284 research outputs found
Sub-millimeter nuclear medical imaging with high sensitivity in positron emission tomography using beta-gamma coincidences
We present a nuclear medical imaging technique, employing triple-gamma
trajectory intersections from beta^+ - gamma coincidences, able to reach
sub-millimeter spatial resolution in 3 dimensions with a reduced requirement of
reconstructed intersections per voxel compared to a conventional PET
reconstruction analysis. This '-PET' technique draws on specific beta^+
- decaying isotopes, simultaneously emitting an additional photon. Exploiting
the triple coincidence between the positron annihilation and the third photon,
it is possible to separate the reconstructed 'true' events from background. In
order to characterize this technique, Monte-Carlo simulations and image
reconstructions have been performed. The achievable spatial resolution has been
found to reach ca. 0.4 mm (FWHM) in each direction for the visualization of a
22Na point source. Only 40 intersections are sufficient for a reliable
sub-millimeter image reconstruction of a point source embedded in a scattering
volume of water inside a voxel volume of about 1 mm^3 ('high-resolution mode').
Moreover, starting with an injected activity of 400 MBq for ^76Br, the same
number of only about 40 reconstructed intersections are needed in case of a
larger voxel volume of 2 x 2 x 3~mm^3 ('high-sensitivity mode'). Requiring such
a low number of reconstructed events significantly reduces the required
acquisition time for image reconstruction (in the above case to about 140 s)
and thus may open up the perspective for a quasi real-time imaging.Comment: 17 pages, 5 figutes, 3 table
Towards a direct transition energy measurement of the lowest nuclear excitation in 229Th
The isomeric first excited state of the isotope 229Th exhibits the lowest
nuclear excitation energy in the whole landscape of known atomic nuclei. For a
long time this energy was reported in the literature as 3.5(5) eV, however, a
new experiment corrected this energy to 7.6(5) eV, corresponding to a UV
transition wavelength of 163(11) nm. The expected isomeric lifetime is
3-5 hours, leading to an extremely sharp relative linewidth of Delta E/E ~
10^-20, 5-6 orders of magnitude smaller than typical atomic relative
linewidths. For an adequately chosen electronic state the frequency of the
nuclear ground-state transition will be independent from influences of external
fields in the framework of the linear Zeeman and quadratic Stark effect,
rendering 229mTh a candidate for a reference of an optical clock with very high
accuracy. Moreover, in the literature speculations about a potentially enhanced
sensitivity of the ground-state transition of Th for eventual
time-dependent variations of fundamental constants (e.g. fine structure
constant alpha) can be found. We report on our experimental activities that aim
at a direct identification of the UV fluorescence of the ground-state
transition energy of 229mTh. A further goal is to improve the accuracy of the
ground-state transition energy as a prerequisite for a laser-based optical
control of this nuclear excited state, allowing to build a bridge between
atomic and nuclear physics and open new perspectives for metrological as well
as fundamental studies
0+ states and collective bands in 228Th studied by the (p,t) reaction
The excitation spectra in the deformed nucleus 228Th have been studied by
means of the (p,t)-reaction, using the Q3D spectrograph facility at the Munich
Tandem accelerator. The angular distributions of tritons were measured for
about 110 excitations seen in the triton spectra up to 2.5 MeV. Firm 0+
assignments are made for 17 excited states by comparison of experimental
angular distributions with the calculated ones using the CHUCK3 code.
Assignments up to spin 6+ are made for other states. Sequences of states are
selected which can be treated as rotational bands and as multiplets of
excitations. Moments of inertia have been derived from these sequences, whose
values may be considered as evidence of the two-phonon nature of most 0+
excitations. Experimental data are compared with interacting boson model and
quasiparticle-phonon model calculations and with experimental data for 229Pa.Comment: 21 pages, 14 figure
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
En-route to the fission-fusion reaction mechanism: a status update on laser-driven heavy ion acceleration
The fission-fusion reaction mechanism was proposed in order to generate
extremely neutron-rich nuclei close to the waiting point N = 126 of the rapid
neutron capture nucleosynthesis process (r-process). The production of such
isotopes and the measurement of their nuclear properties would fundamentally
help to increase the understanding of the nucleosynthesis of the heaviest
elements in the universe. Major prerequisite for the realization of this new
reaction scheme is the development of laser-based acceleration of ultra-dense
heavy ion bunches in the mass range of A = 200 and above. In this paper, we
review the status of laser-driven heavy ion acceleration in the light of the
fission-fusion reaction mechanism. We present results from our latest
experiment on heavy ion acceleration, including a new milestone with
laser-accelerated heavy ion energies exceeding 5 MeV/u
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
- …