35 research outputs found
Modeling oscillatory Microtubule--Polymerization
Polymerization of microtubules is ubiquitous in biological cells and under
certain conditions it becomes oscillatory in time. Here simple reaction models
are analyzed that capture such oscillations as well as the length distribution
of microtubules. We assume reaction conditions that are stationary over many
oscillation periods, and it is a Hopf bifurcation that leads to a persistent
oscillatory microtubule polymerization in these models. Analytical expressions
are derived for the threshold of the bifurcation and the oscillation frequency
in terms of reaction rates as well as typical trends of their parameter
dependence are presented. Both, a catastrophe rate that depends on the density
of {\it guanosine triphosphate} (GTP) liganded tubulin dimers and a delay
reaction, such as the depolymerization of shrinking microtubules or the decay
of oligomers, support oscillations. For a tubulin dimer concentration below the
threshold oscillatory microtubule polymerization occurs transiently on the
route to a stationary state, as shown by numerical solutions of the model
equations. Close to threshold a so--called amplitude equation is derived and it
is shown that the bifurcation to microtubule oscillations is supercritical.Comment: 21 pages and 12 figure
Human Resource Flexibility as a Mediating Variable Between High Performance Work Systems and Performance
Much of the human resource management literature has demonstrated the impact of high performance
work systems (HPWS) on organizational performance. A new generation of studies is
emerging in this literature that recommends the inclusion of mediating variables between HPWS
and organizational performance. The increasing rate of dynamism in competitive environments
suggests that measures of employee adaptability should be included as a mechanism that may
explain the relevance of HPWS to firm competitiveness. On a sample of 226 Spanish firms, the
study’s results confirm that HPWS influences performance through its impact on the firm’s
human resource (HR) flexibility
Development of a secondary triton beam from primary 16,18O beams for (t,3He) experiments at intermediate energies
The in-flight heavy-ion fragmentation technique has been used to produce a secondary beam of tritons (3H) at intermediate energies (Et > 100 MeV / nucleon) from primary 16,18O beams of 150 and 120 MeV/nucleon, respectively. The best results are obtained with a 16O beam of 150 MeV/nucleon, producing a 115 MeV/nucleon triton beam. The triton beam will be used in (t,3He) charge-exchange experiments at the S800 spectrometer at the NSCL. At the target of the S800, a triton rate of 5 × 106 particles per second is achieved, for a primary 16O beam of 100 pnA. The (t,3He) reaction using this beam was tested with a 24Mg target. An excitation-energy resolution of 190 ± 15 keV is achieved
Gamow-Teller strengths in 24 Na using the 24 Mg( t , 3 He) reaction at 115 A MeV
Gamow-Teller transitions from Mg24 to Na24 were studied via the (t,3He) reaction at 115A MeV using a secondary triton beam produced via fast fragmentation of 150A MeV O16 ions. Compared to previous (t,3He) experiments at this energy that employed a primary α beam, the secondary beam intensity is improved by about a factor of five. Despite the large emittance of the secondary beam, an excitation-energy resolution of ∼200 keV is achieved. A good correspondence is found between the extracted Gamow-Teller strength distribution and those available from other charge-exchange probes. Theoretical calculations using the newly developed USDA and USDB sd-shell model interactions reproduce the data well
Weak interaction strengths for supernovae calculations via the (t, He-3) reaction on medium-heavy nuclei.
The Ni-58 (t, He-3)Co-58 reaction at 112 MeV/nucleon was measured to identify strength associated with Gamow-Teller transitions in the T-z = +1 direction. The experiment is a test case for future similar studies. The main aim of such studies is to test theoretical models used to predict Gamow-Teller strength distributions that serve as input for supernovae evolution calculations. The results indicate that the (t, He-3) reaction is indeed a powerful tool to perform such tests
The (t,He-3) and (He-3, t) reactions as probes of Gamow-Teller strength
Charge-exchange reactions are an important tool for determining
weak-interaction rates. They provide stringent tests for nuclear structure
models necessary for modeling astrophysical environments such as neutron stars
and core-collapse supernovae. In anticipation of (t,3He) experiments at 115
MeV/nucleon on nuclei of relevance (A~40-120) in the late evolution of stars,
it is shown via a study of the 26Mg(t,3He) reaction that this probe is an
accurate tool for extracting Gamow-Teller transition strengths. To do so, the
data are complemented by results from the 26Mg(3He,t) reaction at 140
MeV/nucleon which allows for a comparison of T=2 analog states excited via the
mirror reactions. Extracted Gamow-Teller strengths from 26Mg(t,3He) and
26Mg(3He,t) are compared with those from 26Mg(d,2He) and 26Mg(p,n) studies,
respectively. A good correspondence is found, indicating probe-independence of
the strength extraction. Furthermore, we test shell-model calculations using
the new USD-05B interaction in the sd-model space and show that it reproduces
the experimental Gamow-Teller strength distributions well. A second goal of
this work is to improve the understanding of the (t,3He) and (3He,t) reaction
mechanisms at intermediate energies since detailed studies are scarce. The
Distorted-Wave Born Approximation is employed, taking into account the
composite structures of the 3He and triton particles. The reaction model
provides the means to explain systematic uncertainties at the 10-20% level in
the extraction of Gamow-Teller strengths as being due to interference between
Gamow-Teller dL=0, dS=1 and dL=2, dS=1 amplitudes that both contribute to
transitions from 0+ to 1+ states.Comment: 16 pages, 6 figure