Since being delivered to the International Space Station (ISS) by Space Shuttle Endeavour in 2011, the Alpha Magnetic Spectrometer (AMS-02) has recorded more than 200 billion cosmic-ray events with energies extending into the multi-TeV range. Although never designed to be serviceable, a major intervention to the 7.5 tonne detector in 2019/2020, during which astronauts replaced a failing cooling system, extended the lifetime of AMS significantly (CERN Courier March/April 2020 p9). Now, the international collaboration is preparing a new mission to upgrade the detector itself, by adding an additional tracker layer and associated thermal radiators. If all goes to plan, the upgrade will allow physicists to gather key data relating to a mysterious excess of cosmic rays at high energies.
Precise dataset
The increasingly precise AMS-02 dataset reveals numerous unexplained features in cosmic-ray spectra (CERN Courier December 2016 p26). In particular, a high-energy excess in the relative positron flux does not follow the single power-law behaviour expected from standard cosmic-ray interactions with the interstellar medium. While known astrophysical sources such as pulsars cannot yet be ruled out, the spectrum fits well to dark-matter models. If the excess events are indeed due to the annihilation of dark-matter particles, a smoking gun would be a high-energy cut-off in the spectrum. By increasing the AMS acceptance by 300%, the addition of a new tracker layer is the only way that the experiment can gather the necessary data to test this hypothesis before the scheduled decommissioning of the ISS in 2030.
“By 2030 AMS will extend the energy range of the positron flux measurement from 1.4 to 2 TeV and reduce the error by a factor of two compared to current data,” says AMS spokesperson Sam Ting of MIT. “This will allow us to measure the anisotropy accurately to permit a separation between dark matter and pulsars at 99.93% confidence.”
Led by MIT, and assembled and tested at CERN/ESA with NASA support, AMS is a unique particle-physics experiment in space. It consists of a transition radiation detector to identify electrons and positrons, a permanent magnet together with nine silicon-tracker layers to measure momentum and identify different particle species, two banks of time-of-flight counters, veto counters, a ring-image Cherenkov counter and an electromagnetic calorimeter.
The additional tracker layer, 2.6 m in diameter, 30 cm thick and weighing 250 kg, will be installed on the top-most part of the detector. The tracking sensors will populate the opposite faces of an ultralight carbon plane specifically developed for AMS to fulfil thermoelastic stability requirements, surrounded by an octagonal carbon frame that also provides the main structural interface during launch. The powering and readout electronics for the new layer will generate additional heat that is rejected to space by radiators at its periphery. Two new radiators will therefore be integrated into the detector prior to the installation of the layer, while a third, much larger power-distribution radiator (PDS) will also be installed to recuperate the performance of one of the AMS main radiators, which has suffered degradation and radiation damage after 13 years in low-Earth orbit. In January, a prototype of the PDS, manufactured and supported by aerospace company AIDC in Taiwan, was delivered to CERN for tests.
First steps for the upgrade took place in 2021, and the US Department of Energy together with NASA approved the mission in March 2023. The testing of components and construction of prototypes at institutes around the world is proceeding quickly in view of a planned launch in February 2026. The silicon strips, 8 m2 of which will cover both faces of the layer, were produced by Hamamatsu and are being assembled into “ladders” of different lengths at IHEP in Beijing. These are then shipped to INFN Perugia in Italy, where they are joined together to form a quarter plane. Once fully characterised, the eight quarters will be installed at CERN on both faces of the mechanical plane and integrated with electronics, thermal hardware and the necessary brackets. Crucial for the new tracker layer to survive the harsh launch environment and to maintain, once in orbit, the sensor within five microns relative to ground measurements, are the large carbon plane and the shielding cupolas, developed at CERN, as well as the NASA brackets that will attach the layer module to AMS. This hardware represents a major R&D programme in its own right.
By 2030 AMS will extend the energy range of the positron flux measurement from 1.4 to 2 TeV and reduce the error by a factor of two
Following the first qualification model in late 2023, consisting of a quarter of the entire assembled layer, AMS engineers are now working towards a full-size model that will take the system closer to flight. The main tests to simulate the environment that the layer will experience during launch and once in orbit are vibrational and thermal-vacuum, to be performed in Italy (INFN PG) and in Germany (IABG), while the sensors’ position in the layer will be fully mapped at CERN and then tested with beams from the SPS, explains AMS chief engineer Corrado Gargiulo of CERN: “Everything is going very, very fast. This is a requirement, otherwise we arrive too late at the ISS for the upgrade to make sense.”
The new module is being designed to fit snuggly into the nose of a SpaceX Dragon rocket. Once safely delivered to the ISS, a robotic arm will dispatch the module to AMS where astronauts will, through a series of extravehicular activities (EVAs), perform the final mounting. Training for the delicate EVAs is well underway at NASA’s Johnson Space Center. Nearby, at the Neutral Buoyancy Laboratory, the astronauts are trained in a large swimming pool on how to attach the different components under the watchful eyes of safety and NASA divers, among them Gargiulo (see “Space choreography” images). As with the EVAs required to replace the cooling system, a number of custom-built tools and detailed procedures have to be developed and tested.
“If the previous ones were considered high-risk surgery, the EVAs for the new upgrade are unprecedented for the several different locations where astronauts will be required to work in much tighter and less accessible spaces,” explains Ken Bollweg, NASA manager of AMS, who is leading the operations aspect.
