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MiniBooNE - Fermilab E898
Overview for Mineral Oil Suppliers

VERSION 2a - 7/26/01

Randy Johnson
University of Cincinnati

Table of Contents

  1. What Is MiniBooNE?
  2. What Are Neutrinos?
  3. What Are Neutrino Oscillations?
  4. Primer on Neutrino Interactions
  5. Overview of the Experiment
  6. Description of the Detector
  7. Method Used for Distinguishing Neutrino Types
  8. Requirements for the Mineral Oil
  9. Additional Information
  10. Glossary

What is MiniBooNE?

BooNE is an acronym for the Booster Neutrino Experiment. MiniBooNE is the first (smaller) stage of the two stage BooNE project. The purpose of BooNE is to verify or refute the controversial results of a Los Alamos experiment, LSND (Liquid Scintillator Neutrino Detector), which indicated that electron and muon neutrinos have different masses and that these two types of neutrinos could oscillate back and forth between each other. This result is controversial for a number of reasons. First, while other experiments see indications of neutrino oscillations (specifically, experiments that are measuring the number of neutrinos coming from the Sun and experiments that measure the number of the different types of neutrinos coming from cosmic ray showers), there are now too many different results to be compatible with what we call the "Standard Model." Either there must be an extension to this model, or one of the experimental results must have a different explanation. Second, the KARMEN in England examines a region similar to the LSND experiment, but sees no indications of neutrino oscillations. This experiment is less sensitive than LSND, and both could be right.

The purpose of MiniBooNE is to definitively resolve this controversy. If the LSND result is correct, MiniBooNE will have hundreds of events showing neutrino oscillations. If the LSND result is due to something other than neutrino oscillations, then MiniBooNE will prove that.

What Are Neutrinos?

Matter is made up of two types of particles: quarks and leptons. The quarks always congregate in groups, forming things like protons and neutrons in atomic nuclei and the various nuclear fragments that are produced at atom smashers and cosmic ray interactions. There are six different types of quarks which differ from each other in mass and electrical charge. Only the two lowest mass quarks (called the up quark and the down quark) make up the proton and neutron. The proton is made up of two up quarks and one down quark; the neutron, two down and one up.

Like quarks, leptons come in six varieties. Unlike quarks, leptons always appear by themselves. There are three negatively charged leptons, of which the electron is the most familiar. Heavier relatives of the electron are the muon and the tau. With each charged lepton comes a neutral partner, called a neutrino. Neutrinos are primarily produced in processes similar to radioactive beta decay. In such decays, the neutrino and its charged lepton partner are always produced in pairs (electron neutrinos are always produced with electrons, muon neutrinos with muons, etc.). Thus, while it is extremely difficult to see a neutrino itself, we know that it exists by measuring the unobserved energy taken away from a decay. We know the type of neutrino produced by identifying the type of charged lepton coming out of in the decay.

What Are Neutrino Oscillations?

Neutrinos were originally proposed as the vehicle to take away the missing energy in beta decays. At that time, they were assumed to have no mass (like the photon). Since then, all attempts to measure the neutrino mass directly have shown results that are consistent with zero. The upper limits on the mass from direct measurements range from 1/250,000th of an electron mass for the electron neutrino to 40 times the electron mass for the tau neutrino.

If neutrinos do indeed have a mass -- and there is no theoretical reason why they shouldn't -- then quantum mechanics allows a strange phenomenon to occur: a neutrino of one type can change to another type and then back again as it travels through space. Measurement of this oscillation rate between types is both definitive proof that neutrinos have mass and a sensitive measure of the mass difference between the two types.

Primer on Neutrino Interactions

The first and foremost feature of neutrinos is that they interact with other types of matter extremely weakly. The neutrinos coming from the Sun's fusion reactions go through us, go through the earth, and go through most of the universe before they interact. The average neutrino from BooNE would go through a stack of steel that was as long as the distance from here to the moon before it interacted. Thus, to see neutrino interactions you need both lots of neutrinos and large detectors (two methods of overcoming the extremely small interaction probability).

When the neutrinos do interact, they do so in one of two ways. In one way (formally called a neutral current interaction) they bounce off a portion of the detector, leaving a bit of energy behind, and then disappear. In the other way, they reveal their identity by changing into their corresponding charged lepton (electron, muon, or tau) as they scatter. It is this latter type of interaction (called a charged current interaction) that is used to determine the numbers of the different neutrino types hitting the detector.

Overview of the Experiment

MiniBooNE consists of two parts: a beamline which produces a beam of a known type of neutrinos and a detector that detects the different types of neutrinos incident upon it. If neutrinos do not oscillate, only one type of charged lepton will appear in the detector; if they do oscillate, more than one type will be seen.

The beam of neutrinos is produced by the protons from Fermilab's Booster. These protons have been accelerated by the Booster to an energy of 8 GeV. These protons slam into a beryllium target making a lot of nuclear debris. The debris then decays into a lot of junk including neutrinos of mostly the muon type. All of the junk except for the neutrinos is stopped in the earth shielding in front of the detector. The neutrinos, since they interact so weakly, fly right through the dirt and to the detector. Most go through; a few may interact.

The detector does two things for the experiment: it provides a medium in which the neutrinos can interact and it identifies the type of charged lepton that comes out of the interaction.

Description of the Detector

The detector is a 12 m diameter carbon steel sphere which will be filled with mineral oil. Its actual construction is more like a sphere within a sphere. The inner sphere is the "interaction" region and the outer spherical shell is the "veto" region. The walls of both the inner sphere and the outer shell are coverd with photomultiplier tubes (PMTs) which detect both the Cherenkov light and the scintillation light that comes from the charged particles produced in neutrino interactions. The veto region has the added responsibility of detecting charged particles that come in from outside of the detector and particles which escape the interaction volume.

Method Used for Distinguishing Neutrino Types

If a neutrino does interact in the interaction region of the detector, we use the pattern of light detected by the PMTs on the walls of the sphere to determine the type of lepton that was produced and consequently the type of neutrino that interacted. Muons coming from muon neutrinos travel a long way in the detector and make filled-in circles of Cherenkov light on the walls of the detector. They may also travel into the veto volume of the detector and be detected there. Electrons lose energy rather quickly and their Cherenkov light appears as fuzzy rings on the walls. Various background processes produce multiple fuzzy rings. Other background processes produce no Cherenkov light but a lot of scintillation light. To measure the number of muon neutrinos that oscillated to electron neutrino during their flight from the target to the detector, we must determine the ratio of the number of fuzzy rings seen in the detector to the number of filled in circles. The better that measurement can be made, the more accurate MiniBooNE will be.

Requirements for the Detector Mineral Oil

The requirements for the mineral oil used in the MiniBooNE detector is given in the MiniBooNE Mineral Oil Technical Specification. Nothing in this document supercedes that. However, in this section, we will attempt to explain the reason for the various specifications.

First and foremost, the mineral oil must be clear to the light that would be detected by our PMTs (light in the wavelength range of 320 nm to 600 nm). The detector is 12 m in diameter. In order to lose less than 25% of the light generated in the center of the detector by a neutrino interaction, we need to have a light attenuation length of greater than 20 m. Obviously, a longer attenuation length means less light is lost and that is better. It is our expectation that the more uniform the oil (the smaller variation in the lengths of the alkane chain lengths) and the purer the oil (the smaller the concentrations of unsaturated and aromatic hydrocarbons), the longer the attenuation length will be.

The first stage of miniBooNE will run for about two years. If the experiment is succcessful, it may run for up to ten additional years. During this time, we do not want the mineral oil or the detector to degrade. Thus, the requirements on minimal reactivity and solubility of various detector materials with mineral oil.

The more dense the oil, the more interactions we will have. Thus the more dense the oil, the better for the experiment. However, we need to recirculate the oil, which gives us an upper limit on the viscosity of the oil (and hence an implicit maximum density) stated in the technical specification.

Our method of particle identification depends upon knowing the angle at which Cherenkov light is produced by the particles. This angle in turn depends upon the index of refraction of the oil. Larger angles (related to higher indices of refraction) make particle identification easier. Dispersion confuses the angle measurement and makes particle identification more difficult. Therefore ideally, dispersion would be very low.

The index of refraction and the dispersion of a pure mineral oil are related to the oil's density. The higher the density of the oil, the greater the index of refraction and the greater the dispersion. There is nothing that can be done about this relationship. However, impurities in the oils can add absorption, and thus anomolously increase the dispersion.

Finally, as stated above, we will be using the scintillation light produced by charged particles as part of the particle identification scheme. However, we want to control what and how much scintillation light these particles produce. Thus, the initial mineral oil should be as free of scintillating materials as possible. Alkenes and aromatics produce scintillation light to a much greater extent than alkanes. Thus, extremely low concentrations of alkenes and aromatics are important. We also suspect that Vitamin E may produce some scintillation light. While we accept that some concentration of this antioxidant is necessary to stabilize the oil, we would like that concentration to be as low as possible to do the job.

Additional Information

Additional information about MiniBooNE can be found at the experiment's web page. It also contains links to more general web pages about neutrinos and neutrino interactions. The technical specifications for the mineral oil are also on the Web. Additional information can be obtained from Bob Cibic, FNAL Purchasing, (630) 840-3528. In addition, the booklet, Neutrinos Matter, is available from Bob. Comments about this manuscript can be sent to the author.

Glossary

Attenuation Length - The average distance a photon travels through a medium before it is absorbed. A 20 m attenuation length means that 90% of the light incident on a two meter sample will come out the other side.

Beta Decay - Radioactivity is the process which changes one type of atomic nucleus to another. There are three basic types of radiation released during such decays: alpha rays, beta rays, and gamma rays. Alpha rays are helium nuclei, combinations of two protons and two neutrons; beta rays are electrons; and gamma rays are photons. A beta decay is one in which an electron and a neutrino are released from a nucleus and one neutron in the nucleus changes to a proton.

Booster - The Fermilab National Accelerator Laboratory's proton accelerator complex consists of a number of particle accelerators. Protons start as neutral hydrogen atoms. An additional electron is added to each atom, making an H- ion. These ions are first accelerated to 170 keV by the Cockcroft-Walton accelerator (really a very fancy large capacitor). The ions are next accelerated to 400 MeV by a linear accelerator (a very intricate tube down which the ions ride an electromagnetic wave, being accelerated all the way). At the end of the linac (short for linear accelerator), the two electrons are stripped from the H- ions leaving isolated protons. The protons are accelerated to 8 GeV in the Booster, a circular accelerator. Circular accelerators bend particles around circles with magnetic fields and give them a kick each time they go around. The protons coming from the booster are the ones used by miniBooNE. Past the booster, the accelerator complex has two more circular accelerators which further increase the protons energies: the Main Injector which takes the protons from 8 GeV to 120 GeV and the Tevatron which further increases the energies to 1 TeV.

Cherenkov Light - The light given off by a particle that is moving faster than the speed of light in a medium. Note: no particle can move faster than the speed of light in vacuum. However, since light slows down when travelling through media (see index of refraction), a particle travelling close to the speed of light in vacuum can actually travel faster than the speed of light in the medium. The light given off by these rapidly moving particles is a "photonic boom," very similar to the "sonic boom" given off by an aircraft whose speed is faster than the speed of sound in air. Cherenkov light is highly directional, being produced in a cone around the direction of travel of the particle. The angle of the cone is related to how much over the speed of light in the medium the particle is travelling.

Dispersion - The change of the index of refraction with wavelength of light. The dispersion in water droplets gives rainbows; the dispersion in glass gives the color spread of the refracted rays in a prism.

Electron Volt (eV) - The amount of energy an electron or a proton would gain when crossing a capacitor held at a potential of 1 volt. keV (the k stands for kilo, Greek for 1 thousand) is 1 thousand electron volts of energy. The "M" in MeV stands for Mega or million. GeV or Giga electron volts are 1 billion electron volts. 1 TeV or Teva electron volts is 1 trillion electron volts.

Index of Refraction - The ratio of the speed of light in vacuum to the speed of light in a medium. The spped of light in media you can see through is always slower than the speed of light in vacuum. Thus, the index of refraction is always greater than 1. The index of refraction appears in Snell's Law which relates the angle at which light approaches a transparent surface to the angle at which it travels after passing through that surface.

Photomultiplier Tube (PMT) - An extremely sensitive light detector that gives a measurable current pulse out when as little as one photon of light is incident on it. The front end of the PMT is a sensitive surface which converts the photon into a single electron. The electron is accelerated inside the tube until it crashes into a metallic plate (called a dynode) and liberates a few electrons in the collision process. These newly freed electrons are accelerated to and collide with the next dynode each again freeing a few electrons in the process. The process is repeated over and over again until the number of electrons coming out of the back of the PMT is between 10 million and a billion for each photon that makes an electron in the front.

Photon - A single packet of light. This packet behaves like a particle (having a specific energy) and a wave (having a specific wavelength). The mystery of how something can have both wave-like properties and particle-like properties is the mystery of quantum mechanics.

Scintillation Light - When a charged particle passes through a medium, it ionizes (removes electrons from) some of the atoms of that material. The ions then start grabbing back their electron. Some ions emit light in the process of getting their electrons back. This light is called scintillation light. Scintillation light comes out uniformly in all directions.

Standard Model of Particle Physics - The Standard Model of Particle Physics is a model which specifies how the six quarks and the six leptons are grouped into families and how these individual particles interact with each other. It postulates that only the quarks feel the "strong force." This force causew the quarks to clump together and the protons and neutrons to cling together in the nucleus. It also postulates that the electromagnetic force that charged particles feel and the weak forces which all particles feel are really different forms of the same force. To date, all of the data gathered at the various particle accelerators around the world are consistent with the Standard Model.