Primary Standard System Research Articles

Dosimetric tests of an extrapolation chamber in standard computed tomography beams

Computed tomography (CT) diagnostic exams are responsible for the highest dose values to the patients. Therefore, the radiation doses in this procedure must be accurate. For the dosimetry of CT beams, the radiation detector is usually a pencil-type ionization chamber. This type of dosimeter presents a uniform response to the incident radiation beam from all angles, which makes it suitable for such equipment since the X-ray tube executes a circular movement around the table during irradiation. However, there is no primary standard system for this kind of radiation beam yet. In order to search for a CT primary standard, an extrapolation chamber built at the Calibration Laboratory (LCI) of the Instituto de Pesquisas Energéticas e Nucleares (IPEN) was tested. An extrapolation chamber is a parallel-plate ionization chamber that allows the variation of its sensitive air volume. This chamber was used previously for low-energy radiation beams and showed results within the international recommended limits. The aim of this work is to perform some characterization tests (saturation curve, polarity effect, ion collection efficiency and linearity of response) considering the chamber depth of 1.25 mm in the radiation qualities for computed tomography beams at the LCI. The results showed to be within the international recommended limits.

Performance of an extrapolation chamber in computed tomography standard beams

Among the medical uses of ionizing radiations, the computed tomography (CT) diagnostic exams are responsible for the highest dose values to the patients. The dosimetry procedure in CT scanner beams makes use of pencil ionization chambers with sensitive volume lengths of 10 cm. The aim of its calibration is to compare the values that are obtained with the instrument to be calibrated and a standard reference system. However, there is no primary standard system for this kind of radiation beam. Therefore, an extrapolation ionization chamber, built at the Calibration Laboratory (LCI), was used to establish a CT primary standard. The objective of this work was to perform some characterization tests (short- and medium-term stabilities, saturation curve, polarity effect and ion collection efficiency) in the standard X-ray beams established for computed tomography at the LCI.

Some characterization tests for an extrapolation chamber in CT standard beams in a specific chamber depth

The use of Computed Tomography (CT) for diagnostic images has been growing due to technological advances of this equipment. Therefore, there is a concern regarding the dose received by the patients undergoing this procedure, and this procedure needs to be performed with the highest precision and accuracy possible.The radiation detector used for CT beams is a pencil-type ionization chamber. However, there is no primary standard system for this kind of radiation beam. In this work a homemade extrapolation chamber was used to establish a CT primary standard [1]. This detector was tested for low-energy radiation beams and showing results within the internationally acceptable limits [2].This work had the objective to study the response stability of an extrapolation chamber, as well as obtaining the energy dependence and the angular dependence in standard CT beams of the Calibration Laboratory of IPEN (LCI).

Assessments of the characteristics of an extrapolation chamber for 60 to 200 kVp X-rays: Experiment and Monte Carlo simulation

According to the principles of radiation protection, the patient and personnel dose should be decreased to a reasonably achievable level. Therefore, it is essential to use reliable dosimeters in the radiation dosimetry course. One of these dosimeters is an extrapolation chamber. It is a plane-parallel ion chamber (PPIC), which its sensitive volume can be varied. In this paper, the performance of an extrapolation chamber has been evaluated in 60–200 kVp X-rays. It was submitted to several necessary tests, and also simulated with MCNP code. All of the results were within the recommendations of the International Electrotechnical Commission (IEC). The chamber responses were linear with electrode separation. The comparison between experiment and simulation agrees in an overall sense within 5.4% in the worst case. The results indicated the usefulness of the chamber in this energy range. They show that the extrapolation chamber can be also used for computed tomography (CT) beams, considering that a primary standard system doesn't exist for these beams yet.

Establishment of Thailand’s national primary vacuum standard by a static expansion method

A novel primary standard system for a vacuum pressure calibration has been established at the National Institute of Metrology, Thailand (NIMT). The system, which cover the pressure range from 1.3 • 10-1 Pa up to 1.3 • 103 Pa is based on the static expansion method. By the series of re-expansion, the range could be extended to lower calibration pressures. The facility consists of two small and one large vessels, which provide an expansion ratio by a factor of approximately 102 to 103 for a single expansion and more for a multiple one. Six platinum resistance thermometers are attached to the vessel to ensure the correction of the non-isothermal effect during an expansion. The system could deliver the expanded (k = 2) relative uncertainties under 0.6% of reading at the lowest operating pressure. The measurement results of the system were verified by comparing with the calibration results of the capacitance diaphragm gauge (CDG) from Physikalisch-Technische Budensanstalt (PTB). The En ratios represent an excellent agreement between NIMT and PTB calibration results.

Evaluation of an extrapolation chamber for dosimetry in computed tomography beams using Monte Carlo code (MCNP5)

Computed tomography (CT) is responsible for the highest dose values to the patients during the exams. Therefore, the radiation doses in this procedure must be accurate. However, there is no primary standard system for this kind of radiation beam yet. A homemade extrapolation ionization chamber was preliminary evaluated for the establishment of a CT beam primary standard. The aim of this study was to determine the influence that the various components of the extrapolation chamber may present on the energy deposited in its sensitive volume. Different materials for its collecting electrode were also studied for this type of radiation beam. The evaluation of the homemade extrapolation chamber showed the highest influence of 25.9% for the collecting electrode, and the best material for the collecting electrode for CT radiation beams was graphite.

Testing an extrapolation chamber in computed tomography standard beams

The computed tomography (CT) is responsible for the highest dose values to the patients. Therefore, the radiation doses in this procedure must be accurate. However, there is no primary standard system for this kind of radiation beam yet. In order to search for a CT primary standard, an extrapolation ionization chamber built at the Calibration Laboratory (LCI) of the Instituto de Pesquisas Energéticas e Nucleares (IPEN), was tested in this work. The results showed to be within the international recommended limits.

Development and characterization of a new graphite ionization chamber for dosimetry of 60Co beams

Ionization chambers are the most employed dosimeters for precise measurements, as those required in radiotherapy. In this work, a new graphite ionization chamber was developed and characterized in order to compose a primary standard system for the beam dosimetry of the 60 Co sources. This dosimeter is a cylindrical type ionization chamber, with walls and collecting electrode made of high-purity graphite, and the insulators and stem made of Teflon ® . The walls are 3.0 mm thick, and it has a sensitive volume of 1.40 cm 3 . The characterization was divided in two steps: experimental and Monte Carlo evaluations. This new dosimeter was evaluated in relation to its saturation curve, ion collection efficiency, polarity effect, short- and medium-term stabilities, leakage current, stabilization time, linearity of response and angular dependence. All results presented values within the established limits. The second part of the characterization process involved the determination of the correction factors, obtained by Monte Carlo simulations. Comparing these correction factors values with those from other primary standard laboratories, the highest differences were those for the wall and stem correction factors. The air kerma rate of the 60 Co source was determined with this new dosimeter and with the IPEN standard system, presenting a difference of 1.7%. These results indicate that this new dosimeter may be used as a primary standard system for 60 Co gamma beams.

Böhm extrapolation chamber: Study of its behavior in beta radiation fields at the Calibration Laboratory of IPEN

The Calibration Laboratory (LCI) at the Instituto de Pesquisas Energéticas e Nucleares (IPEN) is going to establish a Böhm extrapolation chamber as a primary standard system for the dosimetry and calibration of beta radiation sources and detectors. This chamber was already tested in beta radiation beams with an aluminized Mylar entrance window, and now, it was characterized with an original Hostaphan entrance window. A comparison between the results of the extrapolation chamber with the two entrance windows was performed. The results showed that this extrapolation chamber presents the same effectiveness in beta radiation fields as a primary standard system with both entrance windows, showing that any one of them may be utilized.

Determination of transmission factors in tissue using a standard extrapolation chamber

A commercial ionization chamber, Böhm extrapolation chamber, PTW, model 23392, recommended for measurements in low energy X-rays and beta radiation fields, was tested in three different 90Sr+90Y beams to verify its performance as a primary standard system for the calibration and dosimetry of beta radiation sources and detectors. Characterization tests were performed, as determination of the chamber null depth using two methods (the results presented a difference of only 0.9%), transmission factors in tissue, in comparison with those of the certificate (the maximum difference was 2.1%), and absorbed dose rates of the 90Sr+90Y sources, in comparison with the values provided by the calibration certificates (the maximum difference was 4.90%). The results obtained confirmed that this extrapolation chamber presents a very good behavior in beta radiation fields as a primary standard system.

The humidity effect on air flow rates in a critical flow venturi nozzle

A Critical Flow Venturi Nozzle (CFVN) is usually calibrated using dry air. Yet CFVNs in industrial and calibration service centers are often used to measure flow rates of humid air. Therefore, ISO 9300 provides the calculation method for the humidity effect on discharge coefficients of CFVN. However, since this method is only due to a theoretical analysis, it is important to confirm and check the ISO calculation method for the humidity effect on CFVN with its isentropic analysis by means of an experimental method. In this experiment, three CFVNs with diameters of 0.4, 0.8 and 1.6 mm were calibrated with dry air (with the dew point −40 °C), in a primary air flow standard system with a mercury sealed piston prover, installed at the Korea Research Institute of Standards and Science (KRISS). Another piston prover, a portable dry piston prover, was used as a reference meter and was also calibrated in the primary standard system using dry air. The repeatability of this dry piston prover was confirmed with the deviation being less than 0.05%. The CFVNs were tested with this dry piston prover, using humid air. For air types with high humidity, the humidity effect on flow rates through the CFVNs showed quite significant difference between the experimental results and those from the ISO method with isentropic analysis. But for air types with low humidity, its effect was relatively insignificant.

Realization of ITS-90 Above the Silver Point at the NIM

The high-temperature primary standard system was gradually improved at the National Institute of Metrology (NIM) in China after 2004. A new primary standard pyrometer (PSP) was developed, one with a size-of-source effect of 1 × 10−4, and regional thermostats for interference filters, photoelectric detectors, and I/V converters. The relative spectral responsivity of the entire PSP was calibrated by means of a new facility. A new LED-based measurement facility and novel systematic error correction model were utilized to characterize the PSP nonlinearity and extend the photocurrent to PSP temperature readings of about 2680 °C. As an improved scheme, the fixed-point blackbody pyrometer assembly was utilized to realize and disseminate the International Temperature Scale of 1990 above the silver point. This scheme can avoid the influences of instability and inhomogeneity of tungsten strip lamps and corrects pyrometer drifts, thereby improving the realization uncertainty and simplifying the transfer chain. The expanded uncertainties of the scale realization ranged from 0.04 °C at the silver point to 0.48 °C at 2474 °C.

Step-down procedure of sonic nozzle calibration at low Reynolds numbers

The purpose of this investigation is to develop flow measuring technology for miniscule flow-rates, in order to calibrate sonic nozzles at lower pressures than the existing pressure ranges, without resorting to an expensive primary standard system. Once these sonic nozzles are calibrated successfully, calibration services for flow meters in industry for lower pressure ranges than the atmospheric pressure will be readily available. For this experiment, eight sonic nozzles (throat diameter of 0.2 mm) were manufactured according to the specifications of ISO 9300. Then four nozzles out of these were selected. The criterion for such a selection was the smallest standard deviation among the others, when the ( C d ⋅ A ) ratios for various combinations of two nozzles were calculated. Regarding the experiment: First, all the four nozzles were calibrated for the pressure range 200–600 kPa by means of the primary standard system in KRISS (Korea Research Institute of Standards and Science). Then the step-down procedure was taken in order to reduce the stagnation pressure down to 3 kPa, so that the discharge coefficients for the pressure could be obtained in the range 3–200 kPa. Through this study, it was proven that the step-down procedure could be repeated as many times as desired with only small additional uncertainty.

Self-calibratable rotary encoder

In the National Metrology Institute of Japan (NMIJ)/AIST, a rotary encoder calibration system was developed as the national primary standard for an angle, and a highly precise standard calibration service has been supplied from 2003. The source of uncertainty that influences most greatly in calibration is the attachment error (eccentricity) of a rotary encoder. Even if users get the calibrated rotary encoder, the error when attaching in using apparatus cannot be estimated. In NMIJ, we developed the new rotary encoder that can be calibrated by rotary encoder itself. Its mechanism is very simple; some reading heads are arranged at the equal angle interval around a scale disk. The calibration curve that contains not only all graduation lines deviation but eccentricity can be obtained by analyzing in series the signals acquired from heads. In order to check the performance of this epoch-making rotary encoder, we set up this system on the national primary standard and compared this self-calibration data with the data by using the primary standard system. The accuracy for 0.5′′ or less could be acquired. Since this method is applicable also to a small rotary encoder, it is expected that the self-calibratable rotary encoder be used in many angle control instruments.

Calibration of Coriolis mass flowmeters using a dynamic weighing method

The suitability of various gravimetric calibration methods was investigated for Coriolis mass flowmeters, the operating principle of which is completely different from conventional flowmeters. In this study a new 600 kg weighing system which may be utilized for either static or dynamic weighing was constructed, and the performance of the system was evaluated by comparing its test results with those of the primary standard system. When a standing-start-and-finish method was used, bulk meters and turbine meters over-estimated K factors over the flowrate range higher than 5 kgs −1. Coriolis flowmeters, however, have shown K-factors independent of the calibration method over the entire flowrate range. The method of dynamic weighing has shown less than ±0.1% of uncertainty at the 95% confidence level.