Scintillation cameras with options for detecting positron annihilation quanta in the coincidence acquisition mode may be the most innovative diagnostic devices introduced in nuclear medicine during ...the last few years. Besides conventional low-energy imaging in the collimated single-photon mode, these options offer a relatively inexpensive opportunity to perform uncollimated PET by switching into the coincidence acquisition mode. Instead of collimators, scatter frames (with 2 optional configurations: axial or open scatter frame) can be mounted to reduce the amount of quanta reaching the detectors from parts of the patient's body outside the field of view. This study investigates the coincidence imaging properties of the scintillation camera by measuring spatial resolution, scatter fraction, sensitivity, and count-rate response for 18F.
A needle in air and a plastic tube in water, each filled with 18F, were oriented axially and transversally to measure the transverse and axial spatial resolutions, respectively. Using either the axial or the open scatter frame, a standard cylinder filled homogeneously with activity was studied over several half-life periods to deduce the respective scatter and random fractions of the system by means of a sinogram technique. The activity of the cylinder was kept low to determine the sensitivity to coincidence events for both scatter frames.
Depending on the distance between the line source and the axis of rotation and on the choice of the axial acceptance angle used to reconstruct the coincidence events (single-slice rebinning algorithm), the axial resolution was found to be between 6 and 10 mm (full width at half maximum) with the axial scatter frame mounted. The transversal resolution was 6-6.5 mm on the axis of rotation, independent of the scatter frame used. The scatter fraction amounted to roughly 25% for the axial and 38% for the open scatter frames. The sensitivity when measuring true coincidence pairs ran to nearly 650 Hz/kBq/mL, when acquisition was performed with the axial scatter frame using a 30%-wide photopeak energy window. When acquiring with the open scatter frame, the sensitivity increased to nearly 3000 Hz/kBq/mL. Using the axial scatter frame, the homogeneously filled cylinder could be scanned with a maximum true coincidence rate of 2000 Hz for an activity of 55-60 MBq. Although this maximum true coincidence counting rate did not change significantly when the acquisition was performed with the open scatter frame, the respective activity in the standard cylinder was decreased to 10-15 MBq.
The spatial resolution of the scintillation camera is sufficient for high-resolution coincidence imaging. Compared with a dedicated PET scanner, the scatter fraction is relatively high and should therefore be corrected adequately. The relatively low sensitivity and the rather low maximum true coincidence counting rate are considerably inferior compared with a conventional PET scanner. However, these drawbacks can be partially compensated for, facilitating its clinical use.
Using the ALADIN forward-spectrometer at SIS we have studied multi-fragment decays of
197Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of
E
A
= 600
MeV
. The ...new data presented in this work comprise the measured cross sections of multifragment processes, the
N
Z
ratios of the produced fragments, and the differential distributions of fragment multiplicities and of observables built on the charge correlations within the event.
The
197Au+Cu data are analyzed with the statistical multifragmentation model. It is shown that all observables can be simultaneously reproduced on an absolute scale, apart from an overall normalization constant which relates the number of model events to the measured cross section. A continuous distribution of excited residual nuclei, used as input for the calculations, was derived from the criterion of a best fit to the data. It exhibits a correlation between decreasing mass number
A and increasing excitation energy
E
x
A
and a saturation of the excitation energy at
E
x
A
≈ 8
MeV
.
Kinetic energies of light fragments ( A</=10) from the decay of target spectators in 197Au+197Au collisions at 1000 MeV per nucleon have been measured with high-resolution telescopes at backward ...angles. Except for protons and apart from the observed evaporation components, the kinetic-energy spectra exhibit slope temperatures of about 17 MeV, independent of the particle species, but not corresponding to the thermal or chemical degrees of freedom at breakup. It is suggested that these slope temperatures may reflect the intrinsic Fermi motion and thus the bulk density of the spectator system at the instant of becoming unstable.
Proton–proton correlations and correlations of protons, deuterons and tritons with
α particles from spectator decays following
197
Au +
197
Au collisions at 1000 MeV per nucleon have been measured ...with two highly efficient detector hodoscopes. The constructed correlation functions, interpreted within the approximation of a simultaneous volume decay, indicate a moderate expansion and low breakup densities, similar to assumptions made in statistical multifragmentation models.
Energy spectra for intermediate mass fragments produced in central Au+Au collisions at E/A=100 MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion ...energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the cm system, but they decrease with Z sub , suggesting that all fragments do not participate equally in the collective expansion.
Fragment distributions have been measured for Au+Au collisions at ital E/ital A=100 and 1000 MeV. A high detection efficiency for fragments was obtained by combining the ALADIN spectrometer and the ...MSU-Miniball/WU-Miniwall array. At both energies the maximum multiplicity of intermediate mass fragments (IMF) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at ital E/ital A=100 MeV may be related to a reduction of the density in momentum space which is caused by the collective expansion.
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