Re-entrant Ionization Chambers Research Articles

Calibration of sodium iodide detectors and reentrant ionization chambers for 212Pb activity in different geometries by HPGe activity determined samples

Lead-212 is a promising radionuclide for cancer therapy, but no primary 212Pb activity standardization has been published. A need therefore exists for accurate estimation of injected doses of 212Pb activity in equilibrium with progeny, when it comes to preclinical and clinical trials. In this study, 212Pb activity was determined using a high purity germanium (HPGe) detector, which allowed the determination of geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) and sodium iodide (NaI) detectors.

Radionuclide calibrator responses for 224Ra in solution and adsorbed on calcium carbonate microparticles

A suspension of 224Ra adsorbed onto CaCO3 microparticles shows promise for α-therapy of intracavitary micro-metastatic diseases. To facilitate accurate activity administrations, geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) have been developed for 224RaCl2 solutions and 224Ra adsorbed onto CaCO3 microparticles in suspension in ampoules, vials, and syringes. Ampoules and vials give IC responses consistent with each other to <1%. Microparticles attenuation leads to a ≈1% to ≈2.5% reduction in response in the geometries studied.

The next generation of current measurement for ionization chambers

Re-entrant ionization chambers (ICs) are essential to radionuclide metrology and nuclear medicine for maintaining standards and measuring half-lives. The requirements of top-level metrology demand that systems must be precise and stable to 0.1 % over many years, and linear from 10−14 A to 10−8 A. Thus, laboratories depend on bespoke current measurement systems and often rely on sealed sources to generate reference currents. To maintain and improve present capabilities, metrologists need to overcome two looming challenges: ageing electronics and decreasing availability of sealed sources. Possible solutions using Ultrastable Low-Noise Current Amplifiers (ULCAs), resistive-feedback electrometers, and (quantum) single-electron pumps are reviewed. Broader discussions of IC design and methodology are discussed. ULCAs show promise and resistive-feedback systems which take advantage of standard resistor calibrations offer an alternative.

Primary standardization of 224Ra activity by liquid scintillation counting

A standard for activity of 224Ra in secular equilibrium with its progeny has been developed, based on triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) counting. The standard was confirmed by efficiency tracing and 4παβ(LS)-γ(NaI(Tl)) anticoincidence counting, as well as by 4πγ ionization chamber and NaI(Tl) measurements. Secondary standard ionization chambers were calibrated with an expanded uncertainty of 0.62% (k = 2). Calibration settings were also determined for a 5 mL flame-sealed ampoule on several commercial reentrant ionization chambers (dose calibrators).

An update on 'dose calibrator' settings for nuclides used in nuclear medicine.

Most clinical measurements of radioactivity, whether for therapeutic or imaging nuclides, rely on commercial re-entrant ionization chambers ('dose calibrators'). The National Institute of Standards and Technology (NIST) maintains a battery of representative calibrators and works to link calibration settings ('dial settings') to primary radioactivity standards. Here, we provide a summary of NIST-determined dial settings for 22 radionuclides. We collected previously published dial settings and determined some new ones using either the calibration curve method or the dialing-in approach. The dial settings with their uncertainties are collected in a comprehensive table. In general, current manufacturer-provided calibration settings give activities that agree with National Institute of Standards and Technology standards to within a few percent.

Half-life measurement of the medical radioisotope 177Lu produced from the 176Yb(n,γ) reaction

177 Lu is a medium energy beta-emitter commonly used in Nuclear Medicine for radiotherapeutic applications. In this work, the half-life of 177 Lu has been measured using a re-entrant ionisation chamber over a period of 82 days (approximately 12 half-lives). Unlike the majority of previous studies, the material used in this work was produced via the 176 Yb(n,γ)177 Yb reaction followed by the β-decay to 177 Lu, producing insignificant quantities of 177m Lu. This has resulted in the most precise half-life measurement of 177 Lu to date. A half-life of 6.6430 (11) days has been determined. This value is in statistical agreement with the currently recommended half-life of 6.6463 (15) days (z-score = 1.8).

Source self-attenuation in ionization chamber measurements of 57Co solutions

Source self-attenuation for solutions of 57Co of varying density and carrier concentration was measured in nine re-entrant ionization chambers maintained at NIST. The magnitude of the attenuation must be investigated to determine whether a correction is necessary in the determination of the activity of a source that differs in composition from the source used to calibrate the ionization chamber. At our institute, corrections are currently made in the measurement of 144Ce, 109Cd, 67Ga, 195Au, 166Ho, 177Lu, and 153Sm. This work presents the methods used as recently applied to 57Co. A range of corrections up to 1% were calculated for dilute to concentrated HCl at routinely used carrier concentrations.

Uncertainty evaluation in activity measurements using ionization chambers

SummaryPressurized re-entrant (or 4π) ionization chambers (ICs) connected to current-measuring electronics are used for activity measurements of photon emitting radionuclides and some beta emitters in the fields of metrology and nuclear medicine. As a secondary method, these instruments need to be calibrated with appropriate activity standards from primary or direct standardization. The use of these instruments over 50 years has been well described in numerous publications, such as the Monographie BIPM-4 and the special issue of Metrologia on radionuclide metrology (Ratel 2007 Metrologia 44 S7–16, Schrader1997 Activity Measurements With Ionization Chambers (Monographie BIPM-4) Schrader 2007 Metrologia 44 S53–66, Cox et al 2007 Measurement Modelling of the International Reference System (SIR) for Gamma-Emitting Radionuclides (Monographie BIPM-7)). The present work describes the principles of activity measurements, calibrations, and impurity corrections using pressurized ionization chambers in the first part and the uncertainty analysis illustrated with example uncertainty budgets from routine source-calibration as well as from an international reference system (SIR) measurement in the second part.

Precise practical measurements of 90Y used in radiopharmaceuticals using re-entrant ionization chambers

For diagnostic nuclear medicine, radiopharmaceuticals are labeled with γ-ray emitting nuclides. Routine activity measurements for these drugs are mainly performed with re-entrant ionization chambers. For radiotherapeutic applications, β-ray emitting nuclides are also used. Since very few gamma transitions are involved in these nuclides, an ionization chamber responds only to weak bremsstrahlung. In order to apply the re-entrant chamber method to routine activity assay for these β-ray emitting nuclides, detailed studies were carried out. It was determined that the response was strongly affected by the choice of thickness of the inner wall of the well in the measurement of high energy beta particle emitting nuclides. The geometrical dependences were successfully reduced. Accordingly, routine assay of radioactivity of 90Y therapeutic pharmaceuticals can be performed within an acceptable uncertainty with ordinary re-entrant dose calibrators that are commercially available and equipped in most hospitals.

Results of an international comparison of 57Co

As part of a Cooperative Research Project (CRP) aimed at improving the state of radioactivity measurement in nuclear medicine, the International Atomic Energy Agency (IAEA) organized a comparison of (57)Co solutions among the participants of the project. The comparison solutions were prepared from a single master stock solution and distributed to the participating laboratories, who measured the activity concentration of the solution using either the laboratory's radionuclide activity calibrator or primary standardization methods. A total of 9 sets of results were received, with 5 laboratories reporting results of primary measurements, one reporting results of secondary measurements calibrated against primary standards, and three laboratories reporting values based on measurements in commercial re-entrant ionization chambers using manufacturer-recommended calibration figures. Most of the laboratories reporting primary standardizations also provided results from secondary standardizations. The Comparison Reference Value was calculated from the mean of the five primary standardizations and was found to be 35.54 MBq g(-1), with a standard deviation of the mean of 0.17 MBq g(-1). Degrees of equivalence were calculated for each reporting laboratory and demonstrated that equivalence to within about 4% could be achieved, even in the case of those laboratories that used instruments calibrated by third parties.

Standardisation of positron-emitters in gas with the NPL primary gas counting system

A primary standard for positron-emitters in gas has been developed. The method involves internal gas proportional counting and the use of the PENELOPE Monte Carlo code to determine corrections for counting losses. The development work was carried out using (11)C, although the method can be applied to other positron emitters. The results were compared with measurements of (11)C (in solution) carried out using a secondary standard re-entrant ionisation chamber previously calibrated with reference to absolute counting techniques.

Measurement of Bremsstrahlung as a practical alternative to use of a liquid scintillation counter for the assessment of low activities of 90Y

The use of 90Y for nuclear medicine therapies has steadily increased over the last 10 years. High administered activities are measured in a calibrated re-entrant ionization chamber, while the most sensitive method of assessment of low activities uses a liquid scintillation counter. This method requires the samples to have an acceptably low quench, and therefore heavily coloured samples must undergo chemical processing before assessment. An alternative method has been investigated to measure low activities of 90Y in a sodium iodide well counter by detection of associated Bremsstrahlung. Test samples of 90Y with activities 0.5-730 Bq were measured in a well counter and a liquid scintillation counter, with counting times of 4 h per sample. Both counters were investigated for a relationship between count rate and activity. The efficiency of the well counter was found to be 0.08 count x s(-1) x Bq(-1), for a specified vial and sample volume. This is poor compared with the liquid scintillation counter efficiency of 1.0 count x s (-1) x Bq(-1). The uncertainty in measurement of a sample with unknown activity was calculated for a 4 h count time: +/-8.0% at 730 Bq and +/-45% at 6 Bq for the well counter; and +/-8.0% and +/-8.1%, respectively, for the liquid scintillation counter. These errors are dominated by the initial measurement of activity to determine counting efficiency, using a calibrator with an accuracy of +/-8%. If long counting times of both samples and background radiation are practicable, it has been found that a well counter can successfully be used to assess low activities of 90Y.

Calculation of calibration figures and the volume correction factors for 90Y, 125I, 131I and 177Lu radionuclides based on Monte-Carlo ionization chamber simulation method

Many metrology laboratories are dealing with activity measurements of different radionuclides with special interest in nuclear medicine as well as in radiopharmaceutical industry. In improving the accuracy of radionuclide activity measurements, a key role plays the calculation of calibration figures and the volume correction factors for the radionuclide under study. It is well known that the chamber calibration factors depend on the measurement geometry including the volume of the source and the type of the measurement vessel. In this work, the activity standards in the form of radioactive solutions are delivered in sealed Jena glass 5 ml FIOLAX ®-klar ampoule. Calculation of the calibration figures (or efficiencies) for 90Y, 125I, 131I and 177Lu radionuclides on 5 ml ampoule are presented in this paper. Additionally, their appropriate volume correction factors are determined. These calibration figures for the ISOCAL IV pressurized well re-entrant ionization chamber (IC) are pointed out based on the Monte-Carlo (MC) simulation method of such chamber using the PENELOPE-2005 MC computer simulation code. The chamber is filled with nitrogen gas pressurized to approximately 1 MPa. In determining the volume correction factors, the variation of calibration factors versus the mass of radioactive solution filling the 5 ml ampoule glass is investigated. From the point of view that impurity of 177 m Lu isomer is always accompanying the 177Lu radionuclide, for making possible the correction due to presence of this impurity, the calibration factor and the volume correction factors for 177 m Lu are reported as well.

Intercomparison of ionisation chamber measurements from 125I seeds

The reference air kerma rates of a set of individual 125I seeds were calculated from current measurements of a calibrated re-entrant ionisation chamber. Single seeds were distributed to seven Australian brachytherapy centres for the same measurement with the user's instrumentation. Results are expressed as the ratio of the reference air kerma rate measured by the Australian Nuclear Science & Technology Organisation (ANSTO) to the reference air kerma rate measured at the centre. The intercomparison ratios of all participants were within ±5% of unity.

Experimental determination of calibration settings for plastic syringes containing solutions of 90Y using commercial radionuclide calibrators

Calibration factors for several makes (Capintec, AtomLab, and PTW) and models of commercially available re-entrant ionization chambers (“dose calibrators”) were determined for solutions of 90Y using 10 ml plastic syringes that are used in the administration of the recently approved radioimmunotherapy drug Zevalin ®. Effects of filling volume on chamber response were studied for the anticipated prescription volume range of 3–9 ml in those syringes. A series of syringes were prepared over that range with accurately known volumes using a “standard” solution containing 90YCl 3 in 1 mol l −1 HCl and approximately 50 μg of nonradioactive YCl 3 per gram of solution. The respective calibration factors for the Capintec and AtomLab chambers required to give the correct activity reading were found to be “55±2×10” and 393±6, where the uncertainties are expanded ( k=2) uncertainties. The results indicated no significant effect of filling volume on the calibration setting to within the uncertainty on the activity measurement for all but the PTW chamber. In that case, a variation of about 5% was observed over the filling range. A direct comparison was also made between the actual 90Y Zevalin ® drug and the “standard” solution, with no differences in determined calibration settings observed within the uncertainties on the activity calibration.

Energy response simulation of 4pi gamma ionization chambers using Monte Carlo technique.

The energy response of 4pi gamma re-entrant well-pressurized ionization chambers was calculated using the MCNP4B Monte Carlo computer code. Two different chambers were simulated-one filled with argon and the other with nitrogen gases at 2 MPa pressure. The MCNP4B results were in good agreement with the experimental measurements and the EGS4 Monte Carlo theoretical results. Energy response formulas for both chambers were obtained with respect to the 1.173 MeV gamma energy line of 60Co. Detailed energy response in the low energy region was facilitated by proper curve fitting of the simulation results. The MCNP4B code was also used to examine the effect of the axial displacement of the standard gamma source in the chamber well. The highest chamber response occurs when the source is located at the center position of the well and it decreases with increasing distance from the center. The effect of the source axial position displacement on the chamber energy response is noticeable at medium and high-energy photon energies.

Radioassays and experimental evaluation of dose calibrator settings for 18F

The positron emitter 18F continues to be one of the most important imaging radionuclides in diagnostic nuclear medicine. Assays of radiopharmaceuticals containing this nuclide are often performed in the clinic using commercial reentrant ionization chambers, or “dose calibrators”. Meaningful quantitative clinical studies require accurate knowledge of the injected activity which requires proper calibration of these instruments. Radioassays were performed at the National Institute of Standards and Technology (NIST) on a solution of 18F produced at the National Institutes of Health (NIH) using 4πβ liquid scintillation (fS) counting with 3H-standard efficiency tracing. Cocktails containing water fractions of approximately 0.9 and 9% (both as saline) were used. The massic activity values were measured to be 2.52±0.06 and 2.50±0.03 MBq g −1, respectively, for the 0.9 and 9% water cocktails as of the reference time. The uncertainties on the activity measurements are expanded ( k=2) uncertainties. The largest uncertainty component was found to be the repeatability on a single LS source, with the cocktails containing 0.9% water fraction exhibiting a larger variability by nearly a factor of two. Reproducibility between LS cocktails with the same water fraction was also found to be a large uncertainty component, but with a value less than half that due to measurement repeatability. Radionuclidic impurities consisted of 48V and 46Sc, at levels of 0.11±0.08% (expanded uncertainties) and approximately 2×10 −3% (upper limit) relative to the activity of the 18F, as of the reference time. Dose calibrator dial settings for measuring solutions of 18F were experimentally determined for Capintec CRC-12 and CRC-35R dose calibrators in three measurement geometries: a 5-ml standard NIST ampoule (two ampoules measured), a 12-ml plastic syringe containing 9 ml of solution and a 10-ml Mallinckrodt molded dose vial filled with 5 ml of solution. The experimental dial settings (and the corresponding expanded uncertainties) for these geometries were found to be 477±7, 474±6, 482±6 and 463±7 for the two ampoules, the syringe and the dose vial, respectively, in the CRC-12. The dial settings determined for the CRC-35R were 472±7, 470±7, 464±6 and 456±6 for the two ampoules, the syringe, and the dose vial, respectively. The uncertainties in the dial settings are expanded uncertainties. Comparisons between the empirically determined dial settings and the manufacturer's recommended setting of “439” indicate that use of the manufacturer's setting overestimates the activity by between 3 and 6%, depending upon the geometry used.

Experimental determinations of commercial ‘dose calibrator’ settings for nuclides used in nuclear medicine

Commercial reentrant ionization chambers (‘dose calibrators’) have become the de facto standard instrument for making radioactivity measurements in the radiopharmaceutical industry. Accurate activity measurements, however, depend on the application of the correct calibration factor (dial setting) for the radionuclide in the exact measurement geometry for that particular instrument. Two methods are described for the experimental determination of dial settings for the Capintec CRC-12 dose calibrator. Using these methods, we have determined new calibration factors for 18F, 62Cu, 186Re and 188Re in several geometries. These new factors are presented, along with previously determined settings, for nuclides commonly used in nuclear medicine. The differences between the calibration factors for the different geometries, as well as the differences between the manufacturer's recommended setting underscore the need for empirically determined dose calibrator settings.

Standardisation and decay data of 186Re

A solution of 186Re was standardised using the 4π(PC)-γ-coincidence technique. Two gamma channels were employed to provide separate detection efficiency data arising from the two decay modes (beta and electron capture): the resulting data were analysed using a two-dimensional extrapolation. Calibration factors for a high pressure re-entrant ionisation chamber as well as for the NPL secondary standard radionuclide calibrator were also determined. An efficiency-calibrated germanium spectrometer was used to measure the emission probabilities of the X- and gamma-rays.

The standardization of 62Cu and experimental determinations of dose calibrator settings for generator-produced 62CuPTSM

The Positron Emission Tomography agent 62Cu Pyruvaldehyde Bis( N 4-methyl)thiosemicarbazone (PTSM) has been standardized by the National Institute of Standards and Technology (NIST) using 4 πβ liquid scintillation counting with 3H-standard efficiency tracing. Using a measurement model developed at NIST for the determination of experimental dose calibrator dial settings for short-lived radionuclides, the correct dial settings for 62CuPTSM in a clinically useful geometry (a 35 ml plastic syringe containing 33 ml of solution) as well as the standard NIST 5 ml glass ampoule have been elucidated. This measurement model is fully described, as is the treatment of associated uncertainties. The correct instrument settings for the NIST Capintec CRC-12 re-entrant ionization chamber (‘dose calibrator’) were found to be ‘499±6’ and ‘489±8’, respectively, for the syringe and ampoule geometries. The stated uncertainties are expanded ( k=2) uncertainties. The expanded uncertainties in the measured activity arising from these new dial settings are ±0.8% and ±1.4%, respectively, for the syringe and ampoule. The measured activities using these settings are lower than those obtained from the manufacture’s recommended setting of ‘448’ by 9.7% for the syringe and 7.7% for the ampoule, and underscore the need for experimental verification of the dose calibrator settings for each radionuclide and each geometry used in the clinic.