How ArgoNeuT Works
ArgoNeuT uses Liquid Argon to detect and record neutrino interactions. There are three main systems in ArgoNeuT: the time projection chamber, the purity system, and the recirculation system. The experiment is located underground in the direct path of the NuMI neutrino beam.
The Time Projection Chamber (TPC)
The Time Projection Chamber (TPC) is the heart and soul of ArgoNeuT. 175 L of liquid argon is contained in the TPC, which consists of three wire planes oriented at 60° relative to one another. Each plane has 240 wires spaced 4 mm apart. Two of these planes (for a total of 480 reading channels) record the position and trajectory of particles from any event. The chamber is subjected to an electric field of 500 V/cm, and the ionization electrons produced by a particle track drift towards the first wire plane (the shield plane). When an ionization electron moves towards the induction plane, a current is induced across the wires in one direction. After the electron moves past the induction plane, another current is induced flowing in the opposite direction. The collection plane then collects the ionization electrons from the interactions. The data from the induction and collection planes can be combined to create a 3D model of the event; afterwards time is factored in to obtain the drift coordinate of the track. On a three-dimensional coordinate plane, the first two variables (x and y) represent data from the induction and collection planes, and the third variable (z) represents the drift distance in relation to time. The TPC records 2048 samples in 400 microseconds per NuMI beam spill, allowing for multiple snapshots of the same event.
ArgoNeuT triggers on the beam spill signal from the NuMI beam. To augment this information, ArgoNeuT is instrumented with upstream and downstream trigger paddles built at the University of Texas. Information about these paddles can be found here. Information about whether or not these paddles recorded any hits during the duration of the beam spill will be recorded and used later in the analysis of the data.
The Purity System
The Filtering System
ArgoNeuT has two filters that remove impurities from the liquid argon. Impurities such as oxygen, which is highly electronegative, could absorb ionized particles produced by an event and impact the energy registered by the system. Each filter contains copper granules, which are oxidized by the incoming oxygen impurities to form copper (II) oxide. The purified argon then flows back through the system into the TPC. Once the copper is saturated with oxygen, the filter is heated to around 250° C to regenerate and remove the oxygen from the copper granules, allowing the pellets to process more argon.
The Purity Monitor
For the liquid argon detector to work at maximum efficiency, the purity of the argon must be carefully measured and kept at a level of at least 10 ppb. Any impurities would capture the ionization electrons from the liquid argon and prevent them from drifting towards the wires. To measure the purity, a xenon flash lamp shines UV photons onto a gold plated cathode. Photoelectrons are produced by the photoelectric effect and they are detected by a wire plane cathode. Then they are drifted up the electric field until they are detected again by the wire plane anode. The cathode signals are then compared with the anode signals in terms of both magnitude and shape to observe whether the photoelectrons pass through the liquid argon without being attenuated.
The Recirculation System
Argon becomes a liquid at 87K, so it must be kept at a constant low temperature. When the argon in the Cryostat evaporates, it travels up a system of pipes to a cryocooler-a machine that extracts heat from an object to bring its temperature down to less than 150K. The newly liquefied argon flows back down another system of pipes to the TPC.
Because the boiling point of argon is so low, it will begin to evaporate at the slightest increase in temperature above 87K. If the cooling system were to break down, the argon in the TPC would start to evaporate, causing a pressure buildup. Intermittently along the piping line lie a series of valves designed to open at a certain pressure. Once these valves are open, the argon gas will be released into a new set of pipes that lead out into the atmosphere.
The NuMI Beam System
In the Fermilab complex, there is already a series of experiments involved in neutrino detection. The neutrinos for some of these experiments are produced by focusing 120-GeV protons off of the Main Injector at a graphite target. These protons collide with the target cylinder and produce many different types of pions and kaons. The positive pions can be focused at the neutrino detector by gigantic magnets called horns. The pions decay into muons and muon neutrinos a few meters after they are created. This beam of muon neutrinos is used for the ArgoNeuT experiment, as well as for other neutrino oscillation experiments such as MINOS, which features two detectors 735 km apart. ArgoNeuT sits in front of the MINOS near detector, about 1 km from the target. The far one lies in Soudan, MN. Each is responsible for detecting the amount of neutrinos in each flavor, and the ratio of flavors for each detector is compared to test for neutrino oscillation.
What ArgoNeuT is Looking For
The Types of Neutrino Interactions
Neutrino interactions are highly infrequent and not easily observable. They can either be a CC (charged-current) or NC (neutral-current) events. CC events are W boson exchanging interactions in which a charged lepton is produced; NC (neutral current) events are Z boson exchanging interactions in which no charged lepton is produced. Electron neutrinos interact differently than muon neutrinos, as delineated by the interactions below.
Muon Neutrino Interactions
Muon neutrino + neutron &rarr muon + proton (CCQE)
Muon neutrino + nucleon &rarr muon neutrino + nucleon (NCE)
Muon neutrino + nucleon &rarr muon + nucleon + positive pion (CCpi+)
Muon neutrino + neutron &rarr muon + proton + neutral pion (CCpi0)
Muon neutrino + proton &rarr muon neutrino + proton + neutral pion (NCpi0)
Electron Neutrino Interactions
Electron neutrino + neutron &rarr electron + proton (CCQE)
Low Energy Cross-Sections in Liquid Argon
The techniques utilized in ArgoNeuT can be applied to larger detectors that will further the scientific community's knowledge about neutrinos and their properties. Cross sections are relevant to future long baseline neutrino oscillation events, and future liquid argon detectors, such as MiniBooNE, LAr5, and DUSEL.
Demonstration of Gamma vs. Electron Discrimination in Event Detecting
In the study of neutrino oscillations, it is very important to distinguish the muon neutrino from the electron neutrino. Because neutral pions decay into two gamma rays, a muon neutrino reaction that produces a neutral pion can be confused with an electron neutrino reaction that produces an electron. To tell the two types of events apart, ArgoNeuT measures the value of the change in energy over the change in distance (dE/dx). Because gamma rays then decay into an electron and positron pair, the measurement of a gamma°«s dE/dx is approximately twice that of an electron (~4.2 MeV/cm: ~2.1 MeV/cm). Gamma rays lose twice the energy per unit of distance compared to electrons. Because they can discriminate between photons and electrons, the use of liquid argon in detectors increases their accuracy.
Axial Mass Measurement and Possible &DeltaSUsing CCQE and NCE
When a neutrino interacts with a nucleus, a certain amount of momentum is transferred (Q2). Axial mass is the least well known parameter that governs this momentum transfer. Liquid argon could also help meaure the strange content of nucleons.
Demonstraiting the Effectiveness of the Liquid Argon Purification Techniques
The filtering system was specially desgined to maximize purity and minimize costs. If this particular system of liquid argon purification is sustainable, then the technique can be used on bigger and better detectors.
Long Term Goals
Some of the long term goals for both ArgoNeut and the field of liquid argon neutrino detection are continued measurements of neutrino oscillation parameters, testing for CP violation in the lepton sector, detecting dark matter directly, and searching for proton decay.