Ulisse Gendotti

Measurement of the orthopositronium confinement energy in mesoporous thin films

In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50-400 K. The sample with smaller pore size exhibits a higher minimal energy (73{+-}5 meV), compared to the sample with bigger pores (48{+-}5 meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.

Positronium reemission yield from mesostructured silica films

The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142 ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2gamma and 3gamma energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for Ps annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in CTACl-TEOS films.

Fast neutron detection with pressurized 4He scintillation detectors

Measurement result and performance parameters are presented for fast neutron detectors exploiting the scintillation of natural helium at high pressure. This detection medium has a very low electron density, minimizing the sensitivity to gamma radiation and thus enabling neutron detection also in high gamma radiation environment. Contrary to proportional counters, scintillation detection enables fast (nanosecond) timing and pulse shape discrimination, a technique that enables a lower neutron detection threshold. In this work, the basic principles of the detector are described, followed by a study of gamma rejection capabilities. Methods to calibrate the detector are discussed. Finally, a brief description of a 4He scintillation based detector system including data acquisition electronics is given.

Positronium portal into hidden sector: a new experiment to search for mirror dark matter

The understanding of the origin of dark matter has great importance for cosmology and particle physics. Several interesting extensions of the standard model dealing with solution of this problem motivate the concept of hidden sectors consisting of SU(3)C × SU(2)L × U(1)Y singlet fields. Among these models, the mirror matter model is certainly one of the most interesting. The model explains the origin of parity violation in weak interactions, it could also explain the baryon asymmetry of the Universe and provide a natural ground for the explanation of dark matter. The mirror matter could have a portal to our world through photon-mirror photon mixing (). This mixing would lead to orthopositronium (o−Ps) to mirror orthopositronium oscillations, the experimental signature of which is the apparently invisible decay of o−Ps. In this paper, we describe an experiment to search for the decay o−Ps→invisible in vacuum by using a pulsed slow positron beam and a massive 4π BGO crystal calorimeter. The developed high efficiency positron tagging system, the low calorimeter energy threshold and high hermiticity allow the expected sensitivity in mixing strength to be 10−9, which is more than one order of magnitude below the current Big Bang Nucleosynthesis limit and in a region of parameter space of great theoretical and phenomenological interest. The vacuum experiment with such sensitivity is particularly timely in light of the recent DAMA/LIBRA observations of the annual modulation signal consistent with a mirror type dark matter interpretation.

Positron annihilation in latex-templated macroporous silica films: pore size and ortho-positronium escape

Depth profling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that o-Ps escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps reemission yield decreases from ~ 0:25 to ~ 0:11 as the pore size increases from 32 to 75 nm. The o-Ps reemission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9:1±0:4 g cm-1. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2)ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1:4 ± 4 nm) and from macropores with large effective size (~ 32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19±2 ns. Our results imply that o-Ps effciently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores become small), this capture into the micropores assists the capture into the macropores.

Suitability of high-pressure xenon as scintillator for gamma ray spectroscopy

In this paper we report the experimental study of high-pressure xenon used as a scintillator, in the context of developing a gamma ray detector. We measure a light yield near 2 photoelectrons per keV for xenon at 40 bar. Together with the light yield, we also measured an energy resolution of 9% (FWHM) at 662 keV, dominated by the statistical uctuations in the number of photoelectrons.

Mesoporous silica films with varying porous volume fraction: Direct correlation between ortho-positronium annihilation decay and escape yield into vacuum

The behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction.

Fast neutron detection in homeland security applications

Calculations are presented to support the claim that fast neutron detection systems can achieve higher performance in detecting weak neutron sources than conventional thermal neutron detection systems involving moderators. Minimum Detectable Limits (MDL) are used as a comparative metric, more representative than the metric of absolute sensitivity, which does not take into account the influence of natural backgrounds. Monte Carlo simulations are used to show that heavily shielded neutron sources emit a substantial fraction of fast neutrons. Arguments are presented to support the claim that fast neutron detection systems are superior at defeating heavy neutron shielding than thermal systems.

A Scheme To Produce The Antihydrogen Ion H̄+ For Gravity Measurements

We propose to use the charge exchange reaction of antiprotons with positronium atoms in order to produce antihydrogen atoms, H, and H+ ions. The ions can be cooled down to μK temperatures and then ionized to recover an ultra slow neutral H atom. Its acceleration is then measured by time of flight. Results on the conversion of slow positrons into positronium are presented. This is a first step towards the creation of a dense cloud of positronium atoms to be used as a target for the antiprotons. The source of positrons is based on a 6 MeV industrial electron linac with 0.2 mA average current to be installed in CEA‐Saclay. Equipped with a tungsten target and a moderator, it is aimed at producing rates of order 108 s−1 slow positrons.

Real-time data analysis using the WaveDREAM data acquisition system

The WaveDREAM data acquisition (DAQ) system, based on the DRS4 switched capacitor array waveform digitizing chip, provides Giga-samples per second (GSPS) digitization in a region of interest together with continuous sampling of the input signals at 120 Mega-samples per second (MSPS). In the Field-programmable gate array (FPGA), the 120 MSPS signal can be used to build arbitrarily complex trigger logic and to perform real-time data analysis with hard real-time constraints in the microseconds range. As an application example, the system is configured and optimized for the readout of an array of high-pressure 4He fast neutron detectors. At each end of the detector, photomultiplier tubes (PMTs) collect and amplify the light. Variable gain amplifiers (VGAs) scale the PMT signals such that they optimally fit into the dynamic range of the DRS4 integrated circuit (IC). The trigger in the FPGA employs a coincidence logic between the two PMT signals of a detector to effectively filter out PMT dark counts. Real-time data analysis includes energy deposit measurements and a pulse shape discrimination (PSD) algorithm to reject events originated by gamma radiation, thus greatly reducing the data rate to be processed offline. The high-resolution GSPS signal from the DRS4 IC is used to obtain precise event timing information, thereby enabling neutron time of flight (ToF) measurements with nanosecond time precision. In addition, this signal can be used during offline data processing to strengthen the analysis results. Thus, the WaveDREAM DAQ provides both efficiency (real-time data analysis) and precision (GSPS signal for offline analysis) at low cost.