Latest measurements from the AMS experiment unveil new territories in the flux of cosmic rays

The Alpha Magnetic Spectrometer (AMS[1]) collaboration has today presented its latest results. These are based on the analysis of 41 billion particles detected with the space-based AMS detector aboard the International Space Station. Credit: Image courtesy of CERN

The Alpha Magnetic Spectrometer (AMS[1]) collaboration has today presented its latest results. These are based on the analysis of 41 billion particles detected with the space-based AMS detector aboard the International Space Station. The results, presented during a seminar at CERN[2], provide new insights into the nature of the mysterious excess of positrons observed in the flux of cosmic rays. The findings are published today in the journal Physical Review Letters.

Cosmic rays are particles commonly present in the universe. They consist mainly of protons and electrons, but there are also many other kinds of particles, including positrons, travelling through space. Positrons are the antimatter counterparts of electrons, with the same mass but opposite charge. The presence of some positrons in space can be explained from the collisions of cosmic rays, but this phenomenon would only produce a tiny portion of antimatter in the overall cosmic ray spectrum. Since antimatter is extremely rare in the universe, any significant excess of antimatter particles recorded in the flux of energetic cosmic rays indicates the existence of a new source of positrons. Very dense stars, such as pulsars, are potential candidates.

The AMS experiment is able to map the flux of cosmic rays with unprecedented precision and in the results published today, the collaboration presents new data at energies never before recorded. The AMS collaboration has analysed 41 billion primary cosmic ray events among which 10 million have been identified as electrons and positrons. The distribution of these events in the energy range of 0.5 to 500 GeV shows a well-measured increase of positrons from 8 GeV with no preferred incoming direction in space. The energy at which the positron fraction ceases to increase has been measured to be 275±32 GeV.

"This is the first experimental observation of the positron fraction maximum after half a century of cosmic rays experiments," said AMS spokesperson Professor Samuel Ting. 

"Measurements are underway by the AMS team to determine the rate of decrease at which the positron fraction falls beyond the turning point."

This rate of decrease after the "cut-off energy" is very important to physicists as it could be an indicator that the excess of positrons is the signature of dark matter particles annihilating into pairs of electrons and positrons. Although the current measurements could be explained by objects such as pulsars, they are also tantalizingly consistent with dark matter particles with mass of the order of 1 TeV. Different models on the nature of dark matter predict different behaviour of the positron excess above the positron fraction expected from ordinary cosmic ray collisions. Therefore, results at higher energies will be of crucial importance in the near future to evaluate if the signal is from dark matter or from a cosmic source.

"With AMS and with the LHC to restart in the near future at energies never reached before, we are living in very exciting times for particle physics as both instruments are pushing boundaries of physics," said CERN Director-General Rolf Heuer.

AMS also reported a new observation that both the electron flux and the positron flux change their behaviour at about 30 GeV, the fluxes being significantly different from each other both in their magnitude and energy dependence. In particular, between 20 and 200 GeV, the rate of change of the positron flux is surprisingly higher than that for electrons. 

This is important proof that the excess seen in the positron fraction is due to a relative excess of high-energy positrons, and not the loss of high-energy electrons. This new result is very important for a better understanding of the origin of cosmic ray electrons and positrons, and may be the sign of an unknown phenomenon.

In his seminar, Professor Ting also presented some interesting new results to be published in the near future. These show that, at high energies and over a wide energy range, the combined flux of electrons plus positrons can be described by a single constant spectral index, with no existence of structure as suspected by previous measurements of other experiments.

Notes:

[1] The AMS detector is operated by a large international collaboration led by Nobel laureate Samuel Ting. AMS involves about 600 researchers from China, Denmark, Finland, France, Germany, Italy, Korea, Mexico, the Netherlands, Portugal, Spain, Switzerland, Taiwan, and the United-States. The AMS detector was assembled at CERN, tested at ESA's ESTEC centre in the Netherlands and launched on 16 May 2011 onboard NASA's Space Shuttle Endeavour. It is installed on the International Space Station where it tracks incoming charged particles such as protons, electrons and antimatter particles such as positrons, mapping the flux of cosmic rays with unprecedented precision.

[2] CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. Its headquarters are in Geneva. Its Member States are currently: Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Romania is a Candidate for Accession and Serbia is an Associate Member State in the pre-stage to Membership. India, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have Observer status.

Source: CERN

Where do astronauts go when they need 'to go?'

NASA researchers sought to design a way to contain urine in the inevitable event that future astronauts would need 'to go' while wearing their spacesuits.

Alan Shepard became the first American to fly in space on May 5, 1961. Although NASA engineers had put considerable planning into his mission, dubbed Freedom 7, noticeably missing from this extensive preparation was a way for him to urinate in his spacesuit. 

During a lengthy launch delay, the inevitable happened, and Shepard's urine short-circuited his electronic biosensors. In less than a year, engineers had remedied this seeming oversight for John Glenn's Mercury orbital flight. The system developed for Glenn stood the test of time, remaining in use until the early days of the Space Shuttle program.

In a new article, Hunter Hollins of the National Air and Space Museum reviews the history of urine collection in space and the considerations necessary to accommodate this basic physiological function. That first successful urine collection device, used in 1962, has been on display at the National Air and Space Museum since 1976.

The new article, titled "Forgotten Hardware: How to Urinate in a Spacesuit," appears in the June 2013 edition of Advances in Physiology Education, a journal published by the American Physiological Society

No Need "To Go?"

Hollins writes that though the general public was interested in how astronauts would tackle taking care of this basic need in space (a letter stored in NASA's Historical Reference Collection from a Pennsylvania schoolgirl questioned where the first man in space would use the toilet), NASA's scientists and technicians seemed to ignored the problem before Shepard's mission. Combined with a lack of funding and little crosstalk between the organizations that would end up comprising NASA, scientists in the organization also assumed that the first astronauts would be able to "hold it" during their very short missions.

However, though Shepard's spaceflight was scheduled to last only 15 minutes, he spent eight hours in his spacesuit due to launch delays. During a four-hour stint on the launch pad, he relieved himself in the suit, damaging the electronic medical data sensors attached to his body.

After this understandable event, NASA researchers sought to design a way to contain urine in the inevitable event that future astronauts would need to go while wearing their spacesuits.

New Device a Relief for Astronauts

Working around the spacesuit itself was one barrier to successful urine collection. The pressure suits worn by astronauts help keep their occupants alive during spaceflight by ensuring that pressures inside stay within a healthy physiological range. However, the bulky, uncomfortable suits left little room for devices to capture urine.

The first iteration of urine collection devices proposed for space were in-dwelling catheters, a tube threaded through the penis to collect urine continuously from the bladder. However, such catheters are extremely uncomfortable and greatly increase the risk of infection.

After Gus Grissom's Mercury-Redstone 4 mission followed Shepard's in 1961 -- in which Grissom urinated between two pairs of rubber pants -- NASA researchers set about developing a more suitable urine collection device. They ended up basing theirs on the simple personal urinals already available at the time for people with medical problems, such as impaired bladder control, or those without access to public urinals, such as police officers on a long shift.

In the end, the resulting device resembled a condom made out of more durable materials and open on one end, with a tube connected to a storage container. On Glenn's Mercury-Atlas 6 mission, he voided a full bladder into the new device, confirming its utility.

Tweaks Still Necessary

Astronauts regularly used this type of device with minimal modifications until the early days of the Space Shuttle program, Hollins writes. However, those and modern urine collection devices still aren't perfect. Hollins notes that in a survey done in 2010, the majority of U.S. Air Force pilots flying high altitude spy planes reported problems with the urine collection devices they wore, including poor fit, leaking, and skin damage from extended contact with urine.

"It is the job of the engineer/physiologist to ensure that the man-machine interface promotes the health and safety of the human body," Hollins says.

Source: American Physiological Society (APS)