Electron screening in backward elastic scattering

Abstact: The elastic scattering cross sections, σ (E,θ), for the systems He+Ta and He+W have been measured at θ_lab=165° and E _lab=76.1 keV to 3.988 MeV using targets with a thickness of a few atomic layers. The results are smaller than the results given by the Rutherford scattering law, σ_R(E,θ), due to the effects of electron screening and can be described by σ(E,θ)/σ_R(E,θ)=(1+U_e/E)⁻¹, where U _e is an atomic screening potential energy. The deduced average value, U _e=28 ± 3 keV, is consistent with the Moliére- and Lenz-Jensen-models as well as electron binding energies. Received: 25 May 1998     

Investigation of the D(<Superscript>3</Superscript>He, <Emphasis Type="Italic">p</Emphasis>)<Superscript>4</Superscript>He Reaction in the Astrophysical Energy Region of 18–30 keV

The D(³He, p)⁴He reaction is first investigated in a solid target of deuterated zirconium (ZrD) in the ³He⁺ ion energy range E_He = 18–30 (E = 7.2?12.0keV keV in the center-of-mass system) with a step of 2 keV. The electron screening potential U_e = (617.8 ± 154.7) eV and the D(³He, p)⁴He reaction enhancement factors are experimentally determined in the given energy range. The measured electron screening potential is six times higher than in gaseous targets. This can be due to the ZrD lattice effects, which have not been studied either theoretically or experimentally so far. The D(³He, p)⁴He reaction has been investigated at the pulsed plasma Hall accelerator (Tomsk).     

Measurement of astrophysical S factors and electron screening potentials for d(d, n)3He reaction In ZrD2, TiD2, D2O, and CD2 targets in the ultralow energy region using plasma accelerators

The paper is devoted to study electron screening effect influence on the rate of d(d, n)³He reaction in the ultralow deuteron collision energy range in the deuterated polyethylene (CD₂), frozen heavy water (D₂O) and deuterated metals (ZrD₂ and TiD₂). The ZrD₂ and TiD₂ targets were fabricated via magnetron sputtering of titanium and zirconium in gas (deuterium) environment. The experiments have been carried out using high-current plasma pulsed accelerator with forming of inverse Z pinch (HCEIRAS, Russia) and pulsed Hall plasma accelerator (NPI at TPU, Russia). The detection of neutrons with energy of 2.5MeV from dd reaction was done with plastic scintillation spectrometers. As a result of the experiments the energy dependences of astrophysical S factor for the dd reaction in the deuteron collision energy range of 2?C7 keV and the values of the electron screening potential U _e of interacting deuterons have been measured for the indicated above target: U _e (CD₂) ? 40 eV; U _e (D₂O) ? 26 eV; U _e (ZrD₂) = 157 ± 43 eV; U _e (TiD₂) = 125±34 eV. The value of astrophysical S factor, corresponding to the deuteron collision energy equal to zero, in the experiments with D₂O target is found: S _b (0) = 58.6 ± 3.6 keV b. The paper compares our results with other available published experimental and calculated data.     

Experimental determination of the electron screening potential energy for the d(d, n)3He Reaction in ZrD2 and D2O in the ultralow energy region

This work is devoted to measuring of the values of the astrophysical S-factors and electron screening potential energy for a d(d,n)³He reaction occurring at ultralow energies in zirconium deuteride ZrD₂ (3.5–7.0 keV) and heavy water D₂O (2.2–6.0 keV). The experiment was performed on the Hall pulsed plasma accelerator at the TPU Nuclear Physics Institute (Tomsk) with ZrD₂ and D₂O targets produced by the magnetron sputtering of zirconium in a deuterium environment and heavy water freezing-out on a copper support, respectively. A χ ² analysis of the dependence of the neutron yields and astrophysical S-factors for the dd reaction on the deuteron collision energy E revealed that the upper bounds of the electron screening potential energy for interacting deuterons in ZrD₂ and D₂O and of the astrophysical S-factors at the deuteron collision energy E = 0 were U _e (ZrD₂) < 30 eV, U _e (D₂O) < 25 eV, S(0) = (57.2 ± 3.9) keV · b (ZrD₂), S(0) = (58.6 ± 3.6) keV · b (D₂O) at the 90% confidence level.     

X-rays from antiprotonic helium in helium gas

X-ray transitions to the 4F, 3D, and 2P atomic levels of p?He have been observed with antiprotons stopped in He gas at 4 and 1.1 atm NT. The population by radiative transitions of the 3D level in gas Y_M(4atm) = (28±14)% and Y_M(1.1 atm) = (43±22)% exceeds by more than one order of magnitude that measured in liquid He. The annihilation width of the 3D level Γ^a_3D = 2.8±1.0 × 10^?3eV is determined from the ratio between the numbers of X-rays feeding and depopulating the 3D level. The strong-interaction shift of the 2P level ε(2P) = ?14±6 eV is obtained by inputting the $\text{p}\text{He}$ experimental X-ray yields into a cascade calculation, the results of which are in good agreement with well-established data from muonic, pionic, and kaonic helium.     

Enhanced electron screening in d (d, p)t for deuterated Ta*

The recent observation of a large electron screening effect in the d(d, p)t reaction using a deuterated Ta target has been confirmed using somewhat different experimental approaches: U _e = 309±12 eV for the electron screening potential energy. The high U _e value arises from the environment of the deuterons in the Ta matrix, but a quantitative explanation is missing. Received: 21 December 2001 / Accepted: 29 January 2002     

Iodine as a donor in CdS

Iodine doped single crystals of CdS were grown from the vapor phase. High temperature Hall effect measurements for the crystals equilibrated with Cd and S₂ vapors at temperatures between 700 and 1000°C gave the free electron concentration as a function of p_Cd or p_S2 and temperature. The results can be explained on the basis of a model in which the CdS is saturated with iodine at low p_Cd (=high p_S2) but unsaturated at high p_Cd.The solubility of iodine in CdS is given by c_t=1·73×10²²p_S2^?1/8 exp (?1·045 eV/kT) cm^?3 atm^?1/8=4·62×10¹⁹p_Cd^1/4 exp (?0·195 eV/kT) cm^?3 atm^1/4The formation of pairs (I_SV_Cd)′ from I_S^· and V_Cd″ is governed by the equilibrium constant K_{P(I, V)}=4 exp (≤1·1 eV/kT)If Cd diffusion occurs primarily by free vacancies, the Cd^* tracer self diffusion leads to a vacancy mobility of (1·2±0·5)×10^?5 cm² sec^?1 at 900°C, in agreement with results reported by Woodbury [12], but (7±3) times larger than reported by Kumar and Kroger [10].     

Eine PrÄzisionsmessung des VerhÄltnisses der Spinresonanzfrequenz von Protonen zur Zyklotronfrequenz freier Elektronen im gleichen Magnetfeld

A new precision measurement of the ratio of the proton NMR frequency to the cyclotron frequency of free electrons in the same magnetic field, Ω _p /Ω_e=γ _p mc/e, has been performed. A cyclotron resonance linewidth of 5 ppm only and a reduction of the large space charge shifts, amounting up to the order of 50 ppm in earlier published measurements, enabled a high accuracy. The final result is Ω _p /Ω _e =1.5209945×10^?3±0.5 ppm, where the NMR is referred to a long cylindrical sample of H₂O, 0.2 m CuSO₄. Corrected to the free proton, we get Ω _p /Ω _e =1.5210329×10^?3±0.6 ppm. Combination of our result with an EPR-measurement ofLambe yields a free electrong-factor ofg=2(1+(α/2π)?(0.30±0.09) (α/π)²) in good agreement with the theoretical valueg=2(1+(α/2π)?0.328(α/π)²±...).     

First direct observation of strong interaction spin-orbit effects in antiprotonic atoms

The strong ¯p-nucleus spin-orbit interaction was investigated in a measurement of the strong-interaction effects of the 9→8 transition in ¯p ¹⁷⁴Yb at the Low-Energy Antiproton Ring (LEAR) at CERN. This measurement was part of an experimental programme where, for the first time, the fine-structure components of the last observable X-ray transition in a ¯p atom, which carries information on the strong ¯p-nucleus interaction, were resolved and studied individually. The observed splitting ΔE _exp=2408±26 eV consists of the electromagnetic fine-structure splitting ΔE _FS=2350 eV and an additional splitting Δ?=58±26 eV. In addition, one finds a significant difference in the level widths of Δ=195±59 eV with the larger value_?=1216±41 eV for the lower fine-structure level. This experiment follows an earlier measurement on ¯p ¹³⁸Ba, where the transition 8→7 is influenced by the strong interaction. In this case, however, the fine-structure components could not be resolved. The results for¹⁷⁴Yb may be attributed to a spin-orbit (LS) term in the complex strong-interaction potential.     

Isotopieverschiebungskonstanten von Th,U, Pu und Am

The isotope shifts of unperturbed electron configurations have been determined from isotope shift measurements in the spectra of Th, U, Pu, and Am. The screening of the 7s electron charge density at the nucleus by 6d, 7s, and 7p electrons is discussed. It turns out that the same screening factors as for the 6s electron in lighter elements can be used. The screening of the 7s electron charge density at the nucleus by one 5f electron amounts to about 25%: [δT(f ⁿ s)?δT(f ⁿ )]/[δT(f ⁿ }s)?δ(f ^n} )]=0.7₅. The charge density at the nucleus due to the filleds (andp _1/2) shells is considerabely screened by anf electron. The isotope shiftδT(f^n}-1 d ^m +2)?δT(f ⁿ d ^m ) produced by this effect is of the same order of magnitude as the isotope shiftδT(f ⁿ d ^m s)-δT(f ⁿ d ^m ) due to ans electron. The experimental isotope shift constants are found to be:Β C _exp(Th²³⁰–Th²³²)=880±120;Β C _exp(U²³³–U²³⁵)=1000±180;Β C _exp(U²³⁴–U²³⁶)=1070±200;Β C _exp(U²³⁶–U²³⁸)=1080±180;Β C _exp(Pu²³⁸–Pu²⁴⁰)=1200±120;Β C _exp(Pu²³⁹–Pu²⁴¹)=1060±100;Β C _exp(Pu²⁴⁰–Pu242)=900 ±90;Β C _exp(Am²⁴¹–Am²⁴³)=890±50 [10^?3cm^?1]. The ratiosΒ C _exp/C _th are discussed.     

Subject index

We have investigated the Penning ionization of NO₂(²A₁) by He(2³S) by means of electron—ion coincidence measurements. It is possible to identify two entrance channels. The quartet state is essentially repulsive and gives rise to an electron energy spectrum similar to that found in photoionization. The doublet entrance channel is strongly attractive with a well depth D_e of 4.8 {_?0.3^+0.1 eV. Ionization out of this channel leads to very broad features in the electron energy spectrum.     

Investigation of the 6p 2(3 P 0)n p Rydberg series of bismuth by multiphoton excitation

Rydberg states of the odd-parity series 6p ²(³ p ₀)n p of BiI are excited by a three-photon process. A two-photon dissociation of Bi₂ into excited atomic states followed by a one-photon absorption leads to highly excited atomic Rydberg states up ton = 32. States of the even-parity Rydberg series 6p ²(³ p ₀)nsJ=1/2,ndJ=3/2 andndJ=5/2 are also observed. In order to avoid the background caused by ionization of the bismuth molecules we performed a two-color excitation with pulsed dye lasers. With this experiment the 6p ²(³ p ₀)npJ=3/2 Rydberg series could be resolved up ton=75. The increasing quantum defect of this series is due to a perturbing state close to the first ionization limit. By a MQDT analysis we obtain the energy of the perturbing state and a value of 58,761.68±0.1 cm^?1 for the first ionization limit of atomic bismuth.     

Destruction of26Al in explosive nucleosynthesis

The²⁶Al(n, p)²⁶Mg reaction has been studied using neutron spectra which closely resembled Maxwell-Boltzmann (M.-B.) distributions with thermal energies ofkT=40 ×10^?6, 31 and 71 keV and also forE _n =270–350 keV. These energies correspond to stellar temperaturesT ₉=4.6×10^?7, 0.36, 0.82 and 3.1–4.1, whereT ₉ is in units of 10⁹ K. The partial cross sections for thep ₀-(p ₁-)transition are found to equal 26±10(1,850 ±150), 13±6(124±17), 16±13(84±14) and 21±8(72±15) mb for the above neutron spectra, respectively. The astrophysical reaction rate is determined for the combinedp ₀- andp ₁-transitions to beN _A 〈σν〉=(0.324±0.026, 20.5±2.7, 22.6±4.3 and 38.7±11.1) ×10⁶ cm³ mole^?1 s^?1. The results are compared with previous investigations and with statistical model calculations.     

Investigation of linear momentum transfer on the 35 MeV/u Ar+U system

All significant experimental results on the ³ H-³ He mass difference are reviewed. The weighted average of mass doublet measurements is 18596.7±3.6 eV, of tritium β measurements in magnetic spectrometers is 18610±7 eV and in implanted Si(Li) detectors is 18578±6 eV. The data within each group are consistent but there are discrepancies among the groups; possible explanations are proposed. Our best estimate for the ³ H-³ He atomic mass difference is 18599.4 ± 3.0 eV.     

On the kinematics of (e, e′p) reactions

The problem of interpreting experimental data on quasielastic electron scattering on nuclei in A(e, e′p)(A?1) reactions is considered. It is shown that the existing discrepancies in experimental data on the reaction ⁴He(e, e′p)T are associated with the fact that the residual-nucleus momentum p _m as determined from the law of energy-momentum conservation cannot be treated as that which is equal to the momentum p of the primary intranuclear nucleon. Under the assumption that the momentum transferred from the electron to the intranuclear nucleon is redistributed during the divergence of the products of the (e, e′p) reaction in question, the method for extracting p is modified by introducing a kinematical correction, whereby the situation is considerably improved. For a first approximation, the correction can be evaluated on the basis of data on A(e, e′) inclusive reactions. The argument behind this evaluation is illustrated by considering the example of the reaction ⁴He(e, e′p)T.     

Investigation of the low lying levels in9B

The reaction¹⁰B(³He, α)⁹B(p)⁸Be(g.s.) has been studied using a 1.8 MeV³He⁺ beam. Coincidence spectra were measured at θ_α=?150? and θ_α=22.5?, 30?, 37.5?, 45?, 52.5?, 60? and 67.5? for the indentification of the levels in⁹B which decay by proton emission. Possible new excited levels in⁹B were observed at (1.9±0.1), (2.1±0.1), (2.6±0.1) and (3.1±0.1) MeV.     

Attosecond manipulation of ionization and efficient broadband supercontinuum generation in stretched molecules

<正>We theoretically investigate the high-order harmonic generation from stretched molecules in a linearly polarized intense field.By adopting an infrared pulse combined with an ultraviolet(UV) attosecond pulse,the ionization process can be controlled effectively.In this excitation scheme,the harmonic spectrum beyond I_p+3.17U_P is significantly enhanced by two orders,where I_p and U_p=e~2E_0~2/(4m_eω~2) are the ionization and ponderomotive potential,then smooth broadband supercontinuum with the bandwidth of about 120 eV is obtained,which leads to an isolated sub-60- as attosecond pulse with a high signal-noise ratio.Moreover,the bandwidth of the supercontinuum is weakly dependent on the location and pulse duration of the UV pulse.     

The energy of thermal electrons in electron beam created helium discharges

We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s³S?np³P He I series. Values of kT_e between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He?Hg electron beam created plasma is calculated. The results are applied to the Hg⁺ laser pumping scheme using a electron beam created He?Hg plasma.     

The identification of two entrance channels in the reaction of metastable helium He(23S,21S) with SO2

The energy spectrum of electrons emitted in the reaction of He(2³S) and He(2¹S) with SO₂ indicates that in addition to the covalent entrance channel a strongly attractive ionic channel plays a role. We find upper limits for the well depth of the potential curve that corresponds to the latter channel of D_e ? 3.5 eV and D_e ? 4.3 eV for He(2³S) and He(2¹S) as the reaction partner, respectively. The partitioning between the two entrance channels can be understood by assuming the coupling matrix between both channels to depend strongly on the orientation of the SO₂ molecule with respect to the incident metastable He*.     

Complete density matrix measured for Na(32 P) excited by electron impact

Anisotropies observed in collisional deexcitation of laser excited Na atoms are analysed in terms of the density matrix describing the Na(3P) atom after the process Na(3² S)+e ^?(E)→Na(3² P)+e ^?(E?δE). Collision energiesE range from 5.1 to 22.1 eV, and scattering angles fromθ _col=0 to 25°. The results are presented with reference to a coordinate frame in which the angular momentum transferred during collision is parallel to the quantization axis. This choice allows a convenient exploitation of reflection symmetry and offers most direct relation of measurable quantities to the sublevel excitation amplitudes as e.g. computed by close coupling and distorted wave methods. Comparison between experiment and theory provides a sensitive probe of the strengths and shortcomings of the theory when predicting these alignment and orientation parameters. Since we discuss a complete set of data containing all accessible information on the atomic orbital the present measurements allow the detection of a deviation from coherence in the excitation for small scattering anglesθ _col≈5–10° atE=5.1 eV which we attribute to electron exchange.