A quality control system especially designed for dosimetry in scanning proton beams has been designed and tested. The system consists of a scintillating screen (Gd2O2S:Tb), mounted at the beam-exit side of a phantom, and observed by a low noise CCD camera with a long integration time. The purpose of the instrument is to make a fast and accurate two-dimensional image of the dose distribution at the screen position in the phantom. The linearity of the signal with the dose, the noise in the signal, the influence of the ionization density on the signal, and the influence of the field size on the signal have been investigated. The spatial resolution is 1.3 mm (1 s.d.), which is sufficiently smaller than typical penumbras in dose distributions, The measured yield depends linearly on the dose and agrees within 5% with the calculations. In the images a signal to noise ration (signal/l s.d.) of 10(2) has been found, which is in the same order of magnitude as expected from the calculations. At locations in the dose distribution possessing a strong contribution of high ionization densities (i.e., in the Bragg peak), we found some quenching of the light output, which can be described well by existing models if the beam characteristics are known. For clinically used beam characteristics such as a Spread Out Bragg peak, there is at most 8% deviation from the NACP ionization chamber measurements. The conclusion is that this instrument is a useful tool for quick and reliable quality control of proton beams. The long integration-time capabilities of the system make it worthwhile to investigate its applicability in scanning proton beams and other dynamic treatment modalities. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(98)02104-X].

Fast 2D phantom dosimetry for scanning proton beams

The availability at the Paul Scherrer Institute (PSI) of a spot-scanning technique with an isocentric beam delivery system (gantry) allows the realization of intensity-modulated proton therapy (IMPT). The development of 3D dosimetry is an important tool for the verification of IMPT therapy plans based on inhomogeneous 3D conformal dose distributions. For that purpose new dosimeters are being developed. The concept is to use a system of many millimetre sized scintillating volumes distributed in a polyethylene block, which are read on a CCD camera over a bundle of optical fibres and which can be irradiated from any direction orthogonal to the fibre axis. The purpose of this work is to investigate the composition of such small sensitive volumes. A mixture of inorganic phosphors and optical cement allows an optimal coupling between the scintillating volume and the optical fibre. Five different inorganic phosphors, available as powder, have been examined by considering their response along the Bragg curve. In particular, two phosphors have shown interesting behaviours: Gd2O2S:Tb and (Zn, Cd)S:Ag. Both phosphors have a high emission efficiency but contrasting behaviour in the Bragg peak region. The efficiency of Gd2O2S:Tb decreases with increasing stopping power (quenching of luminescence) while that of (Zn, Cd)S:Ag increases. Because of these contrasting behaviours it is possible to prepare a mixture of the two scintillating powders in a certain ratio in order to modulate the height of the measured Bragg peak relative to the entrance value so that it is in agreement with the ionization chamber measurements. We propose to use a mixture for the sensitive volume consisting of the following weight fractions: 48% Gd2O2S:Tb, 12% (Zn, Cd)S:Ag and 40% optical cement.

Development of an inorganic scintillating mixture for proton beam verification dosimetry

The 4π-fragment-spectrometer FOBOS developed for heavy-ion research at beam energies of 10–100 AMeV has been commissioned for physical experiments at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna. Based on the logarithmic detector principle, it is able to register charged fragments from protons up to heavy residual nuclei in a large dynamical range. Position-sensitive avalanche counters, axial ionization chambers and CsI(Tl) scintillation detectors are arranged in three concentric detector shells. An array of phoswich detectors is used as a more granular forward detector at narrow polar angles. The modular concept of FOBOS allows for different experimental application in the field of exclusive fragment spectroscopy at medium multiplicities. For illustration, the fragment spectroscopy studies concerning the spontaneous fission process and the fragmentation of hot nuclei by means of the FOBOS set-up are considered.

The 4π-fragment-spectrometer FOBOS

Secondary beams of4He and6He were produced through the transfer reactions with the 18.5 MeV/u11B and 35.5 MeV/u7Li primary beams. The fusion-fission cross sections have been measured for the4,6He +209Bi reactions at energies Ecm>1.5*Bfus. In the present experiment it was found that the fission excitation function for the neutron rich nuclei6He is significantly higher than for the4He nuclei.

Fission of209Bi induced by6He ions

Range–energy relations and range straggling of 1,2,3 H and 4,6 He isotopes with the energy ≈50 A MeV are measured for the CsI(Tl), BGO and GSO(Ce) scintillators with an accuracy better than 0.2% and 5%, respectively. The Si-Sci/PD telescope was exposed to secondary beams from the mass separator ACCULINNA. The experimental technique is based on the registration of the “jump” in the amplitude of the photodiode signal for ions passing through the scintillation crystal. Light response of the scintillators for ions 1⩽ Z ⩽4 is measured in energy range (5–50) A MeV, the results are in good agreement with calculations based on Birks model. The energy loss straggling for particles with Δ E / E =0.01–0.50 and mass up to A =10 in 286 μm Δ E silicon detector is studied and compared with theoretical prescriptions. The results allow a precise absolute calibration of the scintillation crystal and to optimize the particle identification by the Δ E – E (Sci/PD) method.

Range-energy relation, range straggling and response function of CsI(Tl), BGO and GSO(Ce) scintillators for light ions

Particle dependent response of a CsI(Tl) scintillation detector with an entrance surface of 314 cm2 has been investigated using a secondary beam facility at the energy range from 2 to 77 MeV/A. The mass and charge identification of particles with Z = 1–18 has been performed by means of time-of-flight and energy measurements. The dependence of light output on E, Z and A has been studied. Using a pulse shape analysis possibility to identify the masses of momentum separated particles with charges of Z = 1–7 is presented.

A. Matthies

Papers

The response of a large CsI(Tl) detector to light particles and heavy ions in the intermediate energy range

First experiments with FOBOS