A measurement of the diffuse reflectivity of 1056 Tyvek in air and water

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1 Nuclear Instruments and Methods in Physics Research A 553 (2005) A measurement of the diffuse reflectivity of 1056 Tyvek in air and water J.C. Arteaga-Vela zquez,c.va zquez-lo pez, A. Zepeda Department of Physics, CINVESTAV-IPN, Mexico, Distrito Federal, Apartado Postal , Mexico Abstract The Pierre Auger Collaboration is constructing the first of two observatories designed to solve the mystery of the ultra high energy cosmic rays. Each observatory will consist of an array of 1600 water Cherenkov detectors spread on a surface of 3000 km 2. The main part of these detectors is a polyethylene tank filled up with water and with three photomultipliers on the top. Tyvek will be used as the inner lining material of these Cherenkov detectors, because of its high reflectivity in the wavelengths of interest. Therefore, the optical properties of this material constitute an important element in the detection of cosmic rays. In this work, we report the results of our measurements of diffuse reflectivity of a sample of Tyvek, in air and water, as a function of the reflection angles for two different wavelengths: one at 488 nm (with an Argon laser) and another at nm (using a Helium Neon laser). We also show that there exist optical anisotropies of Tyvek which are revealed by the shape of its diffuse reflectivity curves when rotating the sample about its normal. r 2005 Elsevier B.V. All rights reserved. PACS: m; Ka Keywords: Cosmic rays; Tyvek reflectivity; Cherenkov detectors 1. Introduction The primary goal of the Pierre Auger Observatory is to solve the puzzle of the ultra high energy cosmic rays. For this purpose, two similar instruments will be located in both the northern and Corresponding author. addresses: jarteaga@fis.cinvestav.mx (J.C. Arteaga- Velázquez), cvazquez@fis.cinvestav.mx (C. Vázquez-Lo pez), zepeda@fis.cinvestav.mx (A. Zepeda). southern hemispheres to study those particles by observing the extensive air showers which they produce in the atmosphere. In order to measure the lateral shower distribution, each instrument will count tracks with an array of 1600 water Cherenkov detectors separated by 1.5 km and distributed on a surface of about 3000 km 2 [1]. Each Cherenkov detector will consist of a polyethylene tank (10 m 2 1:2 m deep) filled with water, an inner liner, called Tyvek, with high diffuse reflectivity in the nm wavelength /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.nima

2 J.C. Arteaga-Velázquez et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) band and three photomultipliers on the top of the tank [1]. Tyvek will be used to increase the efficiency of these Cherenkov detectors. Therefore, the optical properties of this material will constitute an important element in the detection of cosmic rays. Tyvek is a white material made of high-density polyethylene microfibers randomly oriented. It is chemically inert and resists water penetration [2]. In this work, we study the diffuse reflectivity of Tyvek type 1056D and the influence of the anisotropies of the material on these measurements. 2. Experimental set-up and procedure The experimental design is shown in Fig. 1. In this experiment, the parameters of interest were: the incidence angle, y i, the reflection angle, y r and the amount of reflected light, I r, at the incidence plane. In order to get the data, we built an automatic system which allowed us to save time and have a good angular resolution (the details are shown in reference [3]). With this device, we measured I r as a function of y r with steps of 1:8 and for the following incidence angles: 0,15,30, 45, 60 and 75. We worked at two different Fig. 1. Experimental set-up. wavelengths: one in the blue region (at l ¼ 488 nm with an Argon laser of 11 mw of nominal power) and another one in the red region (at l ¼ 632:8nm with a He Ne laser of 2 mw of nominal power). As photodetector, an NPN silicon phototransistor was used. Its assembly was designed to allow us to immerse it in water. The responsivity of the detector in the blue and red lines are 0.18 and 0.32 A/W, respectively. In this experimental design, the position of the laser beam is fixed and the sample is allowed to rotate. The distance from the sample to the laser source was 35 cm, while that to the detector was 2.5 cm. At y i ¼ 0, the diameter of the laser spot on the sample was 2 mm. For the measurements in pure water we used a cylindrical container whose inner surface was sprayed with black paint in order to avoid undesirable reflections. This container was provided with a glass window to allow the incidence of the laser beam on the sample. On the other hand, for the measurements of the anisotropical effects we kept y i constant (at 75 ) and rotated the sample, by an angle a, about its normal. 3. Analysis and results The graphs for the diffuse reflectivity of a sample of Tyvek, in air and water (and inside the cylindrical container without water), are shown in Fig. 2, for y i ¼ 45. In this figure, units are arbitrary but the same scale is used for all the cases. The difference in intensity when changing the laser beam arises from the differences of the spectral responsivity of the detector and the nominal laser power at l ¼ 488 nm and l ¼ 632:8 nm. The decrease of the diffuse reflectivity when the sample is immersed in water is mainly associated with the increase of the refractive index of the incidence medium, from 1 in air to 1.34 (for l ¼ 488 nm) or (for l ¼ 632:8nm) in water [4]. In our graphs, it is observed that the peak is positioned very closely to the specular reflection angle: y r ¼ y i. Also, we appreciate that the signals inside the container without water are slightly attenuated when they are compared with those in air. This effect is due to reflection and absorption of the laser beam in the window of the

3 314 J.C. Arteaga-Velázquez et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) Fig. 2. Angular distribution of the diffuse light reflected by a sample of Tyvek type 1056D (a) in air, (b) inside the cylindrical container and (c) in water, for l ¼ 488 nm, nm and y i ¼ 45. The circles represent the experimental data, while the solid lines, the fitting by using the relation (1). The values of fitting parameters obtained are indicated in each figure, respectively. container. In order to fit the data, we proposed the following relation: I r ðy i ; y r Þ¼I s exp½ ðy r þ y s Þ 2 =ð2s 2 s ÞŠ þ I d cosðy r þ y d Þ ð1þ by similarity with the relations proposed by Hasenbalg et al. [5], and Filevich et al. [6], where it is assumed a cosine function (suggested by Lambert s Law) to fit the diffuse light, and a Gaussian term to fit the peak (which has a specular contribution). Here, y s gives us the position of the I s component, while y d is a phase displacement of the I d component. These parameters were estimated by minimizing the sum of squared deviations between experimental data and the model function. For y i X60, we observe some deviations of the experimental data from the model, originated by the diffuse component, which deviates from the Lambert s law at larger incidence angles. I s and I d are plotted in Fig. 3. From these curves we can see that: (i) the variation of I s with y i is consistent with Fresnel equations (this has been already pointed out by Hasenbalg [5]). (ii) The I s component has a minimum at y i ¼ 30 for l ¼ 488 nm, and at y i ¼ 45 for l ¼ 632:8 nm. (iii) On the other hand, the I d component has a local maximum where the I s component has its minimum. By using the data shown in Fig. 3 we computed the quantity I s =I d and we observed that, for a given wavelength, this quantity is bigger for Tyvek in water than for Tyvek in air. In general, it is observed that for y i ¼ 0 and 15 the width of the Gaussian term (s s ) is approximately 31, while, for y i between 30 and 75 it ranges from 20 to 9 (at l ¼ 632:8 nm) or 14 (at l ¼ 488 nm). From the fits performed on the data it is found that, in general, the position of the Gaussian term is very close to the specular angle and that when increasing y i, the cosenoidal terms tends to move away from the origin (that is y d tends to increase). The optical anisotropies of Tyvek are shown in Figs. 4 and 5 for l ¼ 488 nm and y i ¼ 75. Clearly, the shape of the graphs changes under a rotation, a, around the normal of the sample. This effect can be associated to the fibrous composition of the material and the presence of surface regions with

4 J.C. Arteaga-Velázquez et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) Fig. 3. (a) I s and (b) I d as a function of y i. preferential orientations of the fibers. The height ði p Þ and half width ðs p Þ of the peaks in the reflectivity graphs are shown in Fig. 5 as a function of a. An anticorrelation between I p and s p is clearly observed from the figure, which means that: the larger the specular component ði p Þ, the smaller the diffuse part ðs p Þ. Note that I p can decrease up to 43% of its maximum value. 4. Conclusions Fig. 4. Optical anisotropic effects on the diffuse reflectivity of Tyvek. By building a simple automatized system involving the control of a stepping motor and data acquisition, the diffuse reflectivity of Tyvek has been carefully determined. The comparison of relevant parameters associated to the shape of the

5 316 J.C. Arteaga-Velázquez et al. / Nuclear Instruments and Methods in Physics Research A 553 (2005) Acknowledgements We thank the technical assistance of Blanca Zendejas and the partial support of CONACyT. We are also grateful with DUPONT of Mexico for the technical information about Tyvek kindly provided by this company. One of us, J.C. Arteaga, thanks the organizers of the RICH 2004 Conference for all their support during this workshop. Fig. 5. Behavior of the parameters I p and s p under a rotation of the sample by an angle a. reflectivity curves led us to conclude that the shape of the reflectivity pattern of Tyvek in water differs from that measured in air. The influence of optical anisotropies has been determined and should not be neglected in the simulation studies of the Cherenkov radiation involving Tyvek. This effect must be studied for different surface regions, with a wider and uniformly illuminated laser spot, in order to get representative values for the parameters describing the angular distributions of diffuse light. References [1] Pierre Auger Design Report, Second ed. March Available from [2] Dupont. Tyvek is a Dupont registered trademark. [3] J.C. Arteaga Velázquez, C. Vázquez-Lo pez, A. Zepeda, Superficies y Vacío 11 (50) (2000). [4] D.J. Segelstein, The complex refractive index of water, M.S. Thesis, Department of Physics, University of Missouri- Kansas City, [5] F. Hasenbalg, D. Ravignani, GAP Available from [6] A. Filevich et al., GAP