Evidence of delayed light emission of tetraphenyl-butadiene excited by liquid-argon scintillation light

Tetraphenyl-butadiene is the wavelength shifter most widely used in combination with liquid argon. The latter emits scintillation photons with a wavelength of 127 nm that need to be downshifted to be detected by photomultipliers with glass or quartz windows. Tetraphenyl-butadiene has been demonstrated to have an extremely high conversion efficiency, possibly higher than 100% for 127 nm photons, while there is no precise information about the time dependence of its emission. It is usually assumed to be exponentially decaying with a characteristic time of the order of one ns, as an extrapolation from measurements with exciting radiation in the near UV. This work shows that tetraphenyl-butadiene, when excited by 127 nm photons, re-emits photons not only with a very short decay time, but also with slower ones due to triplet states de-excitations. This fact can strongly contribute to clarifying the anomalies in liquid-argon scintillation light reported in the literature since the 1970s, namely, the inconsistency in the measured values of the long decay time constant and the appearance of an intermediate component. Similar effects should be also expected when the TPB is used in combination with helium and neon, which emit scintillation photons with wavelengths shorter than 127 nm.

Figure 1

Response function of TPB for 127 nm photons at LAr temperature. It has been fitted with a function made of four decaying exponentials convoluted with a Gaussian (see text). The result of the fit is represented by a red (grey) line.

Figure 2

Average waveforms for LAr scintillation light when excited by ${}^{60}\mathrm{Co} \gamma \mathrm{s}$ at different ppm levels of nitrogen contamination. In red (grey) the results of the fits are shown. The fit function is a convolution of the TPB response function (3000 ppm average waveform) with the sum of two decaying exponentials, assumed to be the response of LAr.

Figure 3

Scheme of the experimental setup used to irradiate TPB films with electrons and $\alpha$ particles.

Figure 4

Red (lower grey) line: average waveform obtained by irradiating a TPB film with electrons. Blue (upper grey) line: average waveform obtained by irradiating a TPB film with $\alpha$ particles. Black line: average waveform obtained irradiating TPB with LAr scintillation light quenched by 3000 ppm of nitrogen.

Figure 5

Normalized histograms of the arrival time of single photons. The black curve refers to photon excitation of TPB, the red curve (dark grey) to α excitation, and the green one (light grey) to electron excitation.

Figure 6

Histogram of arrival times of single photons for TPB excited by VUV photons. The blue (grey) line represents the result of the fit with the function of Eq. (3).

Figure 7

Graphical solution of Eq. (6). In black the function $S\left(t\right)$ and in red (grey) $T\left(t\right)$. The solution is represented by the crossing point of the two functions, which is found around 120 ns.