Friday, June 5, 2020

What Is A Neutrino? Discovery, Experiments, Neutrino Detectors For Detection Of Neutrino



Discovery Of Neutrino

In the early 20th century, accurate measurements of the energy of beta decay products found that if only the nuclei and electron were involved, energy and momentum were lost in the decay. To reconcile this observation with the universal conservation of energy, Wolfgang Pauli, in 1930 felt obliged to invent a particle without mass or electric charge that could participate along with the nuclei and electron in the decays.

Postulating a particle with little evidence for existence caused Pauli some worry. He said, "I've done a terrible thing, I postulated a particle that cannot be detected. This particle was later dubbed "Neutrino (or little neutral one)" by Enrico Fermi in his theory of beta decay.

Neutrinos are the most abundant particle in the universe after photons. But how can we see them? How can we detect something that is not affected by the electromagnetic force and has no mass? How can we see a particle that supposedly interacts with nothing?

Detection Of Neutrino

In 1962 John Bahcall, a theoretical physicist from the California Institute of Technology was introduced to Raymond Davis Jr. by William Fowler, who was looking for a system to detect solar neutrinos. Ray Davis asked Bahcall to calculate what amount of neutrinos the Sun could produce and can be captured by his detector?

So, Bahcall calculated the neutrino capture rate in the current solar model by hand. In the early 1960s, John Bahcall and Ray Davis started to consider how to verify, how the Sun shines?

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1964 is the birth of neutrino's physics with a back-to-back paper by Ray Davies and John Bahcall. The idea was to design a solar neutrino trap. Earth's surface is constantly being bombarded by many forms of radiation like cosmic rays and solar particles. The detector had to be underground, to prevent interference from other atmospheric particles. With this in mind, it was built inside the Homestake Gold Mine in Lead South Dakota at the depth of 1,478 meters. The detector was a huge 380 cubic meter tank filled with a common dry-cleaning fluid "tetrachloroethylene" which was chosen because of the amount of chlorine in the compound.

Upon collision with the neutrino, a chlorine atom transforms into a radioactive isotope of Argon which can be extracted and counted. It was necessary to use so much target material because there is a very small probability of a successful neutrino capture. Ray Davis designed a way to collect the Argon that formed.

Every few weeks, he bubbled helium through the tank to collect the radioactive argon. Counting the number of atoms, allowed him to determine, how many nuclei had undergone the reaction induced by a neutrino. So it was able to determine how many neutrinos had been captured.

Homestake Experiment

The Homestake experiment started to work in 1968 and it was an extraordinary success. For the first time, neutrinos from the Sun were detected. But the first results observed that the sun's output of neutrinos was less than expected. In fact, only one-third of those expected from Bahcall's calculations. This discrepancy between the number of predicted neutrinos and the number measured soon became known as the "Solar Neutrino Problem".

The theoretical calculations were refined and checked many times, over the next two decades by many scientists, but no significant errors were found in a model by John Bahcall. The Homestake experiment continued to operate for decades refining its measurements without any significant change in its observations. It was more than 20 years before the new detectors were designed to measure solar neutrinos including lower energies making them more sensitive.

Get The Complete Information About Homestake Experiment - Source: sns.ias.edu

Gallex- Gallium Experiment

Gallex was built inside the Gran Sasso mountains in Italy using liquid Gallium. Soviet-American Gallium Experiment (SAGE) also with gallium in the Baksan Caucasus Mountains in Russia. Even earlier Kamiokande followed by Super-Kamiokande, both using ultra-pure water were built neared Kamioka in Japan and unlike other detectors, were able to make real-time momentum measurements of the neutrino flux, confirming the Sun as the dominant source.

In all cases, observed flux was between 50 and 60 percent of that predicted by the standard solar model. So their results still showed fewer neutrinos than original calculations. The consistency of measurements could only mean one of two things:

1.) The standard solar model was wrong and its success in describing other aspects of solar evolution was an accident.

2.) There were other effects causing the flux of neutrinos from the Sun to diminish before reaching the earth.

This meant that a better theory of fundamental physics was required to solve the mystery of the missing neutrinos.

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In early 2000, there's no experiment that was able to measure electron neutrinos and also all the families of neutrinos. The dissolution to the solar neutrino problem was the neutrinos changed. Neutrinos change type when they come from the Sun to the earth. We don't know why? But, neutrinos and all particles coming three families, on the way from the Sun to the earth. Neutrinos that were produced in one family electron neutrinos changed to the other families muon or tau neutrinos, this is called neutrino oscillations or neutrino flavor conversions.

This experiment was only sensitive to electron neutrinos and that's the reason, why radius experiment was measuring fewer neutrinos than predicted by John N. Bahcall calculations.

The GALLEX Solar Neutrino Experiment At Gran Sasso Underground Laboratory - Source: harvard.edu

The discovery derived by solving the solar neutrino problem is that neutrinos are massive. We don't know why neutrinos have different masses?

It is taken more than 40 years to find the right amount of neutrinos coming from the Sun. It has changed our description of the universe. Science is that unexpected full of big surprises.

Light can give us a picture of the universe at night. We see photons coming from the stars and galaxies. Photons are the most abundant particles in the universe. Academy see what's going on inside stars? How can we get a glimpse of the unknown? Good news an uncommon eye is opening up new frontiers.

IceCube - Neutrino Detector In The South Pole

Why doing astronomy with neutrinos, when they are so hard to detect. Well, the reason is simple, first of all, the neutrino has no electric charge, so it just is basically the same as a photon- the particle of light. So you are doing the same astronomy, the critical difference between neutrinos and light is- neutrinos go to walls and light doesn't. So the inferences that they may reach us from places in the universe that we have never seen before. So scientists built IceCube to do astronomy with neutrinos. Now the simplest way of thinking about astronomy is that you go out at night, you look at the sky and you see beams of light coming from stars. This is the perfect analogy.

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IceCube is basically a big eye that looks at the sky and instead of seeing beams of light, it sees the beams of neutrinos. Now, why were scientists interested in neutrinos? Why not use slightly cheaper and easier?

Well scientist are very likely to see very different things and in fact, at a moment they have detected the first beams and they are trying to figure out what they are actually seeing but educated guesses are that they are seeing very powerful cosmic accelerators may be a supernova, remnants gamma-ray bursts, active galactic nuclei, all the things that are part of the high-energy universe.

Complete Information From Official Website Of IceCube

What is so special about the South Pole?

The South Pole ice itself is the detector. Between one and a half and two and a half kilometers below the surface, groups of light sensors are in a position to see the light produced by particles passing through the ice. Over 5,160 of these light sensors have been deployed, instrumenting a volume of one square kilometer under the ice. High-energy neutrinos produce charged leptons when they interact with the ice. These leptons produce an explosion of light and Ice Cube captures it in its sensors.

What's critical in the design is? how far light travels through the ice?

The light sensors have to be spaced according to the absorption length, the average distance light travels in the ice. In tap water, a light will travel two meters and in distilled water, a light will travel eight meters. In ice beneath the South Pole, light travels more than 100 meters. In some places even more than 200 meters.

The ice in the South Pole is one of the clearest solids that exist. It may not be possible to build a solid in a laboratory as transparent as this ultra-pure ice which in the end is just snow that condensed and fell on Antarctica about 100,000 years ago at the depth of IceCube.

Scientists were doing an analysis where they were looking for extremely high-energy neutrinos. They actually knew exactly what they were looking for, they were looking for something that's called Cosmogenic neutrinos. It doesn't matter what is this? They didn't find any, but when they looked at the data they found something that they had never seen before.

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Over the year scientists have looked at thousands and thousands of events on the online display and knew they had never seen events like those - in fact, they were so special. After seeing them it was clear what was special about these events and they define the new analysis that could go and look for more of that of these. So by now, it has 26 more friends. Scientists claiming that we have evidence for neutrinos that come from space.

Where do they come from, that's next frontier. Some of them are not emitted in the direction of the center of the plane of our own galaxy. So they come from outside the galaxy. There are hints in the data that some of them actually may come from our own galaxy. The problem is there are not enough statistics, there are not enough events to come to a conclusion. Then scientists started the busy approach in a different direction. They started to look for more events and finding more of these very special events.

By the way, what's special about these events is that they have enormous energies. That's why they look different from anything scientists had seen before. Scientists detected a neutrino every six minutes but they are rather uninteresting and produced in our Earth's atmosphere. These events have 10 times bigger energy. So clearly scientists want to get more of these and so they build a bigger detector and are now figuring out how to do that any turns out.

It was not that difficult because scientists found out while building IceCube that the South Pole ice is much more clear than they guessed and so this allows them to build a much bigger detector by basically doubling the number of sensors that they have to deploy in the ice. So they were busily designing the next step.

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The matter is what makes it more special than any other form of energy?

Trillions of particles pass through us every second. One of them is so faint that it is extremely hard to detect. Could it be the answer to this question?

Types Of Beta Decay

There are three types of beta decay:

Standard beta decay

Standard beta decay involves the nucleus decaying by emitting an electron and a neutrino.

Double-Beta Decay

The second is Double-Beta Decay- where two electrons and two neutrinos are emitted simultaneously. Very few nuclei can undergo this decay. Since it requires that the isotope "One-Beta " decay from the original be more unstable making this simpler decay even more unlikely. The nuclei for which this decay has been observed are extremely stable with lifetimes greater than the age of the universe.

Beta Decay: Full Details - Source: oregonstate.edu

Neutrinoless Double-Beta Decay

Third as the yet unobserved channel is Neutrinoless Double-Beta Decay- for this decay to exist the neutrinos must annihilate each other. A process that requires the neutrino to be its own antiparticle and hence a different type of matter for all other particles.

J.J. Gomez-Cadenas, Principal Investigator NEXT IFIC CSIC Universidad de Valencia said, "total beta decay is a very rare decay process, so in some sense, it's very strange. But it has nothing special. It happens in a number of nuclei. What is very relevant is?  This decay is produced without the emission of neutrinos, why is that so? Because, it marks the existence of a very peculiar property of the neutrino, the fact that neutrinos, its own anti-particle."

Alessandro Bettini, Director of Canfranc Underground Lab said, "the neutrinos and anti-neutrinos in the standard model are supposed to be two different particles. If this process happens, neutrinos and anti-neutrinos are the same particles. Because the nucleus that decayed the neutrino exists inside the nucleus. Is it an exchange between two parts of the nucleus to make possible this decay. This decay was foreseen by Ettore Majorana, in 1938 already. Neutrino would be in this case also anti-neutrino that is like matter to be also anti-matter and that in the universe you have only practically matter. So that might be an explanation of why the matter is so dominating in the universe."

J.J. Gomez-Cadenas added, "we wanted to be able to make an experiment that could display a large mass and with very cleansing. A number of ideas around that eventually came out of a discussion with Dave Nygren, a professor from Lawrence Berkeley National Lab, who had the idea for a long time of using the chamber. So we started to discuss, we came up with the idea we could build the chamber for the confront experiment. Next is a simple idea is a pressure chamber. Basically, it's a pot at high pressure in which we fill a lot of Cinnamon. Cinnamon is a gas that has the capability of scintillating, producing ultraviolet light. We are able to use this ultraviolet light to give us the signals that we will use to detect the event of the disintegration of a particle. So one next great idea is the fact that the same gas that we use as a target. Because it can decay as a double beta."

Neutrinoless Double Beta Decay.pdf - Source: indico.cern.ch

NEXT- Neutrino Experiment With A Xenon TPC


NEXT is contained within a pressure vessel made of a low radioactivity titanium steel alloy. Xenon gas is continuously purified and flowed through the vessel. The central part of the vessel contains a system to produce a directed electric field. The field causes electrons liberated from their atoms by passing charged particles to move towards the amplification region. A plane of sensors at the opposite end of the vessel measures the deposited energy. While another more finely instrumented and directly behind the amplification region records the event topology. An array of large-high sensitivity photomultipliers accurately records the amount of light produced. Allowing for a reconstruction of the energy deposited in the gas. In the other plane, an array of small closely packed silicon of photomultipliers is tasked with recording information about where the light was produced.

Justo Martin-Albo, Physicist of NEXT Team said, "In an empty knowledge Double-Beta decay seen on 2 electrons of fixed energy at a million. So what we would expect to detect in NEXT? If double beta decay happens, it is to measure an event with the right energy, 2.5 MeV, and that it looks like two electrons coming from a common point. This with the detector we would take a picture of this event. It would look like a long track with two blocks at the end."

The NEXT team designed a matrix of thousands of silicon photomultipliers that allows the scientists to take a picture of the electron trajectory. The sensors are at a strip of flexible Crypton that avoids the introduction of radioactivity inside the detector. It is very difficult to design and build radio pure electronics that have no interference with a detection signal.

The electronics of the photomultipliers widened the signal, so it can be reconstructed later point by point. The electronics of the silicon photomultipliers integrates the signal every micro-second so it can make a million images per second.

Nadia Yahlali, Physicists of NEXT Team said, "the electrons that are produced in the ionization of the Xenon gas by charged particles excited as seen on atoms which decay emitting VUV light near 172 nanometers. The silicon photomultipliers are not sensitive at this short wavelength. We have therefore to use an organic wavelength shifter called TPB to shift the VUV light into blue light to which the silicon PMs are more sensitive. We do this process by vacuum evaporation in a cleanroom. The vacuum evaporation of TPB allows us to obtain very clean and uniform coatings on the silicon peels, which make them able to record the Xenon scintillation light and perform the tracking function in the TPC. "

The Canfranc Laboratory was first conceived by A. Morales and other researchers at the University of Zaragoza in 1985. This first proposal to build experimental areas inside the old train tunnel under the Pyrenees was dismissed but later a larger facility was built.

Placing detectors for rare events underground shields them from much of the radiation coming from the Sun and atmosphere, stopping the interesting signals from being overwhelmed much like the light from distant stars can be overwhelmed by the light from the Sun or moon. Next, we'll search for neutrinoless double-beta decay in the Canfranc Underground Laboratory.

We know that the universe is made of matter and not of antimatter. We also know that the early universe should have been made up of equal parts, matter, and antimatter. But where did all the antimatter go? Could this ghost of a particle hold the key? Could it amount of neutrino caused the universe to favor matter?

The discovery that the neutrino is its own anti-particle may take a great effort. It may require to have masses of up to one tonne of Xenon. No other experiment is capable we think, of the exploit at the same time the great energy of resolution, the good topological signal in the large mass that we can recoil next. Therefore we do think that as the problem of discovering the neutrino is its own anti-particle becomes more and more difficult. NEXT, get more and more chances of being the discovering experiment.

An interesting question is why do we do this kind of science? Why do we search for this rare event and how do we know that we're gonna succeed?

It's very interesting to compare what we are trying to do in the NEXT. Searching for neutrinoless double beta decay experiments with the recent success of the IceCube experiment.

In the end, you put the faith, the passion, the years of work and you end up discovering something that you didn't even expect. Science is all about is not finding what you expect to find is to finding something you didn't have any idea you were going to find is to find the unexpected.

Get Complete Information From Official Website

The NEXT double beta decay experiment.pdf - Source: iopscience.iop.org