Recoil Ions Research Articles

Ellipticity-dependent sequential over-barrier ionization of cold rubidium

We perform high-resolution measurements of momentum distribution on Rb$^{n+}$ recoil ions up to charge state $n=4$, where laser-cooled rubidium atoms are ionized by femtosecond elliptically polarized lasers with the pulse duration of 35 fs and the intensity of 3.3$\times$10$^{15}$ W/cm$^2$ in the over-barrier ionization (OBI) regime. The momentum distributions of the recoil ions are found to exhibit multi-band structures as the ellipticity varies from the linear to circular polarizations. The origin of these band structures can be explained quantitatively by the classical OBI model and dedicated classical trajectory Monte Carlo simulations with Heisenberg potential. Specifically, with back analysis of the classical trajectories, we reveal the ionization time and the OBI geometry of the sequentially released electrons, disentangling the mechanisms behind the tilted angle of the band structures. These results indicate that the classical treatment can describe the strong-field multiple ionization processes of alkali atoms.

Production of Bh266 in the Cm248(Na23,5n)Bh266 reaction and its decay properties

The nuclide Bh266 was produced in the Cm248(Na23,5n)Bh266 reaction at beam energies of 125.9, 130.6, and 135.3 MeV. Decay properties of Bh266 were investigated with a rotating wheel apparatus for α and spontaneous fission (SF) spectrometry under low background conditions attained by a gas-jet transport system coupled to the RIKEN gas-filled recoil ion separator. Based on genetically correlated α−α and α-SF decay chains, a total of 23 chains were assigned to Bh266 and its daughter nuclide Db262 and granddaughter Lr258. The half-life of Bh266 was measured to be T1/122=10.0−1.7+2.6s which is an order of magnitude longer than the literature data. The α-particle energies of Bh266 disperse widely in the range of Eα=8.62–9.40MeV. The maximum production cross section for the Cm248(Na23,5n)Bh266 reaction was determined to be σ=57±14 pb at 130.6 MeV, whereas the upper limit for the Cm248(Na23,4n)Bh267 reaction was σ≤14 pb at 121.2 MeV. These cross sections are discussed by comparing with the literature data as well as the theoretical calculations.1 MoreReceived 11 June 2020Accepted 30 July 2020DOI:https://doi.org/10.1103/PhysRevC.102.024625©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayFissionLow & intermediate energy heavy-ion reactionsNuclear fusionNuclear reactionsPropertiesA ≥ 220Nuclear Physics

Angular emission distribution of O 1s photoelectrons of uniaxially oriented methanol

The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hν = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3–OH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide.

Laser-induced nonsequential double ionization of helium: classical model calculations

We have investigated the double ionization of helium induced by 45 fs, 394 nm linearly polarized laser pulses for intensities (2.5–150) × 1014 W cm−2. The time evolution of the process was determined by applying the classical ensemble approximation (CEA) model. The results of the calculations obtained for the double-to-single ionization ratio, the momentum distribution of the He2+ recoil ion and the correlated two-electron momentum distribution are compared with the recent experimental data of Henrichs et al (2018 Phys. Rev. A 98 043405). The standard CEA model failed to reproduce the basic features of the experiment. A considerable improvement in the description of the experimental data was achieved by including a two-step process in the model: the formation of a transient double excited state in the first step, and the simultaneous release of the two electrons via ‘over the barrier ionization’ in the second step.

Fragmentation of C2H42+ produced by 300 eV electron impact

The fragmentation of C2H42+ produced by 300 eV electron impact has been investigated. Both 2-body and 3-body fragmentation channels were observed with an ion–ion coincidence technique. Using the recoil ion momentum spectroscopy, the momentum vector of each ionic fragment was measured and the kinetic energy release (KER) distributions for different fragmentation channels were obtained. For 2-body fragmentation channels, the branching ratios and the peak values of the KER distributions were compared with previous photon ionization experiments. The electronic state information of the parent ion C2H42+ was inferred by comparing the present measured KER values and the predictions in previous works. For 3-body fragmentation channels, the dissociation mechanisms were analyzed according to the signature on the Newton diagram.

Ion Recoil from Photon Beam Observed

Ion Recoil from Photon Beam Observed

Development of an Experimentally Validated MCNP6 Model for 11C Production via the 14N(p,α) Reaction Using a GE PETtrace Cyclotron

The General Electric (GE®) PETtrace 860 cyclotron at the Missouri University Research Reactor (MURR) facility is used extensively for medical and research radioisotope production. However, no model exists of its performance, and the proton beam’s energy and spatial distribution are unmeasured. Here, an MCNP6 model was developed to improve upon the performance of the cyclotron target systems that are routinely utilized for research and medical radioisotope production. Since the cyclotron beam energy and profile have a significant impact on the efficiency and character of radioisotope production, the MURR cyclotron proton beam energy was measured using high-purity copper stacked foil activation to be 14.6 ± 0.2 MeV, a significant reduction from the stated 16.4 MeV. Phosphor plate imaging was also used to radiographically image the distribution of radioisotope production within the copper foils and characterize the beam spatial and intensity profile. Total target activity was quantified by trapping the 11C on a solid adsorbent and measuring the amount in an ion chamber. Effective target densities were calculated for irradiations conducted with beam currents between 5 and 40 μA. The measured beam and target characteristics were used to develop an MCNP6 model of 11C production. Through use of the model, it was determined that the targets were, at most, 41% efficient as a thick target design resulting in up to 11.80-MeV average protons impinging on the target walls leading to potential contamination from hot ion recoils.

Momentum Spectroscopy for Multiple Ionization of Cold Rubidium in the Elliptically Polarized Laser Field**Supported by the National Natural Science Foundation of China (Grant Nos. 11827806, 11874368, 61675213, 11822401, and 11674034).

Employing recently developed magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) combined with cold atoms, strong laser pulse, and ultrafast technologies, we study momentum distributions of the multiply ionized cold rubidium (Rb) induced by the elliptically polarized laser pulses (35 fs, 1.3 × 1015 W/cm2). The complete vector momenta of Rbn+ ions up to charge state n = 4 are recorded with extremely high resolution (0.12 a.u. for Rb+). Variations of characteristic multi-bands are displayed in momentum distributions because the ellipticity varies from the linear to circular polarization, are interpreted qualitatively with the classical over-barrier ionization model. Present momentum spectroscopy of cold heavy alkali atoms presents novel strong-field phenomena beyond the noble gases.

Proton range verification with ultrasound imaging using injectable radiation sensitive nanodroplets: a feasibility study

Technologies enabling in vivo range verification during proton therapy are actively sought as a means to reduce the clinical safety margins currently adopted to avoid tumor underdosage. In this contribution, we applied the semi-empirical theory of radiation-induced vaporization of superheated liquids to coated nanodroplets. Nanodroplets are injectable phase-change contrast agents that can vaporize into highly echogenic microbubbles to provide contrast in ultrasound images. We exposed nanodroplet dispersions in aqueous phantoms to monoenergetic proton beams of varying energies and doses. Ultrasound imaging of the phantoms revealed that radiation-induced droplet vaporization occurred in regions proximal to the proton Bragg peak. A statistically significant increase in contrast was observed in irradiated regions for doses as low as 2 Gy and found to be proportional to the proton fluence. The absence of enhanced response in the vicinity of the Bragg peak, combined with theoretical considerations, suggest that droplet vaporization is induced by high linear energy transfer (LET) recoil ions produced by nuclear reactions with incoming protons. Vaporization profiles were compared to non-elastic cross sections and LET characteristics of oxygen recoils. Shifts between the ultrasound image contrast drop and the expected proton range showed a sub-millimeter reproducibility. These early findings confirm the potential of superheated nanodroplets as a novel tool for proton range verification.

Diversifying beam species through decay and recapture ion trapping: a demonstrative experiment at TITAN-EBIT

Recapturing the recoiling daughters from radioactive decay can be a simple way to diversify beam availability at rare isotope beam facilities. In the decay and recapture ion trapping (DRIT) technique, a parent species is stored in an ion trap and left to decay, and the daughter ions are recaptured by the trap and become available for use. We successfully demonstrated the technique using the electron beam ion trap (EBIT) at the TITAN facility. A pure cloud of 30Mg ions was stored in the EBIT for about one half-life and sent to a Penning trap mass spectrometer, which confirmed the production of 30Al daughter ions. Systematic measurements and simulations suggest high recapture efficiencies of the recoil ion and little influence of the recoiling energy in the observed losses. With the secondary beam, we also performed precision mass measurements of the parent 30Mg8+ and the daughter 30Al11+ ions. Our results agree with the literature and improve its precision. The success of this experiment shows that EBITs can produce high-quality beams through the DRIT technique.

Decay microscope for trapped neon isotopes

We review the design, simulation, and tests, of a detection system for measuring the energy distribution of daughter nuclei recoiling from the beta-decay of laser trapped neon isotopes. This distribution is sensitive to several new physics effects in the weak sector. Our `decay microscope' relies on imaging the velocity distribution of high energy recoil ions in coincidence with electrons shaken-off in the decay. We demonstrate by way of Monte-Carlo simulation, that the nuclear microscope increases the statistical sensitivity of kinematic measurements to the underlying energy distribution, and limits the main systematic bias caused by discrepancy in the trap position along the detection axis.

β-delayed-neutron studies ofSb135,136andI140performed with trapped ions

Beta-delayed-neutron ($\beta$n) spectroscopy was performed using the Beta-decay Paul Trap and an array of radiation detectors. The $\beta$n branching ratios and energy spectra for $^{135,136}$Sb and $^{140}$I were obtained by measuring the time of flight of recoil ions emerging from the trapped ion cloud. These nuclei are located at the edge of an isotopic region identified as having $\beta$n branching ratios that impact the r-process abundance pattern around the A~130 peak. For $^{135,136}$Sb and $^{140}$I, $\beta$n branching ratios of 14.6(11)%, 17.6(28)%, and 7.6(28)% were determined, respectively. The $\beta$n energy spectra obtained for $^{135}$Sb and $^{140}$I are compared with results from direct neutron measurements, and the $\beta$n energy spectrum for $^{136}$Sb has been measured for the first time.

Orientation-dependent dissociative ionization of H2 in strong elliptic laser fields: Modification of the release time through molecular orientation

We investigate the photoelectron angular emission distributions obtained by strong field dissociative ionization of H$_2$ using cold target recoil ion momentum spectroscopy. In case of employing laser light with an ellipticity close to 0.9 and an intensity of 1.0 $\times$ 10$^{14}$ W/cm$^2$, we find that the most probable release-time of the electron does not generally coincide with the time when the laser field maximizes. The release-time is affected by the molecular orientation. In addition, we observe that the width of the release-time distribution depends on molecular orientation. We attribute this observation to the two-center-interference.

Development of a recoil ion source providing slow Th ions including 229(m)Th in a broad charge state distribution

Ions of the isomer 229mTh are a topic of high interest for the construction of a “nuclear clock” and in the field of fundamental physics for testing symmetries of nature. They can be efficiently captured in Paul traps which are ideal for performing high precision quantum logic spectroscopy. Trapping and identification of long-lived 232Th+ ions from a laser ablation source was already demonstrated by the TACTICa collaboration on Trapping And Cooling of Thorium Ions with Calcium. The 229mTh is most easily accessible as α-decay daughter of the decay of 233U. We report on the development of a source for slow Th ions, including 229mTh for the TACTICa experiment. The 229mTh source is currently under construction and comprises a 233U monolayer, from which 229mTh ions recoil. These are decelerated in an electric field. Conservation of the full initial charge state distribution of the 229mTh recoil ions is one of the unique features of this source. We present ion-flight simulations for our adopted layout and give a final source design. This source will provide Th ions in their original charge state at energies suitable for capture in a linear Paul trap for spectroscopy investigations.

Study of Quasielastic Barrier Distributions as a Step towards the Synthesis of Superheavy Elements with Hot Fusion Reactions.

The excitation functions for quasielastic scattering of ^{22}Ne+^{248}Cm, ^{26}Mg+^{248}Cm, and ^{48}Ca+^{238}U are measured using a gas-filled recoil ion separator. The quasielastic barrier distributions are extracted for these systems and are compared with coupled-channel calculations. The results indicate that the barrier distribution is affected dominantly by deformation of the actinide target nuclei, but also by vibrational or rotational excitations of the projectile nuclei, as well as neutron transfer processes before capture. From a comparison between the experimental barrier distributions and the evaporation residue cross sections for Sg (Z=106), Hs (108), Cn (112), and Lv (116), it is suggested that the hot fusion reactions take advantage of a compact collision, where the projectile approaches along the short axis of a prolately deformed nucleus. A new method is proposed to estimate the optimum incident energy to synthesize unknown superheavy nuclei using the barrier distribution.

Carbon nanotubes as anisotropic target for dark matter

Directional detection of Dark Matter (DM) particles could be accomplished by studying either ion or electron recoils in large arrays of parallel carbon nanotubes (CNT). For instance, a MeV mass DM particle could scatter off a lattice electron, resulting in the transfer of sufficient energy to eject the electron from the CNT surface. The electron can eventually be detected whenever an external electric field is added to drive it from the open ends of the array. This detection scheme would offer an anisotropic response and could be used to select an orientation of the target with respect to the DM wind. A compact sensor, in which the cathode element is substituted with a dense array of parallel CNT, could serve as the basic detection unit which - if adequately replicated - would allow to explore a significant region of light DM mass and cross-section. A similar detection scheme could be used to detect DM particles with mass in the GeV range scattering off the surface of a CNT and ejecting a carbon ion. We report about the Monte Carlo simulations of such a system and the R&D towards a detector prototype.

Angular distribution in two-body dissociation of

SynopsisThe two-body dissociation of multiply charged SO2 molecules is studied using a recoil ion momentum spectrometer. These molecular ions were generated in collision of the neutral parent with an energetic highly charged ion. From the measured momenta of the recoil ions, angular distributions are generated with respect to the incoming beam direction and are found to vary strongly with kinetic energy release.

State-selective single electron capture in slow O 6+-He collisions

SynopsisThe state-selective single-electron capture processes were measured by cold-target recoil-ion momentum spectroscopy in O 6+-He collisions between 18keV and 96keV. From the longitudinal momentum spectrum of recoil ions, the single electron captured into n=3 states of projectile ion is predominant and n=4 states have a small contribution. The oscillation structure of angular differential cross sections was observed. The present experimental results are compared with other experiments at low energy and the predictions of the molecular Columbic barrier model.

Coincident detection of recoil ions and photoelectrons after multiphoton-ionisation of cold 6Li atoms trapped in a dipole trap

SynopsisWe report on performing high resolution recoil ion momentum spectroscopy in a reaction microscope (ReMi) on a target of dipole-force trapped cold 6Li atoms in a non-interacting |1〉-|2〉 spin mixture and coincident detection of recoil ions and photoelectrons. Photo-ionization occurs from the electronic ground state of the 6Li atoms and IR photons significantly contribute to the ion yield via 3-photon resonances. High resolution time-of-flight spectra are obtained by modulating the trap depth synced with the arrival of the fs laser pulses.

Three body dissociation of

SynopsisMultiply charged ion impact induced three body fragmentation pathways of (q=3, 4) molecular ions have been studied using the technique of recoil ion momentum spectroscopy. An attempt has been made to distinguish the three-body breakup based on mechanism of dissociation, whether sequential or concerted.