The CIEMAT/NIST method

by Agustín Grau Malonda, Instituto de Estudios de la Energía, 2001

The CIEMAT/NIST method is a relative or tracing activity measurement method suitable for activity determination of pure beta, beta-gamma, pure EC and EC-gamma decaying radionuclide using a commercial liquid scintillation (LS) spectrometer.

The commercial LS spectrometers have two photomultipliers working in coincidence to remove the thermal noise generated at the photocathode output of each one of the photomultipliers.

The CIEMAT/NIST method requires one to know the experimental counting efficiency of one tracer radionuclide (tritium seems to be the most adequate) for different degrees of chemical quench and to compute the counting efficiency at the photocathode output for different values of the free parameter. To obtain an universal curve, which allows one to determine the counting efficiency for any radionuclide, a model for the counting efficiency as a function of the free parameter must be developed for each radionuclide.

CIEMAT/NIST method diagram

The following diagram describes the procedure to obtain the counting efficiency for the pure beta ray emitter ¹⁴C. We consider ¹⁴C in this example but any other radionuclide can be used for the counting efficiency determination. The first step is to obtain the experimental calibration curve giving the counting efficiency e vs. chemical quench parameter q (curve(a)), and to compute the counting efficiency e vs. the free parameter l for tritium (curve(b)). Since both curves have in common the counting efficiency, we can obtain the relation between the free parameter and the chemical quench parameter (curve(c)). This curve is universal in the sense that it is independent of the radionuclide but depends on the liquid scintillation spectrometer, the vial and the liquid scintillator used. The next step is to compute the counting efficiency vs. the free parameter for the radionuclide to be standardized, in our case ¹⁴C. By applying the universal curve (c) to the ¹⁴C curve (d), and taking the free parameter as a common quantity, we obtain curve (e), the calibration curve giving the counting efficiency vs. quench parameter for ¹⁴C.

Diagram describing the application of the CIEMAT/NIST method to the standardization of ¹⁴C

(a) Experimental efficiency vs. quench curve for tritium

(b) Computed efficiency vs. free parameter for tritium

(c) Universal curve

(d) Computed efficiency vs. free parameter for ¹⁴C

(e) Computed efficiency vs. quench for ¹⁴C

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