Search for a Neutron Dark Decay in

Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of ^{6}He into ^{4}He+n+χ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity ^{6}He^{+} beam at Grand Accélérateur National d'Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br_{χ}≤4.0×10^{-10} (95% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of O(10^{-5}), improving over global constraints by one to several orders of magnitude depending on m_{χ}.

Evidence of Partial Seniority Conservation in the πg_{9/2} Shell for the N=50 Isotones.

The reduced transition probabilities for the 4_{1}^{+}→2_{1}^{+} and 2_{1}^{+}→0_{1}^{+} transitions in ^{92}Mo and ^{94}Ru and for the 4_{1}^{+}→2_{1}^{+} and 6_{1}^{+}→4_{1}^{+} transitions in ^{90}Zr have been determined in this experiment making use of a multinucleon transfer reaction. These results have been interpreted on the basis of realistic shell-model calculations in the f_{5/2}, p_{3/2}, p_{1/2}, and g_{9/2} proton valence space. Only the combination of extensive lifetime information and large scale shell-model calculations allowed the extent of the seniority conservation in the N=50 g_{9/2} orbital to be understood. The conclusion is that seniority is largely conserved in the first πg_{9/2} orbital.

Experimental Evidence for Common Driving Effects in Low-Energy Fission from Sublead to Actinides.

Isotopic distributions of fragments from fission of the neutron-deficient ^{178}Hg nuclide are reported. This experimental observable is obtained for the first time in the region around lead using an innovative approach based on inverse kinematics and the coincidence between the large acceptance magnetic spectrometer VAMOS++ and a new detection arm close to the target. The average fragment N/Z ratio and prompt neutron M_{n} multiplicity are derived and compared with current knowledge from actinide fission. A striking consistency emerges, revealing the unexpected dominant role of the proton subsystem with atomic number between the Z=28 and 50 magic numbers. The origin of nuclear charge polarization in fission and fragment deformation at scission are discussed.

First Direct Measurement of Isotopic Fission-Fragment Yields of

A direct and complete measurement of isotopic fission-fragment yields of ^{239}U has been performed for the first time. The ^{239}U fissioning system was produced with an average excitation energy of 8.3 MeV in one-neutron transfer reactions between a ^{238}U beam and a ^{9}Be target at Coulomb barrier energies. The fission fragments were detected and isotopically identified using the VAMOS++ spectrometer at the GANIL facility. The measurement allows us to directly evaluate the fission models at excitation energies of fast neutrons, which are relevant for next-generation nuclear reactors. The present data, in agreement with model calculations, do not support the recently reported anomaly in the fission-fragment yields of ^{239}U, and they confirm the persistence of spherical shell effects in the Sn region at excitation energies exceeding the fission barrier by a few mega-electron volts.

Pseudospin Symmetry and Microscopic Origin of Shape Coexistence in the

Lifetime measurements of excited states of the light N=52 isotones ^{88}Kr, ^{86}Se, and ^{84}Ge have been performed, using the recoil distance Doppler shift method and VAMOS and AGATA spectrometers for particle identification and gamma spectroscopy, respectively. The reduced electric quadrupole transition probabilities B(E2;2^{+}→0^{+}) and B(E2;4^{+}→2^{+}) were obtained for the first time for the hard-to-reach ^{84}Ge. While the B(E2;2^{+}→0^{+}) values of ^{88}Kr, ^{86}Se saturate the maximum quadrupole collectivity offered by the natural valence (3s, 2d, 1g_{7/2}, 1h_{11/2}) space of an inert ^{78}Ni core, the value obtained for ^{84}Ge largely exceeds it, suggesting that shape coexistence phenomena, previously reported at N≲49, extend beyond N=50. The onset of collectivity at Z=32 is understood as due to a pseudo-SU(3) organization of the proton single-particle sequence reflecting a clear manifestation of pseudospin symmetry. It is realized that the latter provides actually reliable guidance for understanding the observed proton and neutron single particle structure in the whole medium-mass region, from Ni to Sn, pointing towards the important role of the isovector-vector ρ field in shell-structure evolution.

Evidence for Coexisting Shapes through Lifetime Measurements in

The lifetimes of the first excited 2^{+}, 4^{+}, and 6^{+} states in ^{98}Zr were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in ^{98}Zr were populated using the fission reaction between a 6.2 MeV/u ^{238}U beam and a ^{9}Be target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;2_{1}^{+}→0_{1}^{+}) value in ^{98}Zr, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

_{36}

Prompt γ-ray spectroscopy of the neutron-rich ^{96}Kr, produced in transfer- and fusion-induced fission reactions, has been performed using the combination of the Advanced Gamma Tracking Array and the VAMOS++ spectrometer. A second excited state, assigned to J^{π}=4^{+}, is observed for the first time, and a previously reported level energy of the first 2^{+} excited state is confirmed. The measured energy ratio R_{4/2}=E(4^{+})/E(2^{+})=2.12(1) indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N=60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z=36, of similar amplitude to that observed at N=60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g_{9/2}, d_{5/2}, and s_{1/2} orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R_{4/2} as a function of the neutron number up to N=60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N=60 isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at N=60 and Z=36.

Modern Rutherford experiment: tunneling of the most neutron-rich nucleus.

A modern variation of the Rutherford experiment to probe the tunneling of exotic nuclear matter from the measurement of the residues formed in the bombardment of (197)Au by extremely neutron-rich (8)He nuclei is presented. Using a novel off-beam technique the most precise and accurate measurements of fusion and neutron transfer involving reaccelerated unstable beams are reported. The results show unusual behavior of the tunneling of (8)He compared to that for lighter helium isotopes, highlighting the role of the intrinsic structure of composite many-body quantum systems and pairing correlations.

Development of a 1+/N+ setup for the production of multicharged radioactive alkali ions in SPIRAL.

In the framework of the production of radioactive ion beams by the isotope separator online method, a new system has been developed at GANIL/SPIRAL I to produce multicharged alkali ions. The principle, referred to as the "direct 1+/N+ method," consists of a surface ionization source associated with a multicharged electron-cyclotron-resonance ion source without an intermediate mass separator. This new system has been tested online using a (48)Ca primary beam at 60.3 A MeV. The experimental evidence of the direct 1+/N+ process has been obtained for a potential difference between the two sources of 11 V and with a 1+/N+ charge breeding efficiency of 0.04% for (47)K(5+). This value is significantly lower than the value of 6% obtained for stable K ions with the standard 1+/N+ method. A possible explanation is given in the text.

Chemical & Nano-mechanical Study of Artificial Human Enamel Subsurface Lesions.

White lesions represent an early phase of caries formation. 20 human sound premolars were subjected to pH cycling procedure to induce subsurface lesions (SLs) in vitro. In addition, 2 teeth with naturally developed white spot lesions (WSLs) were used as references. All specimens characterized by confocal Raman microscopy being used for the first time in examining white & subsurface lesions and providing a high resolution chemical and morphological map based on phosphate peak intensity alterations at 960 cm-1. Nanoindentation technique was used to measure Hardness (H) and Young's modulus (E) of enamel. Phosphate map of examined samples exhibited presence of intact surface layer (ISL) followed by severe depletion in (PO43-) peak in the area corresponding to the body of the lesion. In all examined groups, the mechanical properties of enamel were decreased in lesion area and found to be inversely related to penetration depth of indenter owing to enamel hierarchical structure. By combining the above two techniques, we linked mechanical properties of enamel to its chemical composition and ensured that the two methods are highly sensitive to detect small changes in enamel composition. Further work is required to bring these two excellent tools to clinical application to perceive carious lesions at an early stage of development.

The Neutrons for Science Facility at SPIRAL-2.

The neutrons for science (NFS) facility is a component of SPIRAL-2, the new superconducting linear accelerator built at GANIL in Caen (France). The proton and deuteron beams delivered by the accelerator will allow producing intense neutron fields in the 100 keV-40 MeV energy range. Continuous and quasi-mono-kinetic energy spectra, respectively, will be available at NFS, produced by the interaction of a deuteron beam on a thick Be converter and by the 7Li(p,n) reaction on thin converter. The pulsed neutron beam, with a flux up to two orders of magnitude higher than those of other existing time-of-flight facilities, will open new opportunities of experiments in fundamental research as well as in nuclear data measurements. In addition to the neutron beam, irradiation stations for neutron-, proton- and deuteron-induced reactions will be available for cross-sections measurements and for the irradiation of electronic devices or biological cells. NFS, whose first experiment is foreseen in 2018, will be a very powerful tool for physics, fundamental research as well as applications like the transmutation of nuclear waste, design of future fission and fusion reactors, nuclear medicine or test and development of new detectors.

Scanning electron microscopic evaluation of ultrasonic debridement comparing sodium hypochlorite and Bardac-22.

This study compared in vitro the debridement ability of a 3% sodium hypochlorite with a new quaternary ammonium derivative, a 0.5% Bardac-22 solution, in the apical third of freshly extracted teeth with straight root canals. Root canals were instrumented sequentially from size 10 to size 35 using alternatively hand and ultrasonic files. When the hand file was loose in the canal, the ultrasonic instrument of the same size was operated at working length for 30 s, delivering 15 ml of the selected solution. Both solutions were able to flush out pulpal debris from the canal space. At the apical end of the preparation, sodium hypochlorite left a thicker smear layer than Bardac-22 did. The quaternary ammonium derivative easily dissolved the inorganic portion of the smear layer, but did not dissolve proteins as well as sodium hypochlorite.

Lack of correlation among three methods for evaluation of apical leakage.

The purpose of this study was to compare three methods of evaluation of the apical seal. Thirty-six teeth were prepared with a ProFile device and filled according to three filling methods: Thermafil (n = 12), warm vertical condensation (n = 12), and the single cone technique (n = 2). The apical seal was recorded with three methods, each successively used on the same teeth: a fluid filtration method, an electrochemical method, and a dye penetration study. The three methods of evaluation did not rank the apical leakage of the three filling techniques in the same order. The fluid filtration method showed that the vertical condensation was superior to Thermafil, which was in turn superior to the single cone technique (p = 0.04). The dye penetration study showed that the Thermafil was a better technique than the vertical condensation and the single cone technique (p = 0.005). The electrochemical method showed no statistical difference among the three filling techniques. No correlation was found among the results obtained with the three methods of evaluation. This study shows that several studies are necessary before comparing the apical seal obtained with various filling techniques.

Evaluation of temporary restorations' microleakage by means of electrochemical impedance measurements.

The aim of this study was to quantify the changes in the watertightness of three temporary filling materials over 1 wk with a new electrochemical technique: the impedance a measurement technique. Forty sound extracted human maxillary teeth were selected and prepared for the measurements. They were divided into three groups in addition to positive and negative controls. The resistance, and therefore the watertightness, of the intact crown and the resistance after preparation of an endodontic access cavity were registered. After a randomization procedure, 12 teeth were obturated with Cavit G, 12 teeth with Fermit-N, and 12 teeth with Intermediate Restorative Material (IRM). The changes in the resistance were measured first just after obturation (time 0), then after days 1, 2, 3, 4, and 7. The results showed that the IRM group was significantly more watertight than the Fermit-N group (p < 0.05) and much more than the Cavit G group (p < 0.005).

Influence of concentration and application time of maleic acid on dentin permeability.

OBJECTIVES: Since dentin permeability ultimately affects bond strength, the purpose of this study was to compare the effects of different concentrations of maleic acid, and different exposure times on dentin permeability. METHODS: One hundred and thirty freshly extracted human non-carious third molars were used. Hydraulic conductance was determined after various treatments. The teeth were divided into five groups. In the first group, the smear layer was untreated. In three other groups, 0.10, 0.25, or 0.5 M aqueous solution of maleic acid was applied on dentin slices for 15, 30, or 60 s. In the last group, the Scotchbond Multi-Purpose dental adhesive system (3M Dental Products) conditioner was applied for 15, 30, or 60 s. RESULTS: After a 60 s exposure time, the Scotchbond Multi-Purpose dentin conditioner or the 0.1 M solution of maleic acid was as effective at increasing dentin permeability as the 0.25 M and 0.5 M maleic acid solutions. The maximum dentin permeability was reached after 15 s with 0.25 M and 0.5 M solutions of maleic acid, after 30 s with Scotchbond Multi-Purpose and after 60 s with the 0.1 M solution of maleic acid. During the first 30 s, the 0.1 M solution of malic acid did not increase dentin permeability and did not completely remove the smear layer; a SEM examination of the dentin slices corroborated the functional results. SIGNIFICANCE: The manufacturer's recommendation of 15 s of etching using Scotchbond Multi-Purpose conditioning gel produced a moderate amount of etching which increases dentin permeability to about one-half its maximum value.

Influence of human tooth cryopreservation on dentin bond strength.

OBJECTIVES: The goal of this study was to evaluate the effects of cryopreservation of teeth on dentin bond strength as a function of remaining dentin thickness. METHODS: Flat occlusal surfaces of human dentin were prepared in 54 freshly extracted teeth and 54 thawed, cryopreserved teeth. In each group, 18 bonds were performed in superficial dentin, 18 in mid-coronal, and 18 in deep dentin. A resin composite cylinder, 3 mm in diameter and in height, was bonded orthogonally to the surface. After storage in distilled water at room temperature for 1 wk, the bonded cylinders underwent shear testing at a crosshead speed of 0.5 mm min-1. The mean remaining dentin thickness was calculated after longitudinally sectioning the debonded samples through the center of the bonded area. Non-parametric statistical analyses were used to correlate the shear bond strength with the remaining dentin thickness among the storage modes and within the different dentin regions. RESULTS: The lowest shear bond strength values were found in the deep dentin of both fresh and cryopreserved dentin, while the values in deep and mid-coronal dentin were not significantly different in fresh and cryopreserved dentin. In the superficial and mid-coronal dentin of cryopreserved samples, the shear bond strength values were identical. There was a significant difference between the shear bond strength values in the superficial dentin of fresh teeth compared to the values for cryopreserved teeth. SIGNIFICANCE: According to the experimental conditions, tooth cryopreservation shows some promise as a substitute for freshly extracted teeth, provided that the experiments are performed in midcoronal and deep dentin.

Use of new minimum intervention dentistry technologies in caries management.

Preservation of natural tooth structure requires early detection of the carious lesion and is associated with comprehensive patient dental care. Processes aiming to detect carious lesions in the initial stage with optimum efficiency employ a variety of technologies such as magnifying loupes, transillumination, light and laser fluorescence (QLF® and DIAGNOdent® ) and autofluorescence (Soprolife® and VistaCam®), electric current/impedance (CarieScan(®) ), tomographic imaging and image processing. Most fluorescent caries detection tools can discriminate between healthy and carious dental tissue, demonstrating different levels of sensitivity and specificity. Based on the fluorescence principle, an LED camera (Soprolife® ) was developed (Sopro-Acteon, La Ciotat, France) which combined magnification, fluorescence, picture acquisition and an innovative therapeutic concept called light-induced fluorescence evaluator for diagnosis and treatment (LIFEDT). This article is rounded off by a Soprolife® illustration about minimally or even non-invasive dental techniques, distinguishing those that preserve or reinforce the enamel and enamel-dentine structures without any preparation (MIT1- minimally invasive therapy 1) from those that require minimum preparation of the dental tissues (MIT2 - minimally invasive therapy 2) using several clinical cases as examples. MIT1 encompasses all the dental techniques aimed at disinfection, remineralizing, reversing and sealing the caries process and MIT2 involves a series of specific tools, including microburs, air abrasion devices, sonic and ultrasonic inserts and photo-activated disinfection to achieve minimal preparation of the tooth. With respect to minimally invasive treatment and prevention, the use of lasers is discussed. Furthermore, while most practices operate under a surgical model, Caries Management by Risk Assessment (CaMBRA) encourages a medical model of disease prevention and management to control the manifestation of the disease, or keep the oral environment in a state of balance between pathological and preventive factors. Early detection and diagnosis and prediction of lesion activity are of great interest and may change traditional operative procedures substantially. Fluorescence tools with high levels of magnification and observational capacity should guide clinicians towards a more preventive and minimally invasive treatment strategy.

Microleakage of Cavit, CavitW, CavitG and IRM by impedance spectroscopy.

The aim of this study was to quantify the sealing ability of four temporary filling materials over 9 days using a new electrochemical technique. Fifty-two extracted human maxillary bicuspids were selected and prepared for the measurements. They were divided into four groups of 12 teeth each, in addition to two positive and two negative controls. After preparation of the endodontic access cavity the sealing ability was registered. After a randomization procedure one group was obturated with IRM, another group with Cavit, a third group with CavitW and the last group with CavitG. The sealability was measured just after obturation (time 0) and after days 1, 2, 3, 4, 7 and 9. The results showed that the IRM group was significantly more watertight than the different Cavit formulations. Throughout the experiment no significant difference was noticed between the Cavit and CavitW groups (P > 0.05). The CavitG group was significantly less watertight throughout the measurements (P < 0.05).