in evidenza
Rivelare neutroni, senza 3He
La figura mostra l’evoluzione della disponibilità
e dell’utilizzo di 3He nel tempo. I dati del passato
sono dall’Office of Science and Technology
Policy (USA), mentre le proiezioni nel futuro
sono di Jehanne Gillo del DoE. Si vede una
drammatica caduta delle disponibilità a partire
dal 2000. È l’effetto di due fenomeni geopolitici: a.
3
He si ottiene dal decadimento del tritio, la cui
produzione artificiale crollò dopo il trattato START,
b. lo sviluppo del terrorismo rese necessario il
diffuso monitoraggio tramite rivelatori di neutroni.
Il focus point affronta le problematiche connesse.
Alan J. Hurd and Richard T. Kouzes. Focus Point
on 3He replacement in neutron detection: Current
status and perspectives. Why new neutron detector
materials must replace helium-3, Eur. Phys. J. Plus,
129 (2014) 236
Helium-3 has such unique physical and
nuclear properties that to a physicist it seems
appalling the isotope was once indiscriminately
released to the atmosphere as a waste gas.
Not gravitationally bound to our planet, a He-3
atom is effectively lost to the human race once
released. Consequently, when a confluence
of independent factors in national security
and research in the last decade created a
“custody battle” over this scarce isotope, an
intense search for substitutes and alternative
technologies ensued for various applications.
Reprinted from fig. 1 of Alan J. Hure et al., Eur.
Phys. J., Plus 129 (2014) 236, ©2014, SIF, Springer.
A molecular hydrogen absorber at a lookback
time of 12.4 billion years, corresponding
to 10% of the age of the Universe today, is
analyzed to put a constraint on a varying
proton-electron mass ratio, μ. A high resolution
spectrum of the J1443+2724 quasar, which
was observed with the Very Large Telescope, is
used to create an accurate model of 89 Lyman
and Werner band transitions whose relative
frequencies are sensitive to μ, yielding a limit
on the relative deviation from the current
laboratory value of Δμ/μ = (−9.5 ± 5.4stat ±
5.3syst)×10−6.
Uccelli azzurri e cristalli porosi
I colori delle ali di farfalle ed uccelli sono
spesso dovuti all’interferenza entro strutture
spugnose tridimensionali, come dimostrato da
parecchi studi osservativi. L’articolo propone
un particolare modello numerico della micro
struttura delle piume dell’uccellino in figura,
basato sull’analogia con un “disordered air rod
photonic crystal”
T. Ueta et al., Numerical study on the structural
color of blue birds by a disordered porous
photonic crystal model, EPL, 107 (2014) 34004
It has been observationally confirmed that
the color of birds, such as kingfisher and redflanked bluetail, is a structural color owing to
the interference of the light within a sponge
structure inside a barb. In this study, we
consider the air rod photonic crystal to which
disorder is introduced into the translation
vectors and the radius as a model of the
structural color of red-flanked bluetail; the
optical property of the model is numerically
analyzed and is compared with that of the
structural color.
In this paper, we propose a sponge structure
model as a model of a barb of the red-flanked
bluetail, Tarsiger cyanurus, whose sponge layer
is rather thin and whose bubbles within the
sponge structure are long in the direction of
the barb.
Il rapporto tra la massa del protone e
dell’elettrone 12 miliardi di anni fa
Foto ad attosecondi della
propagazione dell’onda di elettroni
Gli autori hanno lanciato pacchetti di elettroni
prodotti per effetto fotoelettrico da impulsi di
luce nell’estremo ultravioletto di durate sotto il
femtosecondo in un cristallo di W coperto di uno
strato di Mg di spessore variabile e controllato.
Il tempo di arrivo alla superfcie dei pacchetti è
misurato con precisione dell’attosecondo.
S. Neppl et al., Direct observation of electron
propagation and dielectric screening on the
atomic length scale, Nature, 517 (2015) 342
The propagation and transport of electrons in
crystals is a fundamental process pertaining
to the functioning of most electronic
devices. Microscopic theories describe this
phenomenon as being based on the motion
of Bloch wave packets. These wave packets
are superpositions of individual Bloch states
with the group velocity determined by the
dispersion of the electronic band structure
near the central wavevector in momentum
space….
Here we present a direct observation of
electron wave packet motion in a realspace and real-time experiment, on length
and time scales shorter than the Bloch
oscillation amplitude and period. We show
that attosecond metrology now enables
quantitative insight into weakly disturbed
electron wave packet propagation on the
atomic length scale without being hampered
by scattering effects, which inevitably occur
over macroscopic propagation length scales.
Adapted by permission from Macmillan Publisher
Ltd.: S. Neppe et al., Nature, 517 (2015) 342,
© NPG, 2015.
Per determinare il rapporto mp /me a miliardi
di anni fa gli astronomi cercano spostamenti
inaspettati di righe di assorbimento dell’H2.
Questa molecola è abbastanza abbondante da
produrre segnali rilevabili anche ad alto redshift. J. Bagdonaite et al. hanno analizzato le
linee di assorbimento nello spettro del quasar
(J1443+2724), che si trova al red-shift z=4.22, da
parte di una galassia davanti ad esso.
J. Bagdonaite et al., Constraint on a Varying
Proton-Electron Mass Ratio 1.5 Billion Years after
the Big Bang., Phys. Rev. Lett., 114 (2015) 071301
84 < il nuovo saggiatore
Tarsiger cyanurus. Reprinted from
T. Ueta et al., EPL 107 (2014) 34004,
©2014 EPLA.
a cura di Alessandro Bettini