A liquid argon time projection chamber, constructed for the Argon Response to Ionization and Scintillation (ARIS) experiment, has been exposed to the highly collimated and quasi-monoenergetic LICORNE neutron beam at the Institute de Physique Nuclaire Orsay in order to study the scintillation response to nuclear and electronic recoils. An array of liquid scintillator detectors, arranged around the apparatus, tag scattered neutrons and select nuclear recoil energies in the [7, 120] keV energy range. The relative scintillation efficiency of nuclear recoils was measured to high precision at null field, and the ion-electron recombination probability was extracted for a range of applied electric fields. Single-scattered Compton electrons, produced by gammas emitted from the de-excitation of 7 Li * in coincidence with the beam pulse, along with calibration gamma sources, are used to extract the recombination probability as a function of energy and electron drift field. The ARIS results have been compared with three recombination probability parameterizations (Thomas-Imel, Doke-Birks, and PARIS), allowing for the definition of a fully comprehensive model of the liquid argon response to nuclear and electronic recoils down to a few keV range. The constraints provided by ARIS to the liquid argon response at low energy allow the reduction of systematics affecting the sensitivity of dark matter search experiments based on liquid argon.
Prompt-fission γ -ray spectra (PFGS) have been measured for the 238 U(n, f ) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission γ rays were measured using a number of LaBr 3 scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission γ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for 238 U(n, f )andalsofor 252 Cf (sf ) as a reference. Spectral characteristics of PFG emission, such as mean γ multiplicity and average total γ -ray energy per fission, as well as the average γ -ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at E n = 1.9 MeV were found to be the mean γ multiplicity M γ = 6.54 ± 0.19, total released energy per fission E γ,tot = 5.25 ± 0.20 MeV, and the average γ -ray energy ǫ γ = 0.80 ± 0.04 MeV. Under similar conditions, the values at E n = 4.8 MeV were measured to be M γ = 7.31 ± 0.46, E γ,tot = 6.18 ± 0.65 MeV, and ǫ γ = 0.84 ± 0.11 MeV.
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