Solid neon moderated electrostatic or magnetic positron beam

A high intensity variable energy positron beam has been constructed at Brookhaven National Laboratory (BNL). Positrons from a 97 mCi²²Na source are moderated by a thin layer of solid neon. A magnetic guiding system delivered up to 5×10⁶ e⁺/sec to an experiment. Currently, tests are under way to facilitate the operation with the magnetic guiding system or—for other experimental sites—with an electrostatic beam transport. The electrostatic lenses are fabricated from μ-metal. No compensation of the earth's magnetic field is required. Several experiments can utilize the beam on a time-sharing basis. In the near future, the source will be replaced by⁶⁴Cu, which has a much higher activity. Beam intensities up to 10⁹ e⁺/sec are expected.     

Laser cooling of a small number of Be+ ions in a penning trap

Be⁺ ions stored in a Penning trap were cooled by a laser beam perpendicular to the magnetic field. The cooled ions are strongly coupled and phase transitions of up to 100 ions were observed. In experiments with only a few ions stored in the trap, a stepwise decrease in fluorescence intensity was observed. All steps are of the same size and so every step is attributed to a single ion. The discrete changes in fluorescence occurred more frequently when the background pressure was increased, caused by collisions between stored ions and background neutral molecules.     

Decay reactions of rare gas cluster ions: Kinetic energy release distributions and binding energies

We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar ₅ ⁺ to Ar ₁₅ ⁺, it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions. Received 20 November 2000     

Two-pion production in proton-proton collisions with a polarized beam

The two-pion production reaction pp → ppπ⁺π⁻ was measured with a polarized proton beam at T _p ≈ 750 and 800MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed-pulse technique for Quirl and central calorimeter provided positive π⁺ identification in addition to the standard particle identification, energy determination as well as time-of-flight and angle measurements. Thus all four-momenta of the emerging particles could be determined with 1–4 overconstraints. Total and differential cross-sections as well as angular distributions of the vector analyzing power have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find significant analyzing-power values up to A _y = 0.3. The data taken in the energy region of the excitation of the Roper resonance confirm that its dominant ππ decay channel is N ^* → Nσ.     

A high-resolution time-of-flight mass spectrometer for the detection of ultracold molecules

We have realized a high-resolution time-of-flight mass spectrometer combined with a magneto-optical trap. The spectrometer enables excellent optical access to the trapped atomic cloud using specifically devised acceleration and deflection electrodes. The ions are extracted along a laser beam axis and deflected onto an off-axis detector. The setup is applied to detect atoms and molecules photoassociated from ultracold atoms. The detection is based on resonance-enhanced multi-photon ionization. Mass resolution up to m/Δm_rms=1000 at the mass of ¹³³Cs is achieved. The performance of this spectrometer is demonstrated in the detection of photoassociated ultracold ⁷Li¹³³Cs molecules near a large signal of ¹³³Cs ions. PACS 07.75.+h; 32.80.Rm; 37.10.Gh     

Search for tensor couplings in the weak interaction

The existence of exotic couplings in the weak interaction can be tested by a precise measurement of the β-ν angular correlation parameter a in nuclear beta decay. In the case of ⁶He, the ratio of tensor and axial-vector couplings can be determined. The goal of the LPC Trap experiment is to improve the limits on the existence of tensor currents. The LPCTrap setup is installed on the low energy beam line LIRAT of the SPIRAL/GANIL facility. The ⁶He⁺ ions are confined in a novel transparent Paul trap allowing the detection in coincidence of both the β-particle and the recoil ion. The detection setup enables to detect the position and energy of these two particles. The a parameter can then be extracted from the time of flight spectrum of the recoil ions. The first ⁶He⁺ decay events were observed in May 2005 during a commissioning run. Over 10⁵ coincidence events have been recorded during a second run in July 2006.     

Direct measurement of lifetimes of infrared multiple-photon dissociated molecules

A beam of mass selected SF ₅ ⁺ ions is crossed with a pulsed CO₂ laser beam. The distribution of lifetimes of infrared multiple-photon pumped SF ₅ ⁺ ion-molecules prior to dissociation into SF ₄ ⁺ +F has been measured directly using a time-of-flight ion-beam technique. The data provide new information on the distribution of the total internal energy of molecules excited by absorption of infrared radiation.     

Determination and analysis of opiates in complex mixtures by surface-ionization mass spectrometry

The results of studies of the analytical opportunities of surface-ionization mass spectrometry (SIMS) for high-sensitivity and selective detection and analysis of opiate mixtures—natural opium, crude heroin, and narcotic analgesic omnopon—are presented. The experiments were carried out using an MX-1320 chemical mass spectrometer modernized for studying surface ionization (SI). It was ascertained that the opiate mixtures are ionized by highly efficient surface ionization. The bands of M⁺ (for papaverine), (M-H)⁺, (M-H-2nH)⁺, (M-R)⁺, and (M-R-2nH)⁺ ions, where M is a molecule, H is a hydrogen atom, R is a radical, are observed in the mass spectra; they are the sum of the SI mass spectra of components of the mixtures. The series of bands of ions with m/z of 144 and 146 that is characteristic of SI-morphinelike molecules is the main one in the mass spectrum of crude heroin, while for omnopon and natural opium this series is the main at low temperatures of a thermionic emitter (up to ∼900 K). At high temperatures of the thermionic emitter, the band series with m/z of 218 and 220 is the main series. Studies of natural opium, crude heroin, and omnopon have shown that the SIMS method allows analyzing mixtures without preliminary chromatographic separation thereof. The study also contains the results of comparative analyses of opiate mixtures by SIMS and chromate- and mass spectrometry (HP-6890) with electronic ionization.     

Frequency measurements in the b <Superscript>3</Superscript>Π(0<Subscript>u</Subscript><Superscript>+</Superscript>) - X <Superscript>1</Superscript>Σ<Subscript>g</Subscript><Superscript>+</Superscript> system of K<Subscript>2</Subscript>

Absolute transition frequencies of the b ³Π(0_u ⁺) - X ¹Σ_g ⁺ system of K₂ were measured in a molecular beam with Lamb dip absorption spectroscopy applying a frequency comb from a femtosecond pulsed laser. Both, K atoms and K₂ molecules are present in the beam and are expected to interact by collisions. The atoms can be deflected optically out of the beam, and thus the collision rate between K atoms and K₂ molecules is changed by about an order of magnitude. The molecular transition frequencies for low collisional rate are compared with those for high one. Limits for the collisional frequency shift within the beam are determined.     

A New High-intensity, Low-momentum Muon Beam for the Generation of Low-energy Muons at PSI

At the Paul Scherrer Institute (PSI, Villigen, Switzerland) a new high-intensity muon beam line with momentum p < 40 MeV/c is currently being commissioned. The beam line is especially designed to serve the needs of the low-energy, polarized positive muon source (LE-μ⁺) and LE-μ SR spectrometer at PSI. The beam line replaces the existing μ E4 muon decay channel. A large acceptance is accomplished by installing two solenoidal magnetic lenses close to the muon production target E that is hit by the 590-MeV PSI proton beam. The muons are then transported by standard large aperture quadrupoles and bending magnets to the experiment. Several slit systems and an electrostatic separator allow the control of beam shape, momentum spread, and to reduce the background due to beam positrons or electrons. Particle intensities of up to 3.5 × 10⁸ μ⁺/s and 10⁷ μ⁻/s are expected at 28 MeV/c beam momentum and 1.8 mA proton beam current. This will translate into a LE-μ⁺ rate of 7,000/s being available at the LE-μ SR spectrometer, thus achieving μ⁺ fluxes, that are comparable to standard μ SR facilities.     

Accelerator mass spectrometry of the heaviest long-lived radionuclides with a 3-MV tandem accelerator

A 3-MV pelletron tandem accelerator is the heart of the Vienna environmental research accelerator (VERA). The original design of the beam transport components allows the transport of ions of all elements, from the lightest to the heaviest. For light ions the suppression of neighboring masses was sufficient to measure isotopic ratios of ¹⁴C/¹²C and ²⁶Al/²⁷Al as low as 10⁻¹⁵ and ¹⁰Be/⁹Be down to 10⁻¹³. To suppress neighboring masses for the heaviest radionuclides in the energy range of 10–20 MeV, the resolution of VERA was increased both by improving the ion optics of existing elements at the injection side and by installing a new high-resolution electrostatic separator at the high-energy side. Interfering ions which pass all beam filters are identified with a Bragg-type ionization detector and a high-resolution time-of-flight system. Two ultra-thin diamond-like carbon (DLC) foils are used in the start and stop detector, which substantially reduces losses due to beam straggling. This improved set up enables us to measure even the heaviest long-lived radionuclides, where stable isobaric interferences are absent (e.g. ²³⁶U and ²⁴⁴Pu), down to environmental levels. Moreover, the advantage of a ‘small’ and well manageable machine like VERA lies in its higher stability and reliability which allows to measure these heavy radionuclides more accurately, and also a large number of samples.     

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions C_nHm ⁺ (n =2-22) upon highly charged Xe^q+ (q =20-44) impact on C₈₄ surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn ⁺ is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models. Received 4 February 2000     

Production of electron-positron pairs by a photon propagating in a strongly magnetized thermal bath

The production of electron-positron pairs by a photon propagating in a thermal bath in both zero and strong (B ≫ 4.41 × 10¹³ G) magnetic fields has been considered. The mean free path has been calculated for the high-energy photon propagating through a thermodynamically equilibrium photon gas along the magnetic field lines so that the γ → e ⁻ e ⁺ decay is kinematically forbidden. It has been shown that the strong magnetic field suppresses the probability of the γγ′ → e ⁻ e ⁺ process. The analyzed process can be useful for analysis of possible mechanisms of the generation of the e ⁻ e ⁺ plasma in the regions of the polar caps of magnetars.     

Magnetic moment measurements of picosecond states: New results and open problems

The temperature dependence of the precession of the angular correlation of decay gamma rays from swift¹⁵⁰Sm (2 ₁ ⁺ ) ions traversing a gadolinium foil has been found to be proportional to the foil magnetization, supporting the assertion that the transient hyperfine magnetic field acting on these ions is proportional to the magnetization of the hosts (iron or gadolinium). Similar experiments on¹⁹⁴Pt (2 ₁ ⁺ ) ions traversing iron and gadolinium foils are consistent with both the magnetic moment obtained from Rutgers experiments on iron and with a hyperfine field at Pt ions larger for gadolinium than for iron foils, in agreement with the Chalk River parametrization for heavy nuclei traversing gadolinium foils. Finally, the magnetic moments of the 2 ₁ ⁺ states in^144–150Nd,^145,150Sm and¹⁵²Gd have been measured. These data support the evidence of shell closure atZ=64 forN≤88 andZ=50 forN>90.     

Systematic field-theory for the hard-core one-component plasma

A continuous liquid flow in a vacuum (a liquid beam) of an aqueous solution of adenine salt containing hydrochloric acid or sodium hydroxide was irradiated with an intense pulsed IR laser at 3 μm, which is resonant to a vibrational mode related to the OH stretch vibration of H₂O. Neutral species isolated into the vacuum were ionized by a pulsed UV laser at 270 nm, and the product ions were mass-analyzed by a time-of-flight mass spectrometer. It is found that AH ₂ ^{2 + .} 2Cl^- and [A-iH] ^{i - .} iNa⁺ (i = 1-3) are isolated in the vacuum from the aqueous acidic and alkaline solutions, respectively, under irradiation of the IR laser, and undergo four-photon ionization involving decomposition and proton transfer of the intermediate species under irradiation of the UV laser. Received 1st May 2002 Published online 13 September 2002     

Orientation of Cs 6p 2P3/2 atomic photofragments in a magnetic field: interference in the photodissociation of diatomic molecules

In a magnetic field, Cs₂ molecules were excited from the ground X¹Σ⁺_g(v_X=0,J_X=55) level to the D ¹Σ⁺_u(v=46,J=54) level by dissociation laser light linearly polarized parallel to the field, for which the magnetic sublevels were degenerated and thus all the transitions between them were simultaneously stimulated. Probe laser light excited the dissociated Cs 6p²P_3/2 atomic fragments to 6p²D_3/2 level and the resultant 6p²P_1/2 - 6d²D_3/2 emission was detected as the function of the wavelength of the probe light. The populations of the 6p²P_3/2,mj magnetic sublevels were determined from the relative strengths of the 6p²P_3/2,mj - 6d²D_3/2,m'j transitions induced by the probe light. Non-zero orientation O₀ was found in the ensemble of dissociated Cs 6p²P_3/2 atomic fragments. The orientation O₀ increased as the magnetic field strength increased. It was demonstrated both experimentally and theoretically that the orientation O₀ was induced through the interference in the excitation and dissociation paths in the presence of an external magnetic field, even when all degenerated transitions between the magnetic sublevels of the molecules are simultaneously excited by the light linearly polarized parallel to the field.     

A massive photon in a strong magnetic field: Generation and decay channels

The conditions for the formation of an effective photon mass in a strong magnetic field are considered. It is shown that massless photons are sterile. The channels γ* → e⁺e⁻ and γ* → v_iv_i for the decay of a massive photon and their possible contribution to the mechanism of energy losses by collapsing astrophysical objects are discussed. Moscow State Industrial University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 35–39, February, 1997.     

Fission and gamma-ray decay of the238U shape isomer

The decay of the²³⁸U superdeformed shape isomer has been reinvestigated by detecting for the first time simultaneously the fission and the gamma-back decay. An electrostatic deflection system has been used to transport the^238mU recoils, produced in a²³⁸U(d, pn) reaction with a pulsed beam of 18 MeV deuterons, in front of a detector set-up consisting of three ion-implanted solidstate detectors and a Ge(Li) gamma-detector. The gamma-back decay has been measured in coincidence with conversion electrons of the 2⁺ 0⁺ transition deexciting the first rotational state in²³⁸U. Two gamma-transitions of 2.513 MeV and 1.878 MeV have been observed with half-lives consistent with the result obtained for the decay by delayed fissionT _1/2=(298±18) ns.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Multiexponential decay of negative molecular ions as a result of target molecule distribution over vibrational states

The lifetimes of long-lived negative molecular ions SF₆⁻-, C₆H₅NO₂⁻, and C₆F₆⁻ are measured with a static mass spectrometer. A great spread in published data for the lifetimes of these ions is explained using a concept of multiexponential decay of molecular ions. The influence of the Boltzmann distribution of neutral target molecules over vibrational states on the lifetime of negative molecular ions is studied in terms of the Illenberger-Smirnov-Kompaneits simple statistical model. It is shown that this distribution has a profound effect on the multiexponential decay of molecular anions and, as a consequence, on the lifetime of negative molecular ions measured on different mass spectrometers.