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Nuclear Stewardship Research.
This report covers the period from June 2005 through May 2006. During this, the third year of our program, our research has focused mainly on applying the surrogate reaction technique and our newly developed surrogate ratio method to deduce neutron induced fission cross sections on uranium nuclei. The year has been marked by continued scientific progress, by the arrival of new personnel, by a growth in the numbers of students working in the group and by a continuation of our experimental program and close collaboration with staff and scientists from Lawrence Livermore National Laboratory and from Lawrence Berkeley National Laboratory
Twirling of Actin by Myosins II and V Observed via Polarized TIRF in a ModiïŹed Gliding Assay
The force generated between actin and myosin acts predominantly along the direction of the actin filament, resulting in relative sliding of the thick and thin filaments in muscle or transport of myosin cargos along actin tracks. Previous studies have also detected lateral forces or torques that are generated between actin and myosin, but the origin and biological role of these sideways forces is not known. Here we adapt an actin gliding filament assay in order to measure the rotation of an actin filament about its axis (âtwirlingâ) as it is translocated by myosin. We quantify the rotation by determining the orientation of sparsely incorporated rhodamine-labeledactin monomers, using polarized total internal reflection (polTIRF) microscopy. In order to determine the handedness of the filament rotation, linear incident polarizations in between the standard s- and p-polarizations were generated, decreasing the ambiguity of our probe orientation measurement four-fold. We found that whole myosin II and myosin V both twirl actin with a relatively long (~ ”m), left-handed pitch that is insensitive to myosin concentration, filament length and filament velocity
Twirling of actin by myosins II and V observed via polarized TIRF in a modified gliding assay
The force generated between actin and myosin acts predominantly along the
direction of the actin filament, resulting in relative sliding of the thick and
thin filaments in muscle or transport of myosin cargos along actin tracks.
Previous studies have also detected lateral forces or torques that are
generated between actin and myosin, but the origin and biological role of these
sideways forces is not known. Here we adapt an actin gliding filament assay in
order to measure the rotation of an actin filament about its axis (twirling) as
it is translocated by myosin. We quantify the rotation by determining the
orientation of sparsely incorporated rhodamine-labeled actin monomers, using
polarized total internal reflection (polTIRF) microscopy. In order to determine
the handedness of the filament rotation, linear incident polarizations in
between the standard s- and p-polarizations were generated, decreasing the
ambiguity of our probe orientation measurement four-fold. We found that whole
myosin II and myosin V both twirl actin with a relatively long (micron),
left-handed pitch that is insensitive to myosin concentration, filament length
and filament velocity
Diffusive hidden Markov model characterization of DNA looping dynamics in tethered particle experiments
In many biochemical processes, proteins bound to DNA at distant sites are
brought into close proximity by loops in the underlying DNA. For example, the
function of some gene-regulatory proteins depends on such DNA looping
interactions. We present a new technique for characterizing the kinetics of
loop formation in vitro, as observed using the tethered particle method, and
apply it to experimental data on looping induced by lambda repressor. Our
method uses a modified (diffusive) hidden Markov analysis that directly
incorporates the Brownian motion of the observed tethered bead. We compare
looping lifetimes found with our method (which we find are consistent over a
range of sampling frequencies) to those obtained via the traditional
threshold-crossing analysis (which can vary depending on how the raw data are
filtered in the time domain). Our method does not involve any time filtering
and can detect sudden changes in looping behavior. For example, we show how our
method can identify transitions between long-lived, kinetically distinct states
that would otherwise be difficult to discern
Isospin Purity of T=1 States in the A=38 Nuclei Studied Via Lifetime Measurements in K38
The Doppler Shift Attenuation Method was used to measure lifetimes for levels in 38K at excitation energies of 1698, 2404, 2830, 2996, and 3671 keV, populated using the 40Ca(d, α) 38K reaction at a beam energy of 4.5 MeV. Values of 109(29), 95(22), 457(63), 130(40), and 160(50) fs, respectively, were measured and are compared with previous values obtained using different stopping powers. The matrix element for the transition between the JÏ = 2+ T=1 and 0+ T=1 states in this Tz = 0 nucleus is compared with the analogous transition in the other nuclei in the T = 1 triplet, 38Ca (Tz = â1) and 38Ar (Tz = +1), and with the results of shell-model calculations
Isospin purity of <i>T</i>=1 measurements in the <i>A</i>=38 nuclei studied via lifetime measurements in <sup>38</sup>K
The Doppler Shift Attenuation Method was used to measure lifetimes for levels in 38K at excitation energies of 1698, 2404, 2830, 2996, and 3671 keV, populated using the 40Ca(d, α) 38K reaction at a beam energy of 4.5 MeV. Values of 109(29), 95(22), 457(63), 130(40), and 160(50) fs, respectively, were measured and are compared with previous values obtained using different stopping powers. The matrix element for the transition between the JÏ = 2+ T=1 and 0+ T=1 states in this Tz = 0 nucleus is compared with the analogous transition in the other nuclei in the T = 1 triplet, 38Ca (Tz = â1) and 38Ar (Tz = +1), and with the results of shell-model calculations
Triplet energy differences and the low lying structure of Ga 62
Background: Triplet energy differences (TED) can be studied to yield information on isospin-non-conserving interactions in nuclei.
Purpose: The systematic behavior of triplet energy differences (TED) of T=1, J\u3c0=2+ states is examined. The A=62 isobar is identified as having a TED value that deviates significantly from an otherwise very consistent trend. This deviation can be attributed to the tentative assignments of the pertinent states in Ga62 and Ge62.
Methods: An in-beam \u3b3-ray spectroscopy experiment was performed to identify excited states in Ga62 using Gamma-Ray Energy Tracking In-Beam Nuclear Array with the S800 spectrometer at NSCL using a two-nucleon knockout approach. Cross-section calculations for the knockout process and shell-model calculations have been performed to interpret the population and decay properties observed.
Results: Using the systematics as a guide, a candidate for the transition from the T=1, 2+ state is identified. However, previous work has identified similar states with different J\u3c0 assignments. Cross-section calculations indicate that the relevant T=1, 2+ state should be one of the states directly populated in this reaction.
Conclusions: As spins and parities were not measurable, it is concluded that an unambiguous identification of the first T=1, 2+ state is required to reconcile our understanding of TED systematics
Time-odd components in the mean field of rotating superdeformed nuclei
Rotation-induced time-odd components in the nuclear mean field are analyzed
using the Hartree-Fock cranking approach with effective interactions SIII,
SkM*, and SkP. Identical dynamical moments are obtained for
pairs of superdeformed bands Tb(2)--Dy(1) and
Gd(2)--Tb(1). The corresponding relative alignments strongly
depend on which time-odd mean-field terms are taken into account in the
Hartree-Fock equations.Comment: 23 pages, ReVTeX, 6 uuencoded postscript figures include
\u3cem\u3eÎł\u3c/em\u3e-ray Spectroscopy of \u3csup\u3e166\u3c/sup\u3eHf: X(5) in \u3cem\u3eN\u3c/em\u3e \u3e 90?
Excited states in 166Hf were populated in the ÎČ+/â decay of 166Ta and studied through off-beam Îł âray spectroscopy at the Yale moving tape collector. New coincidence data found no support for two previously reported excited 0+ states and led to a substantially revised level scheme. Similarities between the revised level scheme of 166Hf and the X(5) critical point symmetry are discussed, and the extent of X(5) behavior in this mass region is explored through the W and Os isotopes. Among X(5) candidates with N \u3e 90, good agreement is observed for most energies and interband B(E2) strengths, while all exhibit similar disagreements with other key observables, in particular, yrast B(E2) values and spacing in the excited K = 0+ sequence
A Shell Model Study of the High Spin States of \u3csup\u3e88\u3c/sup\u3eY
Experiments were carried out at the Wright Nuclear Structure Laboratory at Yale University using the 21MV ESTU Tandem Van de Graaff accelerator with the purpose of studying 88Y. A beam of 18O impinged at laboratory energies of 60, 65 and 70 MeV on a 600 ÎŒg/cm 2 74Ge target with a thick (10mg/cm 2) 197Au backing. This experiment was performed with the specific aim of accessing medium spin states of the nucleus of interest. A second experiment was undertaken to populate the nucleus of interest in higher spin states by impinging the same 18O beam on a thin 62 ÎŒg/cm 2 76Ge target with a 20 ÎŒg/cm2 carbon backing at a laboratory beam energy of 90 MeV. Gamma rays emitted following the decay of excited states in 88Y and other nuclei populated in the reactions were measured using the YRAST ball detector array, consisting of 10 Compton suppressed HPGe clover detectors. In conjunction with the experimental study presented here, nuclear shell model calculations using a truncated valence space have also been performed in an attempt to describe the single-particle make-up of the states observed. Preliminary results from these experiments and theoretical calculations are presented
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