- The document characterizes the properties of ortho-Positronium (o-Ps) in various liquid scintillators using a coincidence system to measure the time between a prompt gamma ray and delayed annihilation gammas.
- It finds that common scintillators like PC, PXE, LAB, and OIL have o-Ps formation probabilities around 50% and lifetimes of about 3 ns.
- The effects of doping scintillators with Gd and Nd are also studied and additional measurements of Li-loaded and Tl-loaded scintillators are planned.
A Comparison of Microsecond vs. Millisecond Dwell Times on Particle Number Co...PerkinElmer, Inc.
With the growing popularity of nanomaterials in a wide variety of products and processes, the need to measure and characterize these materials has also grown. For metallic or metal-containing engineered nanoparticles (NPs), single particle ICP-MS (SP-ICP-MS) has been developed and is growing in popularity due to its ability to rapidly detect and characterize a large number of particles, determine particle size and size distributions, the particle number concentration in a sample, and the elemental mass concentration of both dissolved and particulate species. SP-ICP-MS has been applied to a wide variety of sample types1-5, demonstrating its versatility.
The effects of oxygen isotope substitution 16O ↔ 17O ↔ 18O in the coordination sphere of the
pertechnetate anion ( ) on the NMR 99Tc chemical shifts and 99Tc–17O and 17O–99Tc spin coupling
constants have been studied by 17O and 99Tc NMR. The isotope shifts 16/17Δ and 16/18Δ in 99Tc NMR and the
spin coupling constants of the Tc 16O3
18O–, Tc 16O3
17O–, Tc 16O2
17O 18O–, and
isotopomers have been measured. For the Tc 16O3
18O– and Tc 16O3
17O– anions in an ammonium pertechne
tate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are
described by linear relationships 16/18Δ = –0.616 + 6.45 × 10–4T (ppm) and 16/17 Δ = –0.302 +2.67 × 10–4T
(ppm), respectively. For the Tc16O3
17O– anion in a sodium pertechnetate solution, the magnitude of the
1Δ(16/17O) isotope shift nonlinearly decreases with increasing temperature. The nonlinear temperature
dependence of the J(99Tc–17O) spin coupling constant and the extreme point on the curve of the 1Δ(16/18O)
isotope shift versus temperature for the isotopomers in an NaTcO4 solution are presumably related to equilib
rium between contact and waterseparated ion pairs.
A Comparison of Microsecond vs. Millisecond Dwell Times on Particle Number Co...PerkinElmer, Inc.
With the growing popularity of nanomaterials in a wide variety of products and processes, the need to measure and characterize these materials has also grown. For metallic or metal-containing engineered nanoparticles (NPs), single particle ICP-MS (SP-ICP-MS) has been developed and is growing in popularity due to its ability to rapidly detect and characterize a large number of particles, determine particle size and size distributions, the particle number concentration in a sample, and the elemental mass concentration of both dissolved and particulate species. SP-ICP-MS has been applied to a wide variety of sample types1-5, demonstrating its versatility.
The effects of oxygen isotope substitution 16O ↔ 17O ↔ 18O in the coordination sphere of the
pertechnetate anion ( ) on the NMR 99Tc chemical shifts and 99Tc–17O and 17O–99Tc spin coupling
constants have been studied by 17O and 99Tc NMR. The isotope shifts 16/17Δ and 16/18Δ in 99Tc NMR and the
spin coupling constants of the Tc 16O3
18O–, Tc 16O3
17O–, Tc 16O2
17O 18O–, and
isotopomers have been measured. For the Tc 16O3
18O– and Tc 16O3
17O– anions in an ammonium pertechne
tate solution, the temperature dependences of the isotope shift in the temperature range 278–333 K are
described by linear relationships 16/18Δ = –0.616 + 6.45 × 10–4T (ppm) and 16/17 Δ = –0.302 +2.67 × 10–4T
(ppm), respectively. For the Tc16O3
17O– anion in a sodium pertechnetate solution, the magnitude of the
1Δ(16/17O) isotope shift nonlinearly decreases with increasing temperature. The nonlinear temperature
dependence of the J(99Tc–17O) spin coupling constant and the extreme point on the curve of the 1Δ(16/18O)
isotope shift versus temperature for the isotopomers in an NaTcO4 solution are presumably related to equilib
rium between contact and waterseparated ion pairs.
indices 2 / dental crown & bridge courses /certified fixed orthodontic cour...Indian dental academy
The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.
Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
0091-9248678078
The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.
Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
00919248678078
indices 2 / dental crown & bridge courses /certified fixed orthodontic cour...Indian dental academy
The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.
Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
0091-9248678078
The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.
Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit www.indiandentalacademy.com ,or call
00919248678078
IELTS Essay Topics with Answers (writing task 2)Ben Worthington
I look at 5 IELTS Task 2 questions from the topics education, globalisation and gender equality. Audio: http://bit.ly/1kq7NRl
In the audio I explain the process more, but it's an exercise in brain storming for ideas.
For an introduction on how to start IELTS Writing Task 2 click here:
www.ieltspodcast.com/ielts-writing-task/ielts-writing-start-ielts-writing-task-2/
Gestures and movement provide the visuals that accompany your words. Learning to use them effectively will help you convey your message with confidence and your audience will see your message instead of just hearing it
Laser ablation - optical cavity isotopic spectrometer (LAOCIS) for Mars roversAlexander Bolshakov
Proceedings of SPIE, v. 8385, "Sensors and Systems for Space Applications V"; Baltimore, MD, 2012. ABSTRACT: A concept of a compact device for analyzing key isotopic composition in surface materials without sample preparation is presented. This design is based on an advanced modification of Laser Induced Breakdown Spectroscopy (LIBS). First, we developed Laser Ablation Molecular Isotopic Spectrometry (LAMIS) that involves measuring isotope-resolved molecular emission, which exhibits significantly larger isotopic spectral shifts than those in atomic transitions. Second, we used laser ablation to vaporize the sample materials into a plume in which absorption spectra can be measured using a tunable diode laser. The intrinsically high spectral resolution of the diode lasers facilitates measurements of isotopic ratios. The absorption sensitivity can be boosted using cavity enhanced spectroscopy. Temporal behavior of species in a laser ablation plasma from solid samples with various isotopic composition was studied. Detection of key isotopes associated with signs of life (carbon, nitrogen, hydrogen) as well as strontium and boron in laser ablation plume was demonstrated; boron isotopes were quantified. Isotope-resolved spectra of many other molecular species were simulated. The experimental results demonstrate sensitivity to 86 Sr, 87 Sr, and 88 Sr with spectrally resolved measurements for each of them. It is possible to measure strontium isotopes in rocks on Mars for radiogenic age determination. Requirements for spectral resolution of the optical measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra.
28 Measurement of η photoproduction on the proton from threshold to 1500-MeV ...Cristian Randieri PhD
Measurement of η photoproduction on the proton from threshold to 1500-MeV - The European Physical Journal A, Hadrons and Nuclei, August 2007, Vol. 33, N. 2, pp. 169-184, ISSN: 1434-6001, doi: 10.1140/epja/i2007-10439-9
di O. Bartalini, V. Bellini, J. P. Bocquet, P. Calvat, M. Capogni, L. Casano, M. Castoldi, A. D'Angelo, J. P. Didelez, R. Di Salvo, A. Fantini, D. Franco, C. Gaulard, G. Gervino, F. Ghio, G. Giardina, B. Girolami, A. Giusa, M. Guidal, E. Hourany, R. Kunne, A. Lapik, P. Levi Sandri, A. Lleres, F. Mammoliti, G. Mandaglio, D. Moricciani, A. N. Mushkarenkov, V. Nedorezov, L. Nicoletti, C. Perrin, C. Randieri, D. Rebreyend, F. Renard, N. Rudnev, T. Russew, G. Russo, C. Schaerf, M. L. Sperduto, M. C. Sutera, A. Turinge, V. Vegna (2007).
Abstract
Beam asymmetry and differential cross section for the reaction gamma+p->eta+p were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the eta meson, eta->2g and eta->3pi0, were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.
24 Polarization observable measurements for γp → K+Λ and γp → K+Σ for energie...Cristian Randieri PhD
Polarization observable measurements for γp → K+Λ and γp → K+Σ for energies up to 1.5 GeV - The European Physical Journal A, Hadrons and Nuclei, January 2007, Vol. 31, N. 1, pp. 73-93, ISSN: 1434-6001, doi: 10.1140/epja/i2006-10167-8
di A. Lleres, O. Bartalini, V. Bellini, J. P. Bocquet, P. Calvat, M. Capogni, L. Casano, M. Castoldi, A. D'Angelo, J. P. Didelez, R. Di Salvo, A. Fantini, C. Gaulard, G. Gervino, F. Ghio, B. Girolami, A. Giusa, M. Guidal, E. Hourany, V. Kouznetsov, R. Kunne, A. Lapik, P. Levi Sandri, D. Moricciani, A. N. Mushkarenkov, V. Nedorezov, L. Nicoletti, C. Perrin, C. Randieri, D. Rebreyend, F. Renard, N. Rudnev, T. Russew, G. Russo, C. Schaerf, M. L. Sperduto, M. C. Sutera, A. Turinge (2007)
Abstract
Beam asymmetries and hyperon recoil polarizations for the reactions γ p → K +Λ and γ p → K +Σ0 have been measured from the threshold production to 1500MeV with the GRAAL facility located at the ESRF in Grenoble. These results complement the database for the beam asymmetry, covering for the first time the production threshold region. Recent theoretical analyses are presented for which the beam asymmetry data bring interesting new information and allow to better determine some resonance parameters. Most importantly, these results strengthen the need of a new D13 state around 1900MeV.
42 Beam Asymmetry Σ of the π- Photoproduction off Neutron - International Jou...Cristian Randieri PhD
Beam Asymmetry Σ of the π- Photoproduction off Neutron - International Journal of Modern Physics E, June 2010, Vol. 19, N. 5-6, pp. 965-976, doi: 10.1142/S0218301310015412
di G. Mandaglio, V. Bellini, J. P. Bocquet, L. Casano, A. D'Angelo, R. Di Salvo, A. Fantini, D. Franco, G. Gervino, F. Ghio, G. Giardina, B. Girolami, A. Giusa, A. S. Ignatov, A. M. Lapik, P. Levi Sandri, A. Lleres, F. Mammoliti, M. Manganaro, D. Moricciani, A. N. Mushkarenkov, V. G. Nedorezov, C. Randieri, D. Rebreyend, N. V. Rudnev, G. Russo, C. Schaerf, M. L. Sperduto, M. C. Sutera, A. Turinge, V. Vegna (2010)
Abstract
We present the analysis of data performed in order to identify the events of the γ + n → π- + p reaction obtained by bombarding a liquid Deuterium target with a polarised γ beam of 0.55-1.5 GeV at the Graal-experiment. We show the effect of different kinematic and hardware constraints used to reduce the contamination coming from the concurrent reaction channels. By the simulation we estimate the contamination degree due to the other reaction channels so we can test the reliability of our method. We describe a new three-dimensional cut based on the Fermi momentum reconstruction and its effect on the suppression of the concurrent double charged pion photoproduction. We present the preliminary beam asymmetry Σ of the π- fotoproduction off quasi-free neutron up to about θc.m., π- = 165° together with some theoretical multipolar analysis. For a comparison we also report the data present in literature on the same reaction for Eγ =850-1740 MeV and θc.m., π- ≤105°.
33 Measurement of beam-recoil observables Ox, Oz and target asymmetry T for t...Cristian Randieri PhD
Measurement of beam-recoil observables Ox, Oz and target asymmetry T for the reaction γρ → K+Λ - The European Physical Journal A, Hadrons and Nuclei, February 2009, Vol. 39, N. 2, pp. 149–161, ISSN: 1434-6001, doi: 10.1140/epja/i2008-10713-4
di A. Lleres, O. Bartalini, V. Bellini, J. P. Bocquet, P. Calvat, M. Capogni, L. Casano, M. Castoldi, A. D’Angelo, J. P. Didelez, R. Di Salvo, A. Fantini, D. Franco, C. Gaulard, G. Gervino, F. Ghio, B. Girolami, A. Giusa, M. Guidal, E. Hourany, R. Kunne, V. Kuznetsov, A. Lapik, P. Levi Sandri, F. Mammoliti, G. Mandaglio, D. Moricciani, A. N. Mushkarenkov, V. Nedorezov, L. Nicoletti, C. Perrin, C. Randieri, D. Rebreyend, F. Renard, N. Rudnev, T. Russew, G. Russo, C. Schaerf, M. L. Sperduto, M. C. Sutera, A. Turinge, V. Vegna (2009)
Abstract
The double polarization (beam-recoil) observables Ox and Oz have been measured for the reac- tion γp → K+Λ from threshold production to E ∼ 1500MeV. The data were obtained with the linearly polarized beam of the GRAAL facility. Values for the target asymmetry T could also be extracted despite the use of an unpolarized target. Analyses of our results by two isobar models tend to confirm the necessity to include new or poorly known resonances in the 1900MeV mass region.
SIMONA CAVALU_EPR study of non covalent spin labeled serum albumin and haemog...Simona Cavalu
Abstract
Electron Paramagnetic Resonance (EPR) was used to investigate the Tempyo spin label (3-carbamoyl-2,2,5,5-
tetramethyl-3-pyrrolin-1-yloxy) as a report group for the interactions and the conformational changes of lyophilized
bovine serum albumin (BSA) and bovine hemoglobin (BH), as function of pH values in the range 2.5–11. The EPR
spectra are similar with those of other non-covalently spin label porphyrins in frozen solution at very low temperatures.
This behavior indicated a possible spin–spin interaction between the hemic iron and the nitroxide group. The changes
in the EPR spectra as function of the pH are discussed in terms of conformational changes of the proteins. Spectral
simulations and magnetic EPR parameters reveal the following: (i) one single paramagnetic species, with Gaussian
line shape, was used for the best fits of experimental spectra in the case of serum albumin samples; and (ii) a
weighted sum of Lorentzian and Gaussian line shape in the case of hemoglobin samples. The representation of
correlation time vs. pH, reveals a dependence of degree of immobilization of spin label on the conformational changes
of proteins in acidic and basic environment.
ML-3 - Persistent Phosphors under PressurePhilippe Smet
Slides from plenary talk at the Third Conference on Mechanoluminescence and Novel structural health diagnosis, Hong Kong, December 15-17 2017. Covers absolute trapping capacity of persistent phosphors, the number and nature of energy traps and ML in selected compounds (BaSi2O2N2:Eu and CaZnOS:Mn).
An alternative to the "big molecules" view of proteins is the "small things" view in which protein have a shape and material properties. This talk is about investigating these properties.
43 Beam asymmetry Σ measurements on the π- Photoproduction off neutrons - Phy...Cristian Randieri PhD
Beam asymmetry Σ measurements on the π- Photoproduction off neutrons - The American Physical Society, Physical Review C, Nuclear Physics, October 2010, Vol. 82, N. 4, pp. 045209-1-045209-7, ISSN: 0556-2813, doi: 10.1103/PhysRevC.82.045209
di G. Mandaglio, F. Mammoliti, M. Manganaro, V. Bellini, J. P. Bocquet, L. Casano, A. D'Angelo, R. Di Salvo, A. Fantini, D. Franco, G. Gervino, F. Ghio, G. Giardina, B. Girolami, A. Giusa, A. Ignatov, A. Lapik, P. Levi Sandri, A. Lleres, D. Moricciani, A. N. Mushkarenkov, V. Nedorezov, C. Randieri, D. Rebreyend, N. V. Rudnev, G. Russo, C. Schaerf, M. L. Sperduto, M. C. Sutera, A. Turinge, V. Vegna, W. J. Briscoe, I. I. Strakovsky (2010)
Abstract
The Σ beam asymmetry in the photoproduction of negative pions from quasi-free neutrons in a deuterium target was measured at Graal in the energy interval 700 - 1500 MeV and a wide angular range, using polarized and tagged photons. The results are compared with recent partial wave analyses.
Calculation of isotopic dipole moments with spectroscopic accuracyAntônio Arapiraca
Trabalho apresentado no XVII Symposium on High Resolution Molecular Spectroscopy (HighRus-2012), 2012, Zelenogorsk-Russia. Anals of XVII Symposium on High Resolution Molecular Spectroscopy (HighRus-2012), 2012.
Impact of Equation of State on Simulating CO2 Pipeline Decompression, Solomon Brown, University College London. Presented at CO2 Properties and EoS for Pipeline Engineering, 11th November 2014
45 Evidence for a narrow N* (1685) resonance in quasifree Compton scattering ...Cristian Randieri PhD
Evidence for a narrow N* (1685) resonance in quasifree Compton scattering on the neutron - The American Physical Society, Physical Review C, February 2011, Vol. 83, N. 2, pp. 022201-1-022201-4, ISSN: 0556-2813, doi: 10.1103/PhysRevC.83.022201
di V. Kuznetsov, M. V. Polyakov, V. Bellini, T. Boiko, S. Chebotaryov, H.-S. Dho, G. Gervino, F. Ghio, A. Giusa, A. Kim, W. Kim, F. Mammoliti, E. Milman, A. Ni, I. A. Perevalova, C. Randieri, G. Russo, M. L. Sperduto, C. M. Sutera, A. N. Vall (2011)
Abstract
The study of quasifree Compton scattering on the neutron in the energy range of Eγ=0.75–1.5 GeV is presented. The data reveal a narrow peak at W~1.685 GeV. This result, being considered in conjunction with the recent evidence for a narrow structure at W~1.68 GeV in η photoproduction on the neutron, suggests the existence of a nucleon resonance with unusual properties: a mass M~1.685GeV, a narrow width Γ⩽30 MeV, and the much stronger photoexcitation on the neutron than on the proton.
1. Measurement of ortho-Positronium Properties in Liquid
Scintillators
S. Perasso∗
, G. Consolati†
, D. Franco∗
, S. Hans∗∗
, C. Jollet‡
, A. Meregaglia‡
, A.
Tonazzo∗
and M. Yeh∗∗
∗
Laboratoire AstroParticule et Cosmologie, Paris
†
Department of Aerospace Science and Technology, Politecnico di Milano
∗∗
Brookhaven National Laboratory
‡
IPHC, Université de Strasbourg
Abstract. Pulse shape discrimination in liquid scintillator detectors is a well-established technique for the discrimination of
heavy particles from light particles. Nonetheless, it is not efficient in the separation of electrons and positrons, as they give
rise to indistinguishable scintillator responses. This inefficiency can be overtaken through the exploitation of the formation of
ortho-Positronium (o-Ps), which alters the time profile of light pulses induced by positrons.
We characterized the o-Ps properties in the most commonly used liquid scintillators, i.e. PC, PXE, LAB, OIL and
PC + PPO. In addition, we studied the effects of scintillator doping on the o-Ps properties for dopants currently used in
neutrino experiments, Gd and Nd. Further measurements for Li-loaded and Tl-loaded liquid scintillators are foreseen. We
found that the o-Ps properties are suitable for enhancing the electron-positron discrimination.
Keywords: Positronium, Pulse Shape Discrimination, Scintillator, Neutrino Physics
PACS: 14.60.Cd
INTRODUCTION
Pulse shape discrimination (PSD) is a fundamental technique for background rejection in scintillator detectors. It
relies on the different time profiles of the emitted scintillator light to solicitations from particles with different energy
losses. For this reason, it is particularly effective for discriminating heavy particles (ions, alphas or protons) from light
particles (electrons, positrons). Even so, electrons and positrons generate similar light pulses and therefore PSD fails
in discriminating them.
PSD discrimination between electrons and positrons can be recovered exploiting the deformation of the positron
induced light pulse shape caused by the formation of positronium, a meta-stable positron-electron bound state.
Depending on the system total spin, the positronium can be formed in the singlet state, called para-Positronium (p-Ps,
B.R. 25%), or in the triplet state, called ortho-Positronium (o-Ps, B.R. 75%). Due to the charge-conjugation invariance,
p-Ps decays into two 511 keV gammas, with a mean life in vacuum of 125 ps, where o-Ps decays into three gammas of
total energy equal to 2me, with a mean life in vacuum of 142 ns. Nevertheless, in matter the o-Ps mean life is strongly
reduced by the interactions with the surrounding medium. Chemical reactions (oxidation, compound compositions),
magnetic effects (spin flip) or pick-off (positron annihilation with an anti-parallel spin electron of the medium) cause
the o-Ps to decay into two gammas with a mean life of a few ns. The decay into three gammas is usually reduced to a
negligible fraction.
An o-Ps mean life of a few ns is not enough for separating the positron energy deposition from the annihilation
gammas in scintillator detectors, but it can still induce a positron pulse shape deformation strong enough to enhance
electron-positron discrimination. This enhancement could be exploited in reactor neutrino experiments [1, 2, 3], in
which anti-neutrino detection is performed via inverse beta decay
¯νe + p → e+
+n. (1)
The positron energy deposition and its following annihilation provide the prompt event and the neutron absorption the
delayed event. An o-Ps enhanced PSD could allow to suppress the accidental background and to reject the residual
correlated background, given by the cosmogenic β−−n emitters 9Li and 8He.
The same technique can be also exploited for background rejection in underground low background experiments,
like Borexino [4] and SNO+ [5], in which several β+ emitters (8B, 9Be, 10C) produced by cosmic muon spallation,
arXiv:1306.6001v1[physics.ins-det]25Jun2013
2. FIGURE 1. Positron annihilation time spectrum for the PXE sample, with the fitted function superimposed.
constitute an unavoidable source of background. It has already been successfully applied in Borexino for the detection
of solar pep neutrinos, where the rejection of cosmogenic 11C is essential for the signal extraction.
The characterization of o-Ps properties, i.e. the measurement of its formation probability and lifetime, has been
performed for the most commonly used liquid scintillators, namely PC, LAB, PXE and OIL, and for the mixture
PC + PPO [6]. Furthermore, the effect of scintillator doping on the o-Ps properties has been studied for different
dopants: Gd, commonly used in reactor neutrino experiments to enhance the neutrino signal identification, and Nd,
originally considered as neutrinoless double beta decay signal source in the SNO+ experiment. Forthcoming measures
will characterize also Li-loaded and Tl-loaded liquid scintillators, the latter being chosen as new dopant for the SNO+
scintillator.
EXPERIMENTAL APPARATUS
The o-Ps properties (lifetime and formation probability) in scintillator are measured by means of a 0.8 MBq 22Na
source in a fast coincidence system. The radio-isotope 22Na decays β+ to 22Ne with a B.R. of 89.2% (Q-value = 2.842
MeV). In 99.944% of cases, the decay is to an excited state of 22Ne, which de-excites emitting a 1.274 MeV gamma.
The o-Ps properties are extrapolated from the measurement of the time distance between the de-excitation gamma
(prompt signal) and the positron annihilation gammas (dealyed signal).
A droplet of inert solvent containing the source is deposited between two identical supports, 1 cm in radius, each
composed by two 7.5 µm thick Kapton foils. Kapton is an aromatic polyimide material where positrons are known not
to form Positronium [7, 9]. The source sandwich is placed into a vial containing the scintillator sample.
The vial is in turn positioned between two plastic scintillator detectors, each coupled to a PMT. The threshold
on the first detector is set to 900 keV to isolate the 22Ne de-excitation gamma, while the second detector selects
gammas between 350 keV and 500 keV, in order to accept the annihilation gammas but reject the de-excitation gamma
backscattering.
A constant fraction discriminator generates a fast timing signal at each trigger from the two detectors. A time to
amplitude converter generates a signal linearly increasing with time, starting at the prompt signal and stopping at
the delayed signal. The output is digitized by a 4096 channels ADC. Figure 1 shows an example of resulting time
spectrum.
The system is calibrated using a 60Co source, and each ADC channel is found to corresponds to 10.6 ps. The
resolution is measured with the fast coincidence (182 ps) gammas from 207Bi and found to be 0.28 ns at FWHM.
CHARACTERIZATION OF O-PS IN LIQUID SCINTILLATORS
Positronium has been characterized in common liquid scintillators (PC, PXE, LAB, OIL, PC + PPO) at room
temperature. Each sample has been flushed with Nitrogen and then measured with a ∼ 1 × 106 event statistics,
3. TABLE 1. Measured values of o-Ps formation prob-
ability and lifetime for the different samples of liquid
scintillators.
Scintillator o-Ps fraction lifetime [ns]
PC 0.485 ± 0.005 2.96 ± 0.03
PXE 0.466 ± 0.005 2.74 ± 0.03
LAB 0.542 ± 0.005 3.08 ± 0.03
OIL 0.506 ± 0.005 3.04 ± 0.03
PC + 1.5 gl PPO 0.512 ± 0.005 3.12 ± 0.03
repeatedly from 3 to 5 times, in order to account for possible variations in the experimental conditions (temperature,
Oxygen concentration).
The obtained time distributions are fitted using the RooFit toolkit [8], embedded in the ROOT package and based
on MINUIT. The fit function is
F(t) = χ(t > t0)· ∑
k=1,2
Ak
τk
·e−t/τk +C (2)
where χ is the step function and Ai and τi are the amplitude and mean life of the i-th component: the first accounts
for the effective contribution of direct annihilation and p-Ps, while the second accounts for the o-Ps. The constant
term accounts for accidentals. The function F(t) is convoluted with the detector resolution function, the sum of two
gaussians with the same mean value, but different sigmas (σ1 = 110 ps and σ2 = 160 ps) and relative weights (g1 = 0.8
and g2 = 0.2). The obtained normalized χ2 are in the range between 0.85 and 0.94 and the statistical errors are at the
per mil level.
The o-Ps mean life is given directly by the fit parameter τ2, while the formation probability f2 is computed as the
fraction of o-Ps relative to the annihilations in the scintillator
f2 =
A2
A1 +A2 −AK
. (3)
The fraction of direct annihilations in Kapton, AK, has been extrapolated from dedicated measurements with different
number of Kapton layers and estimated to be (20.6 ± 0.2)%.
The systematic uncertainty is evaluated from the dispersion of the measured mean lives from the weighted average
of the corresponding scintillator sample. The distribution of all the dispersions is fitted to a gaussian, whose standard
deviation is taken as the systematic error. The same procedure is followed for the formation probability. The resulting
systematic errors are 0.03 ns and 0.5%.
The final results are summarized in Table 1. All the scintillators exhibit an o-Ps formation probability around 50%
and a mean life of ∼3 ns.
CHARACTERIZATION OF DOPED LIQUID SCINTILLATORS
The same instrumental setup and analysis tool have been used to characterize the o-Ps in doped liquid scintillators.
The dopants tested so far are Gd, a neutron capture enhancing isotope, and Nd, originally considered for SNO+. Both
dopants have been tested at different concentrations between 0.01% and 0.5% in the common solvent LAB. Further
measurements are foreseen with Tl-loaded and Li-loaded liquid scintillator.
The positron annihilation time spectrum is fitted with the superposition of three exponentials and a constant: the first
two exponentials describe the direct annihilation and the p-Ps components, the third one is an effective contribution
from two and three gamma o-Ps decays and the consent accounts for accidentals. The detector resolution is modeled
with a single gaussian with σ = 120 ps.
The lifetime of o-Ps decay into two gammas, τ2, can be extracted from the fitted parameter τ according to the
relation τ2 = τ3τ/(τ3 − τ), τ3 being the o-Ps lifetime in vacuum (142 ns). Then, the fraction of decays into three
gammas is f3 = τ/τ3 and the one of decays into two gammas is f2 = 1 - f3. The o-Ps probability formation, f, is
computed taking into account the different detection efficiencies for the two gamma channel (ε2) and the three gamma
4. TABLE 2. Measured values of o-Ps formation proba-
bility and lifetime for the Gd-loaded and Nd-loaded liq-
uid scintillators at different concentrations.
Gd concentration o-Ps fraction lifetime [ns]
0 0.544 ± 0.008 3.05 ± 0.03
0.01% 0.554 ± 0.008 3.07 ± 0.03
0.05% 0.540 ± 0.008 3.05 ± 0.03
0.08% 0.537 ± 0.008 3.04 ± 0.03
0.1% 0.529 ± 0.008 3.09 ± 0.03
0.45% 0.406 ± 0.008 3.02 ± 0.03
Nd concentration o-Ps fraction lifetime [ns]
0 0.537 ± 0.013 3.15 ± 0.04
0.05% 0.527 ± 0.013 3.11 ± 0.04
0.1% 0.494 ± 0.013 3.17 ± 0.04
0.3% 0.460 ± 0.013 3.15 ± 0.04
0.5% 0.402 ± 0.013 3.15 ± 0.04
channel (ε3) and is given by
f =
A3τ3
(AA +A3 −AK)·τ3 +(AA −AK)(ε3/ε2 −1)τ
(4)
where AA = A1 + A2 and AK is the fraction of positron annihilations in Kapton. Since the two efficiencies could not
be precisely measured, f is computed for the two extreme cases ε3 = 0 and ε3 = ε2. The average value is taken as
measure of f, while the semi-difference is added to the systematics. The other contribution to systematics comes from
the deviation of measurements from the weighted averages, described in the previous section. Finally, the statistical
uncertainty on f resulting from the error propagation in Eq. (4) is taken into account. The systematic uncertainties on
lifetime is evaluated accordingly, with an additional contribution coming from the setup calibration. To summarize,
the relative uncertainty on the o-Ps formation probability is 1.9%, while the one on the mean life is ∼1%.
Table 2 reports the results for the Gd-loaded and Nd-loaded liquid scintillators at the different concentrations. Both
of the dopants show the same effect on the o-Ps properties: the mean life remains stable under dopant concentration
variations, while the formation probability decreases from ∼50% at concentrations of the order of few tenths per mil
to ∼40% at concentrations around 0.5%.
ACKNOWLEDGMENTS
We acknowledge the financial support from the ANR NuToPs project (ANR-11-JCJC-SIMI4) and from the UnivEarthS
Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02).
REFERENCES
1. Y. Abe et al. [Double Chooz Collaboration], Phys. Rev. D 86, 052008 (2012)
2. F. P. An et al. [DAYA-BAY Collaboration], Phys. Rev. Lett. 108, 171803 (2012)
3. J. K. Ahn et al. [RENO Collaboration], Phys. Rev. Lett. 108, 191802 (2012)
4. G. Alimonti et al., Nucl. Instrum. Meth. A 600, 568 (2009)
5. M. C. Chen, Nucl. Phys. Proc. Suppl., 145 (2005) 65
6. D. Franco, G. Consolati and D. Trezzi, Phys. Rev. C 83, 015504 (2011)
7. S. MGuire and D. J. Keeble, J. Phys. D Appl. Phys. 39, 3388 (2006)
8. W. Verkerke and D. P. Kirkby, eConf C 0303241, MOLT007 (2003) [physics/0306116].
9. K. Plotlowski, T. J. Panek and J. Kansy, Nuovo Cim. D 10, 933 (1988)