Triple Coincidence Research Articles

New experimental investigation of the structure ofBe10andC16by means of intermediate-energy sequential breakup

Be10 and C16 spectroscopy has been investigated by analyzing their breakup events on CH2 and CD2 targets. Breakup fragments have been detected by means of the CHIMERA detector. In particular, we investigated cluster decays of Be10 in He4+He6 and of C16 in He6+Be10 and He4+He6+He6. From the relative energy analysis of breakup fragments, we investigate the spectroscopy of excited states of projectile nuclei. In the Be10 case we observe known states at 9.51, 10.16, 10.6, and 11.8 MeV. Further, we suggest the existence of a new state at 13.5 MeV, possibly 6+ as indicated from angular correlation analysis. The relative energy (Erel+Eth) spectrum of C16, reconstructed starting from He6+Be10 correlations, shows a peak at about 20.6 MeV, probably related to the existence of an high-lying excited state. Non-vanishing yields are also seen in the triple coincidences He4+He6+He6.

Recovering the triple coincidence of non-pure positron emitters in preclinical PET**Parts of this work were presented at the 2012 Annual Congress of the European Association of Nuclear Medicine.

Non-pure positron emitters, with their long half-lives, allow for the tracing of slow biochemical processes which cannot be adequately examined by the commonly used short-lived positron emitters. Most of these isotopes emit high-energy cascade gamma rays in addition to positron decay that can be detected and create a triple coincidence with annihilation photons. Triple coincidence is discarded in most scanners, however, the majority of the triple coincidence contains true photon pairs that can be recovered. In this study, we propose a strategy for recovering triple coincidence events to raise the sensitivity of PET imaging for non-pure positron emitters. To identify the true line of response (LOR) from a triple coincidence, a framework utilizing geometrical, energy and temporal information is proposed. The geometrical criterion is based on the assumption that the LOR with the largest radial offset among the three sub pairs of triple coincidences is least likely to be a true LOR. Then, a confidence time window is used to test the valid LOR among those within triple coincidence. Finally, a likelihood ratio discriminant rule based on the energy probability density distribution of cascade and annihilation gammas is established to identify the true LOR. An Inveon preclinical PET scanner was modeled with GATE (GEANT4 application for tomographic emission) Monte Carlo software. We evaluated the performance of the proposed method in terms of identification fraction, noise equivalent count rates (NECR), and image quality on various phantoms. With the inclusion of triple coincidence events using the proposed method, the NECR was found to increase from 11% to 26% and 19% to 29% for I-124 and Br-76, respectively, when 7.4–185 MBq of activity was used. Compared to the reconstructed images using double coincidence, this technique increased the SNR by 5.1–7.3% for I-124 and 9.3–10.3% for Br-76 within the activity range of 9.25–74 MBq, without compromising the spatial resolution or contrast. We conclude that the proposed method can improve the counting statistics of PET imaging for non-pure positron emitters and is ready to be implemented on current PET systems.

A tandem time-of-flight spectrometer for negative-ion/positive-ion coincidence measurements with soft x-ray excitation.

We present a newly constructed spectrometer for negative-ion/positive-ion coincidence spectroscopy of gaseous samples. The instrument consists of two time-of-flight ion spectrometers and a magnetic momentum filter for deflection of electrons. The instrument can measure double and triple coincidences between mass-resolved negative and positive ions with high detection efficiency. First results include identification of several negative-ion/positive-ion coincidence channels following inner-shell photoexcitation of sulfur hexafluoride (SF6).

Using triple gamma coincidences with a pixelated semiconductor Compton-PET scanner: a simulation study

The Voxel Imaging PET (VIP) Pathfinder project presents a novel design using pixelated semiconductor detectors for nuclear medicine applications to achieve the intrinsic image quality limits set by physics. The conceptual design can be extended to a Compton gamma camera. The use of a pixelated CdTe detector with voxel sizes of 1 × 1 × 2 mm3 guarantees optimal energy and spatial resolution. However, the limited time resolution of semiconductor detectors makes it impossible to use Time Of Flight (TOF) with VIP PET. TOF is used in order to improve the signal to noise ratio (SNR) by using only the most probable portion of the Line-Of-Response (LOR) instead of its entire length. To overcome the limitation of CdTe time resolution, we present in this article a simulation study using β+-γ emitting isotopes with a Compton-PET scanner. When the β+ annihilates with an electron it produces two gammas which produce a LOR in the PET scanner, while the additional gamma, when scattered in the scatter detector, provides a Compton cone that intersects with the aforementioned LOR. The intersection indicates, within a few mm of uncertainty along the LOR, the origin of the beta-gamma decay. Hence, one can limit the part of the LOR used by the image reconstruction algorithm.

Calculation of extrapolation curves in the 4π(LS)β-γ coincidence technique with the Monte Carlo code Geant4

At LNE-LNHB, a liquid scintillation (LS) detection setup designed for Triple to Double Coincidence Ratio (TDCR) measurements is also used in the β-channel of a 4π(LS)β–γ coincidence system. This LS counter based on 3 photomultipliers was first modeled using the Monte Carlo code Geant4 to enable the simulation of optical photons produced by scintillation and Cerenkov effects. This stochastic modeling was especially designed for the calculation of double and triple coincidences between photomultipliers in TDCR measurements. In the present paper, this TDCR-Geant4 model is extended to 4π(LS)β–γ coincidence counting to enable the simulation of the efficiency–extrapolation technique by the addition of a γ-channel. This simulation tool aims at the prediction of systematic biases in activity determination due to eventual non-linearity of efficiency–extrapolation curves. First results are described in the case of the standardization 59Fe. The variation of the γ-efficiency in the β-channel due to the Cerenkov emission is investigated in the case of the activity measurements of 54Mn. The problem of the non-linearity between β-efficiencies is featured in the case of the efficiency tracing technique for the activity measurements of 14C using 60Co as a tracer.

γ-ray spectroscopy of fission fragments produced inPb208(O18,f)

Prompt gamma-ray spectroscopy of fission fragments produced in the heavy-ion induced fusion-fission reaction $^{208}\mathrm{Pb}(^{18}\mathrm{O},f)$ at $E=90$ MeV has been performed. The relative isotopic yields of the fission fragments and the fragment mass distribution have been studied. Structures in the mass distribution have been discussed in the light of earlier results. Relative yields of several odd-$A$ isotopes of Mo, Ru, Pd, and Cd and the odd-$A$ isotones with $N=62$ and 64 have been studied along with the yields of the neighboring even-$Z$, even-$N$ fragments and correlated to nuclear structural effects. The average total neutron multiplicity during fission has been measured to be $5.48\ifmmode\pm\else\textpm\fi{}0.59$. The level schemes of the two neutron-rich nuclei $^{110}\mathrm{Pd}$ and $^{116}\mathrm{Cd}$ have been studied from $\ensuremath{\gamma}$-ray triple coincidence data. A large number of transitions, previously reported only from $\ensuremath{\beta}$-decay studies, have been observed in $^{110}\mathrm{Pd}$ for the first time. The yrast band in $^{116}\mathrm{Cd}$ has been extended up to spin $({16}^{+})$. In addition, a rotational sequence built upon an excited ${5}^{\ensuremath{-}}$ state in $^{116}\mathrm{Cd}$ has been observed up to $({13}^{\ensuremath{-}})$. The level schemes have been discussed in the context of existing results, both experimental and theoretical, in the literature.

Kinematically complete experiment for ionization-excitation of helium

SynopsisWe present a newly built reaction microscope for electron induced ionization-excitation of helium. Kinematically complete measurement with 2p state excitation is achieved by detecting the radiated photon, one electron and the He+ ion in the final state in triple coincidence. Near threshold behavior of the ionization- excitation to 2p state is investigated.

Negative-ion/positive-ion coincidence spectroscopy with a novel spectrometer

SynopsisA novel instrument for negative-ion/positive-ion coincidence spectroscopy (NIPICO) is presented. The instrument consists of a double TOF spectrometer with a common extraction region. Electrons are deflected by means of a magnetic field. We demonstrate detections of double (NIPICO) and triple (NIPIPICO) coincidences.

Dissociative ionization dynamics of triatomic molecules induced by soft X-rays

SynopsisSoft X-rays (50eV to 170eV) have been used in conjunction with a triple coincidence time and position sensitive detection apparatus to observe the Coulomb explosion of OCS and CS2. By varying the X-ray energy we can access ionization channels which result in two or three body break up, for 3+, such as O++C++S+ and for 4+ such as S++C++S2+. We can compare the kinetic energy release to the value calculated for a purely Coulombic break up from near equilibrium. The extent to which channels are concerted or stepwise is also determined.

An (e, 2e + ion) study of low-energy electron-impact ionization and fragmentation of carbon dioxide

SynopsisWe study the ionization and fragmentation of carbon dioxide (CO2) induced by low-energy (E0 = 66 eV) electron-impact using a reaction microscope. The momentum vectors of all three charged final state particles, two outgoing electrons and one fragment ion, are detected with an (e, 2e + ion) triple coincidence method. The fragmentation pathways of carbon dioxide for formation of the CO+2, CO+, O+ and C+ ions are investigated by measuring the ion kinetic energy spectra and the binding energy spectra where a binding energy resolution of 2.0 eV has been achieved in the experiment.

Ion impact induced fragmentation of carbon dioxide

SynopsisMomentum vectors of fragment ions produced in the Coulomb explosion of COq+2 (q = 3 — 5) in collision with highly charged ions are determined by the triple coincidence imaging technique. Kinematics as well as molecular structures of various explosion channels are reconstructed from the measured momentum vectors.

Monte Carlo study of scattered and random coincidences for MADPET-4.

MADPET-4 is a high resolution PET insert under development for use in a 7-T MR. To fully exploit the capabilities of the insert, a good understanding of the physical interactions, which take place in the active and passive components of the insert, is necessary. The goal of this study was to investigate the effects of different physical interactions in an accurate model of MADPET-4 using Monte Carlo (MC) simulations. The main focus of the study was on the impact of the different active and passive components of the system on the amount of random and scattered events, including scattering in the passive components and between the crystals. The influence of the low energy threshold (50-350 keV) and different geometrical conditions in the coincidence sorting process was of particular interest. The effect of including triple coincidences was also considered in the present study. Results showed that the maximum sensitivity achieved at the minimum energy threshold (50 keV), using a geometrical condition of 4-9 sectors, was ~3.43% without passive components and dropping to ~1.36% including the passive components. Interestingly, the minimum noise ratio was obtained with a geometrical condition of 9 sectors difference at an energy threshold of 50 keV. Including triple coincidences showed no benefit for energy thresholds above 250 keV. However, low energy thresholds provided a relative gain in sensitivity, reaching 15-20% for 200 keV and a maximum of 30-40% at 50 keV.

Loss of the associated α-particles in the tagged neutron generators

The reported loss of α-particles in the 14MeV tagged neutron generators has been investigated using two neutron generators equipped with α-particle counters and two neutron detectors. One neutron detector was put right in the middle of the tagged neutron cone and another one was put outside the cone. By measuring the difference between double (neutron–neutron) and triple (α-neutron-neutron) coincidences it is possible to deduce the α-particle loss since the number of triple coincidences should be equal to the number of double coincidences. In all measurements performed a deficit of triple with respect to double coincidences has been observed. This deficit was smallest for the threshold of α-particle Constant Fraction Discriminator (αCFD) being 0 and maximum allowed voltage of α-particle detector being −1.7kV. The smallest measured deficit value was equal to 13±1%. From the observed results it was concluded that the deficit was due to a number of non-detected α-particles that loose sufficient quantity of energy while traveling to the detector because of collisions with particles present in the neutron tube and/or in the tritium target. These α-particles will not be detected as they fall under the threshold of αCFD discriminator. Magnetic fields present in the system worsen the situation since they are forcing α-particles to travel larger distances because of toroidal movement and undergoing additional collisions. Tagged neutron technique has many kind of applications and it is particularly important for high accuracy nuclear cross-sections measurements when α-particles losses must be carefully assessed.

Triple coincidence PALS setup based on fast pulse digitizers

We have assembled positron annihilation lifetime spectroscopy system that consists of three BaF2 detectors with XP2020URQ PMTs which are directly coupled to DRS4 evaluation board. Both time and energy information of the detected gamma rays are reconstructed. In the off-line analysis, by using selected cuts on energies of gamma rays, double, as well as triple coincidence events can be extracted. The setup is very suitable for the determination of contributions of positronium three gamma annihilations which can be of interest in the investigations of various porous materials.

Determination of the Parameters of Plutonium Blocks Placed Inside Containers

Measurements were performed with 1.05 and 3.15 kg plutonium blocks with known (nominal) 239Pu fractions 88.7, 91.0, 93.2, and 95.2% in order to check the possibility of determining their mass and enrichment by using a neutron detector – an active well coincidence counter (AWCC). Analysis of double and triple neutron-neutron coincidences was performed taking spatial effects into account. The intensity of the source of spontaneous fissions in the block and thereby the effective mass of 240Pu were determined on the basis of the requirement that the neutron multiplicities obtained from double and triple coincidences are the same. The 239Pu content was determined to within 0.003–0.004 (1σ), which makes it possible to reliably determine the 239Pu content in the blocks, equal to 91% (power-grade) and 93% (weapons-grade). The plutonium mass in a block is determined to within ±2–3% from the obtained effective neutron multiplication factor.

Three-body fragmentation of CO2 driven by intense laser pulses.

Dissociative ionization dynamics were studied experimentally for CO2 driven by intense laser pulses. Three-dimensional momentum vectors of correlated atomic ions were obtained for each three-body fragmentation event using triple ion coincidence measurement. Newton diagram demonstrated that three-body fragmentation of CO2 (n+) (n = 3-6) can occur through Coulomb explosion process and sequential fragmentation process depending on the fragmentation channels. The experimental data from these two processes were disentangled by using correlation diagram of correlated ions. Based on the accurate Coulomb explosion data, we reconstructed the bond angle distributions of CO2 (n+) at the moment of fragmentation, which are close to that of neutral CO2 before laser irradiation.

Recovery and normalization of triple coincidences in PET.

Triple coincidences in positron emission tomography (PET) are events in which three γ-rays are detected simultaneously. These events, though potentially useful for enhancing the sensitivity of PET scanners, are discarded or processed without special consideration in current systems, because there is not a clear criterion for assigning them to a unique line-of-response (LOR). Methods proposed for recovering such events usually rely on the use of highly specialized detection systems, hampering general adoption, and/or are based on Compton-scatter kinematics and, consequently, are limited in accuracy by the energy resolution of standard PET detectors. In this work, the authors propose a simple and general solution for recovering triple coincidences, which does not require specialized detectors or additional energy resolution requirements. To recover triple coincidences, the authors' method distributes such events among their possible LORs using the relative proportions of double coincidences in these LORs. The authors show analytically that this assignment scheme represents the maximum-likelihood solution for the triple-coincidence distribution problem. The PET component of a preclinical PET/CT scanner was adapted to enable the acquisition and processing of triple coincidences. Since the efficiencies for detecting double and triple events were found to be different throughout the scanner field-of-view, a normalization procedure specific for triple coincidences was also developed. The effect of including triple coincidences using their method was compared against the cases of equally weighting the triples among their possible LORs and discarding all the triple events. The authors used as figures of merit for this comparison sensitivity, noise-equivalent count (NEC) rates and image quality calculated as described in the NEMA NU-4 protocol for the assessment of preclinical PET scanners. The addition of triple-coincidence events with the authors' method increased peak NEC rates of the scanner by 26.6% and 32% for mouse- and rat-sized objects, respectively. This increase in NEC-rate performance was also reflected in the image-quality metrics. Images reconstructed using double and triple coincidences recovered using their method had better signal-to-noise ratio than those obtained using only double coincidences, while preserving spatial resolution and contrast. Distribution of triple coincidences using an equal-weighting scheme increased apparent system sensitivity but degraded image quality. The performance boost provided by the inclusion of triple coincidences using their method allowed to reduce the acquisition time of standard imaging procedures by up to ∼25%. Recovering triple coincidences with the proposed method can effectively increase the sensitivity of current clinical and preclinical PET systems without compromising other parameters like spatial resolution or contrast.

Sensitivity Increase Through a Neural Network Method for LOR Recovery of ICS Triple Coincidences in High-Resolution Pixelated- Detectors PET Scanners

Scanner sensitivity is often critical in high-resolution Positron Emission Tomography (PET) dedicated to molecular imaging. In neighboring pixelated detectors with individual readout, sensitivity decreases because of multiple coincidences produced by Compton scattering. Correct analysis of those coincidences would enable a substantial sensitivity increase. However, including scattering byproducts in the image often lead to image quality degradation because of inaccurate Line-of-Response (LOR) assessment. In such scanners, to support high count rates, multiple coincidences are usually discarded when image degradation is not acceptable, or blindly accepted for a low computational burden. This paper presents a new, real-time capable method that includes Inter-Crystal Scatter (ICS) triple coincidences in the image without significant quality degradation. The method computes the LOR using a neural network fed by preprocessed raw data. As a proof of principle, this paper analyzes the simplest ICS scenario, triple coincidences where one photoelectric 511-keV event coincides with two more whose energy sum is also 511 keV. The paper visits the algorithm structure, presents Monte Carlo assessment with the LabPET model, and displays images reconstructed from real data. With an energy window of 360-660 keV and a singles energy threshold of 125 keV, the inclusion of triple coincidences yielded a sensitivity increase of 54%, a resolution degradation similar to that of other sensitivity-increasing methods, and only a slight contrast degradation for real LabPET data, with potential for numerous further improvements.

On the relativistic electron impact K-shell (e, (3-1)e) differential cross sections for atoms

In this paper, we present the results of our calculation of four-fold differential cross section (4DCS) for the K-shell electron impact double ionization (DI) on various atoms in the relativistic energy regime. The 4DCS is computed from the five-fold differential cross sections (5DCS) by integrating 5DCS over either the angles of one of the ejected electrons or the scattered electron (this process is referred as an (e, (3-1)e) process). Hence, it requires, possibly feasible double coincidence techniques to supplement our findings experimentally instead of the difficult triple coincidence technique, which is required for the complete description of the DI process. The angular profile of 5DCS is scanned with the angles of one of the ejected electrons for both the (e, (3-1)e) cases. A dominant peak in the momentum transfer direction is found when the scattered and one of the ejected electrons are detected in coincidence. The effects of nucleus (Z), energies of the ejected electrons, and the scattering angle on the 4DCS have been discussed. The angular correlation of the ejected electrons has also been discussed when only the ejected electrons are detected. We consider the spin aspects by scanning the singlet and triplet contributions in the angular profile of 4DCS. We also find that the interference term of the longitudinal and transverse interaction is responsible for the strengthening of the secondary peak. We observe a clear manifestation of the effect of nucleus on the spin asymmetry for behavior targets.

LiF/ZnS neutron multiplicity counter

The availability of 3He in recent years is becoming restricted with an order of magnitude price increase for this material. Alternatives to the use of 3He for the detection of thermal neutrons are under investigation. One of the most challenging applications for 3He alternatives is in neutron multiplicity counters that provide rapid assay of samples which contain an unknown amount of plutonium in a potentially unknown configuration. With appropriate detector design that has minimal gamma-ray sensitivity and a high detection efficiency even for triple coincidence events, the neutron single, double, and triple coincidence events can be used to extract three unknown parameters such as the 240Pu-effective mass, the sample self-multiplication, and the (α,n) rate. This project is aimed at determining if commercially available 3He alternatives can satisfy this challenging application. Using MCNP modeling the best alternative identified used LiF/ZnS neutron-scintillator sheets and wavelength shifting plastic for light pipes. A four-panel demonstrator module has been constructed, tested, and compared with detailed modeling results. However, to attain that desired high-level of performance two primary design challenges must be addressed. They include building a fast electronics system and robust neutron/gamma-ray discrimination based on pulse shape analysis at high rates. A review of the current effort and the most recent findings will be presented.