Detecting Antineutrinos in KamLAND |
|
Detecting Antineutrinos in KamLAND |
|
To detect an antineutrino ( , the
antiparticle of the neutrino), we wait for one to smash
into a proton (p) in our detector. The collision destroys the incident
antineutrino and proton, but creates a positron (e+) and a neutron
(n). As will
be described below, the positron and neutron make two distinct flashes
of light in the detector, which are the "signature" of an antineutrino.
|
 |
 |
As the positron travels through KamLAND's liquid scintillator, it emits light in all directions. The faster it moves, the more light it emits. If you could see it, it would resemble a microscopic shooting star. After moving only a few centimeters, the positron runs into an electron (e-) and annihilates. The scintillation and annihilation occur so quickly that together they look like one brief flash of light. |
| This light is detected by phototubes (short for "Photo-Multiplier Tube" or "PMT") mounted on the inside surface of the steel sphere that holds the liquid scintillator. When light hits the phototube, it makes a small electric pulse, as shown at the right. |  |
 a positron event in KamLAND |
This picture shows the pattern of phototube pulses for a positron in KamLAND. Since the positron's energy is closely related to the energy of the antineutrino, we can determine the original antineutrino's energy just by counting the number of pulses recorded by the phototubes. The color of each "dot" in the display corresponds to the time at which the pulse occurs. If you look closely, you can perhaps make out that the positron was closer to the left-side of the detector. |
| A positron event by itself is not enough to distinguish an antineutrino event. All energetic charged particles -- including electrons, muons, and protons -- create flashes of light in the scintillator. Although we only expect to detect at most two neutrinos per day, KamLAND records about 30 such random flashes every second! |
 |
Remember that at the same time the positron was created by the antineutrino-proton collision, a neutron was also created. This neutron bounces around for about 200 microseconds (on average) before it runs into another proton, and the two bind together to become a deuteron atom. In the binding process, another flash of light is released, always with the same brightness. This flash is also detected by the phototubes. |
| As you can see from the event display on the right, a neutron event looks a lot like the positron event shown above. We need to use computers to distinguish the two. But when a neutron-like event occurs within a few hundred microseconds after a positron-like event, we can be almost certain that the two were created by an antineutrino! |
 a neutron event in KamLAND |
 antineutrino spectrum |
So to summarize, an electron-antineutrino collides with a proton in the liquid scintillator and creates a positron and a neutron. The positron creates a flash of light, the brightness of which determines the energy of the antineutrino. A short time later, the neutron creates another flash of light, which distinguishes the event from the flashes of other particles. The goal in the end is to measure the rate of antineutrinos arriving at KamLAND, and to determine their energy distribution, an example of which is shown on the left. |