Problem with electroscopes

Electroscope
B.Rossi, “Cosmic rays”, 1964

Electroscope, a scientific instrument used to detect the presence of an electric charge on a body. The charge accumulated in its leaves makes them open because of the electrostatic force. When electroscope was charged, then during the time its charge becomes smaller and smaller. After some time the charge becomes lost and electroscope leaves become closed. The question was why the charge was lost? Because the isolation was not perfect? People improved isolation, also filled the electroscope with different kind of gases, but still, the charge was lost very fast. At the end of the XIX century, mankind discovered ionization radiation. The newly discovered phenomenon was suspected for discharging electroscopes: ions from ionized gas neutralize the charge on electroscope leaves. Indeed, when electroscope was put inside lead housing, then its discharging was reduced. The new question was about the source of ionization radiation.

Balloon flights

Victor Franz Hess back from his balloon flight in August 1912

Scientists thought that radiation is emitted by radioactive materials placed in the ground. Electroscopes were taken into high mountains or even to the top of Eiffel Tower in the hope it will discharge slower on higher high. A breakthrough was 7 August 1912 y. This day electroscopes were taken up over 5000 meters in the balloon by expedition directed by Austrian physicist Victor F. Hess. The result of experiment shows that to 600m ionization was lower than on the ground, but then started to grow, and on altitude 5000m is 4 times greater. Hess announced that radiation comes “from above”. Soon his conclusion was confirmed by other experimentalists e.g W. Kohlhörster checked that on altitude 9200m ionization is 30 times greater than on ground level.

Verification of balloon flights results

Robert Andrews Millikan (Wikimedia Commons)

Scientists wanted to verify the theory announced by balloon experimentalists. Robert Millikan made the experiment in which he sank electroscopes in two lakes: Muir (3600m above sea level) and Arrowhead (1500m above sea level), 2050m high of air should absorb the same amount of radiation as 1.8m of water of Muir lake. It turned out, that electroscopes sank in Arrowhead lake shows the same level of ionization as electroscopes sank in lake Muir, but 1.8 m deeper. Millikan announced that radiation really comes from above, and the atmosphere between the two lakes is not it’s the source. Millikan introduced the name “space radiation” for this freshly discovered phenomenon.

Investigation continue

Erich Regener, on the left, on board Credit: Archiv der Max-Planck-Gesellschaft, Berlin-Dahlem

Erich Regener, a German physicist, made a series of measures of ionization on high altitudes and deep inside water. Thanks to Regener we have detailed characteristics of space radiation intensity against altitude and water depth. After Milikan and Regener research the world has some pictures of space radiation as a phenomenon which is present everywhere, its intensity does not depend on weather and coms to Earth from every direction.

G-M tube and new possibilities

Early Geiger counter, made by Hans Geiger, 1932,Science Museum London

In 1929y new device was invented: Geiger–Müller tube, a counter of ionization radiation. Scientists got a simple and reliable tool to detect radiation. Instead of long term observation of electroscopes, now become possible to detect short effects of cosmic rays. Kohlhörster and Bothe presented a theory that cosmic radiation is composed of charged particles. It based on an experiment with two G-M tubes which were isolated with a plate of gold ( gold is a very good absorber of particles), two tubes detected coincident effects in the same time, what means that particle entered to the one tube, then left it and move across the gold plate and finally was detected by the second tube. Bruno Rossi made an experiment in which turned out that cosmic radiation penetrates 1m of lead, it means that energy of cosmic radiation is much higher than every known kind of radiation (β radiation can penetrate only a few mm of lead).

Cosmic rays interact with matter

Rossi has continued his experiments with lead and G-M tubes, he put three tubes placed in triangle inside lead housing, when housing has open top, then coincidence of detection radiation by all tubes happened very rarely, but when the housing has closed top , then coincidence detection happen frequently. It was a premise that cosmic radiation interacts with matter and can produce secondary radiation. Combined G-M tubes with Wilson chamber gave possibility to effectively observe track of cosmic rays.

G-M tube and Wilson chamber

Wilson chamber photography
Carl D. Anderson (1905–1991) – Anderson, Carl D. (1933). “The Positive Electron”. Physical Review 43 (6): 491–494

In 1933 Blacket and Occhialini publicized research based on observation of particles traces in Wilson chamber, one of discovery was a “showers of particles” – set of particles produced as a result of interaction hi-energetic cosmic rays particle with the matter. Because it is a low probability that particle from space can reach Earths surface without hitting atmosphere particles the question about the composition of secondary and primary radiation arise. Soon these problems lead to discovery new elementary particles of matter e.g positron, which track is visible on the picture above.

Magnetic field of Earth influence on space radiation particles

Carl Størmer and his assistant Bernt Johannes Birkeland photographing the Aurora Borealis in Alta, Norway.

Earth has a weak but width magnetic field. In theory, the magnetic field should produce a force on moving charged particles, scientists expected that space radiation particles are moved toward magnetic poles of Earth what should produce different ionization rates on different latitude, predicted phenomenon got name “latitude effect”. Another interesting thing was the prediction of “west-east asymmetry”. Norwegian Carl Størmer wanted to explain the phenomenon of aurora borealis and has prepared a metamathematical model of move charged particles in Earth magnetic field, which shows that positively charged particles should rather come from the west, neither negatively charged particles should coming from east.

Evidence for latitude effect and west-east asymmetry

In 1931 Bruno Rossi has constructed “radiation scope” which have possibility to measure radiation coming from particular directions. In 1933 “west-east asymmetry” was confirmed. It was surprising that only 26% of particles comes from east, what means that most of space radiation particles have positive charge. In 1936 Arthur Compton collected all informations about radiation levels on Earth and publicized map of world with ionization levels, indeed ionization is weakest near to magnetic equator (latitude effect), moreover map shows also “longitude effect”: radiation is significantly lower in the area of Indian Ocean.

World map of radiation in sea level
Map of “latitude effect” by Arthur Compton, polish edition of B.Rossi “Cosmic Rays”, 1964

Nature of secondary and primary radiation

Additionally to Wilson Chamber researcher have started to use radiation-sensitive emulsions to detect ionization particles. One of the most interesting problems was the nature of “particles showers”. New elementary particles were discovered muon (1936), pi meson (1947). Muons, which are responsible for penetrating radiation on sea level, have a very short time of life and cannot come from space to sea level, so it becomes obvious that particles observed on low altitudes are produced in Earth atmosphere as a result of hitting primary radiation particles with atmosphere nucleons. To understand the morphology of primary radiation, automatic balloons with emulsions were sent to altitudes higher then 20000m, moreover plenty of laboratories were built on high mountains over the world. Intensive researches have effects with discovered a number of new elementary particles. Analyses of emulsions from balloons show that on high altitudes most of the particles are protons and helium nucleus, moreover hi energetic electrons and heavy nucleons were detected.

The Air Showers

Vision of Particles Shower
Vision of Particles Shower(Credits:NASA)

In the 1934 Rossi made experiments with G-M tubes which were distant horizontally. Experiments showed that from time to time set of detectors detected coincidence of passing particles at the same time, moreover Rossi discovered that on higher altitudes rate of the coincidences are bigger than on sea level. In the 1938 Pierre Auger started systematic research on the new phenomenon and discovered that coincidences of particles detections are detected even if detectors are distant about 75m. The coincidences were results of a concurrent pass of a large group of particles which were created during complex process which was started by entering of primary radiation particle into the atmosphere. The primary radiation particle hits atmospheres particle and started a chain of reaction which produce secondary radiation particles, which hits other particles and produce new elements which finally reach the ground in almost the same moment. Researches said, that energy of rare primary particles responsible for wide Air Shower can even reach 3*1020eV. At the moment mankind can only speed-up particles to energy 7 * 1012eV in LHC accelerator.

Influence The Sun activity on space radiation

In 1937 American physicist Scott E. Forbush observed that after event of solar flare space radiation level observed on The Earth decrease. This new phenomenon got name “Forbush Decrease” , and needed to wait for detailed explanation until the beginning of “Space Age”. The flow of the phenomenon is:

  1. Event on The Sun
  2. After roughly one day, short, 0.1% increase of The Earths magnetic field strength
  3. Few hours duration of decrease The Earths magnetic field strength
  4. Few days of returning back to previous, stable state
Forbush effect
Percent change of space radiation level (up) and changes in magnrtic field (down) during event on February 1958. B.Rossi “Cosmic Rays”, 1964

Enter into Space Age

Sputnik II radiation counter,
S.G.Aleksandrow, Radzieckie sztuczne satelity i statki kosmiczne, 1964
Sputnik II radiation counter,
S.G.Aleksandrow, Radzieckie sztuczne satelity i statki kosmiczne, 1964

4 October 1957, Sputnik 1 – the first artificial satellite was thrown into space. Mankind has entered into Space Age. A month later, on 3 November, Sputnik 2 was thrown to orbit, this time satellite was equipped with apparatus to measure radiation. Because of the cold war, Soviet Union had a connection with Sputnik 2 only on its territory and soviets did not know that apparatus on their spacecraft detected unexpected, very high levels of radiation on altitudes greater than 1000km over middle latitudes. On 1 February 1958 USA threw satellite Explorer 1 which delivered information about the area of very high radiation on high altitudes, this allows Americans to announce the discovery of radiation belts which surrounds Earth.

Van Allen Belts

Van Allen Belts
NASA’s Goddard Space Flight Center/Historic image of Van Allen Belts courtesy of NASA’s Langley Research Center

Satellites instruments showed that Earth is surrounded by belts of hi-energy particles trapped by a magnetic field. James Van Allen was a scientist who designed the instruments and studying the radiation data from Explorer 1, the belts are named with his name. The inner belt consists mostly of protons, outer belt consist electrons. Because of very hi radiation levels, radiation belts are a threat for electronic devices of spacecraft.

Artificial belts of electrons

Image of Starfish Prime
Image of Starfish Prime

Before space flights, scientists predicted, using Carl Størmer research, that charged particles may be trapped by Earth magnetosphere. U.S.A made nuclear explosions in high altitudes to check if is possible to create artificial radiation belts. Secret experiments Argus and Hardtack in 1958, and the most powerful Starfish in 1962 proof possibility of pollute surroundings of the earth with belts of artificially produced electrons.

Solar wind

Solar wind
A wind of particles from the Sun called the solar wind buffets the blue funnel-shaped magnetosphere around the Earth. ESA

In the middle of the XX century, Ludwig Biermann based on his observations presented a theory that tails of comets are created by a stream of particles constantly emitted by the Sun. Eugene Parker presented an explanation of how the Sun heats up its “Corona”, expands it and releases its outer part into space. These released parts of The Sun’s Corona are responsible for making tails of comets and are named “solar wind”. The theory was confirmed during the mission of spacecraft Mariner 2 (started in 1962).

Pioneer 6-7-8-9

On 16 December 1965 U.S.A threw into space, spacecraft Pioneer 6, its mission was to research solar wind and solar activity on solar orbit between orbits of Venus and Earth. Later Pioneer 7(1966), 8(1967) and 9(1968) were thrown and all of them together made the constellation for complex research of solar wind and warning system about solar activity. Measurements made by these spacecraft greatly increased our knowledge of the effects of solar activity on Earth. New information was gathered about the solar wind, solar cosmic rays, the structure of the Sun’s plasma and magnetic fields, the physics of particles in space, and the nature of storms on the Sun which produce solar flares.

Models of space radiation

Space radiation is a threat for astronauts and for electronic equipment of spacecraft. It is important to know the intensity of ionization radiation for a particular space mission to predict dose of absorbed energy. The dose of absorbed radiation limits the lifetime of the mission. Based on a series of satellite missions NASA has developed an empirical model for estimation radiation of trapped particles inside Van Allen belts. The most widely known models are named AE8 and AP8 (E-electrons, P -protons). Although they based on experiments from the sixties and seventies of the twentieth century, AE8 and AP8 are still widely used during missions planning.

South Atlantic Anomaly

SAA on altitude 500km
The anomaly at an altitude of approximately 560 kilometers, ROSAT Guest Observer Facility. Retrieved October 16, 2007.

Since from 1964y with the start of U.S program, OGO(Orbiting Geophysical Observatories) scientists started to collect information about Earths magnetosphere and its models have become more accurate. It turned out that over south Atlantic is an area where the magnetic field is weak and cosmic particles can enter deeply into the atmosphere.

Threat for spacecraft assets

Inside South Atlantic Anomaly(SAA) particles from space can reach altitude 200km and become a serious threat for spacecraft electronic equipment. Small British satellite UoSat3 started in 1989 had detected undesirable events in its electronics during orbiting The Earth. As a result of the satellite mission map with events where prepared. It turned out that 75% of events happened over an area of SAA.

Map with SEU detected by UoSat3
Single Event Upset for Uosat-3 spacecraft (ESA)

References

  • Arnold Hanslmeier (2004) The Sun and space wheather. Kluwer Academic Publisher, New York.
  • Bruno Rossi (1968) Promieniowanie kosmiczne. PWN, Warsaw.
  • NASA (2007) ‘The Pioneer Missions’ [online]. Available at: <https://www.nasa.gov/centers/ames/missions/archive/pioneer.html>. Accessed: 09.06.2019
  • Olgierd Wołczek (1971) Strumienie cząstek kosmicznych. Wiedza Powszechna, Warsaw.
  • S.G. Aleksandrow, R.E. Fiedorow (1964) Radzieckie sztuczne satelity i statki kosmiczne. PWN, Warsaw.