The BiPo detector for ultralow radioactivity measurements

The development of BiPo detectors is dedicated to the measurement of extremely high radiopurity in 208Tl and 214Bi for the SuperNEMO double beta decay source foils. A modular prototype, called BiPo-1, with 0.8 m2 of sensitive surface area, has been running in the Modane Underground Laboratory since February, 2008. The goal of BiPo-1 is to measure the different components of the background and in particular the surface radiopurity of the plastic scintillators that make up the detector. The first phase of data collection has been dedicated to the measurement of the radiopurity in 208Tl. After more than one year of background measurement, a surface activity of the scintillators of A(208Tl) = 1.5 \muBq/m2 is reported here. Given this level of background, a larger BiPo3 detector having 3.25 m2 of active surface area, will able to qualify the radiopurity of the SuperNEMO selenium double beta decay foils with the required sensitivity of A(208Tl) < 3-4 \mu Bq/kg (90% C.L.) with a six month measurement. This detector is actually under construction and will be installed in the Canfranc Underground Laboratory mid 2011.Comment: Proceeding - Low Radioactivity Techniques (LRT) Workshop - SNOLab - 201

BiPo prototype for SuperNEMO radiopurity measurements

The BiPo project is dedicated to the measurement of extremely low radioactive contaminations of SuperNEMO beta-beta source foils (208Tl < 2 microBq/kg and 214Bi < 10 microBq/kg). A modular BiPo1 prototype with its 20 modules and its shielding test facility is running in the Modane Underground Laboratory since February, 2008. The goal of this prototype is to study the backgrounds and particularly the surface contamination of plastic scintillators. After 2 months, a preliminary upper limit on the sensitivity of a 10 m2 BiPo detector in 208Tl contamination of selenium source foils can be extrapolated to: A(208Tl) < 7.5 microBq/kg (90 % C.L.).Comment: 4 pages, 10 figures, Moriond EW 2008 proceedin

Results of the NEMO-3 Double Beta Decay Experiment

The NEMO-3 experiment is searching for neutrinoless double beta decay for 2 main isotopes (100Mo and 82Se) and is studying the two-neutrino double beta decay of seven isotopes. The experiment has been taking data since 2003 and, up to the end of 2009, showed no evidence for neutrinoless double beta decay. Two 90 % CL lower limits on the half-lives of the transitions were obtained : T_{1/2}^{0\nu} > 1.0 10^{24} yr for 100Mo and T_{1/2}^{0\nu} > 3.2 10^{23}$ yr for 82Se. The corresponding limits on the effective Majorana neutrino mass are respectively | m_nu | < 0.47 - 0.96 eV and | m_nu | < 0.94 - 2.5 eV. The measurements of the two-neutrino double beta decays for all the isotopes have also reached the highest precision to date.Comment: Proceeding - 22nd Rencontres de Blois - 201

BiPo prototype measurements for SuperNEMO

International audienceThe BiPo pro ject is dedicated to the measurement of extremely low radioactive contaminations of SuperNEMO ββ source foils ( 208 Tl < 2 µBq/kg and 214 Bi < 10 µBq/kg). A modular BiPo1 prototype with its 20 modules and its shielding test facility is running in the Modane Underground Laboratory since February, 2008. The goal of this prototype is to study the back-grounds and particularly the surface contamination of plastic scintillators. After 2 months, a preliminary upper limit on the sensitivity of a 10 m 2 BiPo detector in 208 Tl contamination of selenium source foils can be extrapolated to: A( 208 Tl) < 7.5 µBq/kg (90 % C.L.)

Effective adhesion strength of specifically bound vesicles

A theoretical approach has been undertaken in order to model the thermodynamic equilibrium of a vesicle adhering to a flat substrate. The vesicle is treated in a canonical description with a fixed number of sites. A finite number of these sites are occupied by mobile ligands that are capable of interacting with a discrete number of receptors immobilized on the substrate. Explicit consideration of the bending energy of the vesicle shape has shown that the problem of the vesicle shape can be decoupled from the determination of the optimum allocation of ligands over the vesicle. The allocation of bound and free ligands in the vesicle could be determined as a function of the size of the contact zone, the ligand-receptor binding strength and the concentration of the system constituents. Several approximate solutions for different regions of system parameters are determined and in particular, the distinction between receptor-dominated equilibria and ligand-dominated equilibria is found to be important. The crossover between these two types of solutions is found to occur at a critical size of the contact zone. The presented approach enables the calculation of the effective adhesion strength of the vesicle and thus permits meaningful comparisons with relevant experiments as well as connecting the presented model with the proven success of the continuum approach for modeling the shapes of adhering vesicles. The behavior of the effective adhesion strength is analyzed in detail and several approximate expressions for it are given.Comment: 19 pages, 6 figures. To appear in Phys. Rev.

Membrane dynamics shape TCR-generated signaling

Despite intensive investigation, the mechanisms of T cell receptor (TCR)-mediated signal generation remain poorly understood. Here we review various dynamic processes at the cell membrane that might critically control this signaling. Firstly, we summarize recent reports providing new information on the sensitivity of TCR/ligand interaction to the membrane environment and particularly to applied forces. Secondly, we review recent evidence that forces and displacements are continuously generated at cell surfaces. Thirdly, we summarize recent experimental evidence demonstrating the capacity of forces to generate signals. Lastly, we provide a quantitative model to exemplify the capacity of dynamic processes to modulate TCR properties such as specificity that were previously difficult to explain with conventional models. It is concluded that the described dynamic processes must be integrated into current models of TCR signaling

Biomolecule association rates do not provide a complete description of bond formation.: Biomolecule association rates

International audienceThe efficiency of many cell-surface receptors is dependent on the rate of binding soluble or surface-attached ligands. Much effort was exerted to measure association rates between soluble molecules (three-dimensional k(on)) and, more recently, between surface-attached molecules (two-dimensional [2D] k(on)). According to a generally accepted assumption, the probability of bond formation between receptors and ligands is proportional to the first power of encounter duration. Here we provide new experimental evidence and review published data demonstrating that this simple assumption is not always warranted. Using as a model system the (2D) interaction between ICAM-1-coated surfaces and flowing microspheres coated with specific anti-ICAM-1 antibodies, we show that the probability of bond formation may scale as a power of encounter duration that is significantly higher than 1. Further, we show that experimental data may be accounted for by modeling ligand-receptor interaction as a displacement along a single path of a rough energy landscape. Under a wide range of conditions, the probability that an encounter of duration t resulted in bond formation varied as erfc[(t(0)/t)(1/2)], where t(0) was on the order of 10 ms. We conclude that the minimum contact time for bond formation may be a useful parameter to describe a ligand-receptor interaction, in addition to conventional association rates