Proton ranges in water between 145 MeV to 227 MeV initial energy have been measured at a clinical superconducting synchrocyclotron using the acoustic signal induced by the ion dose deposition (ionoacoustic effect). Detection of ultrasound waves was performed by a very sensitive hydrophone and signals were stored in a digital oscilloscope triggered by secondary prompt gammas. The ionoacoustic range measurements were compared to existing range data from a calibrated range detector setup on-site and agreement of better than 1 mm was found at a Bragg peak dose of about 10 Gy for 220 MeV initial proton energy, compatible with the experimental errors. Ionoacoustics has thus the potential to measure the Bragg peak position with submillimeter accuracy during proton therapy, possibly correlated with ultrasound tissue imaging.

Recently, proton therapy treatments delivered with ultra-high dose rates have 19 been of high scientific interest, and the Faraday cup is a promising dosimetry 20 tool for such experiments. Different institutes use different Faraday cup designs, 21 and either a high voltage guard ring, or the combination of an electric and a 22 magnetic field is employed to minimize the effect of secondary electrons. The 23 authors first investigate these different approaches for beam energies of 70 MeV, 24 150 MeV, 230 MeV and 250 MeV, magnetic fields between 0 mT and 24 mT 25and voltages between -1000V to 1000V. When applying a magnetic field, the 26 measured signal is independent of the guard ring voltage, indicating that this 27 setting minimizes the effect of secondary electrons on the reading of the Faraday 28 cup. Without magnetic field, applying the negative voltage however decreases 29 the signal by an energy dependent factor up to 1.3% for the lowest energy tested 30 and 0.4% for the highest energy, showing an energy dependent response. Next, 31 the study demonstrates the application of the Faraday cup up to ultra-high dose 32 rates. Faraday cup measurements with cyclotron currents up to 800nA (dose 33 rates of up to approximately 1000 Gy/s) show that the Faraday cup is indeed 34 dose rate independent. Then, the Faraday cup is applied to commission the 35 primary gantry monitor for high dose rates. Finally, short-term reproducibility 36 of the monitor calibration is quantified within single days, showing a standard 37 deviation of 0.1% (one sigma). In conclusion, the Faraday cup is a promising, 38 dose rate independent tool for dosimetry up to ultra-high dose rates. Caution is 39 however necessary when using a Faraday cup without magnetic field, as a guard 40 ring with high voltage alone can introduce an energy dependent signal offset.

Spherical Proportional Counters (SPCs) are a novel gaseous detector technology employed by the NEWS-G low-mass dark matter search experiment for their high sensitivity to single electrons from ionization. In this paper, we report on the first characterization of the single electron response of SPCs with unprecedented precision, using a UV-laser calibration system. The experimental approach and analysis methodology are presented along with various direct applications for the upcoming next phase of the experiment at SNOLAB. These include the continuous monitoring of the detector response and electron drift properties during dark matter search runs, as well as the experimental measurement of the trigger threshold efficiency. We measure a mean ionization energy of W = 27.6 ± 0.2 eV in Ne + CH4 (2%) for 2.8 keV X-rays, and demonstrate the feasibility of performing similar precision measurements at sub-keV energies for future gas mixtures to be used for dark matter searches at SNOLAB.

No abstract

We describe a focused beam-stop array (BSA) for the measurement of object scatter in imaging systems that utilize x-ray beams in the megavoltage (MV) energy range. The BSA consists of 64 doubly truncated tungsten cone elements of 0.5 cm maximum diameter that are arranged in a regular array on an acrylic slab. The BSA is placed in the accessory tray of a medical linear accelerator at a distance of approximately 50 cm from the focal spot. We derive an expression that allows us to estimate the scatter in an image taken without the array present, given image values in a second image with the array in place. The presence of the array reduces fluence incident on the imaged object. This leads to an object-dependent underestimation bias in the scatter measurements. We apply corrections in order to address this issue. We compare estimates of the flat panel detector response to scatter obtained using the BSA to those derived from Monte Carlo simulations. We find that the two estimates agree to within 10% in terms of RMS error for 30 cm x 30 cm water slabs in the thickness range of 10-30 cm. Larger errors in the scatter estimates are encountered for thinner objects, probably owing to extrafocal radiation sources. However, RMS errors in the estimates of primary images are no more than 5% for water slab thicknesses in the range of 1-30 cm. The BSA scatter estimates are also used to correct cone beam tomographic projections. Maximum deviations of central profiles of uniform water phantoms are reduced from 193 to 19 HU after application of corrections for scatter, beam hardening, and lateral truncation that are based on the BSA-derived scatter estimate. The same corrections remove the typical cupping artifact from both phantom and patient images. The BSA proves to be a useful tool for quantifying and removing image scatter, as well as for validating models of MV imaging systems.