Neutrinos: Physics

Science topics

Neutrinos: Physics

Earth science Image courtesy of Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo Particle physics an introduction Astrophysics Ages 14+

What do continental drift, nuclear power Neutrinos are strange particles – small but fascinat- stations and supernovae have in common? ing. This article describes Neutrinos, as Susana Cebrián explains. their origin, properties and detection in an accessible way and with sound facts. It makes good background What are neutrinos? Austrian physicist Wolfgang Pauli reading for physics teach- Neutrinos, meaning ‘little neutral in 1930 to explain observations of ers, but can also be a start- ones’, are everywhere, all around us. radioactive beta decay. It was not until ing point for students do- These tiny elementary particles travel the first nuclear power stations were ing a presentation project through space at close to the speed of built, though, that a sufficiently high on the topic or to stimu- light and have no charge. They were flux of neutrinos (actually their anti- late further discussions, once thought to have no mass either, particles, antineutrinos; see Landua e.g. about particle physics but scientists now suggest that they & Rau, 2008, for more information on in general, the standard do have a mass; it is estimated to be antiparticles) from decaying fission model, detector physics, less than a billionth of the mass of fragments was available to confirm CERN, astrophysics or ra- a hydrogen atom, but the research their existence. In 1956, Clyde Cowan diation. continuesw1. and Frederick Reines built two large The article is mainly use- The existence of neutrinos, among water-filled tanks underground, mere ful for physics lessons, but the most abundant particles in the metres away from the nuclear power it contains links to earth Universe, was first postulated by plant at Savannah River near Aiken, science. To make the topic accessible to younger students (about the age of 14) as well, I would suggest The Super-Kamiokande the teacher selects parts of detector tank is filled the article to discuss. with water

REVIEW Gerd Vogt, Austria

www.scienceinschool.org Science in School I Issue 19 : Summer 2011 I 55 Image courtesy of PBS NOVA; image source: Wikimedia Commons

South Carolina, USA, in which the Three generations of matter Bosons The standard model of particle physics. antineutrinos interacted with the (forces) Matter particles come in two different water’s protons (see diagram below). Mass types, leptons and quarks, forming a set of Frederick Reines was awarded the Charge Spin 12 particles, divided into three genera- Nobel Prize in Physics in 1995w2 for Name strong force tions, each consisting of two leptons (one this experiment. Clyde Cowan could of which is a neutrino) and two quarks. not share the prize, because he had Matter particles can ‘communicate’ with each other in different ways by exchang- passed away in 1974. electromagnetic Quarks force ing different types of messenger particles Neutrinos come in three types or named bosons (a different boson for each flavours , according to the standard of the fundamental interactions), which model of particle physics (see im- can be imagined as little packets of en- age, right): the electron neutrino, muon ergy with specific properties. The masses

neutrino, and tau neutrino, which have Leptons of certain particles are still being investi- all been confirmed experimentally. For gated by the scientific community; these the detection of the muon neutrino, are values from 2008 Leon M Lederman, Melvin Schwartz and Jack Steinberger were awarded the Nobel Prize in Physics in 1988w2. Super-Kamiokande experimentw4, always found many fewer electron A fourth, ‘sterile’ type has been in which muon neutrinos gener- neutrinos arriving from the Sun than proposed, which is immune to the ated in the atmosphere were found expected. In 2001, the Solar Neutrino weak force of the standard model, and to ‘disappear’, presumably into tau Observatoryw7 in Canada demonstrat- recent data including a refined calcu- neutrinos. A recent experiment has ed that they changed into neutrinos of lation of measurements performed at now successfully observed such an other flavours on their way to Earth the Institut Laue-Langevinw3 in Greno- event from the other perspective – as (Bahcall, 2004). Further experiments ble, France, in the 1980s support this an appearing tau neutrino rather than to analyse neutrino oscillations are idea (Hand, 2010; Reich, 2011). Were a disappearing muon neutrino: after underway, for example in France and sterile neutrinos to be found, a new three years in which a beam of muon Japan, where accelerators and nuclear realm of physics beyond the standard neutrinos was released at CERNw5 in power stations provide large numbers model would open up. Geneva, Switzerland, a tau neutrino of antineutrinos for observationw8. Even the three confirmed types of was detected in 2010 by the OPERA neutrinos are special: they oscillate detector at the Gran Sasso National Where do neutrinos come from? from one flavour to another – electron, Laboratoryw6 in Italy, 730 km away Neutrinos first originated some muon and tau neutrinos change from (see images on page 57). The detec- 14 billion years ago, 10-43 seconds after one to another. This phenomenon was tioin of the oscillations also solved a the Big Bang. A mere second later, first observed in 1998 by the Japanese 40-year-old mystery: scientists had they were already rapidly moving

Scintillator coupled to photomultipliers The Reines and Cowan experiment: electron

antineutrinos (ne) interact with the water’s protons (p+) in a big tank filled with water Positron e+ Flux of 0 and cadmium chloride (CdCl ); as a result, n Neutron n Electron e- 2 antineutrons e positrons (e+, the antiparticles of electrons) from nuclear Cd + g ray photons 0 power plant Proton p and neutrons (n ) are produced. Positrons g ray photon from e+ annihilation are annihilated when they encounter elec- from - n0 capture trons from the medium (e ) and neutrons are absorbed by the cadmium (Cd) nuclei.

Water with CdCl2 Both these reactions result in the release of gamma ray photons (g) which are detected Scintillator coupled to photomultipliers by means of scintillators. These transform the signal into flashes of visible light, which can then be detected and processed by photom- n + p+ → e++ n0 e ultiplier tubes n0 +113Cd → 114Cd* → 114Cd + g e+ + e-→2g Image courtesy of Susana Cebrián

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Image courtesy of CERN Image courtesy of Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

The OPERA detector at the Gran Sasso The Super-Kamiokande tank, nearly full of water, seen from above National Laboratory in Italy

away from the rest of the hot and rying information about these areas. heavier ones (see image on page 58, dense primary particle soup; scientists In this sense, neutrinos are cosmic and to learn more, see Westra, 2006, are still seeking to detect neutrinos messengers, and neutrino astronomy and Boffin & Pierce-Price, 2007); more that survive from the Big Bang. is becoming increasingly important. than 1010 solar neutrinos hit a square It is neutrinos’ weak interaction So far, only two sources of extrater- centimetre of Earth every second. with matter that makes them al- restrial neutrinos have been observed: Unlike photons, which take about most impossible to detect, but it is the Sun and supernovae. Raymond 100 000 years to travel from the core also what makes them of interest to Davis Jr and Masatoshi Koshiba won of the Sun to its outer photosphere scientists. Unlike most other particles, the third neutrino Nobel Prize in before speedily travelling to Earth, neutrinos are able to escape from Physics in 2002w2 for their detection of neutrinos released in the same fusion dense regions such as the core of the solar and supernova neutrinos. Like process do the entire trip in a mere Sun or the Milky Way, and they can other stars, the Sun emits electron 8 minutes. This is why solar neutri- travel long distances from far-away neutrinos at several steps of the proc- nos are useful messengers carrying galaxies without being absorbed, car- ess by which light nuclei fuse into information about the current fusion

On their way from CERN in Geneva, Image courtesy of CERN Switzerland, to Gran Sasso, Italy, some of the muon neutrinos in a dedicated beam turn into tau neutrinos, and are detected by OPERA Monte-Prato Monte-Giovo Monte-Maggiorasca Tuscany Emilia-Romagna Florence Alessandria Piedmont Monte-Emilius Arezzo Mont-Blanc Perugia Umbria Laboratory of Gran Sasso of Gran Laboratory Geneva 11.4 km CERN

Neutrino beam 732 km 5 km www.scienceinschool.org Science in School I Issue 19 : Summer 2011 I 57 0 50 km Hydrogen Neutrino

Deuterium

Positron Helium-3

Hydrogen Electron

Gamma ray Helium-4 Fusion reaction Electron Positron Proton Neutrino Neutron Gamma ray

Image courtesy of Mark Tiele Westra

processes inside the Sun, such as only scientists interested in neutrino neutrinos’), who are interested in neu- the chemical composition of its corew9. detectors. On Earth, both natural and trinos that originate in outer space, Supernova neutrinos are the result artificial neutrino sources exist: ra- but it provides neutrino physicists of the violent end of some stars, which dioactive materials from inside Earth with another means of studying their explode and produce even more can undergo beta decay, producing favourite particles. neutrinos than photons (see Székely geo-neutrinos. In addition, nuclear & Benedekfi, 2007): in 1987, several fission reactors produce neutrinos, How to detect neutrinos detectors registered an unusually and dedicated particle accelerators Neutrinos are very useful for study- strong signal (several events within a are being used as neutrino sources ing astronomical and cosmological few seconds, as opposed to the usual for research. These are interesting to phenomena, and neutrino detectors frequency of about one per day), at- particle physicists, of course, to fur- are being built worldwide, deep tributed to neutrinos from supernova ther characterise neutrinos, but also underground to filter out the ‘noise’ of SN1987A in the Large Magellanic to earth scientists and maybe even other particles. The recently finished Cloud. To allow astronomers to pre- politicians (see ‘Neutrinos as nuclear IceCubew11 is the largest detector yet: pare to observe these events, several police’ and ‘Powering Earth’). a cubic kilometre of ice at the South neutrino detectors are now linked Finally, when cosmic rays hit Earth’s Pole, acting as a telescope to search together as the Supernova Early atmosphere, atmospheric neutrinos for neutrinos from astrophysical Warning Systemw10, because during are emitted as decay products of pions sources (see images below). When a these stellar explosions, the neutrinos and muons. This most abundant neutrino hits a proton of Antarctic ice, are released before the photons that source of naturally occurring neutri- a muon is released. Like any charged the astronomers seek to detect. nos on Earth is a nuisance to neutrino particle travelling at more than the Astronomers, however, are not the astronomers (see ‘How to detect speed of light in a specific medium

The IceCube neutrino telescope array A B Images courtesy of NSF is located at the South Pole (A; South Pole Station to the left of the runway, IceCube to the right). It consists of thousands of autonomous digital optical modules (B), which record the arrival time of each neutrino. They are deployed in deep holes in the ice, drilled with hot water (C). When a neutrino hits a proton of Antarctic ice, a Cherenkov cone of blue light is generated (D), and the path of light is reconstructed from the times of neutrino detection

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Fusion in the Sun: two hydrogen nuclei fuse to form a deuterium nucleus, a positron and a neutrino. The positron quickly encounters an electron, they annihilate each other, and only energy remains. The deuterium nucleus goes on to fuse with another hydrogen nucleus to form helium-3. In the final step, two helium-3 nuclei fuse to form helium-4 and two hydrogen nuclei

(though less than the speed of light come from underground. Neutrinos metre antineutrino detectors could be in vacuum), the muon will generate a are the only particles able to penetrate used to non-intrusively monitor and conical trail of blue light – Cherenkov the Earth unhindered, so any muon safeguard nuclear reactorsw12. radiation, the photonic equivalent of coming from that direction must have At present, reactors are monitored a supersonic boom, which can also be been newly generated in the detector indirectly (for example using satel- seen in some nuclear reactors. from a cosmic neutrino. lites, gas and dust emissions, and Thousands of optical sensors, in Other detectors use different ma- seismic and infrasound signatures for a three-dimensional grid 1.5-2.5 terials and strategies – but all put as weapons testing), which can result in km deep in the ice, detect this light; much material in the neutrinos’ way misleading data. Neutrino detectors combined, the data can be used to as possible, trying to make them inter- would provide real-time informa- determine the energy of the neutrino act and reveal themselves. tion about the reactor core power and and the direction it came from. To possibly even its isotopic composition. distinguish muons that are generated Neutrinos as nuclear police An array of about 500 such detectors from cosmic neutrinos from the mil- Detection of nuclear weapons worldwide would be able to calculate lions more muons that are produced and material is important for many the power output of individual reac- by cosmic rays in the atmosphere reasons, including the prevention of tors, allowing the detection of clan- above the detector, IceCube uses Earth nuclear proliferation and terrorism. destine nuclear weapons testing. as a filter, only looking at muons that Scientists now propose that cubic-

Ice top IceCube lab 81 stations, each with An artist’s impression of the mate- 2 ice top Cherenkov detector tanks 2 optical sensors per tank rial around supernova SN1987A: 50 m 324 optical sensors two outer rings, one inner ring and IceCube array the deformed, innermost expelled 86 strings including 8 DeepCore strings

material Image courtesy of ESO / L Calçada 60 optical sensors on each string D 5160 optical sensors December, 2010: project completed, 86 strings

1450 m DeepCore 8 strings-spacing optimised for lower energies 480 optical sensors Eiffel Tower 324 m 2450 m 2820 m

Bedrock www.scienceinschool.org Science in School I Issue 19 : Summer 2011 I 59 Image courtesy of Karlsruhe Institute Technology (KIT)

The KATRIN spectrometer in Karlsruhe, Germany, is one of the experiments aims to ‘weigh’ neutrinos

Powering Earth 10 000 000 000 000 000 neutrinos have subscribe to Nature today: www.nature.com/subscribe Neutrinos are also detected in passed through you without your geophysics. Natural radioactive decay noticing. Tiny, yet with the power to Landua R, Rau M (2008) The LHC: of uranium, thorium and potassium confirm or overthrow a number of a step closer to the Big Bang. in Earth’s crust and mantle sustains scientific theories. Science in School 10: 26-33. the flow of molten material in convec- www.scienceinschool.org/2008/ tive currents, which drive continental Acknowledgement issue10/lhcwhy drift, seafloor spreading, volcano The editors would like to thank Dr Reich ES (2011) The amazing disap- eruptions and earthquakes. Christian Buck, neutrino physicist pearing antineutrino. Nature. doi: There are several models for this from the Max-Planck-Institut für Kern- 10.1038/news.2011.202 decay, depending on the composition physik (Max Planck Institute for Nu- Székely P, Benedekfi O (2007) Fu- of Earth’s crust. Geo-neutrinos pro- clear Physics), Heidelberg, Germany, sion in the Universe: when a giant duced during decay may help answer for advice in developing the article. star dies... Science in School 6: 64-68. the question of crust composition. www.scienceinschool.org/2007/ Geo-neutrinos were first detected in References issue6/fusion w13 2005 by the KamLAND experiment Bahcall J (2004) Solving the mystery of Westra MT (2006) Fusion in the in Japan, although an abundance of the missing neutrinos. Nobelprize. Universe: the power of the Sun. nuclear power stations limited the org. http://nobelprize.org/nobel_ Science in School 3: 60-62. studies, because the antineutrinos prizes/physics/articles/bahcall www.scienceinschool.org/2006/ they release have similar energetic Boffin H, Pierce-Price D (2007) Fusion issue3/fusion signatures to those of geo-neutrinos. in the Universe: we are all stardust. Web references In 2009, an international team from Science in School 4: 61-63. www. w6, w14 the Borexino project was more scienceinschool.org/2007/issue4/ w1 – Scientists from the University successful because there are fewer nu- fusion College of London have estimated the mass of the neutrino: clear power stations nearby, so even- Hand E (2010) Hunt for the sterile www.ucl.ac.uk/news/ tually, statistically significant numbers neutrino heats up. Nature 464: 334- news-articles/1006/10062204 of geo-neutrinos should be collected 335. doi: 10.1038/464334a to determine the relative amounts of Hopes for the most accurate meas- Download the article free of charge uranium, thorium and potassium. urements yet lie with the KATRIN from the Science in School website In the time that you have experiment, which is in Karlsruhe, (www.scienceinschool.org/2011/ been reading this article, about Germany: www-ik.fzk.de/~katrin issue19/neutrinos#resources), or 60 I Science in School I Issue 19 : Summer 2011 www.scienceinschool.org Science topics

w2 – Read about the Cowan and http://arxiv.org/pdf/0811.2424 duced good overview brochures on Reines experiments here: For more information about fusion neutrinos, freely available online. http://library.lanl.gov/cgi-bin/ See: www-boone.fnal.gov/about/ in the Sun and elsewhere in the getfile?00326606.pdf nusmatter (Neutrinos Matter) Universe, see the Science in School and www.interactions.org/pdf/ To find out more about the Nobel article series: www.scienceinschool. neutrino_pamphlet.pdf (Neutrino Prizes in Physics 1988, 1995 and org/fusion Odyssey) 2002, see: http://nobelprize.org/ w10 – The Supernova Early Warning nobel_prizes/ physics/laureates The IceCube project website hosts System website can be found here: a wonderful teaching activity on w3 – The Institut Laue-Langevin oper- http://snews.bnl.gov ‘popcorn neutrinos’, in which ates one of the most intense neutron w11 – For more information about on- students can investigate the sources in the world. It is a member concepts behind beta decay. See: going research projects at IceCube, of EIROforum, the publisher of www.icecube.wisc.edu see: http://icecube.wisc.edu Science in School. See: www.ill.eu or use the direct link: Olivier Hainaut from the European w4 – For more information on the http:// tinyurl.com/45ytuq7 Southern Observatory (ESO), the Japanese Super-Kamiokande ex- For slides of three talks on neutrino world’s most productive astronomi- periment, see: www-sk.icrr.u-tokyo. research for school students, avail- ac.jp/sk/index-e.html cal observatory, which is a mem- able in German, see: www.mpi-hd. ber of EIROforum, talked to the mpg.de/hfm/wh/pams/PamS0708. w5 – To find out more about CERN, IceCube scientists during the 24-h htm the world’s largest particle phys- live webcast ‘Around the world in Particle Adventure is a fun interac- ics laboratory and one of the eight 80 telescopes’ during the Interna- tive online tour of particle physics: members of EIROforum, see: tional Year of Astronomy 2009. Start http://particleadventure.org www.cern.ch watching the online video from The Contemporary Physics Educa- To read more about the successful minute 1:27: www.eso.org/public/ tion Project offers student and detection of a muon neutrino turned videos/10msouthpole teacher worksheets in English and into a tau neutrino in CERN’s press For the ESO website, see: Spanish for classroom activities on release, see http://public.web. www.eso.org particle physics, including one on cern.ch/press or use the direct link: conservation laws, following http://tinyurl.com/64crhr5 For further astronomy Pauli’s footsteps in postulating Or watch a video on the resources, see: the neutrino (Activity 5). See: topic: www.youtube.com/ Starr C, Harwood R (2009) Educa- www.cpepweb.org/Class_act.html watch?v=M3aB_zUZ1c8 tion resources for the International The UK Science and Technology w6 – The Gran Sasso National Labora- Year of Astronomy. Science in School Council has compiled a resource tory in Abruzzo near L’Aquila, Italy, 13. www.scienceinschool.org/2009/ guide for teaching particle physics. is the largest underground labora- issue13/iya See: www.stfc.ac.uk/Public and Schools/2563.aspx tory in the world for experiments w12 – To learn more about the in particle physics, particle astro- idea of detecting secret fission physics and nuclear astrophysics, reactors using neutrinos, see: Susana Cebrián is a professor at the including the OPERA and Borexino http://physicsworld.com/cws/ University of Zaragoza, Spain, work- experiments. To learn more, see: article/news/44411 ing on several experiments in the field www.lngs.infn.it of astroparticle physics at the Spanish w13 – If you would like to find w7 – For more information about Canfranc Underground Laboratory. out more about the KamLAND the Canadian Solar Neutrino experiment in Japan, see: Observatory experiment, http://kamland.lbl.gov see: www.sno.phy.queensu.ca w14 – Find out the latest about w8 – In France and Japan, detec- the Borexino experiment: tors have been installed to further http://borex.lngs.infn.it analyse neutrino oscillations. See: http://doublechooz.in2p3.fr and Resources http://jnusrv01.kek.jp The Booster Neutrino Experiment To learn how to use this code, see w9 – To find out more about current and Interactions.org have pro- page 1. solar neutrino research, see: www.scienceinschool.org Science in School I Issue 19 : Summer 2011 I 61