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DOE OSTI2023

Dissociation dynamics of ion-pair states accessed by low-energy electron collisions

Abstract Ion-pair (IP) states are the superexcited states of the neutral molecule that dissociate as a pair of positive and negative ions. These states are generally present near the ionization continuum of the molecule. IP states can be accessed by using photoexcitation or by using the electron collision technique. Different photoexcitation techniques are widely used over the years to study the threshold (threshold IP production spectroscopy) and the dynamics (IP imaging spectroscopy) of the IP states. However, the electron collision technique is ignored over the years and only a few studies are available. In this review, we will discuss different experimental techniques to probe IP states by using electron collision and also the dynamics of the IP states that are accessed by the electron collision.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2021

Enhancing the sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment to low energy signals

Two-phase xenon detectors, such as that at the core of the forthcoming LZ dark matter experiment, use photomultiplier tubes to sense the primary (S1) and secondary (S2) scintillation signals resulting from particle interactions in their liquid xenon target. This paper describes a simulation study exploring two techniques to lower the energy threshold of LZ to gain sensitivity to low-mass dark matter and astrophysical neutrinos, which will be applicable to other liquid xenon detectors. The energy threshold is determined by the number of detected S1 photons; typically, these must be recorded in three or more photomultiplier channels to avoid dark count coincidences that mimic real signals. To lower this threshold: a) we take advantage of the double photoelectron emission effect, whereby a single vacuum ultraviolet photon has a $\sim20\%$ probability of ejecting two photoelectrons from a photomultiplier tube photocathode; and b) we drop the requirement of an S1 signal altogether, and use only the ionization signal, which can be detected more efficiently. For both techniques we develop signal and background models for the nominal exposure, and explore accompanying systematic effects, including the dependence on the free electron lifetime in the liquid xenon. When incorporating double photoelectron signals, we predict a factor of $\sim 4$ sensitivity improvement to the dark matter-nucleon scattering cross-section at $2.5$ GeV/c$^2$, and a factor of $\sim1.6$ increase in the solar $^8$B neutrino detection rate. Dropping the S1 requirement may allow sensitivity gains of two orders of magnitude in both cases. Finally, we apply these techniques to even lower masses by taking into account the atomic Migdal effect; this could lower the dark matter particle mass threshold to $80$ MeV/c$^2$.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2021

Enhancing the sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment to low energy signals

Akerib, DS↗

DOE OSTI2014

Measurements of radiation effects on a 4 Mb PSRAM memory

The results of a static test of total ionizing dose (TID) effects on an ISSI 4Mb PSRAM memory are reported in this work. The irradiation was performed at the IEAv’s Laboratory of Ionizing Radiation with 1.17 and 1.32 MeV gamma-rays from a {sup 60}Co source at a dose rate of 2.5 krad/h up to an accumulated dose of 215.7 krad. The TID threshold for bit flip found in this experiment was 52.5 krad. From a sampling of 4096 memory addresses it was estimated a bit flip rate of approximately 50% at an accumulated dose of 215.7 krad.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2020

Transient work function gating: A new photoemission regime

We present the theoretical basis for a new photoemission regime, transient work function gating (TWFG), which temporally and energetically gates photoemission and produces near-threshold photoelectrons with thermally limited emittance, percent-level quantum efficiency, and control over temporal coherence. The technique consists of actively gating the work function of a generalized photocathode using a non-ionizing long-wavelength optical field to produce an adiabatic modulation of the carrier density at their surface. We examine TWFG as a means to circumvent the long-standing trade-off between low emittance and high quantum efficiency, untethered to particle source or photocathode specifics. TWFG promises new opportunities in photoemission physics for next generation electron and accelerator-based x-ray photon sources.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Search for dark matter annual modulation with DarkSide-50

Dark matter may induce an event in an Earth-based detector, and its event rate is predicted to show an annual modulation as a result of the Earth’s orbital motion around the Sun. We searched for this modulation signature using the ionization signal of the DarkSide-50 liquid argon time projection chamber. No significant signature compatible with dark matter is observed in the electron recoil equivalent energy range above 40 eV ee , the lowest threshold ever achieved in such a search. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

True Breakdown Voltage and Overvoltage Margin of GaN Power HEMTs in Hard Switching

This work studies the dynamic breakdown voltage (BV) and overvoltage margin of a 650-V-rated commercial GaN power HEMT in hard switching. The dynamic BV measured in the hard switching circuits is over 1.4 kV, being 450 V higher than the static BV measured in the quasi-static I-V sweep. The device can survive at least 1 million hard-switching overvoltage pulses with 1.33 kV peak overvoltage (~95% dynamic BV). Recoverable device parametric shifts are observed after the 1-million pulses, featuring small reductions in threshold voltage and on-resistance. These shifts are different from the ones after the hard-switching pulses without overvoltage and are attributable to the trapping of the holes produced in impact ionization. Furthermore, these results suggest that the BV and overvoltage margin of GaN HEMTs in practical power switching can be significantly underestimated using the static BV. Index Terms— GaN, HEMT, power electronics, hard switching, breakdown voltage, overvoltage, ruggedness, reliability.

42 ENGINEERING↗

DOE OSTI2018

Direct detection of MeV-scale dark matter utilizing germanium internal amplification for the charge created by the ionization of impurities

Light, MeV-scale dark matter (DM) is an exciting DM candidate that is undetectable by current experiments. A germanium (Ge) detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a promising new technology with experimental sensitivity for detecting MeV-scale DM. We analyze the physics mechanisms of the signal formation, charge creation, charge internal amplification, and the projected sensitivity for directly detecting MeV-scale DM particles. We present a design for a novel Ge detector at helium temperature (~4 K) enabling ionization of impurities from DM impacts. With large localized E-fields, the ionized excitations can be accelerated to kinetic energies larger than the Ge bandgap at which point they can create additional electron–hole pairs, producing intrinsic amplification to achieve an ultra-low energy threshold of ~0.1 eV for detecting low-mass DM particles in the MeV scale. Correspondingly, such a Ge detector with 1 kg-year exposure will have high sensitivity to a DM-nucleon cross section of ~5 × 10 -45 cm 2 at a DM mass of ~10 MeV/c 2 and a DM-electron cross section of ~ 5 × 10 -46 cm 2 at a DM mass of 1 MeV/c 2 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Probability Density Function for the spatial and intensity distribution of neutron-induced defects in Silicon

The ability to model semiconductor device degradation under neutron irradiation depends upon having a robust modeling capability for the neutron-induced collision cascades as well as a means to analytically fit the resulting probability distributions of defect production and ionizing energy deposition for purposes of extrapolation to low-probability, high-consequence scenarios. In this paper, the widely-utilized binary collision approximation codes MARLOWE and SRIM are deployed in conjunction with a critical examination of their parameterizations as benchmarked against higher-fidelity molecular dynamics simulations. A simple 3-parameter form described by the Generalized Logistic Distribution is shown to be a good fit to Frenkel pair and ionization intensity distributions in bulk silicon. The BCA codes are then applied to simulate cascades in 5 nm layers of a representative gate-all-around nanosheet transistor, where joint probability distributions of threshold levels of damage to multiple layers are evaluated.

36 MATERIALS SCIENCE↗

DOE OSTI2013

A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system

Purpose: To develop a calibration phantom for {sup 192}Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system.Methods: A phantom, herein designated BrachyPhantom, consists of a Solid Water™ 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder's axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber's reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate a factor, P{sub sw}{sup lw}, to correct the reading of the ionization chamber to a full scatter condition in liquid water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument reading was then converted to absorbed dose to water using the N{sub gas} formalism and the P{sub sw}{sup lw} factor. Likewise, the absorbed dose to water was calculated using the source strength, S{sub k}, values provided by 15 institutions visited in this work.Results: A value of 1.020 (0.09%, k= 2) was found for P{sub sw}{sup lw}. The expanded uncertainty in the absorbed dose assessed with the BrachyPhantom was found to be 2.12% (k= 1). To an associated S{sub k} of 27.8 cGy m{sup 2} h{sup −1}, the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the S{sub k} values did not exceed ±3% and ±1.6%, respectively.Conclusions: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ±3%, respectively, and in the long run save time for the medical physicist.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2013

Quantitatively identical orientation-dependent ionization energy and electron affinity of diindenoperylene

Molecular orientation dependences of the ionization energy (IE) and the electron affinity (EA) of diindenoperylene (DIP) films were studied by using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The molecular orientation was controlled by preparing the DIP films on graphite and SiO{sub 2} substrates. The threshold IE and EA of DIP thin films were determined to be 5.81 and 3.53 eV for the film of flat-lying DIP orientation, respectively, and 5.38 and 3.13 eV for the film of standing DIP orientation, respectively. The result indicates that the IE and EA for the flat-lying film are larger by 0.4 eV and the frontier orbital states shift away from the vacuum level compared to the standing film. This rigid energy shift is ascribed to a surface-electrostatic potential produced by the intramolecular polar bond (>C{sup −}-H{sup +}) for standing orientation and π-electron tailing to vacuum for flat-lying orientation.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2019

Total-Ionizing-Dose Effects and Low-Frequency Noise in 16-nm InGaAs FinFETs With HfO 2 /Al 2 O 3 Dielectrics

Here, total-ionizing-dose mechanisms are investigated in 16-nm InGaAs FinFETs with an HfO 2 /Al 2 O 3 gate-stack. Transistors are irradiated up to 500 krad(SiO 2 ) and annealed at high temperatures. Irradiated devices show negative threshold-voltage V th shifts, subthreshold stretch-out, and leakage current increases. These result from positive charge trapping in the gate oxide and shallow trench insulators, and increases in the interface and border-trap charge densities. Low-frequency noise measurements at different temperatures indicate a significant increase of noise magnitude in irradiated devices at an activation energy of ~0.4 eV. Density functional theory (DFT) calculations strongly suggest that transistor V th shifts are due primarily to hole trapping at oxygen vacancies in HfO 2 , and the increased noise is due primarily to O vacancies in Al 2 O 3 . Additional contributions to the noise from defects in the GaAs buffer layer are also likely, primarily at low temperatures.

total dose effects↗

DOE OSTI2021

Augmented signal processing in Liquid Argon Time Projection Chambers with a deep neural network

The Liquid Argon Time Projection Chamber (LArTPC) is an advanced neutrino detector technology widely used in recent and upcoming accelerator neutrino experiments. It features a low energy threshold and high spatial resolution that allow for comprehensive reconstruction of event topologies. In current-generation LArTPCs, the recorded data consist of digitized waveforms on wires produced by induced signal on wires of drifting ionization electrons, which can also be viewed as two-dimensional (2D) (time versus wire) projection images of charged-particle trajectories. For such an imaging detector, one critical step is the signal processing that reconstructs the original charge projections from the recorded 2D images. For the first time, we introduce a deep neural network in LArTPC signal processing to improve the signal region of interest detection. By combining domain knowledge (e.g., matching information from multiple wire planes) and deep learning, this method shows significant improvements over traditional methods. This work details the method, software tools, and performance evaluated with realistic detector simulations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2013

DarkSide search for dark matter

The DarkSide staged program utilizes a two-phase time projection chamber (TPC) with liquid argon as the target material for the scattering of dark matter particles. Efficient background reduction is achieved using low radioactivity underground argon as well as several experimental handles such as pulse shape, ratio of ionization over scintillation signal, 3D event reconstruction, and active neutron and muon vetos. The DarkSide-10 prototype detector has proven high scintillation light yield, which is a particularly important parameter as it sets the energy threshold for the pulse shape discrimination technique. The DarkSide-50 detector system, currently in commissioning phase at the Gran Sasso Underground Laboratory, will reach a sensitivity to dark matter spin-independent scattering cross section of 10 -45 cm 2 within 3 years of operation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

SU-E-T-194: Commissioning of Monaco Treatment Planning System On An Elekta VersaHD Linear Accelerator

Purpose: The Monaco treatment planning system (TPS) uses a Monte-Carlo algorithm based dose computation engine to model the photon beams of a linear accelerator. The aim is to perform verification of Monaco TPS beam modeling of a Elekta VersaHD linac with 6MV, 6MV FFF, 10 MV, 10MV FFF, 18MV photon beams and 160 multileaf collimators (MLC) with a projected width of 5-mm at the isocenter. Methods: A series of dosimetric tests were performed to validate Monaco calculated beams including point dose measurement in water with and without heterogeneity and 2-dimensional dose distributions on a Delta4 bi-planar diode dosimeter array (Scandidos, Uppsala, Sweden). 3D conformal beams of different field sizes, source-to-surface distances, wedges, and gantry angles were delivered onto a phantom consisting of several plastic water and Styrofoam slabs. Point dose measurements were verified with a PTW 31013 Semiflex 0.3 cc ionization chamber (PTW, Freiburg, Germany). In addition, 8 step and shoot intensity modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (VMAT) beams included in the Monaco TPS commissioning suite were verified against measurements on Delta4 to test and fine tune parameters in the beam model. IMRT verification was computed using gamma analysis with dose difference and distance-to-agreement criteria of 3%/3mm with a dose threshold of 10%. Results: Point dose measurements agreed within 2% in the homogeneous phantom and within 3% in the heterogeneous phantom for all photon energies. IMRT beams yielded a passing percentage of 99.1±1.1% in the gamma analysis which is well above the institutional passing threshold of 90%. Conclusion: Monaco TPS commissioning was successfully performed for all the photon energies on the Elekta VersaHD linac prior to clinical usage.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2022

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO 2 laser

Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long-wavelength infrared (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity ~10 12 W/cm 2 , the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm 2 threshold when a train of 3 ps 10.6 μm laser pulses separated by 18 ps is used. The breakdown first appears at the best focus but propagates backward toward the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 10 9 cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown, and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10% of the shots. When a cm-size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm -3 ) breakdown sites appear, each initially producing a near-critical density plasma. On a 10 ns–1 μs timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2022

Cosmic Reionization on Computers: Physical Origin of Long Dark Gaps in Quasar Absorption Spectra

I explore the properties of “dark gaps” - regions in quasar absorption spectra without significant transmission - with several simulations from the Cosmic Reionization On Computers (CROC) project. The CROC simulations in the largest available boxes (120 cMpc) come close to matching both the distribution of mean opacities and the frequency of dark gaps, but alas not in the same model: the run that matches the mean opacities fails to contain enough dark gaps and vice versa. Nevertheless, the run that matches the dark gap distributions serves as a counterexample to claims in the literature that the dark gap statistics requires a late end to reionization - in that run reionization ends at z = 6.7 (likely too early). While multiple factors contribute to the frequency of large dark gaps in the simulations, the primary factor that controls the overall shape of the dark gap distribution is the ionization level in voids—the lowest-density regions produce the highest transmission spikes that terminate long gaps. As the result, the dark gap distribution correlates strongly with the fraction of the spectrum above the gap detection threshold and the observed distribution is matched by the simulation in which this fraction is 2%. Hence, the gap distribution by itself does not constrain the timing of reionization, although it may do so in combination with the distribution of mean opacities.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2{sup 1}P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.

74 ATOMIC AND MOLECULAR PHYSICS↗