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Cherenkov Aerogel Detector Optimization

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Introduction

CUA's Cherenkov Aerogel detector has been housed in Jefferson Lab's Hall C for three years and, compounded with the lab's recent 12 GeV upgrade, can be used to detect pions and kaons. As an electron beam accelerated by 12 GeV collides with particles, charged pions and kaons collide with the detector's aerogel tiles. When this happens, the charged particles move faster than the speed of light through the aerogel; this process causes the emission of a photon at a specific frequency. The Cherenkov radiation then goes into a lightbox where it can be detected by a photomultiplier tube (PMT). PMTs take advantage of the photoelectric effect and magnify the emitted photons into an electric signal that can then be measured.

Hall C's aerogel detector has already detected kaons, but it can still be improved in some ways. The aerogel tiles currently being used in the detector are Japanese tiles coming from various manufacturers. CUA has the option to replace the Japanese tiles with new ones from a different manufacturer, but the viability of this replacement still needs to be determined. If the new tiles have improved transmittance and light yield, they could prove to facilitate pion and kaon detection.

Refractive Index

Before the tiles' light yield could be experimentally determined, their refractive indices needed to be verified. Four Japanese tiles were tested, two from Matsushita Electic Works and two from Japan Fine Ceramics Center. The tiles had theoretical refractive indices of 1.03, 1.02. 1.015, and 1.010. The first two were from Matsushita, while the last two were from JFCC.

Methodology

In order to measure the tiles' refractive indices, an optical method was used. Using a SKIL 8201-CL vertical laser, the deviation of the incoming light due to the aerogel could be measured. The laser had a wavelength of 645nm corresponding to red light. Using a protractor, the aerogel tiles were lined up at a 45-degree angle to the incident ray such that only the corner refracted the incoming light. A barrier was set up to view the original and refracted rays 14.125 in away from the tile's corner. The distance between the original and refracted rays was then measured on the barrier. The total angle of refraction could then be calculated using these measurements. Using the following equation, θ= γ − (π/2 + α) + sin^−1[sin α sin γ + cos α(sqrt n^2 − sin^2γ)], a mathematical relationship between the total deflection angle, θ, the angle of incidence γ, and a very small angle α that was taken into consideration should the corners of the tile not be exactly 90°. It should be noted that because α is such a small angle, it will have little impact over the results due to the 45° angle of incidence.

Transmittance Measurements

After experimentally determining the refractive indices for the tiles, their transmittance was to be measured. Using a photodiode and the same 645 nm laser as with the refractive index measurements, the output current could be measured with a multimeter. The aerogel tiles were each placed directly in between the photodiode and the laser and the output current was measured. The ratio of the initial current to the output current with the aerogel tile can give a rough estimate of the tile's transmittance. While this method was much less precise than others, it provided a very quick way to determine the tiles' transmittance.

The transmittance was then measured using a PerkinElmer Lambda 950 UV/VIS/IR spectrophotometer. The tiles were placed inside the spectrometer at three different positions such that we could measure the transmittance in the middle, left, and right sides of the tiles. The wavelengths of light used ranged from 200 nm to 800 nm.

PMT Testing

After the transmittance and refractive indices for the tiles were measured, PMTs needed to be tested to ensure that they were working properly. This testing involved placing the PMT in a dark box with an LED and monitoring the output signals on an oscilloscope. The tested PMT was a Photonis XP4500B. The PMT's output was first connected to a discriminator to eliminate signal noise and then to the oscilloscope. For this model, the operating voltage ranges from ~1800-2500V, with this setup running at 2100V on the high voltage output. The

Presentations

6/25/18 Progress Meeting: [1]

7/2/18 Progress Meeting: [2]

7/11/18 Progress Meeting: [3]