Study of a Monte Carlo rearrangement model for the activity determination of electron-capture nuclides by means of liquid scintillation counting.

A new Monte Carlo approach for the computation of the electron spectra of electron-capture nuclides is applied to obtain efficiencies in liquid scintillation counting for CIEMAT/NIST applications. The new method is applied to the radionuclides (109)Cd and (125)I by using a stochastic atomic rearrangement model, taking into account rearrangement processes including L-, M-, and N-subshells. The counting efficiencies were computed with the new code MICELLE which also comprises an approach for calculating the counting efficiency of a radionuclide in a gel phase sample. The calculated counting efficiencies are compared with experimental results.

The ionization quenching function for coincident electrons.

When two electrons interact simultaneously with a liquid scintillator, they generate two distributions of solvent excited molecules. These distributions can overlap totally, partially or not overlap at all. The overlap produces an increase of the excitation density, and consequently a modification of the ionization quenching. A theoretical description that calculates the mean overlap and the ionization quench function for multiple electron coincidences is presented. We analyse the overlapping factor for electrons of similar energies. Two overlap models, two dimensional (2D) and three dimensional (3D), have been developed. Both Birks and Voltz formulae have been applied to a KLM atomic rearrangement model to compute the ionization quenching function. The counting efficiency for (125)I obtained by applying the overlapping correction is about 1% lower than that computed without this correction.

Determination of the shape factor of (90)Sr by means of the cutoff energy yield method.

Usually, Kurie plots are used to analyze beta-spectra shape-factor functions measured by means of semiconductor and magnetic spectrometers. A drawback of these techniques is the occurrence of self-absorption within the samples through which the emission spectrum is altered. In liquid-scintillation samples self-absorption does not occur, but the poor energy resolution makes the analysis of the spectra difficult. To overcome this problem, two resolution-invariant observables are used for determining the shape-factor function of (90)Sr: (1) the maximum point energy and (2) the cutoff energy yield. The measured shape-factor function of (90)Sr agrees with the one which is predicted by theory for the first-forbidden unique transition.

Synergic quenching effects of water and carbon tetrachloride in liquid scintillation gel samples.

Although both water and carbon tetrachloride induce chemical quenching when incorporated to Insta Gel, the quenching power of carbon tetrachloride is nearly two orders of magnitude greater than that of water. This huge difference shows how different the chemical quenching mechanisms among quenchers can be. One remarkable fact of all pure beta-ray nuclides is that calibration curves do not depend on the water to scintillator volume ratio or on the nature of the quencher. The fact is clearly surprising, because two tritium samples of different water to scintillator and quencher to scintillator volume ratios may have identical counting efficiencies. As we shall prove, this fact is only possible when the external quench parameter of both samples is the same. We study the synergies in quenching generated when carbon tetrachloride is added to a sample in gel phase.

Computational aspects in modelling the interaction of low-energy X-rays with liquid scintillators.

The commercial liquid scintillators available nowadays are mostly complex cocktails that frequently include non-negligible amounts of heavier elements than the commonly expected carbon or hydrogen. In May 1993, nine laboratories agreed to participate in the frame of the EUROMET project in a comparison of the activity concentration measurement of 55Fe. One particular aspect of the results that caught one's eye was a small systematic difference between the activity concentrations obtained with Ultima Gold and Insta Gel. The detection of the radiation emitted by EC nuclides involves, in addition to the atomic rearrangement generated by the capture of the electron by the nucleus, a frequently ignored secondary atomic rearrangement process due to photoionization. Such a process can be neglected for scintillators that only contain hydrogen and carbon, e.g., toluene, but must be taken into account when the EC nuclide solution is incorporated to cocktails with heavier elements, e.g., Ultima Gold. All along the present year, an improved version of the program EMI has been developed. This code adds the photoionization reduced energy correction to the previous versions, and successfully explains the systematic difference between the measured activity concentrations of 55Fe in Ultima Gold and Insta Gel.

The LSC efficiency for low-Z electron-capture nuclides.

In this paper, we apply the CIEMAT/NIST method to the three low-Z electron-capture nuclides 54Mn, 55Fe and 65Zn by using the KLM and KL1L2L3M atomic rearrangement models, respectively. The counting efficiencies are computed with the new code EMILIA, which comprises an improved model for the interaction of low-energy X-rays with the scintillation cocktail. The calculated counting efficiencies are compared with experimental results.

The photoionization-reduced energy in LSC.

The atomic rearrangement cascade that follows the electron-capture decay process in low-Z radionuclides involves X-rays which have high photoelectric interaction probabilities. When the K-shell binding energy of the ionized atom (e.g., hydrogen) is significantly lower than the energy of the X-ray photon, the detector response to a photon-equivalent energy electron and the whole photoionization process are very similar. This is not the case when the scintillator cocktail contains larger atoms (e.g., oxygen and phosphorus in Ultima Gold). For larger Z atoms, the reduced energy of the whole photoionization process is less than the reduced energy of the interacting photon due to the nonlinear effects of ionization quenching. This paper shows the convenience of including a more detailed simulation of the photoionization process in the atomic rearrangement detection model for electron-capture nuclides such as (55)Fe, (51)Cr and (54)Mn. The need for more elaborate atomic rearrangement models is a consequence of the analysis of (125)I data.

Comparison of calculated spectra for the interaction of photons in a liquid scintillator. Example of 54Mn 835 keV emission.

The CIEMAT/NIST and TDCR methods in liquid scintillation counting, initially developed for the activity standardization of pure-beta radionuclides, have been extended to the standardization of electron capture and beta-gamma radionuclides. Both methods require the calculation of the energy spectrum absorbed by the liquid scintillator. For radionuclides emitting X-rays or gamma-rays, when the energy is greater than a few tens of keV the Compton interaction is important and the absorption is not total. In this case, the spectrum absorbed by the scintillator must be calculated using analytical or stochastic models. An illustration of this problem is the standardization of 54Mn, which is a radionuclide decaying by electron capture. The gamma transition, very weakly converted, leads to the emission of an 835 keV photon. The calculation of the detection efficiency of this radionuclide requires the calculation of the energy spectrum transferred to the scintillator after the absorption of the gamma ray and the associated probability of absorption. The validity of the method is thus dependent on the correct calculation of the energy transferred to the scintillator. In order to compare the calculation results obtained using various calculation tools, and to provide the metrology community with some information on the choice of these tools, the LS working group of the ICRM organised a comparison of the calculated absorbed spectra for the 835 keV photon of 54Mn. The result is the spectrum of the energy absorbed by the scintillator per emission of an 835 keV gamma ray. This exercise was proposed for a standard 20 ml LS glass vial and for LS cocktail volumes of 10 and 15 ml. The calculation was done for two different cocktails: toluene and a widely used commercial cocktail, Ultima Gold. The paper describes the results obtained by nine participants using a total of 12 calculation codes.

Nanodosemeters based on gel scintillators.

The feasibility of a nanodosemeter based on a liquid scintillator cocktail of four components (ethoxylated nonylphenol, pseudocumene, water and a lipophilic mixture) is studied. The dosemeter can work in distinct gel phases, for which the radioactive substance can be confined inside aqueous nanoscale structures of different size. For water volumes ranging 0-15%, it results in a gel with micelles of 4 nm radius. For water volumes ranging 30-50%, the resulting liquid-crystal gel contains nanostructures of approximately 20 nm radius. The low-energy electron emission arising from the decay of (3)H and (55)Fe is counted in a commercial liquid-scintillation counting spectrometer for both homogeneous and gel samples. The counting efficiency gap between the two phases is used to compute the average energy deposited inside the micelle.

Half-life determination of 40K by LSC.

The long-lived radioisotope 40K is one of the isotopes applied by geologists to date rocks hundreds or even thousands of millions of years old. Knowing the half-life of 40K, the potassium-argon (K-Ar) method gives an estimate of the date of the rock's formation by measuring the quantity of the daughter stable isotope 40Ar. As in the case of other radiometric methods, the results of the K-Ar dating method are generally accompanied by an error estimate, which includes the counting process, the uncertainty in the half-life of 40K and the beta to capture branching ratio. The objectives of this paper are basically three. First, we describe a procedure to incorporate the largest amount of the potassium cations into the liquid scintillator cocktail. This time, gels are able to provide much higher counting rates than the background. Second, Cerenkov counting gives the best shapefactor for 40K beta-ray transitions. Third, the CIEMAT/NIST method determines the activity of the samples and an averaged half-life for the radionuclide.

Inelastic scattering and stopping power of low-energy electrons (0.01-10 keV) in toluene.

In this study, the stopping power for electrons in toluene is reported for incident energies ranging from 10 to 10,000 eV. The present results have been obtained by combining the calculated inelastic electron scattering cross-sections with an experimental energy loss procedure. Calculations have been carried out by means of a quasifree absorption model whose reliability has been checked by comparison with empirical electron scattering total cross section data. Results have been compared with the high-energy stopping power data available in the literature. For energies below 1 keV these are the first results of the stopping power for electrons in toluene.

Improved Cerenkov counting techniques based on a free parameter model.

In the past few years, two Cerenkov methods were developed to make activity measurements of high-energy beta emitters in liquid scintillation counters with two or three photomultiplier tubes (PMTs) possible. Both methods are based on a free parameter model and make use of the Frank and Tamm theory for the emission of Cerenkov light. In this article, additional effects are discussed and further improvements are presented. The dependence of the refractive index of water on the wavelength can now be taken into account, which has also an influence on the upper limit of the wavelength region for the production of Cerenkov light. In addition, the dependence of the PMT response on the wavelength is taken into account. Finally, it is possible to take a potential asymmetry of efficiencies in a system with three PMTs into account. To this end, three free parameters are assigned to each individual PMT and then determined by means of a downhill simplex optimization algorithm. The computed counting efficiencies for a triple-to-double coincidence ratio (TDCR) system were compared with experimental data for (32)P, (89)Sr, and (90)Y.

Monte Carlo simulation of Auger-electron spectra.

A procedure to calculate the complex spectra of electron-capture nuclides which simultaneously eject several electrons and X-rays with different energies is presented. The model is applied to compute spectra of the radionuclides (125)I, (123)I and (111)In. The spectra are then compared with experimental spectra obtained by means of liquid scintillation counting. To this end, the computed spectra were transformed to allow for the nonlinear response function for a liquid scintillator, chemical quenching, as well as the Wallac-type amplifier used for the measurements. The calculated spectra are important for applications of free parameter models in liquid scintillation counting and also for studying the impact of electron-capture nuclides on DNA.

Standardization of electron-capture radionuclides by liquid scintillation counting

Liquid scintillation counting has proven to be an excellent technique to standardize beta-ray emitters with good accuracy. However, the success achieved in beta-ray nuclide standardization is not extensible for radionuclides decaying by electron-capture. Last comparison of 55Fe revealed discrepancies of 5% among laboratories. This paper presents two new versions of the computer program EMI. First we consider a better simulation of the photoelectric interaction of X-rays with the scintillator cocktail (EMI 1.2); and second, a more sophisticated KL1L2L3M atomic rearrangement model (EMI 2.0). We emphasize the importance of improving the simulation of the electron capture process to achieve better standardization of electron-capture nuclides by the CIEMAT/NIST method.