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Register a new userMeasurement of Liquid Argon Scintillation Light Properties by means of an Alpha Source placed inside the CERN 10-PMT LAr Detection System
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A particle detection system that exploits the scintillation light produced by ionizing particles in liquid argon (LAr) has been assembled at CERN. The system is based on 10 large-area photomultiplier tubes (PMT) immersed in a 1500-liter dewar filled with liquid argon and equipped with an extendible feed-through and mechanical support for an alpha source (241Am). The position of the source can be changed with respect to the PMT plane in vertical and horizontal directions. Arrays of silicon photomultiplier (SiPM) photodetectors, integrated in the source support, are used for the data acquisition trigger and to define the t0 of the light generation. PMT and SiPM signals can be recorded at different distances and different positions allowing the measurement of some of the LAr scintillation light properties. The system was studied and characterized in detail, and physics results on LAr scintillation properties are expected soon.
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We have investigated the possibility of calibrating the PMTs of scintillation detectors, using the primary scintillation produced by X-rays to induce single photoelectron response of the PMT. The high-energy tail of this response, can be approximated to an exponential function, under some conditions. In these cases, it is possible to determine the average gain for each PMT biasing voltage from the inverse of the exponent of the exponential fit to the tail, which can be done even if the background and/or noise cover-up most of the distribution. We have compared our results with those obtained by the commonly used single electron response (SER) method, which uses a LED to induce a single photoelectron response of the PMT and determines the peak position of such response, relative to the pedestal peak (the electronic noise peak, which corresponds to 0 photoelectrons). The results of the exponential fit method agree with those obtained by the SER method when the average number of photoelectrons reaching the first dynode per light/scintillation pulse is around 1.0. The SER method has higher precision, while the exponential fit method has the advantage of being useful in situations where the PMT is already in situ, being difficult or even impossible to apply the SER method, e.g. in sealed scintillator/PMT devices.
Liquid argon is used as active medium in a variety of neutrino and Dark Matter experiments thanks to its excellent properties of charge yield and transport and as a scintillator. Liquid argon scintillation photons are emitted in a narrow band of 10~nm centered around 127 nm and with a characteristic time profile made by two components originated by the decay of the lowest lying singlet and triplet state of the excimer Ar$_2^*$ to the dissociative ground state. A model is proposed which takes into account the quenching of the long lived triplet states through the self-interaction with other triplet states or through the interaction with molecular Ar$_2^+$ ions. The model predicts the time profile of the scintillation signals and its dependence on the intensity of an external electric field and on the density of deposited energy, if the relative abundance of the unquenched fast and slow components is know. The model successfully explains the experimentally observed dependence of the characteristic time of the slow component on the intensity of the applied electric field and the increase of photon yield of liquid argon when doped with small quantities of xenon (at the ppm level). The model also predicts the dependence of the pulse shape parameter, F$_{prompt}$, for electron and nuclear recoils on the recoil energy and the behavior of the relative light yield of nuclear recoils in liquid argon, $mathcal{L}_{eff}$
The propagation velocity of scintillation light in liquid argon $v_{g}$ at $lambda sim 128$~nm wavelength, has been measured for the first time in a dedicated experimental setup at CERN. The obtained result $frac{1}{v_{g}} = 7.46 pm 0.08$~ns/m , is then used to derive the value of the refractive index (n) and the Rayleigh scattering length ($mathcal{L}$) for liquid argon in the VUV region. For $lambda = 128$~nm we found $n= 1.358 pm 0.003$ and $mathcal{L}= 99.1 pm 2.3$~cm. The measured values are of interest for a variety of experiments searching for rare events like neutrino and dark matter interactions. The derived quantities also represent key information for theoretical models describing the propagation of scintillation light in liquid argon.
Measurements were made of scintillation light yield of alpha particles from the $^{222}$Rn decay chain within the DarkSide-50 liquid argon time projection chamber. The light yield was found to increase as the applied electric field increased, with alphas in a 200 V/cm electric field exhibiting a 2% increase in light yield compared to alphas in no field.
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, reemits 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 clarify the anomalies in liquid Argon scintillation light reported in literature since seventies, 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, that emit scintillation photons with wavelengths shorter than 127 nm.
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