Dosimetry Of Neutron Beams Research Articles

Effects of ionization chamber orientation in photon and neutron beams

Boron neutron capture therapy (BNCT) is a biologically targeted radiotherapy modality that utilizes a neutron beam. Ionization chambers are used in neutron beam dosimetry which is part of the commissioning process and periodic quality assurance. Ionization chambers used in BNCT are a bit larger in size compared to conventional external beam radiotherapy counterparts and a fixed horizontal beam direction poses its challenges among other considerations. Because of this, more flexible ionization chamber positioning would be preferred even in a large water phantom. In this work, we explain the effects and uncertainties that depend on chamber orientation. We compared depth dose curves measured with Exradin T2 and M2 ion chambers with two radiation sources, 6X photon and epithermal neutron beam, and in two orientations; one where the ionization chamber rod was positioned in perpendicular and the other where it was positioned in parallel orientation compared to the beam central axis. The perpendicular orientation is the reference orientation used in calibration. However, the parallel orientation allowed us to measure depth dose closer to the surface and the measurements had smaller systematic uncertainties because of a more straightforward alignment process. We can convert the data between the two different orientations with two simple correction parameters; correction factor which adjusted amplitude (k) and shift which adjusted the change of the effective point position in depth direction (Δ). All correction factor results were within 1.000±0.015 and all shift results were smaller or the same order of magnitude compared to the positioning accuracy, 0.05mm - 0.5mm. Even without the usage of corrections, the measurements were already almost in agreement. Because of this, we would recommend taking into consideration 2% additional uncertainty if measurements are performed in parallel orientation and no such correction parameters are used.

Spectrometry and dosimetry of neutron beams produced by 7Li (p,n) reactions in the proton energy range of 3–5 MeV

Abstract Neutron spectrometry was carried out with a state of art neutron spectrometer system named ROSPEC based on multiple proportional counters, for neutron beams generated by bombarding proton beams on a 7Li target with projectile energy in the range of 3–5 MeV. Along with the neutron spectra, neutron fluence rates and ambient dose equivalent rates at the position of the detector were also measured. Neutron spectra measured by ROSPEC were compared with theoretically simulated neutron spectra. The neutron peak energy correspond to n o group of neutrons was found to be within 10%–15% of the expected peak values (theoretical values). The study also determines the neutron fluence rate and ambient dose equivalent rates per unit beam current for neutron energy below and above 50 keV for the studied proton beams energies. The characterization of the quasi mono-energetic neutron beams could be useful for the purpose of radiation protection of the users of the beam line, Boron neutron Capture Therapy (BNCT), and study of response characteristics of the neutron detectors.

Thermal neutron detector based on COTS CMOS imagers and a conversion layer containing Gadolinium

In this work we will introduce a novel low cost position sensitive thermal neutron detection technique, based on a Commercial Off The Shelf CMOS image sensor covered with a Gadolinium containing conversion layer. The feasibility of the neutron detection technique implemented in this work has been experimentally demonstrated. A thermal neutron detection efficiency of 11.3% has been experimentally obtained with a conversion layer of 11.6 μm. It was experimentally verified that the thermal neutron detection efficiency of this technique is independent on the intensity of the incident thermal neutron flux, which was confirmed for conversion layers of different thicknesses. Based on the experimental results, a spatial resolution better than 25 μm is expected. This spatial resolution makes the proposed technique specially useful for neutron beam characterization, neutron beam dosimetry, high resolution neutron imaging, and several neutron scattering techniques.

Study of the response of phenol compounds exposed to thermal neutrons beams for Electron Paramagnetic Resonance dosimetry

This paper reports the results regarding a new organic compound (IRGANOX® 1076 phenols) with and without low content (5% by weight) of gadolinium oxide (Gd2O3) for Electron Paramagnetic Resonance (EPR) dosimetry of neutron beams. The dependence of EPR signal as function of neutron dose was investigated in the fluence range studied between 1011cm−2 to 1014cm−2. We evaluated also the effect of gadolinium on 60Co gamma photon sensitivity of this organic compound. Our analysis showed that a low concentration of gadolinium oxide (of the order of 5% of the total mass of the dosimeter) can enhance the thermal neutron sensitivity more than 10 times with a small reduction of photon tissue equivalence. The free radicals produced after irradiation of photons and neutrons are stable for more than first 300 h after irradiation. The presence of additives does not substantially modify the fading of the EPR signal induced by photons and neutrons in the first 15 days after exposure.

Wedge factor dependence with depth and field size for fast neutron beams

The dependence of the wedge factors (WFs) on field size (FS) and depth for a fast neutron beam has been investigated. In a previous study (Popescu et al 1999 Med. Phys. 26 541), a method was presented that allows a simple and accurate way of calculating the wedge-factor dependence on FS and depth in the case of a photon beam. The validity of a similar approach is tested in the present study for neutron beam dosimetry. The clinical neutron therapy system at the University of Washington (UW) has a flattening filter assembly consisting of two filters: a small field filter and a large field filter. Despite this complication, the approach presented in Popescu et al (1999 Med. Phys. 26 541) can be used to describe the WF dependence on FS and depth (d).

Use of low-pressure tissue equivalent proportional counters for the dosimetry of neutron beams used in BNCT and BNCEFNT.

The absorbed dose in a phantom or patient in boron neutron capture therapy (BNCT) and boron neutron capture enhanced fast neutron therapy (BNCEFNT) is deposited by gamma rays, neutrons of a range of energies and the 10B reaction products. These dose components are commonly measured with paired (TE/Mg) ion chambers and foil activation technique. In the present work, we have investigated the use of paired tissue equivalent (TE) and TE+ l0B proportional counters as an alternate and complementary dosimetry technique for use in these neutron beams. We first describe various aspects of counter operation, uncertainties in dose measurement, and interpretation of the data. We then present measurements made in the following radiation fields: An epithermal beam at the University of Birmingham in the United Kingdom, a d(48.5) + Be fast neutron therapy beam at Harper Hospital in Detroit, and a 252Cf radiation field. In the epithermal beam, our measured gamma and neutron dose rates compare very well with the values calculated using Monte Carlo methods. The measured 10B dose rates show a systematic difference of approximately 35% when compared to the calculations. The measured neutron+gamma dose rates in the fast neutron beam are in good agreement with those measured using a calibrated A-150 TEP (tissue equivalent plastic) ion chamber. The measured 10B dose rates compare very well with those measured using other methods. In the 252Cf radiation field, the measured dose rates for all three components agree well with other Monte Carlo calculations and measurements. Based on these results, we conclude that the paired low-pressure proportional counters can be used to establish an independent technique of dose measurement in these radiation fields.

Use of SSNTDs in neutron beam dosimetry

A new Biological Irradiation Facility (BIF) has been constructed at the Budapest Research Reactor Center. The 10 cm diameter beam is led through a variable filter and collimator system, providing different neutron spectra from the nearly pure fission one to a well moderated, 1/E spectrum. To use the irradiator for biological irradiation and for investigating the response of neutron detector materials one must know the neutron spectrum, the beam intensity and profile in advance, and moreover, each irradiation should be adequately monitored to be able to correct the dose conversion factors against the burnup of the fuel, change in the reactor thermal power etc. For the purpose, a set of track detectors were developed. All the detectors are chemically etched and evaluated by a new image analyzer system called VIRGINIA (also presented on this conference). Detectors developed are: CR-39 (TASTRAK) track detectors with proton converter (polyethylene, PE) turning this combination into a threshold detector when applying thin gold foils as proton energy degraders; fission track detectors using depleted uranium and thorium with LEXAN polycarbonate and CR-39 using the 6Li and 10B contents of commercially available TLD materials as alpha converters. We present the basic fundamentals of the proton recoil detector, including the response calculation model.

Concluding Remarks - Present Status and Future in Neutron Beam Dosimetry

Concluding Remarks - Present Status and Future in Neutron Beam Dosimetry

Concluding Remarks - Present Status and Future in Neutron Beam Dosimetry

Abstract

Dosimetry of d(14)+Be neutrons with the two-peak method of LiF TLD-700

The utility of LiF TLD-700 for the dosimetry of neutron beams are assessed. The fact that for in-phantom irradiations the 200 degrees C peak of TLD-700 has a much lower sensitivity to neutrons than to gamma rays and the higher temperature peak has comparable sensitivities to both radiations is of relevance from the point of view of the use of the two-peak method for the dosimetry of therapeutically used fast-neutron beams. The difference in the neutron sensitivities of the two peaks of TLD-700 is greater than that of the peaks of any other TLD including CaF:Tm. Since the gamma ray absorbed dose content in cyclotron-produced neutron beams is small, the supralinearity of the high-temperature glow peak does not pose a serious problem. Also by using thin ribbons, slow heating rates (about 3.5 degrees C s-1) during the TL readout and by applying predetermined individual calibration factors for each peak of each detector, the intensities of the glow peaks could be measured with a reasonable precision.

European protocol for neutron dosimetry for external beam therapy

A common basis for neutron dosimetry will be one of the essential requirements for the comparison of clinical results obtained at the increasing number of fast-neutron radiotherapy facilities. Co-operation between the physicists involved in fast-neutron radiotherapy has been established in Europe by the High LET Therapy Project Group under the sponsorship of the European Organization for Research on Treatment of Cancer, EORTC. In September 1978, the clinical group decided that the physicists should intensify their contacts and organize a formal co-operation. For this purpose a European Clinical Neutron Dosimetry Group, abbreviation ECNEU, was established. In the United States all groups involved in or planning fast neutron therapy have directly or indirectly participated in the Fast Neutron Beam Dosimetry Physics Task Group 18 of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM). Protocols for neutron dosimetry for radiotherapy have been drafted for the European ...

Fast neutron beam dosimetry: comparison of the ion chamber and proportional counter approaches

Data were presented comparing the absorbed dose in tissue exposed to beams of fast neutrons as measured by ion chambers and tissue-equivalent proportional counters. The fast neutron beams studied include 14 MeV monoenergetic beams and those generated by high energy deuterons incident on beryllium. The ion chamber approaches include the tissue-equivalent plastic, tissue-equivalent gas and polyethylene wall-ethylene gas systems. The proportional counters were of the tissue-equivalent wall-tissue-equivalent gas type. Sources of uncertainty were discussed, including values of stopping power ratios, W for the gases, differences in atomic composition between the wall and the gases, and separation of the photon component of the total dose.

Neutron beam dosimetry at the NRL cyclotron

A 35 MeV deuteron beam impinging upon a thick Be target is being employed to generate a neutron beam for radiobiological experiments of relevance to later possible fast neutron therapy trials. The primary calibration of the beam has been based upon tissue-equivalent plastic ionization chambers, calibrated in turn with 60Co gamma -rays. CaF2:Mn and 7LiF (TLD-700) thermoluminescent dosemeters have been employed to determine the gamma -ray dose component in the neutron beam, by a method depending upon the ratio of fast neutron sensitivities of the two phosphors.