Gamma-ray Coincidence Research Articles

Multiple triaxial bands inNd138

High-spin states in Nd-138 were investigated by using the Ca-48 + Zr-94 reaction and gamma-ray coincidences were acquired with the GASP spectrometer. A rich level scheme was developed including 14 new bands of quadrupole transitions at very high spins. Linking transitions connecting 11 high-spin bands to low-energy states have been observed. Calculations based on the cranked Nilsson-Strutinsky formalism have been used to assign configurations to the observed bands. The main result of these calculations is that all 14 bands exhibit a stable triaxial deformation up to the highest observed spins, giving strong support to the existence of a triaxial minimum with normal deformation and positive asymmetry parameter in nuclei with a few holes in the N = 82 shell closure.

THE BROAD-LINED Type Ic SN 2012ap AND THE NATURE OF RELATIVISTIC SUPERNOVAE LACKING A GAMMA-RAY BURST DETECTION

We present ultraviolet, optical, and near-infrared observations of SN 2012ap, a broad-lined Type Ic supernova in the galaxy NGC 1729 that produced a relativistic and rapidly decelerating outflow without a gamma-ray burst signature. Photometry and spectroscopy follow the flux evolution from -13 to +272 days past the B-band maximum of -17.4 +/- 0.5 mag. The spectra are dominated by Fe II, O I, and Ca II absorption lines at ejecta velocities of 20,000 km/s that change slowly over time. Other spectral absorption lines are consistent with contributions from photospheric He I, and hydrogen may also be present at higher velocities (> 27,000 km/s). We use these observations to estimate explosion properties and derive a total ejecta mass of 2.7 Msolar, a kinetic energy of 1.0x10^{52} erg, and a 56Ni mass of 0.1-0.2 Msolar. Nebular spectra (t > 200d) exhibit an asymmetric double-peaked [OI] 6300,6364 emission profile that we associate with absorption in the supernova interior, although toroidal ejecta geometry is an alternative explanation. SN 2012ap joins SN 2009bb as another exceptional supernova that shows evidence for a central engine (e.g., black-hole accretion or magnetar) capable of launching a non-negligible portion of ejecta to relativistic velocities without a coincident gamma-ray burst detection. Defining attributes of their progenitor systems may be related to notable properties including above-average environmental metallicities of Z > Zsolar, moderate to high levels of host-galaxy extinction (E(B-V) > 0.4 mag), detection of high-velocity helium at early epochs, and a high relative flux ratio of [Ca II]/[O I] > 1 at nebular epochs. These events support the notion that jet activity at various energy scales may be present in a wide range of supernovae.

Recent Direct Reaction Experimental Studies with Radioactive Tin Beams

Direct reaction techniques are powerful tools to study the single-particle nature of nuclei. Performing direct reactions on short-lived nuclei requires radioactive ion beams produced either via fragmentation or the Isotope Separation OnLine (ISOL) method. Some of the most interesting regions to study with direct reactions are close to the magic numbers where changes in shell structure can be tracked. These changes can impact the final abundances of explosive nucleosynthesis. The structure of the chain of tin isotopes is strongly influenced by the Z=50 proton shell closure, as well as the neutron shell closures lying in the neutron-rich, N=82, and neutron-deficient, N=50, regions. Here we present two examples of direct reactions on exotic tin isotopes. The first uses a one-neutron transfer reaction and a low-energy reaccelerated ISOL beam to study states in 131Sn from across the N=82 shell closure. The second example utilizes a one-neutron knockout reaction on fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements of gamma rays in coincidence with charged particles proved to be invaluable.

Simulation of triple coincidences in PET

Although current PET scanners are designed and optimized to detect double coincidence events, there is a significant amount of triple coincidences in any PET acquisition. Triple coincidences may arise from causes such as: inter-detector scatter (IDS), random triple interactions (RT), or the detection of prompt gamma rays in coincidence with annihilation photons when non-pure positron-emitting radionuclides are used (β+γ events). Depending on the data acquisition settings of the PET scanner, these triple events are discarded or processed as a set of double coincidences if the energy of the three detected events is within the scanner’s energy window. This latter option introduces noise in the data, as at most, only one of the possible lines-of-response defined by triple interactions corresponds to the line along which the decay occurred. Several novel works have pointed out the possibility of using triple events to increase the sensitivity of PET scanners or to expand PET imaging capabilities by allowing differentiation between radiotracers labeled with non-pure and pure positron-emitting radionuclides. In this work, we extended the Monte Carlo simulator PeneloPET to assess the proportion of triple coincidences in PET acquisitions and to evaluate their possible applications. We validated the results of the simulator against experimental data acquired with a modified version of a commercial preclinical PET/CT scanner, which was enabled to acquire and process triple-coincidence events. We used as figures of merit the energy spectra for double and triple coincidences and the triples-to-doubles ratio for different energy windows and radionuclides. After validation, the simulator was used to predict the relative quantity of triple-coincidence events in two clinical scanners assuming different acquisition settings. Good agreement between simulations and preclinical experiments was found, with differences below 10% for most of the observables considered. For clinical scanners and pure positron emitters, we found that around 10% of the processed double events come from triple coincidences, increasing this ratio substantially for non-pure emitters (around 25% for 124I and > 50% for 86Y). For radiotracers labeled with 18F we found that the relative quantity of IDS events in standard acquisitions is around 18% for the preclinical scanner and between 14 and 22% for the clinical scanners. For non-pure positron emitters like 124I, we found a β+γ triples-to-doubles ratio of 2.5% in the preclinical scanner and of up to 4% in the clinical scanners.

Two-neutrino double-βdecay ofNd150to excited final states inSm150

Double-beta decay is a rare nuclear process in which two neutrons in the nucleus are converted to two protons with the emission of two electrons and two electron anti-neutrinos. We measured the half life of the two-neutrino double-beta decay of $^{150}$Nd to excited final states of $^{150}$Sm by detecting the de-excitation gamma rays of the daughter nucleus. This study yields the first detection of the coincidence gamma rays from the 0$^+_1$ excited state of $^{150}$Sm. These gamma rays have energies of 333.97 keV and 406.52 keV, and are emitted in coincidence through a 0$^+_1\rightarrow$2$^+_1\rightarrow$0$^+_{gs}$ transition. The enriched Nd$_2$O$_3$ sample consisted of 40.13 g $^{150}$Nd and was observed for 642.8 days at the Kimballton Underground Research Facility, producing 21.6 net events in the region of interest. This count rate gives a half life of $T_{1/2}=(1.07^{+0.45}_{-0.25}(stat)\pm0.07(syst.))\times 10^{20}$ years. The effective nuclear matrix element was found to be 0.0465$^{+0.0098}_{-0.0054}$. Finally, lower limits were obtained for decays to higher excited final states. Our half-life measurement agrees within uncertainties with another recent measurement in which no coincidence was employed. Our nuclear matrix element calculation may have an impact on a recent neutrinoless double-beta decay nuclear matrix element calculation which implies the decay to the first excited state in $^{150}$Sm is favored over that to the ground state.

An Effective Localization Method for Robotic Endoscopic Capsules Using Multiple Positron Emission Markers

The wireless capsule endoscope (WCE) is a first-line medical tool for the diagnosis of many gastrointestinal (GI) tract diseases such as obscure GI bleeding, Crohn's disease, small bowel tumors, and Celiac disease. Over the past few years, significant research attention has been paid to upgrading the WCE from a diagnostic-only tool to an active medical robot having not only diagnostic capabilities but therapeutic functionalities, such as biopsy, microsurgery, and targeted drug delivery, as well. One of the major limitations that impedes the development of such a robotic-type endoscope is the lack of a highly accurate localization system. In this paper, we present an experimental evaluation of a new realtime localization method (patent pending) based on tracking three positron emission markers embedded in the cover of an endoscopic capsule. Coincidence gamma rays emitted from the markers are detected by surrounding gamma ray detectors. The position and orientation information of the capsule can then be extracted by an effective tracking algorithm. The experiments were conducted in two different commercial positron emission technology (PET) scanners: Philips Allegro and Philips TF64. The experimental results show that the proposed localization method could provide less than 0.5-mm position error and 2.4° orientation error in a localization time interval of 50 ms with an average computational time of 6 ms per time interval. Zero power consumption and zero space occupation inside the capsule are additional advantages of this localization method.

Revised level structure ofTe120

The level scheme of the nucleus Te-120, populated in the reaction Se-80(Ca-48, alpha 4n), was reinvestigated using gamma-ray coincidence data measured with the Gammasphere spectrometer. Previously, five high-spin rotational bands were discovered in this nucleus. The present reinvestigation revealed that the decay of band b1 is more complex than suggested in the earlier work and that it cannot be uniquely determined. Furthermore, a number of new transitions are added to the level scheme. The implications for the spin assignments and excitation energies of the five bands and for comparisons with cranked Nilsson-Strutinsky calculations are discussed.

National Array of Neutron Detectors (NAND): A versatile tool for nuclear reaction studies

The first phase of the National Array of Neutron Detectors (NAND) consisting of 26 neutron detectors has been commissioned at the Inter University Accelerator Centre (IUAC), New Delhi. The motivation behind setting up of such a detector system is the need for more accurate and efficient study of reaction mechanisms in the projectile energy range of 5–8MeV/n using heavy ion beams from a 15 UD Pelletron and an upgraded LINAC booster facility at IUAC. The above detector array can be used for inclusive as well as exclusive measurements of reaction products of which at least one product is a neutron. While inclusive measurements can be made using only the neutron detectors along with the time of flight technique and a pulsed beam, exclusive measurements can be performed by detecting neutrons in coincidence with charged particles and/or fission fragments detected with ancillary detectors. The array can also be used for neutron tagged gamma-ray spectroscopy in (HI, xn) reactions by detecting gamma-rays in coincidence with the neutrons in a compact geometrical configuration. The various features and the performance of the different aspects of the array are described in the present paper.

Concept and simulation study of a novel localization method for robotic endoscopic capsules using multiple positron emission markers.

Over the last decade, wireless capsule endoscope has been the tool of choice for noninvasive inspection of the gastrointestinal tract, especially in the small intestine. However, the latest clinical products have not been equipped with a sufficiently accurate localization system which makes it difficult to determine the location of intestinal abnormalities, and to apply follow-up interventions such as biopsy or drug delivery. In this paper, the authors present a novel localization method based on tracking three positron emission markers embedded inside an endoscopic capsule. Three spherical(22)Na markers with diameters of less than 1 mm are embedded in the cover of the capsule. Gamma ray detectors are arranged around a patient body to detect coincidence gamma rays emitted from the three markers. The position of each marker can then be estimated using the collected data by the authors' tracking algorithm which consists of four consecutive steps: a method to remove corrupted data, an initialization method, a clustering method based on the Fuzzy C-means clustering algorithm, and a failure prediction method. The tracking algorithm has been implemented inMATLAB utilizing simulation data generated from the Geant4 Application for Emission Tomography toolkit. The results show that this localization method can achieve real-time tracking with an average position error of less than 0.4 mm and an average orientation error of less than 2°. The authors conclude that this study has proven the feasibility and potential of the proposed technique in effectively determining the position and orientation of a robotic endoscopic capsule.

High-spin structures of 124−131Te: Competition of proton- and neutron-pair breakings

The 124-131Te nuclei have been produced as fission fragments in two fusion reactions induced by heavy-ions (12C + 238U at 90 MeV bombarding energy and 18O + 208Pb at 85 MeV) and studied with the Euroball array. Their high-spin level schemes have been extended to higher excitation energy from the triple gamma-ray coincidence data. The gamma-gamma angular correlations have been analyzed in order to assign spin and parity values to many observed states. Moreover the half-lives of isomeric states have been measured from the delayed coincidences between the fission-fragment detector SAPhIR and Euroball, as well as from the timing information of the Ge detectors. The behaviors of the yrast structures identified in the present work are first discussed in comparison with the general features known in the mass region, particularly the breakings of neutron pairs occupying the nuh11/2 orbit identified in the neighboring Sn nuclei. The experimental level schemes are then compared to shell-model calculations performed in this work. The analysis of the wave functions shows the effects of the proton-pair breaking along the yrast lines of the heavy Te isotopes.

Collective and noncollective excitations in122Te

High-spin states in Te-122 were populated in the reaction Se-82(Ca-48, alpha 4n) Te-122 at a beam energy of 200 MeV and gamma-ray coincidences weremeasured with the Gammasphere spectrometer. The previously known level scheme was extended to considerably higher spin. Maximally aligned states and several high-energy transitions feeding into some of these levels were observed. In addition, seven collective high-spin bands were discovered for the first time in this nucleus. The experimental results are compared with cranked Nilsson-Strutinsky model calculations and possible configuration assignments to the new high-spin structures are discussed.

Measurements of alpha–gamma coincidences with an optimized dual-parameter multichannel system

Measurements of alpha–gamma coincidences have usually been carried out using a single channel to detect alpha-particles of a given energy, and a multichannel analyser for the detection of the corresponding coincident gamma-rays. An alpha–gamma coincidence chamber coupled to the electronic chain ending with a dual-parameter multichannel analyser has been developed and optimized. This system simultaneously stores alpha-particle, gamma-ray, and alpha–gamma coincidence spectra, which allows a general analysis to be made of the degree of coincidence between each alpha-particle and each gamma-ray emission. With this technique, a two-dimensional spectrum was obtained and analysed using “contour graphics”. An application to the study of the decay scheme of 241Am is described.

High-spin structures of3688Kr52and3789Rb52: Evolution from collective to single-particle behavior

The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were built from triple gamma-ray coincidence data and gamma-gamma angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two levels schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.

The GASPARD project

New recoil particle arrays are being developed for optimal study of direct reactions at forthcoming facilities for radioactive beams such as SPIRAL2 at GANIL. The GASPARD project is based on a concept of a high granularity 4π particle silicon detector fully integrable in major gamma-ray devices currently under development. The detection of gamma-rays in coincidence with particles allows the use of thicker targets as well as a large gain in excitation energy resolution as compared to particle detection only. Detection of gamma-rays also provides crucial spectroscopic information on the populated states. Other important features of GASPARD are the implementation of state-of-the-art particle identification techniques and integration of special targets such as the pure and windowless hydrogen target being currently developed. General aspects concerning the design are presented as well as most recent results of the related research and development program on particle identification using pulse shape discrimination.

High resolution phoswich gamma-ray imager utilizing monolithic MPPC arrays with submillimeter pixelized crystals

We report the development of a high spatial resolution tweezers-type coincidence gamma-ray camera for medical imaging.This application consists of large-area monolithic Multi-Pixel Photon Counters (MPPCs) and submillimeter pixelized scintillator matrices.The MPPC array has 4 × 4 channels with a three-side buttable, very compact package.For typical operational gain of 7.5 × 105 at + 20 °C, gain fluctuation over the entire MPPC device is only ± 5.6%, and dark count rates (as measured at the 1 p.e. level) amount to ⩽ 400 kcps per channel.We selected Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) and a brand-new scintillator, Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) due to their high light yield and density.To improve the spatial resolution, these scintillators were fabricated into 15 × 15 matrices of 0.5 × 0.5 mm2 pixels.The Ce:LYSO and Ce:GAGG scintillator matrices were assembled into phosphor sandwich (phoswich) detectors, and then coupled to the MPPC array along with an acrylic light guide measuring 1 mm thick, and with summing operational amplifiers that compile the signalsinto four position-encoded analog outputs being used for signal readout.Spatial resolution of 1.1 mm was achieved with the coincidence imaging system using a 22Na point source.These results suggest that the gamma-ray imagers offer excellent potential for applications in high spatial medical imaging.

Simulations of Multi-Gamma Coincidences From Neutron-Induced Fission in Special Nuclear Materials

A study is presented on the detection of illicit special nuclear materials (SNM) in cargo containers using a conceptual neutron-based inspection system with xenon-doped liquefied argon (LAr(Xe)) scintillation detectors for coincidence gamma-ray detection. For robustness, the system is envisioned to exploit all fission signatures, namely both prompt and delayed neutron and gamma emissions from fission reactions induced in SNM. However, this paper focuses exclusively on the analysis of the prompt gamma ray emissions. The inspection system probes a container using neutrons produced either by (d, D) or (d, T) in pulsed form or from an associated particle neutron generator to exploit the associated particle imaging (API) technique, thereby achieving background reduction and imaging. Simulated signal and background estimates were obtained in MCNPX (2.7) for a 2 kg sphere of enriched uranium positioned at the center of a 1m × 1m × 1m container, which is filled uniformly with wood or iron cargos at 0.1 g/cc or 0.4 g/cc. Detection time estimates are reported assuming probabilities of detection of 95% and false alarm of 0.5%.

Low-energy level schemes of66,68Fe and inferred proton and neutron excitations acrossZ=28andN=40

Background: The nuclei in the region around ${}^{68}$Ni display an apparent rapid development of collectivity as protons are removed from the ${f}_{7/2}$ single-particle state along the $N=40$ isotonic chain. Proton and neutron excitations across the $Z=28$ and $N=40$ gaps are observed in odd-A ${}_{27}$Co and ${}_{26}$Fe isotopes. Little spectroscopic information beyond the excited 2${}^{+}$ and 4${}^{+}$ is available in the even-even ${}_{26}^{66,68}\mathrm{Fe}$ nuclei to compare with shell model calculations.Purpose: Our goal is to determine the low-energy level schemes of ${}^{66,68}$Fe and compare the observed excitations with shell model calculations to identify states wherein a contribution from excitations across $Z=28$ and $N=40$ are present.Method: The low-energy states of ${}^{66,68}$Fe were populated through the beta decay of ${}^{66,68}$Mn produced at the National Superconducting Cyclotron Laboratory. Beta-delayed gamma-ray transitions were detected and correlated to the respective parent isotope to construct a low-energy level scheme.Results: The low-energy level schemes of ${}^{66,68}$Fe were constructed from observed gamma-ray coincidences and absolute gamma-ray intensities. Tentative spin and parity assignments were assigned based on comparisons with shell model calculations and systematics. The two lowest 0${}^{+}$ and 2${}^{+}$ states were characterized in terms of the number of protons and neutrons excited across the respective shell gaps.Conclusion: The removal of two protons from ${}^{68}$Ni to ${}^{66}$Fe results in an inversion of the normal configuration and the one characterized by significant excitation across the $Z=28$ and $N=40$ gaps. Approximately, one proton and two neutrons are excited across their respective single-particle gaps in the ground state of ${}^{66}$Fe.

Electromagnetic Transition Rate Measurements in the N=80 Isotone, 138Ce

A study of intrinsic state halflife measurements in the N=80 nucleus 138Ce has been made using the 130Te(12C,4n)138Ce fusion evaporation reaction at beam energy of 56 MeV. The fast-timing gamma-ray coincidence method was used with a mixed LaBr3(Ce)-HPGe array to establish the lifetimes of the yrast 6+ state at 2294 keV, the Iπ=5− state at 2218 keV, the Iπ=11+ state at 3943 keV and the 14+ state at that at 5312 keV, all of which are in the sub nanosecond regime. Reduced transition probabilities have been calculated for the electromagnetic decays from these states.

The correct and incorrect way to calibrate a Compton suppression counting system for gamma-ray efficiency

Gamma-ray efficiency calculations for a germanium detector have been made for a Compton suppression system. Results have shown that for radionuclides that have gamma rays in coincidence the photopeaks can be severely depressed leading to erroneous results. While this can be overcome in routine neutron activation analysis using a comparator method, special consideration must be given to determine the suppression for coincident gamma rays when calculating the efficiency curve and for radionuclide activities. This is especially important for users of the k 0 method and for fission product identification using Compton suppression methods.

Determination of zinc in geological samples using Compton suppression with thermal and epithermal instrumental neutron activation analysis

The determination of Zn in geological samples using instrumental neutron activation analysis is usually done using the 64Zn(n,γ)65Zn reaction and its 244 day half-life. However this analysis has proven to be potentially difficult. This is due to its relatively low neutron absorption cross section and gamma ray intensity, and the relatively high neutron absorption cross section and gamma intensity of 46Sc, which has an energy peak that is only 5 keV greater than 65Zn. The use of a high resolution detector makes it possible to differentiate between the 65Zn and 46Sc photopeaks peaks. However, the dominating 46Sc gamma ray can even make peak fitting routines unsuccessful in the proper determination of 65Zn. The use of a Compton suppression system suppresses the 46Sc peak, which has two coincident gamma-rays, and this greatly improves the ratio of the height of the 46Sc 1120.5 keV photopeak to the 65Zn 1115.4 keV photopeak. Irradiating the sample with epithermal neutrons also improves the measurement since 65Zn has a higher cross section for epithermal neutrons rather than thermal neutrons, whereas 46Sc has a higher thermal cross section. Another technique to determine zinc is the use of 68Zn(n,γ)69mZn reaction with its 13 h half-life using epithermal neutrons and Compton suppression INAA. However, the 438 keV gamma ray of 69mZn has no interference with any adjoining photopeak. A critical comparison of these two methods is given.