Absolute Activity Measurement Research Articles

Study of XK and γ photon emission following decay of 154Eu

A joint project has been established between VNIIM (D.I. Mendeleyev Institute for Metrology) and LNHB (Laboratoire National Henri Becquerel) to determine as accurately as possible the X- and γ-ray emission probabilities of 154Eu. Point sources were prepared by VNIIM, and absolute measurements of activity per unit mass were undertaken by both laboratories using coincidence, anti-coincidence and 4π– γ counting methods. Other point sources and one aliquot were also prepared for precise γ-ray spectrometry measurements. Absolute photon emission probabilities were determined with a maximum uncertainty of 0.5% for the most intense lines, supporting the development of this nuclide as a multigamma standard.

Feasibility study of absolute activity measurement with metallic magnetic microcalorimeters

We report on a feasibility study on precise determination of mass-specific activity of low-energy emitting radioisotopes. Conventional methods of activity measurement suffer from source self-absorption and a strong decrease in detection efficiency for low-energy electrons and photons. We propose a new method based on metallic magnetic microcalorimeters with the source embedded in the detector target in a 4π geometry. First results with a 55Fe source show that electrons and photons are detected with a detection efficiency close to unity and with little loss of energy for electrons. The aim of this study is to provide standards of activity with very low uncertainties in the framework of radiation metrology.

Towards absolute activity measurements by ionisation chambers using the PENELOPE Monte-Carlo code

The Monte-Carlo code PENELOPE for Ionisation Chamber Simulation Method has been applied for the calculation of ionisation-chamber (IC) calibration factors. Measuring only a few radionuclides well selected within the relevant energy range, and determining an adjustable parameter, commonly used radionuclides can be measured without any specific calibration. The simulation revealed a discontinuity in the IC response as a function of photon energy and its dependence on the chemical composition of the radioactive solution.

The design of a liquid scintillation counter based on the triple to double coincidence ratio method

In Liquid Scintillation Counting (LSC), the Triple to Double Coincidence Ratio method (TDCR) is an absolute activity measurement method, especially developed for pure β- and pure EC-emitters activity standardisation. In this method, the detection efficiency is calculated using a physical and statistical model of the photon distribution emitted by the source. The application of this method requires the use of a specific three-photodetector Liquid Scintillation (LS) counter, which is not commercially available. When designing such a counter, the fundamental basis of the LSC phenomena and TDCR method must be taken into account. The key points of this design are presented in this paper.

Absolute measurement of the activity of [formula omitted] using a proportional counter

A measuring set-up comprising a proportional counter of calculable 222 Rn efficiency and quantifiable active volume ( δ V<0.1%) is described. On account of the special design of the end caps, the counter is suitable for absolute activity measurements on gaseous radiation sources. The 222 Rn efficiency is determined by computer simulation of the measured α-spectra. The procedures necessary for absolute measurements by means of the counter are described, and the suitability of the counter for absolute measurements of the 222 Rn activity is proved by experiments. Thus, a new method for the realization of the unit of activity of 222 Rn is obtained, which is independent of the unit of activity of 226 Ra .

A new absolute method for the standardization of radionuclides emitting low-energy radiation

Microcalorimeters (or bolometers) operated at temperatures below 100 mK allow individual counting of photons and electrons with a very low energy detection threshold. The physics is based on the pulse temperature increase of the target (or absorber) of the detector due to the complete absorption of both electrons and photons. Since this target can be constructed with a perfect 4- π geometry, a bolometer offers potentially a new method for absolute activity measurements of radionuclides emitting low-energy radiation. In this paper we present our first results of a feasibility study of activity standardization of a 55Fe solution with a prototype 4-π bolometer.

Software coincidence counting

A system designed for absolute activity measurement is described using a digital method. The system is based on data recording from a coincidence measurement and subsequent software processing of the data records. The data acquisition device collects amplitudes of individual pulses from analogue-to-digital converters and supplies them with time information. Software processing of data records from this system offers many benefits in comparison to conventional coincidence counting, for example it enables to perform time and pulse height analysis and setting of coincidence parameters by using a wide variety of evaluation methods to one data record. The digital system was tested with a 4 πβ– γ coincidence detectors arrangement consisting of a proportional counter and two NaI(Tl) scintillation detectors. The results obtained with a 60Co source are presented.

Cryogenic detectors below 100 mK for X-ray measurements in metrology

Due to the intrinsic performances of cryogenic detectors such as energy resolution, LPRI has decided to use these devices to improve the quality of the radioactive measurements usually obtained with classical semiconductor detectors. A bolometer with a 10 μg tin absorber has been developed at IAS (Institut d’Astrophysique Spatiale) and has been tested in the cryogenic installation of LPRI; an energy resolution (full width half maximum, FWHM) of 39 eV has been obtained on the K α line of Mn. Besides these good spectrometry results, an absolute activity measurement using bolometers is proposed by adapting an absorber geometry for 4 π counting.

Standardization of 169Yb and γ-ray emission probability data

Within the framework of EUROMET project No. 410, standardization of a 169Yb solution has been carried out and the gamma-ray emission probabilities at its decay have been measured. 169Yb activity was measured by the 4πβ–γ-coincidence extrapolation method. Special attention was paid to the measurement conditions associated with the existence of 316 keV metastable level of 169Tm ( T≈0.66 μs ). The 169Yb gamma-ray emission probabilities have been determined from the results of absolute activity measurements and data obtained using a coaxial Ge(Li) spectrometer.

Measurements of linear absorption coefficients of liquid scintillators using synchrotron radiation

The use of liquid scintillation counting as a fundamental radionuclide standardisation method requires a correct description of the physical phenomena induced by the interaction of the ionising radiation with the liquid scintillator. In particular the standardisation of radionuclides decaying by electron capture as 55Fe and other electron-capture nuclides, requires the knowledge of total linear absorption coefficients of the liquid scintillator used for absolute activity measurement. Total linear absorption coefficient measurements have been undertaken using synchrotron radiation. Linear absorption coefficients of two widely used commercial liquid scintillators were measured in the 5.5–23 keV energy range. Small discrepancies were noted between theoretical and measured values of the Ultima-Gold™ mass attenuation coefficients for low energy photons.

Quantitation in planar renal scintigraphy: which mu value should be used?

The attenuation coefficient value mu used by different authors for quantitation in planar renal scintigraphy varies greatly, from the theoretical value of 0.153 cm-1 (appropriate for scatter-free data) down to 0.099 cm-1 (empirical value assumed to compensate for both scatter and attenuation). For a 6-cm-deep kidney, such variations introduce up to 30% differences in absolute measurement of kidney activity. Using technetium-99m phantom studies, we determined the mu values that would yield accurate kidney activity quantitation for different energy windows corresponding to different amounts of scatter, and when using different image analysis approaches similar to those used in renal quantitation. With the 20% energy window, it was found that the mu value was strongly dependent on the size of the region of interest (ROI) and on whether background subtraction was performed: the mu value thus varied from 0.119 cm-1 (loose ROI, no background subtraction) to 0.150 cm-1 (kidney ROI and background subtraction). When using data from an energy window that could be considered scatter-free, the mu value became almost independent of the image analysis scheme. It is concluded that: (1) when performing background subtraction, which implicitly reduces the effect of scatter, the mu value to be used for accurate quantitation is close to the theoretical mu value; (2) if the acquired data were initially corrected for scatter, the appropriate mu value would then be the theoretical mu value, whatever the image analysis scheme.

A new liquid scintillation counter for the absolute activity measurement of radionuclides

The detection efficiency of a scintillation counter can be deduced from the knowledge of the statistical distribution of the light emitted by the scintillator. When using traditional photomultiplier tubes (PMT), the resolution of the of photons' spectrum is limited by the multiplication factor of the first dynode. Even with a high-gain PMT, the resolution of the single electron peak is low and the spectrum analysis is difficult. This is no longer the case with a new kind of commercially available photodetector, the hybrid photomultiplier tube (HPMT) in which the mean number of information quanta created by one photoelectron is much greater than those created in a PMT. The photoelectron spectrum resolution is thus dramatically improved and it is possible to deduce the statistical distribution of the number of photons emitted by the scintillator. This paper describes the use of this new photodetector in a liquid scintillation counter, designed for the standardisation of pure beta and electron capture radioactive solutions. The spectrum deconvolution process used to deduce the detection efficiency of the counter is also described.

Photophysics of Calcium Green 1 in vitro and in live cells

The absolute measurement of calcium activity in cells is hindered by the presence of different forms of the probe (adsorbed, protonated). Time-resolved fluorescence spectroscopy can be a sensitive technique to separate contaminating forms of the probe. The fluorescent probe Calcium Green 1 was characterised, invitro, in terms of calcium and also pH dependence, albumin adsorption, extent of hydrolysis, viscosity and medium polarity. A 1:1 stoichiometry was obtained for the calcium complexation with a Kd value that agrees with reported values. A pKa of the BAPTA moiety was measured as 6.7. The fluorescence lifetimes are slightly dependent on polarity but not on viscosity. It was observed that most of the interfering forms of the probe share the common lifetimes of 0.5 and 3.5 ns. An alternative mechanism for Calcium Green 1 sensing is proposed based on a ground state conformational equilibrium affecting the photoinduced electron transfer.

An improved coincidence counting technique for standardization of radionuclides

A data acquisition system based on a coincidence controller with buffer memory has been developed for standardization of radionuclides. The energy and output time of the shaped linear pulses generated from an ADC are digitized and latched by a 10 MHz clock oscillator, then these data are stored in the random access memory (RAM) of the coincidence controller. The coincidence resolving time and gamma windows can be selected by an off-line computer analysis program and the efficiency function is obtained by a computer discrimination method. This method is well suited for absolute activity measurement of radionuclides with relatively long-lived intermediate states as well as prompt decay.

The determination of the effective attenuation coefficient from effective organ depth and modulation transfer function in gamma camera imaging

Absolute measurement of activity implies a determination of effective depths and effective attenuation coefficients. In order to define restoration filters, it is necessary to measure the transfer function, i.e. position a line source at an effective depth for the specific measurement situation. A phantom was designed which can simulate an organ with a certain thickness at a certain depth. The phantom was used to measure transfer functions and a comparison was made with transfer functions from a line source to determine effective depths. Effective attenuation coefficients were calculated for , and for different organ thicknesses and depths of simulated organs. The effective attenuation coefficient for was found to be , in good agreement with previously published values. For , the attenuation coefficient decreased with the depth of an organ due to the use of two energy windows in the measurements and a corresponding change in mean photon energy by depth. For , the attenuation coefficient decreased with increasing organ thickness due to the increasing fraction of scattered radiation in the 40% energy window used. Using attenuation coefficients of 0.124, 0.184 and for , and respectively, the derived equations can be used to calculate the position of a conventional line source for measurements of transfer functions for a specific organ with a certain thickness at a certain depth for definition of different types of restoration filter.

Generalized 4π(LS)β—γ tracer method for standardization of pure beta emitters

The basic mathematical relationships, as well as experimental results, are presented for a generalized tracer method, with a multiparameter coincidence and anti-coincidence system, for absolute activity measurements of pure beta-emitters. A liquid scintillation counter, with two photomultipliers working in coincidence, was used for beta-particle detection and two scintillation counters with NaI(Tl) crystals for gamma-photon detection. A double extrapolation procedure has been applied: first, extrapolation as a function of tracer counting efficiency, and second, extrapolation as a function of emitter to tracer concentration ratio in the mixed radioactive solution. It should be emphasized that application of the second extrapolation procedure minimizes the counting uncertainty given by the first procedure. The experimental results are illustrated by 63Ni and 147Pm activity measurements with the same tracer, 60Co. The uncertainties obtained were within a limit of 1% for a 0.99 confidence level. The radioactive solutions in question ( 63Ni, 147Pm) were measured afterwards by the TDCR method (triple to double coincidence ratio) and the results compared. Good agreement (within the uncertainties of both methods) was obtained.

Standardization of125I by the sum-peak method and the results of a bilateral comparison between ENEA (Italy) and INP (Albania)

An intercomparison on absolute activity measurements of125I sources by the sumpeak method was carried out in 1993 between the Primary Standard Laboratories for Ionizing Radiations of Italy (ENEA) and Albania (INP). The sources were prepared by ENEA starting from a125I solution. The sources were measured at ENEA and successively at INP. The results were in good agreement, the deviation of the INP results from the ENEA results being about −0.5% with an overall uncertainty of both results about 0.3% (1σ). This paper describes the instrumentation and the experimental procedures used in this comparison by the two Institutes.

Absolute activity measurements with the windowless 4π-CsI(Tl)-sandwich spectrometer

The windowless 4π-CsI(Tl)-sandwich spectrometer consists of two scintillation crystals sandwiching radioactive sources deposited on thin plastic foils. This configuration has a solid angle very close to 4π sr. The detectors are sensitive to charged particles with energies > 15 keV and measure photons of 15–200 keV with a probability > 98%. Disintegration rates of samples of radionuclides with complex decay modes can be determined directly from the measured count rates with uncertainties below 0.3%. Radionuclide solutions of 57Co, 109Cd, 125I, 152Eu and 192Ir were standardised, partly in the framework of international comparisons. A detailed description of the spectrometer and the measurement procedure is given.

Absolute alpha activity measurements of some plants growing in monazite bearing soils in Sri Lanka

Abstract Deposits of monazite bearing soils occur along the Southwest, West and East Coasts of Sri Lanka. High levels of gamma activity in some plant species growing in the West Coast have been reported. The high levels were due to the presence of the daughter nuclides of 232 Th, most of which are alpha emitters. Absolute alpha activity measurements of ash samples of some plants growing in monazite bearing soils were carried out using alpha sensitive polymeric nuclear track detector CR-39. The values ranged from 60–1900 mBq/g and were in good agreement with the values obtained from conventional scintillation counting method. The activity concentration of 228 Th in the ash samples was also calculated by measuring the activity concentration of emanated thoron trapped inside a glass bottle, with the use of a CR-39 track detector.

Absolute measurement of activity and total photon yield in the decay of 201Tl

Using absolute measurement methods of 201Tl activity was determined with the total error of ±2.8% ( P = 0.99). The total yield of 167.4 and 165.9 keV photons was found to be (0.106 ± 0.004). This yield was measured with a Ge(Li) detector, the efficiency of which was calibrated by a set of sources of known radioactivity.