Detection of far ultraviolet radiation by wavelength-shifting tetraphenyl butadiene

Far ultraviolet radiation (FUV) is used by a number of detectors employed 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 dosimetry.\footnote{J. C. McComb, {\em et al.}, J. Appl. Phys. {\bf 115}, 144504 (2014).} Particularly difficult to detect due to its strong absorption by most materials, FUV can be converted to visible light by use of wavelength-shifting scintillators. Tetraphenyl butadiene (TPB) is a widely-used wavelength shifting compound that fluoresces at wavelengths $\lambda \approx 420 \, $ nm when excited by FUV. We have used two methods to coat glass disks with thin films of TPB: direct vapor-phase deposition; and spin-coating of TPB suspended in polystyrene/toluene solutions. Using the NIST SURF III Synchrotron Ultraviolet Radiation Facility,\footnote{see {\tt http://j.mp/surf111} and {\tt http://j.mp/N3utr0n}} we have measured the absolute fluorescence efficiency of TPB in the range $120 \, \mathrm{nm} < \lambda < 400 \, \mathrm{nm}$. Our results are compared with previous studies using FUV discharge sources .\footnote{V. M. Gehman, {\em et al.}, Nuc. Instr. Meth. Phys. Res. A {\bf 654}, 116 (2011).}