S. N. Atutov

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.

Generation of a frequency comb with a sharp edge of adjustable intensity and frequency

A frequency comb is generated by successive frequency shifts of a single mode laser by an acousto-optic modulator coupled to a passive ring cavity. The cavity design and the laser-cavity coupling have a very high efficiency and allow for a perfect control of both the frequency and the intensity of the sharp edge. A second acousto-optic modulator can be used to double the comb bandwidth. The obtained spectrum is particularly suitable for laser cooling when a large velocity capture range is required as it is the case, for example, of ions confined in a storage ring.

Efficiency of photodesorption of Rb atoms collected on polymer organic film in vapor-cell

Abstract The efficiency of photodesorption of Rb atoms previously collected on polymer organic film has been studied in detail. This study was carried out in a Pyrex glass cell of which the inner surface was covered with (poly)dimethylsiloxane (PDMS) film and illuminated by a powerful flash lamp. The desorption dynamic of the Rb atoms density in the cell caused by the illumination and percentage of desorbed atoms was studied by using of Rb resonance lamp and free running diode laser as sources of probing light. It was determined that 85 percent collected chemical active Rb atoms and stored during 16 s in the closed cell, 75 percent in the pumped cell can be desorbed by single flash of the lamp. The number of stored atoms decays with a characteristic time of 60 min in isolated cell and with a time 12.4 min in a pumped cell. We believe that this efficient method of collection and fast realization of atoms or molecules could be used for enhancement of sensitivity of existed sensors for the trace detection of various elements (including toxic or radioactive ones) which is important to environmental applications, medicine or in geology. The effect might help to construct an efficient light-driven atomic source for a magneto-optical trap in a case of extremely low vapor density or very weak flux of atoms, such as artificial radioactive alkali atoms.

Francium sources at Laboratori Nazionali di Legnaro: Design and performance

A facility for the production of radioactive francium is operating at the laboratories of the Istituto Nazionale di Fisica Nucleare (INFN) in Legnaro, Italy. The goal is to collect a cold sample of radioactive atoms in a magneto-optical trap for studies in atomic, nuclear, and particle physics. Production of francium is achieved via the fusion-evaporation reaction Au197(O18,kn)215−kFr generated by a ∼100‐MeV O6+18 beam on a thick gold target. The production target is heated to ∼1200K and kept at a potential of +3kV to enhance Fr diffusion and surface desorption. Average production rates are 0.7×106ions∕s for Fr210 with a primary beam flux of 1012particles∕s, with peaks of 2×106ions∕s. Details are given on the design and construction of the production targets and on the measurements that characterize their performance.

The Legnaro francium magneto-optical trap

Laser cooling and trapping of radioactive atoms represent the new frontier in atomic physics and a new powerful tool in nuclear physics. We are setting up at the INFN-Legnaro National Laboratories a laser cooling facility that has as a first goal the realization of a 210Fr magneto-optical trap. The general outline of the experiment and the improvements of the final trap efficiency are discussed. Some preliminary results are presented.

A francium MOT for atomic parity violation measurements

The apparatus for the production and trapping of francium is described and its performances are reported. Latest results on magneto-optical trapping of Francium are summarized: trapping of 209, 210, 211 Francium isotopes, measurements of their trapping frequencies, measurements of diffusion parameters of Francium ions in yttrium. Future experiments on fundamental physics are presented.

EXPLOSIVE EVAPORATION OF Rb OR K CLUSTERS BY LOW POWER LASER RADIATION IN THE PRESENCE OF EXCITED ATOMS

Explosive evaporation of metallic Rb or K clusters in the presence of excited atoms stimulated by resonant CW laser radiation has been observed in a glass cell. Evaporation occurs at low laser-power density. The effect consists of the generation of optically thick and sharply localized alkali vapor clouds propagating in the cell against the laser beam. The clouds are charged and exhibit a strong luminescence of Rb or K spectral lines. The explosive evaporation of metallic clusters is explained by the laser excitation of alkali atoms, which collide onto the surface of the clusters and transfer their internal energy, which thereby causes other atoms to be evaporated and to continue the avalanche process.

Laser cooling and trapping of radioactive atoms

Laser cooling and trapping techniques made possible during the last two decades important achievements in the atomic physics and quantum mechanics fields. These same techniques can be usefully applied to radioactive atoms by opening new fields of investigations. Nuclear processes can be studied with the atomic physics tools. We focused our attention on Francium radioactive atoms. A magneto-optical trap has been set up at the INFN Legnaro laboratories. Preliminary tests with other stable alkali atoms aimed at an improvement of the MOT collection efficiency are reported. Fast and efficient trap loading of rubidium has been obtained through the light-induced atomic desorption from an organic coating. A larger number of sodium atoms, as compared to monochromatic trapping laser, has been trapped by using a broad-band laser.

Light-induced atomic desorption: recent developments

Light induced atomic desorption (LIAD) is an impressive manifestation of a new class of phenomena involving alkali atoms, dielectric films and light. LIAD consists of a huge emission of alkali atoms (experimentally proved for sodium, potassium, rubidium and cesium) from siloxane films when illuminated by laser or ordinary light. Most of the experiments have been performed in glass cells suitably coated by a thin film (of the order of 10 micrometer) either of poly - (dimethylsiloxane) (PDMS), a polymer, or of octamethylcyclotetrasiloxane (OCT), a crown molecule. LIAD is a combination of two processes: direct photo-desorption from the surface and diffusion within the siloxane layer. The photo-desorbed atoms are replaced by fresh atoms diffusing to the surface. Moreover, from the experimental data it comes out that the desorbing light increases atomic diffusion and hence the diffusion coefficient. To our knowledge this is the first time that such an effect is clearly observed, measured and discussed: LIAD represents a new class of photo-effects characterized by two simultaneous phenomena due to the light: surface desorption and fastened bulk diffusion.

Development of a broadband laser in the UV region

A frequency comb in the UV region with a bandwidth greater than 100 MHz is generated by successive frequency shifts of a single mode laser by an acousto-optic modulator coupled to a passive ring cavity. The obtained spectrum is particularly suitable for laser cooling when a large velocity capture range is required as in the case, for example, of ions confined in a storage ring.