A 4pi(LS)beta-gamma coincidence system using a TDCR apparatus in the beta-channel

Liquid scintillation counting of electrons from internal conversion for quantifying 109Cd activity using a 3-photomultiplier tube system and a specific spectral unfolding

As part of the international key comparison CCRI(II)-K2.Cd-109.2021, the BIPM has implemented a specific method to measure the activity of ¹⁰⁹Cd solution - a key radionuclide in the calibration of gamma-ray spectrometers. The counting of electrons from internal conversion was carried out using a liquid scintillation counter based on 3-photomultiplier tubes. In this technique, a major part of the uncertainty comes from the overlap between the conversion electrons peak and the peak at lower energy from the other products of the decay. As a result, the energy resolution that the liquid scintillation system can achieve is the most critical challenge to achieving a precise measurement. The study demonstrates the advantage of producing a sum of the signal from the three photomultipliers to enhance the energy resolution and limit the peak overlap. In addition, the spectrum has been processed by a specific unfolding approach to properly separate the spectral components. Thanks to the method introduced in this study, an activity estimation has been realized with a relative standard uncertainty of 0.5%.

Characterization of a 4παβ(LS)-γ(HPGe) prototype system for low-background measurements

A ground-level prototype system for low-background measurements was developed and tested. The system consists of a high-purity germanium (HPGe) detector used for detecting γ rays and coupled to a liquid scintillator (LS) used for detecting α and β particles. Both detectors are surrounded by shielding materials and anti-cosmic detectors ("veto") used to suppress background events. The energy and timestamp of detected α, β and γ emissions are recorded event-by-event and analyzed offline. By requiring timing coincidence between the HPGe and LS detectors, background events originating from outside the volume of the measured sample can be effectively rejected. The system performance was evaluated using liquid samples containing known activities of an α emitter (²⁴¹Am) or a β emitter (⁶⁰Co) whose decays are accompanied by γ rays. The LS detector was found to provide a solid angle of almost 4π for α and β particles. Compared to the traditional γ-singles mode, operating the system in coincidence mode (i.e., α-γ or β-γ) reduced the background counts by a factor of ∼100. Consequently, the minimal detectable activity for ²⁴¹Am and ⁶⁰Co was improved by a factor of 9, being 4 mBq and 1 mBq for an 11-d measurement, respectively. Furthermore, by applying a spectrometric cut in the LS spectrum that corresponds to α emission from ²⁴¹Am, a background reduction factor of ∼2400 (compared to γ-singles mode) was achieved. Beyond low-background measurements, this prototype exhibits additional compelling features, such as the ability to focus on certain decay channels and study their properties. This concept for a measurement system may be of interest to laboratories that monitor environmental radioactivity, studies involving environmental measurements and/or trace-level radioactivity.

Development of a movable 4πβ(LS)-γ coincidence counting system for activity standardization of β-γ emitters

A new movable 3PM-γ coincidence system, based on 4πβ(LS)-γ coincidence counting, for activity measurement of β-γ emitters has been designed at the Korea Research Institute of Standards and Science (KRISS). The system incorporates 3 PM tubes on the plane and two detectors placed above and below the center of the plane. The 3 PM tubes for β-counters in the plane are movable up to 100 mm from a liquid scintillation vial, thus enabling the variation of β-detection efficiencies by a geometrical technique. A NaI(Tl) γ-counter was set above for the present work. The β-event is determined by counting the logical sum of three double coincidences. All the necessary electronics, i.e., logical sum, adjusting the duration of dead-time of each counting channel and coincidence resolving times, and analyzing coincidence relation, were specially designed to be fabricated in an integrated circuit. Details of the detectors, the electronics, the overall movable 3PM-γ coincidence system are presented, as well as the results of investigations to assess its operating characteristics. Validation measurements have been performed with ⁶⁰Co and ⁵⁷Co sources. The highest β-detection efficiency achieved with ⁶⁰Co and ⁵⁷Co was 97% and 95%, respectively. The activity concentration determined with a new system agreed with calibrated values within the uncertainty range. Further results from validation measurements and the corresponding uncertainty budgets are presented.

Comparison of digital coincidence modules used at POLATOM and PTB for TDCR and 4πβ(LS)-γ coincidence counters

Activity measurements of ³H, ²⁴¹Am and ⁶⁰Co solutions were performed to compare digital coincidence modules used at PTB and POLATOM for TDCR and 4πβ(LS)-γ coincidence counting. The activities determined with various coincidence modules connected in parallel to the same counter at PTB were found to be consistent. Observed discrepancies caused by differences in the coincidence resolving time did not exceed 0.14%. Accidental coincidences simulated by a frequency generator were registered, and the coincidence resolving time was determined.

A new 4π(LS)-γ coincidence counter at NCBJ RC POLATOM with TDCR detector in the beta channel

A new 4π(LS)-γ coincidence system (TDCRG) was built at the NCBJ RC POLATOM. The counter consists of a TDCR detector in the beta channel and scintillation detector with NaI(Tl) crystal in the gamma channel. The system is equipped with a digital board with FPGA, which records and analyses coincidences in the TDCR detector and coincidences between the beta and gamma channels. The characteristics of the system and a scheme of the FPGA implementation with behavioral simulation are given. The TDCRG counter was validated by activity measurements on (14)C and (60)Co solutions standardized in RC POLATOM using previously validated methods.

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.

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

The implementation of the TDCR method (Triple to Double Coincidence Ratio) is based on a liquid scintillation system which comprises three photomultipliers; at LNHB, this counter can also be used in the beta-channel of a 4pi(LS)beta-gamma coincidence counting equipment. It is generally considered that the gamma-sensitivity of the liquid scintillation detector comes from the interaction of the gamma-photons in the scintillation cocktail but when introducing solid gamma-ray emitting sources instead of the scintillation vial, light emitted by the surrounding of the counter is observed. The explanation proposed in this article is that this effect comes from the emission of Cherenkov photons induced by Compton diffusion in the photomultiplier windows. In order to support this assertion, the creation and the propagation of Cherenkov photons inside the TDCR counter is simulated using the Monte Carlo code GEANT4. Stochastic calculations of double coincidences confirm the hypothesis of Cherenkov light produced in the photomultiplier windows.

Standardization of 67Ga using a 4π(LS)β–γ anti-coincidence system

(67)Ga is an interesting radionuclide as it is widely used in nuclear medicine. The meta-stable level related to the 93.3keV gamma-transition represents the main difficulty when using the coincidence method to standardize this radionuclide. The 4pi(LS)beta-gamma anti-coincidence system implemented at LNHB is based on the use of electronic modules specifically designed for radioactivity metrology. On the contrary to classical coincidence systems, activity measurements of (67)Ga are carried out as for prompt beta-gamma emitters; indeed, when using a live-timed anti-coincidence system with extendable dead times, the problem due to the excess of counting generated by the meta-stable level is avoided. Considering that the standardization of (67)Ga does not depend on the decay scheme parameters (except for the half-life), the measurement of the gamma-emission intensities has been performed. The standardization of this radionuclide was also a good opportunity for a new participation of our laboratory in the SIR of (67)Ga (International Reference System); the result obtained with the 4pi(LS)beta-gamma anti-coincidence system is compared with those submitted by other National Metrology Institutes (NMIs). The non-extendable dead times used in most of the participations could be one of the causes responsible for the abnormal dispersion of the results. The optimization of the standard solution of (67)Ga for the radioactive source preparation is also discussed.