V.R. Bom

Real-time prompt γ monitoring in spot-scanning proton therapy using imaging through a knife-edge-shaped slit.

In this paper we report on Monte Carlo simulations to investigate real-time monitoring of the track depth profile in particle therapy by measuring prompt gamma ray emissions: a high sensitivity imaging system employing a knife-edge-shaped slit combined with a position-sensitive gamma detector was evaluated. Calculations to test this new concept were performed for a head-sized software phantom. Clear spatial correlation is shown between the distribution of gamma rays detected with energies above 1.5 MeV and the distribution of prompt gamma rays emitted from the phantom. The number of neutrons originating from nuclear reactions in the phantom that are detected at these high energies is small. Most importantly it is shown that under common therapy conditions enough data may be collected during one spot-step (of the order of 10 ms) to locate the distal dose edge with a 1σ accuracy of better than 1 mm. This indicates that simple slit cameras have high potential for accurate real-time particle therapy adjustment and may become a practical way to improve particle therapy accuracy.

Experimental results and Monte Carlo simulations of a landmine localization device using the neutron backscattering method

Experiments were carried out to investigate the possible use of neutron backscattering for the detection of landmines buried in the soil. Several landmines, buried in a sand-pit, were positively identified. A series of Monte Carlo simulations were performed to study the complexity of the neutron backscattering process and to optimize the geometry of a future prototype. The results of these simulations indicate that this method shows great potential for the detection of non-metallic landmines (with a plastic casing), for which so far no reliable method has been found.

Comparative study of silicon detectors

We studied three different types of silicon sensors: PIN diodes, circular drift detectors, both made at the Delft University of Technology (DUT), and Hamamatsu S5345 avalanche photodiodes. Measurements have been carried out in the same optimized experimental setup, both at room temperature and at low temperatures. Comparison is made for direct X-ray detection and CsI(Tl) scintillation light readout.

Measurement of the spin and temperature dependence of ddµ molecule formation rate in solid and liquid deuterium

The ddµ molecule formation rate is experimentally measured for the two hyperfine states of the dµ-atom in the temperature range of 5–30 K. Results are consistent with a preliminary measurement by the TRIUMF group and contradict theoretical predictions. The work has been performed on the JINR phasotron (Dubna).

Land mine detection with neutron back scattering imaging using a neutron generator

The neutron back scattering technique may be used to search for low-metallic land mines. An advantage of this technique is the speed of detection: the scanning speed may be made comparable to that of a metal detector. The method is sensitive to soil moisture. A limitation of the method is therefore that the soil must be sufficiently dry. The neutrons are produced with a pulsed neutron generator. An image of the back scattered thermal neutron radiation is obtained with a two dimensional position sensitive detector. Getting optimal settings for the detector system is described. The mine detection time as function of the neutron pulse parameters is investigated. Results with various dummy mines are presented

A scintillating GEM for 2D-dosimetry in radiation therapy

The first results of a study on the properties of a gaseous scintillation detector based on a Gas Electron Multiplier (GEM) are reported. The detector is designed for use in position-sensitive dosimetry applications in radiation therapy. A double GEM system, operating in a 90 10% Ar-CO2 gas mixture at a gas amplification factor of similar to 3000, emits a sufficient amount of detectable light to perform measurements of similar to 1 Gy doses in two dimensions. The light yield does not suffer from quenching processes when particles with high stopping power are detected. This operation mode of GEMs offers the dosimetric advantages of a gas-filled detector and the 2D read-out can be performed with a CCD camera. Compared to the existing dosimeters, this system is relatively simple and no complex multi-electrode read-out is necessary

A semiconductor beta ray spectrometer

Abstract The design and construction of a beta spectrometer which uses a hyper-pure germanium crystal for energy determination is described. Due to the good energy resolution that is achieved, end points of continuous beta spectra can be measured accurately, even if the nuclides have short half-lives. The spectrometer is therefore especially suited for atomic mass determination in on-line mass separator experiments. A simple wire chamber is used to discriminate beta particles from gamma radiation. Disadvantages arise from the large amounts of scattered beta particles, deforming the continua; a method is described to minimize this scattering. The result of a first measurement is presented.

Comparison of pinhole collimator materials based on sensitivity equivalence

Pinhole SPECT often provides an excellent resolution sensitivity trade-off for radionuclide imaging compared to SPECT with parallel holes, particularly when imaging small experimental animals like rodents. High absorption pinhole materials are often chosen because of their low edge penetration and therefore good system resolution. Capturing more photons in the edges however results in decreased system sensitivity if the pinhole diameter remains the same, which may partly undo the beneficial effect on the resolution. In the search for an optimal trade-off we have compared pinhole projection data and reconstructed images of different materials with pinhole aperture diameters adjusted to obtain equal sensitivity. Monte Carlo calculations modeling the transmission, penetration and scattering of gamma radiation in single pinholes of uranium, gold, tungsten and lead were performed for a range of pinhole opening angles, diameters and gamma ray energies. In addition, reconstructed images of a hot rod phantom were determined for a multipinhole SPECT system and for a system that can image the 511 keV annihilation photons of positron emitting tracers with clustered pinholes. Our results indicate that, under the condition of equal sensitivity, tungsten and for SPECT also lead pinholes perform just as well as gold and uranium ones, indicating that a significant cost reduction can be achieved in pinhole collimator manufacturing while the use of rare or impractical materials can be avoided.

A LaF3 : Nd(10%) scintillation detector with microgap gas chamber read-out for the detection of γ-rays

Abstract A LaF 3 : Nd(10%) scintillator crystal has been placed in a microgap gas chamber to obtain a position-sensitive detector for γ-rays. Directly on the crystal a thin layer of nickel is evaporated, and on top of that a thin semi-transparant CsI photocathode. γ-rays absorbed in the scintillator crystal produce a UV-light flash, which liberates electrons in the photocathode. These electrons are multiplied and detected in the microgap chamber. By comparing the spectrum measured when this detector is irradiated with 511 keV γ-rays with a spectrum that is computed in a Monte Carlo simulation, it is concluded that the probability that a UV-light photon created in the scintillator produces a photo-electron in the photocathode, is about 2.5%.

DUNBLAD, the Delft University neutron backscatter landmine detector

The neutron backscattering technique may be applied to search for non-metallic land mines in relatively dry soils. A novel, ergonomic detector system has been constructed. Tests with real land mines in a realistic environment show that anti-tank mines can reliably be found, but that anti-personnel mines may escape detection. The performance could be improved when an image of the mine signal could be obtained. One approach is to use an array of position sensitive 3He detectors placed close to the soil. A first test with a pulsed neutron generator shows that further improvements can be made by applying a time window on the neutron transit time. The possibilities of neutron backscattering imaging systems are investigated using Monte Carlo simulations with GEANT. A neutron backscattering imaging device with a 2D sensitive detection plane is currently under development.