Reflectivity Spectra for Commonly Used Reflectors (2014)

Reflectivity Spectra for Commonly Used Reflectors Martin Janecek Abstract—Monte Carlo simulations play an important role in developing and evaluating the performance of radiation detection systems. To accurately model a reflector in an optical Monte Carlo simulation, the reflector’s spectral response has to be known. We have measured the reflection coefficient for many commonly used reflectors for wavelengths from 250 nm to 800 nm. The reflectors were also screened for fluorescence and angular distribution changes with wavelength. The reflectors examined in this work include several polytetrafluoroethylene (PTFE) reflectors, Spectralon, GORE diffuse reflector, titanium dioxide paint, magnesium oxide, nitrocellulose filter paper, Tyvek paper, Lumirror, Melinex, ESR films, and aluminum foil. All PTFE films exhibited decreasing reflectivity with longer wavelengths due to transmission. To achieve reflectivity in the 380 to 500 nm range, the PTFE films have to be at least 0.5 mm thick—nitrocellulose is a good alternative if a thin diffuse reflector is needed. Several of the reflectors have sharp declines in reflectivity below a cut-off wavelength, including (420 nm), ESR film (395 nm), nitrocellulose (330 nm), Lumirror (325 nm), and Melinex (325 nm). PTFE-like reflectors were the only examined reflectors that had reflectivity above 0.90 for wavelengths below 300 nm. Lumirror, Melinex, and ESR film exhibited fluorescence. Lumirror and Melinex are excited by wavelengths between 320 and 420 nm and have their emission peaks located at 440 nm, while ESR film is excited by wavelengths below 400 nm and the emission peak is located at 430 nm. Lumirror and Melinex also exhibited changing angular distributions with wavelength. Index Terms—Fluorescence, Lambertian reflection, reflection coefficient, specular reflection. I. I NTRODUCTION S cintillating crystals emit light in a broad range of wavelengths, with many of them having their peak emissions located between 375 and 480 nm [1-6]. In order to convert this optical signal into a sizable electrical signal, the light has to be directed into a photodetector by the means of surrounding the scintillating crystal with a reflective material. This reflector should maximize the light collection and the reflector has to be chosen to provide high reflection coefficients at the scintillator’s emission wavelengths. Monte Carlo simulations play an important role in developing and evaluating the performance of radiation detection systems. The simulations offer a way of evaluating the system as a whole by exploring system parameters without the added cost of constructing the system or any of its Manuscript received September 23, 2011; revised December 14, 2011; accepted January 04, 2012. This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The author is with the Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (phone: 510-486-5579; fax: 510-486-4768; e-mail: mjanecek@lbl.gov). Fig. 1. Schematic of an integrating sphere. Note that the light enters at a slight angle in relation to the examined sample (~8o) in order to be able to detect the specular reflection from the sample. The inside surface of the integrating sphere is coated with a diffuse reflector creating a uniform angular distribution at the photodetector. To force the light to reflect multiple times within the integrated sphere, baffles (i.e., light barriers) are located between the illuminated sample and the photodetector as well between the in-port and the photodetector, eliminating any direct optical paths. components. For these simulations, the reflection’s angular distribution [7] and the reflection coefficient (as a function of wavelength) has to be known for the results to be accurate. Reflectivity measurements are generally performed with an integrating sphere, where the entire angular distribution of the reflected light is collected and thus contributes to the photodetector response. As the light is reflected many times within the integrating sphere before detection, it is important to coat the inside of the sphere with a highly reflective “white” material. The reflection data acquired using this technique do not always provide sufficient information to perform an accurate Monte Carlo simulation — some reflectors are fluorescent or change their angular reflectance distributions with incidence angle. Integrating sphere materials include barium sulfate (BaSO 4 ) [8-14], magnesium oxide (MgO) [8, 11-17], and polytetrafluorethylene (PTFE) based reflectors [18-20], and these reflector materials have been extensively studied. Other reflectors that are frequently used in optical systems include titanium dioxide (TiO 2 ) paint [11, 16, 21], Tyvek ® paper [22, 23], ESR (Enhanced Specular Reflector) film [24, 25], and Spectralon [19, 26]. In addition, some commonly used reflectors have not been reported on in the literature, including nitrocellulose, GORE ® , Lumirror ® , and Melinex ® . The aim of this work is to measure the reflection coefficients as a function of wavelength for the most common reflectors used in the radiation detection field, and to screen them for fluorescence and angular distribution changes with wavelength.