Escape Peak Research Articles

Capturing the kinetics of escape

Experiments with an optically trapped glass nanosphere have revealed the elusive Kramers turnover, at which the particle’s odds of escape peak. Experiments with an optically trapped glass nanosphere have revealed the elusive Kramers turnover, at which the particle’s odds of escape peak.

Response and calibration of organic scintillators for gamma-ray spectroscopy up to 15-MeV range

We present a calibration method for an organic scintillator suitable for gamma energies of up to 15 MeV. We use a combination of experiments and simulations to identify prominent features in the light output spectra to provide an accurate calibration method over this energy range. Gamma energies in the range of 0.511 MeV to 15.1 MeV were used to perform two separate calibrations to the measured light output expressed in the units of electron equivalent energy. The first method strictly uses the well-known technique of identifying the Compton edges, while the second utilizes a combination of Compton edges and double escape peaks for several gamma energies. We performed a least-squares test to the high end of the light output spectra for a 15.1 MeV gamma ray to assess the accuracy of the two methods. The calibration technique that employs a combination of Compton edges and escape peaks yielded a significant improvement in accuracy when compared to a calibration that relies solely on the use of Compton edges.

Improved approach for the determination of low‐Z elements in uranium samples using a vacuum chamber TXRF spectrometer

Total reflection X‐ray fluorescence (TXRF) method for the determination of low atomic number (Z) elements Na (Z = 11) to Ca (Z = 20) using Cr Kα excitation, vacuum atmosphere sample chamber, and ultrathin window detector of a low Z–high Z TXRF spectrometer has been developed. The earlier reported problem in determination of Al, due to interference of U Mα escape peak with Al Kα, was taken care by more intense solvent extraction‐based separation of uranium matrix from the sample solutions. Any dissolved tri‐n‐butyl phosphate having traces of uranium left after solvent extraction was removed from the sample solutions by equilibrating with n‐dodecane. This approach resulted in the betterment of analytical results. The method developed was validated by analyzing three Certified Reference Materials for trace metals in uranium oxide, developed by Department of Atomic Energy, Government of India. The TXRF determined results were found to have an average precision of 8.0% (1σ, n = 3) and the average deviation of the TXRF determined values from the certified concentrations was 7.3%. Copyright © 2017 John Wiley & Sons, Ltd.

γ-Ray spectroscopy using a binned likelihood approach

The measurement of a reaction cross section from a pulse height spectrum is a ubiquitous problem in experimental nuclear physics. In γ-ray spectroscopy, this is accomplished frequently by measuring the intensity of full-energy primary transition peaks and correcting the intensities for experimental artifacts, such as detection efficiencies and angular correlations. Implicit in this procedure is the assumption that full-energy peaks do not overlap with any secondary peaks, escape peaks, or environmental backgrounds. However, for complex γ-ray cascades, this is often not the case. Furthermore, this technique is difficult to adapt for coincidence spectroscopy, where intensities depend not only on the detection efficiency, but also the detailed decay scheme. We present a method that incorporates the intensities of the entire spectrum (e.g., primary and secondary transition peaks, escape peaks, Compton continua, etc.) into a statistical model, where the transition intensities and branching ratios can be determined using Bayesian statistical inference. This new method provides an elegant solution to the difficulties associated with analyzing coincidence spectra. We describe it in detail and examine its efficacy in the analysis of 18O(p,γ)19F and 25Mg(p,γ)26Al resonance data. For the 18O(p,γ)19F reaction, the measured branching ratios improve upon the literature values, with a factor of 3 reduction in the uncertainties.

Artificial peaks in energy dispersive X-ray spectra: sum peaks, escape peaks, and diffraction peaks

Sum peaks, escape peaks, and diffraction peaks are considered artificial or spurious peaks in energy dispersive X-ray spectrometry. Experimental examples are given, which showed that escape and diffraction peaks can add up to become sum peaks. These artificial peaks are not weak, and great care must be taken to differentiate them from peaks due to impurity or trace elements. The relationship between the intensity of a sum peak and the original peaks is illustrated using computer simulation as well as probability theory. Copyright © 2016 John Wiley & Sons, Ltd.

Experimental measurements of Doppler broadening of escape peaks in HPGe detectors

Single-escape, double-escape, and annihilation peak widths from eleven spectra taken with differing HPGe coaxial detectors and differing primary gamma sources were measured. For each of these, the Doppler broadening component of the peak width was derived. Significant Doppler broadening was observed only in the single escape peak and in the annihilation peak width. Averaged values and their uncertainties useful for modeling each type of peak width in nuclear radiation spectra are presented.

Monte Carlo Simulations of the GE Discovery Alcyone CZT SPECT Systems

Compact SPECT systems with cadmium zinc telluride (CZT) solid-state detectors with improved energy resolution and shorter acquisition times have recently been introduced. These systems have, however, different energy characteristics compared to NaI(Tl) crystal-based cameras; therefore, a need exists to develop new simulation models for these cameras. We modeled the charge transport within the CZT detectors for a GE Discovery 530c/570c SPECT system with multiple pinhole collimators employing the SIMIND Monte Carlo program and validated simulations against measurements. The incomplete charge collection between the anode and cathode in the pixilated CZT was modeled with the Hecht equation. The simulation also included charge-sharing effects across pixels due to physical interactions and charge diffusion. To validate our CZT-model, $^{99{\rm m}}{\rm Tc}$ and $^{123}{\rm I}$ point sources and a $^{201}{\rm Tl}$ line source were acquired and measured. Measured energy spectra were compared with simulated energy spectra. The Monte Carlo simulated energy spectra agreed well with the experimental measurements within the photopeak, overestimated the k-edge x-ray escape peaks of cadmium and telluride, and slightly underestimated the remainder of the tail. Comparisons of system sensitivity and spatial resolution were also conducted for an array of point source locations with results showing excellent agreement. Lastly, to demonstrate a clinically realistic case, a simulation of an anthropomorphic phantom with a cardiac insert and an inferior defect was performed. Simulated projections were processed using the GE Xeleris software confirming the accuracy of the SIMIND geometry. We conclude that it is feasible to simulate the GE Discovery 530c/570c SPECT system using the SIMIND code.

Energy resolution studies of Ce- and Pr-doped aluminum and multicomponent garnets: The escape and photo-peaks

The photopeak and escape peak processes were studied in heavy Ce3+-doped LuAG and GGAG, Pr3+-doped LuAG and undoped BGO scintillating crystals. Energy resolution measurements were performed with 511keV photons of 22Na radioisotope. In the pulse-height spectra the escape peaks were resolved clearly on thin samples (up to 3mm). If sample thickness increases the escape peak cannot be resolved anymore (for thicknesses approx. above 5mm, especially). Consequently, energy resolution increases by about 40% compared with that measured at 1mm thick samples.

Benchmarking the repeatability of a pneumatic cyclic neutron activation analysis facility using 16O(n,p)16N for nuclear forensics

A target was prepared for cyclic neutron activation analysis by heat sealing lithium-carbonate in polyethylene. The target was cyclically irradiated 50 times using a Thermo-Scientific accelerator based deuterium–tritium fusion neutron generator. During counting periods, gamma-rays emitted by 16N were detected using three high-purity germanium detectors acquiring data in list-mode. Total counts acquired in each spectrum were compared between the three detectors to examine variability in geometric positioning of the target and variability of the generator intensity throughout the experiment. These two effects were determined to be the primary sources of variation in the measured counts. Variation in target positioning and generator intensity were found to increase the standard deviation by 34% and 33%, respectively. Transit times to the detector were found to be slower and more variable than transit to the generator but were well below the half second threshold needed to measure short-lived radionuclides with half-lives on the order of seconds. The standard deviation in irradiation time was found to be less than 1 milliseconds. The impact on statistical variability in the measured counts was negligible relative to the two primary sources of variation. Spectra acquired from each cycle were summed together. The sum of the peak areas from the 6.1MeV gamma-ray and its corresponding single and double escape peaks were used to measure the half-life of 16N. The result of 7.108(15)seconds derived from data suggests that the currently published value of 7.13(2)seconds has minimal systematic bias induced by background.

Nonproportionality in the scintillation light yield of bismuth germanate

We present measurements of nonproportionality in the scintillation light yield of bismuth germanate (BGO) for gamma-rays with energies between 6keV and 662keV. The scintillation light was read out by avalanche photodiodes (APDs) with both the BGO crystals and APDs operated at a temperature of ≈90K. Data were obtained using radioisotope sources to illuminate both a single BGO crystal in a small test cryostat and a 12-element detector in a neutron radiative beta-decay experiment. In addition one datum was obtained in a 4.6T magnetic field based on the bismuth K x-ray escape peak produced by a continuum of background gamma rays in this apparatus. These measurements and comparison to prior results were motivated by an experiment to study the radiative decay mode of the free neutron. The combination of data taken under different conditions yields a reasonably consistent picture for BGO nonproportionality that should be useful for researchers employing BGO detectors at low gamma ray energies.

Analysis of the effects of pair production for the suppressed clover detector

Full energy peak, single escape peak and double escape peak areas have been extracted for an escape suppressed clover detector. Results have been obtained for the single crystal and addback modes of operation as well as the active and passive suppression cases at several gamma energies. We have compared the ratio of single escape peak areas in addback mode with that of single crystal mode to study if the single escape peak gains or loses counts due to addback mode. Detailed analysis has been performed for quantifying the advantages of using addback mode and active suppression. Comparison is made for different types of clover detectors with different volumes.

Measurements of charge sharing in small pixel CdTe detectors

CdTe detectors with a thickness of 1mm and a pixel pitch of 250µm have been flip-chip-bonded to the HEXITEC read-out ASIC. The detectors record both the position and energy of X-ray interactions producing an X-ray spectrum for each of the 6400 pixels in the energy range 2–200keV. In this arrangement, detectors have been shown to produce excellent spectroscopy with FWHM better than 1.2keV at an interaction energy of 59.5keV. Due to the use of small pixels, a significant number of events experience charge sharing where the total energy of the event is distributed between multiple pixels. Under normal operating conditions, the proportion of events that experience charge sharing was measured to be 36.4%. Without correction these events lead to a reduction in the spectroscopic performance of the detector, the production of pronounced fluorescence and X-ray escape peaks, and the detection sensitivity. In this paper, the effect of different correction algorithms and operating conditions on the detector performance is presented.

SU-E-T-186: Feasibility Study of Glass Cherenkov Detector for Prompt Gamma Detection in Proton Therapy

Purpose: To simulate a Cherenkov glass detector system utilizing prompt gamma (PG) technique to quantify range uncertainties in proton radiation therapy. Methods: A simulation of high energy photons typically produced in proton interactions with materials incident onto a block of Cherenkov glass was performed with the Geant4 toolkit. The standard electromagnetic package was used along with several decay modules (G4Decay, G4DecayPhysics, and G4RadioactiveDecayPhysics) and the optical photon components (G4OpticalPhysics). Our setup included a pencil beam consisting of a hundred thousand 6 MeV photons (approximately the deexcitation energy released from 16O) incident onto a 2.5 ⊗ 2.5 ⊗ 1.5 cm3 of a Cherenkov glass (7.2 g of In2O3 + 90 g cladding, density of 2.82 g/cm3, Zeff = 33.7, index of refraction 1.56). The energy deposited from incident 6 MeV photons as well as secondary electrons and resulting optical photons were recorded. Results: The energy deposited by 6 MeV photons in glass material showed several peaks that included the photoelectric, the single and double escape peaks. About 11% of incident photons interacted with glass material to deposit energy. Most of the photons collected were in the region of double escape peak (approximately 4.98 MeV). The secondary electron spectrum produced from incidentmore » photons showed a high energy peak located near 6 MeV and a sharp peak located ∼120 keV with a continuous distribution between these two points. The resulting Cherenkov photons produced showed a continuous energy distribution between 2 and 5 eV with a slight increase in yield beginning about 3 eV. The amount of Cherenkov photons produced per interacting incident 6 MeV photon was ∼240.7. Conclusion: This study suggests the viability of utilizing the Cherenkov glass material as a possible prompt gamma photon detection device. Future work will include optimization of the detector system to maximize photon detection efficiency.« less

TU-F-18A-07: To Explore the More Realistic Energy Responses of the In-Depth Photon Counting Detectors

Purpose: We study the effect of the secondary photon events on modeling the energy response functions of the In-depth photon counting X-ray detectors (PCXD) and the potential impact of the spectral distortion on material decompositions. Methods: Square-shape wafers of three potential PCXD materials were constructed (5-by-20-by-30 mm^3 for Si, 4-by-20-by-5 mm^3 for GaAs and 4-by-20-by-3 mm^3 for CdTe), with pixel size of 5-by-4 mm^2 for Si and 5-by-5 mm^2 for GaAs and CdTe. The depth direction (z-direction) was segmented into 5 layers with exponentially increasing thicknesses of each layer. X-rays from 10keV to 120keV with 20000 photons per keV bin was simulated to characterize the energy response function of each PCXD using Geant4. Secondary photons events were recorded and we omitted the photons exiting the detector. The Energy Response Functions (ERFs) from the Monte Carlo (MC) simulations were compared with those from a semi-ideal model developed earlier. Results: For Si, detection of secondary events in the center detector were minimal due to the long aspect ratio of the detector, which results in the agreement between the theoretical prediction and the MC simulation with and without the secondary photons. For CdTe, the secondary photons captured by the center pixel were important, leading to obvious disagreement between the analytical and the simulated ERF. After correction for secondary events, the two curves were more similar except for the escape peaks which are not correctly portrayed by the semi-ideal model. For GaAs, the behavior is in between Si and CdTe. Conclusion: Given the complexity of the In-Depth PCXD's geometry, the uniform semi-ideal model does not fully characterize the ERF at each layer. Therefore, more realistic models need to be explored for better modeling of the spectral distortion.

Energy-dispersive X-ray diffraction mapping on a benchtop X-ray fluorescence system

A method for energy-dispersive X-ray diffraction mapping is presented, using a conventional low-power benchtop X-ray fluorescence spectrometer, the Seiko Instruments SEA6000VX. Hyper spectral X-ray maps with a 10 µm step size were collected from polished metal surfaces, sectioned Bi, Pb and steel shot gun pellets. Candidate diffraction lines were identified by eliminating those that matched a characteristic line for an element and those predicted for escape peaks, sum peaks, and Rayleigh and Compton scattered primary X-rays. The maps showed that the crystallites in the Bi pellet were larger than those observed in the Pb and steel pellets. The application of benchtop spectrometers to energy-dispersive X-ray diffraction mapping is discussed, and the capability for lower atomic number and lower-symmetry materials is briefly explored using multi-crystalline Si and polycrystalline sucrose.

Approaches for theoretical and experimental determinations ofK-shell decay rates and fluorescence yields in Ge

Fluorescence yields are one of the fundamental parameters in atomic physics and related areas. Despite the increase of experimental work in the last decade, to obtain values of $K$-shell fluorescence yields available data are scarce or outdated for many elements. The available theoretical results cannot fill the gap since quite often they are derived from semi-empirical calculations based on old models. This is the case of Ge that has many applications in science. In this work, we present the results of a collaboration between an experimental and a theoretical group to obtain the decay rates and fluorescence yields for Ge. The calculations were performed within the Dirac-Fock method, including relativistic and QED corrections, using a state-of-the-art approach. The experimental work was carried out at the SOLEIL synchrotron and the fluorescence yields were measured by two distinct methods: the reflection geometry method and the escape peak method. The results show a very good agreement between the experiment and theory (1.1%), well within the experimental uncertainty (2.4%).

Detector response function of an energy-resolved CdTe single photon counting detector.

While spectral CT using single photon counting detector has shown a number of advantages in diagnostic imaging, knowledge of the detector response function of an energy-resolved detector is needed to correct the signal bias and reconstruct the image more accurately. The objective of this paper is to study the photo counting detector response function using laboratory sources, and investigate the signal bias correction method. Our approach is to model the detector response function over the entire diagnostic energy range (20 keV <E< 140 keV) using a semi-analytical method with 12 parameters. The model includes a primary photo peak, an exponential tail, and four escape peaks. Four radioactive isotopes including Cdmium-109, Barium-133, Americium-241 and Cobalt-57 are used to generate the detector response function at six photon energies. The 12 parameters are obtained by non-linear least-square fitting with the measured detector response functions at the six energies. The correlations of the 12 parameters with energy are also investigated with the measured data. The analytical model generally describes the detector response function and is in good agreement with the measured data. The trend lines of the 12 parameters indicate higher energies tend to cause grater spectrum distortion. The spectrum distortion caused by the detector response function on spectral CT reconstruction is analyzed theoretically, and a solution to correct this spectrum distortion is also proposed. In spectral and fluorescence CT, the spectrum distortion caused by detector response function poses a problem and cannot be ignored in any quantitative analysis. The detector response function of a CdTe detector can be obtained by a semi-analytical method.

Measurement of response function of CdTe detector using diagnostic X-ray equipment and evaluation of Monte Carlo simulation code

An X-ray spectrum measured with CdTe detector has to be corrected with response function, because the spectrum is composed of full energy peaks (FEP) and escape peaks (EP). Recently, various simulation codes were developed, and using them the response functions can be calculated easily. The aim of this study is to propose a new method for measuring the response function and to compare it with the calculated value by the Monte Carlo simulation code. In this study, characteristic X-rays were used for measuring the response function. These X-rays were produced by the irradiation of diagnostic X-rays with metallic atoms. In the measured spectrum, there was a background contamination, which was caused by the Compton scattering of the irradiated X-ray in the sample material. Therefore, we thought of a new experimental methodology to reduce this background. The experimentally derived spectrum was analyzed and then the ratios of EP divided by FEP (EP/FEP) were calculated to compare the simulated values. In this article, we showed the property of the measured response functions and the analysis accuracy of the EP/FEP, and we indicated that the values calculated by Monte Carlo simulation code could be evaluated by using our method.

A high resolution SPECT detector based on thin continuous LYSO

Single-photon emission computed tomography (SPECT) detectors with improved spatial resolution can be used to build multi-pinhole SPECT systems that have a higher sensitivity or a higher spatial resolution. In order to improve the spatial resolution we investigate the performance of a 2 mm thick continuous Lutetium Yttrium Orthosilicate (LYSO) scintillator and compare it to the performance of a 5 mm thick continuous NaI(Tl) scintillator. The advantages of LYSO are its high stopping power and its non-hygroscopicity. Drawbacks are the lower light output and the intrinsic radioactivity. The hypothesis of this study is that such a thin LYSO scintillator will have a small light spread and, as a consequence, will also have an improved spatial resolution when coupled to a Hamamatsu H8500 position sensitive photomultiplier tube. To optimize the spatial resolution and the useful detector area we used a mean nearest neighbor event-positioning method. Beam source measurements (99mTc, 140 keV) were done to investigate the energy resolution and the spatial resolution of both detectors. The effect of the intrinsic radioactivity of the LYSO scintillator in the energy window was quantified. The mean energy resolution is 9.3% for the NaI(Tl) scintillator and 21.3% for the LYSO scintillator. The LYSO spectrum shows an X-ray escape peak which decreases the detection efficiency with 9.1%. The spatial resolution of the LYSO detector (0.93 mm full width at half maximum (FWHM)) is superior to the spatial resolution of the NaI(Tl) detector (1.37 mm FWHM). The intrinsic radioactivity in the energy window (42% window centered at 140 keV) is low (125.6 cps, 0.024 cps mm−3). LYSO is a promising scintillator for small-animal SPECT imaging, where spatial resolution is more important than energy resolution.

Determinations of low atomic number elements in real uranium oxide samples using vacuum chamber total reflection x-ray fluorescence

Conditions for the total reflection x-ray fluorescence (TXRF) analysis of real uranium samples for low atomic number elements using vacuum chamber TXRF spectrometer were optimized. It was observed that for analysis of low atomic number elements, almost complete removal of uranium matrix is required. Two certified reference materials of uranium containing trace elements in different concentrations were dissolved in minimum amount of high purity nitric acid. The uranium matrix from these solutions was separated by solvent extraction using tri-n-butyl phosphate as extracting reagent. Low atomic number elements in TXRF spectrum of the aqueous phase could be seen only after six tri-n-butyl phosphate equilibrations in extraction. The TXRF determinations of the certified low atomic number elements Mg and Al were made in these aqueous solutions after addition of Sc as an internal standard. The TXRF determined values for Mg were in good agreement with the certified values, whereas TXRF determined Al values differed from the certified values appreciably, probably due to the interference of Al Kα peak with escape peak of U Mα and the neighboring Si Kα peak. Copyright © 2013 John Wiley & Sons, Ltd.