Fragmentation studies in
antiproton-nucleus annihilation

ASACUSA is conducting a study of the charged multiplicity distribution in antiproton-nucleus anni- hilation at rest on various materials by using a high resolution annihilation vertex detector.

The multiplicity distribution of charged annihilation products (such as pions, protons and nuclear fragments) is not well known for antiprotons annihilating at rest on nuclei. This information is an essential ingredient to Monte Carlo simulations (such as GEANT4), to model the background contribution in antihydrogen and low energy antiproton experiments. The current simulations are performed with the CHIPS, FRITIOF or FLUKA packages (ref. 1). The former two were developed for high energy hadronic interactions, based on the interaction between parton constituents extrapolated to low energy antiproton annihilation, while FLUKA models the hadronic interaction in terms of resonance production and decay.

There are only few measurements of annihilations at rest on nuclei (such as Al, Cu, Ag and Au), performed recently at the Antiproton Decelerator (ref. 2). However, none of the models is able to describe the results satisfactorily, with FLUKA performing better than CHIPS and FRITIOF (fig. 1).

Figure 1: Particle multiplicity in antiproton annihilation at rest, as a function of atomic number for minimum ionizing particles (MIPS) and heavy ionizing ones, such as nuclear fragments (HIPS).

ASACUSA is using slow antiprotons from the MUSASHI trap (see antihydrogen) which provides very low energy continuous antiproton beams (∼150 eV). The setup (fig. 2) consists of three main parts: a four-way cross that hosts einzel lenses to focus the beam), the Timepix3 detector (from the Medipix3 Collaboration) and the target foil with the annihilation detector surrounding the Timepix3. The latter (ref. 3) is an ASIC hybrid detector module consisting of 256 x 256 square pixels (thickness of 500 μm), with a pitch of 55 μm. We are using a quad array (total active area of 28 x 28 mm2), developed and provided by the Medipix collaboration at CERN. It was assembled and mounted on a special chipboard with a flex-rigid design provided by NIKHEF. The ASIC is able to measure simultaneously the Time-over-Threshold and Time-of-Arrival within 1.56 ns. The readout, based on a Xilinx 7-series FPGA, was provided by NIKHEF.
 
A drawing of the Timepix3 layout and a photograph are shown in fig. 3. By combining the Timepix3 and annihilation detector we can detect pions, protons, alphas and heavy nuclear fragments. The position of the annihilation vertex can be determined with a precision in the 10 – 20 μm range. Figure 4 shows the tracks with their energy deposits from an antiproton annihilating into three pions on the surface of a Timepix3 detector (ref. 4), and the reconstruction of the annihilation vertex.