3He Gas Research Articles

(Invited) Hexagonal Boron Nitride Epilayers

As a member of the III-nitride wide bandgap semiconductor family, BN has received much less attention in comparison with other nitride semiconductors. The stable phase of BN synthesized at any temperature under ambient pressure is hexagonal. Due to its wide bandgap (> 6 eV) and layered structure, hexagonal BN (h-BN) is an ideal platform for probing fundamental 2D properties of wide bandgap semiconductors. In this talk, a brief overview of the synthesis and electrical and optical properties of wafer-scale h-BN epilayers will be presented [1-7]. It was shown that the unique 2D structure of h-BN induces exceptionally high density of states, large exciton binding energy and high optical absorption and emission intensity. By growing h-BN under high V/III ratios, epilayers exhibiting pure free exciton emission have been obtained [4]. Photocurrent excitation spectroscopy results directly provided a room temperature bandgap value for h-BN in between 6.4 and 6.5 eV and the free exciton binding energy (Ex) of 0.73 eV [5]. Layer-structured B-rich BGaN alloys, heterostructures, and quantum wells have also been successfully grown and their properties will be discussed. Thermal neutron detectors fabricated from 100% B-10 enriched h-BN epilayers of thicknesses exceeding 50 mm have attained the highest detection efficiency to date among solid-state detectors at about 58% [6, 7]. These solid-state neutron detectors have become increasingly desirable for a wide range of applications from fissile materials sensing to well logging, because 3He gas detectors are inherently bulky, require high pressurization and high voltage application, slow response time, and expensive. It is our belief that h-BN will lead to many potential applications from deep UV optoelectronics, radiation detectors, to novel layered-structured photonic and electronic devices. [1] R. Dahal, J. Li, S. Majety, B.N. Pantha, X. K. Cao, J. Y. Lin, and H.X. Jiang, Appl. Phys. Lett. 98, 211110 (2011). [2] S. Majety, J. Li, X. K. Cao, R. Dahal, B. N. Pantha, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 100, 061121 (2012). [3] H. X. Jiang, and J. Y. Lin, Semicon. Sci. Technol. 29, 084003 (2014). [4] X. Z. Du, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 106, 021110 (2015); 108, 052106 (2016). [5] T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 122101 (2016). [6] A. Maity, T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 072101 (2016). [7] A. Maity, S. J. Grenadier, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 111, 033507 (2017) and J. Appl. Phys. 123, 044501 (2018).

Effects of helium annealing in low-temperature and solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors

In this paper, low-temperature, solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) with annealing temperatures as low as 300°C were fabricated using different annealing gases of He, N2, and O2 and their electrical characteristics and long-term stability were analyzed. The field-effect mobility was improved by nearly two times for the a-IGZO TFTs annealed in helium ambient (He sample) in comparison with those annealed in oxygen and nitrogen ambient environments. The subthreshold slope and threshold voltage were also improved for the a-IGZO TFTs annealed in the helium ambient environment due to the low defect density of states in the sub-bandgap region. However, X-ray photoelectron spectroscopy measurements indicate that the electrical characteristics of the low-temperature solution-processed a-IGZO TFTs show severe channel-length dependencies due to the oxygen vacancy variations in the active region. In addition, long-term stability measurements up to seven days reveal that due to the inherently low electron density and high defect density of states in the active region, an increase in the carrier density in the active region induces a large negative shift of the threshold voltage without changing the field-effect mobility or subthreshold slope. It is understood that low-temperature solution-processed a-IGZO TFTs strongly depend on the formation of oxygen vacancies, which can be resolved by controlling the indium mole fraction under a helium annealing condition.

Temperature dependence of pressure broadening and shift of Rb D2 line

In this work the collisional broadening and shift rates of the Rb D2 (52S1/2–52P3/2) transition induced by 4He gas had been measured. A novel full-quantum calculations based on the Baranger theory were also presented to understand the observation. The calculation results were compared with other experimental measurements and predictions from the semi-classical Anderson–Talman theory and other full-quantum theoretical calculations. Compared with prior calculations based on the scattering matrix, our calculations are more self-consistent and show better agreement with experimental results.

Enhanced polarization of low pressure [formula omitted]He through metastability exchange optical pumping at high field

We report high steady-state nuclear polarization of 1torr 3He gas nuclei via metastability exchange optical pumping at magnetic fields above 2T. The introduction of highly polarized 3He gas into Brookhaven’s Electron Beam Ion Source would enable a new, polarized 3He ion source for use at the Relativistic Heavy Ion Collider and a future Electron–Ion Collider facility. By adapting recent developments in high field metastability exchange optical pumping for higher pressure gas, we have successfully polarized 1torr 3He sealed cells in the EBIS solenoid. Through careful manipulation of the RF discharge parameters, polarizations above 80% were attained at 2, 3 and 4T, with 89% being reached at 3T with a 664s relaxation time.

Thick-target inverse kinematic method in order to investigate alpha-clustering in212Po

The inverse-kinematic thick-target method has been used in order to investigate 212Po alpha-structure by the elastic scattering of 208Pb on 4He target. A 208Pb beam, accelerated by the Superconducting Cyclotron (CS) of Laboratori Nazionali del Sud - INFN, at the incident energy of 10.1 A MeV was impinging onto a specifically designed 4He gas cell, two meter long. The gas cell wasacting both as target and as beam degrader, stopping the beam before reaching the alpha-particle detection system placed at 0° with respect to the beam axis. In order to disentangle the elastic contribution from other reaction channels (e.g. inelastic scattering) a microchannel plate was used to measure the Time of Flight(ToF) of both the 208Pb beam particles and the ejectiles along the gas cell. The 208Pbstopping power in the 4He gas target was also measured, as a key ingredient in order to establish theinteraction point inside the gas cell, in turn determining the solid angle covered by the detector. In the following, the experimental technique will be described, and the results of a preliminary data analysis will be shown.

A novel broad-band neutron spin filter based on dynamically polarized protons using photo-excited triplet states

The use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter.

Cirtopic white LED with a 490 nm emission peak based on He-Zn annealed ZnO nanorods/polymer blend p-n heterojunction

A white light emitting diode (WLED) based on ZnO nanorods (ZNRs)/polymer blend p-n heterojunction has been fabricated. Different from the common commercial GaN WLEDs with a blue emission, the ZNRs/polymer blend WLED exhibits a blue-green emission peaked at 490 nm. Thus, the ZNRs/polymer blend WLED has a better cirtopic effect and smaller blue-light hazard. To get the 490 nm emission, intrinsic n-type ZNRs were annealed in a mixed gas of He and Zn vapor, and poly (9, 9-dioctylfluorene) (PFO) was mixed into the p-type polymer blend. Poly (3-hexylthiophene) (P3HT) was also mixed into the blends to get red emission. Results show the ZNRs/polymer blend heterojunction exhibits high-quality white-light emission with superior chromaticity coordinate (0.30, 0.33) as well as good cirtopic-effect spectrum. In comparison with the commercial GaN WLED, the ZNRs/polymer blend WLED is significantly potential for human health and work efficiency.

Bosonization of Cooper pairs and novel Bose-liquid superconductivity and superfluidity in high-[formula omitted] cuprates and other exotic systems

Various BCS-like pairing theories are still invariably used to describe the unusual superconducting/superfluid states and properties of condensed matters without clarifying the fermionic or bosonic nature of Cooper pairs and the relevant mechanisms of superconductivity and superfluidity. We show that the bosonization of Cooper pairs and the novel superconductivity and superfluidity resulting from the condensation of an attractive Bose gas of Cooper pairs into a superfluid state in high-Tc cuprates and other exotic systems occur under the condition εF≲2εA (where εF is the Fermi energy, εA is the energy of the effective attraction between fermions). We argue that the simple BCS criterion (i.e. formation of an energy gap on the Fermi surface) is insufficient for the occurrence of unconventional superconductivity (superfluidity) and the superconducting/superfluid transition in attractive Bose systems is more λ-like than the usual Bose–Einstein condensation. The new and specific criteria for unconventional superconductivity and superfluidity are formulated. Mean-field theory of the three- and two-dimensional superfluid Bose condensates of Cooper pairs describes reasonably well the novel superconducting states (i.e., two distinct superconducting phases below the bulk superconducting transition temperature Tc and a vortex-like state above Tc) and properties (such as λ-like transition at Tc, first-order phase transition and gapless bosonic excitations somewhat below Tc or even far below Tc) of high-Tc cuprates in full agreement with the experimental findings. The unusual superconducting/superfluid states and properties of heavy-fermion and organic compounds, Sr2RuO4, 3He, 4He and atomic Fermi gases are explained more clearly by this theory.

Order-disorder phase transitions in Zn2(C8H4O4)2.C6H12N2 in atmospheres of noble gases

The molar heat capacity of Zn2(C8H4O4)2.C6H12N2 (Zn-DMOF) was measured for the first time at 4.8–302 K in atmospheres of noble gases 3He, 4He, and 40Ar under pressures P up to 1.52·105 Pa. It is shown that phase transitions reported earlier at Tc = 13, 58.9, and 131.2 K are second-order phase transitions associated with the ordering/disordering of organic ligands in Zn-DMOF. The transition entropy (ΔSm) in the temperature range of phase transitions associated with the ordering/disordering of C6H12N2 molecules decreases at Tc = 13 K and increases at Tc = 58.9 K as P increases from 0.51·105 to 1.52·105 Pa. The values of ΔSm do not depend on P in the region of the phase transition at Tc = 131.2 K associated with the ordering/disordering of C8H4O42− anions.

Study of α-cluster Structure in 22Mg Using a Radioactive Ion Beam

The α-clusterization of an atomic nucleus has been studied for a long time, even from the earliest days of nuclear physics. Our understanding of the α-cluster structure is, however, mainly limited to the self-conjugate A = 4n (n = 2, 3, 4, · · · ) light nuclei and some of the neutron-rich radionuclides such as 10Be, 11B, and 14C. In order to study the α-cluster structure of a neutron-deficient 22Mg nucleus, we have measured the 18Ne(α, α)18Ne scattering in inverse kinematics by using radioactive 18Ne beams and the 4He gas target at the Center for Nuclear Study radioactive ion beam separator of the University of Tokyo. Recoiling α particles from the scattering were detected by using silicon strip detectors, which constitute ΔE-E telescopes for particle identification. By adopting a thick target method, we were able to investigate a wide range of excitation energies Ex = 9.9 - 16.5 MeV in 22Mg in this work.

Neutron Rem Detector Response Changes due to Activation of Components.

The relative response of neutron rem detectors has previously been shown to increase after prolonged exposure to a neutron flux. This increase has been referred to as the "soak factor." The cause of the increased response has been previously unexplained. This note reports on a search for the underlying cause of the increased response. Testing involved gamma-ray spectroscopy of activated neutron rem detector components and testing of instrument response at various rates of neutron flux and accumulated fluence. The proposed primary cause of the increased response is activation of copper in the brass collar at the fill end of the gas-filled detector tube. This component allows for attachment of the extension housing on the tube containing BF3 or He gas. Under a neutron flux, some of this copper activates to Cu. In addition, Mn was detected in activated components but does not seem to be a significant contributor to the detector response. Cadmium isotopes did not appear to be significant. The activated component causes an increase in indicated neutron dose rate due to decay photons from activated components. Photons normally have little impact on neutron rem detector readings because the electrical pulses produced in the probes are below the lower-level discriminator. However, the photon pulses do impact the overall count rate when they occur simultaneously with normally uncounted, lower amplitude, wall-effect neutron pulses.

Modeling of the 3He Density Evolution Inside the CABRI Transient Rods During Power Transients

CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under reactivity-initiated accident conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber (3He) situated in transient rods inside the reactor core. The shapes of power transients depend on the total amount of reactivity injected and on the injection speed. The injected reactivity can be calculated by conversion of the 3He gas density into units of reactivity. So, it is of upmost importance to properly model gas density evolution in transient rods during a power transient. The 3He depressurization was studied by computational fluid dynamics (CFD) calculations validated by measurements using pressure transducers. The CFD calculations show that the density evolution is slower than the pressure drop. Studies also show that it is harder to predict the depressurization during the power transients because of neutron/3He capture reactions that induce a gas heating. Surrogate models were built based on CFD calculations and validated against preliminary tests in the CABRI transient system. Two methods were identified to evaluate the gas density evolution: CFD calculations and reverse point kinetics (PKs). The first one consists in adding a heat source in transient rods based on the experimental power conversion. The second one consists in using the measured power by boron ionization chambers to evaluate the net reactivity by a reverse PKs method and to subtract the reactivity feedbacks calculated with the DULCINEE multiphysics code.

Recent progress in the commercialization of the Li Foil multi-wire proportional counter neutron detectors

The scarcity and rising cost of 3He gas has introduced a push in alternative neutron detector technologies. The Li foil multi-wire proportional counter (Li Foil MWPC) neutron detectors have shown promise as a 3He replacement technology. Large, gas-filled proportional counters with five layers of 75 µm thick 6Li foils have been built and characterized, yielding over 55% thermal neutron detection efficiency, with the possibility of increasing the efficiency above 70% with ten 96% enriched 6Li foil layers. Most recently, a backpack radiation detector (BRD) was fabricated, equipped with four Li Foil MWPC devices. The neutron sensitivity-to-weight ratio of the Li Foil MWPC devices was first optimized using MCNP6 by simulating the angular response to an ANSI-moderated-252Cf source positioned 1.5 m away from the devices. The simulated neutron sensitivity results were compared and benchmarked to that of the commercially-available Thermo-Fisher PackEye v. 1.0 that contains two 14 in. long, 2 in. diameter, 2.5 atm 3He tubes. The experimental efforts studied the performance of the Li Foil MWPCs in the backpack instrument configuration. Fabrication advancements were implemented to the Li Foil MWPC devices to increase the device manufacturability and improve the commercialization capabilities. These advancements included weight reduction, new hermetic sealing techniques, optimizing active area, and anode wire cost reduction. The Li Foil MWPC BRD was evaluated as a part of a PNNL test campaign for neutron sensitivity where 0.36 cps/ng of 252Cf at 1.5 m was measured. The gamma-ray rejection ratio (GRR) of the BRD was also evaluated, yielding a measured value of 3.1 × 10−8 for a 137Cs exposure rate of 50 mR/Hr. Further evaluations were performed under the PNNL test campaign including detector response to neutron sources of 252Cf, AmBe, AmLi and WGPu, detector angular response, and the gamma absolute rejection ratio in the presence of neutrons (GARRn).

Measuring low-energy ([formula omitted]) reaction cross sections using an extended gas target and gas recirculator

Direct measurements of (α,p) reactions of astrophysical interest with radioactive beams presents serious challenges because of the difficult nature of helium targets and the typical low intensities of the beams. To address this, a new technique has been developed for measurements of low-energy (α,p) reactions with heavy ion beams using an extended 4He gas target and a newly developed gas recirculating system. The system was used to measure the 4He(19F, 1H)22Ne reaction as a demonstration. Excitation functions of the 19F(α,p)22Ne and 19F(α,p′)22Ne∗ reactions were successfully measured to show the viability of this technique. Details of the approach and future plans are given.

A radial time projection chamber for [formula omitted] detection in CLAS at JLab

A new Radial Time Projection Chamber (RTPC) was developed at the Jefferson Laboratory to track low-energy nuclear recoils to measure exclusive nuclear reactions, such as coherent deeply virtual Compton scattering and coherent meson production off 4He. In 2009, we carried out these measurements using the CEBAF Large Acceptance Spectrometer (CLAS) supplemented by the RTPC positioned directly around a gaseous 4He target, allowing a detection threshold as low as 12 MeV for 4He. This article discusses the design, principle of operation, calibration methods and performances of this RTPC.

3He diffusion MRI in human lungs

Hyperpolarized 3He gas allows the air spaces of the lungs to be imaged via MRI. Imaging of restricted diffusion is addressed here, which allows the microstructure of the lung to be characterized through the physical restrictions to gas diffusion presented by airway and alveolar walls in the lung. Measurements of the apparent diffusion coefficient (ADC) of 3He at time scales of milliseconds and seconds are compared; measurement of acinar airway sizes by determination of the microscopic anisotropy of diffusion is discussed. This is where Dr. JJH Ackerman’s influence was greatest in aiding the formation of the Washington University 3He group, involving early a combination of physicists, radiologists, and surgeons, as the first applications of 3He ADC were to COPD and its destruction/modification of lung microstructure via emphysema. The sensitivity of the method to early COPD is demonstrated, as is its validation by direct comparison to histology. More recently the method has been used broadly in adult and pediatric obstructive lung diseases, from severe asthma to cystic fibrosis to bronchopulmonary dysplasia, a result of premature birth. These applications of the technique are discussed briefly.

Magnetotransport of Monolayer Graphene with Inert Gas Adsorption in the Quantum Hall Regime

The surface of graphene is easily accessible from outside, and thus it is a suitable material to study the effects of molecular adsorption on the electric transport properties. We investigate the magnetotransport of inert-gas-adsorbed monolayer graphene at a temperature of 4.4 K under a magnetic field ranging from 0 to 7 T. We introduce 4He or Ar gas at low temperature to graphene kept inside a sample cell. The magnetoresistance change ΔRxx and Hall resistance change ΔRxy from the pristine graphene are measured as a function of gate voltage and magnetic field for one layer of adsorbates. ΔRxx and ΔRxy show oscillating patterns related to the constant filling factor lines in a Landau-fan diagram. Magnitudes of these quantities are relatively higher around a charge neutral point and may be mass-sensitive. These conditions could be optimized for development of a highly sensitive gas sensor.

A new magnetic field system for 3He polarization

The nuclear spin-polarized 3He gas has been in depth studied and widely used in various scientific experiments. The polarized 3He gas can be used as a polarized neutron target to study the reaction of neutrons with charged particles or photon beams. On the other hand, spin polarized 3He gas is a good probe for detecting the new interactions in the supernormal model, and has many other applications as follows:the spin-dependent interaction can be studied quantitatively by measuring the NMR frequency shift but the spin-dependent interaction can also be studied by measuring the relaxation time of polarized 3He gas; the polarized 3He gas can be applied to magnetometers and magnetic resonance imaging (MRI); the highly polarized 3He gas can be used as a neutron spin filter for neutron polarization and polarization analysis because of the high correlation between the absorption cross section of the neutron in polarized 3He nucleus and the spin orientation. At present, the three major domestic sources of neutron, CMRR, CARR, and CSNS, are used to study the neutron polarization and polarization analysis techniques based on spin polarized 3He gas. The longitudinal (or spin-lattice) relaxation time (i.e., T1) of 3He is a key parameter that limits the polarizability of 3He gas. In order to reduce the effect of magnetic field gradient on the longitudinal relaxation time of polarized 3He gas, large-sized Helmholtz coils are usually constructed to provide the main magnetic field where the uniformity in the magnetic field central region reaches 10-4 cm-1. To obtain enough magnetic field uniformity, some magnetic field systems even exceed 1.5 m in size. However, it is expected to have a small magnetic field configuration from the view of practicality and convenience. For the common size (3He cells, Merritt coil and Saddle coil can effectively reduce the size of the magnetic field apparatus. However, for electron scattering experiments of 3He cells, the chamber length can be 40 cm. The system length exceeds 1 m even by using the Merritt coil. In this work, a new six-coil system for 3He polarization is obtained. Within the coils, the magnetic field gradient satisfies the requirement that √|▽Bx|2+|▽By|2/B0 -4 cm-1 in more than 30% area, which is better than all the existing coils used in polarized 3He experiments and can be applied to the future 3He instruments. For other experiments that require magnetic field to have a large uniform area, the new six-coil system is also a good option.

Development of high-pressure multi-wire proportional chamber neutron detector for the China Spallation Neutron Source multipurpose reflectometer

As a multipurpose reflectometer device, the two-dimensional (2D) position resolution neutron detector with a 200 mm×200 mm effective area is developed for China Spallation Neutron Source (CSNS) in Dongguang, China. Due to the requirements for the specific parameters of the multipurpose reflectometer, it should be designed to have a more than 50% (@2 Å) detection efficiency, better than 2 mm position resolution and 3 times n/γ resolution ability during the whole operation period of 10 years. The high pressure multi-wire proportional chamber (MWPC) neutron detector filling 3He gas is used as a key detector. Some simulation results and the experimental results show that the optimized thickness of the neutron entrance window should be 9 mm with using the 7075 aluminum alloy, the high pressure chamber should be sealed by the aluminous ring and a gas mixture should be filled with 6 bar 3He+2.5 bar C3H8. The assembled detector can achieve a more than 54% (@2 Å) detection efficiency in the normal operation.With the 100 μm wide collimator slit, the position resolution for X-rays is about 0.235 mm. Therefore, the position resolution for neutron is about 1.4 mm when 2.5 bar propane is used as the stopping gas for proton and triton. In the chamber, the water vapor, the oxygen and the organic impurity gases will reduce the gas gain, cause the detector electrodes to break down and the detector to speed up aging. To solve the outgassing effect of the detector components and keep the stable operation, the recycled device is designed to have the purification function for the working gases. It could purify the working gas at a flow rate of 2 L/min to remove the oxygen, the water vapor and the organic impurity gases. The detector gain increases about 27% with the purification function. Finally, the n/γ resolution and 2D imaging ability of the detector are tested with the 252Cf neutron source in Institute of High Energy Physics, Chinese Academy of Sciences, the peak ratio of neutron to gamma is obtained to be above 5 from the energy spectra and the detecor has good 2D imaging ability. The performance of the high-pressure MWPC neutron detector could meet the requirements for the multipurpose reflectometer, and the detector will be mounted in the CSNS in this year.

Assessment of the 3He pressure inside the CABRI transient rods - Development of a surrogate model based on measurements and complementary CFD calculations

CABRI is an experimental pulse reactor, funded by the French Nuclear Safety and Radioprotection Institute (IRSN) and operated by CEA at the Cadarache research center. It is designed to study fuel behavior under RIA conditions. In order to produce the power transients, reactivity is injected by depressurization of a neutron absorber (3He) situated in transient rods inside the reactor core. The shapes of power transients depend on the total amount of reactivity injected and on the injection speed. The injected reactivity can be calculated by conversion of the 3He gas density into units of reactivity. So, it is of upmost importance to properly master gas density evolution in transient rods during a power transient. The 3He depressurization was studied by CFD calculations and completed with measurements using pressure transducers. The CFD calculations show that the density evolution is slower than the pressure drop. Surrogate models were built based on CFD calculations and validated against preliminary tests in the CABRI transient system. Studies also show that it is harder to predict the depressurization during the power transients because of neutron/3He capture reactions that induce a gas heating. This phenomenon can be studied by a multiphysics approach based on reaction rate calculation thanks to Monte Carlo code and study the resulting heating effect with the validated CFD simulation.