Detection of far ultraviolet radiation by wavelength-shifting tetraphenyl butadiene

Far ultraviolet (FUV) radiation has been used in low-energy particle physics\footnote{V. Chepel and H. Ara\'{u}jo, J. Inst. {\bf 8}, R04001 (2013).}, dark matter searches\footnote{E. Aprile, {\em et al.}, Astroparticle Phys. {\bf 35}, 573 (2012).}, and neutron detection\footnote{J. C. McComb, {\em et al.}, J. Appl. Phys. {\bf 115}, 144504 (2014).}, in conjuction with wavelength-shifting (WLS) materials. Tetraphenyl butadiene (TPB) has been found to have high conversion efficiency compared to other WLS fluorophores. We have spin-coated TPB films with high uniformity and optical quality on glass windows and compared the absolute efficiencies of both the spin-coated and vapor deposited films over the incident radiation wavelengths $120 \, \mathrm{nm} < \lambda < 400 \, \mathrm{nm}$ at the NIST SURF III Synchrotron Ultraviolet Radiation Facility. While photon efficiencies of 0.7 and 1.35, have been reported\footnote{V. M. Gehman, {\em et al.}, Nuc. Instr. Meth. Phys. Res. A {\bf 654}, 116 (2011).}, our preliminary results indicate the absolute efficiencies to be between 0.2 and 0.5. The Neutron Observatory\footnote{{\tt http://j.mp/N3utr0n}} was used to compare conversion efficiencies in well-characterized detector platforms.