AQUA
ADVANCED QUALITY ASSURANCE:
PROTON RANGE RADIOGRAPHY PRR

Measuring position and residual energy (or range) of a beam of mono-energetic protons after an absorber one can deduce the integrated density along the proton trajectory and reconstruct a 2-D density map in the target [1]. The technique has the advantage of reflecting the true density distribution in the target, as against the more complex dependence of absorption on atomic composition for X-rays.

r(l) and S(l,El) are density and stopping power of the medium after a penetration l and residual energy El

The basic principle of the Proton Range Radiography (PRR) method is to measure the residual range of the beam traversing an absorber, deduce the energy loss in the target and build a two-dimensional map of the integral density. The precision in the measurement of the residual range determines the sensitivity to density variations in the target. The AQUA PRR system makes use of a scintillator stack to measure directly the residual proton range, a method developed some time ago at PSI [2]. A pair of position-sensitive detectors (Gas Electron Multipliers, GEM) record position and direction of protons emerging from the target.

The AQUA Proton Range Radiography prototype (PRR10) includes 28, 3 mm thick, 10 cm x10 cm plastic scintillators read out with wavelength shifter fibres by solid state sensors (Hamamatsu Multi - Pixel Photon Counters). Two thicker scintillators in front are used for triggering and timing of the readout. Pulses proportional to the energy loss in each counter are shaped and encoded by individual fast ADCs; for each event, the full pulse height profile is recorded on a PC and analyzed to determine the proton range. Position and incidence angles of the particles are provided by a pair of Gas Electron Multipier (GEM) trackers, with 10 cm x10 cm active area.

A scintillator module with readout electronics	The Proton Range Radiography setup with two GEM chambers

The completed Proton Range Radiography (PRR10) in operating conditions.

The prototype PRR has been exposed to proton beams of various energies at the Paul-Scherrer-Institute (PSI). Major results are described in Refs [3] and [4].

Bragg energy loss profiles recorded for proton beam energies between 50 and 230 MeV. The penetration is given in scintillation counters units (3 mm).	Using a simple recognition of the rightmost scintillator with signal over threshold, one obtains the “digital” range determination shown for 72 and 99 MeV; a gaussian fit to the distribution provides a resolution of about 0.52 units of scintillator thickness, or 1.4 mm rms.

The target used for measurements, a pattern of holes of diameter between one and 10 mm cut in a 20 mm plexiglas plate at depths of 5 and 20 mm.	PRR image of the phantom made with a wide 100 MeV proton beam. Color shades correspond to the recorded average density in the holes

Based on the experience acquired in the construction and operation of the PRR10, a larger acceptance and faster readout system, PRR30, is in the final phase of realization.

[1] K.M. Hanson et al, Phys. Med. Biol. 26 (1981) 965
[2] P. Pemler et al, Nucl. Instr. and Meth. A432 (1999) 483
[3] U. Amaldi et al, Nucl. Instr. and Meth. A617(2010)248
[4] U. Amaldi et al, Nucl. Instr. and Meth. A629(2011)337

FS 14.11.2011