Monte Carlo Simulations of the Current Obtained with Ionisation Chamber Detectors in Mixed Fields of Neutrons and Gammas

It needs no introduction that good measurements regarding BNCT dosimetry are of vital interest. Above all, calculations for patient treatment planning are initially based on these measurements. Closely related, well understood dosimetry of mixed neutron and gamma fields is necessary to explain the outcomes of the many experiments performed. It is believed that the sometimes confusing and incomprehensible outcomes in BNCT research are due to incorrect dosimetry, i.e. misleading measurements. A popular detector used to describe the absorbed neutron and gamma doses is the ionisation chamber. To understand better the behaviour and intricacies of this detector, the collected and measured current is directly simulated with MCNPX. This Monte Carlo code is able to track neutrons, gammas and electrons all around and in the ionisation chamber. The calculated dose deposited by the electrons in the gas is proportional to the current measured. Protons and alphas emanating from the wall and/or gas materials due to nuclear reactions can also cause ionisations and thus add to the current. A custom-made program has been written to simulate this contribution. The issue in this study is that a disagreement between simulated and measured current can be caused by the computer code and/or measurement set-up and/or unknown influences of source and/or materials. Therefore, the model of the ionisation chamber as well as the neutron and gamma source descriptions are validated step-by-step. After having obtained enough confidence in the model it can be concluded that ionisation chamber measurements can be significantly affected by neutron interactions (this is energy dependent). Neutrons can increase the measured current due to unknown and unconsidered beta-, proton- and/or alpha-producers in the wall material and gas; this dose component does not exist without the presence of the ionisation chamber.JRC.F.4-Safety of future nuclear reactor

Determination of the Neutron Capture Sensitivity of Ionisation Chambers Based on Neutron Capture Reaction Rates

An epithermal neutron beam from the High Flux Reactor is used for clinical BNCT at Petten. For the validation of the treatment planning simulations an accurate experimental determination of the different dose components in the mixed beam of neutrons and photons is important. Dosimetry is performed with paired ionisation chambers, among other techniques. Experiments have been made using Mg/Ar and TE/TE ionisation chambers. Calibration factors for both chambers have been obtained from measurements in a 60Co gamma-ray beam. Ionisation chambers are sensitive to neutron capture reactions in the structure of the chambers. This paper describes a novel approach to the correction for the parasitic sensitivity to neutron capture. The k’ values for ionisation chambers in epithermal neutron beams have been determined previously using thermal neutron flux values. This procedure neglects the contribution of the epithermal neutrons which are present in the low energy part of the intermediate-energy neutron spectrum. In this work k’ factors have been determined using reaction rate relations of neutron activation reactions.JRC.F.3-High Flux and Future Reactor

Simulation of MG-AR and TE-TE Ionisation Chambers with MCNPX in a Straightforward Gamma and Beta Irradiation Field

New methods are required for a better interpretation of the response of ionisation chambers in order to improve further the determination of the absorbed doses in a mixed neutron-gamma BNCT field. Simulations might help to understand in particular the behaviour of the sensitivity factors of an ionisation chamber in the mixed field. The study presented here is a continuation of previous work with the final aim to obtain validated computer models of the Mg-Ar and TE-TE ionisation chambers. With these two chambers the so-called paired ionisation chamber technique is performed by which the neutron and gamma dose components can be separated and determined. By knowing exactly the neutron and gamma source in a simulation, the chamber response can be investigated. However, the validation of a simulated ionisation chamber set-up, starting directly with the mixed neutron/gamma fields is too complicated regarding the number of possibilities that could cause a discrepancy between measurements and simulations. Therefore, two simplified and well-known irradiation fields are considered first and will be discussed in this paper: an existing 60Co calibration source and a 90Sr check source which has been designed and constructed to perform ionisation chamber stability measurements. Both irradiation sources as well as the two ionisation chambers are modelled with MCNPX. This code has been used because it is capable of simulating neutrons among many other types of particles and rays. The model of the two ionisation chambers is investigated comparing the measured charge collection rate when the detectors are exposed in the 60Co gamma-ray field and in the 90Sr beta field with the calculated results. For the 60Co experiments the calculations agree within 3% with the measured values. For the 90Sr source the simulated Mg-Ar charge is 8% higher than the measurement and the simulated TE-TE charge is 6% lower than the measured charge from the ionisation chamber.JRC.F.4-Safety of future nuclear reactor

Validating a MCNPX Model of Mg(Ar) and TE(TE) Ionisation Chambers Exposed to 60Co Gamma-rays

For an accurate determination of the absorbed doses in complex radiation fields (e.g. mixed neutron-gamma fields), a better interpretation of the response of ionisation chambers is required. This study investigates a model of the ionisation chambers using a different approach, analysing the collected charge per minute as a response of the detector instead of the dose. The MCNPX Monte Carlo code is used. In this paper, the model is validated using a well-known irradiation field only: a 60Co source. The detailed MCNPX models of a Mg(Ar) and TE(TE) ionisation chamber is investigated comparing the measured charge per minute obtained free-in-air and in a water phantom with the simulated results. The difference between the calculations and the measurements for the TE(TE) chamber is within +2% whereas for the Mg(Ar) chamber is around 17%. The systematic discrepancy in the case of Mg(Ar) chamber is expected to be caused by an overestimation of the sensitive volume.JRC.F.3-Energy securit

Feasibility Study into the Use of an Aluminum Ionization Chamber as a Gamma Dosimeter in a Mixed Neutron and Gamma-ray Field

Generally, the determination of the gamma-ray dose in a mixed neutron-gamma field is obtained by using ¿neutron-insensitive¿ detectors. For this purpose, graphite, magnesium and aluminum ionization chambers are available. It is known that graphite chambers suffer from porosity and magnesium chambers encounter oxidation and manufacturing problems. So far, the aluminum chamber is mostly applied in fast neutron fields. This study presents the results of an aluminum chamber, flushed with argon gas, when applied in a mixed neutron and gamma field. A computer model of the ionization chamber is developed for an accurate interpretation of the responses. Special interest is given to the charge that can be measured after the irradiation has stopped, which is due to decay of 28Al. Methods and Materials: The Monte Carlo code MCNPX is used to simulate the neutrons, gammas and charged particles in and around the Al-Ar chamber. The detector is modeled in detail and all possible reactions which can occur in the materials of the chamber are incorporated. The response of the Al-Ar chamber is compared with the results of a Mg-Ar one in terms of collected charge. Results: All individual components contributing to the signal of the detector are identified and calculated. Although the decay-charge produced by aluminum is much higher, in comparison to magnesium, a better estimation of the gamma dose is expected when the decay-charge in aluminum can be accurately determined. Another advantage is that the higher activation in Al can be used for identifying the neutron contribution. Despite the great detail in the model used, there is a ~25% discrepancy between the experimental and simulated total charges for both the Mg-Ar and Al-Ar chambers, which requires evidently further investigation. Conclusions: The Al-Ar chamber can be used complementary to the Mg-Ar chamber as gamma dosimeter in a mixed field of neutrons and gammas.JRC.F.4-Safety of future nuclear reactor

A Preliminary Inter-centre Comparison Study for Photon, Thermal Neutron and Epithermal Neutron Responses of Two Pairs of Ionisation Chambers Used for BNCT

The dual ionisation chamber technique is the recommended method for mixed field dosimetry of epithermal neutron beams. Its importance has been long recognised and it has featured highly in the dosimetry exchange programme of the MIT BNCT group. This paper presents initial data from an ongoing inter-comparison study involving two identical pairs of ionisation chambers used at the BNCT facilities of Petten, NL and of the University of Birmingham, UK. The goal of this study is to evaluate the photon, thermal neutron and epithermal neutron responses of both pairs of TE(TE) (Exradin T2 type) and Mg(Ar) (Exradin M2 type) ionisation chambers in similar experimental conditions. At this stage, the work has been completed for the M2 type chambers and is intended to be completed for the T2 type chambers in the near future. Photon calibration: The photon responses of the ionisation chambers were obtained in 6 and 10 MV clinical photon beams at the University Hospital Birmingham. Photon calibration factor ratios, in terms of dose to water (Petten/Birmingham) of 1.077 ± 0.006 and 1.029 ± 0.005 were found for the M2 and T2 type chambers, respectively. Thermal neutron response: The thermal neutron sensitivities of the M2 type ionisation chambers were determined using the thermal neutron beam available at the Low Flux Reactor, Petten. Ratios of the integrated charge measured for each chamber indicate a ratio (Petten/Birmingham) of 0.980 ± 0.007 for the M2 chambers. BNCT epithermal neutron beam: Measurements in a reference PMMA cubic phantom were performed using the M2 type ionisation chambers in the epithermal neutron beam of the High Flux Reactor, Petten. At a depth of 2.5 cm, a ratio of the integrated charge for the chambers yields a sensitivity ratio (Petten/Birmingham) of 0.985 ± 0.008.JRC.F.4-Safety of future nuclear reactor

A Preliminary Inter-centre Comparison Study for Photon, Thermal Neutron and Epithermal Neutron Responses of Two Pairs of Ionisation Chambers Used for BNCT

The dual ionisation chamber technique is the recommended method for mixed field dosimetry of epithermal neutron beams. This paper presents initial data from an ongoing inter-comparison study involving two identical pairs of ionisation chambers used at the BNCT facilities of Petten, NL and of the University of Birmingham, UK. The goal of this study is to evaluate the photon, thermal neutron and epithermal neutron responses of both pairs of TE(TE) (Exradin T2 type) and Mg(Ar) (Exradin M2 type) ionisation chambers in similar experimental conditions. At this stage, the work has been completed for the M2 type chambers and is intended to be completed for the T2 type chambers in the near future.JRC.F.4-Safety of future nuclear reactor

Irradiation of an Explanted Pig Liver at the HFR Petten

As part of a study between the University Hospital Essen and the JRC Petten to assess the feasibility to perform BNCT on an explanted organ (liver) at the HFR, a liver was taken from a pig in the operating theatre for animals at the Central Animal Facility of the Medical Faculty in Essen, and transported by car to Petten. On arrival 3 hours later, the liver was placed into the special PMMA holder and loaded into the custom-built Liver-Irradiation-Facility (LIF). Air supply provided by cold gun sprays gave a temperature of 5¿10 °C around the liver holder throughout the irradiation, which lasted 3 hours exactly. The liver was then brought back to Essen. It was noted that the liver was more radioactive than expected, in comparison to a patient irradiation. The measured radiation level directly following radiation was almost 200µSv/h on contact but after only about 15 minutes, halved to 100µSv/h, due to activated 24Na. The exercise established where improvements are needed, including: writing of Standard Operating Procedures; documentation files fulfilling the legal requirements for human irradiation; a treatment plan; better temperature control, including calibration of the cold guns; but also the need for ready availability of equipment, such as ice and cleansing material (tissue, alcohol, etc.). The overall exercise is one of the first of many procedures, i.e. testing of the transport logistics and the irradiation device (LIF), and should be seen as one of a number of steps needed prior to a full human treatment.JRC.F.7-Energy systems evaluatio

The Activation Contamination in the Metal-based Ionization Chambers as Gamma Dosimeters in the Mixed Filed Dosimetry

This study aims to determine the activation contamination in the metal-based ionization chambers, i.e. Mg(Ar)- and Al(Ar)-chambers. The contamination was estimated by directly measuring the signal caused by the radioactive nuclides after the irradiation. The INAA gamma ray spectra clearly showed that the ionization chambers were activated. The activation contamination could comprise more than 2% of the total measured current of the Mg(Ar)-chamber, and more than 50% of the Al(Ar)-chamber, both irradiated free-in-air at the THOR epithermal neutron beam. The activation contamination provides concrete evidence of the necessity of the thermal neutron sensitivity correction.JRC.F.6-Energy systems evaluatio

The Activation Contamination in the Metal-Based Ionization Chambers as Gamma Dosimeters in the Mixed Filed Dosimetry

This study aims to determine the activation contamination in the metal-based ionization chambers, i.e. Mg(Ar)- and Al(Ar)-chambers. The contamination was estimated by directly measuring the signal caused by the radioactive nuclides after the irradiation. The INAA gamma ray spectra clearly showed that the ionization chambers were activated. The activation contamination could comprise more than 2% of the total measured current of the Mg(Ar)-chamber, and more than 50% of the Al(Ar)-chamber, both irradiated free-in-air at the THOR epithermal neutron beam. The activation contamination provides concrete evidence of the necessity of the thermal neutron sensitivity correction.JRC.F.4-Safety of future nuclear reactor