Street Broad Scientific
Review
Neutrinos and Their Detection Jason Liang
Introduction Neutrinos are among the most mysterious and ethereal particles in the universe. They are so weakly interacting that they were not found until 1956; even today huge detectorshave to be built to produce only a few measurable interactions per day. However, many physicists today think that this peculiar property of neutrinos can be exploited togain insights into other phenomena. Since neutrinos only react rarely and weakly, they can provide scientists with information about events that other methods of remote sensing, such as photons, cannot, such as the earliest ages of the universe and the conditions inside the supernovae. In this literature review, I will briefly give an overview of neutrinos and the physics associated with them and then explain the different neutrino detection methods. Neutrinos Prediction and Discovery In 1930 Wolfgang Pauli was the first to hypothesize the existence of neutrinos in order to explain missing momentum and energy in certain nuclear reactions, specifically the missing energy of the electron in beta decay. His predicted particle had no charge and spin 1/2 . In 1933, Enrico Fermi used neutrinos in his theory of beta decay. They were also soon used to explain the decay of particles produced by cosmic rays (Kaneyuki). However, it was not until more than 20 years later in 1956 that neutrinos were detected by Clyde Cowan and Frederick Reines at the Savannah River nuclear reactor. This is because neutrinos are leptons and are not affected by the strong nuclear force. Also, they are not affected by the electromagnetic force and have too little mass to be considerably affected by gravity. As a result only the weak nuclear force measurably affects them so they are extremely hard to detect. Neutrinos come in three flavors: electron, muon, and tau, corresponding to the three lepton flavors. When electron neutrinos react with an atomic nucleus, an electron is produced. Similarly, muons are produced when a muon neutrino interacts, and tau particles are produced in the case of a tau neutrino. The Solar Neutrino Problem The first solar neutrino detector was built by Raymond Davis at the Homestake Mine in South Dakota in the late
1960s. After many years of collecting data, Davis found that the measured neutrino flux at Homestead was only less than a third of the flux predicted from the standard solar model. General consensus was that his experimental methods were sound, and similar experiments confirmed this, but no one knew where the missing neutrinos were. This prompted a deluge of revisions to the standard solar model of the Sun’s interior to correct for the predicted neutrino flux (Bachall). The Atmospheric Neutrino Problem Another paradox was discovered during the 1980s and 1990s when several detectors measured the number of cosmic ray neutrinos produced by cosmic ray showers in the Earth’s atmosphere. Theory predicted that cosmic ray showers would produce two muon neutrinos for every electron neutrino, with those particles resulting from the decay sequence of a pion:
π + → µ+ ν µ → ν e + e + + ν µ + ν µ
However, at SuperKamiokande, only 1.3 muon neutrinos were detected for every electron neutrino (Kajita). This contradiction with theory was called the atmospheric neutrino problem. In particular, the ratio of muon to electron neutrinos was less for neutrinos going upward (through the bulk of the Earth) than for neutrinos coming downward. The only explanation for this that did not result in major contradictions in other areas of physics was that neutrinos could oscillate, or change flavors. Most of the detectors then in use, including SuperKamiokande, could not detect tau neutrinos because to be detectable in those detectors, tau neutrinos must interact with a nucleus via the weak interaction and produce tau particles, which requires an immense amount of energy that the neutrino usually does not have. Thus, the phenomenon of muon neutrinos changing into tau neutrinos would explain the discrepancy in the ratio of electron neutrinos to muon neutrinos. Other more complicated three-flavor oscillations also explain the experimental results (Kajita). Neutrino Oscillation and Mass Oscillation implies that neutrinos have mass, which contradicts the results obtained from most forms of the Standard Model. Even so, in the past 20 years it has Volume 1 | 2011-2012 | 41
Street Broad Scientific become accepted that neutrinos actually have an extremely small but nonzero mass. Neutrino mass seems to be the most likely explanation for the atmospheric neutrino paradox. Quantum mechanics can provide the explanation for neutrino oscillation because all particles with mass have wave properties. In the case of neutrinos, the wavefunction determines the probability of the neutrino being a muon neutrino instead of being an electron neutrino. Neutrinos of different masses have dierent frequencies for their wavefunctions, and as the wavefunctions propagate through space the probability oscillates (Figure 1).
Figure 1. Oscillation of neutrino wavefunctions (Source: College of William and Mary)
The frequency of the oscillation depends on the several factors, including the energy of the neutrino and the difference of the squares of the masses (Kaneyuki). A slow oscillation corresponds to a small difference in mass of the neutrinos, while a fast oscillation indicates a large dierence. This has allowed scientists to place limits on the masses of the different flavors of neutrinos. Thus far, the upper limit of the mass is 2.2 eV for the electron neutrino, 0.17 MeV for the muon neutrino, and 15.5 MeV for the tau neutrino (Meszaros).
Review do not corroborate very well (Figure 2). Also, since the original solar neutrino experiments, the Sudbury Neutrino Detector has confirmed that the total flux of all three types of neutrinos from the Sun is equal to the theoretically predicted value, showing that the detection methods of SuperKamiokande and other detectors could not have been the sources of the deficiency (Freedman). Neutrino Detection Methods Introduction Neutrinos do not respond to the strong or electromagnetic forces, so detection methods cannot detect them directly but rather rely on detecting the products of their reaction with other particles. There are two main methods of detecting neutrinos: via scattering or capture. Detecting neutrinos with the scattering method involves imaging the Cherenkov radiation that particles emit after their collision with a neutrino. Neutrino capture (or charged-current) detection methods record the energies of electrons or antielectrons that are emitted when a neutrino collides with a proton and the proton undergoes beta decay or inverse beta decay. As a result, this method can only detect electron neutrinos of electron flavor. Radiochemical Detectors The first detector specically designed to find solar neutrinos was a chlorine detector built by Raymond Davis of Brookhaven National Laboratory in the 1960s. His detector consisted of 100000 gallons of perchloroethylene (C2Cl4) in an underground tank at the Homestake Gold Mine in Lead, South Dakota (Freedman). When a neutrino struck the nucleus of a chlorine atom, one of the neutrons was converted into a proton, turning the chlorine into a radioactive atom of 37Ar (Bahcall). This reaction,
νe +37 Cl → e− +37 Ar
Figure 2. Comparison of models incorporating neutrino oscillation to models which do not (Source: Kajita)
As for proof that neutrino mass can provide the explanation for the atmospheric neutrino problem, models of the number of neutrino detection events versus the angle of the incoming neutrino which incorporate neutrino oscillation agree well with the data that were collected, while regular models which do not take oscillation into account, 42 | 2011-2012 | Volume 1
could capture neutrinos with a threshold energy of 0.814 MeV at the rate of 10-35 neutrinos per atom per second, which could detect all the major sources of neutrinos except the common pp neutrinos. To determine the eciency of the collection method, samples of 36Ar and 38Ar were added before the radioactive argon was collected. The argon was separated chemically fromthe perchloroethylene every few months by circulating helium gas through the detector, which collected the argon onto a charcoal trap where it was absorbed (Bahcall). The number of atoms was then counted by measuring the number of electrons produced by the decay of 37Ar, which has a halflife of 35 days. This process removed the argon with an efficiency of 95%. This experiment provided the first evidence of neutrino oscillation, since the measured neutrino flux was 2.1±.9 SNU (where 1 SNU = 10-36
Review captures per target atom per second) while the predicted flux was 7.9 Âą1.33 SNU (Bachall), indicating a deficit in the flux of electron neutrinos. Scattering Detectors The main type of scattering detector is the water Cherenkov detector. When particles in a medium move faster than the speed of light in that medium a shock wave of photons is created, similar to the creation of a sonic boom by an object moving faster than the speed of sound, which results in a cone of blue light in the direction of the particle (Scholberg). (This does not violate special relativity because this theory states that nothing can ever travel faster than the speed of light in a vacuum, not just the speed of light in a particular medium.) Photomultiplier tubes lining the inside of the detector then amplify the photons into measurable electrical pulses and record them. When an energetic neutrino enters the detector and collides with protons, positrons and other particles are produced. These particles recoil, and many of them recoil faster than the speed of light in water, producing Cherenkov radiation. The number of photons produced by a charged particle moving through the detector is proportional to the amount of energy lost by the particle. This is the principle upon which water Cherenkov detectors operate. A drawback of these detectors is that any particle that can scatter electrons is detected, not just neutrinos, so there have to be strict lters on which events are analyzed (Scholberg). One of the oldest continuously operating detectors is Kamiokande (currently SuperKamiokande), located in Kamioka, Japan. Kamiokande was originally built in the 1980s as a proton decay detector. To screen out other particles, such as muons from cosmic rays, which could produce similar ashes of light, Kamiokande is located one kilometer under Mount Ikenoyama. It is a cylinder which contains 50 kilotons of ultrapure water to help light maintain its intensity over a long distance and more than 10000 inward-facing photomultiplier tubes each half a meter in diameter to pick up interactions inside the cylinder. Muons produce events which are similar to the ones which neutrinos cause, so almost 2000 photomultiplier tubes face outward to detect light from charged particles, which allows muons and neutrinos to be differentiated because muons have charge while neutrinos do not (Kajita). The main producers of Cherenkov radiation in water Cherenkov detectors are electrons. The threshold energy for these types of detectors is 0.8 MeV because the electron must have a velocity greater than c/n (where n is the index of refraction of water), about 225000 km/s, to produce Cherenkov photons. By analyzing the properties of the light cones, such as their size, shape, and intensity, scientists can infer many properties of the incoming neutrinos. For example, they can also tell what kind of neutrino produced the light
Street Broad Scientific cone (Figure 3). The rings of light produced by muon neutrinos are relatively sharp, while electron neutrinos result in a more diuse ring because electron neutrinos produce electrons, which then scatter photons more effectively than the muons produced by muon neutrinos (Kaneyuki). Also, the direction of the light cone is the direction of the particle produced by the neutrino, which is extremely close to the incident direction of the neutrino and thus points to the direction of the neutrino source. Lastly, the number of photons produced lets scientists know the energy of the particle and allows the energy of the neutrino to be inferred. The results of SuperKamiokande provided significant evidence for neutrino oscillation and led to its widespread acceptance by the scientific community as the solution to the solar neutrino problem and atmospheric neutrino problem. As noted in Figure 2, the data collected by SuperKamiokande fit models incorporating oscillation but correlatedpoorly with models that did not. Another type of scattering detector is the organic scintillation detector. Scintillation detectors are composed of hydrocarbons, specically alkanes (Scholberg). Photomultiplier tubes observe photons produced by the de-excitation of molecular orbitals. Just like in water Cherenkov detectors, the energy loss is proportional to the number of photons collected. However, light emission from de-excitation is uniform in all directions, so unlike water Cherenkov detectors scintillation detectors cannot determine the direction of the incoming neutrino. An example of this kind of detector is Borexino in Italy.
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Street Broad Scientific
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Heavy Water Detection There are several detectors that can detect neutrinos using both the scattering and capture methods. One example is a heavy water detector in which each hydrogen atom in water is replaced by a deuterium atom. This is the detection method that the Sudbury Neutrino Observatory (SNO) in Canada uses (Freedman). SNO can easily detect all three types of neutrinos, while detectors utilizing electron capture can only detect one flavor, and detectors using neutrino-electron scattering are far more sensitive to one type of neutrino than to other types. When a neutrino of any flavor passes through the detector, it can eject the neutron out of the deuterium nucleus in a neutral-current reaction. The ejected neutron is soon captured by another nucleus, producing a gamma ray which can be detected by photomultiplier tubes. The SNO can also detect neutrinos via scattering and absorption using charged currents, which are the methods that other detectors such as Super-Kamiokande use and which are limited to mainly detecting electron neutrinos. The neutral-current reactions are: νe + d → p + p + e −
ν+d→ν+p+n
The main result of this experiment was the confirmation of the theory of neutrino oscillation (Freedman). SNO could detect all three flavors of neutrinos unlike previous detectors, and when the total neutrino flux was added up the result matched the solar neutrino flux predicted by the standard solar model. Liquid Argon Detectors The last major type of neutrino detector is the liquid argon detector, which uses a time-projection chamber (Figure 4). The time-projection chamber forms a threedimensional image of the recoil electrons produced by the charged-current reaction by using a constant electric field to drift electrons into a recording plane, with the time of drift and final position of the electron determining the original position (Bahcall). The reaction is
νe +40 Ar → e− +40 K ∗
with a threshold energy of 5.885 MeV. The excited state of potassium emits gamma rays almost immediately, which allows the time of the event to be recorded. This is the detection method used by the ICARUS detector in the Gran Sasso lab.
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Figure 4. A diagram of an argon detector. (Source: Symmetry Magazine)
Conclusions In the past 20 years, there have been many groundbreaking studies in the field of neutrino physics. This is exemplified by the discovery of neutrino oscillations and the realization that neutrinos have mass at SuperKamiokande in Japan. In addition to research being done at SuperKamiokande, many new detectors are in the first stages of planning and construction, including a new large-scale liquid argon detector in the United States. These potential “neutrino telescopes” will allow scientists access to new and innovative methods of neutrino detection and analysis, which may result in groundbreaking discoveries relating to the most high-energy events in the universe, such as the Big Bang and supernovae. References Cited Bahcall, John H. Neutrino Astrophysics. Cambridge: Cambridge University Press, 1989. Print. Freedman, Roger A., Robert M. Geller, and William J. Kaufman III. Universe. 9th ed. New York: W. H. Freeman and Company, 2011. Print. Kajita, Kakaaki, Edward Kearns and Yoji Totsuka. “Detecting Massive Neutrinos.” Scientic American August 1999: 48-55. Print. Kaneyuki, Kenji and Kate Scholberg. “Neutrino Oscillations.” American Scientist May-June 1999: 222-231 Print. Scholberg, Kate. “Supernova Neutrino Detection.” Annual Review of Nuclear and Particle Science 2012. Print.