Positron Accumulation And Lifetime

Because of method's simplicity, we initially attempted to load positrons by following, as much as possible, the method described in Ref. [17] of field ionizing high-Rydberg positronium. We summarize here the accumulation rate obtained with this method. We also plan to accumulate positrons by the method outlined in Ref. [2], where positrons are loaded through Coulomb collisions with trapped 9Be+ ions. Results of this method will be discussed in a future publication.

The basic idea from Gabrielse's group is that in high magnetic field a fraction of the moderated positrons that leave the moderator crystal

Figure 5. Comparison of the number of trapped positrons obtained from the calibrated annihilation signal with the number obtained from the volume of the "dark'' plasma column. Measurements were taken with the positrons ejected from different traps, with different voltage pulse heights (500 - 750 V), and with the Nal crystal located 2.5 cm (squares and circles) and 5 cm (triangles) from the Ti foil.

Figure 5. Comparison of the number of trapped positrons obtained from the calibrated annihilation signal with the number obtained from the volume of the "dark'' plasma column. Measurements were taken with the positrons ejected from different traps, with different voltage pulse heights (500 - 750 V), and with the Nal crystal located 2.5 cm (squares and circles) and 5 cm (triangles) from the Ti foil.

combine with an electron to form positronium in a very high Rydberg state at the moderator crystal's surface. After leaving the crystal, the positronium travels into the trap as long as the electric fields between the moderator and trap are not large enough to field-ionize the Rydberg state. The trap potentials are adjusted to give a larger electric field inside the trap capable of field-ionizing the positronium and therefore capturing the positron. Positrons were accumulated with roughly the same trap potential shape but with different overall well depths (or electric field strengths). Figure 6 shows one of the smallest trap potential and the resulting electric field used to accumulate positrons by this method in our setup. During accumulation, 9Be+ ions were stored in the trap but were at low density because the laser-cooling and rotating wall were turned off. Similar to [17], we were able to accumulate positrons with a reverse bias of a few volts on the moderator crystal which would prevent low-energy

Figure 6. Schematic diagram of the experimental Penning trap and the on-axis potential well and electric fields used for trapping positrons. This was one of the smaller well depths used to accumulate positrons.

positrons from entering the trap. Figure 7 shows the accumulation of positrons for two different trap depths. The solid curves are fits to the rate equation ^ = a — ^ for the number of accumulated positrons N. Here a is the accumulation rate and r = 200 hours is the positron lifetime obtained from the fit to the lifetime data of Fig. 8. Similar to [17] we observe an increase in the number of accumulated positrons as the maximum electric field strength within the trap is increased. However, our maximum accumulation rate (trap voltage ~200 V) occurs at an electric field strength that is 5 to 10 times greater than observed in [17].

With this method we were able to accumulate a few thousand positrons. However, our accumulation rate is approximately 3 orders of magnitude lower than that obtained in [16, 17] and limited the total number of

2800

2400

2400

Trap voltage

—1-"-1-1-J—

□ 200 V

■ 70 V

35 e+/h

/ ---

Accumulation time (h)

24 48 72 96

Accumulation time (h)

Figure 7. Number of positrons vs. accumulation time for 2 different trap depths. The 200 V trap corresponds to -40 , -38, -200, -220, -200, 0 V and the 70 V trap to -15, -13, -70, -77, -70, 0 V on the moderator crystal and experimental trap electrodes. Here the potential of the moderator crystal is listed first followed by the experimental trap electrode potentials in order of their proximity to the moderator crystal.

positrons loaded into the trap. While both experiments are performed in a high magnetic field (5.3 T in [17] and 6 T in our setup), there were substantial differences in the two setups. In particular, reference [17] used tungsten moderator crystals at cryogenic (4 K) temperatures, compared with the room-temperature Cu moderator used here. They observed that their accumulation rate depended sensitively on the gas absorbed on the surface of the moderator crystal. Heating the moderator while the rest of the trap is at 4.2 K significantly reduced the accumulation rate. Cycling the apparatus to 300 K and back to 4 K restored the accumulation rate. Our Cu moderator crystal was baked with the rest of the trap at 350 °C for about 2 weeks, which may have desorbed much of the adsorbed gases.

Figure 8 shows the measured lifetime of the positrons, 9Be+ ions, and light mass impurity ions. The 9Be+ ion and positron lifetimes were measured simultaneously on the same plasma by first accumulating positrons and then blocking the 22Na source and measuring the number of 9Be+ ions and positrons that remained after each day for a week. The trap voltage during the lifetime measurement was -40 V. When the ion and positron numbers were not being measured, the laser cooling and rotating wall were turned off. The measured lifetime of the positrons was 8

Figure 8. Life-time of positrons (solid circles, number shown = actual number x 100), 9Be+ ions (solid boxes) and light impurity ions (triangles)

days and is nearly identical to the measured lifetime. This indicates that the measured positron lifetime could be limited by the charged particle's trapping lifetime of our trap rather than by annihilation with background gas. We measure the background pressure in our trap to be between 10-9 and 10~8 Pa. The trap was baked at 350 °C for about 2 weeks and was pumped by a sputter-ion pump and a titanium sublimation pump. For comparison we also show the measured lifetime of light-mass impurity ions. These ions such as Hj, Hj or He+ disappear relatively quickly due to reactions with background gas molecules.

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