222 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
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
Nuclear photonics at ultra-high counting rates and higher multipole excitations
Next-generation gamma beams beams from laser Compton-backscattering
facilities like ELI-NP (Bucharest)] or MEGa-Ray (Livermore) will drastically
exceed the photon flux presently available at existing facilities, reaching or
even exceeding 10^13 gamma/sec. The beam structure as presently foreseen for
MEGa-Ray and ELI-NP builds upon a structure of macro-pulses (~120 Hz) for the
electron beam, accelerated with X-band technology at 11.5 GHz, resulting in a
micro structure of 87 ps distance between the electron pulses acting as mirrors
for a counterpropagating intense laser. In total each 8.3 ms a gamma pulse
series with a duration of about 100 ns will impinge on the target, resulting in
an instantaneous photon flux of about 10^18 gamma/s, thus introducing major
challenges in view of pile-up. Novel gamma optics will be applied to
monochromatize the gamma beam to ultimately Delta E/E~10^-6. Thus
level-selective spectroscopy of higher multipole excitations will become
accessible with good contrast for the first time. Fast responding gamma
detectors, e.g. based on advanced scintillator technology (e.g. LaBr3(Ce))
allow for measurements with count rates as high as 10^6-10^7 gamma/s without
significant drop of performance. Data handling adapted to the beam conditions
could be performed by fast digitizing electronics, able to sample data traces
during the micro-pulse duration, while the subsequent macro-pulse gap of ca. 8
ms leaves ample time for data readout. A ball of LaBr3 detectors with digital
readout appears to best suited for this novel type of nuclear photonics at
ultra-high counting rates.Comment: 4 pages, 1 figure, 1 tabl
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