The influence of the type of filling gas on the response of ionisation chambers to a mixed high-energy radiation field

Radiation protection dosimetry in radiation fields behind the shielding of high-energy accelerators such as CERN is a challenging task and the quantitative understanding of the detector response used for dosimetry is essential. Measurements with ionisation chambers are a standard method to determine absorbed dose (in the detector material). For applications in mixed radiation fields, ionisation chambers are often also calibrated in terms of ambient dose equivalent at conventional reference radiation fields. The response of a given ionisation chamber to the various particle types of a complex high-energy radiation field in terms of ambient dose equivalent depends of course on the materials used for the construction and the chamber gas used. This paper will present results of computational studies simulating the exposure of high-pressure ionisation chambers filled with different types of gases to the radiation field at CERN's CERN-EU high-energy reference field facility. At this facility complex high-energy radiation fields, similar to those produced by cosmic rays at flight altitudes, are produced. The particle fluence and spectra calculated with FLUKA Monte Carlo simulations have been benchmarked in several measurements. The results can be used to optimise the response of ionisation chambers for the measurement of ambient dose equivalent in high-energy mixed radiation fields.     

用于同步辐射X射线注量绝对测量的自由空气电离室

根据X射线束光子通量、注量和空气比释动能等物理量的关系,通过空气比释动能的测量,转换确定了所需要的光子通量、注量等。依据X射线自由空气电离室原理,针对同步辐射光子束的物理条件,确定了自由空气电离室的设计方案并完成了精密加工和安装调试。电离室主要部件尺寸公差、机械性能和电性能满足计量标准的要求,为计量标准的建立提供了基础。     

Absorbed dose measurements of a handheld 50 kVP X-ray source in water with thermoluminescence dosemeters

Absorbed dose rate measurements of a 50 kV(p) handheld X-ray probe source in a water phantom are described. The X-ray generator is capable of currents of up to 40 microA, and is designed for cranial brachytherapy and intraoperative applications with applicators. The measurements were performed in a computer-controlled water phantom in which both the source and the detectors are mounted. Two different LiF thermoluminescence dosemeter (TLD) phosphors were employed for the measurements, MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P). Two small ionisation chambers (0.02 and 0.0053 cm(3)) were also employed. The TLDs and chambers were positioned in watertight mounts made of water-equivalent plastic. The chambers were calibrated in terms of air-kerma rate, and conventional protocols were used to convert the measurements to absorbed dose rate. The TLDs were calibrated at National Institute of Standards and Technology (NIST) in terms of absorbed dose rate using a (60)Co teletherapy beam and narrow-spectrum X-ray beams. For the latter, absorbed dose was inferred from air-kerma rate using calculated air-kerma-to-dose conversion factors. The reference points of the various detectors were taken as the center of the TLD volumes and the entrance windows of the ionisation chambers. Measurements were made at distances of 3-45 mm from the detector reference point to the source center. In addition, energy dependence of response measurements of the TLDs used was made using NIST reference narrow spectrum X-ray beams. Measurement results showed reasonable agreement in absorbed dose rate determined from the energy dependence corrected TLD readings and from the ionisation chambers. Volume averaging effects of the TLDs at very close distances to the source were also evident.     

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 (60)Co 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 +7%. The systematic discrepancy in the case of Mg(Ar) chamber is expected to be caused by an overestimation of the sensitive volume.     

Simulation and measurements of the response of an air ionisation chamber exposed to a mixed high-energy radiation field

CERN's radiation protection group operates a network of simple and robust ionisation chambers that are installed inside CERN's accelerator tunnels. These ionisation chambers are used for the remote reading of ambient dose rate equivalents inside the machines during beam-off periods. This Radiation Protection Monitor for dose rates due to Induced Radioactivity ('PMI', trade name: PTW, Type 34031) is a non-confined air ionisation plastic chamber which is operated under atmospheric pressure. Besides its current field of operation it is planned to extend the use of this detector in the Large Hadron Collider to measure radiation under beam operation conditions to obtain an indication of the machine performance. Until now, studies of the PMI detector have been limited to the response to photons. In order to evaluate its response to other radiation components, this chamber type was tested at CERF, the high-energy reference field facility at CERN. Six PMI detectors were installed around a copper target being irradiated by a mixed hadron beam with a momentum of 120 GeV c(-1). Each of the chosen detector positions was defined by a different radiation field, varying in type and energy of the incident particles. For all positions, detailed measurements and FLUKA simulations of the detector response were performed. This paper presents the promising comparison between the measurements and simulations and analyses the influence of the different particle types on the resulting detector response.     

Ionisation chamber containing boron as a neutron detector in medical accelerator fields

A combination of the recombination principle of H(10) measurements with the use of the ionisation chambers containing boron has been presented, in order to increase the relative sensitivity of the chamber to neutrons by a factor close to the radiation quality factor of photoneutrons. Three types of the chambers were investigated. Two of them were filled with BF(3) and the third one contained electrodes covered with B(4)C. All the chambers were placed in paraffin moderators. The response of the chambers was investigated, depending on gas pressure and polarising voltage. The results showed that it was possible to obtain nearly the same response of the chamber to H(10) for photons and neutrons in a restricted energy range; however, further investigations are needed to make an optimum design.     

Stability of A-150 plastic ionisation chamber response over a approximately 30-y period

At the Northern Illinois University Institute for Neutron Therapy at Fermilab, the clinical tissue-equivalent ionisation chamber response is measured every treatment day using a cesium source that was configured to match readings obtained at the National Bureau of Standards. Daily measurements are performed in air using the air-to-tissue dose conversion factors given in AAPM Report #7. The measured exposure calibration factors have been tabulated and graphed as a function of time from 1978 to present. For A-150 plastic ionisation chambers, these factors exhibit a sinusoidal variation with a period of approximately 1 y and amplitude of +/- 1%. This variation, attributable to the hygroscopic nature of A-150 plastic, is correlated with the relative humidity of the facility, and is greater than the humidity corrections for gas described in the literature. The data suggest that chamber calibration should be performed at least weekly to accommodate these variations.     

Limitations of silicon diodes for clinical electron dosimetry

This work investigates the relevance of several factors affecting the response of silicon diode dosemeters in depth-dose scans of electron beams. These factors are electron energy, instantaneous dose rate, dose per pulse, photon/electron dose ratio and electron scattering angle (directional response). Data from the literature and our own experiments indicate that the impact of these factors may be up to +/-15%. Thus, the different factors would have to cancel out perfectly at all depths in order to produce true depth-dose curves. There are reports of good agreement between depth-doses measured with diodes and ionisation chambers. However, our measurements with a Scantronix electron field detector (EFD) diode and with a plane-parallel ionisation chamber show discrepancies both in the build-up and in the low-dose regions, with a ratio up to 1.4. Moreover, the absolute sensitivity of two diodes of the same EFD model was found to differ by a factor of 3, and this ratio was not constant but changed with depth between 5 and 15% in the low-dose regions of some clinical electron beams. Owing to these inhomogeneities among diodes even of the same model, corrections for each factor would have to be diode-specific and beam-specific. All these corrections would have to be determined using parallel plane chambers, as recommended by AAPM TG-25, which would be unrealistic in clinical practice. Our conclusion is that in general diodes are not reliable in the measurement of depth-dose curves of clinical electron beams.     

Angle dependence of signal currents from cylindrical ionisation chambers

A signal current from a cylindrical ionisation chamber with an ionisation volume of 62.7 cm3, 40 mm in diameter and 50 mm long, peaked when the chamber was lixed at 0 degrees and at 90 degrees in 137Cs and 60Co gamma ray fields for source-chamber distances of 1 m and 2 m. A smaller ionisation chamber showed a small peak at 0 degrees in both fields but not at 90 degrees. However, calculations indicated that the signal current from the smaller chamber would also show a peak at 90 degrees in a 137Cs point-source gamma ray field. Peaks occur because gamma rays attenuate along the cylindrical side wall or along the end walls when a chamber is tilted slightly from 0 degrees or 90 degrees and the direction of the gamma ray beam agrees with the plane of one of these walls. These facts suggest the need for care in the common practice of measuring and calculating responses for cylindrical ionisation chambers fixed perpendicular to gamma ray beams.     

Method for determination of gamma and neutron dose components in mixed radiation fields using a high-pressure recombination chamber

A new method is proposed for the determination of dose components in mixed radiation fields (gamma + neutrons) using a recombination chamber. The method involves the determination of the ratio of ionisation currents measured at two different voltages applied to the chamber without the need of determining the saturation current, neither in the radiation field investigated nor during calibration. Therefore, the chamber can be filled with a gas under a pressure much higher than that used in presently available recombination chambers. This paper presents theoretically derived formulae supporting the method and the experimental results of dose component measurements using a high-pressure recombination chamber filled with methane. The method can be used for determining neutron and gamma dose components in the environment, especially in the vicinity of nuclear centres.     

Field calibration studies for ionisation chambers in mixed high-energy radiation fields

The monitoring of ambient doses at work places around high-energy accelerators is a challenging task due the complexity of the mixed stray radiation fields encountered. At CERN, mainly Centronics IG5 high-pressure ionisation chambers are used to monitor radiation exposure in mixed fields. The monitors are calibrated in the operational quantity ambient dose equivalent H*(10) using standard, source-generated photon- and neutron fields. However, the relationship between ionisation chamber reading and ambient dose equivalent in a mixed high-energy radiation field can only be assessed if the spectral response to every component and the field composition is known. Therefore, comprehensive studies were performed at the CERN-EU high-energy reference field facility where the spectral fluence for each particle type has been assessed with Monte Carlo simulations. Moreover, studies have been performed in an accessible controlled radiation area in the vicinity of a beam loss point of CERN's proton synchrotron. The comparison of measurements and calculations has shown reasonable agreement for most exposure conditions. The results indicate that conventionally calibrated ionisation chambers can give satisfactory response in terms of ambient dose equivalent in stray radiation fields at high-energy accelerators in many cases. These studies are one step towards establishing a method of 'field calibration' of radiation protection instruments in which Monte Carlo simulations will be used to establish a correct correlation between the response of specific detectors to a given high-energy radiation field.     

The Determination of the Neutron Sensitivity of Suspensions of Chambers for Transport Nuclear Reactor Control and Protection Systems

A method of determining the neutron sensitivity of ionisation chambers and suspensions of ionisation chambers, used in transport nuclear-power plant control and protection systems is considered. Measured values of the sensitivity for type KNK-53M and KNK-16 ionisation chambers are given.     

Simulation studies on a prototype ionisation chamber for measurement of personal dose equivalent, Hp(10)

A prototype ionisation chamber for direct measurement of the personal dose equivalent, Hp(10), similar to the one developed by the Physikalisch-Technische Bundesantalt (PTB), was designed and constructed by the Metrological Laboratory of Ionizing Radiation (LMRI) of Nuclear and Technological Institute (ITN). Tests already performed have shown that the behaviour of this chamber is very similar to the PTB chamber, mainly the energy dependence for the X-ray radiation qualities of the ISO 4037-1 narrow series N-30, N-40, N-60, N-80, N-100 and N-120 and also for gamma radiation of 137Cs and 60Co. However, the results obtained also show a dependence on the energy and angles of incident radiation and a low magnitude of the electrical response of the ionisation chamber. In order to optimise the performance of the chamber, the LMRI initiated numerical simulation of this ionisation chamber by Monte Carlo method using the MCNPX code.     

A toolkit for epithermal neutron beam characterisation in BNCT

Methods for dosimetry of epithermal neutron beams used in boron neutron capture therapy (BNCT) have been developed and utilised within the Finnish BNCT project as well as within a European project for a code of practise for the dosimetry of BNCT. One outcome has been a travelling toolkit for BNCT dosimetry. It consists of activation detectors and ionisation chambers. The free-beam neutron spectrum is measured with a set of activation foils of different isotopes irradiated both in a Cd-capsule and without it. Neutron flux (thermal and epithermal) distribution in phantoms is measured using activation of Mn and Au foils, and Cu wire. Ionisation chamber (IC) measurements are performed both in-free-beam and in-phantom for determination of the neutron and gamma dose components. This toolkit has also been used at other BNCT facilities in Europe, the USA, Argentina and Japan.     

An ionisation chamber for measuring gamma exposure rates in mixed radiation fields

The development of an ionisation chamber for measuring gamma exposure rates in graphite thermal reactors is described. Through careful design, the absolute thermal neutron sensitivity has been minimized.     

Development of a gamma compensated boron lined ionisation chamber for reactor safety and control applications

Boron lined ionisation chambers with an overall diameter of 85 mm and maximum length of 165 mm have been developed and tested. The chamber consists of 34 numbers of parallel plate aluminium electrodes spaced at a distance of 2 mm and mounted on SS rods and radiation resistant polyetheretherketone (PEEK) spacers. One surface of the signal electrode and both the surfaces of the +HT electrodes are dip coated with boron. It is filled with nitrogen gas at a pressure of 128 cm of Hg. Tests at the ⁶⁰Co source facility at gamma fields ranging from 200 R/h to 830 kR/h showed that the chamber required 500 V to obtain 90% of the saturation current at 830 kR/h. The gamma compensation factor was measured as 0.12–7% for various gamma fields for polarising voltages of +400 and −350 V. Neutron measurements at the Apsara Thermal Column showed that the linearity of the chamber response as a function of reactor power was within 2%. The neutron sensitivity was measured as 3.9 fA/nv.     

Treasure of the Past IX: Exposure Standardization of Iodine-125 Seeds Used for Brachytherapy

A method for calibrating iodine-125 seeds in terms of exposure has been established. The standard free-air ionization chamber, used for measuring soft x rays, was chosen for the measurements. Arrays of four to six seeds were used to enhance the ionization-current-to-background-current ratio. Seeds from an array were measured individually in a re-entrant chamber. The quotient of the exposure rate for the array by the sum of the ionization currents in the re-entrant chamber is the calibration factor for the re-entrant chamber. Calibration factors were established for three types of iodine-125 seeds. The overall uncertainty for the seed exposure calibrations is less than 6%.     

Operational experience and relation to deposition process for NEG-coated chambers installed on the ESRF electron storage ring

A majority of the ESRF insertion device sections have been equipped with NEG-coated low gap vacuum chambers made of extruded aluminium and PVD coated with a low-temperature non-evaporable getter coating at ESRF. The low initial outgassing and quick conditioning allowed some modifications for chamber installation and accelerator restart procedures. The operational behaviour of those chambers has been studied not only by means of vacuum gauges but also with ionisation chambers measuring the bremsstrahlung sent to the connected synchrotron beamlines. The NEG-coating film thickness was optimised taking into account vacuum performance and also coating process parameters and duration. The 2nd more flexible and powerful NEG-coating device is under commissioning in the NEG-coating facility at ESRF.     

A small-sized MWPC with 1 mm wire spacing as a beam/target chamber for nuclear experiments

We developed a small-sized multiwire proportional chamber with 1 mm wire spacing for a study of backward Λ production in high energy hadron-nucleus reactions. Two chambers were installed in a cylindrical drift chamber of the large aperture spectrometer FANCY in order to determine beam trajectories precisely. A position resolution of about 0.6 mm was obtained with a counting efficiency of 97%.     

Detection properties of a measuring system for a continuous soil radon concentrations monitoring.

The continual soil-gas radon concentration measurements are absolutely crucial for a reliable assessment of radon entry characteristics into the indoor building environment. For this purpose, a new detection system (a continuous monitor RM-3) was developed and tested. The detection principle of the monitoring device is based on an airflow ionisation chamber operating in a current mode. A comprehensive series of testing and calibration experiments have been carried out in a laboratory environment. An output signal of the device caused by the radon concentration in a sensitive detection volume significantly depends on a detector ventilation rate, the gas flow rate through the ionisation chamber. A set of calibration experiments was accomplished with the artificial radon source application and close circuit experimental arrangements. The system detection properties including applied experimental conditions and key results of pilot in situ measurements are reported in detail.