Noble Gas Targets Research Articles

Proportional scintillation in liquid xenon: demonstration in a single-phase liquid-only time projection chamber

The largest direct dark matter search experiments to date employ dual-phase time projection chambers (TPCs) with liquid noble gas targets. These detect both the primary photons generated by particle interactions in the liquid target, as well as proportional secondary scintillation light created by the ionization electrons in a strong electric field in the gas phase between the liquid-gas interface and the anode. In this work, we describe the detection of charge signals in a small-scale single-phase liquid-xenon-only TPC, that features the well-established TPC geometry with light readout above and below a cylindrical target. In the single-phase TPC, the proportional scintillation light (S2) is generated in liquid xenon in close proximity to 10 μm diameter anode wires. The detector was characterized and the proportional scintillation process was studied using the 32.1 keV and 9.4 keV signals from 83mKr decays. A charge gain factor g 2 of up to (1.9 ± 0.3) PE/electron was reached at an anode voltage 4.4 kV higher than the gate electrode 5 mm below it, corresponding to (29 ± 6) photons emitted per ionization electron. The duration of S2 signals is dominated by electron diffusion and approaches the xenon de-excitation timescale for very short electron drift times. The electron drift velocity and the longitudinal diffusion constant were measured at a drift field of 470 V/cm. The results agree with the literature and demonstrate that a single-phase TPC can be operated successfully.

Correlated characterization of 20Ne-implanted targets using nuclear reaction analysis, Rutherford backscattering spectrometry, and ion transport modeling

We present the preparation and characterization of a large sample of implanted noble gas targets for use in precision nuclear astrophysics measurements with intense proton beams. Tantalum and titanium backings were prepared using wet-acid etching, outgassed via resistive heating, and implanted with 20Ne+ beams from differing ion sources. These experimental targets were investigated using both nuclear reaction analysis techniques on the 1169-keV resonance in 20Ne(p, γ)21Na and Rutherford backscattering spectrometry analysis with 2-MeV α-particle beams. Results from these analyses reveal small target-to-target variations in stoichiometry, while exhibiting excellent agreement independent of ion-beam analysis method. We also present a self-consistent validation of the nuclear reaction analysis results using ion transport simulations in TRIM-2013 that rely on input parameters from SIMNRA scattering-yield fits to Rutherford backscattering spectra. In addition to a complete description of the implantation profile, this analysis method provides an alternate solution for characterizing a large sample of implanted targets when no suitable resonances are available for nuclear reaction analysis.

Single-Capture Cross Sections from Biological Molecules and Noble Gases by Bare Ion Impact

In this work, we report theoretical electron capture cross sections for single-electron removal from molecules of biological interest and noble gases by bare ion (H+ and He2+) impact at energies ranging from 25 to 10,000 keV/amu. We use a distorted wave (DW) method where the intermediate continuum state of the active electron with the target ion has been taken into account. This method is developed within the framework of the independent electron model taking particular care of the representation of the bound continuum target states. Two different approximations have been considered for molecular targets: molecular representation of the bound-state target wavefunction and Bragg’s additivity rule. The molecular orbital for targets are described within the framework of the complete neglect of differential overlap (CNDO) method based on the linear combination of atomic orbital (LCAO) approximation. Using the DW method, we have also calculated the K-, L- and M- shell electron capture in collisions of bare ions with three noble gases He, Ne, and Ar respectively. Contributions from different molecular orbitals and different shells to the total cross sections (TCS) are studied. The preference of electron capture occurs in accordance as the binding energy of the active electron in molecular orbital and atomic shell. The maximum contributions to TCS for SC comes from the less bound electrons in repetitive orbitals, whereas the tightly bound electrons dominate the TCS at higher projectile energy regime. Variation of TCS with impact energy are compared with the available experimental observation and other theoretical findings. We find that the present theoretical method is satisfactory in both intermediate and high-energy region for molecules as well as noble gas targets to give reliable outcomes compared to other theoretical methods.

Blister resistant targets for nuclear reaction experiments with [formula omitted]-particle beams

Solid targets for nuclear measurements that use α-particle beams commonly experience a form of degradation known as blistering. The effect can prevent the use of solid targets for high intensity α-particle experiments, often necessitating complex gas target systems. To combat this problem, three different blister resistant target backings were designed for use in direct reaction measurements with high intensity α-particle beams. The blister resistant target designs utilize gas diffusive properties of fused silica, sintered metal, and porous evaporated metal. Each target was implanted with 22Ne ions and bombarded with α-particle beam to test blister resistance. Targets were characterized and monitored using the 22Ne(p,γ)23Na reaction to determine the degradation of implanted material, and compare them to typical implanted noble gas targets. We find that all targets studied exhibit resistance to blistering, with the porous evaporated metal targets displaying the least amount of target material degradation.

Highlights from LHCb

The recent highlights from LHCb in soft QCD and Heavy Ion physics are presented. This includes measurements from collisions of proton and lead [see formula in PDF] ion beams with other beams as well as noble gas targets. An outlook on future analyses of [see formula in PDF] collisions is presented.

New high resolution measurements of open and hidden charm production in proton-nucleus collisions at [formula omitted] with LHCb

Open and hidden charm production in nucleus-nucleus collisions is considered as a key probe of Quark Gluon Plasma (QGP) formation. In the search of specific QGP effects, proton-nucleus collisions are used as the reference as they account for the corresponding Cold Nuclear Matter (CNM) effects. The LHCb experiment, thanks to its System for Measuring Overlap with Gas (SMOG) can be operated in a fixed target mode with the LHC beams, at an intermediate center-of-mass energy between nominal SPS and RHIC energies. In 2015, for the first time, reactions of incident LHC proton beams on noble gas targets have been recorded by the LHCb experiment at a center-of-mass energy of 110 GeV and within the center-of-mass rapidity range −2.77<y⁎<−0.2. The first high resolution measurements on open and hidden charm production obtained under these conditions are presented.

Absolute cross sections for electron loss, electron capture, and multiple ionization in collisions of Li2+ with argon

Exclusive absolute cross sections for the electron loss and capture processes, accompanied by target multiple ionization and pure target multiple ionization, as well as total electron loss and capture cross sections, in collisions of Li2+ with Ar have been measured in the 0.5–3.5 MeV energy range. The experimental data of the total electron loss cross section are compared with theoretical results based on the plane-wave Born approximation and the free-collision model, and with the available experimental data. Some discrepancies are observed when comparing the experimental data with the theoretical models, which can be attributed to the competitive mechanisms that lead to electron loss. The dependences of the single-capture and transfer-ionization processes on the projectile charge state are similar to those observed for collisions between other low-charged light ions and noble-gas targets. The same behaviour is observed when one compares the present data for the single- and double-ionization cross sections with those for He2+ projectiles on Ar. These facts indicate that the dynamics of the collision does not seem to depend on the projectile species, so that few-electron projectiles may act as structureless point charges in the intermediate- to high-velocity regime.

(e,2e) ionization studies of the stable noble gases in a coplanar symmetric geometry

A comprehensive set of coplanar symmetric ($e$,2$e$) ionization data are presented for the stable noble gas targets helium, neon, argon, krypton, and xenon. The incident electron energy ranged from 1.2 to 200 eV above the ionization potential of the target atoms, so as to span from near threshold to the high-energy regime. Data are presented with high precision, and cover a wide angular range from forward scattering where the electrons were detected at angles from 30\ifmmode^\circ\else\textdegree\fi{} to the incident electron beam, through to the backscatter region where the deflection angle was up to 130\ifmmode^\circ\else\textdegree\fi{}. The data are normalized to unity at each energy, and are compared to previously published theoretical calculations where available. The data show complex changes in the structure of the measured cross sections, depending upon both the energy and selected target. Similarities and differences in the cross sections were found between the different targets, which provide a challenge to future models of the ionization process.

Universal behavior of the electron-electron interaction in the simultaneous projectile and target ionization

The antiscreening contribution to the simultaneous projectile and target ionization is analyzed for He+ and C3+ projectiles impinging on noble gas targets. The energy dependence of the average number of target active electrons which contribute to the antiscreening is described by means of a simple function, which is "universal" for noble gases but whose parameters are projectile dependent. A method is presented to estimate the number of active electrons for each target subshell in the high-velocity regime.

A molecular beam scattering investigation of methanol–noble gas complexes: Characterization of the isotropic potential and insights into the nature of the interaction

Integral cross section experiments involving rotationally hot CH3OH projectiles and noble gas (Ng=Ne, Ar, Kr and Xe) targets are reported for the first time. Measured data have been exploited to characterize the phenomenological radial interaction in the CH3OH–Ng weakly bound complexes. Potential energy surfaces for all the systems have been formulated on the basis of a pairwise additive multicenter model. The comparison of model predictions with the most relevant experimental findings suggests that in CH3OH–Ng complexes, at variance with the behavior of the analogous complexes involving water or ammonia, the interaction is mainly due to van der Waals and induction components.

Collinear generation of ultrashort UV and XUV pulses

We demonstrate the collinear generation of few-femtosecond ultraviolet and attosecond extreme ultraviolet pulses via a combination of third-harmonic and high harmonic generation in noble gases. The ultrashort coherent light bursts are produced by focusing a sub-1.5-cycle near-infrared/visible laser pulse in two subsequent quasi-static noble gas targets. This approach provides an inherently synchronized pair of UV and XUV pulses, where the UV radiation has a photon energy of approximately 5 eV and a pulse energy of up to 1 microJ and the XUV radiation contains up to 3.5 10(6) XUV photons per shot with a photon energy exceeding 100 eV. This source represents a novel tool for future UV pump/XUV probe experiments with unprecedented time-resolution.

Hard collisions of few keV diatomic molecular ions with atomic gas targets: Collision induced dissociation and target ionization

Target ionization in close encounters between few keV simple diatomic molecular ions and noble gas targets have been studied experimentally. Some of the projectile molecular ions fragment as a result of these violent collisions while others remain bound despite undergoing a "hard" collision. The measured momenta shed light on the mechanisms responsible for this behavior.

Collinear Generation of Ultrashort UV and XUV Pulses for Pump/probe Spectroscopy

AbstractThe collinear generation of sub-4 fs ultraviolet (UV) and attosecond extreme ultraviolet (XUV) pulses via subsequent third-harmonic and high harmonic generation in noble gases is demonstrated. The ultrashort coherent light bursts are produced by focusing a sub-1.5 cycle near-infrared/visible (NIR) laser pulse in two subsequent quasi-static noble gas targets. With this simple approach, inherently synchronized UV pulsed radiation with a photon energy of ˜5 eV and a pulse energy of ˜1 μJ, and XUV pulses with a cut-off photon energy of more than 120 eV and up to ˜107 XUV photons can be generated. This source represents a novel tool for future UV pump/XUV probe experiments with unprecedented time-resolution.

Monte Carlo studies of combined shielding and veto techniques for neutron background reduction in underground dark matter experiments based on liquid noble gas targets

We present results of a detailed simulation study of neutron background reduction in dark matter detectors based on observation of nuclear recoils below 100 keV. Background rates are estimated for neutrons from muons and from natural radioactivity, and with separate Monte Carlo simulations for the underground rock, shielding, and detector components. With optimum shielding combinations, further reduction of background can be achieved with muon and neutron vetos, simulations of the latter giving first estimates of the rejection efficiency as a function of veto geometry. Design options for neutron vetos are also discussed. A detailed tabulation of backgrounds is given, from which it is concluded that the total neutron background can be reduced below the level 10 −4–10 −3 kg −1 d −1 needed to detect dark matter particle-nucleon cross sections in the range 10 −10–10 −9 pb, with further reductions attainable in principle with lower-activity detector materials.

Close-coupling and distorted-wave calculations for electron-impact excitation of the(5p56p)states of xenon

We report on a series of calculations for electron-impact excitation of the $(5{p}^{5}6p)$ states in xenon from the ground state $(5{p}^{6})^{1}S_{0}$. As in previous calculations for other noble-gas targets, we find strong evidence of channel coupling for all incident energies considered (between threshold and $200\phantom{\rule{0.3em}{0ex}}\text{eV}$). Although qualitative agreement with the experimental results of Fons and Lin [Phys. Rev. A 58, 4603 (1998)] is achieved, severe quantitative discrepancies of sometimes more than a factor of 2 remain.

Collisional destruction of anionic carbon and silicon clusters by helium, neon, and argon atoms at intermediate velocities

We measured the total destruction cross sections of small ${X}_{n}^{\ensuremath{-}}$ clusters $(X=\mathrm{Si}$ and C; $n=2,$ 3, and 4) in noble gas targets (Y = He, Ne, and Ar), with relative velocities $v$ in the 0.10--0.95 a.u interval for silicon, and in the 0.20--1.20 a.u. interval for carbon. The measured values increase monotonically with target and projectile sizes, as expected on a geometrical basis. In particular the dependence of the cross sections on n is linear for carbon clusters, agreeing with a quantitative model based on the linear structure of these clusters. Ratios of the measured cross sections for the several ${\mathrm{X}}_{n}^{\ensuremath{-}}$ clusters, ${\ensuremath{\sigma}}_{X,n}^{\mathrm{Ne}}(v)/{\ensuremath{\sigma}}_{X,n}^{\mathrm{He}}(v),$ ${\ensuremath{\sigma}}_{X,n}^{\mathrm{Ar}}(v)/{\ensuremath{\sigma}}_{X,n}^{\mathrm{He}}(v),$ and ${\ensuremath{\sigma}}_{X,n}^{\mathrm{Ar}}(v)/{\ensuremath{\sigma}}_{X,n}^{\mathrm{Ne}}(v),$ were verified to fall into universal curves, i.e., each cross section is the product of target- and projectile-dependent factors.

PIC-MCC simulation of a r.f. planar magnetron discharge and comparison with experiment

A two-dimensional time-resolved PIC-MCC simulation was developed for the self-consistent modelling of planar r.f. magnetron reactor. Computation results such as target voltage and sputtering ‘race-track’ are in good agreement with experimental results recorded on our device. New information on the ion and electron transport through the r.f. sheath is provided. We discuss the contribution of the ‘classical’ γ-Townsend secondary emission and of the r.f. volume heating to the discharge maintaining. The present model developed for noble gas (xenon) and metallic target (copper) will be extended to dielectric targets (ceramics).

Intermediate- to high-energy positrons scattered by alkali-metal atoms

We present calculations of the differential, integrated elastic, and total cross sections for positrons scattered from alkali-metal atoms. The energy of the positrons ranges from 10 eV to 1000 eV. In the calculations we use parameter-free model potentials for the correlation-polarization and absorption interactions. The absorption potential used for positron scattering is based on a quasifree model that we recently proposed and tested for the noble-gas targets. For positron--alkali-metal scattering the model potentials have produced reliable scattering cross sections over the extended range of impact energies when compared against the available experimental data.

Incorporation of Helium into Endohedral Complexes of C60 and C70 Containing Noble-Gas Atoms: A Tandem Mass Spectrometry Study

Endohedral complexes of fullerenes and noble-gas atoms were originally prepared by a mass spectrometric method involving high-energy collisions of fullerene ions and noble-gas targets and, following that, by a high-pressure, high-temperature method involving neutral fullerenes and a noble gas as reactants. The latter method has made available sufficient quantities of the endohedral complexes of C60 and C70 with the various noble gases (He, Ne, Ar, and Kr) that the tandem mass spectrometric method can be used to test the possibility of preparing a new class of endohedral complexes that contain two noble-gas atoms. These new capture complexes do indeed form upon activation of the radical cation of the noble-gas endohedral complex by high-energy collisions with helium atoms. These complexes then decompose to give equivalent parallel losses of C2 units accompanied by expulsion of none, one, or both of the captured noble-gas atoms. The capture event causes the gain in excess of 34 eV of additional internal energy, but the capture complexes survive on the time scale (∼50 μs) for the transit through the tandem mass spectrometer. Their survival, along with their fragmentation characteristics, is consistent with formation of an endohedral complex containing both noble-gas atoms although the fullerene cage may now be distorted especially when the fullerene is C60. By analogy with the properties of empty fullerene and endohedral metallofullerene complexes, end points exist for the equivalent number of C2 units that can be expelled to give a closed-structure product that contains either one or two noble-gas atoms. These end points correlate directly with the sizes of the fullerene and the noble-gas atom(s), giving support to our conclusion that endohedral complexes containing two noble gases can be prepared.

Threshold and channel-coupling effects in positron elastic scattering from noble gases

We examine angular-distribution measurements of Kauppila et al and others of positrons elastically scattered from noble-gas targets at energies near the positronium (Ps) threshold. We use an R-matrix theory approach in which the R-matrix elements are used as fitting parameters. An attempt is made to find a set of parameters also consistent with the energy dependence of the (angle-integrated) elastic and Ps-formation cross sections. We show that close to threshold, both below and above, small differences between the experimental angular distributions and calculations by McEachran et al of differential cross sections, which neglect the Ps channel, are due to coupling to the Ps channel. In accordance with calculations of McAlinden and Walters, we find evidence for the dominance of channel coupling near the Ps threshold between the (incoming, outgoing) orbital angular momentum channels (1,0) and (2,1), respectively.