An Exploration of the Optical Detection of Ionizing Radiation Utilizing Modern Optics Technology

Transcription

1 An Exploration of the Optical Detection of Ionizing Radiation Utilizing Modern Optics Technology SAND T PRESENTED BY Sean D. Fournier Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy s National Nuclear Security Administration under contract DE-NA

2 2 Overview Introduction Optical Filtering Techniques Light Detection for Imaging Image Processing Optical Detection of Ionizing Radiation Tests Conclusions

3 Introduction Molecular Spectroscopy, Ionizing Radiation, Nitrogen Florescence, and the Optical Detection of Radiation (ODR)

4 4 Molecular Spectroscopy Excited molecules can de-excite through several mechanisms including a process called fluorescence Fluorescence involves the emission of a photon The wavelength of this light relates to the electronic energy structure of the molecule Nitrogen is very abundant in the air and can be excited to these energetic states by ionizing radiation The fluorescence of nitrogen can be observed at distances much farther than traditional ionizing radiation Maxim Bilovitskiy

5 5 Ionizing Radiation Air Attenuation Length of Radiation α β n γ UV 0.05 meters 5 meters 50 meters 150 meters 2500 meters

6 6 Nitrogen Florescence Wavelength Excited Transition Relative Photons per Decay (nm) System ν' ν'' Intensity Po-210 P P ± ± P ± ± P ± ± N ± ± 0.05 Measurement results from (Brett, et al., 2017) Model results from (Thompson, Barritt, & C., 2016)

7 7 Optical Detection of Radiation (ODR) Remote detection of large sources Hot-particle detection Radiation facility dose mapping Imaging medical treatment beams

8 8 Equipment Used in ODR Camera Lens Filters Mirrors Andor ikon-m DU934P Highly Sensitive to UV, liquid cooled Nikon UV mm f/4.5 UV Capable Lens Chroma SP650 and ET360BP5.5nm Specifically Engineered for ODR application BB211-E01 Zerodur 2 Broadband Dielectric Mirror, nm

9 Optical Filtering Techniques Optical Background, Transmissivity, and Off-band Blocking Determination

10 10 Sources of Optical Background ODR is a background-limited measurement technique Optical background in any natural situation is orders of magnitude more intense than the fluorescence light being imaged Sources of optical background are Sun and moonlight Starlight Manmade lighting CFL Halogen Incandescent LED

11 11 Optical Filtering Heavy optical filtering is necessary to reduce the background (off-band) light reaching the sensor Filtering is achieved using interference (dichroic) filters Very thin coatings are sputtered onto a glass substrate Thickness on the order of ¼ the wavelength of light being passed (10s - 100s of nm) Filters can be custom engineered to pass very narrow bands of light and very effectively block all off-band light down to several orders of magnitude

12 12 Determination of Transmissivity Transmissivity = the fraction of light transmitted over the light incident at a specific wavelength Measured in the lab with a light source and a spectrometer Filters are placed in the path of the light using off-axis parabolic mirrors coupled to fiber optics This reduces the shift in wavelength that can happen when using lens-based fiber collimators Transmissivity is used in the calculation of overall detection efficiency for the ODR system TT ii = II ff ii II rr ii 100%

13 13 Effect of Angle on Transmissivity

14 14 Optimized Filter Configurations and Lessons Learned A consistent test rig is critical to performing reliable transmissivity measurements. Filter vendors may not provide data for the full spectral region Stacking identical filters in series reduces the background but does not greatly reduce the overall transmissivity in the passband as long as the individual transmissions are large (>80%) Off-axis light is transmitted with a passband that is shifted to the shorter wavelengths Off-parallel filter stacking can be used to sharpen passbands

15 Light Detection for Imaging Techniques and Technology

16 16 Light Collection and Detection Light Collection Refractive (Lens-Based) Fused Silica has a very high transmission in the UV Reflective (Mirror-Based) Metallic or custom coated dichroic mirrors can reflect UV at high rates while passing off-band light Light Detection EMCCD Electron-multiplied Charge Coupled Device Very high Quantum Efficiency (conversion of light to signal) Very low dark noise when cooled to low temperatures iccd Intensified Charge Coupled Device Very sensitive to low signals Fast gating of intensifier can be used in special fast applications

17 17 Optimization of Acquisition Settings: Laboratory Setup Available acquisition settings were optimized in a controlled dark-box experiment with the compact ODR system Camera placed ~ 4 feet away from a ~200 uci Po-210 source Narrow bandpass filter placed in front of lens Passes 340 nm with a FWHM of 10 nm 78% peak transmittance Nikon UV-105 f4.5 lens used to focus Designed for UV photography Very high transmittance (~60%) Andor ikon-m EMCCD camera with liquid cooler Automation script written to speed up analysis

18 18 Optimization of Acquisition Settings: Experiment Acquisition Chip Time Temperature Readout (seconds) ( C) Binning x x x x x x

19 19 Optimization of Acquisition Settings: Electronic Binning Binning improved signal detection but magnified the effect of direct radiation strikes on the CCD Optimal binning for most applications seems to be around 4x4 to 8x8

20 20 Optimization of Acquisition Settings: CCD Temperature EMCCD cooling is critical to reduce the dark noise. Optimal temperature is achieved around -80 C but if cooler temperatures can be reached (such as in the lab, background is further reduced

21 21 Optimization of Acquisition Settings: Acquisition Time Longer acquisition times lead to increased detection sensitivity. However, longer acquisition times allow for more direct radiation strikes on the CCD. This negatively impacts the image quality.

22 22 Optimization of Acquisition Settings: Summary Electronic binning should be employed to increase image sensitivity at the cost of some resolution Cooling is absolutely necessary to reduce dark noise and liquid coolers effectively remove heat quickly. Longer acquisition times copen the chip up for more direct radiation strikes. Shielding is necessary for long acquisition times or measurements in higher-thanbackground radiation fields We designed a custom Tungsten polymer shield for our cameras to reduce the dose to the CCD Designed to be 1/10 th thickness for 500 kev gamma rays

23 23 SNL ODR Configurations: Pathfinder Telescope

24 24 SNL ODR Configurations: Demonstrator Telescope

25 25 SNL ODR Configurations: Compact System

26 Image Processing Methods and Lessons Learned

27 27 Contrast and Signal to Noise Ratio 1 of 4 Noise Regions CC = SS NN NN SSSSSS = SS σσ NN Signal Region

28 28 Image Conditioning Techniques Export as uncompressed, full scale TIFF Contrast Enhancement Shift Black White scale to small percentage Median Filtering Remove speckle due to hot pixel or direct radiation strikes Source and Background identification Algorithm or User based Sample Pixel values from each region of interest Quantitative Analysis Contrast SNR Counts per Minute Lookup Tables: False-color heatmap Blue background Red most intense signal Opacity and Overlay Align against target image taken with backlight

29 Optical Detection of Ionizing Radiation Tests Review of Measurements Conducted by SNL Team

30 30 Po-210 Static Eliminator Source in a Dark Box 200 uci Po-210 static eliminator source Dark box enclosure Compact filter & lens based optics Used to optimize camera settings These sources can be useful as quality control sources for field tests

31 31 Po-210 Static Eliminator Source in a Dark Room 40 mci Po-210 source placed on the inside of a ~2in Diameter tube Very intense signal observed (as little as 10s exposures) Very good for quick alignment and QC checks for ODR system Proved good for measurements at long distances Using mirrors in the lab, this source was measured at 40 meters Camera Comparison Pathfinder at 40m in 300s

32 32 Cs-137 Shepherd Irradiator Cs-137 source used to calibrate high-dose health physics instrumentation NIST traceable dose fields inside well characterized chamber Port hole allows clear optical path inside chamber Showed great promise for future ODR calibration

33 33 Irradiated Y-90 foils at the Annular Core Research Reactor Can ODR be used to measure pure beta radiation? Research purity Yttrium foils were irradiated at ACRR to produce high levels of Y-90

34 34 Irradiated Y-90 foils at the Annular Core Research Reactor Camera and optics off-axis with pit to decrease dose to CCD Camera shielded with lead bricks

35 35 Irradiated Y-90 foils at the Annular Core Research Reactor

36 Conclusions Future Optimization and Applications

37 37 Future Optimizations Light-output modeling capability for SNL s ODR applications Solar-blind optical filter design and testing More detailed worn on ODR of gamma radiation ODR measurements of neutron radiation In-Situ ODR spectroscopy Radiation energy identification through line yield ratios

38 38 Future Potential Applications Sterilization Industry Evaluating radiation environments and checking for shielding leaks Pulsed power and radiography In-Situ dosimetry real-time feedback of radiation environments Medical Physics Treatment beam quality assurance Real-time beam profiling and patient dose evaluation Radiochemical processing Non-invasive hot-particle localization Decommissioning before/after verification Nuclear facility monitoring Live monitoring of radioactive material to meet safeguards requirements

39 39 Conclusion Background reduction is absolutely key to performing ODR measurements Measurement of custom filter stacks across whole-spectrum needed. Manufacturers often do not quote transmission beyond the narrow spectral region the filters were designed to be used in Angular response of filters is important to consider in optics designs when narrow bandpass filters are used. Transmission shifts towards shorter wavelengths the steeper the angle This effect can actually be used to narrow the composite passband when identical narrow bandpass filters are used Contrast enhancement, median filter, and careful source ID algorithms are needed for image processing Direct radiation strikes on the CCD are a real problem and should be mitigated by Shielding the CCD chip from external radiation Median filtering Rotating orientation of sensor with turning mirrors

40 40 Conclusion Deep thermoelectric (TE) cooling is needed to reduce dark noise TE coolers are enhanced by using liquid heat exchangers Allows for closed-off camera systems such as shields or dark boxes since fans are no longer needed Electronic binning has a major impact to image sensitivity Must be weighed against the need for resolution Must be weight against the effect of magnifying direct radiation strikes on the CCD Careful attention to losses in transmissivity by optical components Every lens surface, mirror, filter introduces losses to the overall detection efficiency Losses combine multiplicatively Quantitative ODR may be possible in the near future but the following are needed Robust dose modeling Accurate fluorescence predictions Light transmission modeling Careful measurement of overall system detection efficiency

41 References and Extra Slides

42 42 References

43 43 References

44 44 Determination of Off-Band Blocking Fraction Using the same apparatus, set the acquisition time such that the off-band region is measurable Compare filter configurations in the offband regions to determine effectiveness We learned that using multiples of the same filter helped reduce the off-band blocking fraction This method is not quantitative because the spectrometer is saturated

45 45 Effect of Angle on Transmissivity

46 46 Effect of Angle on Transmissivity

47 47 Effect of Angle on Transmissivity

48 48 Effect of Angle on Transmissivity

49 49 Benefits of off-normal filter stacking

50 50 Image Analysis Process