251 research outputs found
Stochastic equation for a jumping process with long-time correlations
A jumping process, defined in terms of jump size distribution and waiting
time distribution, is presented. The jumping rate depends on the process value.
The process, which is Markovian and stationary, relaxes to an equilibrium and
is characterized by the power-law autocorrelation function. Therefore, it can
serve as a model of the 1/f noise as well as a model of the stochastic force in
the generalized Langevin equation. This equation is solved for the noise
correlations 1/t; the resulting velocity distribution has sharply falling
tails. The system preserves the memory about the initial condition for a very
long time.Comment: 7 pages, 5 Postscript figure
Muscle pathology in myotonic dystrophy: light and electron microscopic investigation in eighteen patients
Myotonic dystrophy (DM) is the most common muscular dystrophy in adults.
Two known genetic subtypes include DM1 (myotonic dystrophy type 1) and
DM2 (myotonic dystrophy type 2). Genetic testing is considered as the only
reliable diagnostic criterion in myotonic dystrophies. Relatively little is known
about DM1 and DM2 myopathology. Thus, the aim of our study was to characterise
light and electron microscopic features of DM1 and DM2 in patients with
genetically proven types of the disease. We studied 3 DM1 cases and 15 DM2
cases from which muscle biopsies were taken for diagnostic purposes during
the period from 1973 to 2006, before genetic testing became available at our
hospital. The DM1 group included 3 males (age at biopsy 15–19). The DM2
group included 15 patients (5 men and 10 women, age at biopsy 26–60). The
preferential type 1 fibre atrophy was seen in all three DM1 cases in light microscopy,
and substantial central nucleation was present in two biopsies.
Electron microscopy revealed central nuclei in all three examined muscle biopsies.
No other structural or degenerative changes were detected, probably due to the
young age of our patients. Central nucleation, prevalence of type 2 muscle fibres,
and the presence of pyknotic nuclear clumps were observed in DM2 patients in
light microscopy. Among the ultrastructural abnormalities observed in our DM2
group, the presence of internal nuclei, severely atrophied muscle fibres, and lipofuscin
accumulation were consistent findings. In addition, a variety of ultrastructural
abnormalities were identified by us in DM2. It appears that no single ultrastructural
abnormality is characteristic for the DM2 muscle pathology. It seems, however,
that certain constellations of morphological changes might be indicative of
certain types of myotonic dystrophy. (Folia Morphol 2011; 70, 2: 121–129
In-plane uniaxial anisotropy rotations in (Ga,Mn)As thin films
We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial
magnetic easy axis is consistently associated with particular crystallographic
directions and that it can be rotated from the [-110] direction to the [110]
direction by low temperature annealing. We show that this behavior is
hole-density-dependent and does not originate from surface anisotropy. The
presence of uniaxial anisotropy as well its dependence on the
hole-concentration and temperature can be explained in terms of the p-d Zener
model of the ferromagnetism assuming a small trigonal distortion.Comment: 4 pages, 6 Postscript figures, uses revtex
Beam profile investigation of the new collimator system for the J-PET detector
Jagiellonian Positron Emission Tomograph (J-PET) is a multi-purpose detector
which will be used for search for discrete symmetries violations in the decays
of positronium atoms and for investigations with positronium atoms in
life-sciences and medical diagnostics. In this article we present three methods
for determination of the beam profile of collimated annihilation gamma quanta.
Precise monitoring of this profile is essential for time and energy calibration
of the J-PET detector and for the determination of the library of model signals
used in the hit-time and hit-position reconstruction. We have we have shown
that usage of two lead bricks with dimensions of 5x10x20 cm^3 enables to form a
beam of annihilation quanta with Gaussian profile characterized by 1 mm FWHM.
Determination of this characteristic is essential for designing and
construction the collimator system for the 24-module J-PET prototype.
Simulations of the beam profile for different collimator dimensions were
performed. This allowed us to choose optimal collimation system in terms of the
beam profile parameters, dimensions and weight of the collimator taking into
account the design of the 24 module J-PET detector.Comment: 14 pages, 9 figure
Searches for discrete symmetries violation in ortho-positronium decay using the J-PET detector
In this paper we present prospects for using the J-PET detector to search for
discrete symmetries violations in a purely leptonic system of the positronium
atom. We discuss tests of CP and CPT symmetries by means of ortho-positronium
decays into three photons. No zero expectation values for chosen correlations
between ortho-positronium spin and momentum vectors of photons would imply the
existence of physics phenomena beyond the Standard Model. Previous measurements
resulted in violation amplitude parameters for CP and CPT symmetries consistent
with zero, with an uncertainty of about 10-3. The J-PET detector allows to
determine those values with better precision thanks to a unique time and
angular esolution combined with a high geometrical acceptance. Achieving the
aforementioned is possible due to application of polymer scintillators instead
of crystals as detectors of annihilation quanta.Comment: in Nukleonika 201
Studies of unicellular micro-organisms Saccharomyces cerevisiae by means of Positron Annihilation Lifetime Spectroscopy
Results of Positron Annihilation Lifetime Spectroscopy (PALS) and microscopic
studies on simple microorganisms: brewing yeasts are presented. Lifetime of
ortho - positronium (o-Ps) were found to change from 2.4 to 2.9 ns (longer
lived component) for lyophilised and aqueous yeasts, respectively. Also
hygroscopicity of yeasts in time was examined, allowing to check how water -
the main component of the cell - affects PALS parameters, thus lifetime of o-Ps
were found to change from 1.2 to 1.4 ns (shorter lived component) for the dried
yeasts. The time sufficient to hydrate the cells was found below 10 hours. In
the presence of liquid water an indication of reorganization of yeast in the
molecular scale was observed.
Microscopic images of the lyophilised, dried and wet yeasts with best
possible resolution were obtained using Inverted Microscopy (IM) and
Environmental Scanning Electron Microscopy (ESEM) methods. As a result visible
changes to the surface of the cell membrane were observed in ESEM images.Comment: Nukleonika (2015
Potential of the J-PET detector for studies of discrete symmetries in decays of positronium atom - a purely leptonic system
The Jagiellonian Positron Emission Tomograph (J-PET) was constructed as a
prototype of the cost-effective scanner for the simultaneous metabolic imaging
of the whole human body. Being optimized for the detection of photons from the
electron-positron annihilation with high time- and high angular-resolution, it
constitutes a multi-purpose detector providing new opportunities for studying
the decays of positronium atoms. Positronium is the lightest purely leptonic
object decaying into photons. As an atom bound by a central potential it is a
parity eigenstate, and as an atom built out of an electron and an anti-electron
it is an eigenstate of the charge conjugation operator. Therefore, the
positronium is a unique laboratory to study discrete symmetries whose precision
is limited in principle by the effects due to the weak interactions expected at
the level of (~10) and photon-photon interactions expected at the level
of (~10). The J-PET detector enables to perform tests of discrete
symmetries in the leptonic sector via the determination of the expectation
values of the discrete-symmetries-odd operators, which may be constructed from
the spin of ortho-positronium atom and the momenta and polarization vectors of
photons originating from its annihilation. In this article we present the
potential of the J-PET detector to test the C, CP, T and CPT symmetries in the
decays of positronium atoms.Comment: 27 pages, 6 figure
PALS investigations of free volumes thermal expansion of J-PET plastic scintillator synthesized in polystyrene matrix
The polystyrene dopped with 2,5-diphenyloxazole as a primary fluor and
2-(4-styrylphenyl)benzoxazole as a wavelength shifter, prepared as a plastic
scintillator was investigated using positronium probe in wide range of
temperatures from 123 to 423 K. Three structural transitions at 260 K, 283 K
and 370 K were found in the material. In the o-Ps intensity dependence on
temperature, the significant hysteresis is observed. Heated to 370 K, the
material exhibits the o-Ps intensity variations in time.Comment: in Nukleonika 201
A feasibility study of ortho-positronium decays measurement with the J-PET scanner based on plastic scintillators
We present a study of the application of the Jagiellonian Positron Emission
Tomograph (J-PET) for the registration of gamma quanta from decays of
ortho-positronium (o-Ps). The J-PET is the first positron emission tomography
scanner based on organic scintillators in contrast to all current PET scanners
based on inorganic crystals. Monte Carlo simulations show that the J-PET as an
axially symmetric and high acceptance scanner can be used as a multi-purpose
detector well suited to pursue research including e.g. tests of discrete
symmetries in decays of ortho-positronium in addition to the medical imaging.
The gamma quanta originating from o-Ps decay interact in the plastic
scintillators predominantly via the Compton effect, making the direct
measurement of their energy impossible. Nevertheless, it is shown in this paper
that the J-PET scanner will enable studies of the o-Ps decays with
angular and energy resolution equal to and
keV, respectively. An order of magnitude shorter decay
time of signals from plastic scintillators with respect to the inorganic
crystals results not only in better timing properties crucial for the reduction
of physical and instrumental background, but also suppresses significantly the
pileups, thus enabling compensation of the lower efficiency of the plastic
scintillators by performing measurements with higher positron source
activities
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