Radiometric Calibration of the SwRI Ultraviolet ...
Property of Preston L Karnes, SwRI
Radiometric Calibration of the SwRI Ultraviolet Reflectance Chamber (SwURC) Far-Ultraviolet Reflectometer Preston L. Karnes1, Kurt D. Retherford1, Gregory S. Winters1, Eric C. Bassett1, Stephen M. Escobedo1, Edward L. Patrick1, Amanda Richter1, Michael W. Davis1, Paul F. Miles1, Joel W. Parker2, G. Randall Gladstone1, Thomas K. Greathouse1, Eric R. Schindhelm2, Lori M. Feaga3 and S. Alan Stern2 1
Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238 2
Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302
3
Department of Astronomy, University of Maryland, College Park, MD 20742 ABSTRACT
The Southwest Research Institute Ultraviolet Reflectance Chamber (SwURC) is a highly capable UV reflectometer chamber and data acquisition system designed to provide bidirectional scattering data of various surfaces and materials. The chamber provides laboratory-based UV reflectance measurements of water frost/ice, lunar soils, simulants, and analogs to support interpretation of UV reflectance data from the Lyman Alpha Mapping Project (LAMP) Lunar Reconnaissance Orbiter (LRO). A deuterium lamp illuminates a monochromator with a nominal wavelength range of 115 nm to 210 nm. The detector scans emission angles -85° to +85° in the principal plane. Liquid nitrogen passed through the sample mount enables constant refrigeration of tray temperatures down to 78 K to form water ice and other volatile samples. The SwURC can be configured to examine a wide range of samples and materials through the use of custom removable sample trays, connectors, and holders. Calibration reference standard measurements reported here include Al/MgF 2 coated mirrors for specular reflection and Fluorilon for diffuse reflectances. This calibration work is a precursor to reports of experiments measuring the far-UV reflectance of water frost, lunar simulants, and Apollo soil sample 10084 in support of LRO-LAMP.
INTRODUCTION Southwest Research Institute (SwRI), Space Science and Engineering Division, has a successful history of building UV spectrographs for NASA spacecraft and sounding rockets. One of these spacecraft, the Lunar Reconnaissance Orbiter, carries a SwRI-built spectrograph known as “LAMP” (Lyman Alpha Mapping Project). LRO entered lunar orbit in 2009 and is currently mapping lunar surfaces, including features near the poles known as permanently shadowed regions (PSR). These low temperature PSRs contain water ice and other compounds that accumulated at and below the surface from the lack of sunlight and atmosphere. The SwRI Ultraviolet Reflectance Chamber (SwURC) was conceived during the LAMP instrument design and build period as a laboratory reflectometer to be used to corroborate observational science data from LAMP, and to provide comparison data acquired with known materials and under known conditions1. Implementation of new instruments within SwURC increases the accuracy and range of data analysis. New additions include a 5901 Channeltron electron multiplier, 500W halogen lamp, 10” nipple, 9906 Net Controls motor and controller, and a 200 amu Extorr RGA. A Hamamatsu R10824 photomultiplier tube was removed in place for the more
Property of Preston L Karnes, SwRI
accurate 5901 Channeltron electron multiplier. The Channeltron offers a wider range of unsaturated data while using a higher voltage (maximum 3000V) and lower current (60 to 167 μA). A 500W halogen lamp on an extendable shaft enables a more than applicable bake-out if necessary. A variable transformer is used to control the power consumed by the lamp, not to exceed 120˚C to avoid damaging interior instruments. The halogen lamp is assembled on one of the five ports available on the 10” nipple flange. A camera view window and extra electrical feedthroughs were also implemented on the flange, leaving two ports open for future use. In order to correct repeatability issues, a 9906 Net Controls 6 Amp bidirectional motor and controller was installed without the use of a coupling. The direct connection of the motor shaft to the detector arm eliminates the 'flop' when changing directions or homing. A 100 amu Extorr RGA was replaced by a 200 amu Extorr RGA enabling twice the resolution and accuracy of the residual gases found within the chamber. Southwest Research Institute Ultraviolet Reflectance Chamber can be used to test ultraviolet reflectance of any ultra-high vacuum compatible substance, but is mainly utilized to analyze moon dust simulants. The simulants offer a known reference for interpreting incoming LAMP data from the LRO. Composition and gases from the soil directly affect the ultraviolet reflectance rate enabling a known substance comparison to that of the lunar surface distinguishing specific elements and compounds from the bulk. SwURC is capable of reaching pressures of 10-7 Torr with a bake-out using a 500 Watt halogen lamp and has a safe temperature range of 78 to 393 K. 78 K is reached with the help of Cryogen-flow tubes and a thermally conductive sample tray holster by freezing samples in a dry nitrogen purge at atmospheric pressure.
Figure 1: Outside View of SwURC
Property of Preston L Karnes, SwRI
EXPERIMENTS Experiments are conducted following the limits and movements shown in Figure 2 below. A 30 W Deuterium lamp feeds the entry slit of a VUV f/4.5, 200 mm focal length monochromator, using a reflective concave mirror. The monochromator is mounted on the chamber illuminating the horizontally mounted sample tray at a fixed 45° angle. The detector rotates freely over a 270° range, so both forward and backscatter measurements are possible. -30º to -60º (0º places the detector perpendicular, directly above the sample tray) is in the ultraviolet beam field of view; therefore, a scan cannot be performed within this interval. In order to conduct a direct beam (calibration) scan, the sample tray must be retracted for the detector to rest at 135º and removing the tray from the field of view. A scan can either be completed by setting a stationary wavelength and scanning through multiple detector angles, or by moving the detector to a specified angle and sweeping through the wavelengths instead. The stationary detector with variable wavelengths is more popular of the two, but both reveal useful data. A typical scan first consists of a 135º direct beam scanning through specified wavelengths and a set dwell time (length of time at each wavelength) with the sample tray retracted. Second, the sample tray extends for measurement of reflectance at each detector angle with the option of having a separate dwell time than the direct beam. Lastly, another direct beam ends the scan in order to verify the accuracy of the detector before and after each run. The output of the preamp feeds both a pulse counter and a digital voltmeter (DVM). Materials with very low reflectance, such as lunar regolith simulant, require data to be recorded in the pulse-counting mode. Highly reflective surfaces, such as Al/MgF 2 coated polished UV mirrors, require a usable signal from both the pulse counter and the DVM. This dual acquisition occurs as the signal pulse rate increases to the point when there is no dead or recovery time between pulses and the signal becomes a DC photocurrent. When executing a scan, SwURC always acquires/stores data from both the pulse counter and the DVM.
Figure 2: Detector Movement and Beam Schematic
Property of Preston L Karnes, SwRI Analysis
Initial calibration of the Channeltron electron multiplier was accomplished through multiple tests and adjustments. At first, the detector was installed as direct with the beam as possible by eying its position in respect to the slit. Multiple connections on the mount were adjusted manually until the detector viewed directly into the slit opening. Once the detector was roughly near its correct position, fine tuning via LabVIEW was initiated. The monochromator was set to Lyman-Alpha, 121.6 nm, wavelength for each scan then the detector would be adjusted according to the peak count rate. After the peak count rate equaled a steady maximum average for multiple scans, calibration of the detector's bracket mounting was accomplished. To ensure full accuracy, the monochromator was calibrated before and after the install by matching the peak at Lyman-Alpha with its correct wavelength, 121.6 nm. The Agilent 53220A frequency counter calibration was performed using the total counts produced by a 1 kHz square wave and adjustments were made accordingly in the LabVIEW programming and analysis of receiving and recording the count rate. The 9906 Net Controls motor calibration consisted of multiple commands and runs before being installed in the chamber for further accuracy adjustments. A collection of virtual instruments were written for controlling the motor solely with LabVIEW. Once the motor was installed in the chamber, multiple sweeps of the detector varying by ±10° were completed while keeping the monochromator stationary at 121.6 nm. The results of these tests, along with multiple software changes, verified the offset from home to achieve accurate motor positioning calibration. The baseline for reflectance calibration is conducted by placing the detector to collect the 45º reflected beam off an Al/MgF2 mirror. The incident and reflectance angle are both 45º from the vertical axis resulting in a minimally obstructed beam of reflectance entering the point of view of the Channeltron. Any data comparison between old and new scans must be conducted on an altered scale due to the differences in voltage usage and multiplier algorithms of each detector. The method of comparison is performed with the use of IDL (Interactive Data Language) programs written for raw ratio comparisons and observations. Adjustments were made to the detector in accordance to the data analysis of the noise ratio, or variances between scans. The graphical user interface utilized for device control and data collection of SwURC is accomplished via LabVIEW 2012. An example of the front panel can be found in Figure 3 below. Data collection involves precise timing of software and hardware/mechanical actions via USB, RS-232, and NI driver bus. Status comparisons, delays, registers, sequenced events, case structures, and Boolean logic were all used in order to ensure proper timing with the hardware and recording. The main LURE_Lab_System virtual instrument is a collection of many virtual instruments operating in synchronization with the front panel graphical user interface. All connecting, booting, and communication with instruments is done through LabVIEW as a stand alone program. Constant read-outs are sent to each device providing real-time status displays while performing automatic accuracy and safety checks. Real-time displays of count rate and detector position versus time are displayed under the data tab.
Property of Preston L Karnes, SwRI
Figure 3: LabVIEW Front Panel
Results
A direct beam monochromator scan was performed with the implementation of the Channeltron detector. Spectral analysis of the plot found in Figure 4 reveals the peak at 121.6° Lyman-Alpha with a quick degradation after 165 nm. After multiple tests and specification analysis, the optimal range for the Channeltron when used with a 30 Watt deuterium lamp is from 115 to 165 nm (first order UV).
Figure 4: Preliminary Channeltron Direct Beam Spectrum
Property of Preston L Karnes, SwRI
The additions to SwURC posed a risk of contamination and were cleaned thoroughly before installation. Before the chamber was brought to vacuum, following the hardware changes, a complete cleaning began using 2-propanol and Texwipes. Every surface, touched or untouched, was wiped down with emphasis on the new instruments and chamber door. A dry nitrogen purge was also active throughout the installation process in order to help prevent water vapor from collecting on the surfaces inside the chamber. Once back to vacuum, an RGA analysis was performed to verify no foreign elements were present inside the chamber. Hydrogen, water vapor, and oxygen were the peak elements found by the scan, as shown if Figure 5 below. Figure 5: Cleanliness of SwURC by RGA Analysis
Conclusions Overall, the level of accuracy for the detector motor increased from ±5º to ±0.002º with the addition of the 9906 Net Controls 6 Amp motor and controller. The direct placement of the detector mount on the motor shaft eliminates the error involved with using a coupling, as implemented before. The 5901 Channeltron detector enables a more focused/accurate view of the first order UV spectrum when comparing to the previously used Hamamatsu R10824 photomultiplier tube (PMT). Verification and calibration testing will continue to ensure accurate, optimal results for each experiment. Future plans with SwURC include the addition of a Convectron pressure gauge for controlling an interlock, verifying safe operating conditions for the deuterium lamp and high voltage detector without the addition of extra ions. A stationary Channeltron is also planned to be mounted directly on the chamber at 135° with respect to the vertical axis. Plans to test Apollo 11 lunar samples, additional lunar simulants, meteorite samples, UV optical surfaces and coatings, and various planetary icy mixtures, gases and minerals are in the near future. SwURC is also available to provide useful research data for groups outside Southwest Research Institute.
Property of Preston L Karnes, SwRI
References
[1]Gregory S. Winters, Kurt D. Retherford, Michael W. Davis, Stephen M. Escobedo, Eric C. Bassett, Edward L. Patrick, Maggie E. Nagengast, Matthew H. Fairbanks, Paul F. Miles, Joel W. Parker, G. Randall Gladstone, David C. Slater, S. Alan Stern, “The Southwest Research Institute ultraviolet reflectance chamber (SwURC) a far ultraviolet reflectometer,” Proc. SPIE 8495, 84950N (2012)
Radiometric Calibration of the SwRI Ultraviolet Reflectance Chamber (SwURC) Far-Ultraviolet Reflectometer Preston L. Karnes1, Kurt D. Retherford1, Gregory S. Winters1, Eric C. Bassett1, Stephen M. Escobedo1, Edward L. Patrick1, Amanda Richter1, Michael W. Davis1, Paul F. Miles1, Joel W. Parker2, G. Randall Gladstone1, Thomas K. Greathouse1, Eric R. Schindhelm2, Lori M. Feaga3 and S. Alan Stern2 1
Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238 2
Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302
3
Department of Astronomy, University of Maryland, College Park, MD 20742 ABSTRACT
The Southwest Research Institute Ultraviolet Reflectance Chamber (SwURC) is a highly capable UV reflectometer chamber and data acquisition system designed to provide bidirectional scattering data of various surfaces and materials. The chamber provides laboratory-based UV reflectance measurements of water frost/ice, lunar soils, simulants, and analogs to support interpretation of UV reflectance data from the Lyman Alpha Mapping Project (LAMP) Lunar Reconnaissance Orbiter (LRO). A deuterium lamp illuminates a monochromator with a nominal wavelength range of 115 nm to 210 nm. The detector scans emission angles -85° to +85° in the principal plane. Liquid nitrogen passed through the sample mount enables constant refrigeration of tray temperatures down to 78 K to form water ice and other volatile samples. The SwURC can be configured to examine a wide range of samples and materials through the use of custom removable sample trays, connectors, and holders. Calibration reference standard measurements reported here include Al/MgF 2 coated mirrors for specular reflection and Fluorilon for diffuse reflectances. This calibration work is a precursor to reports of experiments measuring the far-UV reflectance of water frost, lunar simulants, and Apollo soil sample 10084 in support of LRO-LAMP.
INTRODUCTION Southwest Research Institute (SwRI), Space Science and Engineering Division, has a successful history of building UV spectrographs for NASA spacecraft and sounding rockets. One of these spacecraft, the Lunar Reconnaissance Orbiter, carries a SwRI-built spectrograph known as “LAMP” (Lyman Alpha Mapping Project). LRO entered lunar orbit in 2009 and is currently mapping lunar surfaces, including features near the poles known as permanently shadowed regions (PSR). These low temperature PSRs contain water ice and other compounds that accumulated at and below the surface from the lack of sunlight and atmosphere. The SwRI Ultraviolet Reflectance Chamber (SwURC) was conceived during the LAMP instrument design and build period as a laboratory reflectometer to be used to corroborate observational science data from LAMP, and to provide comparison data acquired with known materials and under known conditions1. Implementation of new instruments within SwURC increases the accuracy and range of data analysis. New additions include a 5901 Channeltron electron multiplier, 500W halogen lamp, 10” nipple, 9906 Net Controls motor and controller, and a 200 amu Extorr RGA. A Hamamatsu R10824 photomultiplier tube was removed in place for the more
Property of Preston L Karnes, SwRI
accurate 5901 Channeltron electron multiplier. The Channeltron offers a wider range of unsaturated data while using a higher voltage (maximum 3000V) and lower current (60 to 167 μA). A 500W halogen lamp on an extendable shaft enables a more than applicable bake-out if necessary. A variable transformer is used to control the power consumed by the lamp, not to exceed 120˚C to avoid damaging interior instruments. The halogen lamp is assembled on one of the five ports available on the 10” nipple flange. A camera view window and extra electrical feedthroughs were also implemented on the flange, leaving two ports open for future use. In order to correct repeatability issues, a 9906 Net Controls 6 Amp bidirectional motor and controller was installed without the use of a coupling. The direct connection of the motor shaft to the detector arm eliminates the 'flop' when changing directions or homing. A 100 amu Extorr RGA was replaced by a 200 amu Extorr RGA enabling twice the resolution and accuracy of the residual gases found within the chamber. Southwest Research Institute Ultraviolet Reflectance Chamber can be used to test ultraviolet reflectance of any ultra-high vacuum compatible substance, but is mainly utilized to analyze moon dust simulants. The simulants offer a known reference for interpreting incoming LAMP data from the LRO. Composition and gases from the soil directly affect the ultraviolet reflectance rate enabling a known substance comparison to that of the lunar surface distinguishing specific elements and compounds from the bulk. SwURC is capable of reaching pressures of 10-7 Torr with a bake-out using a 500 Watt halogen lamp and has a safe temperature range of 78 to 393 K. 78 K is reached with the help of Cryogen-flow tubes and a thermally conductive sample tray holster by freezing samples in a dry nitrogen purge at atmospheric pressure.
Figure 1: Outside View of SwURC
Property of Preston L Karnes, SwRI
EXPERIMENTS Experiments are conducted following the limits and movements shown in Figure 2 below. A 30 W Deuterium lamp feeds the entry slit of a VUV f/4.5, 200 mm focal length monochromator, using a reflective concave mirror. The monochromator is mounted on the chamber illuminating the horizontally mounted sample tray at a fixed 45° angle. The detector rotates freely over a 270° range, so both forward and backscatter measurements are possible. -30º to -60º (0º places the detector perpendicular, directly above the sample tray) is in the ultraviolet beam field of view; therefore, a scan cannot be performed within this interval. In order to conduct a direct beam (calibration) scan, the sample tray must be retracted for the detector to rest at 135º and removing the tray from the field of view. A scan can either be completed by setting a stationary wavelength and scanning through multiple detector angles, or by moving the detector to a specified angle and sweeping through the wavelengths instead. The stationary detector with variable wavelengths is more popular of the two, but both reveal useful data. A typical scan first consists of a 135º direct beam scanning through specified wavelengths and a set dwell time (length of time at each wavelength) with the sample tray retracted. Second, the sample tray extends for measurement of reflectance at each detector angle with the option of having a separate dwell time than the direct beam. Lastly, another direct beam ends the scan in order to verify the accuracy of the detector before and after each run. The output of the preamp feeds both a pulse counter and a digital voltmeter (DVM). Materials with very low reflectance, such as lunar regolith simulant, require data to be recorded in the pulse-counting mode. Highly reflective surfaces, such as Al/MgF 2 coated polished UV mirrors, require a usable signal from both the pulse counter and the DVM. This dual acquisition occurs as the signal pulse rate increases to the point when there is no dead or recovery time between pulses and the signal becomes a DC photocurrent. When executing a scan, SwURC always acquires/stores data from both the pulse counter and the DVM.
Figure 2: Detector Movement and Beam Schematic
Property of Preston L Karnes, SwRI Analysis
Initial calibration of the Channeltron electron multiplier was accomplished through multiple tests and adjustments. At first, the detector was installed as direct with the beam as possible by eying its position in respect to the slit. Multiple connections on the mount were adjusted manually until the detector viewed directly into the slit opening. Once the detector was roughly near its correct position, fine tuning via LabVIEW was initiated. The monochromator was set to Lyman-Alpha, 121.6 nm, wavelength for each scan then the detector would be adjusted according to the peak count rate. After the peak count rate equaled a steady maximum average for multiple scans, calibration of the detector's bracket mounting was accomplished. To ensure full accuracy, the monochromator was calibrated before and after the install by matching the peak at Lyman-Alpha with its correct wavelength, 121.6 nm. The Agilent 53220A frequency counter calibration was performed using the total counts produced by a 1 kHz square wave and adjustments were made accordingly in the LabVIEW programming and analysis of receiving and recording the count rate. The 9906 Net Controls motor calibration consisted of multiple commands and runs before being installed in the chamber for further accuracy adjustments. A collection of virtual instruments were written for controlling the motor solely with LabVIEW. Once the motor was installed in the chamber, multiple sweeps of the detector varying by ±10° were completed while keeping the monochromator stationary at 121.6 nm. The results of these tests, along with multiple software changes, verified the offset from home to achieve accurate motor positioning calibration. The baseline for reflectance calibration is conducted by placing the detector to collect the 45º reflected beam off an Al/MgF2 mirror. The incident and reflectance angle are both 45º from the vertical axis resulting in a minimally obstructed beam of reflectance entering the point of view of the Channeltron. Any data comparison between old and new scans must be conducted on an altered scale due to the differences in voltage usage and multiplier algorithms of each detector. The method of comparison is performed with the use of IDL (Interactive Data Language) programs written for raw ratio comparisons and observations. Adjustments were made to the detector in accordance to the data analysis of the noise ratio, or variances between scans. The graphical user interface utilized for device control and data collection of SwURC is accomplished via LabVIEW 2012. An example of the front panel can be found in Figure 3 below. Data collection involves precise timing of software and hardware/mechanical actions via USB, RS-232, and NI driver bus. Status comparisons, delays, registers, sequenced events, case structures, and Boolean logic were all used in order to ensure proper timing with the hardware and recording. The main LURE_Lab_System virtual instrument is a collection of many virtual instruments operating in synchronization with the front panel graphical user interface. All connecting, booting, and communication with instruments is done through LabVIEW as a stand alone program. Constant read-outs are sent to each device providing real-time status displays while performing automatic accuracy and safety checks. Real-time displays of count rate and detector position versus time are displayed under the data tab.
Property of Preston L Karnes, SwRI
Figure 3: LabVIEW Front Panel
Results
A direct beam monochromator scan was performed with the implementation of the Channeltron detector. Spectral analysis of the plot found in Figure 4 reveals the peak at 121.6° Lyman-Alpha with a quick degradation after 165 nm. After multiple tests and specification analysis, the optimal range for the Channeltron when used with a 30 Watt deuterium lamp is from 115 to 165 nm (first order UV).
Figure 4: Preliminary Channeltron Direct Beam Spectrum
Property of Preston L Karnes, SwRI
The additions to SwURC posed a risk of contamination and were cleaned thoroughly before installation. Before the chamber was brought to vacuum, following the hardware changes, a complete cleaning began using 2-propanol and Texwipes. Every surface, touched or untouched, was wiped down with emphasis on the new instruments and chamber door. A dry nitrogen purge was also active throughout the installation process in order to help prevent water vapor from collecting on the surfaces inside the chamber. Once back to vacuum, an RGA analysis was performed to verify no foreign elements were present inside the chamber. Hydrogen, water vapor, and oxygen were the peak elements found by the scan, as shown if Figure 5 below. Figure 5: Cleanliness of SwURC by RGA Analysis
Conclusions Overall, the level of accuracy for the detector motor increased from ±5º to ±0.002º with the addition of the 9906 Net Controls 6 Amp motor and controller. The direct placement of the detector mount on the motor shaft eliminates the error involved with using a coupling, as implemented before. The 5901 Channeltron detector enables a more focused/accurate view of the first order UV spectrum when comparing to the previously used Hamamatsu R10824 photomultiplier tube (PMT). Verification and calibration testing will continue to ensure accurate, optimal results for each experiment. Future plans with SwURC include the addition of a Convectron pressure gauge for controlling an interlock, verifying safe operating conditions for the deuterium lamp and high voltage detector without the addition of extra ions. A stationary Channeltron is also planned to be mounted directly on the chamber at 135° with respect to the vertical axis. Plans to test Apollo 11 lunar samples, additional lunar simulants, meteorite samples, UV optical surfaces and coatings, and various planetary icy mixtures, gases and minerals are in the near future. SwURC is also available to provide useful research data for groups outside Southwest Research Institute.
Property of Preston L Karnes, SwRI
References
[1]Gregory S. Winters, Kurt D. Retherford, Michael W. Davis, Stephen M. Escobedo, Eric C. Bassett, Edward L. Patrick, Maggie E. Nagengast, Matthew H. Fairbanks, Paul F. Miles, Joel W. Parker, G. Randall Gladstone, David C. Slater, S. Alan Stern, “The Southwest Research Institute ultraviolet reflectance chamber (SwURC) a far ultraviolet reflectometer,” Proc. SPIE 8495, 84950N (2012)
