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The Basic Properties of Liquid Xenon as Related to its Application in Radiation Detectors
Abstract
An overview of the thermodynamic, electrical, and electronic properties of liquid xenon is given as they pertain to the application of this liquid as detection medium in liquid ionization chambers. Special emphasis is placed on the transport properties of the charge carriers produced by high energy radiation and their reactions with impurities. The process of purification is discussed from a physico-chemical point of view.
... The statistical limitation on the energy resolution, neglecting readout electronics, is then : √ N/N = 3.5 %. In comparison, in liquid Xenon, the free ion yield for a field of 7 kV·cm −1 reaches approximately 6.1 electrons per 100 eV of absorbed energy [33], resulting in an amplitude charge signal of 4.9 fC for a 500 keV photon and an energy resolution limited to 0.6 %. ...
The CaLIPSO project is an innovative high-energy photon detector concept using trimethylbismuth as sensitive medium in a liquid ionization chamber. The detector, designed for high precision brain PET imaging, works as a time-projection chamber and detects Cherenkov light and charge signal. We measured the free ion yield of trimethylbismuth, which represents the number of electron-ion pairs released by the incident photon. To do so, we developed a low-noise measuring system to determine the current induced by a \Co source in the liquid with an accuracy better than 5 fA for an electric field up to 7 kV/cm. We used tetramethylsilane as benchmark liquid to validate the apparatus and we measured a zero-field free ion yield of 0.53 ± 0.03 in agreement with measurements in literature. However, we found a zero-field free ion yield of 0.083 ± 0.003 for trimethylbismuth, which is a factor 7 lower than the typical values for similar dielectric liquids. Quantum chemistry computations on heavy atoms tend to demonstrate a high capacity of trimethylbismuth to capture electrons which could explain the weak value. The consequences of a low free ion yield in terms of high-energy photon detection and brain PET imaging are finally discussed.
... In comparison, in liquid Xenon, the free ion yield for a field of 7 kV·cm −1 reaches approximately 6.1 electrons per 100 eV of absorbed energy [32], resulting in an amplitude charge signal of 4.9 fC for a 500 keV photon and an energy resolution limited to 0.6 %. ...
CaLIPSO project is an innovative {\gamma} detector concept using trimethylbismuth as sensitive medium in a liquid ionization chamber. The detector, designed for high precision brain PET imaging, works as a time-projection chamber and detects both ionization and Cherenkov light signals. In order to characterize the detector, we measured the free ion yield of trimethylbismuth and tetramethylsilane, used as a reference liquid, by measuring the current induced by a $^{60}$Co source in an ionization chamber. We measured a zero-field free ion yield of $0.53 \pm 0.03$ for tetramethylsilane, in agreement with measurements in literature. For trimethylbismuth, the zero-field free ion yield is $0.083 \pm 0.003$, which is a factor 7 lower than the typical values for similar dielectric liquids. Quantum chemical computations on heavy atoms tend to demonstrate a high capacity of TMBi to capture electrons which could explain the discrepancy. The consequences of a low free ion yield in terms of {\gamma} detection and brain PET imaging are finally discussed.
CaLIPSO est un concept de détecteur de photons pour la Tomographie par Émission de Positrons dédiée au cerveau. Il s'agit d'une technique d'imagerie médicale reposant sur la détection en coïncidence de deux photons de 511 keV. Pour la première fois, le triméthylbismuth liquide est utilisé comme milieu de détection. Chaque photon de 511 keV libère un électron primaire qui émet des photons Cherenkov et ionise le milieu. CaLIPSO fonctionne sur le principe d'une chambre à projection temporelle et détecte à la fois la lumière Cherenkov et le signal de charge. Le nombre total de charges libérées étant proportionnel à l’énergie déposée par le photon incident, nous avons pu mesurer le rendement de production de charge du triméhylbismuth. Pour cela, nous avons développé un système d'ultra-purification du liquide associé à un système de mesure bas bruit du courant induit par une source de photons γ avec une précision < 5 fA pour un champ électrique allant jusqu'à 7 kV/cm. Le tétraméthylsilane a été utilisé comme liquide de référence pour valider la mesure. Nous avons obtenu un rendement de production de charge du triméthylbismuth inférieur d'un facteur 6 aux valeurs typiques des liquides diélectriques similaires. Des calculs de chimie quantique sur atomes lourds ont permis de montrer que ce comportement est dû à la géométrie de la molécule de triméthylbismuth. L'atome de bismuth se comporte comme un centre de capture des électrons qui induit un mécanisme de recombinaison supplémentaire des électrons près de leurs cations parents. Enfin, afin de vérifier cette hypothèse et de quantifier la mobilités des charges dans les liquides, nous avons développé un système de mesure d'impulsions de charge individuelles qui a été validé avec succès avec le tétraméthylsilane.
The XENON100 dark matter experiment uses liquid xenon in a time projection
chamber (TPC) to measure xenon nuclear recoils resulting from the scattering of
dark matter Weakly Interacting Massive Particles (WIMPs). In this paper, we
report the observation of single-electron charge signals which are not related
to WIMP interactions. These signals, which show the excellent sensitivity of
the detector to small charge signals, are explained as being due to the
photoionization of impurities in the liquid xenon and of the metal components
inside the TPC. They are used as a unique calibration source to characterize
the detector. We explain how we can infer crucial parameters for the XENON100
experiment: the secondary-scintillation gain, the extraction yield from the
liquid to the gas phase and the electron drift velocity.
An international project to search μ→eγ decay includes the use of a liquid xenon gamma ray detector. So, a liquid level meter working at a low temperature with low outgassing is needed and the prototype is constructed. The meter shows the liquid level by measuring the capacitance between electrodes with small intervals immersed in the liquid. The operation was successful with the estimated precision of 1 mm in RMS or better.
We have developed an efficient cryogenic system with heat exchange and
associated gas purification system, as a prototype for the XENON1T experiment.
The XENON1T detector will use about 3 ton of liquid xenon (LXe) at a
temperature of 175K as target and detection medium for a dark matter search. In
this paper we report results on the cryogenic system performance focusing on
the dynamics of the gas circulation-purification through a heated getter, at
flow rates above 50 Standard Liter per Minute (SLPM). A maximum flow of 114
SLPM has been achieved, and using two heat exchangers in parallel, a heat
exchange efficiency better than 96% has been measured.
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