On December 8, the Alpha Magnetic Spectrometer (AMS) Collaboration, in which Shandong University being an active part, released the results from five years of experiment on the International Space Station (ISS) at CERN. The experiment has measured the positron spectrum and positron fraction, the antiproton/proton ratio, the behavior of the fluxes of electrons, positrons, protons, helium and other nuclei in cosmic rays with an unprecedented accuracy, which has changed people’s understanding of cosmic rays.

The AMS Experiment is an international scientific project headed by the Nobel Prize winner in Physics Samuel Chao Chung Ting. It is one of the largest scientific plans worldwide around the turn of the century. Being the most powerful and sensitive particle detector ever deployed in space, it is so far the only creative scientific experiment mounted on the ISS permanently. Up to now, AMS has collected data from more than 90 billion cosmic rays and published its major physics results inPhysical Review Letters.

Over the past century, there have been many measurements of the electron, positron and proton spectra which had large errors and created many diverse theoretical models. Currently, AMS has observed that with a data set of 16,500,000 electrons and 1,080,000 positrons, the electron flux and positron flux display different behaviors in their magnitude and energy dependence. Above 60 GV, positrons, protons and antiprotons display identical rigidity dependence, where rigidity is the momentum per unit charge, but electrons exhibit a totally different energy dependence. AMS precision measurements have revealed new and distinct information that has changed our understanding of cosmic rays.

There has been much interest over the last few decades in understanding the origin and nature of dark matter. When particles of dark matter collide, they produce energy that transforms into ordinary particles. As seen from the current results of the AMS positron flux and positron fraction, after rising from 8 GeV above the rate expected from cosmic ray collisions, the spectrum and fraction exhibit a tendency to sharply drop off at high energies. The positron data is in excellent agreement with the dark matter model predictions with a dark matter mass of 1 TeV. An alternative speculation for the newly measured positron spectrum and fraction is that this rise and drop off may come from other astrophysical phenomena such as pulsars. By continuing to collect data through the lifetime of the Space Station (2024), AMS will be able to distinguish between these two new sources.

Antiprotons are very rare in the cosmos. There is only one antiproton in 10,000 protons therefore a precision experiment requires a background rejection close to 1 in a million. It has taken AMS five years of operations to obtain a clean sample of 349,000 antiprotons. Of these, AMS has identified 2200 antiprotons with energies above 100 billion electron volts. Experimental data on cosmic ray antiprotons are crucial for understanding the origin of antiprotons in the cosmos and for providing insight into new physics phenomena. At the same time, protons are the most abundant particles in cosmic rays. AMS has measured the proton flux to an accuracy of 1% with 300 million protons and found that the proton flux cannot be described by a single power law, as had been assumed for decades, and that the proton spectral index changes with momentum. This result has changed the universal understanding of cosmic rays.

At the same time, AMS contains seven instruments with which to independently identify different elementary particles as well as nuclei. Helium, lithium, carbon, oxygen and heavier nuclei up to iron have been studied by AMS. From the AMS measurements, for carbon-to-helium and for carbon-to-oxygen these ratios are, indeed, independent of rigidity flat, as expected. Unexpectedly, the proton-to-helium flux ratio drops quickly but smoothly with rigidity. Other secondary cosmic rays being measured by AMS include boron and beryllium. The ratio of beryllium to boron provides information on the age of the cosmic rays in the galaxy. From this, AMS has determined that the age of cosmic rays in the galaxy is 12 million years. Above 65 GeV, the B/C ratio measured by AMS is well described by a single power law B/C= kRd with d= -0.333±0.015. This is in agreement with the Kolmogorov turbulence model of magnetized plasma where d= -1/3 asymptotically. Of equal importance, the B/C ratio does not show any significant structures in contrast to many cosmic ray models.

The Big Bang origin of the Universe requires that matter and antimatter be equally abundant at the very hot beginning of the universe. The explanation for the apparent absence of primordial antimatter is known as Baryogenesis. Baryogenesis requires both a strong symmetry breaking and a finite proton lifetime. Despite the outstanding experimental efforts over many years, no evidence of strong symmetry breaking nor of proton decay have been found. Therefore, the observation of a single anti-helium event in cosmic rays is of great importance. AMS has collected 3.7 billion helium events (charge Z = +2). To date we have observed a few Z = -2 events with mass around 3He. In the coming years, with more data, one of our main efforts is to ascertain the origin of the Z = -2 events.

The results from AMS promotes the understanding of the production, acceleration and propagation of cosmic rays. The accuracy and characteristics of the data, simultaneously from many different types of cosmic rays require the development of a comprehensive model. In the coming years, with more data, one of our main efforts is to ascertain the origin of the Z = -2 events.

Shandong University joined the AMS Collaboration in March 2004. Professor Cheng Lin takes charge of the AMS Thermal Control System and takes full responsibility for the research, design, manufacture and experimentation of the system. The Thermal Control System of the particle detector AMS operating on the ISS was completed in 7 years, solving the key problem of particle detection in space. Under the leadership of Professor Cheng Lin, more than 30 scientists from MIT, ETH, NASA etc. proposed a new method to keep the detector's temperature balance based on the periodical change of the temperature difference and the dynamic response characteristics of heat transfer of the media with a large heat capacity. They built a radiator with both high heat transfer and heat storage capacities which is special in its structure, enabling the quick heat dispassion from the power distribution, electronics and detectors operating on AMS. At the meantime, the radiator becomes a heat source by absorbing heat when the ISS is facing the sun, which raises the overall temperature and ensures the efficiency of heat dispassion and the uniformity and stability of the temperature field. The key problem of AMS' operation in space environment is thus solved.

AMS cycles the earth together with the ISS every 90 minutes, undergoing periodical temperature changes from -40°C to +60°C, with the possible lowest and highest being -90°C and +230°C respectively. The Thermal Control System lays the foundation for the normal operation of all parts on AMS. During more than 5 years operation on ISS, AMS has gone through many different extreme conditions and made a success, the Thermal Control System of which makes a great contribution to the thermal control method of large scientific instruments operating in space.

During these 5 years after AMS being mounted on the ISS, under the leadership of Professor Cheng Lin, the team from the Institute of Thermal Science and Technology of Shandong University continue taking charge of the operation and monitoring of the Thermal Control System, undertaking 96% of all the work. They modify and build thermal control models according to the real operation data in order to cope with different extreme conditions. There have been more than 60 people from Shandong University working at CERN, CGS and ESA since 2004, making continuous contributions to the manufacture and data analysis of AMS. The work done by Shandong University is one of the key factors for the success of AMS.

Source: the Institute of Thermal Science and Technology of Shandong University

Written by: Song Jiwei

Edited by: Xie Tingting