Contribución (en porcentaje) de los cuatro procesos de aprendizaje

Noble gases such as argon are scintillators which can be utilized as particle detectors. The interaction medium is the argon which has a density of 1.40 g/cm3. The scintillation light can be read out by a photomultiplier tube (PMT) or semiconductor detectors such as a silicon photomultiplier (SiPM) or an avalanche photo diode (APD) .

4.3.1 Scintillation in Argon

Interactions of ionizing particles in argon excite or ionize argon atoms which then recom- bine under the emission of 128 nm UV light. The scintillation process occurs in gaseous and liquid argon with different properties. Fig. 4.11illustrates the two different processes of ex- citation and ionization. If an argon atom is excited Ar∗it collides with a neighboring argon atom forming a neutral excited dimer or excimer1 Ar∗₂ within _{O(1 − 10 ps). This excimer} decays into two argon atoms under emission of scintillation photons. If an argon atom is ionized, it combines with another Ar atom to a charged excimer Ar+₂. The Ar+₂ recombines with a thermalized electron within_{O(100 ps) leading to a neutral excimer under emission} of recombination luminescence. The neutral excimer then decays with scintillation light emission as for the excitation process. The excitation process is dominant in gaseous ar- gon (GAr) at room temperature and atmospheric pressure. In LAr the ionization process

1

4.3 Liquid Argon as Particle Detector 45

dominates with an excitation-ionization ratio Ar∗/Ar+ of 0.21 [98]. Both processes are density dependent. A more detailed description of the processes can be found in [98, 99,

100].

Figure 4.11 Two mechanisms for scintillation in argon. The excitation channel dominates for GAr whereas the ionization channel dominates for LAr. From [99].

Argon excimers can be produced in a singlet or a triplet state. The population of these states depends on the linear energy transfer (LET) or stopping power of the ionizing par- ticle and is different for e, p, n, α particles and nuclear recoils. The ratios between singlet and triplet state population increases with the LET and is 0.3, 1.3 and 3 for electrons, alphas and nuclear recoils respectively [101]. The lifetime of the two states is different and independent of the LET: The de-excitation of the singlet state is an allowed transition with a lifetime of 2_{− 6 ns whereas the de-excitation of the triplet state is forbidden with a} lifetime of 1100_{− 1600 ns [}102,103]. This is referred to as the fast and the slow component of the scintillation light. The difference in lifetime is uniquely large for argon scintillation and can be utilized to distinguish different types of particle interactions. E.g. electron interaction show a larger slow component than alpha interactions.

The emission of argon scintillation light peaks at 128 nm with _{≈ 6 nm FWHM [}102]. An emission spectrum for LAr is shown in the bottom panel ofFig. 4.12. Most light detectors are encased in glass which is opaque for hard UV light. Therefore the scintillation photons have to be shifted to higher wavelength before detection. The light yield of XUV photons per MeV is roughly 40.000 γ/MeV in pure LAr for interacting electrons with 1 MeV energy. The light yield is dependent on the LET and is quenched for alpha interactions by 11 % [98]. The attenuation of scintillation light in LAr is influenced by scattering and absorption which have to be distinguished. In the application of a large volume detector such as the Gerda cryostat, the absorption will result in light loss whereas the scattering merely changes the direction. In fact the scattering of light may even enhance the light detection from certain locations with circumventing shadowing effects. On the other hand, small scale experiments measuring LAr properties are often long thin tubes in which scattering of light is almost equivalent to losing the light on the vessel walls. Special care has to be taken to interpret literature values of attenuation. The Rayleigh scattering length in LAr is theoretically calculated as 90 cm [105] and measured as 66 cm [106]2_{. The absorption}

2

Figure 4.12 Top: Transmission spectrum of pure LAr. Bottom: Emission spectrum with the dominant peak at 128 nm. From [104].

length varies largely with the type and concentration of impurities and has to be measured in-situ of the specific setup. Measurements with an artificial light source showed that the attenuation is strongly wavelength dependent for XUV light [104]. A transmission spec- trum for XUV light is shown in the top panel ofFig. 4.12. For optical photons e.g. after wavelength shifting of the XUV photons, the LAr is practically transparent.

Impurities in the LAr such as N2, O2, H2O or CO2can significantly alter the light yield, the

triplet lifetime and the attenuation length. Changes in scintillation properties are mainly caused by increasing non-radiative de-excitation of the triplet state and by absorbing the 128 nm scintillation light after production. The effects depend on the impurity concentra- tion and the chemical properties. Investigations have been performed in e.g. in [100,107], [108] and [99] for nitrogen, oxygen and air respectively.

4.3.2 Single Photon Detection

Photomultiplier tubes (PMT) are evacuated glass tubes for the detection of single photons. If a photon hits the photocathode it knocks out an electron via the photoelectric effect. The probability for this process is the quantum efficiency which depends on the cathode material and the photon wavelength. The photo electron is accelerated in an elec- tric field towards the first dynode where it knocks out multiple secondary electrons. The secondary electrons are accelerated towards the next dynode in an successively increasing electric field until they reach the anode. The number of electrons collected at the anode for a single incident photon is called the gain factor of the PMT. The amplified measured signal is called the single photo electron (spe) signal. Multiple initial photons scale the spe signal linearly. The gain and the spe signal are dependent on the number and type of dynodes and the operational voltage. A typical operational voltage is 1000− 2000 V with a gain between 106 and 107 and an spe signal around 1_{− 10 mV.}

A signal can also be created by thermal electron emission, cosmic rays or radioactive decays inside the PMT. The frequency of those events is called dark rate. Specific design con- cepts are necessary for PMT operation at cryogenic temperatures and in a low background