Measurements of attenuation and scattering properties of water-based liquid scintillator

Water-based liquid scintillator (WbLS) is a hybrid detector medium which has been proposed as a fill material for future large-volume rare event searches, including <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>ν</a:mi><a:mo>−</a:mo></a:math> detectors. This family of scintillating suspensions promises waterlike attenuation and scattering behavior, while offering scintillation light yield for sub-Cherenkov-threshold events. These features may allow for improved vertex and energy resolution, and concomitant improvements in background rejection and particle identification. While subscale measurements of light yield, timing and pulse-shape have been performed in several WbLS formulations, detailed measurements of the attenuation and scattering properties of WbLS remain a critical, unresolved step along the pathway to deployment. In pursuit of a better understanding of these parameters, a “long-arm” attenuation and scattering instrument has been developed at Lawrence Livermore National Laboratory, dubbed LASE (Livermore Attenuation and Scattering Experiment). Optical property measurements have been performed using LASE in DI water, Gd-water, WbLS and Gd-WbLS. Measurements of the optical properties of WbLS (1% LAB-PPO scintillator) provided by Brookhaven National Laboratory demonstrate an attenuation minimum of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mrow><c:mn>4.87</c:mn><c:mo>×</c:mo><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>4</c:mn></c:mrow></c:msup></c:mrow><c:mo>±</c:mo><c:mrow><c:mn>1.42</c:mn><c:mo>×</c:mo><c:msup><c:mrow><c:mn>10</c:mn></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>4</c:mn></c:mrow></c:msup><c:mtext> </c:mtext><c:mtext> </c:mtext><c:msup><c:mrow><c:mi>cm</c:mi></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:mrow></c:mrow></c:math> at 450 nm, while Gd-loaded WbLS (1% LAB-PPO scintillator, 0.1% Gd) demonstrated a minimum attenuation coefficient of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mrow><e:mn>4.77</e:mn><e:mo>×</e:mo><e:msup><e:mrow><e:mn>10</e:mn></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>4</e:mn></e:mrow></e:msup></e:mrow><e:mo>±</e:mo><e:mrow><e:mn>1.27</e:mn><e:mo>×</e:mo><e:msup><e:mrow><e:mn>10</e:mn></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>4</e:mn></e:mrow></e:msup><e:mtext> </e:mtext><e:mtext> </e:mtext><e:msup><e:mrow><e:mi>cm</e:mi></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>1</e:mn></e:mrow></e:msup></e:mrow></e:mrow></e:math> at the same wavelength. Measurements of scattering in WbLS show a scattering coefficient of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mrow><g:mn>3.39</g:mn><g:mo>×</g:mo><g:msup><g:mrow><g:mn>10</g:mn></g:mrow><g:mrow><g:mo>−</g:mo><g:mn>4</g:mn></g:mrow></g:msup></g:mrow><g:mo>±</g:mo><g:mrow><g:mn>1.98</g:mn><g:mo>×</g:mo><g:msup><g:mrow><g:mn>10</g:mn></g:mrow><g:mrow><g:mo>−</g:mo><g:mn>5</g:mn></g:mrow></g:msup></g:mrow><g:mtext> </g:mtext><g:mtext> </g:mtext><g:msup><g:mrow><g:mi>cm</g:mi></g:mrow><g:mrow><g:mo>−</g:mo><g:mn>1</g:mn></g:mrow></g:msup></g:mrow></g:math> at the 450 nm attenuation minimum, while Gd-WbLS has a scattering coefficient of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mrow><i:mn>2.8</i:mn><i:mo>×</i:mo><i:msup><i:mrow><i:mn>10</i:mn></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>4</i:mn></i:mrow></i:msup></i:mrow><i:mo>±</i:mo><i:mrow><i:mn>1.63</i:mn><i:mo>×</i:mo><i:msup><i:mrow><i:mn>10</i:mn></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>5</i:mn></i:mrow></i:msup></i:mrow><i:mtext> </i:mtext><i:mtext> </i:mtext><i:msup><i:mrow><i:mi>cm</i:mi></i:mrow><i:mrow><i:mo>−</i:mo><i:mn>1</i:mn></i:mrow></i:msup></i:mrow></i:math> at that wavelength. These scattering and attenuation coefficients are significantly larger than measured for DI water at similar wavelengths (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mrow><k:mn>8.8</k:mn><k:mo>×</k:mo><k:msup><k:mrow><k:mn>10</k:mn></k:mrow><k:mrow><k:mo>−</k:mo><k:mn>5</k:mn></k:mrow></k:msup></k:mrow><k:mtext> </k:mtext><k:mtext> </k:mtext><k:msup><k:mrow><k:mi>cm</k:mi></k:mrow><k:mrow><k:mo>−</k:mo><k:mn>1</k:mn></k:mrow></k:msup><k:mo>±</k:mo><k:mrow><k:mn>3.8</k:mn><k:mo>×</k:mo><k:msup><k:mrow><k:mn>10</k:mn></k:mrow><k:mrow><k:mo>−</k:mo><k:mn>5</k:mn></k:mrow></k:msup></k:mrow></k:mrow></k:math> attenuation at 430 nm, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:mrow><m:mn>2.2</m:mn><m:mo>×</m:mo><m:msup><m:mrow><m:mn>10</m:mn></m:mrow><m:mrow><m:mo>−</m:mo><m:mn>5</m:mn></m:mrow></m:msup></m:mrow><m:mo>±</m:mo><m:mrow><m:mn>1.28</m:mn><m:mo>×</m:mo><m:msup><m:mrow><m:mn>10</m:mn></m:mrow><m:mrow><m:mo>−</m:mo><m:mn>6</m:mn></m:mrow></m:msup></m:mrow><m:mtext> </m:mtext><m:mtext> </m:mtext><m:msup><m:mrow><m:mi>cm</m:mi></m:mrow><m:mrow><m:mo>−</m:mo><m:mn>1</m:mn></m:mrow></m:msup></m:mrow></m:math> scattering). Though the WbLS attenuation and scattering coefficients measured are lower than the corresponding values published for ultrapure oils and LAB-PPO liquid scintillator, the material tested attenuates significantly more than water across the blue and green portions of the visible spectrum. This attenuation is markedly stronger at wavelengths under 430 nm.