Data Analysis - Cherenkov Calibration
Cherenkov Calibration
Written by Cameron Cotton. The information presented here draws heavily from the documents in the reference section, so I thank the authors: Ryan Ambrose, Garth Huber, and Simona Malace.
The Cherenkov Detector (For Beginners)
The Cherenkov Detector uses the phenomenon of Cherenkov Radiation to detect when particles traveling faster than the speed of light of the detector chamber medium (Heavy Gas, Noble Gas, Aerogel) pass through the detector chamber. The primary purpose of the Cherenkov Detector is particle identification, particularly for distinguishing between electrons and pions in our experiments. Since the dipole magnets in front of the HMS and SHMS detectors can "select" the energy of the particles that are bent into the detector, we know the velocities of the electrons and pions that enter the detector. We can then choose the medium in the Cherenkov Detector such that Cherenkov radiation is emitted for the light and fast electrons, but not for the heavy and slow moving pions with the same energy. PMTs are then used to detect the resulting Cherenkov Radiation, taking the original photon and converting it into an electric pulse. An ADC is then used to convert the analog signal into a digital signal that reflects the ampltude of the original electric pulse (1pC = ADC Channel 1, 7pC = ADC Channel 7, etc).
While this is an overly simplistic picture, I hope it makes clear the basic principles behind how a Cherenkov Detector works and the utility it provides to particle physics experiments.
Purpose
The goal of performing a Cherenkov Detector Calibration is to quantify the Cherenkov Detector's response to the passage of various particles. In particular, we seek to map the PMT signal from a single photoelectron (SPE) to a particular ADC channel (pC). This will allow us to convert our Cherenkov Detector data from a physically meaningless quantity (ADC channel), to something much more meaningful (number of photoelectrons, or NPE). Having an accurate mapping between ADC channel and NPE is very useful, as cuts are often made on NPE to distinguish between electron and pion events.
We expect the PMT to respond linearly to the number of photoelectrons (for reasonably low NPE). For example, if there are 3 photoelectrons, we expect 3 times the signal output from the PMT. So, if the SPE peak was found to be at ADC channel 50, we would expect to see the peak for NPE = 3 around ADC channel 150.
Method 1
Method 1 [2] is used when you are able to clearly identify the Single Photoelectron (SPE) peak in the "goodAdcPulseInt" spectrum. Simply fit the pulse integral of the SPE with a gaussian. The calibration is complete once you put 1/fit_result (mean) into the PARAM file. For sample plots, see [2] or my log entry.
Method 2
Method 2 [2] is used when you cannot identify the SPE. First, you should fit the charge integral (goodAdcPulseInt) from electrons only (cuts on Preshower, Shower, and others (see logbook entry in [2] slide 5)). You can then fit the resulting histogram to get the calibration constant for the PARAM file (see details for one possible fit in [2] slide 5, or use a "stretched" Poisson distribution to fit). This method is not as nice as Method 1 and can be very sensitive to exact cuts, so caution should be taken when using this method (do systematic studies). For sample plots, see [2] or my log entry.
