392 research outputs found
Characterization of a ballistic supermirror neutron guide
We describe the beam characteristics of the first ballistic supermirror
neutron guide H113 that feeds the neutron user facility for particle physics
PF1B of the Institute Laue-Langevin, Grenoble (ILL). At present, the neutron
capture flux density of H113 at its 20x6cm2 exit window is 1.35x10^10/cm^2/s,
and will soon be raised to above 2x10^10/cm^2/s. Beam divergence is no larger
than beam divergence from a conventional Ni coated guide. A model is developed
that permits rapid calculation of beam profiles and absolute event rates from
such a beam. We propose a procedure that permits inter-comparability of the
main features of beams emitted from ballistic or conventional neutron guides.Comment: 15 pages, 11 figures, to be submitted to Nuclear Instruments and
Methods
Self-referencing embedded strings (SELFIES): A 100% robust molecular string representation
The discovery of novel materials and functional molecules can help to solve
some of society's most urgent challenges, ranging from efficient energy
harvesting and storage to uncovering novel pharmaceutical drug candidates.
Traditionally matter engineering -- generally denoted as inverse design -- was
based massively on human intuition and high-throughput virtual screening. The
last few years have seen the emergence of significant interest in
computer-inspired designs based on evolutionary or deep learning methods. The
major challenge here is that the standard strings molecular representation
SMILES shows substantial weaknesses in that task because large fractions of
strings do not correspond to valid molecules. Here, we solve this problem at a
fundamental level and introduce SELFIES (SELF-referencIng Embedded Strings), a
string-based representation of molecules which is 100\% robust. Every SELFIES
string corresponds to a valid molecule, and SELFIES can represent every
molecule. SELFIES can be directly applied in arbitrary machine learning models
without the adaptation of the models; each of the generated molecule candidates
is valid. In our experiments, the model's internal memory stores two orders of
magnitude more diverse molecules than a similar test with SMILES. Furthermore,
as all molecules are valid, it allows for explanation and interpretation of the
internal working of the generative models.Comment: 6+3 pages, 6+1 figure
A system in balance? ? Implications of deep vertical mixing for the nitrogen budget in the northern Red Sea, including the Gulf of Aqaba (Eilat)
International audienceWe investigated the implications of deep winter mixing for the nitrogen budget in two adjacent systems, the northern Red Sea proper, and the Gulf of Aqaba. Both are subtropical oligotrophic water bodies. The main difference is that in the gulf deep winter mixing takes place regularly, whereas the northern Red Sea proper is permanently stratified. In the Gulf of Aqaba, we observed significantly lower nitrate deficits, i.e. deviations from the Redfield ratio, than in the northern Red Sea proper. Assuming that other external inputs and losses in N or P are very similar in both systems, the higher nitrate deficit can be explained by either lower nitrogen fixation in the (stratified) northern Red Sea, which seems unlikely. An alternative explanation would be higher rates of benthic denitrification than in the gulf. By comparing the two systems we have indirect evidence that benthic denitrification was much lower in the Gulf of Aqaba due to higher oxygen concentrations. This we attributed to the occurrence of deep winter mixing, and as a consequence, the nitrate deficit was close to zero (i.e. N:P ratio close to "Redfield"). If both nitrogen fixation and benthic denitrification take place, as in the northern Red Sea proper, the result was a positive nitrate deficit (i.e. a deficit in nitrate) in the ambient water. The nitrate deficit in the northern Red Sea was observed in spite of high iron deposition from the surrounding desert. Our results strongly support the concept of nitrogen as the proximate, and phosphate as the ultimate limiting nutrient for primary production in the sea. This must not be neglected in efforts for protecting the adjacent reefs against eutrophication
The point spread function of electrons in a magnetic field, and the decay of the free neutron
Experiments in nuclear and particle physics often use magnetic fields to
guide charged reaction products to a detector. Due to their gyration in the
guide field, the particles hit the detector within an area that can be
considerably larger than the diameter of the source where the particles are
produced. This blurring of the image of the particle source on the detector
surface is described by a suitable point spread function (PSF), which is
defined as the image of a point source. We derive simple analytical expressions
for such magnetic PSFs, valid for any angular distribution of the emitted
particles that can be developed in Legendre polynomials. We investigate this
rather general problem in the context of neutron beta decay spectrometers and
study the effect of limited detector size on measured neutron decay correlation
parameters. To our surprise, insufficient detector size does not affect much
the accuracy of such measurements, even for rather large radii of gyration.
This finding can considerably simplify the layout of the respective
spectrometers.Comment: 24 pages, 12 figure
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