MainPage:Nuclear:Summer2014:AerogelAbsorptionLength

Abstract

The purpose of this experiment is to determine the scattering and absorption lengths or aerogel produced by Matsushita Electric Works for use in Cherenkov detectors at Jefferson Lab. These tiles detect particles because when a particle passes through a medium faster than light travels in that medium it emits light and slows down. Cherenkov detectors detect particles based on the radiation itself, and from the cone of radiation emitted as the particle moves can be used to determine the momentum and hence mass of the particle, identifying the particles. As light passes through the aerogel however light can be scattered and absorbed, which can make these measurements less accurate. Scattering will be measured by shining a collimated led beam through aerogel in a darkbox and measuring its intensity before and after passing through lengths of aerogel varying from 1-5 cm. Absorption length will be caclulated using lengths of aerogel varying from 10-20 cm, the measurement performed similarly to that of scattering length, factoring in the intensity lost due to scattering.

Aerogel Structure

Silica Aerogels are produced by replacing the liquid portions of a gel with gas, resulting in a structure comprised of chains of silica molecules and microscopic gas pockets. The Aerogel which will be tested, was produced by Matsushita Electric Works, through a process which waterproofs silica particles in the gel, resulting in a significantly longer lifetime for the tiles produced. Aerogels in general tend to have a refractive index around 1.02 to 1.06, and the tiles tested will have refractive indices of approximately 1.02 and 1.03. Because of the pores in the aerogel, light experiences Rayleigh Scattering.

Scattering and Absorption Length

Rayleigh scattering occurs when particles much smaller than the wavelength of light interfere with electromagnetic radiation. Because of this, the scattering of light in aerogel is highly dependent on the wavelength of the light passing through the aerogel. In the visable range, the absorption length of light in aerogel remains for the most part constant. Scattering and Absorption Length are the length into a material at which the probability that a particle passing through the material has not been absorbed drops to 1/e, and it characterizes the negative exponential trend of that probability: The Probability that that a photon has not been scattered or absorbed is P=e^x/Λ, where Λ is the scattering or absorption length and x is the distance into the material which the photon has passed.

Materials and Methods

• 20 11x11x1cm Matsushita Electric SP-20 Aerogel tiles
• Case for holding Aerogel
• Photomjltiplier Tube
• Blue, Red, . , . LEDs
• Collimator tube
• PMT Apeture
• Darkbox

The LED light is collimated by a long tube painted black on the inside. This restricts the angles at which the LED light can leave the tube, focusing it into a beam. The led is elevated to a height so that it is aiming through the center of the aerogel tiles. The aerogel tiles are held in a black case, which should minimize any light scattering back to where it would be collected. An Aperture with the same diameter as the collimator is placed over the PMT, aligned with the it so the PMT will not capture any scattered light. The intensity of the light with and without the aerogel will be used to calculate the scattering length. After scattering length is measured, larger numbers of aerogel tiles will be used and intensity lost to scattering will be taken into account to calculate absorption length. In order to reduce error due to the minute differences in the optical properties of the aerogel tiles, different tiles will be tested in combination, and 5 trials will be taken and the results averaged.

Citations

Aschenauer, E.; Bianchi, N.; Capitani, G. P.; Carter, P.; Casalino, C.; Cisbani, E.; Coluzza, C.; De Leo, R.; De Sanctis, E.; De Schepper, D.; Djordjadze, V.; Filippone, B.; Frullani, S.; Garibal-di, F; Hansen, J. O.; Hommez, B.; Iodice, M.; Jackson, H. E.; Kaiser, R.; Kanesaka, J.; Lagamba, L.; Muccifora, V.; Nappi, E.; Nowak, W. D.; O’Neill, T. G;; Potterveld, D.; Ryckbosch, D.; Sakemi, Y.; Sato, F.; Schwind, A.; Suetsugu, K.; Shibata, T. A;. Thomas, E.; Tytgat, M.; Urciuoli, G. M.; Van de Kerckhove, K.; Van de Vyver, R.; Yoneyama, S.; Zhang, L. F. Nucl. Instrum. Methods Phys. Res., Sect. A, 2000, 440, 338

Whippe, C. Matsushita Improves Silica Aerogel. Photonics Spectra.