Cosmogenic activation of xenon and copper

Rare event search experiments using liquid xenon as target and detection medium require ultra-low background levels to fully exploit their physics potential. Cosmic ray induced activation of the detector components and, even more importantly, of the xenon itself during production, transportation and storage at the Earth's surface, might result in the production of radioactive isotopes with long half-lives, with a possible impact on the expected background. We present the first dedicated study on the cosmogenic activation of xenon after 345 days of exposure to cosmic rays at the Jungfraujoch research station at 3470m above sea level, complemented by a study of copper which has been activated simultaneously. We have directly observed the production of 7Be, 101Rh, 125Sb, 126I and 127Xe in xenon, out of which only 125Sb could potentially lead to background for a multi-ton scale dark matter search. The production rates for five out of eight studied radioactive isotopes in copper are in agreement with the only existing dedicated activation measurement, while we observe lower rates for the remaining ones. The specific saturation activities for both samples are also compared to predictions obtained with commonly used software packages, where we observe some underpredictions, especially for xenon activation.Comment: 9 pages, 11 figure

Accelerazione laser plasma di elettroni: primi risultati sperimentali dai LNF

L'accelerazione di elettroni mediante acceleratori convenzionali sta richiedendo negli ultimi decenni l'utilizzo di macchine sempre più grandi e costose. Questo è dovuto al fatto che il campo elettrico accelerante è limitato dalla soglia di rottura dei materiali costituenti le cavità a radio frequenza, che è di circa 10100 MV/m. Una soluzione per aumentare i campi elettrici oltre tale limite (per la realizzazione di acceleratori più compatti e meno costosi) è offerta dalle proprietà dei plasmi, nei quali la materia è già ionizzata e non sussiste quindi il limite di “breakdown”. Nell'ambito della ricerca e dello sviluppo di tecniche innovative per la realizzazione di acceleratori compatti di elettroni, l'INFN ha avviato in collaborazione con il CNR il Progetto Strategico PLASMONX (PLASma acceleration and MONochromatic X-ray production). Questo progetto prevede l'installazione di un sistema laser da oltre 200 TW in grado di produrre impulsi ultracorti di durata fino a 25 fs. Il principale obiettivo di questo progetto consiste nella messa a punto di nuove tecniche di accelerazione basate su plasmi prodotti da laser. In particolare, l'oggetto di questa tesi è l'esperimento di accelerazione di elettroni per autoiniezione, SITE (Self Injection Test Experiment), previsto nella fase finale del “commissioning” del sistema laser. In questo esperimento l'impulso laser crea un'onda elettronica di scia nel regime fortemente non lineare di “bolla”, caratterizzato da una zona approssimativamente sferica e parzialmente svuotata di elettroni, in cui vengono catturati ed accelerati parte degli elettroni del plasma (autoiniezione per wavebreaking dell'onda di plasma). Questa tesi riguarda il lavoro sperimentale in cui sono stato impegnato per diversi mesi presso i LNF dell'INFN, durante le fasi finali della realizzazione del laboratorio laser FLAME (Frascati Laser for Acceleration and Multidisciplinary Experiments) ed i primissimi test di accelerazione. In questo lavoro di tesi ho sviluppato un algoritmo numerico in grado di processare automaticamente centinaia di immagini relative alla rivelazione degli elettroni accelerati. Si è potuto così caratterizzare il loro puntamento e la loro divergenza colpo su colpo. Le distribuzioni di queste grandezze mostrano che l'instabilità di puntamento (sia verticale che orizzontale) è minore di 15 mrad, la divergenza verticale dei pacchetti è minore di 5 mrad, mentre quella orizzontale minore di 10 mrad (tutti valori RMS). L'analisi qualitativa delle immagini della diffusione Thomson della luce laser mostra che, entro le lunghezze d'interazione laser plasma (circa 2 mm), si innescano regimi di self-focus degli impulsi laser, quando la potenza è sufficientemente elevata (circa 30 TW). Sono state effettuate anche delle prime valutazioni energetiche degli elettroni accelerati. Dapprima abbiamo utilizzato un calorimetro costituito da un pacchetto di fogli di radiocromico alternati a fogli di materiali ad alto Z (Al, Fe, Pb). In questo modo abbiamo potuto verificare che l'energia degli elettroni accelerati eccedeva i 50 MeV. Successivamente abbiamo impiegato un'elemento dispersivo costituito da un magnete permanente da circa 1 T. Le misure, seppur preliminari, indicano che sono stati accelerati pacchetti di elettroni ad energie di centinaia di MeV su lunghezze di accelerazione millimetriche

A Dual-phase Xenon TPC for Scintillation and Ionisation Yield Measurements in Liquid Xenon

A small-scale, two-phase (liquid/gas) xenon time projection chamber (Xurich II) was designed, constructed and is under operation at the University of Zurich. Its main purpose is to investigate the microphysics of particle interactions in liquid xenon at energies below 50 keV, which are relevant for rare event searches using xenon as target material. Here we describe in detail the detector, its associated infrastructure, and the signal identification algorithm developed for processing and analysing the data. We present the first characterisation of the new instrument with calibration data from an internal 83m-Kr source. The zero-field light yield is 15.0 and 14.0 photoelectrons/keV at 9.4 keV and 32.1 keV, respectively, and the corresponding values at an electron drift field of 1 kV/cm are 10.8 and 7.9 photoelectrons/keV. The charge yields at these energies are 28 and 31 electrons/keV, with the proportional scintillation yield of 24 photoelectrons per one electron extracted into the gas phase, and an electron lifetime of 200 $\mu$s. The relative energy resolution, $\sigma/E$, is 11.9 % and 5.8 % at 9.4 keV and 32.1 keV, respectively using a linear combination of the scintillation and ionisation signals. We conclude with measurements of the electron drift velocity at various electric fields, and compare these to literature values.Comment: 11 pages, 14 figure

First Operation of a Resistive Shell Liquid Argon Time Projection Chamber -- A new Approach to Electric-Field Shaping

We present a new technology for the shaping of the electric field in Time Projection Chambers (TPCs) using a carbon-loaded polyimide foil. This technology allows for the minimisation of passive material near the active volume of the TPC and thus is capable to reduce background events originating from radioactive decays or scattering on the material itself. Furthermore, the high and continuous electric resistivity of the foil limits the power dissipation per unit area and minimizes the risks of damages in the case of an electric field breakdown. Replacing the conventional field cage with a resistive plastic film structure called 'shell' decreases the number of components within the TPC and therefore reduces the potential points of failure when operating the detector. A prototype liquid argon (LAr) TPC with such a resistive shell and with a cathode made of the same material was successfully tested for long term operation with electric field values up to about 1.5 kV/cm. The experiment shows that it is feasible to successfully produce and shape the electric field in liquefied noble-gas detectors with this new technology.Comment: 13 page

Online 222Rn^{222}Rn removal by cryogenic distillation in the XENON100 experiment

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant $^{222}Rn$ background originating from radon emanation. After inserting an auxiliary $^{222}Rn$ emanation source in the gas loop, we determined a radon reduction factor of $R\,>\,27$ (95% C.L.) for the distillation column by monitoring the $^{222}Rn$ activity concentration inside the XENON100 detector

Search for Electronic Recoil Event Rate Modulation with 4 Years of XENON100 Data

We report on a search for electronic recoil event rate modulation signatures in the XENON100 data accumulated over a period of 4 yr, from January 2010 to January 2014. A profile likelihood method, which incorporates the stability of the XENON100 detector and the known electronic recoil background model, is used to quantify the significance of periodicity in the time distribution of events. There is a weak modulation signature at a period of $431^{+16}_{−14}$ day in the low energy region of (2.0–5.8) keV in the single scatter event sample, with a global significance of 1.9$\sigma$; however, no other more significant modulation is observed. The significance of an annual modulation signature drops from 2.8$\sigma$, from a previous analysis of a subset of this data, to 1.8$\sigma$ with all data combined. Single scatter events in the low energy region are thus used to exclude the DAMA/LIBRA annual modulation as being due to dark matter electron interactions via axial vector coupling at 5.7$\sigma$

Erratum: First axion results from the XENON100 experiment [Phys. Rev. D 90, 062009 (2014)]

n our paper, we presented searches for solar axions and galactic axionlike particles (ALPs) in the data collected by the XENON100 experiment (with an exposure of 224.6 days). We recently found a bug in the code to calculate the exclusion limit for galactic ALPs. This resulted in an underestimation of the ALP expected rate, which in turn led to an overly conservative limit, compared to what it should really be. We corrected the code, and the result of the XENON100 90% C.L. exclusion limit on galactic ALPs (shown in Fig. 1) was reevaluated. The corrected limit is stronger than the one previously published by approximately a factor of 5 across all masses and sets the best published limit on the axion-electron coupling, $g_{Ae}$, in the (1–40)  keV/$c^2$ mass range

Measurement of cosmogenic radioactive products in xenon and copper

Rare events searches, such as direct dark matter detection or neutrinoless double beta decay (0vββ) observation, using liquid xenon as target and detection medium require ultralow background to fully exploit the physics potential. Cosmogenic activation of the detector components, and even more importantly, of the xenon itself might have undesired impact on the background and the final sensitivity of the experiment. Since no measurement of cosmogenic activation of xenon was present in literature so far, we performed such a measurement exposing of a natural xenon sample to the cosmic radiation at the Jungfraujoch research station at an altitude of 3470 m above sea level for 245 days. This study was complemented with a ultra pure copper sample that was activated together with the xenon. We directly observed, with gamma-ray spectrometry, the production of (7)Be, (101)Rh, (125)Sb, (126)I and (127)Xe in xenon, out of which only (125) Sb could potentially lead to a background relevant for multi-ton scale direct dark matter search. The production rates for five out of eight radioactive isotopes in copper are in good agreement with the only dedicated measurement present in literature. The production rates measured for both samples were compared with the predictions obtained with commonly used software packages. The latter showed a systematic under-estimation, especially for xenon