Double Escape Peak Research Articles

Virtual segmentation of a small contact HPGe detector: inference of hit positions of single-site events via pulse shape analysis

Exploring hit positions of recorded events can help to understand and suppress backgrounds in rare event searches. We propose a pulse shape analysis method to discriminate single-site events (SSEs) in the inner and outer layer of a small contact P-type germanium detector (HPGe). SSEs in the inner and outer layer have different pulse shape features, of which the rise time of the (TQ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(T_{Q})$$\\end{document} and current pulse (TI)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(T_{I})$$\\end{document} are selected for discrimination. A 500 Bq Thorium-228 (Th-228) source is used to determine the boundaries between the two layers. The double escape peak events from 2614.5 keV γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma $$\\end{document}-ray are selected as typical SSEs, their numbers in the two layers are used to calculate the volumes and shapes of those layers. Considering the statistical and systematic uncertainties, the inner layer volume is evaluated to be 47.2% ± 0.26%(stat.) ± 0.18%(sys.) ± 0.22%(sys.) of the total sensitive volume. Selecting the inner layer as the analysis volume can reduce the external background in the signal region of Ge-76 neutrinoless double beta (0νββ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta )$$\\end{document} decay. We use the Th-228 data to validate the inner layer model and evaluate the background suppression power in the 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta $$\\end{document} signal region (Qββ=2039\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(Q_{\\beta \\beta }=2039$$\\end{document} keV). The virtual segmentation further reduces the background from the external Th-228 source by about 10%. The virtual segmentation could be used to efficiently suppress surface background like electrons from Ar-42 decay in 0νββ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u \\beta \\beta $$\\end{document} experiments using germanium detectors immersed in liquid argon.

Lineshape response of plastic scintillator to pair production of 4.44 MeV [formula omitted]’s

We measure the distribution of energy deposited in a 40 × 88 mm plastic scintillator by e+e− pair production of 4.44 MeV γ-rays. We observe the double-escape peak of 3.42 MeV from pair production by tagging 511 keV annihilation radiation in two high-Z scintillators. The source is a standard commercial neutron source using α-emitting 241Am encapsulated with 9Be, which has a reaction branch feeding the first Iπ=2+ state of 12C making the 4.44 MeV γ-rays. We demonstrate the extraction of the double-escape peak from the large neutron-produced backgound, and explore some of the features and difficulties of this technique with our apparatus.

Measurement of HFETR reactor power by improved 16N gamma spectrum analysis method

To increase redundancy and diversity of the reactor power measurement, a system for the measurement of reactor power can be established based on neutron activation analysis using 16O(n,p)16N reaction. In this work, gamma spectrums from the coolant of HFETR core are continuously detected by 16N monitor with NaI(Tl) detector and the count rates in the spectrum range of 4.5 MeV-7.5 MeV are obtained. Meanwhile, the overlapping peaks in the given spectrum range are decomposed with Levenberg-Marquardt method during the operation of HFETR. The results confirm that the count rates of full-energy peak, single-escape peak and double-escape peak of 6.13 MeV gamma spectrums is proportionate to the increased reactor power measured by other monitors such as compensated ionization chamber and thermal power. It is possible to achieve the absolute nuclear power measurement by adopting the count rate of 16N 6.13 MeV peak, detection efficiency of detector and 16N source term.

Boosting background suppression in the NEXT experiment through Richardson-Lucy deconvolution

Next-generation neutrinoless double beta decay experiments aim for half-life sensitivities of ∼ 1027 yr, requiring suppressing backgrounds to < 1 count/tonne/yr. For this, any extra background rejection handle, beyond excellent energy resolution and the use of extremely radiopure materials, is of utmost importance. The NEXT experiment exploits differences in the spatial ionization patterns of double beta decay and single-electron events to discriminate signal from background. While the former display two Bragg peak dense ionization regions at the opposite ends of the track, the latter typically have only one such feature. Thus, comparing the energies at the track extremes provides an additional rejection tool. The unique combination of the topology-based background discrimination and excellent energy resolution (1% FWHM at the Q-value of the decay) is the distinguishing feature of NEXT. Previous studies demonstrated a topological background rejection factor of ∼ 5 when reconstructing electron-positron pairs in the 208Tl 1.6 MeV double escape peak (with Compton events as background), recorded in the NEXT-White demonstrator at the Laboratorio Subterráneo de Canfranc, with 72% signal efficiency. This was recently improved through the use of a deep convolutional neural network to yield a background rejection factor of ∼ 10 with 65% signal efficiency. Here, we present a new reconstruction method, based on the Richardson-Lucy deconvolution algorithm, which allows reversing the blurring induced by electron diffusion and electroluminescence light production in the NEXT TPC. The new method yields highly refined 3D images of reconstructed events, and, as a result, significantly improves the topological background discrimination. When applied to real-data 1.6 MeV e−e+ pairs, it leads to a background rejection factor of 27 at 57% signal efficiency.

Differential cross section measurements of the 6.13 MeV gamma ray from proton-induced reaction of fluorine

Fundamental cross section measurements are essential for particle-induced gamma emission (PIGE) analysis. 19F(p,αγ2-0)16O can be utilized in PIGE analysis; however, differential cross section measurements for this reaction do not exist due to the difficulty in deconvolving overlapping peaks in the gamma spectrum. Although gamma yield measurements for this reaction exist (Dababneh et al., 1993), these can only be used for relative measurements. This work presents the 6.13 MeV gamma production differential cross sections for the 19F(p,αγ2-0)16O reaction in the incident proton kinetic energy range of 837 to 1517 keV. The yield of the 6.13 MeV gamma was obtained by subtracting the contribution of the 7.12 MeV double escape peak using the experimentally-determined ratio of the escape peaks. The corrected yields were converted to differential cross sections, which are presented here for the first time, as a contribution to the current knowledge and utilization of PIGE techniques.

Active interrogation of Special Nuclear Material containers using AmBe quasi-forward biased directional source and PGNAA

Special Nuclear Material (SNM) including 95% 235U and 239Pu is capable of being identified by utilizing a quasi-forward directional AmBe source from Patent No: US20190013109A1 using Prompt Gamma Neutron Activation Analysis (PGNAA) simulated with Monte-Carlo N-Particle transport 6.2 (MCNP). HPGe and LaBr3 detector arrays were used to identify and quantify the peak-to-background and peak-to-total ratios of the associated photon spectra from the SNM encased in a polyethylene shield. The conducted simulations varied the volume of the SNM and neutron source strength in the MCNP data card to conduct an uncertainty analysis. Photopeaks identified include K-shell X-rays from 235U and 239Pu, 61 keV from fission, 2.223 MeV prompt gamma from hydrogen, 511 keV annihilation, and a single and double escape peak from the prompt gamma interaction from hydrogen. Relationships between peak-to-total and peak-to-background as a function of SNM and particle history were investigated to aid in the analysis. The capabilities of both detector systems to acquire well resolved photopeak with a 5% relative error or less, with a 1 Ci source activity, and a peak-to-background ratio of 1.15. This was determined to take 326 s for LaBr3 and 163 s for HPGe which is comparable to current methods for material detection which take between 100 and 900 s to acquire.

Precision imaging of 4.4 MeV gamma rays using a 3-D position sensitive Compton camera

Imaging of nuclear gamma-ray lines in the 1–10 MeV range is far from being established in both medical and physical applications. In proton therapy, 4.4 MeV gamma rays are emitted from the excited nucleus of either 12C* or 11B* and are considered good indicators of dose delivery and/or range verification. Further, in gamma-ray astronomy, 4.4 MeV gamma rays are produced by cosmic ray interactions in the interstellar medium, and can thus be used to probe nucleothynthesis in the universe. In this paper, we present a high-precision image of 4.4 MeV gamma rays taken by newly developed 3-D position sensitive Compton camera (3D-PSCC). To mimic the situation in proton therapy, we first irradiated water, PMMA and Ca(OH)2 with a 70 MeV proton beam, then we identified various nuclear lines with the HPGe detector. The 4.4 MeV gamma rays constitute a broad peak, including single and double escape peaks. Thus, by setting an energy window of 3D-PSCC from 3 to 5 MeV, we show that a gamma ray image sharply concentrates near the Bragg peak, as expected from the minimum energy threshold and sharp peak profile in the cross section of 12C(p,p)12C*.

Improving sensitivity of a BEGe-based high-purity germanium spectrometer through pulse shape analysis

We performed Pulse Shape Analysis to separate single-scattered gamma energy deposition events from multiple-scattered photons in a high-sensitivity gamma -ray spectrometer. The spectrometer is based on a Broad Energy High Purity Germanium detector and the developed technique uses multivariate analysis by an application of the Multi-Layer Perceptron Neural Network. A very good separation of the single-site- and multi-site events was achieved leading to a significant reduction of the background level of the investigated spectrometer – the double escape peak, rich in single-site events, was reduced by 95%, while the full energy peaks lost at most 25% of their counts. The peak to Compton ratio, calculated for the 2614.5 keV gamma line from ^{208}Tl, was improved by 114.3%.

SU-F-T-368: Improved HPGe Detector Precise Efficiency Calibration with Monte Carlo Simulations and Radioactive Sources

Purpose: To obtain an improved precise gamma efficiency calibration curve of HPGe (High Purity Germanium) detector with a new comprehensive approach. Methods: Both of radioactive sources and Monte Carlo simulation (CYLTRAN) are used to determine HPGe gamma efficiency for energy range of 0–8 MeV. The HPGe is a GMX coaxial 280 cm3 N-type 70% gamma detector. Using Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University, the radioactive nucleus 24 Al was produced and separated. This nucleus has positron decays followed by gamma transitions up to 8 MeV from 24 Mg excited states which is used to do HPGe efficiency calibration. Results: With 24 Al gamma energy spectrum up to 8MeV, the efficiency for γ ray 7.07 MeV at 4.9 cm distance away from the radioactive source 24 Al was obtained at a value of 0.194(4)%, by carefully considering various factors such as positron annihilation, peak summing effect, beta detector efficiency and internal conversion effect. The Monte Carlo simulation (CYLTRAN) gave a value of 0.189%, which was in agreement with the experimental measurements. Applying to different energy points, then a precise efficiency calibration curve of HPGe detector up to 7.07 MeV at 4.9 cm distance away from the source 24 Al was obtained. Using the same data analysis procedure, the efficiency for the 7.07 MeV gamma ray at 15.1 cm from the source 24 Al was obtained at a value of 0.0387(6)%. MC simulation got a similar value of 0.0395%. This discrepancy led us to assign an uncertainty of 3% to the efficiency at 15.1 cm up to 7.07 MeV. The MC calculations also reproduced the intensity of observed single-and double-escape peaks, providing that the effects of positron annihilation-in-flight were incorporated. Conclusion: The precision improved gamma efficiency calibration curve provides more accurate radiation detection and dose calculation for cancer radiotherapy treatment.

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.

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.

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.

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

Pulse shape discrimination for Gerda Phase I data

The Gerda experiment located at the Laboratori Nazionali del Gran Sasso of INFN searches for neutrinoless double beta (0νββ) decay of 76Ge using germanium diodes as source and detector. In Phase I of the experiment eight semi-coaxial and five BEGe type detectors have been deployed. The latter type is used in this field of research for the first time. All detectors are made from material with enriched 76Ge fraction. The experimental sensitivity can be improved by analyzing the pulse shape of the detector signals with the aim to reject background events. This paper documents the algorithms developed before the data of Phase I were unblinded. The double escape peak (DEP) and Compton edge events of 2.615 MeV γ rays from 208Tl decays as well as two-neutrino double beta (2νββ) decays of 76Ge are used as proxies for 0νββ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92±0.02 of signal-like events while about 80 % of the background events at Q ββ =2039 keV are rejected. For semi-coaxial detectors three analyses are developed. The one based on an artificial neural network is used for the search of 0νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q ββ . The 2νββ events have an efficiency of 0.85±0.02 and the one for 0νββ decays is estimated to be $0.90^{+0.05}_{-0.09}$ . A second analysis uses a likelihood approach trained on Compton edge events. The third approach uses two pulse shape parameters. The latter two methods confirm the classification of the neural network since about 90 % of the data events rejected by the neural network are also removed by both of them. In general, the selection efficiency extracted from DEP events agrees well with those determined from Compton edge events or from 2νββ decays.

SU‐E‐T‐443: Measuring Prompt Gamma Emission During Passive Beam Proton Radiotherapy for Real Time Treatment Verification

Purpose: Recent studies demonstrate the feasibility of prompt‐gamma imaging for pencil beam proton therapy. In contrast to passive‐scatter proton radiotherapy, the existence of high background signal from secondary radiation produced by protons in the beam line components makes the detection of prompt‐gammas challenging. Our primary goal is to produce a more complete measurement of the prompt‐gammas produced in tissue for clinical treatment conditions. Methods: The proton treatment facility at iThemba Labs was studied in detail and all the nozzle components that interact with the proton beam were built and positioned at the locations specified by the manufacturer using the Geant4 Monte Carlo toolkit. The treatment nozzle was validated against depth dose and lateral profiles in a water phantom at therapeutic energies. The model included NaI detectors of different dimensions to detect the prompt‐gamma spectra, and standard gamma emitting sources in the energy range 0.661 to 4.438 MeV were used to determine the Gaussian broadening parameters of the detectors. Finally, prompt‐gamma spectra from the tissue phantom were simulated over a range of energies in order to make comparisons with the experimental spectra currently being measured. Results: The prompt‐gamma energy spectra show prominent peaks caused by collisions of proton with nuclei in the tissue phantom. Single and double escape peaks of 4.44 MeV were clearly seen, while the 6.129 MeV peak was unresolved due to the Doppler broadening effect, peaks at 6.916 and 7.115 MeV were not observed, but the single and double escape peaks of 6.129 MeV were visible. Conclusion: Our study confirms the feasibility of prompt‐gamma imaging during passive proton radiotherapy to assist for on‐line treatment verification. The energy range from 3.0 MeV to 5.0 MeV is a suitable candidate for further investigation and the presence of background radiation can be minimized by introducing proper shielding and collimation.

Effects due to a Pu-C source on a HPGe detector and the corresponding neutron shielding

A gamma spectrum of a Pu-C source is measured using a p-type HPGe detector, whose three peaks (full energy, single-escape and double-escape peak) can be used as a calibration source for the beam energy measurement system of BEPCII. The effect of fast neutron damage on the energy resolution of the HPGe detector is studied, which indicates that the energy resolution begins to deteriorate when the detector is subject to 2×107 n/cm2 fast neutrons. The neutron damage mechanism and detector repair methods are reviewed. The Monte Carlo simulation technique is utilized to study the shielding of the HPGe detector from the fast neutron radiation damage, which is of great significance for the future commissioning of the beam energy measurement system.

A Mathematical Approach to Determine Escape Peak Efficiencies of High-Purity Germanium Cylindrical Detectors for Prompt Gamma Neutron Activation Analysis

Determining single-escape (SE), double-escape (DE), and full-energy (FE) peak efficiencies is helpful in prompt gamma-ray neutron activation analysis, in which identifying complicated gamma peaks as well as quantifying spectra is crucial. The FE peak efficiency was measured at the National Institute of Standards and Technology for a closed-end, n-type high-purity germanium (HPGe) cylindrical detector using standard point sources, whereas SE and DE peak efficiencies and the SE-to-DE peak ratio for the HPGe detector were calculated using compact analytical expressions.

A Pulse Shape Analysis technique for the MAJORANA experiment

In order to achieve background count rates sufficiently low so as to allow the observation of rare events such as neutrinoless double beta (0νββ) decay, background suppression techniques are routinely employed. In this paper we present details of a novel Pulse Shape Analysis algorithm, which allows single-site events such as 0νββ decay to be distinguished from multi-site background events in germanium detectors. The algorithm, which is based on the event-by-event χ 2 fitting of experimental signals to a basis data set of unique single-site pulse shapes, has been developed through simulation studies and tested experimentally using a Broad Energy Germanium detector. It is found experimentally that the technique is able to successfully identify and reject 99% of multi-site events in the single escape peak associated with the gamma decay of 208Tl, whilst maintaining a survival probability of 98% for neutrinoless double-beta-decay-like double escape peak events.

Systematic effects in pulse shape analysis of HPGe detector signals for [formula omitted

Pulse shape analysis is an important background reduction and signal identification technique for next generation of 76Ge 0 ν β β experiments. We present a study of the systematic uncertainties in one such parametric pulse-shape analysis technique for separating multi-site background from single-site signal events. We examined systematic uncertainties for events in full-energy gamma peaks (predominantly multi-site), double-escape peaks (predominantly single-site) and the Compton continuum near Q β β (which will be the dominant background for most 0 ν β β searches). In short, we find total (statistical plus systematic) fractional uncertainties in the pulse shape cut survival probabilities of: 6.6%, 1.5% and 3.8% for double-escape, continuum and γ ‐ ray events, respectively.

Neutron inelastic scattering processes as a background for double-β decay experiments

We investigate several Pb$(n,{n}^{'}\ensuremath{\gamma}$) and Ge$(n,{n}^{'}\ensuremath{\gamma}$) reactions. We measure \ensuremath{\gamma}-ray production from Pb$(n,{n}^{'}\ensuremath{\gamma}$) reactions that can be a significant background for double-\ensuremath{\beta} decay experiments which use lead as a massive inner shield. Particularly worrisome for Ge-based double-\ensuremath{\beta} decay experiments are the 2041-keV and 3062-keV \ensuremath{\gamma} rays produced via Pb$(n,{n}^{'}\ensuremath{\gamma}$). The former is very close to the $^{76}\mathrm{Ge}$ double-\ensuremath{\beta} decay endpoint energy and the latter has a double escape peak energy near the endpoint. We discuss the implications of these \ensuremath{\gamma} rays on past and future double-\ensuremath{\beta} decay experiments and estimate the cross section to excite the level that produces the 3062-keV \ensuremath{\gamma} ray. Excitation \ensuremath{\gamma}-ray lines from Ge$(n,{n}^{'}\ensuremath{\gamma}$) reactions are also observed. We consider the contribution of such backgrounds and their impact on the sensitivity of next-generation searches for neutrinoless double-\ensuremath{\beta} decay using enriched germanium detectors.