A Positron Accumulator for Antihydrogen Synthesis

A positron accumulator based on the modified Penning–Malmberg design of Surko and co-workers at UCSD has been constructed and undergone testing in preparation for the ATHENA experiment now under way at CERN. This experiment aims to produce and characterize atomic anti-hydrogen. The positron accumulator utilises nitrogen buffer gas to cool and trap a continuous beam of positrons emanating from a Na radioactive source. A solid neon moderator slows the positrons from the source down to epithermal energies of a few eV before being injected into the trap. It is estimated that around 10 positrons can be trapped and cooled to ambient temperature within a couple of minutes in this scheme using a 6 mCi source. Preliminary tests have so far demonstrated trapping of approximately 3 x 10 positrons and an efficiency of the Ne moderator of nearly 1 %. INTRODUCTION In order to produce low energy antihydrogen via recombination it is necessary to have copious amounts of cold positrons available. To attain this a positron accumulator based on the design of the Surko Group at the University of California San Diego [1-3] has been constructed and undergone preliminary testing in the United Kingdom before being shipped to CERN in Geneva to be a part of the ATHENA (AnTiHydrogEN Apparatus) experiment [4-5]. The accumulator is an ideal source of positrons in this case as it is capable of supplying large quantities (>10 in the later stages of the experiment) of positrons in short well defined bursts, with a short cycle time, in the order of 5 minutes. EXPERIMENTAL The positron accumulator is an instrument for trapping and cooling a continuous beam of slow positrons. A continuous beam of slow positrons injected into the accumulator is generated by moderating β+ particles from a 6 mCi Na radioactive source and guiding them into the trapping region using a magnetic field. A cryogenic cold head capable of reaching 5.5 K cools down the source and makes it possible to grow a solid neon moderator directly on the source. The slow positrons emanating from the source/moderator are magnetically guided through a kink and a long narrow pumping restriction into the main trapping region (see [6] for details). Placing a NaIdetector close to the gate valve between the source end and the main trapping region made it possible to optimise the moderator growth. During moderator growth this gate valve would be closed and the positrons would annihilate on the closed valve. To calibrate the NaI signal to absolute beam strength a channeltron detector was briefly inserted here and coincidence Figure 1. Schematic of the trapping scheme showing the electrode array and the pressures and electric potentials along the array. measurements performed. The channeltron detector with a retarding grid in front of it was also used to measure the energy distribution of the emitted positrons. The main trapping chamber is situated within a 0.1 T magnet and contains an electrode array. This consists of a set of eight separate goldplated aluminium electrodes with an appropriate potential applied which will confine the positrons in the axial direction after the initial trapping (Fig.1). The 0.1 T axial magnetic field supplies the radial confinement and combined with the electric potentials this constitutes our Penning-Malmberg trap. The physical dimensions of the electrodes are designed to allow a pressure gradient to be developed along their length. Nitrogen gas can be introduced midway along electrode II and is pumped out at either end or through a set of three vents located at the end of the same electrode. The positrons are trapped and cooled within the array via a buffer gas method. The pressure of the nitrogen gas used as buffer gas is tuned such that on average, a positron entering from the source region will experience one inelastic collision with a nitrogen molecule whilst traversing electrode II. Thus confined and unable to escape the array a second collision typically occurs within a millisecond further confining the positron to between electrodes III and VI. Typically, a third collision after some 10ms will then finally restrict the positron to electrodes IV and V. Electrode IV is split into 6 segments, making it possible to use it to compress the plasma by applying a rotating electric field (‘rotating wall’) [7]. This method has been shown recently to work well also for positrons plasmas [8]. In our case the electrodes used have a significantly larger radius (~10 cm) than in earlier experiments but preliminary tests have shown that we can still influence the positron plasma from that distance. The detection system used for our experiment consists of 3 detectors: (1) A segmented Faraday cup detector consisting of 9 plates to get information about the size and position of the plasma. (2) A CsI-diode detector to detect the positron annihilation signal. (3) A ‘Gauss’ Law’ signal from one of the confining electrodes caused by charge being induced on the electrode as the confined plasma is being dumped on the Faraday cup. Presently the whole experimental set-up including the data acquisition from the detectors is being automated to make it possible to run the experiment remotely. RESULTS A study of moderator growth was conducted using a channeltron and a NaI detector in coincidence in order to ascertain the moderator efficiencies for different moderator thicknesses etc. These detectors were placed at the entrance to the main vacuum system. Using this method we have been able to detect more than 1.6 μ 10 e+ s, giving a moderator efficiency of 0.62 % based on a source strength of 7 mCi at the time of the measurement. However, using a measurement of the current of positrons from the source taken during an electronically quiet period as the base for the moderator efficiency a real efficiency of 0.76 % is reached. The study of the energy distribution of 01020340506 10 20 30 40 50 CsI Output (mV) Holding Tme (s) 01020340506 10 20 304 50 CsI Output (mV)