INTRODUCTION:

A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half integer spin. The neutrino (meaning "little neutral one" in Italian) is denoted by the Greek letter ν. All evidence suggests that neutrinos have mass but that their masses are tiny compared to the standards of subatomic particles.

In 1930 Wolfgang Pauli, desperate to preserve the principle of energy conservation, postulated the idea of an unseen particle-the neutrino. Enrico Fermi gave the neutrino its name and wrote down the first description of how neutrinos interact with other particles. Because neutrinos are so light and without electric charge, they are almost inert. In spite of the fact that trillions of neutrinos go through each of us every second, it took nearly 30 years for Pauli’s hypothesis and Fermi’s theory to be confirmed. In 1956, Frederick Reines and his team detected neutrinos produced by a powerful nuclear reactor in Savannah River, South Carolina. He was awarded the Nobel Prize in physics for this discovery. Neutrinos do not carry any electric charge which means that they are not affected by the elecromagnetic force that acts on charged particles, such as electrons and protons. Neutrinos are affected only by the weak subatomic particles, which is of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale. Therefore, a neutrino typically passes through normal matter unimpeded.

SOURCES OF NEUTRINO:

The majority of neutrinos that are floating around where born around 15 billion years ago, soon after the birth of the universe. Shortly after the discovery of radioactivity more than 100 years ago, physicists discovered that Earth is constantly bombarded by cosmic rays from space. Today, the cosmic rays are known to consist of protons, photons, nuclei of atoms from helium to uranium, electrons and positrons, neutrinos, and possibly particles yet to be identified, with energies ranging from millions of electron volts to more than a billion trillion electron volts. Cosmic rays colliding with Earth’s atmosphere produce tremendous numbers of neutrinos which are referred as "atmospheric neutrinos". Other neutrinos are being produced constantly from terrestrial sources such as nuclear power stations and particle accelerator.

Figure 1. Path of neutrino emitted from sun

Abundant Particle

Like photons, they do not carry any electric charge but unlike photons, which have zero mass, neutrinos have a tiny mass. Among the fundamental particles neutrinos are a bit strange in the fact that they interact very feebly with the rest of matter because of which all forms of matter in the universe- the earth and all objects and living things on it- are nearly transparent to them. About 100 trillion of neutrinos from the sun and other cosmic sources pass through our bodies every second. Frederick Reines, who led the discovery of the neutrinos in the year 1956 commented about Wolfang Pauli's prediction in 1930 of its existence, as "The most tiny quantity of reality ever imagined by a human being".

Types of Neutrino

Neutrinos are known to come in three types - electron, muon and tau - associated with three charged particles. Although known to have small masses, the individual masses of the three neutrinos remain unknown. Of the three neutrino types, or "flavours" the heaviest has at least 10 millionth of the electron's mass and it is recently found that each type is found interchange into the other. The ordering of three neutrino masses is unknown and it is referred as "mass hierarchy" question, which the INO (Indian based Neutrino Observatory) is well suited to investigate. They are also found to interchange from one flavor to another as they propagate (Figure 2).

Figure 2. Types of neutrinos

Neutrino Detection

A neutrino can be detected only if it interacts. Neutrinos interact in two ways:

· Charged-current interactions, where the neutrino converts into the equivalent charged lepton (e.g. inverse beta decay, ν_e + p → n + e⁺)– the experiment detects the charged lepton;

· Neutral-current interactions, where the neutrino remains a neutrino, but transfers energy and momentum to whatever it interacted with – we detect this energy transfer, either because the target recoils (e.g. neutrino-electron scattering, ν + e → ν + e) or because it breaks up (e.g. ²H + ν → p + n + ν).

Charged-current interactions occur through the exchange of a W^± particle, neutral-current through the exchange of a Z⁰. In principle, charged-current interactions are easier to work with, because electrons and muons have characteristic signatures in particle detectors and are thus fairly easy to identify. They also have the advantage that they “flavour-tag” the neutrino: if an electron is produced, it came from an electron-neutrino. However, there must be enough available energy to allow for the mass of the lepton to be created from E = mc² – this means that for very low-energy neutrinos (e.g. solar and reactor neutrinos) charged-current interactions are only possible for electron-neutrinos.

Various different detector technologies have been used in neutrino experiments over the years, depending on the requirements of the particular study. Desirable features of a neutrino experiment will typically include several of the following:

· Low energy threshold, so that low-energy neutrinos can be detected and studied (especially for solar neutrinos);

· Good angular resolution, so that the direction of the detected particle can be accurately reconstructed (especially for astrophysical neutrinos);

· Good particle identification, so that electrons and muons can be well separated (essential for oscillation experiments);

· Good energy measurement, so that the energy of the neutrino can be reconstructed (useful for oscillation measurements and astrophysics);

· Good time resolution, so that the time evolution of transient signals can be studied (essential for supernova neutrinos, and important for other astrophysical sources);

· Charge identification, so that leptons and antileptons can be separated (will be essential for neutrino factory experiments).

It is not possible to have all of these things in one experiment – for example, experiments with very low energy threshold tend not to have good angular or energy resolution. Neutrino physicists will select the most appropriate technology for the aims of their particular experiment.

Detection techniques and detectors

(i) Radiochemical experiments

The lowest energy thresholds are provided by radiochemical experiments, in which the neutrino is captured by an atom which then (through inverse beta decay, a charged-current interaction) converts into another element. The classic example of this is the chlorine solar neutrino experiment. Even lower thresholds were achieved by using gallium as the target: the reaction ⁷¹Ga + ν → ⁷¹Ge + e^– has a threshold of only 0.233 MeV, and is even sensitive to pp neutrinos (see figure 6). The produced isotope is unstable, and will decay back to the original element: neutrinos are counted by extracting the product and observing these decays.

Examples of radiochemical experiments: Homestake (Ray Davis; chlorine); SAGE (gallium); GALLEX/GNO (gallium).

(ii) Liquid scintillator experiments

Liquid scintillators have an impressive pedigree as neutrino detectors, since the neutrino was originally discovered using a liquid-scintillator detector. They are primarily sensitive to electron-antineutrinos, which initiate inverse beta decay of a proton: ν_e + p → e⁺ + n. Being organic compounds, liquid scintillators are rich in hydrogen nuclei which act as targets for this reaction. The positron promptly annihilates, producing two gamma rays; the neutron is captured on a nucleus after a short time (a few microseconds to a few hundred microseconds), producing another gamma-ray signal (sometimes the scintillator is loaded with an element such as gadolinium or cadmium, both of which have very high affinities for slow neutrons, to enhance this capture rate). This coincidence of a prompt signal (whose energy gives the antineutrino energy) and a delayed signal (whose energy is characteristic of the nucleus that captures the neutron – 2.2 MeV for capture on hydrogen) allows the experiment to reject background effectively.

Examples of liquid scintillator experiments: Borexino (solar neutrino experiment); KamLAND (reactor neutrino oscillation experiment); MiniBooNE (accelerator neutrino oscillation experiment); SNO+ (liquid-scintillator experiment using the SNO hardware, under construction).

(iii) Tracking experiments

Tracking detectors reconstruct the path of the charged leptons produced in charged-current interactions, either by the ionisation that they cause or by the energy that they deposit. A magnetic field causes the path of the particle to be bent, allowing the momentum of the charged particle, and the sign of its charge, to be reconstructed. These detectors are best suited to higher energy neutrinos, because the distance that a particle will travel through a detector increases as its energy increases, and longer tracks are easier to reconstruct.

Examples of tracking detectors: MINOS (tracking calorimeter for neutrino oscillations); MINERνA (scintillator tracker for studies of neutrino interactions); ICARUS (liquid argon tracker for neutrino oscillations); T2K ND280 near detector (scintillator tracker and gaseous tracker, for characterisation of T2K beam and studies of neutrino interactions).

(iv) Emulsion

Nuclear emulsions are simply the sensitive material of photographic film, made into a slab instead of a thin coat, and exposed to the beam. The ionisation produced by the passage of a charged particle causes chemical changes in the emulsion, which become revealed as visible tracks when the emulsion is developed. A fine-grained emulsion can provide micrometre accuracy in track positions: ideal for reconstructing the decay of an extremely short-lived particle

Examples of Emulsion methods are: The OPERA experiment at the Gran Sasso underground laboratory and the DONUT experiment at Fermilab

(v) Water Cherenkov experiments

Water Cherenkov detectors for neutrinos can be divided into two types:

(a) Densely instrumented artificial tanks (Super-Kamiokande, SNO)

The water is contained in a tank lined with photomultipler tubes. The Cherenkov light produced by the muon or electron is reconstructed as a ring of hit PMTs. The appearance of the ring can be used to identify the originating particle: muons are single particles, and make sharp rings, whereas electrons (and photons) initiate electromagnetic showers, and the nearly parallel electrons and positrons in the shower combine to make a fuzzy ring.

Examples of densely instrumented water Cherenkov experiments: Super-Kamiokande (solar neutrinos, atmospheric neutrinos, far detector for K2K and T2K oscillation experiments); IMB (proton decay experiment, 1979–1989, which was one of the two water Cherenkovs to detect neutrinos from SN 1987A).

(b) Sparsely instrumented natural water (neutrino telescopes)

A very large volume of natural water is instrumented with a sparse array of photomultipliers dispersed throughout the volume (not concentrated at the edges). The cone geometry is not visually apparent, but can be reconstructed using the time at which each hit photomultiplier records its pulse (the opening angle of the cone is known, because these detectors see only high-energy particles). The threshold of these detectors depends on the spacing of the PMTs, but is normally very high (tens or hundreds of GeV); they reconstruct muons, which make a long straight track, much better than electrons, which deposit all their energy in a fairly small volume and are thus seen by fewer PMTs.

Examples of neutrino telescopes: IceCube, ANTARES and Baikal.

Indian based Neutrino Observatory

India was at the forefront of neutrino research in the 1960's. One of the earliest laboratories established, to detect neutrinos in the world was located more than 2 km deep Kolar Gold Field mined in Karnataka. It was at this laboratory that the atmospheric neutrinos were first detected in 1965. Unfortunately this laboratory had to be shut down with the closure of the mines in the 1990's with a growing interest in neutrino physics worldwide, a Indian Neutrino Observatory could provide Indian scientist an opportunity to re-establish their preeminence in neutrinos research.

INO proposed a project to build a world class underground neutrino detector in the Bodi west hills region of Theni district, about 110 km west of Madurai in Tamil Nadu and about 60 km from the Kerala border. Bodi hills was chosen as a suitable site for locating the underground detector as the steep slopes of the Western Ghats provide an ideal and stable rock conditions to build a large underground cavern for long term use (Figure 3).

Figure 3. Preliminary work under way for the proposed neutrino observatory at Pottipuram in the Bodi hills on Jan 12 (photo courtesy: Frontline)

The INO is one of the mega science projects envisaged in the 12th plan to be funded jointly Department of Atomic Energy (DAE) and the Department Of Science And Technology (DST). At present, 26 Indian institutions and about 100 scientists are involved in the project, with the Tata Institute of Fundamental Research (TIFR), Mumbai as the nodal institute. It will be the mega collaborative experimental project to be undertaken in India with a budget of Rs.1584 crores and could set a precedent for large-sclae collaborative efforts in basic sciences (Figure 4).

Figure 4. Rough sketch from INO (Infographics courtesy: Frontline)

INO will house a Iron Calorimeter (ICAL) detector for studding neutrinos, consisting of 50000 tons of magnetized iron plates arranged in stacks with gaps in between where Resistive Plate Chambers (RPCs) would be inserted as active detectors, the total number of 2m X 2m RPCs being around 29000. The sensitive detector will track particles produced by the neutrino interactions inside the detector mass. INO will probe the neutrino oscillation, determination of neutrino masses and mixing parameters, which is one of the most important open problems in physics today. The ICAL detector is designed to address some of these key open problems in a unique way. Over the years this underground facility is expected to develop into a full-fledged underground science laboratory for other studies in physics, biology, geology, hydrology, etc.

Imagined Disaster Situations

There are imagined disaster situation such as radioactive contamination due to “beam misdirection", “radiation high-dose due to neutrino beams from fermilab emerging through the land above the laboratory" and " radioactive particles like carbon 14 and tritium generated by the hadron shower at the point of emergency which would travel great distances along with stream and ground water".

First it must be understood that while the neutrinos are products of radioactive decays, they themselves are not radioactive. So merely detecting them, whichever source they come from at the INO sight does not cause any radioactivity. We all know powerful lasers can damage tissues, cut through matter and can also be used as weapons for destructions. Lasers are intense and focused beams of photons. But that does not deter as from using light in our day today life and in laboratory experiments. The same is true of neutrions. Just because of a focused high energy beam can cause some radiation damage should not mean that scientist should not set up a laboratory to study low energy neutrinos that are harmless.

Applications

1. Rediscovering human understanding of universe

The recent discoveries involving neutrinos, dark matter, and sources of very high energy photons have deepened our understanding of both the universe and the laws that govern it. In addition, these discoveries point to new opportunities for even greater advances. The questions that can be probed include:

• Why do neutrinos have tiny masses, and how do they transform into one another?

• Are the existence and stability of ordinary matter related to neutrino properties?

• Are there additional types of neutrinos?

• What is the mysterious dark matter, and how much of it consists of neutrinos?

• What causes the most powerful explosions in the universe?

• What role do neutrinos play in the synthesis of the elements in the periodic table?

• How do super massive black holes produce very high energy gamma rays?

• Is there a deeper simplicity underlying the forces and particles we see?

2. Communication using neutrinos

The study of neutrinos is important in particle physics because neutrinos typically have the lowest mass, and hence are examples of the lowest-energy particles theorized in extensions of the standard model of particle physics. In November 2012 American scientists used a particle accelerator to send a coherent neutrino message through 780 feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent to immense distances (Figure 5).

Figure 5. Communication through neutrinos

3. Communication with extraterrestrial life

This application is a little far-fetched, but since it is possible to encode messages in neutrinos, theoretically those encoded neutrinos could be beamed into space. Currently, scientists don't have the ability to beam neutrinos that far, and any aliens on the receiving end would have to be able to decode the message (Figure 6)

Figure 6. Communication with extraterrestrial

4. To find Minerals and Oil Deposits

Neutrinos change the way they spin depending on how far they have traveled and how much matter they have passed through. If the properties are studied thoroughly and suitable detectors built, they can reveal the presence of minerals and oil deposits.

5. Study of Dark Matter

Some 50 years after Pauli’s proposal to save the principle of energy conservation, physicists and astronomers proposed another particle to save another important principle of physics—gravity. Fritz Zwicky, Vera Rubin, and other astronomers showed that galaxies and clusters of galaxies do not contain enough matter in the form of stars to be held together by gravity as we understand it. This means either that our present understanding of gravity is incorrect or that there must be a nonluminous form of matter (now called dark matter) that holds these objects and the universe together. The case has grown more interesting in the past decade: By establishing that the total amount of ordinary matter (matter made of neutrons, protons, and electrons) falls short by a factor of seven of being able to account for the needed dark matter, astrophysicists have now raised the stakes. A new form of matter must explain the dark matter. Like the neutrino before it, the hypothesized dark-matter article must be neutral and must interact very weakly with ordinary matter, making it challenging to detect. Could neutrinos be the cosmic dark matter? While we are now confident that they account for at least some part of it, upper limits to the masses of neutrinos from experiments involving the nuclear decay of tritium (a heavy form of hydrogen) already preclude the possibility that neutrinos constitute all of the dark matter. There is now a strong case for the existence of a new particle, which, like the neutrino, must be uncharged and almost inert but may account for the bulk of the dark matter in the universe. This idea has resonated with particle theorists, whose unified theories predict the existence of new types of stable particles with just the properties needed for dark matter.

Figure 7: Neutrino detector in Antartica

REFERENCES:

1. Neutrinos and Beyond, The National Academies Press, Washington, D.C. 2003

2. R. Ramachandran, Neutrino scare, Frontline, March 6, 2015.

3. http://www.ino.tifr.res.in/ino//index.php

4. http://t2k-experiment.org/neutrinos/neutrino-detection/

5. https://icecube.wisc.edu/outreach/neutrinos

6. https://www.stfc.ac.uk

7. www.cosmoquest.org

Received on 20.03.2016 Modified on 24.04.2016

Research J. Pharm. and Tech. 9(4): April, 2016; Page 474-478

DOI: 10.5958/0974-360X.2016.00088.3