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arXiv:1511.07924 · Journal of Physics B Atomic Molecular and Optical Physics

Study on cooling of positronium for Bose–Einstein condensation

K Shu, X Fan, T Yamazaki, T Namba, S Asai, K Yoshioka, Makoto Kuwata‐Gonokami

Published Apr 22, 2016• 28 citations
arXivHTMLPDF

Abstract

Abstract A proposed new method of cooling positronium is to realize the Bose–Einstein condensation (BEC) of positronium. We perform detailed studies of three processes: (1) thermalization processes between positronium and the silica walls of a cavity, (2) Ps–Ps scattering and (3) laser cooling. The thermalization process is shown to be not sufficient for BEC. Ps–Ps collision is shown to have a big effect on the cooling performance. We combine both methods and establish an efficient cooling process for BEC. We also propose a new optical laser system for the cooling.

DOI
10.1088/0953-4075/49/10/104001

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Abstract A proposed new method of cooling positronium is to realize the Bose–Einstein condensation (BEC) of positronium. We perform detailed studies of three processes: (1) thermalization processes between positronium and the silica walls of a cavity, (2) Ps–Ps scattering and (3) laser cooling. The thermalization process is shown to be not sufficient for BEC. Ps–Ps collision is shown to have a big effect on the cooling performance. We combine both methods and establish an efficient cooling process for BEC. We also propose a new optical laser system for the cooling.
A new method to realize Bose-Einstein condensation (BEC) of positronium (Ps) is proposed and a Monte-Carlo simulation was performed to evaluate the cooling efficiency of Ps atoms. Ps-BEC was found to be possible by a method including laser cooling of Ps. Laser specifications for Ps cooling are challenging and development is ongoing. A new temperature measurement method for cold Ps and development of an intense, pulsed, slow-positron microbeam are also necessary. They are under study as outlined in this contribution.
Realization of the Bose–Einstein condensation (BEC) of positronium is a long-standing challenge of positron physics. Since the positron is the antimatter of the electron, the positronium is the antimatter of itself, and its gravity interaction is a sum of matter and antimatter components. In this sense, it can be used to study antimatter gravity. It can also be used as a source of a γ-ray laser. We have proposed a new method to realize a positronium BEC: a combination of thermalization in a cold silica target and laser cooling using 1S-2P transitions. We have started some basic studies based on our new idea. Here we report a preliminary result of our positronium thermalization measurement in cryogenic environment and development status of a new laser system for positronium cooling.
Realization of Bose-Einstein Condensate (BEC) of positronium (Ps), a purely leptonic particle-antiparticle bound system, is a challenging subject [1, 2]. Recently, Ivanov et al. [3] theoretically investigated the Ps-Ps scattering and showed theoretically that the scattering length for the o-Ps atoms is positive, indicating that a stable o-Ps BEC is actually possible. The condition for the BEC of the ideal Bose gas is λth > (2.612/n)1/3, where λth = h/ √ 2πmkT is the thermal de Broglie wavelength of the atom and n is the number density. This gives the critical temperature for the Ps BEC as Tc = 1.46×10−15n2/3, where Tc is in K and n is in m−3. This condition is much less severe than for the usual alkali atom BEC because λth is longer by five orders of magnitude. However, Ps has a finite lifetime: the lifetime of singlet state, para-positronium (p-Ps), is 125 ps and that of triplet state, ortho-positronium (o-Ps), is 142 ns. Thus a many-Ps system must be cooled within the lifetime of o-Ps. The short lifetime of Ps demands quick cooling and high density. The laser cooling which is employed for the accomplishment of alkali atom BEC is likely to be impractical because it takes longer time. It is also subject to relatively large recoil of the Ps atom when hit by a photon. Moreover, unless an effective method for trapping Ps in free space is developed, the system of Ps atoms diffuses and lowers its density. Platzmann and Mills [1] suggested to confine Ps in a small cavity of insulating material and let the Ps atoms give away their kinetic energy to the walls of the cavity. If one injects an intense polarized positron beam onto a material which has a micro cavity near surface, some of the positrons form Ps and are emitted into the cavity and stay there. This is expected because the work function of most of the material for Ps is negative, i.e., Ps atoms emitted into the cavity after thermalization in the material will never penetrate back into the material. They just hit the cavity walls and approach thermal equilibrium by exciting phonons. The probability of the annihilation of o-Ps with a surface electron is small if there are no paramagnetic centers on the cavity walls. It is necessary to use a polarized positron beam to make Ps. Otherwise all the o-Ps atoms are lost through mutual spin conversion into p-Ps [1, 4]. In this report we would like to discuss prospect of a Ps BEC with special attention to the following aspects [4]: (1) use of 18F for a source of an intense spin-polarized positron beams [5], (2) development of high efficiency Wmesh positron moderator, (3) emission of Ps from solid surface (4) the cooling of Ps in a cavity, and (5) the detection of a Ps BEC.
Abstract We present the actual state of affairs and future perspectives in the study of a quantum system of a collection of positronium (Ps) atoms. The interaction of a Ps atom with other atoms and molecules and specially with another Ps atom is described in some detail as Ps‐Ps interaction should play a crucial role in the dynamics of an assembly of Ps atoms. Using a simple model‐exchange potential, we could describe the available experimental results of Ps scattering reasonably well. The present scenario of the observation of Ps2 molecule, Ps Bose‐Einstein condensate (BEC) and the annihilation laser from a Ps BEC is presented. Possibilities of a Ps BEC formation via laser cooling of Ps atoms and via Ps formation in cavities are considered and difficulties with each procedure discussed (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the basis of three dimensional (3-D) Monte Carlo simulation, we studied details of laser cooling (Doppler cooling) of ortho-Positronium ( o -Ps) which would be applied to the achievement of Bose–Einstein condensation of positronium. The cooling is carried out by an ultraviolet laser with a wavelength of 243 nm being equivalent to the 1 S –2 P transition energy of o -Ps. For this purpose, we have constructed a laser cooling apparatus which consists of a slow positron generator and a long pulse, wide linewidth laser. A 3-D Monte Carlo calculation, which incorporated various parameters of laser beam and positronium distributions expected in the current laser cooling apparatus, revealed that 7% of o -Ps was able to be cool down to 1 K and to be confined in a relatively small volume without employing a special trap.
Abstract We report recent progress toward achieving positronium (Ps) Bose–Einstein condensation (BEC) by combining thermalization in nanoporous materials with ultraviolet (UV) laser Doppler cooling. Experiments were performed using silica aerogels held at two distinct thermal conditions—approximately room temperature and cryogenic temperature down to 8 K (measured at the sample-holder)—to study the laser-induced annihilation of Ps atoms excited to the 2P state. Resonant excitation at 243 nm significantly enhanced gamma-ray emission, with the annihilation signal reduced by approximately 50 % at low temperature compared to room temperature. Increasing the delay between Ps formation and laser excitation further reduced the annihilation probability. These observations confirm that Ps thermalization affects 2P-Ps annihilation. The results support a model of surface-induced 2P-state decay and suggest that excited-state Ps serves as a sensitive probe of surface interactions in nanoporous media. This constitutes an important step toward implementing laser cooling of Ps confined in solids—an essential prerequisite for realizing Ps-BEC. Ongoing efforts focus on systematic evaluation of pore structure, surface chemistry, and temperature effects to optimize Ps dynamics toward condensation.