Strategies to Detect Neutrinos

Thomas Kayll

What are Neutrinos and how do we detect them

A neutrino (ν) is a subatomic particle from the lepton family with a lepton number of +1, a charge of 0 and a spin of ½. There are three flavours of neutrino the Muon Neutrino, Tau Neutrino and the Electron Neutrino¹. Neutrinos rarely interact with matter because they are so small and have no charge and are also not affected by the strong nuclear force. So the only way a neutrino can interact with matter is through the weak nuclear force. Neutrinos are about 100,000 times smaller than electrons but there are so many neutrinos being emitted into the universe that even with their incredibly small mass they outweigh the amount of matter in the universe⁹.

Figure 1

Neutrinos were hypothesised in 1930 by Wolfgang Pauli⁸, he theorised that another particle must be emitted in beta decay other than the electron as not all the energy from the decay carried by the electron so Pauli suggested that another particle was emitted and was carrying the rest of the energy given off. It was expected that the electron would carry all the energy but this is not what was found. The law of conservation of energy states that Energy can’t be created or destroyed, but it can be changed into a different form, also that in a closed system it cannot be lost.

The red line represents the energy the electron should have if none was shared from the beta decay of carbon 14 and the blue line represents the actual energy of the electrons.

The first people to detect the neutrino were Reines and Cowan. They did this by using the prediction the nuclear reactors were meant to produce high amounts of neutrino fluxes. When one of the antineutrinos collides with a proton a neutron and a positron are given off⁶.

These positrons then collide with electrons and annihilate via pair-annihilation. When this happens two gamma rays are produced as radiation in opposite direction.

Figure 8

Reines and Cowan soon realized that detecting the gamma bursts wasn’t enough evidence to categorically say they had found neutrinos. So they aimed to detect the neutron given off as well. Reines and Cowan set up a new experiment where they constructed a tank of water and lined it with a scintillating material to detect the gamma radiation. A scintillating material is a material that fluoresces when hit by a photon or a charged particle. This is then picked up and amplified by photomultiplier tubes. They also put cadmium into the tank; cadmium absorbed the neutrons given off in the reaction between the antineutrino and the proton and becomes an exited form of cadmium witch give off gamma radiation^1.1.

Figure 9

The gamma rays form the exited cadmium were detected 5X10^-6 seconds after the positron electron annihilation. This gave enough evidence to prove that neutrinos did exist. Reines and Cowan repeated the experiment in a different location with better cosmic ray shielding. Cosmic rays comprise of very high energy particles such as high energy photons, these particles can interfere with very sensitive electronics used in the experiments and can create false readings. Form this they got more reliable results^1.1.

In a reaction the baryon number, lepton number and the strangeness must stay the same. So in beta decay where an electron is given off an anti lepton must be released to make the lepton number 0 again.

I am going to be looking at how the Super-Kamiokandeis able to detect neutrinos. The Super-Kamiokandeis a large experiment where 50,000 tonnes2 of ultra pure water is held in a stainless steel spherical tank covered in 11,146 photomultiplier tubes all of this is located in a old mine 1,000 meters underground to stop cosmic ray interference. To be detected, a neutrino would interact with a H2O molecule and would cause an Electron to be discharged and this would be travelling faster than the speed of light in water causing Cherenkov radiation to be emitted. Cherenkov radiation is emitted when a particle travels faster than the maximum velocity of a photon in that medium. This radiation produces a ring of light which is detected by the photomultiplier tubes witch amplify the signal, using this we can calculate where the neutrino interacted and what flavour of neutrino it was³.

Photomultiplier tubes are needed as they are able to amplify the signal by around 100 million times. When a photon from the Cherenkov radiation hits the photocathode then a photoelectron is released vie the photoelectric effect , this is then attracted to the first dynode with a pd of approximately 100V this electron gains kinetic energy and then hits the dynode liberating more electrons (typically 3-4) then these are attracted to the next electrode with a pd of 100V and a charge of 200eV and the same happens again until there is a strong enough signal and the electrons hit the anode and then the detected signal is sent off to the computer⁴.

For each electron liberated on the dynodes the energy is

The 100eV the electron carries is enough to liberate around 4 new electrons on the next dynode.

With some electrons not hitting the dynodes and some not liberating exactly 4 new electrons then the figure that the signal is amplified by 100 million times and that (3-4) electrons are liberated by on electron make are correct.

From research it seems that the dynodes have a work function (ψ) of around 5eV this means that about 80eV is lost when the electron hits the surface of the material.

Figure 5

This Is the Super-Kamiokande form the inside. Each dot is a photomultiplier tube, and there are two people checking them on the surface of the water in a dingy.

The first recorded instance of an observation of a neutrino was in 1970 on the 13 of November. The event was observed when a neutrino collided with a proton and created a mu-meson (muon) and a pi+-meson (pion). A pi+-meson is a particle which consists of a quark and an anti-quark. A pi+-meson consists of an up quark and an anti down quark. A muon is a member of the lepton family in the standard model. This all occurred in a hydrogen bubble chamber. A bubble chamber is a vessel that holds super heated liquid (in this case hydrogen); it is used to detect charged particles that enter it. It is able to crate observations of these particles as when a charged particle passes through the chamber it causes an ionisation path which causes the surrounding liquid to vaporise and form bubbles which size are proportional to the specific particles energy loss. This is all captured by cameras which can produce a picture of the event⁵.

Figure 6

This is the original picture of the collision

This is an annotated picture showing the paths of the colliding particles. Muon (μ^–), proton (p), neutrino (νμ) and the pion (π+). When the neutrino and the proton collide the proton moves to the left. The neutrino is turned into a muon which keeps going forwards and the pion is created from the collision5.

The annotation to the right shows what is happening at the sub atomic level with quarks.

In 2011 the OPERA experiment conducted which came across the odd results that neutrinos were travelling faster than the speed of light. The results were declared as anomalous as anything going faster than the speed of light in a vacuum is considered to go against special relativity. The scientists conducting the experiment set investigations into why they got the results they did. From these investigations it was found the there were two faults in how the experiment was set up. One was that a fibre optic cable was improperly connected and that a clock oscillator was set to fast. Taking both of these errors into account meant the reading were not actually faster than the speed of light.

In 2012 it was reported that the speeds of neutrinos are the same as the speed of light. This information was gathered by numerous different scientific groups including OPERA.

There are many different sources of neutrinos such gamma ray bursts, supernovas, neutron stars, nuclear fission and cosmic rays. Neutrinos are defiantly not rare with potentially about 100,000 billion passing through your body every second. All of these sources are some of the most energetic/violent processes in the universe. The main source of our neutrinos that are detected by places like ice cube and Super-Kamiokande is the sun through its nuclear fission which gives off many neutrinos.

Here you can so that a neutrino and a positron are emitted when two H¹ atoms collide and coalesce to form a H² atom.

Ice cube is another neutrino detector in the South Pole that uses the same idea as the super-Kamiokande in that it detects the gamma rays from when a neutrino collides with a water molecule. Ice cube is a hexagon that is around 2,450 meters deep and has 86 lines of sensors with 60 sensors on each line so a total of 5,160 sensors.

Conclusion

From my research into what neutrinos are and how we can detect them I have found out the fundamental nature of neutrinos and how we are able to detect something that rarely interacts with matter. I have learnt that neutrinos are harder to detect than I had imagined and that there are different methods such as detecting the Cherenkov radiation from the neutrinos colliding with water molecules or by seeing their ionising path in a bubble chamber. I have also found out some of the reasons behind why neutrinos are so hard to detect in the first place, for example that neutrinos are extremely small, have very low mass, are not charged and only really interact through the weak nuclear force. Over all, neutrinos are very elusive and one of the weirder particles that we have discovered and there is still a lot we do not know about them.

Bibliography

Date accessed: 23/11/2014

1 URL: http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html

URL: http://hyperphysics.phy-astr.gsu.edu/hbase/particles/cowan.html

Hyper physics is a reliable website source because it is hosted by the physics and astronomy department at Georgia state university and has professors who teach the subjects input also it should be non biased as there is no gain for it providing false information. Hyper physics states that their second experiment at Savannah River Plantwas 12 meters underground and states the cross-section of the reaction to be 6X10-44 and the same figures are stated http://en.wikipedia.org/wiki/Cowan%E2%80%93Reines_neutrino_experiment

Date accessed: 23/11/2014

URL: http://physicsworld.com/cws/article/news/1998/jun/05/super-kamiokande-finds-neutrino-mass

Physics world is a website that publishes the new and old physics topics and has many different topics that it has published. It is a reliable source as it is backed by some very credible companies, such as Angstrom Sciences and Moxtek Inc. It also has scientist informing and righting as well which further proves the reliability of the website.

Date accessed: 21/11/2014

URL: http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Super-Kamiokande.html

Princeton.edu states that the page is sourced from Wikipedia

URL: http://en.wikipedia.org/wiki/Super-Kamiokande. it is reliable as Princeton university would not be publishing wrong information on their site as that would be bad for them so that gives this information some credibility. Also physics world URL:http://physicsworld.com/cws/article/news/1998/jun/05/super-kamiokande-finds-neutrino-mass states that the super-Kamiokande is 1000m underground and holds 50,000 tonnes of water which is the same as on the Princeton page this back up the reliability of the data.

Date accessed: 23/11/2014

URL: http://en.wikipedia.org/wiki/Photomultiplier

The data that I found on Wikipedia on photomultiplier tubes was backed up from the equations I used to try and estimate the number of electrons hitting the anode, which gave similar figures to my calculations. Also the theory behind how photomultipliers work was the same as explained in http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/photomultipliers.html

this website also stated gains around 100 million which is my calculated and Wikipedia’s stated value. All this shows that it is a reliable source.

Date accessed: 26/11/2014

URL: http://abyss.uoregon.edu/~js/ast123/lectures/lec22.html

This is an educational site from the University of Oregon who should not be biased as they have no reason to put incorrect information on their website as it would have a negative effect on them and they wouldn’t gain anything. It is reliable as it is written by scientists. The date stated on the page November the 13^th 1970 is the same as stated on http://commons.wikimedia.org/wiki/File:FirstNeutrinoEventAnnotated.jpg.

Date accessed: 26/11/2014

URL: http://t2k-experiment.org/neutrinos/a-brief-history/

T2k is a website dedicated to neutrinos. The website is primarily about news in the field and the t2k experiment of neutrino oscillation. It is a reliable source as it is written by professionals.” the positron annihilates with an electron to create two gamma rays” this statement says the same thing as http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html says on the topic.

Date accessed: 30/11/2014

URL: http://icecube.wisc.edu/science/depth

Ice cube is a website dedicated to the ice cube particle detector in the south-pole that is trying to detect neutrinos and more. Its primary funding source is the national science foundation, this is a US government organisation that funds and conducts many different projects. Their aim is to keep US science at the forefront of the world in discovery. The web site ice cube should be reliable as it has major government input and would not gain anything from false publication. On ice cube it states that the detector has 5,160 detectors this is the same at http://phys.org/news/2013-11-world-largest-particle-detector-icecube.html. Phys is a large physics news blog with articles written by universities and scientists so it is a reliable website as it is written by people who have extensive knowledge in what they write.

“Advanced Physics”, Steve Adams, Jonathan Allday/oxford university press/November 2^nd 2000, p416

Advanced physics is a book published by oxford university press. It is reliable because Oxford University is a highly regarded university that would get a negative publicity if what they published was incorrect. Oxford should not be biased as it doesn’t have any large

Companies or influential people pressuring them to publish false information.

“Neutrino”, Frank Close/oxford university press/ February 23^rd 2012, p2

The book neutrino talks about what neutrinos are and how we detect them, their history, their discovery, their sources and many different topics related to them. The point of the book is to inform and educate people on neutrinos. Professor Frank Close the author is a professor at Oxford University this shows he knows what he is talking about and that the book is reliable as he is a regarded physicist. Oxford press is a reliable publisher as I have stated in reference 8.

Figures