The LPCTrap setup

The LPCTrap setup

Introduction

   The setup (see figure 1) is devoted to the measurement of the beta-neutrino angular correlation parameter, a, in the ß decay of ⁶He with a relative precision below 1%, to search for the presence of tensor type contributions in the weak interaction. The experiment uses the low-energy ⁶He⁺ beam from LIRAT and a novel ion-trapping technique. Singly charged ⁶He⁺ ions are confined in a transparent Paul trap where they decay. The ß particles and the recoil ions resulting from the decay are recorded in coincidence and the angular correlation coefficient is deduced from the analysis of the time of flight distribution of the recoil ions relative to the ß particles.

Figure 1: The LPCTrap setup at LIRAT.

Experimental technique

     ⁶He⁺ ions, extracted from the SPIRAL target ion source system at an energy of about 10 keV, are transported to the setup after a crude mass separation (delta M/M > 1/250). They are then cooled and bunched by means of the buffer gas technique with a Radio Frequency Quadrupole (RFQ) filled with H₂ gas (left part of figure 2). More details about the operation of the RFQ can be found in [1].

     Cold bunches extracted from the RFQ are injected into the Paul trap (right part of figure 2). The transfer of ions from the RFQ, at high voltage, to the trap chamber at ground, is achieved with two pulse-down electrodes, located between the RFQ and the trap, which reduce the ion energy for an efficient trapping. The trap is made of a set of coaxial rings providing an open access for injection and extraction of ions and for the detection of the decay products. Ions are confined in the Paul trap by a RF field (120 V peak-to-peak, 1.14 MHz). The RF voltage is applied to the inner rings of the trap. The ß particles from in trap decays are detected by a telescope constituted by a double-sided position sensitive silicon strip detector (SSD) followed by a thick plastic scintillator. The recoil ions are counted by a position sensitive micro-channel plate (µCP) [2]. A second µCP, located downstream, serves to monitor the trapped ions and measures continuously the ions remaining in the trap at the end of the cycle. The arrangement of the ß telescope and ion detector, in a back-to-back geometry, is the most sensitive to search for tensor couplings in Gamow-Teller decays.

Figure 2: Details of the LPCTrap setup.

    Ion bunches from the RFQ are continuously injected/extracted into/from the Paul trap, with a repetition rate of the order of 100 ms. The repetition rate is adjustable depending of the effective storage time of ions in the trap. After injection, the RF voltage is permanently applied to the rings of the Paul trap. The production of a signal by the plastic scintillator generates the trigger of an event, which in most cases is a ß particle in singles. Following a trigger the SSD is read out and a start signal is generated for the time measurement, waiting for a signal from the µCP over a 8 µs range. During the flight, the motion of the ion from the trap is somewhat affected by the RF field. The phase of the RF signal is then registered at the trigger time, to study off-line the effect of the RF field on the measured time of flight. The time after injection of the ions in the trap is also registered to study effects related with ion cloud phase space evolution in the trap. A detailed description of the setup is given in [3].

Achievements

    After a commissioning run in 2005 two experiments were performed in 2006 and 2008. In the former, about 10⁵ coincident events were recorded [4]. From a comparison of the time of flight spectrum to Monte Carlo simulations (see figure 3), the angular coefficient parameter could be determined with a statistical precision of 2 %.

Figure 3: Comparison between the experimental time-of-flight spectrum (black) and calculations from Monte Carlo simulations, assuming either a = -1/3 (light gray) or a =3 (dark gray). The insert shows the most sensitive interval to the value of the angular correlation coefficient a (picture taken from [4]).

    During the last experiment  (2008), the decay of about 2 10^{8 6}He⁺ ions was observed with an overall efficiency of 1.5 10^-3. It allowed reaching an unprecedent statistical precision of 0.5% on the measurement of the angular correlation coefficient a.

    Currently, systematic effects are investigated by means of simulations and off-line tests.

Contact: E. Liénard

[1] G. Ban et al., Nucl. Inst. Meth. Phys. Res. A518, 712 (2004).

[2] E. Liénard et al., Nucl. Inst. Meth. Phys. Res. A551, 375 (2005).

[3] D. Rodriguez et al., Nucl. Instr. Meth. Phys. Res. A565, 876 (2006).

[4] X. Fléchard et al., Phys. Rev. Lett. 101, 212504 (2008).