On the photomultiplier-tube asymmetry in TDCR systems

Karsten Kossert (1) , Benoit Sabot (2) , Philippe Cassette (2) , Romain Coulon (3) , Haoran Liu (4)

1 PTB - Physikalisch-Technische Bundesanstalt [Braunschweig]
2 LNHB (CEA, LIST) - Laboratoire National Henri Becquerel (CEA, LIST)
3 BIPM - Bureau International des Poids et Mesures
4 National Institute of Metrology [Beijing]

The responses of the three photomultiplier tubes (PMTs) in a triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) system are often not identical. Such asymmetries can have a significant influence on activity determinations. The problem is often solved by means of a minimization algorithm which can easily be applied when analytical methods are used for the efficiency calculation, as is usually done for pure beta emitters. However, for radionuclides with more complex decay schemes, the counting efficiencies are often calculated with stochastic methods and the computation of the required corrections becomes very challenging. This paper presents a new numerical method to overcome the asymmetry problem for such complex decays and a comprehensive study on 55Fe is described in detail. For the measurements, the asymmetry was varied by means of grey filter films which were placed in front of one of the photomultiplier tubes. In the case of the pure electron-capture (EC) radionuclide 55Fe, the asymmetry can also be taken into account with a very simple correction which is derived assuming monoenergetic emissions. This work is also of great importance for the planned extension of the International Reference System (SIR) at the BIPM which will be used for international comparisons in radionuclide metrology.

Mots clés: Asymmetry; Complex decay schemes; International comparisons; radioactivity; ionizing radiation; instrumentation; nuclear instrumentation; metrology; interlaboratory comparison; photomultiplier tube; triple-to-double coincidence ratio; liquid scintillation; Primary activity measurement; activity determination; signal processing; spectrum analysis; minimization algorithm; beta-rays; pure beta emitter; Decay data measurement; counting efficiency; 55Fe; pure electron-capture; TDCR; Photomultiplier tube

Domaines: Instrumentations et Détecteurs [physics.ins-det]; Physique Nucléaire Expérimentale [nucl-ex]; Electronique

Karsten Kossert, Benoit Sabot, Philippe Cassette, Romain Coulon, Haoran Liu. On the photomultiplier-tube asymmetry in TDCR systems. Applied Radiation and Isotopes, 2020, 163, 109223 (7 p.). ⟨10.1016/j.apradiso.2020.109223⟩. ⟨cea-04551032⟩