STUDYING THE OPTICAL PROPERTIES OF THE NPD MIRRORS

STUDYING THE OPTICAL PROPERTIES OF THE NPD MIRRORS Yenny Martinez Brigham Young University March 20, 2002

ACKNOLEDGEMENTS David Allred, R. Steven Turley, and Spencer Olson n help and insight they give to me throughout the time of this resesarch Matt Squires, Mike Newey, and many other students n assisting me in writing my thesis and finishing the research Shannon Lunt and Elke Jackson n measurements taken via reflectometry

OUTLINE Background Information: n n Why study Mars? Neutral Particle Detector (NPD) mirrors Desired characteristics Calculations made by us Experimental Setup: n n Making the NPD mirrors Measuring the NPD mirrors Conclusion

WHY STUDY MARS? Photographs have been taken of Mars’ surface, revealing the existence of riverbeds and river cannons Water might have been present! Where did it go? BECAUSE MARS LACKS A MAGNETIC FIELD, ITS ATMOSPHERE HAS BEEN DEPLETED BY ITS INTERACTION WITH THE SOLAR River canyons WIND. WITHOUT AN ATMOSPHERE, ANY WATER PRESENT ON MARS MIGHT HAVE EVAPORATED AWAY AS WELL. Riverbeds

WHY STUDY MARS? (CONTINUED) The European Space Agency is preparing a spacecraft for flight to Mars in 2003 Its main instrument on board is the ASPERA-3, which holds a time-of-flight Neutral Particle Detector (NPD) This NPD will measure the momentum of neutral particles in the solar wind that are postulated to be formed from the atmospheric particles. n n n Solar wind particles ionize the atmospheric gas This ionized gas leaves the atmosphere along with the solar wind By stripping electrons off neutral space atoms, the atmospheric atoms become neutral particles

NEUTRAL PARTICLE DETECTOR (NPD) The NPD sensor is a one-dimensional pinhole camera with a 90° field of view Charged particles are deflected, only allowing energetic neutral particles (ENAs) to enter START SURFACE TO STOP DETECTOR -e -e CHARGE PARTICLE DEFLECTION TO START DETECTOR STOP SURFACE ENAs would release secondary electrons upon impact with the start surface n Secondary electrons start the time-of-flight detector SCHEMATIC DIAGRAM OF NPD SENSOR ENAs continue their path and hit a stop surface n Secondary electrons stop the time-of-flight detector

NEUTRAL PARTICLE DETECTOR (NPD) (CONTINUED) Desired characteristics for the NPD mirrors: Surfaces must have ample electrons to emit when hit by the neutral particles Surfaces must poorly reflect ultraviolet light (1216 Å) at the critical angle of 15°

NEUTRAL PARTICLE DETECTOR (NPD) (CONTINUED) Work done by ESA affiliates showed that Cr, W, and their oxides would provide the necessary amount of secondary particles desired A generic algorithm was used to determine suitable multilayer configurations using Cr, W, and Mg. F 2 n n For low reflective surfaces, a transmissive layer (Mg. F 2) is needed between two reflective layers The reflectance is determined by the thickness of the layers and the optical constants of the layers

NEUTRAL PARTICLE DETECTOR (NPD) (CONTINUED) OUR DESIGN: Substrate n n Ti based aircraft alloy Supplied by the ESA 275 Å Chromium 103 Å Magnesium Fluoride 8 Å Tungsten

EXPERIMENTAL SETUP Creating the NPD mirrors n n Evaporation DC-Sputtering Measuring their reflectivity n Variable angle reflectometer Measuring their thickness n n Ellipsometry Monitored in the deposition using quartz crystal monitors

EXPERIMENTAL SETUP (CONTINUED) EVAPORATION: Used to grow Cr and Mg. F 2 layers DC-SPUTTERING: Used to grow W layer

EXPERIMENTAL SETUP (CONTINUED) Reflectivity measurements were made with a variable angle reflectrometer Obtaining good measurements at low angles is difficult n n A slight shift in angle drastically shifts the reflectivity A misaligned system causes the same type of problems

EXPERIMENTAL SETUP (CONTINUED) Ellipsometry was used to measure thicknesses of the individual layers Crystal monitors inside the evaporator dictate the relative thicknesses of the evaporation run

CONCLUSION One of the automated runs on the actual flight mirrors The 15° measurement is 24% reflectivity

CONCLUSION (CONTINUED) Reflectance desired was 20% at 15° Attained reflectances were 21%-24% Possible reasons: n n n Mg. F 2 and absorption of water Smoothness Other factors (measurements, time constraint, …) Further study on NPD mirror simulations n n Using Si. O 2 substrates instead of aircraft alloy A very thorough study of surface and individual layer components