Delayed avalanches in Multi-Pixel Photon Counters

Hamamatsu Photonics introduced a new generation of their Multi-Pixel Photon Counters in 2013 with significantly reduced after-pulsing rate. In this paper, we investigate the causes of after-pulsing by testing pre-2013 and post-2013 devices using laser light ranging from 405 to 820nm. Doing so we investigate the possibility that afterpulsing is also due to optical photons produced in the avalanche rather than to impurities trapping charged carriers produced in the avalanches and releasing them at a later time. For pre-2013 devices, we observe avalanches delayed by ns to several 100~ns at 637, 777nm and 820 nm demonstrating that holes created in the zero field region of the silicon bulk can diffuse back to the high field region triggering delayed avalanches. On the other hand post-2013 exhibit no delayed avalanches beyond 100~ns at 777nm. We also confirm that post-2013 devices exhibit about 25 times lower after-pulsing. Taken together, our measurements show that the absorption of photons from the avalanche in the bulk of the silicon and the subsequent hole diffusion back to the junction was a significant source of after-pulse for the pre-2013 devices. Hamamatsu appears to have fixed this problem in 2013 following the preliminary release of our results. We also show that even at short wavelength the timing distribution exhibit tails in the sub-nanosecond range that may impair the MPPC timing performances.Comment: Submitted to JINST, 14 pages, 16 figure

A fast ethanol assay to detect seed deterioration

The most common way to test seed quality is to use a simple and reliable but time- and space-consuming germination test. In this paper we present a fast and simple method to analyse cabbage seed deterioration by measuring ethanol production from partially imbibed seeds. The method uses a modified breath analyser and is simple compared to gas chromatographic or enzymatic procedures. A modified method using elevated temperatures (40°C instead of 20°C) shortened the assay time and improved its sensitivity. The analysis showed an inverse correlation between ethanol production and seed quality (e.g. the final percentages or speed of germination and the number of normal seedlings). The increase in ethanol production was observed when cabbage seeds were deteriorated by storage under ambient conditions or hot water treatments, both of which reduced the number of normal seedlings. Premature seeds produced more ethanol upon imbibition than mature seeds. Ethanol production occurred simultaneously with oxygen consumption, indicating that lack of oxygen is not the major trigger for ethanol production

Do p+p Collisions Flow at RHIC? Understanding One-Particle Distributions, Multiplicity Evolution, and Conservation Laws

Collective, explosive flow in central heavy ion collisions manifests itself in the mass dependence of $p_T$ distributions and femtoscopic length scales, measured in the soft sector ($p_T\lesssim 1$ GeV/c). Measured $p_T$ distributions from proton-proton collisions differ significantly from those from heavy ion collisions. This has been taken as evidence that p+p collisions generate little collective flow, a conclusion in line with naive expectations. We point out possible hazards of ignoring phase-space restrictions due to conservation laws when comparing high- and low-multiplicity final states. Already in two-particle correlation functions, we see clear signals of such phase-space restrictions in low-multiplicity collisions at RHIC. We discuss how these same effects, then, {\it must} appear in the single particle spectra. We argue that the effects of energy and momentum conservation actually dominate the observed systematics, and that $p+p$ collisions may be much more similar to heavy ion collisions than generally thought.Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Flow effects on jet profiles and multiplicities

We study the effects of low-$p_T$ collective flow on radiative energy loss from high-$p_T$ partons traversing the QCD medium created in high-energy nucleus-nucleus collisions. We illustrate this idea through three examples. Due to longitudinal flow, jet profiles at the LHC present marked asymmetries in the $\eta\times \phi$-plane, and widths in $\eta$ and $\phi$ of particle distributions associated with a high-$p_T$ trigger at RHIC become different. Finally, transverse flow implies an increase of high-$p_T$ $v_2$ at RHIC.Comment: LaTeX, 4 pages, 4 eps figs included using graphics, uses enclosed svepj.clo and svjour.cls; proceedings of Hard Probes 2004, Ericeira, Portugal, November 4th-10th 200

Simulations of a micro-PET System based on Liquid Xenon

The imaging performance of a high-resolution preclinical microPET system employing liquid xenon as the gamma ray detection medium was simulated. The arrangement comprises a ring of detectors consisting of trapezoidal LXe time projection ionization chambers and two arrays of large area avalanche photodiodes for the measurement of ionization charge and scintillation light. A key feature of the LXePET system is the ability to identify individual photon interactions with high energy resolution and high spatial resolution in 3 dimensions and determine the correct interaction sequence using Compton reconstruction algorithms. The simulated LXePET imaging performance was evaluated by computing the noise equivalent count rate, the sensitivity and point spread function for a point source, and by examining the image quality using a micro-Derenzo phantom according to the NEMA-NU4 standard. Results of these simulation studies included NECR peaking at 1326 kcps at 188 MBq (705 kcps at 184 MBq) for an energy window of 450 - 600 keV and a coincidence window of 1 ns for mouse (rat) phantoms. The absolute sensitivity at the center of the field of view was 12.6%. Radial, tangential, and axial resolutions of 22Na point sources reconstructed with a list-mode maximum likelihood expectation maximization algorithm were <= 0.8 mm (FWHM) throughout the field of view. Hot-rod inserts of < 0.8 mm diameter were resolvable in the transaxial image of a micro-Derenzo phantom. The simulations show that a liquid xenon system would provide new capabilities for significantly enhancing PET images

Recent Results from RHIC & Some Lessons for Cosmic-Ray Physicists

The Relativistic Heavy Ion Collider (RHIC) studies nuclear matter under a variety of conditions. Cold nuclear matter is probed with deuteron-gold collisions, while hot nuclear matter(possibly a quark-gluon plasma (QGP)) is created in heavy-ion collisions. The distribution of spin in polarized nucleons is measured with polarized proton collisions, and photoproduction is studied using the photons that accompany heavy nuclei. The deuteron-gold data shows less forward particle production than would be expected from a superposition of $pp$ collisions, as expected due to saturation/shadowing. Particle production in $AA$ collisions is well described by a model of an expanding fireball in thermal equilibrium. Strong hydrodynamic flow and jet quenching shows that the the produced matter interacts very strongly. These phenomena are consistent with new non-perturbative interactions near the transition temperature to the QGP. This writeup will discuss these results, and their implications for cosmic-ray physicists.Comment: 8 pgs; invited talk presented at the XIVth International Symposium on Very High Energy Cosmic Ray Interactions (ISVHECRI 2006

Liquid Xenon Detectors for Positron Emission Tomography

PET is a functional imaging technique based on detection of annihilation photons following beta decay producing positrons. In this paper, we present the concept of a new PET system for preclinical applications consisting of a ring of twelve time projection chambers filled with liquid xenon viewed by avalanche photodiodes. Simultaneous measurement of ionization charge and scintillation light leads to a significant improvement to spatial resolution, image quality, and sensitivity. Simulated performance shows that an energy resolution of <10% (FWHM) and a sensitivity of 15% are achievable. First tests with a prototype TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference, Vancouver, Canada, 201

Characterization of Silicon Photomultipliers for nEXO

Silicon Photomultipliers (SiPMs) are attractive candidates for light detectors for next generation liquid xenon double-beta decay experiments, like nEXO. In this paper we discuss the requirements that the SiPMs must satisfy in order to be suitable for nEXO and similar experiments, describe the two test setups operated by the nEXO collaboration, and present the results of characterization of SiPMs from several vendors. In particular, we find that the photon detection efficiency at the peak of xenon scintillation light emission (175-178 nm) approaches the nEXO requirements for tested FBK and Hamamatsu devices. Additionally, the nEXO collaboration performed radioassay of several grams of bare FBK devices using neutron activation analysis, indicating levels of 40K, 232Th, and 238U of the order of <0.15, (6.9e10-4 - 1.3e10-2), and <0.11 mBq/kg, respectively.Comment: Version as accepted to Transaction of Nuclear Science. 12 pages, 15 figures (one figure removed following peer review), 8 tables (1 table added following peer review