Simulation of Cherenkov photons emitted in photomultiplier windows induced by Compton diffusion using the Monte Carlo code GEANT4

TDCRPy: A python package for TDCR measurements

The TDCR (Triple-to-Double Coincidence Ratio) measurement technique is a primary standardization method used by metrology laboratories to accurately determine the activity of radioactive solutions, particularly for radionuclides unsuitable for traditional coincidence counting methods, such as pure beta emitters. The TDCR method leverages a liquid scintillation counter equipped with three photomultiplier tubes (PMTs). This paper introduces TDCRPy, a novel Python package developed by the BIPM, designed to calculate detection efficiency of liquid scintillation counters using Monte Carlo simulations and decay data evaluations from the Decay Data Evaluation Project (DDEP). The software simulates particle interactions within the liquid scintillation counter, utilizing pre-calculated probability distributions for energy deposition. Comparisons with the PENNUC/NUR code and tests with measurement from the BIPM.RI(II)-K1.Co-60 key comparison demonstrate the potential of TDCRPy. This open-source package is distributed at https://pypi.org/project/TDCRPy and available for collaborative development on GitHub https://github.com/RomainCoulon/TDCRPy, where detailed user documentation can be found.

Activity determination of 56Mn using extended TDCR-Čerenkov method

The accuracy of activity determination for activated nuclide ⁵⁶Mn is the key to the manganese bath method applying to the characterization of radionuclide neutron source. As an alternative to the 4π(Č)-γ method, TDCR-Čerenkov method could also be applied to the measurement of ⁵⁶Mn in the manganese bath device, if the existing calculation model is extended. There are two difficulties when the existing TDCR-Čerenkov method is applied to the activity determination of ⁵⁶Mn. One is that the efficiency computation of gamma transitions, and the other is the interference contributed by Čerenkov photons emitted in the photomultiplier windows induced by Compton scattering. In this study, the above two difficulties are solved by extending the calculation model. For efficiency computation, the decay scheme of ⁵⁶Mn is taken into account in the calculation of efficiency. Among them, the efficiency of gamma transition is calculated from the simulated secondary electronic spectra. In addition, Čerenkov photons emitted in photomultiplier windows are corrected by additional light proof experiment and improved calculation model. The results derived from this extended method are in good agreement with other standardization technique.

Activity standardization of two enriched 40K solutions for the determination of decay scheme parameters and the half-life

In this paper we describe experiments on two enriched ⁴⁰K solutions to accurately determine decay data. The first solution was measured in 2004/2005 by means of a gamma-ray spectrometer with low background and a liquid scintillation (LS) counter to apply the CIEMAT/NIST efficiency tracing method. A combination of results yields an emission probability of the 1461 keV gamma-rays of P_γ = 0.1030(11) which is lower than current results of data evaluations. The activity concentration of the second solution was also determined by means of LS counting, but here, the CIEMAT/NIST efficiency tracing method as well as the TDCR method were applied. Again, the result was combined with that of independent gamma-ray spectrometry and the gamma-ray emission probability was found to be P_γ = 0.1029(9) in good agreement with the result obtained from the first solution. A combination of both experiments yields P_γ = 0.1029(9). The spectra of a TriCarb LS counter were carefully analyzed and a beta minus emission probability [Formula: see text]  = 0.8954(14) was determined. The new results for P_γ and [Formula: see text] indicate that the overall probability of the decay via EC in recent data evaluations is overestimated. The LS counting efficiencies were computed with a stochastic model and up-to-date calculations of the beta spectrum and fractional EC probabilities were used. The final activity result of the second solution is combined with the outcome of a comprehensive isotopic analysis to determine the half-life of ⁴⁰K which is found to be 1.2536(27) ·10⁹ years. All above-stated uncertainties are standard uncertainties (k = 1).

Activity standardization by means of liquid scintillation counting and determination of the half-life of 89Zr

A⁸⁹Zr solution was measured by means of liquid scintillation counting techniques in order to determine the activity concentration. Two methods were used: the CIEMAT/NIST efficiency tracing method with ³H as a tracer, and the triple-to-double coincidence ratio method. The counting efficiencies were computed with a stochastic model. The very detailed investigation showed that a few corrections are particularly important: Asymmetries in the photodetector responses as well as the backscattering of high-energy gamma rays must be taken into account. Corresponding corrections have therefore been applied. In addition, a detailed uncertainty analysis was carried out and the uncertainties compared with those determined by other research groups. The activity concentrations obtained from the two methods agree well and a combined result was used to establish calibration factors for ionization chambers, which are important secondary standardization instruments. The ionization chambers were combined with a new high-precision current measurement device to provide outstanding linearity. Measurement data from one chamber were used to determine the half-life, which was found to be T_1/2=(78.373 ± 0.023) h.

Review of in vivo optical molecular imaging and sensing from x-ray excitation

SIGNIFICANCE

Deep-tissue penetration by x-rays to induce optical responses of specific molecular reporters is a new way to sense and image features of tissue function in vivo. Advances in this field are emerging, as biocompatible probes are invented along with innovations in how to optimally utilize x-ray sources.

AIM

A comprehensive review is provided of the many tools and techniques developed for x-ray-induced optical molecular sensing, covering topics ranging from foundations of x-ray fluorescence imaging and x-ray tomography to the adaptation of these methods for sensing and imaging in vivo.

APPROACH

The ways in which x-rays can interact with molecules and lead to their optical luminescence are reviewed, including temporal methods based on gated acquisition and multipoint scanning for improved lateral or axial resolution.

RESULTS

While some known probes can generate light upon x-ray scintillation, there has been an emergent recognition that excitation of molecular probes by x-ray-induced Cherenkov light is also possible. Emission of Cherenkov radiation requires a threshold energy of x-rays in the high kV or MV range, but has the advantage of being able to excite a broad range of optical molecular probes. In comparison, most scintillating agents are more readily activated by lower keV x-ray energies but are composed of crystalline inorganic constituents, although some organic biocompatible agents have been designed as well. Methods to create high-resolution structured x-ray-optical images are now available, based upon unique scanning approaches and/or a priori knowledge of the scanned x-ray beam geometry. Further improvements in spatial resolution can be achieved by careful system design and algorithm optimization. Current applications of these hybrid x-ray-optical approaches include imaging of tissue oxygenation and pH as well as of certain fluorescent proteins.

CONCLUSIONS

Discovery of x-ray-excited reporters combined with optimized x-ray scan sequences can improve imaging resolution and sensitivity.

Standardization of 68Ge/68Ga using the 4πβ-γ coincidence method based on Cherenkov counting

In the framework of an international BIPM comparison (Bureau International des Poids et Mesures), the activity standardization of ⁶⁸Ge in a solution of ⁶⁸Ge/⁶⁸Ga in equilibrium provided by NIST was carried out at LNHB. This exercise was organized to meet the growing interest in ⁶⁸Ga as a radiopharmaceutical in nuclear medicine services (e.g. as a surrogate of ¹⁸F for PET imaging). Due to the volatility of germanium, the activity standardization of ⁶⁸Ge was investigated at LNHB by means of 4πβ-γ coincidence counting based on Cherenkov measurements. This technique was applied to take advantage of the Cherenkov threshold (~ 260keV in aqueous solutions) in order to prevent counting from electron-capture events associated with ⁶⁸Ge disintegrations. Cherenkov counting was performed using glass and polyethylene vials and the resulting activity concentrations were compared with 4πβ-γ coincidence measurements based on liquid scintillation. The efficiency-extrapolation curve obtained with Cherenkov measurements in glass vials was compared to Monte Carlo simulations based on the Geant4 code.

A radionuclide calibrator based on Cherenkov counting for activity measurements of high-energy pure β¯-emitters

Due to their stability and reproducibility, re-entrant pressurized ionization chambers (also called radionuclide calibrators) are widely used for activity measurements in nuclear medicine services as well as in national metrology institutes to maintain reference standards. Generally, these secondary instruments yield accurate activity measurements for γ-emitting radionuclides. Ionization chambers are easy to use and thus well-adapted to guarantee the metrological traceability between national metrology institutes and end-users. However, the reproducibility of calibration factors can be significantly impaired when measuring high-energy pure β¯-emitters such as radiopharmaceuticals based on ⁹⁰Y. This is because the bremsstrahlung emission contributing to the instrument response is strongly dependent on the geometry of the components surrounding the radioactive solution. The present article describes a new design based on pulse counting to address this problem. It takes advantage of Cherenkov emission resulting from Compton scattering in transparent materials. The interest of Cherenkov counting is to obtain a low-sensitivity detector that enables direct measurements of high-activity solutions (at least up to 10 GBq for ⁹⁰Y-microspheres in aqueous suspensions used in nuclear medicine). A simple design based on a geometry close to an ionization chamber used at LNHB (Vinten 671 type) is described. The feasibility in terms of detection efficiencies (lower than 10^-4 for ⁹⁰Y solutions) of the new radionuclide calibrator is investigated by Monte Carlo calculations using the Geant4 code.

Monte Carlo based approach to the LS–NaI 4πβ–γ anticoincidence extrapolation and uncertainty

The 4πβ–γ anticoincidence method is used for the primary standardization of β−, β+, electron capture (EC), α, and mixed-mode radionuclides. Efficiency extrapolation using one or more γ ray coincidence gates is typically carried out by a low-order polynomial fit. The approach presented here is to use a Geant4-based Monte Carlo simulation of the detector system to analyze the efficiency extrapolation. New code was developed to account for detector resolution, direct γ ray interaction with the PMT, and implementation of experimental β-decay shape factors. The simulation was tuned to 57Co and 60Co data, then tested with 99mTc data, and used in measurements of 18F, 129I, and 124I. The analysis method described here offers a more realistic activity value and uncertainty than those indicated from a least-squares fit alone.

Calculation of extrapolation curves in the 4π(LS)β-γ coincidence technique with the Monte Carlo code Geant4

At LNE-LNHB, a liquid scintillation (LS) detection setup designed for Triple to Double Coincidence Ratio (TDCR) measurements is also used in the β-channel of a 4π(LS)β-γ coincidence system. This LS counter based on 3 photomultipliers was first modeled using the Monte Carlo code Geant4 to enable the simulation of optical photons produced by scintillation and Cerenkov effects. This stochastic modeling was especially designed for the calculation of double and triple coincidences between photomultipliers in TDCR measurements. In the present paper, this TDCR-Geant4 model is extended to 4π(LS)β-γ coincidence counting to enable the simulation of the efficiency-extrapolation technique by the addition of a γ-channel. This simulation tool aims at the prediction of systematic biases in activity determination due to eventual non-linearity of efficiency-extrapolation curves. First results are described in the case of the standardization (59)Fe. The variation of the γ-efficiency in the β-channel due to the Cerenkov emission is investigated in the case of the activity measurements of (54)Mn. The problem of the non-linearity between β-efficiencies is featured in the case of the efficiency tracing technique for the activity measurements of (14)C using (60)Co as a tracer.

First TDCR measurements at low energies using a miniature x-ray tube

Developed for radionuclide standardization using liquid scintillation, the Triple to Double Coincidence Ratio (TDCR) method is applied using coincidence counting obtained with a specific three-photomultiplier system. For activity determination, a statistical model of light emission is classically used to establish a relation between the detection efficiency and the experimental TDCR value. At LNE-LNHB, a stochastic approach of the TDCR modeling was developed using the Monte Carlo code Geant4. The interest of this TDCR-Geant4 model is the possibility to simulate the propagation of optical photons from their creation in the scintillation vial to the production of photoelectrons in photomultipliers. As an alternative to the use of radionuclide sources, first TDCR measurements are presented using a miniature x-ray tube closely coupled to the scintillation vial. The objective of this new set-up was to enable low-energy depositions (lower than 20 keV) in liquid scintillator in order to study the influence of both time and geometrical dependence between PMTs already observed with radioactive sources. As for the statistical TDCR model, the non-linearity of light emission is implemented in the TDCR-Geant4 model using the Birks formula which depends on the kB factor and the scintillation yield. Measurements performed with the x-ray tube are extended to the assessment of these parameters and they are tested afterwards in the TDCR-Geant4 model for activity measurements of (3)H.