S.J.C. do Carmo

Experimental Study of the

An experimental study of the w -values and Fano factors for Ar-Xe gaseous mixtures at atmospheric pressures for 5.895 keV X-rays was carried out with two experimental set-ups based on gas proportional scintillation counter techniques. For the 100% Xe, Ar-40% Xe, Ar-20% Xe and Ar-10% Xe mixtures at 800 Torr the measured w-values were 21.61 eV, 21.63 eV, 21.64 eV and 21.42 eV, respectively, within an accuracy of plusmn0.21 eV. For the same mixtures the Fano factors were 0.254, 0.258, 0.250 and 0.256, respectively, within an accuracy of about plusmn0.004. For lower Xe concentrations of 5%, 3% and 1% the Fano factors were ap0.24, ap0.23 and ap0.22, respectively. These experimental results are compared with those available in the literature.

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A standard gas proportional scintillation counter equipped with a photomultiplier tube has been used to measure the primary scintillation yield in Xe at 800 Torr for 5.9 keV x-rays. The value m = 53?8 was measured for the number of primary scintillation photons produced per absorbed 5.9 keV x-ray photon, which gives a value Ws = 111?16 eV for the average energy expended to produce a primary scintillation photon in Xe for this x-ray energy. The experimental method is based on a comparison between the primary and secondary scintillation signals measured with a digital oscilloscope, where the distinction of the small primary signal from the noise is achieved by always triggering the oscilloscope with the secondary scintillation signal, and making a time average over 128 primary/secondary scintillation pulses. No evidence for recombination scintillation was found. A qualitative discussion of Ws in terms of the ratio between excitation and ionization cross sections for electrons in Xe is also presented.

-Values and Fano Factors of Gaseous Xenon and Ar-Xe Mixtures for X-Rays

An experimental study of the scintillation properties of Ar–Xe mixtures has been carried out with a gas proportional scintillation counter instrumented with a photomultiplier tube. Energy resolutions of 7.7, 7.8, 7.8, 8.3, 8.5, 9.1 and 11.5% were obtained for 5.9 keV X-rays respectively for the 100% Xe, 60%Ar–40%Xe, 80%Ar–20%Xe, 90%Ar–10%Xe, 95%Ar–5%Xe, 97%Ar–3%Xe and 99%Ar–1%Xe mixtures at 800 Torr. Thresholds for scintillation and ionization were also determined. The performance of Ar–Xe mixtures as detection media in large detection area applications has been studied for a 52 mm diameter photomultiplier and a 38 mm diameter radiation window. The curved grid technique was used to minimize spectra distortion due to solid angle effects.

Absolute primary scintillation yield of gaseous xenon under low drift electric fields for 5.9 keV X-rays

The number of cyclotrons capable of accelerating protons to about 20 MeV is increasing throughout the world. Originally aiming at the production of positron emission tomography (PET) radionuclides, some of these facilities are equipped with several beam lines suitable for scientific research. Radiobiology, radiophysiology, and other dosimetric studies can be performed using these beam lines. In this work, we measured the Bragg peak of the protons from a PET cyclotron using a stacked target consisting of several aluminum foils interleaved with polyethylene sheets, readout by in-house made transimpedance electronics. The measured Bragg peak is consistent with simulations performed using the SRIM/TRIM simulation toolkit. Furthermore, we report on experimental results aiming at measuring proton beam currents down to 10 pA using a thin aluminum foil (20-μm-thick). The aluminum was chosen for this task because it is radiation hard, it has low density and low radiation activity, and finally because it is easily available at negligible cost. This method allows for calculating the dose delivered to a target during an irradiation with high efficiency, and with minimal proton energy loss and scattering.

Ar–Xe mixtures and their use in curved grid gas proportional scintillation counters for X-rays

The performance of a gas proportional scintillation counter filled with either pure Xe or Ar-Xe mixtures is described for the 0.1-10 keV X-ray energy range. It is shown that the spectra tail distortion exhibited by Xe filled gaseous detectors for soft X-rays below 2 keV is reduced if Ar-Xe mixtures are used. The peak-to-valley ratio increases by a factor of about 2 or 3 as the Xe concentration is reduced from 100% to 5%. Moreover, the energy resolutions for such mixtures are similar to the ones for pure Xe.

On-line measurements of proton beam current from a PET cyclotron using a thin aluminum foil

A design for a gaseous radiation detector based on charge multiplication is described and its performance assessed. The device uses a microstructure which has an anode surrounded by another close-lying second anode, playing the role of a grid at a potential between anode and cathode that allows a more accurate definition of the multiplication volume. The main characteristics of this gridded microstrip gas chamber (G-MSGC) detector are described. The best energy resolutions for 5.9 keV X-rays obtained were 13.4% for Xe and 12.6% for the P10 mixture.

Performance of Argon-Xenon Mixtures in a Gas Proportional Scintillation Counter for the 0.1–10 keV X-Ray Region

An out-of-yoke irradiation setup using the proton beam from a cyclotron that ordinary produces radioisotopes for positron emission tomography (PET) has been developed, characterized, calibrated and validated. The current from a 20 μm thick aluminum transmission foil is readout by home-made transimpedance electronics, providing online dose information. The main monitoring variables, delivered in real-time, include beam current, integrated charge and dose rate. Hence the dose and integrated current delivered at a given instant to an experimental setup can be computer-controlled with a shutter. In this work, we report on experimental results and Geant4 simulations of a setup which exploits for the first time the 18 MeV proton beam from a PET cyclotron to irradiate a selected region of a target using the developed irradiation system. By using this system, we are able to deliver a homogeneous beam on targets with 18 mm diameter, allowing to achieve the controlled irradiation of cell cultures located in biological multi-well dishes of 16 mm diameter. We found that the magnetic field applied inside the cyclotron plays a major role for achieving the referred to homogeneity. The quasi-Gaussian curve obtained by scanning the magnet current and measuring the corresponding dose rate must be measured before any irradiation procedure, with the shutter closed. At the optimum magnet current, which corresponds to the center of the Gaussian, a homogenous dose is observed over the whole target area. Making use of a rotating disk with a slit of 0.5 mm at a radius of 150 mm, we could measure dose rates on target ranging from 500 mGy/s down to 5 mGy/s. For validating the developed irradiation setup, several Gafchromic® EBT2 films were exposed to different values of dose. The absolute dose in the irradiated films were assessed in the 2D film dosimetry system of the Department of Radiotherapy of Coimbra University Hospital Center with a precision better than 2%. In the future, we plan to irradiate small animals, cell cultures, or other materials or samples.

The Gridded-Microstrip Gas Chamber as a High Energy Resolution Gaseous X-Ray Detector

The performance of a gas proportional scintillation counter filled with either pure Xe or Ar-Xe mixtures is described for the 0.1-10 keV X-ray energy range. It is shown that the spectra tail distortion exhibited by Xe filled gaseous detectors for soft X-rays below 2 keV is reduced if Ar-Xe mixtures are used. The peak-to-valley ratio increases by a factor of about 2 or 3 as the Xe concentration is reduced from 100% to 5%. Moreover, the energy resolutions for such mixtures are similar to the ones for pure Xe.

Development of a PET cyclotron based irradiation setup for proton radiobiology

The number of cyclotrons capable of accelerating protons to about 20MeV is increasing throughout the world. Originally aiming at the production of positron emission tomography (PET) radionuclides, some of these facilities are equipped with several beam lines suitable for scientific research. Radiobiology, radiophysiology, and other dosimetric studies can be performed using these beam lines. In this work, we report on experimental results and SRIM/TRIM simulations, which aim at measuring the Bragg peak of the 18 -MeV proton beam from the PET cyclotron. By using a piece of plastic scintillator and a D-link Ethernet-based camera, we measured the proton beam range and width with a spatial resolution of 0.1mm. The relation between the intensity of the scintillation light, which was generated by irradiation with the proton beam, and the cyclotron stripper current shows good linearity below 0.6 μA. In addition, we plotted the stability of the proton beam range within the scintillator block. The ranges of the proton beam in the plastic scintillator were used to assess its energy after trespassing one or two Havar windows and either a 40-cm-long or a 2.4-m-long aluminum pipe. The initial energy of the proton beam from the PET cyclotron was found to be 18MeV. The dose calculation follows immediately by taking into consideration the total beam current delivered on a target, which was measured by means of a thin aluminum foil trespassed by the beam and readout by in-house made transimpedance amplifiers.

Performance of Argon-Xenon mixtures in a gas proportional scintillation counter for the 0.1-10 keV X-ray region

Haltern 70 amphora sherds obtained from Castro do Vieito were studied by Mossbauer spectroscopy, X-ray diffraction and X-ray fluorescence. In all our samples non magnetic Fe3+ haematite species were found. In some samples, also Fe2+ was found. We conclude that the samples were fired under changing atmosphere, air being admitted at the end of the firing cycle following firing in a reducing atmosphere.