We found that formation of the activated vacancies occurs during irradiation, which leads to vacancy clustering.

OH radical in water studied by Quantum beats on positron annihilation -the effect of water liquid structures-

Singlet radical pairs of ortho-positronium and hydroxyl (OH) radical were generated in positron radiolysis in water. Quantum beats on the energy distribution of positron annihilation γ-rays caused by the reaction of electron-spin-correlated pairs of ortho-positronium and OH radical were newly measured at 32°C by using positron annihilation age-momentum correlation (AMOC) measurement. The quantum beats can provide information of hyperfine coupling constants of the OH radical in water liquid structures, and hence can provide the information of change of the water liquid structures. The quantum beats appeared at 32°C could not be explained easily. The reason was continuous and slow change of water liquid structures at 32°C. The water liquid structures probably can be understood by using the positron annihilation quantum beat measurements.

Measuring Cosmic Elements with Gamma-Ray Telescopes

AbstractGamma-ray telescopes are capable of measuring radioactive trace isotopes from cosmic nucleosynthesis events. Such measurements address new isotope production rather directly for a few key isotopes such as 44Ti, 26Al, 60Fe, and 56Ni, as well as positrons from the β+-decay variety. Experiments of the past decades have now established an astronomy with γ-ray lines, which is an important part of the study of nucleosynthesis environments in cosmic sources. For massive stars and supernovae, important constraints have been set: Co isotope decays in SN1987A directly demonstrated the synthesis of new isotopes in core-collapse supernovae, 44Ti from the 340-year-old Cas A supernova supports the concept of α-rich freeze-out, but results in interesting puzzles pursued by theoretical studies and future experiments. 26Al and 60Fe has been measured from superimposed nucleosynthesis within our Galaxy, and sets constraints on massive-star interior structure through its intensity ratio of ∼15%. The 26Al γ-ray line is now seen to trace current star formation and even the kinematics of interstellar medium throughout the Galaxy. Positron annihilation emission from nucleosynthesis throughout the plane of our Galaxy appears to be mainly from 26Al and other supernova radioactivity, but the striking brightness of the Galaxy's bulge region in positron annihilation γ-rays presents a puzzle involving several astrophysics issues beyond nuclear astrophysics. This paper focuses mainly on a discussion of 26Al and 60Fe from massive-star nucleosynthesis.

Time-of-Flight Positron Emission Tomography Using Optical Fiber Circuit

The measurement method and system architecture of a new time-of-flight positron emission tomography (TOF-PET) system are proposed. This system collects scintillation light using optical fibers connected directly to scintillators and measures the position of positron annihilation. Many scintillators are placed cylindrically whereby a pair of scintillators detects a pair of γ-rays generated at the positron annihilation point. Optical fiber circuits, most of which are bundles of optical fibers bound clockwise or counterclockwise around the cylinder of scintillators, collect light signals generated by γ-rays. These light signals are amplified by several photomultiplier tubes and processed using a single digital oscilloscope to determine the TOF of the positron annihilation γ-rays. One of the most important factors in the performance of the TOF-PET system is the TOF resolution. When fiber circuits are used for transmitting light signals, the dispersion of light signals and the decrease in light intensity are the major factors in the deterioration of the TOF resolution. The result of the preliminary experiment leads to the conclusion that the use of optical fibers degrades the intensity of light but does not severely degrade the TOF resolution.

LaCl3:Ce scintillator for γ-ray detection

In this paper, we report on a relatively new cerium-doped scintillator—LaCl3 for γ-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman method. This material when doped with 10% cerium has high light output (∼50,000 photons/MeV) and fast principal decay time constant (∼20ns). Furthermore, it shows excellent energy resolution for γ-ray detection. For example, energy resolution as low as 3.2% (FWHM) has been achieved with 662keV photons (137Cs source) at room temperature. Also, high timing resolution (264ps—FWHM) has been recorded with LaCl3-PMT and BaF2-PMT detectors operating in coincidence using 511keV positron annihilation γ-ray pairs. Details of crystal growth, scintillation properties, and variation of these properties with cerium concentration are also reported.

High sensitivity of positron annihilation to thermal oxidation of polyethylene

We demonstrate the high sensitivity of positron annihilation to compositional changes related to the thermal degradation of polyethylene (PE). Positron annihilation γ-ray and lifetime measurements were conducted for PE films with and without antioxidant (1000-ppm Ciba® IRGANOX® 1076), subjected to heat treatment at 100 °C for different periods, to a maximum of 30 days. For the film without antioxidant, the positron Doppler parameter (S) and ortho-positronium formation probability (Io−Ps) appreciably decreased with increased heat treatment times, whereas they barely changed for the film with antioxidant. This, together with the Fourier transform infrared measurements, demonstrated that the variations of S and Io−Ps are caused by the thermal oxidation of PE. The S parameter was found to be sensitive to the early stage of degradation, where the carbonyl concentration is inferred to be lower than 100 ppm. The high sensitivity results from the large positron mobility in PE and from the high positron affinity of oxygen-containing polar groups. This work provides the basis for an application of positron annihilation to sensitive detection of the initial degradation of PE and other nonpolar polymers.

A position-sensitive γ-ray detector for positron annihilation 2D-ACAR based on metal package photomultiplier tubes

A new position-sensitive γ-ray detector to be used in a two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) apparatus has been developed. It consists of 36 compact position-sensitive photomultiplier tubes (PS-PMT: HAMAMATSU R5900-00-C8), a light guide, and 2676 Bi 4Ge 3O 12 (BGO) scintillator pieces of size 2.6 mm×2.6 mm×18 mm. A high detection efficiency for 511 keV γ-ray is achieved with the length of BGO scintillators used. The detection area is about 160 mm×160 mm. The 288 anode outputs of the PS-PMTs are wired and connected to resistor chains from which 16 outputs (8 outputs each along the X and Y directions) are taken to identify the incident position of the γ-ray. The spatial resolution is about 3 mm (FWHM). The timing signal taken from the last dynodes of the PS-PMTs gives a timing resolution of 7.7 ns (FWHM) for 511 keV positron annihilation γ-rays.

A new positron lifetime spectrometer using a fast digital oscilloscope and BaF 2 scintillators

A new positron lifetime spectrometer has been constructed and tested. It consists of a state-of-the-art digital oscilloscope (LeCroy Wavepro 960) and scintillation detectors based on Hamamatsu H3378 (R2083Q) photomultiplier tubes and BaF 2 scintillators. A very good time resolution is obtained with scintillators of commonly used sizes (28 mm in diameter, 10–20 mm thick) and wide energy window settings (40–50%). The time resolution of 144 ps full-width at half-maximum (FWHM) is achieved for the positron lifetime measurement, in which time differences are measured between the arrivals of a 1.27 MeV γ-ray from 22Na and one of the two positron annihilation γ-rays of 0.511 MeV. The time resolution is further improved with a ‘double-stop’ setup, where timing information carried by the positron annihilation γ-ray pair is combined to determine the time of the annihilation. The time resolution with this setup is 119 ps FWHM. This improvement is interpreted in terms of the energy deposit in the scintillation detectors by γ-rays. Applications of the new spectrometer to other scintillation timing measurements are also described.

Recent progress in thallium bromide detectors for X- and γ-ray spectroscopy

Recent progress in the development of room-temperature TlBr detectors is presented. Higher-performance TlBr detectors have been fabricated from high-purity TlBr crystals grown by the traveling molten zone method. The TlBr crystals have exhibited large mobility–lifetime products of 2.6×10 −4 and 3.7×10 −5 cm 2/V for electrons and holes, respectively. An energy resolution of 29.8 keV (5.8%) FWHM has been recorded for 511 keV γ-rays with a TlBr detector at room temperature. Timing properties of the TlBr detectors have been studied by recording the coincidence timing spectra between a TlBr detector and a BaF 2 scintillator coupled to a photomultiplier tube. Timing resolution of 21.3 ns FWHM has been measured for 511 keV positron annihilation γ-rays from a 22Na source. By using a single TlBr detector in current mode, an X-ray CT scanner system of a first-generation type has been developed. The performance of the system has been evaluated by observing tomographic images. Images with good spatial resolution have been obtained by the scanner with the TlBr detector.

Damage profile of ion-implanted polycarbonate studied using a variable-energy positron beam

Damage in polycarbonate (PC) irradiated with B+, C+, Si+, Cu+, and Au+ ions at energies between 0.19 and 2.4 MeV was studied using a variable-energy positron beam by measuring the Doppler broadening of positron annihilation γ-rays as a function of incident positron energy. Positron data were plotted, instead of incident positron energy E, as a function of average positron implantation depth normalized to average ion implantation depth. Such plots enable damage distributions produced by ions with different implantation ranges to be compared directly. We found that normalized damage distributions in ion-irradiated PC vary systematically with dimensionless reduced energy proposed by Lindhard et al. (K. Dan. Vidensk. Selsk. Mat.-Fys. Medd. 36 (1968) 10)

Room temperature X- and gamma-ray detectors using thallium bromide crystals

Thallium bromide (TlBr) is a compound semiconductor with wide band gap (2.68 eV) and high X- and γ-ray stopping power. The TlBr crystals were grown by the horizontal travelling molten zone (TMZ) method using purified material. Two types of room temperature X- and γ-ray detectors were fabricated from the TlBr crystals: TlBr detectors with high detection efficiency for positron annihilation γ-ray (511 keV) detection and TlBr detectors with high-energy resolution for low-energy X-ray detection. The detector of the former type demonstrated energy resolution of 56 keV FWHM (11%) for 511 keV γ-rays. Energy resolution of 1.81 keV FWHM for 5.9 keV was obtained from the detector of the latter type. In order to analyze noise characteristics of the detector-preamplifier assembly, the equivalent noise charge (ENC) was measured as a function of the amplifier shaping time for the high-resolution detector. This analysis shows that parallel white noise and 1/ f noise were dominant noise sources in the detector system. Current–voltage characteristics of the TlBr detector with a small Peltier cooler were also measured. Significant reduction of the detector leakage current was observed for the cooled detectors.

Depth-selective defect analysis of Si implanted with As + under channeling conditions using a variable-energy positron beam

Wafers of p-type Si (100) were implanted with 200 keV As + ions with a dose of 1 × 10 13ions/cm 2 from different directions, and Doppler broadening of positron annihilation γ-rays was measured as a function of incident positron energy. The measurements were carried out to study the channeling effect of implanted ions on defect formation. The optimum defect distribution obtained from the positron data revealed that the defects produced by ion implantation under a channeling condition extended deeper than in the case without channeling. It was found that the channeling effect was significant when the implantation angle, defined as the angle between the direction of the ion beam and that normal to the (100) plane, was smaller than 3°.

The study of the positron states in CS2

Mogensen reported that CS2 was the only liquid which showed large decrease of the positronium (Ps) intensity with lowering the temperature. The ratio between the intensity of the longest lifetime and the width of the narrowest component on the angular correlation measurement was not 3∶1 in CS2 liquids. Here I measured the change of intensity and lifetime of the longest lifetime component caused by varying the surface tension, the dielectric constant, and the temperature to clarify the positron state in CS2. I also measured the angular correlation of the positron annihilation γ-rays in CS2. High surface tension and high electron and positron affinities of CS2 cause many phenomena which can not be seen in other liquids.

Deconvolution of positron annihilation γ-ray energy spectra with the maximum entropy principle

Abstract Deconvolution with the maximum entropy principle has been examined for analyzing positron annihilation γ-ray energy spectra. When applied to variable energy positron beam measurements, this technique may make possible the quantitative study of depth-dependent local electron momentum distributions and other effects in solids without introducing spurious features.