First Direct Measurement of an Astrophysical p-Process Reaction Cross Section Using a Radioactive Ion Beam.

We have performed the first direct measurement of the ^{83}Rb(p,γ) radiative capture reaction cross section in inverse kinematics using a radioactive beam of ^{83}Rb at incident energies of 2.4 and 2.7A MeV. The measured cross section at an effective relative kinetic energy of E_{cm}=2.393 MeV, which lies within the relevant energy window for core collapse supernovae, is smaller than the prediction of statistical model calculations. This leads to the abundance of ^{84}Sr produced in the astrophysical p process being higher than previously calculated. Moreover, the discrepancy of the present data with theoretical predictions indicates that further experimental investigation of p-process reactions involving unstable projectiles is clearly warranted.

Improved sub-milimeter range-verification method for proton therapy using a composite hadron tumour marker (HTM).

Objective.The results of a follow-up experiment investigating a novel method for sub-milimetre range verification (RV) in proton therapy (PT) are presented.Approach.The method consists of implanting a hadron tumour marker (HTM) near the planned treatment volume, and measuring theγ-ray signals emitted as a result of activation by the proton beam. These signals are highly correlated with the energy of the beam impinging on the HTM and can provide an absolute measurement of the range of the beam relative to the position of the HTM, which is independent of any uncertainties in beam delivery.Main results.Three candidate HTM materials were identified and combined into a single composite HTM, which makes use of the strongest reaction in each material. The setup of the previous experiment was improved on by using high-purity germanium detectors to measure theγ-ray signal with a higher resolution than was previously achieved. A PMMA phantom was also used to simulate theγ-ray background from tissue activation. HTM RV using the data collected in this study yielded range measurements whose average deviation from the expected value was 0.13(22)mm.Significance.Range uncertainty in PT limits the prescribed treatment plan for cancer patients with large safety margins and constrains the direction of the proton beam in relation to any organ at risk. The sub-milimetre range uncertainty achieved in this study using HTM RV, if implemented clinically, would allow for a reduction in the size of safety margins, increasing the therapeutic window for PT.

Characterization of three layer depth of interaction PET detectors for small animal imaging.

High sensitivity and spatial resolution are two main parameters of each small animal imaging system. Multi-layers phoswich detectors have been developed to measure the depth of interaction and to improve system spatial resolution. In the past, efforts have been made to develop these kinds of detectors with different crystal materials and lengths. In this work, three layers phoswich detectors based on GE Healthcare eXplore VISTA PET scanner geometry composing LYSO, GSO and BGO scintillators with different crystal orders and lengths were investigated to find the optimum case with the highest sensitivity and uniform spatial resolution. All simulations were performed using the Monte Carlo simulation tool, the Geant4 Application for Tomographic Emission (GATE). In order to validate GATE simulation package, the GE eXplore VISTA small animal PET system was modeled and output results were compared with the experimental data. The length of each of the three layers varied, while the total length (LYSO + GSO + BGO) was fixed at 15 mm. The order of these crystal layers was also changed, so that totally we have considered 55 × 6 = 330 different configurations. Using three layer phoswich detector, a 25%-68% improvement in the sensitivity at central slice was found compared to the dual layer VISTA PET scanner, dependent on different detector configurations. In this study, among all posible configurations, detector length permutations with higher efficiency values (12 × 6 = 72 cases) were choosen to evaluate spatial resolution. The radial and tangential spatial resolutions were markedly improved for all studied different detector configurations compared to the VISTA PET scanner. Among all the possible selected detector configurations, LYSO (4 mm) + GSO (4 mm) + BGO (7 mm) and BGO (5 mm) + LYSO (5 mm) + GSO (5 mm) cases gave the best DOI radial and tangential resolutions for an energy threshold of 250 keV, respectively. In the LYSO (4 mm) + GSO (4 mm) + BGO (7 mm) scanner configuration, the radial resolution was kept below 1.154 mm, over 25 mm field of view (FOV). The tangential resolution variations were minimized to less than 1.029 mm, over 25 mm FOV, using the BGO (5 mm) + LYSO (5 mm) + GSO (5 mm) detector type. As a result, our new designed three layers phoswich detectors with excellent DOI resolution will lead to small animal PET scanners with higher sensitivity and uniform spatial resolution across the FOV.