513 research outputs found
Nuclear Photonics
With new gamma-beam facilities like MEGa-ray at LLNL (USA) or ELI-NP at
Bucharest with 10^13 g/s and a bandwidth of Delta E_g/E_g ~10^-3, a new era of
g-beams with energies <=20 MeV comes into operation, compared to the present
world-leading HIGS facility (Duke Univ., USA) with 10^8 g/s and Delta
E_g/E_g~0.03. Even a seeded quantum FEL for g-beams may become possible, with
much higher brilliance and spectral flux. At the same time new exciting
possibilities open up for focused g-beams. We describe a new experiment at the
g-beam of the ILL reactor (Grenoble), where we observed for the first time that
the index of refraction for g-beams is determined by virtual pair creation.
Using a combination of refractive and reflective optics, efficient
monochromators for g-beams are being developed. Thus we have to optimize the
system of the g-beam facility, the g-beam optics and g-detectors. We can trade
g-intensity for band width, going down to Delta E_g/E_g ~ 10^-6 and address
individual nuclear levels. 'Nuclear photonics' stresses the importance of
nuclear applications. We can address with g-beams individual nuclear isotopes
and not just elements like with X-ray beams. Compared to X rays, g-beams can
penetrate much deeper into big samples like radioactive waste barrels, motors
or batteries. We can perform tomography and microscopy studies by focusing down
to micron resolution using Nucl. Reson. Fluorescence for detection with eV
resolution and high spatial resolution. We discuss the dominating M1 and E1
excitations like scissors mode, two-phonon quadrupole octupole excitations,
pygmy dipole excitations or giant dipole excitations under the new facet of
applications. We find many new applications in biomedicine, green energy,
radioactive waste management or homeland security. Also more brilliant
secondary beams of neutrons and positrons can be produced.Comment: 8 pages, 3 figures, 2 table
Introducing the Fission-Fusion Reaction Process: Using a Laser-Accelerated Th Beam to produce Neutron-Rich Nuclei towards the N=126 Waiting Point of the r Process
We propose to produce neutron-rich nuclei in the range of the astrophysical
r-process around the waiting point N=126 by fissioning a dense
laser-accelerated thorium ion bunch in a thorium target (covered by a CH2
layer), where the light fission fragments of the beam fuse with the light
fission fragments of the target. Via the 'hole-boring' mode of laser Radiation
Pressure Acceleration using a high-intensity, short pulse laser, very
efficiently bunches of 232Th with solid-state density can be generated from a
Th layer, placed beneath a deuterated polyethylene foil, both forming the
production target. Th ions laser-accelerated to about 7 MeV/u will pass through
a thin CH2 layer placed in front of a thicker second Th foil closely behind the
production target and disintegrate into light and heavy fission fragments. In
addition, light ions (d,C) from the CD2 production target will be accelerated
as well to about 7 MeV/u, inducing the fission process of 232Th also in the
second Th layer. The laser-accelerated ion bunches with solid-state density,
which are about 10^14 times more dense than classically accelerated ion
bunches, allow for a high probability that generated fission products can fuse
again. In contrast to classical radioactive beam facilities, where intense but
low-density radioactive beams are merged with stable targets, the novel
fission-fusion process draws on the fusion between neutron-rich, short-lived,
light fission fragments both from beam and target. The high ion beam density
may lead to a strong collective modification of the stopping power in the
target, leading to significant range enhancement. Using a high-intensity laser
as envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP),
estimates promise a fusion yield of about 10^3 ions per laser pulse in the mass
range of A=180-190, thus enabling to approach the r-process waiting point at
N=126.Comment: 13 pages, 6 figure
The Refractive Index of Silicon at Gamma Ray Energies
The index of refraction n(E_{\gamma})=1+\delta(E_{\gamma})+i\beta(E_{\gamma})
is split into a real part \delta and an absorptive part \beta. The absorptive
part has the three well-known contributions to the cross section \sigma_{abs}:
the photo effect, the Compton effect and the pair creation, but there is also
the inelastic Delbr\"uck scattering. Second-order elastic scattering cross
sections \sigma_{sca} with Rayleigh scattering (virtual photo effect), virtual
Compton effect and Delbr\"uck scattering (virtual pair creation) can be
calculated by integrals of the Kramers-Kronig dispersion relations from the
cross section \sigma_{abs}. The real elastic scattering amplitudes are
proportional to the refractive indices \delta_{photo}, \delta_{Compton} and
\delta_{pair}. While for X-rays the negative \delta_{photo} dominates, we show
for the first time experimentally and theoretically that the positive
\delta_{pair} dominates for \gamma rays, opening a new era of \gamma optics
applications, i.e. of nuclear photonics.Comment: 4 pages, 3 figure
Signatures of the Unruh effect from electrons accelerated by ultra-strong laser fields
We calculate the radiation resulting from the Unruh effect for strongly
accelerated electrons and show that the photons are created in pairs whose
polarizations are maximally entangled. Apart from the photon statistics, this
quantum radiation can further be discriminated from the classical (Larmor)
radiation via the different spectral and angular distributions. The signatures
of the Unruh effect become significant if the external electromagnetic field
accelerating the electrons is not too far below the Schwinger limit and might
be observable with future facilities. Finally, the corrections due to the
birefringent nature of the QED vacuum at such ultra-high fields are discussed.
PACS: 04.62.+v, 12.20.Fv, 41.60.-m, 42.25.Lc.Comment: 4 pages, 1 figur
Exploring the multi-humped fission barrier of 238U via sub-barrier photofission
The photofission cross-section of 238U was measured at sub-barrier energies
as a function of the gamma-ray energy using, for the first time, a
monochromatic, high-brilliance, Compton-backscattered gamma-ray beam. The
experiment was performed at the High Intensity gamma-ray Source (HIgS) facility
at beam energies between E=4.7 MeV and 6.0 MeV and with ~3% energy resolution.
Indications of transmission resonances have been observed at gamma-ray beam
energies of E=5.1 MeV and 5.6 MeV with moderate amplitudes. The triple-humped
fission barrier parameters of 238U have been determined by fitting EMPIRE-3.1
nuclear reaction code calculations to the experimental photofission cross
section.Comment: 5 pages, 3 figure
On the feasibility of a nuclear exciton laser
Nuclear excitons known from M\"ossbauer spectroscopy describe coherent
excitations of a large number of nuclei -- analogous to Dicke states (or Dicke
super-radiance) in quantum optics. In this paper, we study the possibility of
constructing a laser based on these coherent excitations. In contrast to the
free electron laser (in its usual design), such a device would be based on
stimulated emission and thus might offer certain advantages, e.g., regarding
energy-momentum accuracy. Unfortunately, inserting realistic parameters, the
window of operability is probably not open (yet) to present-day technology --
but our design should be feasible in the UV regime, for example.Comment: 7 pages RevTeX, 4 figure
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