Fundamental symmetries studies with cold trapped francium atoms at ISAC

Francium combines a heavy nucleus (Z = 87) with the simple atomic structure of alkalis and is a very promising candidate for precision tests of fundamental symmetries such as atomic parity non-conservation measurements. Fr has no stable isotopes, and the ISAC radioactive beam facility at TRIUMF, equipped with an actinide target, promises to provide record quantities of Fr atoms, up to 10¹⁰/s for some isotopes. We discuss our plans for a Fr on-line laser trapping facility at ISAC and experiments with samples of cold Fr atoms. We outline our plans for a measurement of the nuclear anapole moment – a parity non-conserving, time-reversal conserving moment that arises from weak interactions between nucleons – in a chain of Fr isotopes. Its measurement is a unique probe for neutral weak interactions inside the nucleus.      

Interaction of frequency modulated light pulses with rubidium atoms in a magneto-optical trap

The spatial displacement of the ⁸⁵Rb atoms in a Magneto-Optical Trap (MOT) under the influence of series of frequency modulated light pulse pairs propagating opposite to each other is measured as a function of the time elapsed after the start of the pulse train, and compared with the results of simulations. Adiabatic excitation and consecutive de-excitation take place between the ground 5²S_1/2 (F=3) and the 5²P_3/2 (F'=2, 3, 4) excited levels as the result of the interaction. The displacement of the ⁸⁵Rb atoms is calculated as the solution of simple equation of motion where the expelling force is that arising from the action of the frequency modulated light pulses. The restoring and friction forces of the MOT are taken into account also. The system of Bloch equations for the density matrix elements is solved numerically for transitions between six working hyperfine levels of the atom interacting with the sequence of the frequency modulated laser pulses. According to these simulations, the momentum transferred by one pulse pair is always smaller than the expected 2ħk, (1) where ħ is the Plank constant and k=2π/λ where λ is the wavelength, (2) having a maximum value in a restricted region of variation of the laser pulse peak intensity and the chirp.     

An atom faucet

We present a simple and efficient source of slow atoms. From a background vapour loaded magneto-optical trap (MOT), a thin laser beam extracts a continuous jet of cold rubidium atoms. The jet that is typical to leaking MOT systems is created without any optical parts placed inside the vacuum chamber. We also present a simple three dimensional numerical simulation of the atomic motion in the presence of these multiple saturating laser fields combined with the inhomogeneous magnetic field of the MOT. At a pressure of P _Rb87 = 10^-8 mbar and with a moderate laser power of 10 mW per beam, we generate a flux Φ = 1.3×10⁸ atoms/s with a mean velocity of 14 m/s and a divergence of 10 mrad. Received 13 January 2001     

Acceleration of cold Rb atoms by frequency modulated light pulses

The displacement of Rb atoms in a magneto-optical trap (MOT) caused by the force of a finite time series of counter-propagating frequency modulated light pulse pairs is measured as a function of the chirp of the pulses. The frequency modulated light pulses induced ⁸⁵Rb 5²S_1/2 F=3 ↔ ⁸⁵Rb 5²P_3/2 F'=2, 3, 4 excitation and de-excitation of the atoms. The result of this excitation de-excitation process is a force causing the acceleration and, consequently, the displacement of the maximum of the spatial distribution of the trap atoms. The time dependence of the populations of the levels of the atom are calculated — including also the ⁸⁵Rb 5²S_1/2 F=2 and F'=1 states — as the result of the interaction with the finite train of counter propagating frequency modulated light pulses by the solution of the Bloch equations. As the result of the measurement the interval of the chirp of the frequency modulated light of given intensity where the transitions take place, are determined. The results of the experiment and the expectation on the basis of model calculations are in qualitative agreement.     

Release time calculations for the SPES direct UCx target

A Direct Target for a mid-term RIB ISOL-type facility is being developed at LNL, in the framework of the R&D for the SPES project [1]. Using a 40 MeV proton beam impinging on a UCx thick target of 2.5 g/cm³ density, a production rate of 10¹³ in target fissions per second is expected [2]. The crucial point, when short-lived isotopes are produced in the target, is to build systems (target + ion source) with good release properties and high efficiency. Monte Carlo simulations were performed using the GEANT4 toolkit [3] and the RIBO code [4] in order to optimize our target geometry and to estimate the average release time.     

Beta-decay experiments of neutral atoms in a magneto-optical trap

Magneto optical traps (MOT) allow the cooling and storing of neutral atoms in a volume of a few cubic millimeters by use of laser beams and a magnetic field. Such devices offer new and exciting opportunities for precision measurements of radioactive isotopes. Here we present experiments performed with a double-MOT system coupled to the on-line separator TISOL at TRIUMF/Vancouver, Canada. For the first time, the Β-decay of free atoms stored in such a device could be observed. We report on coincidence measurements between beta-particles and the argon recoils in the decay of ³⁷K and ³⁸rm{m}}K. The charge state ratios of the recoil-ions were deduced by Time-Of-Flight separation in an acceleration field. The final goal of those investigations is a precision test of the Standard Model by measuring the –nu-correlation parameter a. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magneto-optical trap and mass-separator system for the ultra-sensitive detection of <Superscript>135</Superscript>Cs and <Superscript>137</Superscript>Cs

We report the first magneto-optical trapping of radioactive ¹³⁵Cs and ¹³⁷Cs and a promising means for detecting these isotopes at ultra-sensitive levels by coupling a magneto-optical trap (MOT) to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass-separated, and implanted in a Zr foil within the trapping cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large-diameter beams and a dry-film-coated cell to achieve high trapping efficiency. Trapped-atom numbers in the case of either isotope ranged from 10⁴ to 10⁷, as determined from the MOT fluorescence signal. Over this trapped-atom range, the MOT fluorescence signal was found to increase linearly with the number of atoms implanted in the foil and without isotopic bias to within 4%. In principle, this method can then provide a measurement of the ¹³⁷Cs/¹³⁵Cs ratio accurate to within 4% through the direct ratio of MOT fluorescence signals. The fluorescence signal from stable ¹³³Cs, when implanted and released from the foil, was suppressed relative to MOT signals by more than seven orders of magnitude when the system was tuned to trap ¹³⁵Cs or ¹³⁷Cs. When combined with the isotopic selectivity of ≥10⁵ for the mass separator, the overall suppression of ¹³³Cs is expected to exceed 10¹². At present our system delivers atoms from sample to MOT with an efficiency of 0.5%, has a trapped-atom detection limit of 4000 atoms, and achieves a sample-detection sensitivity of one million atoms. Received: 23 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-505/667-0440, E-mail: mdd@lanl.gov     

Broad-area diode-laser system for a rubidium Bose-Einstein condensation experiment

We report on master-oscillator power amplification using a broad-area laser diode (BAL) emitting at a wavelength of λ =780 nm. The master oscillator is an injection-locked single-mode diode laser delivering a seeding beam of 35 mW, which is amplified in double pass through the BAL up to 410 mW. After beam shaping and spatial filtering by a single-mode fibre we obtain a clean Gaussian beam with a maximum power of 160 mW. There is no detectable contribution of the BAL eigenmodes in the spectrum of the output light. This laser system is employed for operation of a ⁸⁷Rb magneto-optical trap (MOT) and for near-resonant absorption imaging in a Bose-Einstein condensation experiment. Received: 10 April 2000 / Revised version: 13 June 2000 / Published online: 2 August 2000     

Photoionization cross-sections of the first excited states of sodium and lithium in a magneto-optical trap

A two element magneto-optical trap (MOT) for Na and ⁷Li or ⁶Li is used to cool and trap each of them separately. A fraction of the cold atoms is maintained in the first ²P_3/2 excited state by the cooling laser. These excited state atoms are ionized by laser light in the near-UV region, giving rise to a smaller number of trapped atoms and to different loading parameters. Photoionization cross-sections were derived out of these data. They are in reasonable agreement with data previously obtained using thermal samples and with theoretical predictions. Received 21 March 2001 and Received in final form 3 August 2001     

Lineshapes of the even mp1/2 5n(p′/f′) autoionizing resonances of Ar, Kr and Xe

The even parity mp_1/2 ⁵np^′ and mp_1/2 ⁵nf^′ autoionizing resonances of Ar, Kr, and Xe (m=3,4,5) were investigated experimentally and theoretically by one-photon excitation from lower-lying intermediate levels. In particular, high resolution measurements for the Ar(nf^′), Kr(12p^′,8f^′), and Xe(8p^′) resonances are reported; lineshape parameters for these resonances have been derived by a Fano-type analysis, thus yielding reduced resonance widths. The experimental spectra and the resonance parameters are compared with theoretical calculations which are based on the configuration interaction Pauli–Fock approach including core polarization. The measured and calculated lineshapes are in good agreement. In addition, theoretical predictions are presented for other resonances, which have not yet been observed experimentally, and some systematic trends are elucidated.     

Target ion source and charge exchange cell development for the EXCYT facility

The EXCYT facility at the INFN-LNS is based on a K-800 superconducting cyclotron delivering stable ion beams on a Target Ion Source (TIS) assembly to produce the required nuclear species, and on a 15 MV Tandem for post-accelerating the radioactive beams. For some ion beams such as for Li, the extraction efficiency from the TIS is higher when obtained by positive ionisation, while the injection into the Tandem is possible only after a charge exchange to obtain negative ions. In this work we present the procedures together with the results of the production of ^6,7,8,9Li beams extracted at EXCYT during the last year. The production of the radioactive elements was performed by sending a ¹³C⁴⁺ primary beam of 45 MeV/u on a graphite target. The ionisation of the production species was achieved by a tungsten positive surface ioniser. The Li⁺ has been extracted from TIS at different energies to cross-check the transmission and the charge exchange efficiency. To perform the conversion from positive to negative ions we employed a Charge Exchange Cell (CEC) containing Cs vapours. The Li beam interacts with the latter in a two-step reaction, thus converting its charge from +1 to –1. The CEC was already characterised during off-line tests; the results obtained at EXCYT confirmed both the isotopic shift effect and the efficiency values at several given extraction energies. Future improvements of the TIS and the CEC are discussed.     

A continuous cold atomic beam from a magneto-optical trap

We have developed and characterized a new method to produce a continuous beam of cold atoms from a standard vapour-cell magneto-optical trap (MOT). The experimental apparatus is very simple. Using a single laser beam it is possible to hollow out in the source MOT a direction of unbalanced radiation pressure along which cold atoms can be accelerated out of the trap. The transverse cooling process that takes place during the extraction reduces the beam divergence. The atomic beam is used to load a magneto-optical trap operating in an ultra-high vacuum environment. At a vapour pressure of 10^-8mbar in the loading cell, we have produced a continuous flux of 7×10⁷atoms/s at the recapture cell with a mean velocity of 14 m/s. A comparison of this method with a pulsed transfer scheme is presented. Received 19 February 2001     

Experimental study of vapor-cell magneto-optical traps for efficient trapping of radioactive atoms

We have studied magneto-optical traps (MOTs) for efficient on-line trapping of radioactive atoms. After discussing a model of the trapping process in a vapor cell and its efficiency, we present the results of detailed experimental studies on Rb MOTs. Three spherical cells of different sizes were used. These cells can be easily replaced, while keeping the rest of the apparatus unchanged: atomic sources, vacuum conditions, magnetic field gradients, sizes and power of the laser beams, detection system. By direct comparison, we find that the trapping efficiency only weakly depends on the MOT cell size. It is also found that the trapping efficiency of the MOT with the smallest cell, whose diameter is equal to the diameter of the trapping beams, is about 40% smaller than the efficiency of larger cells. Furthermore, we also demonstrate the importance of two factors: a long coated tube at the entrance of the MOT cell, used instead of a diaphragm; and the passivation with an alkali vapor of the coating on the cell walls, in order to minimize the losses of trappable atoms. These results guided us in the construction of an efficient large-diameter cell, which has been successfully employed for on-line trapping of Fr isotopes at INFN’s national laboratories in Legnaro, Italy.     

Dipole traps with mode-locked lasers

We investigate the properties of two separate dipole traps, realised using a cw mode-locked Ti:sapphire laser and a cw mode-locked Nd:YVO₄ laser, red-detuned by 25 nm and 284 nm, respectively. Approximately 10³ laser-cooled ⁸⁵Rb atoms were confined in the traps at ≈50 μK, with no observable heating after initial loading. The lifetimes of the traps were consistent with limitations imposed by wavelength-dependent photoassociation losses and collisions with background vapour. Determination of the ac Stark shift of the 780 nm cooling transition using a weak probe beam showed no observable difference between using narrow-bandwidth or mode-locked trapping light. Techniques for trapping and focusing of atoms based on the dipole force of blue and uv light become much more accessible through efficient doubling, tripling and quadrupling of mode-locked sources. This opens up the possibility of manipulating more technologically interesting species. Received: 30 September 1999 / Revised version: 21 December 1999 / Published online: 24 March 2000     

Production of a bose einstein condensate of metastable helium atoms

We recently observed a Bose-Einstein condensate in a dilute gas of ⁴He in the 2³S₁ metastable state. In this article, we describe the successive experimental steps which led to the Bose-Einstein transition at 4.7 μK: loading of a large number of atoms in a MOT, efficient transfer into a magnetic Ioffé-Pritchard trap, and optimization of the evaporative cooling ramp. Quantitative measurements are also given for the rates of elastic and inelastic collisions, both above and below the transition. Received 15 October 2001     

Detection by two-photon ionization and magnetic trapping of cold Rb2 triplet state molecules

We present detailed experimental spectra and accurate theoretical interpretation of resonance-enhanced two-photon ionization of ultracold rubidium molecules in the 14000–17000 cm^-1 transition energy range. The dimers are formed in a magneto-optical trap by photoassociation followed by radiative decay into the a ³Σ_u⁺ lowest triplet state. The theoretical treatment of the process, which reproduces the main features of the spectra, takes into account the photoassociation and decay steps as well as the resonant ionization through the manifold of intermediate gerade states correlated to the 5S + 4D limit. In particular, the energy of the v=1 level of the potential well has been determined for the first time. In addition, a tight constraint has been put on the position of the a ³Σ_u⁺ repulsive wall. Finally, magnetic trapping of rubidium molecules in the a ³Σ_u⁺ state is demonstrated. Electronic supplementary material Online Material     

Accumulation of chromium metastable atoms into an optical trap

We report the fast accumulation of a large number of metastable ⁵²Cr atoms in a mixed trap, formed by the superposition of a strongly confining optical trap and a quadrupolar magnetic trap. The steady state is reached after about 400 ms, providing a cloud of more than one million metastable atoms at a temperature of about 100 μK, with a peak density of 10¹⁸ atoms m^-3. We have optimized the loading procedure, and measured the light shift of the ⁵D₄ state by analyzing how the trapped atoms respond to a parametric excitation. We compare this result to a theoretical evaluation based on the available spectroscopic data for chromium atoms.     

Plasma plume induced during laser ablation of graphite

The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×10⁴ and 3.8×10⁴ m s⁻¹, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×10²³ m⁻³ 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×10²² m⁻³ at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.     

Laser cooling of thulium atoms

We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10⁶ thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s⁻¹. Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 10⁴ purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 10⁷ atoms at the expense of higher temperature.     

Three-body decay of a rubidium Bose–Einstein condensate

We have measured the three-body decay of a Bose–Einstein condensate of rubidium (⁸⁷Rb) atoms prepared in the doubly polarized ground state F=m _F =2. Our data are taken for a peak atomic density in the condensate varying between 2×10¹⁴ cm^-3 at initial time and 7×10¹³ cm^-3, 16 s later. Taking into account the influence of the uncondensed atoms on the decay of the condensate, we deduce a rate constant for condensed atoms L=1.8 (±0.5) ×10^-29 cm⁶ s^-1. For these densities we did not find a significant contribution of two-body processes such as spin dipole relaxation. Received: 24 November 1998 / Revised version: 26 June 1999 / Published online: 8 September 1999