Detector Design
Relevant muon properties that drove the design of our muon decay detector:
- Muons are charged so we can detect them with a scintillation detector by HV photomultiplier tubes We need a safe HV supply for high school students.
- Muons are unstable, living approximately 2 microseconds before they decay. To measure the muon signals, we had to have fast electronics and computer readout.
- When muons decay, they disappear and an electron and two neutrinos are created. We must be able to detect the electron (30 MeV) from the decay. Capturing the neutrinos is hopeless, since they are neutral and interact even less than muons, easily traveling straight through the Earth.
- Our electronics must allow us to select (trigger) muons and not random electronic noise.
- The detector must be large enough so that we can capture enough decaying muons in a few hours to be able to have a viable lab in high schools.
Relevant muon properties that drove the design of our general muon detectors:
- Muons are charged so we can detect them with a scintillation detector by HV photomultiplier tubes We need a safe HV supply for high school students.
- Our electronics must allow us to select (trigger) muons and not random electronic noise.
- Muons can traverse several detectors without losing an appreciable fraction of their energy. Several detectors stacked on top of each other fire together when a single muon passes through them.
- Often, many muons are created in collisions of cosmic rays with the upper atmosphere. Multiple muons can fire several detectors if they are separated horiziontally from each other.
- The detector must be large enough so that we can capture enough decaying muons in a few hours to be able to have a viable lab in high schools.
Detailed Description Heading link
We designed detectors and electronics to be able to observe muons as well as the electron created when a muon decays. The detector must be sensitive to charges particles and insensitive to electronic noise, so that we can observe a second flash of light when the muon disappears, and creates an electron.
The active element of the detector is a chunk of clear plastic that scintillates (gives off a small amount of blue light) when charged particles go through it. Very little light is emitted, but if we wrap the detector in black paper to shut out normal light, and look at it with a photomultiplier tube (PMT) that amplifies the light into a small electrical current, we can detect when charged particles hit it. The electrical light lasts only 40 ns (ns = 10-9 seconds) so we need sensitive electronics or an oscilloscope to detect the light. (View the signal from our PMT online – password required).
The electronics consists of several stages and was developed by a group of engineers and physicists at Fermilab (Electronics Manual) for QuarkNet. The stages consist of discriminator, coincidence logic, timing buffer, processor, and data output to computer as shown in the DAQ1 figure.
If the input pulse is large enough, the discriminator sends a logic pulse to the coincidence unit. The user can adjust the level so that electrical noise is excluded. There are four possible inputs from PMTs.
The coincident logic can be configured by the user to require that between 1 to 4 of the PMTS from separate scintillation counters fire at approximately the same time. The number of hits required and window of time allowed are both adjustable. Once the coincident level is satisfied (defined as a TRIGGER), we let the electronics continue. In other words, we coincidence logic we can require that the muon pass through two detectors to be able to reduce random hits in electronics, and to define the direction the muon came from. More on the direction later.
After the trigger occurs, the timing buffer waits for 25 microseconds and records the next hit from any single PMT that passes the discriminator. After that time, the microprocessor gathers the information and sends it to the computer via the serial output line. Reported are which counters fired the trigger, times between triggers, which counter fired after the trigger and the time between the trigger and that next hit. A visual scaler (LEDs) displays the number of triggers.
The data acquisition card described above has been upgraded in two times. A DAQ2 version added GPS timing information and more detector timing information so that data collected at different schools can be treated as coming from one very large detector spread out over many schools. The DAQ2.5 upgrade improved the GPS technology and changed the hardware processor to avoid the use of special purpose chips that had become obsolete.
