Helium Target Research Articles

Single-electron capture from helium targets by heavy nuclei of charges 1–7

Single-electron capture by multiply charged nuclei from helium atoms is studied by means of the prior form of the four-body boundary-corrected continuum intermediate state (BCIS-4B) method. Computations concern total cross sections for the state-selective (Qnlm) and state-summed (Qnl,Qn,QΣ) populations at 20–3000 keV/amu. These refer to the collisions of the type AZP++He(1s2)→A(ZP−1)+(nlm)+He+(1s). Here, the projectile AZP+ covers the ions H+, He2+, Li3+, Be4+, B5+, C6+ and N7+. The reported findings are tabulated for each value of the quantum numbers {n,l,m}. The maximum values nmax of the principal quantum number n are 4 (H+, He2+, Li3+), 5 (Be4+), 6 (B5+) and 7 (C6+, N7+). The sum over all the n states is truncated by selecting an appropriate cutoff value nmax and by subsequently applying the Oppenheimer n−3 scaling rule to approximately include the contributions from n>nmax. The obtained results for QΣ and for some final n−specific states favorably describe the corresponding experimental data. This qualifies the prior form of the BCIS-4B method for further useful applications in several interdisciplinary fields, including astrophysics, thermonuclear fusion, plasma physics and ion therapy in medicine.

Cross section sensitivity to perturbation strengths in distorted waves for double electron capture by alpha particles from helium targets

Computer experiments are performed on total cross sections for capture of both electrons from helium targets at 100-10000 keV. Employed are four quantum-mechanical perturbative four-body distorted wave methods (one of the first and three of the second order). The goal is to determine the cross section sensitivity to the perturbation strengths in distorted waves from the second-order methods. The perturbation strength is parametrized by the Sommerfeld factor (the quotient of the nuclear charge and the relative velocity of the colliding particles). At each fixed impact energy, the sought sensitivity is monitored by gradually modifying the nuclear charges in the Sommerfeld factors. These factors reside in the Coulomb distortions of the unperturbed channels states. The focus is on the electronic distortions through the eikonal Coulomb logarithmic phases and the full Coulomb waves. The logarithmic phases are the constituents of the compound phases for the net charges of the two heavy scattering aggregates in relative motions. A striking perturbation strength sensitivity of the obtained total cross sections is recorded.

Spin-polarized 3He shock waves from a solid-gas composite target at high laser intensities

We investigate collisionless shock acceleration of spin-polarized 3 He for laser pulses with normalized vector potentials in the range a0=100−200 . The setup utilized in the 2D-particle-in-cell simulations consists of a solid carbon foil that is placed in front of the main Helium target. The foil is heated by the laser pulse and shields the Helium from the highly oscillating fields. In turn, a shock wave with more homogeneous fields is induced, leading to highly polarized ion beams. We observe that the inclusion of radiation reaction into our simulations leads to a higher beam charge without affecting the polarization degree to a significant extent.

Antideuteron Identification in Space with Helium Calorimeter

The search for low-energy antideuterons in cosmic rays allows the addressing of fundamental physics problems testing for the presence of primordial antimatter and the nature of Dark Matter. The PHeSCAMI (Pressurized Helium Scintillating Calorimeter for AntiMatter Identification) project aims to exploit the long-living metastable states of the helium target for the identification of low-energy antideuterons in cosmic rays. A space-based pressurized helium calorimeter would provide a characteristic identification signature based on the coincident detection of a prompt scintillation signal emitted by the antideuteron energy loss during the slowing-down phase in the gas, and the (≈µs) delayed scintillation signal provided by the charged pions produced in the subsequent annihilation. The performance of a high-pressure (200-bar) helium scintillator prototype, tested in the INFN-TIFPA laboratory, will be summarized.

Quantum-mechanical four-body versus semi-classical three-body theories for double charge exchange in collisions of fast alpha particles with helium targets

AbstractWithin the two-channel distorted wave second-order perturbative theoretical formalism, we study capture of both electrons from helium-like targets by heavy nuclei as projectiles at intermediate and high impact energies. The emphasis is on the four-body single-double scattering (SDS-4B) method and the three-body continuum distorted wave impact parameter method (CDW-3B-IPM). The SDS-4B method deals with the full quantum-mechanical correlative dynamics of all the four interactively participating particles (two electrons, two nuclei). The CDW-3B-IPM is a semi-classical three-body independent particle model (one electron, two nuclei), using a combinatorial calculus to describe double capture by a product of two uncorrelated probabilities, integrated over impact parameters. Both theories share a common feature in having altogether two electronic full Coulomb continuum wave functions. One such function is centered on the projectile nucleus in the entrance channel, whereas the other is centered on the target nucleus in the exit channel. These two methods satisfy the correct initial and final Coulomb boundary conditions in the asymptotic region of scattering, at infinitely large inter-particle separations. Yet, it is presently demonstrated that most of the available experimental data on total cross sections for the double capture from helium by alpha particles distinctly favor the SDS-4B method. This is especially true at intermediate energies. Such energies are critically important in versatile applications under the general umbrella of ion transport in matter, including thermonuclear fusion (plasma physics) and ion therapy (medicine).

Various mechanisms for double capture from helium targets by alpha particles

An analysis is presented using six quantum-mechanical four-body distorted wave (DW) theories for double capture (DC) in ion-atom collisions at intermediate and high energies. They all satisfy the correct boundary conditions in the entrance and exit channels. This implies the usage of short-range perturbation potentials in compliance with the exact behaviors of scattering wave functions at infinitely large separations of particles. Specifically, total cross sections Q are analyzed for collisions of alpha particles with helium targets. Regarding the relative quantitative performance of the studied DW theories at different impact energies E, our main focus is on the sensitivity of Q to various collisional mechanisms. The usual mechanism in most DW theories assumes that both electrons undergo the same type of collisions with nuclei. These are either single or double collisions in one or two steps, respectively, per channel, but without their mixture in either channel. The signatures of double collisions in differential cross sections are the Thomas peaks. By definition, these cannot be produced by single collisions. There is another DC pathway, which is actually favored by the existing experimental data. It is a hybrid, two-center mechanism which, in each channel separately, combines a single collision for one electron with a double collision for the other electron. The ensuing DW theory is called the four-body single-double scattering (SDS-4B) method. It appears that this mechanism in the SDS-4B method is more probable than double collisions for each electron in both channels predicted by the four-body continuum distorted wave (CDW-4B) method. This is presently demonstrated for Q at energies E=[200,8000] keV in DC exemplified by alpha particles colliding with helium targets.

Doubly differential cross sections for ionisation in proton–helium collisions at intermediate energies: energy and angular distribution of emitted electrons

Using the two-centre wave-packet convergent close-coupling approach, we continue our study of the proton–helium collision system. This method uses a correlated two-electron wave function to describe the helium target and discretises the continuum using wave-packet pseudostates. The cross section differential in the electron-emission energy and emission angle is calculated for incident-projectile energies in the intermediate range from 70 to 300 keV, where coupling between various channels and electron–electron correlation effects are important. We also apply an alternative, simpler approach that reduces the target to an effective single-electron system. Overall, the present results from both methods agree well with the available experimental data. This positions both implementations of the two-centre wave-packet convergent close-coupling approach well to further study other doubly differential, as well as fully differential, cross sections of single ionisation in proton–helium collisions.Graphical abstract

Cross sections for single-electron capture from heliumlike targets by fast heavy nuclei

Single charge-exchange in collisions of heavy bare nuclei with the ground state of two-electron atomic targets is described perturbatively as a four-body problem. The employed four-body boundary-corrected continuum intermediate state (BCIS-4B) method considers the correlated and uncorrelated target wave functions ${\ensuremath{\varphi}}_{i}.$ A thorough examination is performed for the formation of any final hydrogen-like $nlm$ state of the captured electron. For arbitrary projectile and target nuclear charges, the nine-dimensional integral in the transition amplitude is reduced to a two-dimensional numerical quadrature. The general analysis is applied to one-electron capture by protons from helium targets beginning with the lower edge (10 keV) of intermediate energies and extending to the higher (12.5 MeV) domain. These include the main peaks (Massey, Thomas) due to single and double scattering, respectively. The results encompass over 70 state-selective and state-summed cross sections $(n\ensuremath{\le}6,\phantom{\rule{0.16em}{0ex}}0\ensuremath{\le}l\ensuremath{\le}n\ensuremath{-}1,\phantom{\rule{0.16em}{0ex}}\ensuremath{-}l\ensuremath{\le}m\ensuremath{\le}l).$ In comparison to measurements, the electronic correlations in ${\ensuremath{\varphi}}_{i}$ greatly improve the overall performance of the BCIS-4B method around the Massey peak, below about 100 keV. Moreover, while largely outperforming the three-body boundary-corrected continuum intermediate state method, the cross sections in the BCIS-4B with the correlated ${\ensuremath{\varphi}}_{i}$ compare excellently overall with the available experimental data at 10 to 12 500 keV. Hence, the BCIS-4B method, with its built-in two main capture mechanisms (one-step Massey and two-step Thomas) is capable of spanning impact energies covering three or more orders of magnitude at which the state-summed cross sections vary over 11 orders of magnitude.

One-electron transfer from helium targets to protons: the BCIS-4B and CDW-3B methods for state-selective and state-summed total cross sections vs measurements

One-electron transfer from helium targets to protons: the BCIS-4B and CDW-3B methods for state-selective and state-summed total cross sections vs measurements

Charge transfer and ionization cross-sections in collisions of singly charged lithium ions with helium and nitrogen atoms

We present a non-perturbative classical treatment of the charge transfer and ionization processes in collisions between singly charged lithium ions with helium and nitrogen atomic targets. Single capture and single ionization total cross sections are calculated using a three-body classical trajectory Monte Carlo (CTMC) method in which the interaction between the collision partners is described by a Garvey-type model potential. The cross sections are evaluated for collision energies between 20 keV and 100 MeV. In particular, we found excellent agreement between our results and the available experimental data for the case of the single capture of He(1s) by Li+ ions. In addition, our CTMC results are in a reasonable agreement with the experimental results for collision energies higher than 200 keV for single capture of N(2p) atoms by Li+. Furthermore, we present single ionization cross sections for both collision systems.

Measurement of the 86Kr(α,n)89Sr cross section at energies relevant for the weak r-process

The r-process has been shown to be robust in reproducing the abundance distributions of heavy elements, such as europium, seen in ultra-metal poor stars. In contrast, observations of elements 26 < Z < 47 display overabundances relative to r-process model predictions. A proposed additional source of early nucleosynthesis is the weak r-process in neutrino-driven winds of core-collapse supernovae. It has been shown that in this site (α,n) reactions are both crucial to nucleosynthesis and the main source of uncertainty in model-based abundance predictions. Aiming to improve the certainty of nucleosynthesis predictions, the cross section of the important reaction 86Kr(α,n)89Sr has been measured at an energy relevant to the weak r-process. This experiment was conducted in inverse kinematics at TRIUMF with the EMMA recoil mass spectrometer and the TIGRESS gamma-ray spectrometer. A novel type of solid helium target was used.

The BCIS-4B method for state-selective and state-summed total cross sections: Proton–helium​ charge exchange at 10–4000 keV

This work is on single electron capture by protons from helium targets at intermediate and high impact energies (10–4000 keV). Assessed is the overall performance of the second-order four-body boundary-corrected continuum intermediate state (BCIS-4B) method by detailed comparisons with all the available measurements. The state-selective Qnl,Qn and state-summed QΣ total cross sections are computed for n≤4, including all the sub-levels. Cross sections Qn are the sum of Qnl for 0≤l≤n−1. The state-summed cross sections QΣ are obtained by using Qn exactly for 1≤n≤3 and approximately for n≥4 through the Oppenheimer n−3 scaling rule. Overall, the measured cross sections are extremely favorably represented by the BCIS-4B method for Qnl,Qn and QΣ, even toward the lowest part of the considered intermediate energy region. Most notably, at all the energies above 10 or 20 keV, e.g. cross sections Q2p,Q3p and Q3d from the BCIS-4B method are in excellent agreement with the corresponding experimental data. This is very important since it is precisely for the non-spherically symmetric states (l≠0) of the captured electron that the existing discrepancies between measurements and other theories are the largest or even enormous.

Development of a cold target recoil ion momentum spectrometer and a projectile charge state analyzer setup to study electron transfer processes in highly charged ion-atom/molecule collisions.

We report the development and performance of a cold target recoil ion momentum spectrometer (COLTRIMS) setup at TIFR, which is built to study various atomic and molecular processes involving the interaction of slow, highly charged ions from an electron cyclotron resonance based ion accelerator. We give a detailed description of the experimental setup, as well as report some initial results on the electron-capture process in collisions of Ar8+ ions with helium and carbon monoxide targets. Here, we present the longitudinal momentum transfer and the sub-shell resolved Q-value spectrum in the case of 2, 4, and 6 keV/u Ar8+ beams in collision with helium. A longitudinal momentum resolution of 0.27a.u. is achieved in the present system. We also report the state-selective scattering angle distributions for all the collision systems under investigation. We further discuss the fragmentation of the CO2+ molecular ions for different electron capture channels for the 5 keV/u Ar8+ beam. The combination of the COLTRIMS, along with the beam cleaner, the electrostatic deflectors, and the charge state analyzer, is shown to have certain advantages.

Helium line ratio imaging in the C-2W divertor.

A 2D imaging instrument has been designed and deployed on C-2W ("Norman") [H. Gota et al., Nucl. Fusion 61, 106039 (2021)] to study the plasma in the expander divertor by simultaneously measuring three neutral helium spectral lines. Ratios of these images, in conjunction with a collisional-radiative model, yield 2D maps of electron temperature and density. Almost the entire radial plasma cross-section (∼60cm) can be mapped with a spatial resolution ≲1cm. These data can, in principle, be acquired at 3kHz. The neutral helium target is provided by a custom-built supersonic gas injector located inside the divertor vessel, which injects helium toward the magnetic axis and perpendicular to the camera sight-cone. Images of helium emission and reconstructed electron density and temperature profiles of the plasma produced from an end gun are presented. Voltages applied to concentric annular electrodes located in the divertors are used to stabilize beam-driven field reversed configuration plasmas. Magnetic field expansion is also employed to thermally isolate electrons from the end electrodes. Measurements of electron temperature and density in the divertor are important in order to study the effects of both the electrostatic biasing and the divertor magnetic field on electron confinement, neutral gas transport, and the overall machine performance.

High-resolution laser resonances of antiprotonic helium in superfluid 4He

When atoms are placed into liquids, their optical spectral lines corresponding to the electronic transitions are greatly broadened compared to those of single, isolated atoms. This linewidth increase can often reach a factor of more than a million, obscuring spectroscopic structures and preventing high-resolution spectroscopy, even when superfluid helium, which is the most transparent, cold and chemically inert liquid, is used as the host material1–6. Here we show that when an exotic helium atom with a constituent antiproton7–9 is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 106, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation7. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses9. The sharp spectral lines may enable the detection of cosmic-ray antiprotons10,11 or searches for antideuterons12 that come to rest in liquid helium targets.

First 3D vector tracking of helium recoils for fast neutron measurements at SuperKEKB

We present results from the first deployment of novel, high definition, compact gas Time Projection Chambers (TPCs) with pixel chip readout as part of the BEAST II beam background measurement project at SuperKEKB. The TPCs provide detailed 3D imaging of ionization from neutron-induced nuclear recoils in a helium and carbon dioxide target gas mixture at standard temperature and pressure. We present the TPC performance and the neutron backgrounds observed during the initial stage of collider commissioning. We find excellent electron background rejection, leading to background-free nuclear recoil measurements above 50 keVee, despite the extreme high-background environment. We measure an angular resolution better than 20° for recoil tracks longer than 1.7 mm, corresponding to an average ionization energy of approximately 100 keVee. We also obtain the full 3D vector direction of helium recoils by utilizing charge profile measurements along the recoil axis, with a correct head/tail assignment efficiency of approximately 80%. With this performance, we present comparisons between measured and simulated event rates, recoil energy spectra, and directional distributions originating from beam–gas and Touschek beam losses at SuperKEKB. We utilize head/tail recognition to distinguish neutron components traveling with positive radial velocity in the Belle II coordinate system from those traveling in the opposite direction. Finally, we present a novel method of discriminating beam–gas interactions from Touschek beam losses that can eliminate the need for dedicated accelerator runs for background measurements. This method is still statistics-limited. However, future studies should be able to verify this method, which in turn could lead to neutron background analysis runs symbiotic with normal Belle II operation. The capabilities demonstrated here also suggest that high definition recoil imaging in gas TPCs is applicable to low energy, low-background experiments, such as directional dark matter searches.

Proton-helium collisions at intermediate energies: Singly differential ionization cross sections

We investigate the four-body proton-helium scattering problem using the two-center wave-packet convergent close-coupling approach. The approach uses a correlated two-electron wave function for the helium target. The continuum is discretized using wave-packet pseudostates. Calculations of ionization cross sections differential in the electron emission energy, in the emission angle, as well as in the scattered-projectile angle are performed in the intermediate energy region where coupling between various channels and electron-electron correlation effects are important. The results agree well with experimental data, where available. Moreover, our calculations reveal an interesting interplay between direct ionization and electron capture into the continuum. In particular, we demonstrate that the ionization cross section differential in the angle of the ejected electron is dominated by electron capture into the continuum for ejection into small angles, while ejection into large angles is purely due to direct ionization. It is concluded that the two-center wave-packet convergent close-coupling approach can provide accurate singly differential cross sections for ionization in proton-helium collisions. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. Somewhat unexpectedly, the results of the effective one-electron method exhibit a very good level of agreement with the full two-electron ones for all three singly differential cross sections. Therefore, we also conclude that, at least for the purpose of calculating the singly differential cross sections for single ionization of helium, an effective one-electron treatment of the target suffices.

Silver nanoparticles for biolistic transformation in Nicotiana tabacum L.

The present study reports the use of silver nanoparticles as a gene carrier, substituting gold microcarrier for biolistic gene delivery in Nicotiana tabacum L. Efficiency of biolistic transformation using silver nanoparticles (100nm) was compared with that of gold microcarriers (0.6 micron) under varying helium pressure (450 psi, 650 psi, 900 psi and 1100 psi) and target distance (6cm and 9cm). Among the different concentrations (0.01-100 mgL-1) of silver nanoparticles tried, 10 mgL-1 produced the highest number of transient GUS expression (30) with statistical significance. Helium pressure of 650 and target distance of 9cm, and 900 psi pressure and 6cm distance resulted in the highest GUS expression with gold microcarriers and silver nanoparticles, respectively. Transformation efficiency was significantly higher with silver nanoparticles than gold microparticles as carriers resulting in a reduction up to 37.5-fold on the cost of consumables. Regeneration efficiencies of tissues bombarded with gold microcarriers and silver nanoparticles were 62.5% and 70.83%, respectively.

Differential study of proton-helium collisions at intermediate energies: Elastic scattering, excitation, and electron capture

In a series of papers we present a comprehensive investigation of the four-body proton-helium differential scattering problem using the two-center wave-packet convergent close-coupling approach in the intermediate energy region where coupling between various channels is important. The approach uses correlated two-electron wave functions for the helium target. For comparison, a recently developed method that reduces the target to an effective single-electron system is also used. In this paper we present calculations of angular differential cross sections for elastic-scattering, target excitation, and electron-capture processes. The results of the two-electron and effective single-electron methods exhibit a good level of agreement. They also agree well with available experimental data. It is concluded that both versions of the wave-packet convergent close-coupling approach are capable of providing a realistic differential picture of all interdependent and interconnected binary processes taking place in proton-helium collisions at intermediate energies.

Neutralization of excited antiprotonic helium ion in collisions with helium atoms

We consider the neutralization of an excited antiprotonic helium ion which is accompanied by antiproton transitions to lower states during collisions with helium atoms $$({\bar{p}}\mathrm {He}^{2+})_{nl} + \mathrm {He} \rightarrow \left[ ({\bar{p}}\mathrm {He}^{2+})_{n_f l_f} e\right] _{1s} + \mathrm {He}^+$$ in a low-temperature medium. Interactions in the input and output channels are taken to be similar to the potentials of the $$\mathrm {H}^+ - \mathrm {He}$$ ( $$X^1\varSigma ^+$$ ) and $$\mathrm {H} - \mathrm {He}^+$$ ( $$A^1\varSigma ^+$$ ) systems, respectively. The potentials in the output channels are shifted by an amount equal to the dexcitation energy of the antiproton. Therefore, the potentials in the input and output channels can intersect for the some antiproton quantum numbers leading to appreciable cross sections of the transitions. We have found that this process can increase the effective annihilation rates of highly excited antiprotonic helium ions in helium targets of low atomic density and temperature that were used in recent experiments.