Structure of negative impurity ions in liquid helium

The experimental mobilities of negative halogen (Cl^?, F^?, and I^?) and metal (Ba^? and Ga^?) impurity ions in superfluid ⁴He are close to each other and much lower than the mobilities of not only He⁺ ions but also electron bubbles. It has been shown that the formation of multiatomic complexes (clusters or bubbles) around ions is responsible for this low mobility. Although the mobilities are similar, the structures of the resulting complexes are qualitatively different in the cases of halogens and metals: solid clusters, which are similar to a well-studied cluster at the He⁺ ion, are formed near halogen ions, which exhibit high electron affinities, whereas metal ions are localized in bubbles, which are similar to electron bubbles. The temperature and pressure dependence of the mobility of these complexes is qualitatively different. Experiments in this area, most likely, performed with a wider variety of negative ions, would enhance the understanding of the structure of charged complexes in liquid helium.     

Recombination channels of molecular ions He 2 + and electrons

The dependence of the intensities of atomic lines and molecular bands emitted in the afterglow of a helium discharge on the electron temperature is used to identify the processes in which states of He and He ₂ ^* are populated. It is established that the formation of He^* atoms (n = 3, 4) in decaying helium plasma occurs principally on account of the associative recombination of vibrationally excited He ₂ ⁺ ions and electrons. Analysis of the experimental reuslts leads to the conclusion that the distribution of the molecular ions He ₂ ⁺ over vibrational states is strongly nonequilibrium.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 67–72, February, 1984.It remains to thank N. P. Penkin for discussions and for his interest in the work.     

A description of the laser-induced annealing and diffusion behaviour of implanted silicon crystals

Abstract

It is shown that as a result of the Greenwood-Foreman-Rimmer loop punching mechanism applied for helium bubble growth in nickel implanted with 5 keV He⁺ ions at 273 K, a considerable amount of helium remains outside the bubbles which are visible in a transmission electron microscope (TEM). It is also shown that even when it is assumed that there is an energy barrier with an upper limit equal to the formation energy of a self-interstitial atom, not all implanted helium can be accumulated in the bubbles below the critical dose for blistering.

The experimental observation of bubble growth in a helium pre-implanted nickel specimen during 1 MeV e^? irradiation may demonstrate that indeed a significant amount of helium remains between the bubbles visible in TEM.     

The Formation of Helium Bubbles in Silicon Surface Layers via Plasma Immersion Ion Implantation

The surface layers of single-crystal silicon Si(001) substrates subjected to plasma-immersion implantation with 2- and 5-keV helium ions to a dose of 5 × 10¹⁷ cm^–2 were probed via grazing incidence small-angle X-ray scattering and transmission electron microscopy. A surface layer formed by helium ions was found to possess a multilayer structure, wherein the upper layer is amorphous silicon, being on top of a sublayer with helium bubbles and a sublayer with a disturbed crystal structure. The in-depth electron density distribution, as well as the concentration and pore-size distribution, were established. The average pore sizes of bubbles at the above implantation energies are 4 nm and 8 nm, respectively.     

Experimental investigation of bubble occurrence and locality distribution of bubble detectors bombarded with high-energy helium ions

Large-sized bubble detectors with microscopic droplets of superheated liquids of dichlorodifluoromethane (Freon-12), dichlorotetrafluoroethane (Freon-114), tetrafluoroethane (Freon-134a), and mixture of Freon-12 and Freon-114, respectively, were irradiated with 150 MeV/amu helium ions at the HIMAC accelerator in NIRS, Chiba, Japan. Distributions of bubbles produced by the helium ions have been studied in each type of the detectors. The origin of the bubbles has been investigated. The detection efficiency of each type of the bubble detectors for helium ions with respect to the energy of the ions has been obtained. The phenomenon of bubble occurrence and its possible applications to the determination of He intensity from accelerators, research of track formation mechanism, energy loss straggling and neutron detection in the space and at higher altitude are discussed.     

The density of helium in bubbles in implanted materials: Results from VUV absorption and eel spectroscopy

Abstract

The novel application of vacuum ultra-violet absorption spectroscopy and electron energy loss spectroscopy to helium bubbles in metals is presented. These measurements, carried out on thin aluminium films containing different concentrations of helium and various bubble size distributions, were aimed at determining the density (and thus pressure) of helium in bubbles by observing the shift and broadening of the IS-2P transition in the helium. The data coupled with a theoretical model developed by the authors (see following paper) indicate densities as high as 10²³ He cm^?3 for specimens containing small bubbles. Data are also presented on the effect that annealing and cooling have on these spectra. The annealing experiments give rise to fairly complex changes in absorption peak structure but with a general shift towards the unperturbed resonance line. The cooling experiment gives rise to a further shift and a narrowing of the absorption spectrum on cooling to 77 K which is tentatively identified as the liquid/solid transition in the helium. Finally, fluorescence spectrum of an Al/He specimen excited with low energy electrons is presented.     

Vacancy trapping and helium decoration in Sb ion-implanted tungsten studied by Mössbauer spectroscopy

Mössbauer effect measurements have been performed using sources of¹¹⁹Sb implanted in W without and with post-implanted helium. Each of the sources was subjected to an isochronal annealing sequence in order to study vacancy trapping, helium decoration and recovery of damage. Four sites have been identified for Sb implanted in tungsten; one of these corresponds with substitutional Sb atoms, two others are assigned to Sb atoms associated with vacancies, while the last one can be either vacancy or impurity associated. The development of site occupation as a function of annealing temperature is in accordance with the one-interstitial model. Injection of 2·10¹⁶ He/cm² leads to nucleation of helium bubbles. Helium atoms that are released from these bubbles at about 1300 K are retrapped by Sb atoms to form new bubbles.     

Field evaporation experiments with a magnetic sector atom-probe FIM

A magnetic sector atom-probe FIM has been successfully operated for dc field evaporation of tip materials such as Rh, W, Ir, Mo and Ti. A limited number of evaporated metal ions were clearly identified forming a line spectrum. Field evaporation of Rh in the presence of ³He and ⁴He gases showed that the formation of the helium compound (RhHe)²⁺ is quite sensitive to He gas pressure; no helium compound were observed below 5 × 10^?7 Torr and all ions detected as helium compound above 5 × 10^?5 Torr at 78 K.     

Dynamical response of helium bubble motion to irradiation with high-energy self-ions in aluminum at high temperature

Brownian-type motion of helium bubbles in aluminum and its dynamical response to irradiation with 100-keV Al⁺ ions at high temperatures has been studied using in situ irradiation and transmission electron microscopy. It is found that, for most bubbles, the Brownian-type motion is retarded under irradiation, while the mobility returns when the irradiation is stopped. In contrast, under irradiation, a small number of bubbles display exceptionally rapid motion associated with the change in bubble size. These effects are discussed in terms of the dynamical interaction of helium bubbles with cascade damage formed by the high-energy self-ion irradiation.     

Implantation and spectroscopy of metal atoms in solid helium

The first implantation of neutral Ba and Cs atoms into solid⁴He is reported. We discuss details of the experimental setup and techniques used to load the helium with atoms at concentrations of 10⁸ cm^–3. If photodissociation of Cs molecules and clusters is performed twice per hour this atomic concentration can be kept without a second implantation for almost a day. From the optical spectra of Ba in solid helium we infer no significant difference in the trapping site with respect to that in liquid helium.     

Inelastic p-He scattering at 141 MeV

Inelastic p-⁴He scattering at 141±2 MeV has been investigated with a high-pressure cloud chamber filled with helium. Results are presented for the total cross sections of each reaction channel and the neutron spectra for the p(⁴He, pn)³He reaction.     

Angular distribution measurements of Li(p,α)He reaction at 140 keV proton energy using nuclear track detectors

Angular distributions of a ⁶Li(p,α) ³He reaction were measured at six angles for 140 keV proton energy using nuclear track detectors (NTDs). The measurements were carried out over 60°–160° lab. angles in 20° increments using a scattering chamber of 80° beam line of the 350 kV accelerator. A semiconductor silicon surface barrier (SSB) detector was placed at +160° and was used as a monitor. The results have shown that the CR-39 detector has excellent capabilities to distinguish 1.4–2.7 MeV α+ ³He particles from the ⁶Li(p,α) ³He reaction and 8–9.4 MeV α-particles from the ⁷Li(p,α) ⁴He reaction through their track diameters. However, it was not possible to distinguish between the 2.3 MeV ³He ions and the 1.7 MeV ⁴He ions from the ⁶Li(p,α) ³He reaction from their track diameter measurements, but it was possible to differentiate between the two, from the darker contrast of the ³He particles caused by its deeper tracks as compared to those of ⁴He.     

Features of the pre-equilibrium charge states of light ions passing through thin celluloid films

A semi-empirical method for establishing of the average charge of ions passing through thin celluloid films is proposed. Calculations for helium (He) and nitrogen (N) ions with different initial velocities (12?18.7 × 10⁸ cm/s for He and 4?12 × 10⁸ cm/s for N) are presented. The equilibrium thickness of a celluloid target in dependence on the initial charge states of incident ions is evaluated.     

Thermal desorption spectroscopy of he from Ni at and below saturation

Trapping of helium after implantation at energies of 8 to 150 keV and fluences up to 10¹⁹ He-ions/cm² in nickel at room temperature is studied by measuring the thermal desorption spectra during linear heating up to 1000°C. At several annealing stages the trapped helium is measured by means of the nuclear reaction ³He(d, α)H and the target surface is observed by laser scattering and with the scanning electron microscope.

The thermal desorption spectra depend strongly on the implantation fluence but only slightly on the implantation energy, indicating a similar trapping of He in the lattice for the implantation energies used here, The temperature at which desorption starts decreases with increasing fluence. Above the critical fluence for blistering an additional low temperature (150°C) desorption maximum is found.

The desorption peak at 150°C can be approximated theoretically with a single jump desorption process of first order and a Gaussian distribution of activation energies around 1 eV. The measurements indicate that at higher temperatures (>300°C) helium desorption is partly due to the opening of helium bubbles at the target surface.     

Determination of the he pressure in bubbles formed during α-implantation of Ni

Abstract

At T?90°C, Ni single crystals were uniformly implanted to a depth of 10 μm with multi-energy He ions (E≤5.2 MeV), to concentrations from 0.06 to 1.1 at.%. From a small-angle X-ray scattering study, helium was found to be trapped in small bubbles with radii of 5.6 to 12 Å. The average distance between bubbles of about 80 Å was found to be independent of the implantation dose. At the higher concentration, CH_c 1.1%, the bubbles were found to be strongly over-pressurized, i.e. the He pressure in the larger bubbles was determined to be as high as 300 kbar with a He atom/vacancy ratio of 2.     

Effect of helium concentration on irradiation damage of Fe-ion irradiated SIMP steel at 300℃ and 450℃

SIMP steel is newly developed fully martensitic steel for lead-cooled fast reactors and accelerator-driven systems.It is important to evaluate its radiation resistance via high flux neutron irradiation, where dense He atoms can be formed via(n, α) transmutation reaction. Co-irradiation with Fe and He ions, instead of neutron, was performed. Specimens were irradiated with 6.4-Me V Fe ions to the damage dose of 5 dpa at a depth of 600 nm. Three different helium injection ratios of 60-appm He/dpa(dpa: displacements per atom), 200-appm He/dpa and 600-appm He/dpa at a depth of 600 nm,were performed. Two different irradiation temperatures of 300℃ and 450℃ were carried out. The effect of helium concentration on the microstructure of Fe-irradiated SIMP steel was investigated. Microstructural damage was observed using transmission electron microscopy. The formed dislocation loops and bubbles depended on the helium injection ratio and irradiation temperature. Lots of dislocation loops and helium bubbles were homogeneously distributed at 300℃, but not at 450℃. The causes of observed effects are discussed.     

Study of background processes with the formation of neutrons in nuclear reactions in the energy range of 26–32 kev

The nature of background processes accompanying astrophysical nuclear reactions induced by hydrogen, helium, and neon ions in deuterated targets with small cross sections has been studied in calculations and experiments. The experiments have been performed at a Hall pulsed plasma accelerator in the ion energy range of 26–32 keV. The yield of background neutrons and γ-quanta with energies below 4 MeV in the proton-induced D(p, γ)³He reaction is primarily due to the presence of a natural impurity of gaseous deuterium in gaseous hydrogen and the chain of D(D, ³He)n → (n, γ) or (n, n'γ) reactions. A small contribution comes from the chain of D(¹H, ¹H)D → D(D, ³He)n → (n, γ) or (n, n'γ) reactions. It has been shown that background neutrons and γ-quanta from the D(4He, γ)6Li reaction are entirely due to the chain of D(⁴He, ⁴He)D → D(D, ³He)n → (n, γ) or (n, n'γ) reactions. It has been shown that the yield of neutrons and γ-ray photons detected at the interaction of neon ions with deuterated targets is also entirely due to the chain of elastic- scattering reactions of neon ions on deuterons in the target and to subsequent inelastic processes of interaction of deuterons accelerated at elastic scattering with other deuterons of the target. The main contribution to the yields of background neutrons and γ-quanta comes from doubly charged neon ions. The main conclusion is that the explanation of the yield of neutrons and γ-quanta at the interaction of hydrogen, helium, and neon ions with deuterated targets does not require “exotic” theoretical models.     

Diffusion transport of negative ions through the interface between cryogenic liquids

A theory of electron bubble transport through the interface between cryogenic liquids is developed based on a new approach to calculating the potential of interaction of a bubble with the interface. The theory is in good agreement with experiments on the electric-field dependence of the potential barrier near the interface between liquid ⁴He, ³He, and vacuum, as well as at the interface between ³He and ⁴He saturated solutions. It is found that the interaction potential dependence on the distance between the electron bubble and the interface is isotopically invariant to three versions of such an interface. The dependence of the lifetime of negative ions in ⁴He and ³He on the temperature and electric field has been determined using the Kramers theory.     

Magnetic hyperfine interaction of swift43 K ions recoiling in helium and xenon

The 738 keV 7/2^– isomeric state in⁴³K (=292 ± 5 ns,g=1.266 ± 0.015) was produced in the reaction⁴He(⁴⁰Ar,p) using the 185 MeV pulsed⁴⁰Ar beam of VICKSI and a 2–7 bar helium target cell. The suitability of this isomeric state for hyperfine studies during recoil in gases and after implantation into solids was investigated via the TDPAD technique. The hyperfine deorientation of highly stripped⁴³K ions in He and Xe was investigated and interpreted with the AbragamPound model. When adding up to 15% Xe to the He target gas, a near-exponential loss of alignment with the Xe partial pressure was observed. This effect can be explained by K-hole production in⁴³K in the Xe-K collision for which a cross section of=5 · 10^–18 cm² was estimated.     

X-ray studies on muon transfer reactions from hydrogen to helium

We have experimentally studied the muon transfer reactions from hydrogen to helium in liquid hydrogen with helium impurity concentration around 100–1000 ppm. The X-ray from the decay of (d⁴He) molecule was clearly observed in the D₂-⁴He system, whereas the corresponding X-ray was very weak in other systems such as D₂-³He and H₂-⁴He. This is well explained by the particle decay mode of the muonic molecule.