The AMS-02 will fly in the Permanent Magnet (PM) configuration. Several motivations contributed to this decision, the most important is the extension of the International Space Station (ISS) operations to 2020 (2028 is also being considered). Thanks to the ISS extension AMS-02 would have the unique opportunity of studying with the highest accuracy cosmic-ray composition for ten years or more, searching for very rare components. The Superconducting Magnet (SCM) would not have had an endurance comparable to the extended lifetime of the ISS.
The Path towards the Decision
The Superconducting Magnet (SCM) development begun in the year 2000, under the assumption that AMS-02 after three years on the ISS would return to Earth. The AMS-02 SCM configuration lifetime has been measured during the ESTEC Thermo-Vacuum Test revealing an endurance of less of 3 years. At the end of this period the Superfluid Helium would have boiled off and the cryomagnet would have reached ambient temperature before returning to ground.
On February 1st 2003, the Columbia accident caused a major modification of the Shuttle manifest, drastically reducing the missions to the minimal number needed to complete the ISS and fulfill the international obligations. Even AMS-02 was removed from the manifest for a few years. This did not stop the magnet and payloads development, but when AMS-02 was restored at the beginning of 2009, it was clear that the ticket was one-way only: once installed on the ISS with the STS-134 mission the experiment would never return to Earth. At that time the launch date was set for 2010 and the ISS was supposed to operate until 2015.
The decision made on early 2010 of operating the ISS until at least 2020 made again the Permanent Magnet a very interesting option and a consensus within the AMS Collaboration and NASA was quickly reached.
The AMS-02 PM configuration is technically simpler to operate with respect to the SCM one. There is no need for the Helium tank, for any cryogenic device, etcetera. There is also some gain in weight and the safety requirements are less stringent. Since the AMS-02 subsystems were originally designed to support both the PM and the SCM options, the two vacuum cases structures are mechanically identical and integration is very similar.
The New Design
Particles and anti-particles are bent by magnetic fields in opposite directions. A direct measurement of the curvature direction allows the separation between matter/antimatter (left-positive, right-negative). From an experimental point of view it is straightforward to say that: the larger is the bending of the particle trajectory, the simpler is the determination of the sign of the curvature.
The trade off in using the PM is the fact that the magnetic field is 5 times weaker than the SCM one. This means that charged particles of the same energy (as the two depicted in the previous figure) are bent less by the PM field. This would limit the matter-to-antimatter separation to lower energies using the PM instead of the SCM.
To cope with this fact, an optimization of the geometry of the AMS Silicon Tracker can be implemented. The basic idea is to extend the lever arm of the tracking measurement adding measurements above and below AMS-02. With a bigger lever arm tiny angular deviation of the particle in the magnetic field can also be appreciated. With such an “extended” Tracker smaller curvatures can be measured. With this technique we can recover the full sensitivity of AMS-02 on matter-antimatter separation.
An approximate formula for the relative error (the percentage of error) on the rigidity evaluation in the Tracker, ΔR/R, is given in the upper plot. You may consider as ΔR/R the percentage of error on the curvature determination. When ΔR/R becomes 100%, the curvature becomes so tiny that we can no longer separate positive from negative tracks. In both formulas ΔR/R increases – corresponding to a worse accuracy on curvature determination – if the magnetic field is reduced. We can improve the resolution by extending the lever arm (L).
In the plot above the expected rigidity resolution for protons are plotted as a function of rigidity. With 9 tracker planes, the resolution of AMS-02 with the permanent magnet is equal (within 10% or less) to that of the superconducting magnet.
The optimization of the Silicon Tracker is obtained by adding 2 more planes at the beginning and at the end of the detector, thus extending the lever arm from ~ 1 m to ~ 4 m. These two planes are built reshuffling some “ladders” already existing on the Silicon Tracker, then no new electronics or Silicon ladders are needed.
The Expected Performances of the New Design
As a side effect of the lever arm extension, the number of cosmic-rays per second passing through the tracking volume is reduced. This is because longer cylinders – with the same diameter – have narrower angular field-of-views. However the number of Cosmic Rays (CRs) collected in 10 years in the PM Scenario is greater than the number of CRs that could be collected in the SCM Scenario. The larger number of particles collected increases the probability of discovering very rare events as traces of primordial antimatter or as products of Dark Matter annihilation.
As an example the number of positrons collected in the PM configuration is expected to be a factor 2 to 6 times greater to the SCM configuration, depending on the energy. Positrons are an important rare component of cosmic-rays. They could be an important marker of the Dark Matter indirect search.
Indeed an important AMS-02 research topic regards the measurement of positron-to-electron ratio. PAMELA has clearly pointed out an excess of positrons at high energies: a similar effect was previously measured with larger statistical uncertainties by HEAT and AMS-01. The debate on the nature of such an excess is still going on. One possible hypothesis is that it is resulting from Dark Matter (DM) annihilation producing electrons and positrons in the final state.
DM particles are everywhere and interact very weakly with normal matter. Direct detection is pursued at underground laboratories and is very difficult. In the galaxy DM particles could interact and sometimes annihilate. Since DM is expected to have a large mass, the energy released in these annihilations is quite large. This energy can be converted in pairs of ordinary matter-antimatter particles (electron-positron, proton-antiproton) having quite a high energy. The excess of positrons in the spectrum seen in PAMELA data could then be explained considering that high energy positrons could be produced by DM annihilations taking place in our galaxy.
From the experimental point of view, in AMS-02 the identification of electrons and positrons against the large background of protons will be done using the Transition Radiation Detector (TRD) and the Electromagnetic CALorimeter (ECAL). The new Tracker configuration will match the acceptance of ECAL, so that all particles passing through the Tracker are also measured by the ECAL maximizing AMS-02 acceptance and identification power for electrons and positrons.
In the upper figure is presented the expected AMS-02 performance in the positron-to-electron ratio. The hypothesis of a DM of mass of 200 GeV has been made. AMS-02 could extend the actual measurements (PAMELA, HEAT, AMS-01) toward higher energies and could point to the discovery of annihilating DM.