Low-energy Protons Research Articles

Simulation of the impact of using a novel neutron conversion screen on detector time characteristics and efficiency

To directly measure the DT neutrons from inertial confinement fusion with a high time resolution, a new type of neutron conversion composed of a CH2 conversion layer, a metal moderation layer, and a CsI secondary electron emission layer is proposed. The conversion screen is based on the principle that recoil protons produced by elastic scattering of the neutrons in CH2 interact with CsI to generate secondary electrons. The moderation layer can filter the energy spectrum of protons to prevent low-energy protons from reaching CsI, which shortens the duration of the secondary electron pulse and improves the temporal resolution of the conversion screen. Based on the Monte Carlo method, both the neutron impulse and background γ-rays response of this conversion screen were calculated. The simulation indicates that the temporal resolution of the conversion screen can reach up to 4.9 ps when the thickness of the gold layer is 100 µm. The detection efficiency of secondary electrons/neutrons can reach 7.4 × 10−3. The detection efficiency of the neutron conversion screen for secondary electrons/γ-rays is an order of magnitude lower than the neutron impulse response, and the response time of γ-rays is 20 ps earlier than the neutron pulses. This means that using this conversion screen is beneficial to distinguish between neutrons and γ-rays and has a good signal-to-noise ratio.

Production of positron-emitting radionuclide yttrium-86 with a computer-aided design target for positron emission tomography

Yttrium-86 (86Y, t1/2 = 14.7 h, 33% β+) is a nonstandard medical nuclide with a longer half-life than other standard nuclides such as gallium-68 for radiolabelling for PET (positron emission computed tomography) imaging. 86Y is compatible with DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and can be used to evaluate DOTA-chelated radiopharmaceuticals with long clearance half-lives, particularly therapeutic radiopharmaceuticals in several days. 86Y can be produced through 86Sr(p, n)86Y reaction, in which 86Sr-enriched SrCO3 powder is bombarded with low-energy protons. To handle the SrCO3 powder, a powder target for 86Y production in 14.6 MeV proton cyclotron was designed while maximizing yield, reliability and efficiency. To that end, the target's primary design parameters were traversed, simulated and optimized using computer algorithms to obtain advantageous parameter combinations. In this way, 86Y was produced on the powder target with high yield and successfully isolated with high purity (>97%) by solid-phase extraction via DGA resin for high-quality PET imaging and drug screening.

Numerical simulation analysis of low energy proton irradiation mechanism of In x Ga1-x As (x = 0.2, 0.3, 0.53) solar cell

In this paper, we present an approach to study the electrical and spectral degradation properties of In x Ga1-x As (x = 0.2, 0.3, 0.53) solar cell under 150 keV proton irradiation based on numerical simulation. First, the structures of In x Ga1-x As solar cell were optimized by adjusting the doping concentration of base layer and modifying the thickness of base and emitter layer. The structural parameter related to providing higher conversion efficiency was figured out. Then, the degradation of electrical parameters such as short-circuit current density (Jsc ), open-circuit voltage (Voc ), maximum output power (P max), fill factor (FF), as well as the spectral response were obtained. The degradation correlation with radiation-induced defects was built to reveal the damage mechanism under 150 keV proton irradiation. This study provides constructive model of In x Ga1-x As to be applied in multi-junction solar cells, and the predicted degradation behavior could contribute to design the radiation-hardened space devices.

Observation-based modelling of the energetic storm particle event of 14 July 2012

Aims. We model the energetic storm particle (ESP) event of 14 July 2012 using the energetic particle acceleration and transport model named ‘PArticle Radiation Asset Directed at Interplanetary Space Exploration’ (PARADISE), together with the solar wind and coronal mass ejection (CME) model named ‘EUropean Heliospheric FORcasting Information Asset’ (EUHFORIA). The simulation results illustrate both the capabilities and limitations of the utilised models. We show that the models capture some essential structural features of the ESP event; however, for some aspects the simulations and observations diverge. We describe and, to some extent, assess the sources of errors in the modelling chain of EUHFORIA and PARADISE and discuss how they may be mitigated in the future. Methods. The PARADISE model computes energetic particle distributions in the heliosphere by solving the focused transport equation in a stochastic manner. This is done using a background solar wind configuration generated by the ideal magnetohydrodynamic module of EUHFORIA. The CME generating the ESP event is simulated by using the spheromak model of EUHFORIA, which approximates the CME’s flux rope as a linear force-free spheroidal magnetic field. In addition, a tool was developed to trace CME-driven shock waves in the EUHFORIA simulation domain. This tool is used in PARADISE to (i) inject 50 keV protons continuously at the CME-driven shock and (ii) include a foreshock and a sheath region, in which the energetic particle parallel mean free path, λ∥, decreases towards the shock wave. The value of λ∥ at the shock wave is estimated from in situ observations of the ESP event. Results. For energies below ∼1 MeV, the simulation results agree well with both the upstream and downstream components of the ESP event observed by the Advanced Composition Explorer. This suggests that these low-energy protons are mainly the result of interplanetary particle acceleration. In the downstream region, the sharp drop in the energetic particle intensities is reproduced at the entry into the following magnetic cloud, illustrating the importance of a magnetised CME model.

High-frequency structure design and RF stability analysis of a 4-vane radio frequency quadrupole with pi-mode stabilizer loops

Compact accelerator-based neutron source facilities are garnering attention and play an important and expanding role in material and engineering sciences, as well as in neutron science education and training. Neutrons are produced by bombarding a low-energy proton beam onto a beryllium or lithium target. In such an accelerator-based neutron source, a radio frequency quadrupole (RFQ) is usually utilized to accelerate a high-intensity proton beam to a few MeV. This study mainly covers the high-frequency structure design optimizations of a 4-vane RFQ with pi-mode stabilizer loops (PISLs) and its RF stability analysis. A 176 MHz RFQ accelerator is designed to operate at a 10% duty factor and could accelerate an 80 mA proton beam from 65 keV to 2.5 MeV within a length of 5.3 m. The adoption of PISLs ensures high RF stability, eases the operation of the accelerator, and implies less stringent alignment and machining tolerances.

Measurements of Low-Energy Protons Using a Silicon Detector for Application to SEE Testing

Measurements of Low-Energy Protons Using a Silicon Detector for Application to SEE Testing

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Proceedings to the 7th Annual Conference of the Particle Therapy Cooperative Group North America (PTCOG-NA)

Peculiar Features of Molecular Hydrogen Ions Formed by Proton Collisions with Hydrogen Atoms of the Second Flavor

We analyze Molecular Hydrogen Ions (MHIs) formed by collisions of low-energy protons with the Second Flavor of Hydrogen Atoms SFHA, whose existence was previously proven by two kinds of atomic experiments and also evidenced by two kinds of astrophysical observations. We find that the resulting MHIs would lack a significant number of terms compared to the MHIs formed by collisions of low-energy protons with the usual hydrogen atoms. We show that, in this situation, the radiative transition between the terms of such MHIs of the lowest quantum numbers would be between the terms 5fσ and 4dσ. We calculate the position of the edge of the corresponding molecular band and find it to be at the frequency 14,700 cm−1 or equivalently at the wavelength of 680 nm, which belongs to the visible range. It should be easier to observe this band compared to the spectral bands that are completely beyond the visible range. We emphasize that these results open up another avenue for finding an additional experimental proof of the existence of the SFHA. Namely, if the SFHA is present in gas (in addition to the usual hydrogen atoms), on which a beam of low-energy protons is incident, then the relative intensity of the band, corresponding to the radiative transitions between the terms 5fσ and 4dσ of the MHIs, would be enhanced compared to the absence of the SFHA.

The free electron model and the electronic energy losses of protons at low velocities interacting with polycrystalline tantalum

In this letter, we report experimental data and theoretical work on the electronic energy loss and energy loss straggling of protons transmitted through self-supported thin films of tantalum in a polycrystalline tetragonal phase . Low-energy protons with energies below 10 keV and Ta films with nominal thickness of 6, 9 and 12 nm are used. The aim of this work is to understand the unexpected values of the Ta stopping power for low-energy proton backscattering reported recently, which are far from the prediction of the standard free electron gas model and semi-empirical approaches. This had led the authors to conclude the failure of the free electron model. Our transmission measurements confirm these experimental results. In this work, a qualitative discussion and quantitative explanation of our experimental results is given, using an approach based on the density functional theory within the framework of the free electron gas (FEG) model. We performed semiclassical deterministic trajectory simulations and employ the local density approximation model, using an inhomogeneous electron density distribution and the polycrystalline character of Ta samples.

The Extended Field-aligned Suprathermal Proton Beam and Long-lasting Trapped Energetic Particle Population Observed Upstream of a Transient Interplanetary Shock

Abstract The properties of the suprathermal particle distributions observed upstream of interplanetary shocks depend not only on the properties of the shocks but also on the transport conditions encountered by the particles as they propagate away from the shocks. The confinement of particles in close proximity to the shocks, as well as particle scattering processes during propagation to the spacecraft, lead to the common observation of upstream diffuse particle distributions. We present observations of a rare extended anisotropic low-energy (≲30 keV) proton beam together with a trapped ≳500 keV proton population observed in association with the arrival of an oblique interplanetary shock at the Advanced Composition Explorer, the Interplanetary Monitoring Platform-8, and the Wind spacecraft on 2001 January 31. Continuous injection of particles by the traveling shock into a smooth radial magnetic field region formed in the tail of a modest high-speed solar wind stream produced an extended foreshock region of energetic particles. The absence of enhanced magnetic field fluctuations upstream of the shock results in the observation of a prolonged anisotropic field-aligned beam of ≲30 keV protons as well as a population of higher-energy (≳500 keV) protons with small pitch-angle cosine (μ ∼ 0) extending far from the shock.

Cosmic rays in molecular clouds probed by H2 rovibrational lines

Context. Low-energy cosmic rays (<1 TeV) play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H2. Their characterisation is therefore important both for the interpretation of observations and for the development of theoretical models. However, the methods used so far for estimating the cosmic-ray ionisation rate in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. Aims. We refine and extend a previously proposed method to estimate the cosmic-ray ionisation rate in molecular clouds by observing rovibrational transitions of H2 at near-infrared wavelengths, which are mainly excited by secondary cosmic-ray electrons. Methods. Combining models of interstellar cosmic-ray propagation and attenuation in molecular clouds with the rigorous calculation of the expected secondary electron spectrum and updated electron-H2 excitation cross sections, we derive the intensity of the four H2 rovibrational transitions observable in cold dense gas: (1−0)O(2), (1−0)Q(2), (1−0)S(0), and (1−0)O(4). Results. The proposed method allows the estimation of the cosmic-ray ionisation rate for a given observed line intensity and H2 column density. We are also able to deduce the shape of the low-energy cosmic-ray proton spectrum impinging upon the molecular cloud. In addition, we present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar cosmic-ray proton spectrum. We finally comment on the capability of the James Webb Space Telescope to detect these near-infrared H2 lines, which will make it possible to derive, for the first time, spatial variation in the cosmic-ray ionisation rate in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will finally be possible to test competing models of cosmic-ray propagation and attenuation in the interstellar medium, as well as compare cosmic-ray spectra in different Galactic regions.

Dose response of visible color center radiophotoluminescence in lithium fluoride crystals irradiated with a reference 60Co gamma beam in the 1–20 Gy dose range

The visible radiophotoluminescence response of radiation-induced stable F2 and F3+ color centers in nominally pure lithium fluoride crystals, irradiated with a reference 60Co gamma beam in the 1–20 Gy dose range, was carefully investigated. After irradiation, the Stokes-shifted emission spectra of the colored crystals were measured as a function of time under continuous laser excitation at 445 nm and in stationary conditions. The F2 radiophotoluminescence intensity remains constant; the signal, spectrally integrated in a 50 nm-wide interval centered on the F2 emission peak at 670 nm, exhibited a linear dependence on the absorbed dose with a good signal-to-noise ratio, stability for multiple readout and long storage times. Combined with our previous investigations after irradiation at similar doses with low-energy protons and clinical X-ray beams, these experimental results confirm the feasibility of passive solid-state dosimeters for radiotherapy based on color center radiophotoluminescence in nominally pure lithium fluoride crystals.

Ultrafast nonadiabatic dynamics of λ-DNA upon low energy proton irradiation

Direct ionizing damage and indirect secondary electron damage play important parts in the cell death under ion beam radiation. Depending on the real-time time-dependent density functional theory, we study the physical and chemical properties of the normal (α-) and mutant DNA (λ-DNA) in the process of low energy proton irradiation. The mutation of base pair causes the change in local chemical environment of DNA molecule and further changes the charge density of trajectory, proton–DNA interaction energy, force and track for moving proton, energy deposition, and the secondary electron evolution. The layered discrete charge in the intruding direction results in a spatial insensitivity of the electrons on the base pair to the energetic ion. The abnormal energy deposition for λ-DNA is ascribed to the mutation of the charge density and the nucleus–nucleus interaction potential. A more stable bond is formed between the mutant base pairs for λ-DNA, and there exists a flow of secondary electrons on the phosphate backbone under the ion beam radiation, resulting in a subsequent indirect chemical damage. These results provide an understanding for the central role of the physical states in radiation-induced cell death and a theoretical reference to improve the success rate of ion beam radiotherapy.

Collision site effect on the radiation dynamics of cytosine induced by proton

By combing the time-dependent density functional calculations for electrons with molecular dynamics simulations for ions (TDDFT-MD) nonadiabatically in real time, we investigate the microscopic mechanism of collisions between cytosine and low-energy protons with incident energy ranging from 150 eV to 1000 eV. To explore the effects of the collision site and the proton incident energy on irradiation processes of cytosine, two collision sites are specially considered, which are N and O both acting as the proton receptors when forming hydrogen bonds with guanine. Not only the energy loss and the scattering angle of the projectile but also the electronic and ionic degrees of freedom of the target are identified. It is found that the energy loss of proton increases linearly with the increase of the incident energy in both situations, which are 14.2% and 21.1% of the incident energy respectively. However, the scattering angles show different behaviors in these two situations when the incident kinetic energy increases. When proton collides with O, the scattering angle of proton is larger and the energy lost is more, while proton captures less electrons from O. The calculated fragment mass distribution shows the high counts of the fragment mass of 1, implying the production of H+ fragment ion from cytosine even for proton with the incident energy lower than keV. Furthermore, the calculated results show that N on cytosine is easier to be combined with low-energy protons to form NH bonds than O.

Rb83/Kr83m production and cross-section measurement with 3.4 MeV and 20 MeV proton beams

$^{83m}\mathrm{Kr}$, with a short lifetime, is an ideal calibration source for liquid xenon or liquid argon detectors. The $^{83m}\mathrm{Kr}$ isomer can be generated through the decay of the $^{83}\mathrm{Rb}$ isotope, which is usually produced by proton beams bombarding natural krypton atoms. In this paper, we report a successful production of $^{83}\mathrm{Rb}/^{83m}\mathrm{Kr}$ with a proton beam energy of 3.4 MeV, and the first measurement of the production rate with such low-energy proton beams. Another production attempt is performed using the newly available 20 MeV proton beam in China, and the measured production rate is consistent with previous measurements. The produced $^{83m}\mathrm{Kr}$ source has been successfully injected into the PandaX-II liquid xenon detector, yielding enough statistics for detector calibration.

Charge-Exchange Byproduct Cold Protons in the Earth’s Magnetosphere

Owing to the spatial overlap of the ion plasma sheet (ring current) with the Earth’s neutral-hydrogen geocorona, there is a significant rate of occurrence of charge-exchange collisions in the dipolar portion of the Earth’s magnetosphere. During a charge-exchange collision between an energetic proton and a low-energy hydrogen atom, a low-energy proton is produced. These “byproduct” cold protons are trapped in the Earth’s magnetic field where they advect via E×B drift. In this report, the number density and behavior of this cold-proton population are assessed. Estimates of the rate of production of byproduct cold protons from charge exchange are in the vicinity of 1.14 cm−3 per day at geosynchronous orbit or about 5 tons per day for the entire dipolar magnetosphere. The production rate of cold protons owing to electron-impact ionization of the geocorona by the electron plasma sheet at geosynchronous orbit is about 12% of the charge-exchange production rate, but the production rate by solar photoionization of the neutral geocorona is comparable or larger than the charge-exchange production rate. The byproduct-ion production rates are smaller than observed early time refilling rates for the outer plasmasphere. Numerical simulations of the production and transport of cold charge-exchange byproduct protons find that they have very low densities on the nightside of geosynchronous orbit, and they can have densities of 0.2–0.3 cm−3 at geosynchronous orbit on the dayside. These dayside byproduct-proton densities might play a role in shortening the early phase of plasmaspheric refilling.

A Pulsed Synchronous Linear Accelerator for Low-Energy Proton

A low-energy proton accelerator named pulsed synchronous linear accelerator (PSLA) is proposed and developed at the Institute of Fluid Physics, which is driven by unipolar-pulsed high voltages. Pulsed-accelerating electric fields and low-energy ion beams are precisely synchronized on temporal and spatial positions for continuous acceleration. The operating mode and the features of the PSLA are introduced. At present, the feasibility of a low-energy proton PSLA has been verified in principle. An average accelerating gradient up to 3 MV/m for protons is achieved.

Data inversion and erroneous annealing of floating gate cell under proton radiation

In this paper, the 60-MeV proton beam is used to carry out the proton irradiation experiment on NAND (not and) flash memory, the single-event-upset cross section data of the floating gate cell are obtained, the annealing rule of the floating gate cell errors is analyzed, and the effect of proton irradiation on the data retention capability of floating gate cells is studied. The obtained results are as follows. The single-event-upset cross section of the floating gate cell increases with the increase of proton energy, and decreases with the increase of proton fluence. The floating gate cell errors continue to increase over time, and this effect is more pronounced when low energy protons are incident. After proton irradiation, the data retention capability of the floating gate cell is significantly degraded. The analysis suggests that the high energy protons are indirectly ionized through the nuclear reaction with the target atom, causing single-event-upset of the floating gate cell. The correlation between the upset cross section and the proton fluence is due to the difference in single-event-effect sensitivity of the floating gate cell. The proton-induced non-ionizing damage can form partially permanent defect damage in the tunnel oxide layer, creating multiple auxiliary trap channels that can leak floating gate electrons, resulting in the increase of floating gate cell errors and the degradation of data retention capability.

High-contrast low-dose proton radiography of thin samples at the Tandetron accelerator

We investigate the ability of using high-resolution position-sensitive pixel detector and standard non-scanning beams of low-energy protons in air as a flexible tool and simplified technique for density-sensitivity imaging of thin samples. Proton radiography can provide high contrast and low radiation dose delivered to the inspected sample. Density-sensitive contrast response can be provided by a single proton per imaging pixel. For this purpose, we use the silicon semiconductor high-resolution pixel detector Timepix3 to evaluate particle radiography of thin samples with monoenergetic low-energy proton beams from the Tandetron light-ion accelerator. Measurements were performed with various well-defined thin samples. A proton micro beam was used to test and evaluate the technique. Spatial information of the samples is provided by the imaging detector. Density-sensitive contrast is obtained from the measurement of small differences in the deposited energy of transmitted protons across the sample. The transmitted protons are detected with high spatial resolution in the pixel detector. The single particle tracks registered in the detector are analysed by detailed pattern recognition algorithms. Various of these track parameters of spectral response are used for imaging contrast. Resulting proton radiographies for various well-defined thin samples are presented.

Low-Energy Protons in Strong-Field Dissociation of H 2 + via Dipole-Transitions at Large Bond Lengths

More than ten years ago, the observation of the low-energy structure in the photoelectron energy spectrum, regarded as an “ionization surprise,” has overthrown our understanding of strong-field physics. However, the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption, one of the well-observed phenomena in light-molecule interaction, still lacks an unambiguous mechanism and remains mysterious. Here, we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules. By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H 2 + driven by a polarization-skewed laser pulse, we reveal that the low-energy protons (below 0.7 eV) are produced via dipole-transitions at large bond lengths. The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H 2 + cation, which is distinct from the well-accepted bond-softening picture. Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field, and the energy gap between the field-dressed transient electronic states may favor the one- or three-photon transitions at the internuclear distance larger than 5 a.u. The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.