Construction of a time-of-flight measurement system to study the low energy positronium production

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Abstract

To investigates the low energy positronium (Ps) production in terms of target temperatures and materials, surface condition and so on, we adopt the time-of-flight (TOF) method and perform extensive Monte Carlo simulation to design an apparatus. Under reasonable assumptions in the simulation with respect to an initial energy distribution of Ps, we examine how cleanly ortho-positronium (oPs) is observed without being subject to large backgrounds of 2γ decay from e+e annihilation. The simulation finds that the contribution of 2γ events is suppressed by 10−6 smaller than the generated Ps by using the 2.5-cm thick W and 4.6-cm Pb collimators with slit gap of 1 mm. We inject slow positron beams on the W target installed in the TOF system and move the position of the target. Then we expect the position resolution of 1.5 mm resulting the accuracy of determining the oPs velocity 20%, corresponding to 1.1×106 cm/s for thermal Ps with 30 meV.

Introduction

Cold positronium (Ps) with the kinetic energy of a few meV will be utilized for various experiments to study atomic properties of Ps, Ps spectroscopy and eventually to realize Ps Bose–Einstein condensation. In order to achieve rapid cooling of ortho-positronium (oPs) within the life-time of oPs (142 ns) by means of laser excitation of the 1s state to 2p state, we should create the oPs at a possible lowest temperature [1]. Previous experiments [2]clarified that the cold Ps might be produced via a thermal desorption process and thus, the Ps temperature seems to be closely related to the temperature of production targets. Using slow positrons provided from TOPPS (Tokyo Metropolitan University Polarized Positron Beam System), we shall examine technical details of cold Ps production mechanisms. In order to construct an apparatus of time-of-flight (TOF), we perform extensive Monte Carlo (MC) simulation studies to check the accuracy of determining Ps temperature at various target temperatures. Our strategy is that, first of all, a target at room temperature is utilized to establish a reliable method to measure the thermal Ps temperature or energy, and then the cold targets cooled by the liquid N2 and liquid He will be installed to study technical possibilities to produce cold Ps. The MC simulation clarifies that the energy of thermal oPs has to be measured distinctively without being disturbed by oPs having the energy of the workfunction of the corresponding target.

In Section 2, we present results obtained from the MC study. The design of the TOF measurement system are given in Section 3. Section 4is devoted to conclusion.

Section snippets

Monte Carlo simulation for designing the TOF system

The energy spectrum of Ps thermally desorbed from an Al(111) surface is studied in Ref. [3]. The longitudinal energy distribution of oPs emitted perpendicular to the surface is given by:dNdE=expEKTS(E,T)/KT.The quantities used in Eq. (1)are defined as follows: N: number of Ps; E: the longitudinal energy of Ps; T: the target temperature; K: the Boltzmann constant; and S(E,T): the sticking coefficient. In our analysis, we assume that the factor S(E,T) is constant [3]. It was pointed out

Position resolution of TOF

To guide and focus the positron beam on the surface of the target, we use three coils just around MGD. The e+ beam is magnetically guided in TOPPS under a uniform magnetic field of 100 G. Using POISCR [7]and POEM [8]simulation programs, we can optimize the output beam profiles. We are going to measure the TOF of oPs precisely. Therefore, the position resolution of this system plays an important role to obtain the oPs velocity or equivalently energy distribution. We gave the 1 mm (W) and 3 mm

Conclusion

We shall soon carry-out the low energy Ps experiment using the TOF apparatus after improving the e+ beam intensity and the bunching system of TOPPS. The detailed MC study shows that we will accumulate the events of about 50,000 for 10 days under assumption of e+ beam intensity 2×104/s. Consequently, it may be concluded that this TOF and TOPPS systems can determine the Ps temperature down to the He temperature with reasonable statistics.

Acknowledgements

We like to thank Dr. M. Chiba for his useful comments and T. Nishimura for his kind help to perform this simulation. N.N. Mondal gratefully acknowledges the scholarship support of MONBUSHO, the Ministry of Education and Scientific Research, Japan.

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