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R F Laitano

Publications and source records attributed to R F Laitano.

11 recordsLinked to original sources
PubMed2006

Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse.

The correction for charge recombination was determined for different plane-parallel ionization chambers exposed to clinical electron beams with low and high dose per pulse, respectively. The electron energy was nearly the same (about 7 and 9 MeV) for any of the beams used. Boag's two-voltage analysis (TVA) was used to determine the correction for ion losses, k(s), relevant to each chamber considered. The presence of free electrons in the air of the chamber cavity was accounted for in determining k(s) by TVA. The determination of k(s) was made on the basis of the models for ion recombination proposed in past years by Boag, Hochhäuser and Balk to account for the presence of free electrons. The absorbed dose measurements in both low-dose-per-pulse (less than 0.3 mGy per pulse) and high-dose-per-pulse (20-120 mGy per pulse range) electron beams were compared with ferrous sulphate chemical dosimetry, a method independent of the dose per pulse. The results of the comparison support the conclusion that one of the models is more adequate to correct for ion recombination, even in high-dose-per-pulse conditions, provided that the fraction of free electrons is properly assessed. In this respect the drift velocity and the time constant for attachment of electrons in the air of the chamber cavity are rather critical parameters because of their dependence on chamber dimensions and operational conditions. Finally, a determination of the factor k(s) was also made by zero extrapolation of the 1/Q versus 1/V saturation curves, leading to the conclusion that this method does not provide consistent results in high-dose-per-pulse beams.

Calibration↗
PubMed2002

Determination of the Kwall correction factor for a cylindrical ionization chamber to measure air-kerma in 60Co gamma beams.

The factor Kwall to correct for photon attenuation and scatter in the wall of ionization chambers for 60Co air-kerma measurement has been traditionally determined by a procedure based on a linear extrapolation of the chamber current to zero wall thickness. Monte Carlo calculations by Rogers and Bielajew (1990 Phys. Med. Biol. 35 1065-78) provided evidence, mostly for chambers of cylindrical and spherical geometry, of appreciable deviations between the calculated values of Kwall and those obtained by the traditional extrapolation procedure. In the present work an experimental method other than the traditional extrapolation procedure was used to determine the Kwall factor. In this method the dependence of the ionization current in a cylindrical chamber was analysed as a function of an effective wall thickness in place of the physical (radial) wall thickness traditionally considered in this type of measurement. To this end the chamber wall was ideally divided into distinct regions and for each region an effective thickness to which the chamber current correlates was determined. A Monte Carlo calculation of attenuation and scatter effects in the different regions of the chamber wall was also made to compare calculation to measurement results. The Kwall values experimentally determined in this work agree within 0.2% with the Monte Carlo calculation. The agreement between these independent methods and the appreciable deviation (up to about 1%) between the results of both these methods and those obtained by the traditional extrapolation procedure support the conclusion that the two independent methods providing comparable results are correct and the traditional extrapolation procedure is likely to be wrong. The numerical results of the present study refer to a cylindrical cavity chamber like that adopted as the Italian national air-kerma standard at INMRI-ENEA (Italy). The method used in this study applies, however, to any other chamber of the same type.

Calibration↗
PubMed2000

Proton stopping powers averaged over beam energy spectra.

Stopping powers averaged over the proton energy spectra at various depths in water and a tissue-like material were calculated for proton beams with initial energies between 50 and 250 MeV. The analysis was made for proton beams with a spread-out Bragg peak (SOBP) to assess in particular the variations of stopping power with depth in the SOBP plateau region. Nuclear interactions were accounted for in determination of the proton energy spectra. In modulated beams, stopping power varies considerably with depth along the plateau region of the spread-out peak. Moreover, stopping powers at the same water depth may differ from each other by up to 20% if referring to modulated or non-modulated beams. Calculations of water-air mass stopping power ratios were also performed, with and without the inclusion of nuclear interactions, for modulated and non-modulated beams. The stopping power ratios do not depend significantly on proton energy, and this makes the effect of accounting for nuclear interactions in proton fluence calculation negligible. For the same reason the stopping power ratios for modulated beams do not appreciably differ from those referred to non-modulated beams with the same Emax at the same depth in water. The present results also show that the dependence of stopping power on energy spread and spatial divergence for proton beams is not negligible in some conditions. Some conclusions are drawn on the possibility of using the residual range as a descriptor of the proton beam quality for different experimental beam conditions.

Algorithms↗
PubMed1996

Characteristics of the absorbed dose to water standard at ENEA.

The primary standard of absorbed dose to water established at ENEA for the Co-60 gamma-ray quality is based on a graphite calorimeter and an ionometric transfer system. This standard was recently improved after a more accurate assessment of some perturbation effects in the calorimeter and a modification of the water phantom shape and size. The conversion procedure requires two corresponding depths, one in graphite and one in water, where the radiation energy spectra must be the same. The energy spectra at the corresponding points were determined by a Monte Carlo simulation in water and graphite scaled phantoms. A thorough study of the calorimeter gap effect corrections was also made with regard to their dependence on depth and field size. A comparison between the ionization chamber calibration procedures based on the standards of absorbed dose to water and of air kerma was also made, confirming the consistency of the two methods.

Calorimetry↗
PubMed1995

Experimental determination of the beam quality dependence factors, kQ, for ionization chambers used in photon and electron dosimetry.

Dosimetry in radiotherapy with ionization chambers calibrated in 60Co gamma beams in terms of absorbed dose to water, DW, can be performed if a factor conventionally denoted as kQ is known. The factor kQ depends on the beam quality and the chamber characteristics. Calculated values of the kQ factors for many types of ionization chamber have been recently published. In this work the experimental determination of the kQ factors for various ionization chambers was performed for 6 MV and 15 MV photon beams and for a 14 MeV electron beam. The kQ factors were determined by a procedure based on relative measurements performed with the ionization chamber and ferrous sulphate solution in 60Co gamma radiation and accelerator beams, respectively. The experimental kQ values are compared with the calculated values so far published. Theoretical and experimental kQ values are in fairly good agreement. The uncertainty in the experimental kQ factors determined in this work is less than about 1%, that is, appreciably smaller than the uncertainty of about 1.5% reported for the calculated values.

Cobalt Radioisotopes↗
PubMed1993

Recent results (1986-1989) on the ENEA quality assurance programme for the radioactivity surveillance network in Italy.

A programme for the Quality Assurance of the Italian network for surveillance of radioactivity in foodstuff and environmental samples was started by the ENEA in 1983. The main purpose of this programme is to establish uniform levels of accuracy and reproducibility in measurement procedures routinely used by the laboratories belonging to the network. This paper describes the technical aspects relevant to the calibration and intercomparison campaigns carried out, in Italy, in the period 1986-1989. The relevant measurements include gamma spectrometry and beta-ray counting in liquid aqueous samples.

Calibration↗
PubMed1988

Activity measurements of Sr-90 and Cs-137 in spiked solutions of milk. Results of a national intercomparison in Italy.

A national intercomparison was organized by ENEA for activity measurements of 90Sr and 137Cs in spiked solutions of milk. The participants in the intercomparison were the laboratories belonging to the Italian network for control of radioactivity in the environment. The purpose of these measurements was to check the reliability of measurement procedures and experimental means as routinely used by each participant. This first intercomparison showed appreciable discrepancies among the participants and this was due both to improper procedures and non correct calibrated spectrometers. Soon after a second intercomparison was carried out to check to what extent the previous results were reproducible among the various participants. Methods and results are discussed.

Animals↗
PubMed1987

Diagnostic x-ray spectra measurements using a silicon surface barrier detector.

A silicon surface barrier detector is used to analyse diagnostic x-ray spectra. This detector, usually employed to detect charged particles, has a very low efficiency for x-ray. This characteristic is advantageous in overcoming experimental problems caused by the high fluence rates typical of diagnostic x-ray beams. The pulse height distribution obtained with silicon surface barrier detectors is very different from the true photon spectra because of the presence of escaped Compton photons and the fact that detection efficiency falls abruptly when photon energy increases. A detailed analysis of the spurious effects involved in detection is made by applying a Monte Carlo method. A stripping procedure is described for implementation on a personal computer. The validity of this method is finally tested by comparison with the experimental results obtained with a Ge detector. The spectra obtained with the Si detector are in fairly good agreement with the analogous spectra measured with a Ge detector. The advantages of using Si as opposed to Ge detectors in x-ray spectrometry can be summarised as: its simplicity of use, its greater economy for use in routine diagnostic x-ray spectroscopy and the possibility that the stripping procedure can be implemented on a personal computer.

Radiography↗
PubMedDate not supplied

Determination of x-ray spectra and of the scattered component up to 300 kV.

Several x-ray spectra, including those of the ISO reference radiations, were measured by 2-hp Ge detectors. Measurements were carried out in different experimental conditions with regard to detector size, beam collimation, and SDD. A stripping procedure to improve the spectrum analysis was developed on the basis of a detailed evaluation (by means of a Monte Carlo method) of the detector's spurious effects. These effects include K-photon escape, Compton photon escape, electron escape, and collimation effect. The stripping procedure also allows us to determine directly the spectra of possible scattered radiation reaching the detector in addition to the primary beam. When the primary beam is heavily filtered, the leakage radiation from the x-ray tube housing scattered onto the detector may not be negligible even when the x-ray tube is provided with appreciable shielding. Possible practical consequence of these effects are discussed. The results obtained on the ISO x-ray spectra are in agreement with previous determinations. The advantage of the present procedure is its more immediate applicability to Ge detectors of any size and with different beam collimation conditions.

Mathematics↗