Radiation therapy accelerators require highly accurate dose deposition and the output must be monitored frequently and regularly. Ionization chambers are the primary tool for this control, but their ...size and fragility make them unsuitable for use during patient treatment. In collaboration with a French center for cancer treatment in Caen, we describe the development and testing of a low-cost radiotherapy dosimeter (SDM) based on scintillating fibers and signal processing to reduce the effect of Cerenkov radiation background. The employment of photodiodes for light collection reduces the cost relative to systems using photomultipliers (PMTs). However, we have also developed a highly sensitive system that uses PMTs for very low-dose deposition for Brachytherapy. Comparison with standard ionization chambers shows about a 1% difference over a range of 6- to 25-MV photons. This dosimeter is 1 mm in diameter and can be placed where ever required including inside the body. Its small size and flexibility make it useful for delimiting critical regions, where organs may be very radiation sensitive.
The properties of fragments and light charged particles emitted in multifragmentation of single sources formed in central 36
A
MeV Gd+U collisions are reviewed. Most of the products are ...isotropically distributed in the reaction c.m. Fragment kinetic energies reveal the onset of radial collective energy. A bulk effect is experimentally evidenced from the similarity of the charge distribution with that from the lighter 32
A
MeV Xe+Sn system. Spinodal decomposition of finite nuclear matter exhibits the same property in simulated central collisions for the two systems, and appears therefore as a possible mechanism at the origin of multifragmentation in this incident energy domain.
The influence of the entrance channel mass asymmetry upon the fragmentation process is addressed by studying heavy-ion induced reactions around the Fermi energy. The data have been recorded with the ...INDRA 4π array. An event selection method called the Principal Component Analysis is presented and discussed. It is applied for the selection of central events and furthermore to multifragmentation of single source events. The selected subsets of data are compared to the Statistical Multifragmentation Model (SMM) to check the equilibrium hypothesis and get the source characteristics. Experimental comparisons show the evidence of a decoupling between thermal and compressional (radial flow) component of the excitation energy stored in such nuclear systems.
We study the anisotropy effects measured with INDRA at GSI in central collisions of
129Xe+
natSn at 50
A
MeV and
197Au+
197Au at 60, 80, 100
A
MeV incident energy. The microcanonical ...multifragmentation model with non-spherical sources is used to simulate an incomplete shape relaxation of the multifragmenting system. This model is employed to interpret observed anisotropic distributions in the fragment size and mean kinetic energy. The data can be well reproduced if an expanding prolate source aligned along the beam direction is assumed. An either non-Hubblean or non-isotropic radial expansion is required to describe the fragment kinetic energies and their anisotropy. The qualitative similarity of the results for the studied reactions suggests that the concept of a longitudinally elongated freeze-out configuration is generally applicable for central collisions of heavy systems. The deformation decreases slightly with increasing beam energy.
A sample of ‘single-source’ events, compatible with the multifragmentation of very heavy fused systems, are isolated among well-measured
155Gd
+
natU 36
A
MeV reactions by examining the evolution of ...the kinematics of fragments with
Z⩾5 as a function of the dissipated energy and loss of memory of the entrance channel. Single-source events are found to be the result of very central collisions. Such central collisions may also lead to multiple fragment emission due to the decay of excited projectile- and target-like nuclei and so-called ‘neck’ emission, and for this reason the isolation of single-source events is very difficult. Event-selection criteria based on centrality of collisions, or on the isotropy of the emitted fragments in each event, are found to be inefficient to separate the two mechanisms, unless they take into account the redistribution of fragments' kinetic energies into directions perpendicular to the beam axis. The selected events are good candidates to look for bulk effects in the multifragmentation process.
Invariant cross sections of intermediate mass fragments in peripheral collisions of 197Au on 197Au at incident energies between 40 and 150 MeV per nucleon have been measured with the 4π ...multi-detector INDRA. The maximum of the fragment production is located near mid-rapidity at the lower energies and moves gradually towards the projectile and target rapidities as the energy is increased. Schematic calculations within an extended Goldhaber model suggest that the observed cross section distributions and their evolution with energy are predominantly the result of the clustering requirement for the emerging fragments and of their Coulomb repulsion from the projectile and target residues. The quantitative comparison with transverse energy spectra and fragment charge distributions emphasizes the role of hard scattered nucleons in the fragmentation process.
A simple formalism describing the light response of CsI(Tl) to heavy ions, which quantifies the luminescence and the quenching in terms of the competition between radiative transitions following the ...carrier trapping at the Tl activator sites and the electron–hole recombination, is proposed. The effect of the δ-rays on the scintillation efficiency is for the first time quantitatively included in a fully consistent way. The light output expression depends on four parameters determined by a procedure of global fit to experimental data.
The light output of the 324 CsI(Tl) scintillators of INDRA has been measured over large ranges in energy: 1–
80
AMeV
and in atomic number of incident ions:
Z=1–60. An analytical expression for the ...non-linear total light response as a function of the energy and the identity of the ion is developed. It depends on four parameters. For three of them, connected to CsI(Tl) intrinsic characteristics, fixed values are proposed. Two applications are presented: energy calibration and fragment identification in telescopes using a CsI(Tl) crystal as residual energy detector.
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