METHOD FOR DETERMINING A BACKGROUND COUNT RATE IN LIQUID SCINTILLATION COUNTING

Abstract

The present invention provides a method for determining a background count rate in liquid scintillation counting. The method comprises measuring external standard spectra of a sample, determining, from the external standard spectra, a triple to double coincidence ratio and a quench parameter, determining, based on the triple to double coincidence ratio and the quench parameter, a background reference parameter, and determining, based on the background reference parameter, the background count rate from a background reference curve.

Claims

1. A method for determining a background count rate in liquid scintillation counting, wherein the method comprises: measuring external standard spectra of a sample; determining, from the external standard spectra, a triple to double coincidence ratio and a quench parameter; determining, based on the triple to double coincidence ratio and the quench parameter, a background reference parameter; and determining, based on the background reference parameter, the background count rate from a background reference curve.

2. The method according to claim 1, wherein the background reference curve is generated by: using a plurality of background samples having different quenches and performing the following steps for each background sample: measuring external standard spectra of the background sample; determining, from the external standard spectra, a triple to double coincidence ratio and a quench parameter; determining, based on the triple to double coincidence ratio and the quench parameter, a background reference parameter; measuring an energy spectrum of the background sample; and determining, from the energy spectrum, a background sample count rate within an energy window; plotting the background sample count rates against the background reference parameters; and fitting a curve to the datapoints to obtain the background reference curve.

3. The method according to claim 2, wherein the background sample contains a liquid scintillation cocktail and a quench agent.

4. The method according to claim 2 wherein the number of background samples is at least 6.

5. The method according to claim 2, wherein a lower limit of the energy window is between 1 and 10 keV, and an upper limit of the energy window is between 30 and 75 keV.

6. The method according to claim 1, wherein the quench parameter is the spectral endpoint of the external standard spectrum.

7. The method according to claim 1, wherein the background reference parameter is calculated by an equation:
Ref=TDCR{circumflex over ( )}a*QP{circumflex over ( )}b, where TDCR is the triple to double coincidence ratio, QP is the quench parameter, and a and b are weight parameters.

8. The method according to claim 1, wherein the background reference parameter is calculated by an equation:
Ref=TDCR{circumflex over ( )}a*QP{circumflex over ( )}b*Ecounts{circumflex over ( )}c, where TDCR is the triple to double coincidence ratio, QP is the quench parameter, Ecounts is an external standard count rate within an energy window, and a, b and c are weight parameters.

9. A method for determining a net sample count rate in liquid scintillation counting, comprising: measuring an energy spectrum of a sample; determining, from the energy spectrum, a gross sample count rate within an energy window; wherein the method comprises: determining a background count rate according to claim 1; and subtracting the background count rate from the gross sample count rate to obtain the net sample count rate.

10. The method according to claim 1, wherein the sample contains carbon-14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG. 1 illustrates an example of a liquid scintillation counting system that employs a coincidence counting technique,

[0057] FIG. 2 illustrates examples of an external standard spectrum,

[0058] FIG. 3 illustrates an example of a background reference curve, and

[0059] FIG. 4 illustrates an example of a quench curve.

DETAILED DESCRIPTION OF THE DRAWINGS

[0060] FIG. 1 has already been described in connection with the background of the invention. Embodiments of the invention will now be described with reference to FIGS. 2 to 4.

[0061] FIG. 2 illustrates examples of an external standard spectrum. The external standard spectrum represents an external standard count rate as a function of energy (pulse height). In FIG. 2, there are shown external standard spectra measured from two different samples, namely sample A and sample B. The external standard spectra have been measured with the liquid scintillation counting system of FIG. 1, which enables to detect double and triple coincidence pulses. The external standard spectra of the sample A are indicated with reference numbers 201 and 202. The external standard spectrum 201 includes all (i.e. double and triple) coincidence counts measured from the sample A, and the external standard spectrum 202 includes only the triple coincidence counts measured from the sample A. The external standard spectra of the sample B are indicated with reference numbers 203 and 204. The external standard spectrum 203 includes all (i.e. double and triple) coincidence counts measured from the sample B, and the external standard spectrum 204 includes only the triple coincidence counts measured from the sample B. The external standard spectra 201, 202, 203 and 204 were achieved by using an external gamma radiation source in the liquid scintillation counting system. The gamma radiation produced a wide spectrum of energies of Compton electrons via the Compton effect.

[0062] Based on the external standard spectra 201, 202, 203 and 204, TDCR values (TDCR A, TDCR B) within an energy window 205 can be calculated and quench parameters (QP A, QP B) can be determined. The quench parameters are the spectral endpoints of the external standard spectra 201 and 203. The TDCR value and the quench parameter are used in determining a background reference parameter that is then used to determine a background count rate for the sample from a background reference curve. The quench parameter can also be used in determining the counting efficiency for the sample from a quench curve.

[0063] FIG. 3 illustrates an example of a background reference curve. The background reference curve 301 represents a background count rate as a function of a background reference parameter. The background reference curve 301 was created by using a set of blank background samples with variable quench. For each background sample, the background reference parameter was determined as well as the actual count rate was measured. The background reference curve 301 was established by plotting all the datapoints to the count rate versus the background reference parameter graph and by fitting a curve to these datapoints.

[0064] The background count rates for the samples A and B can be determined from the background reference curve 301 as follows. First, the background reference parameters (Ref A, Ref B) are calculated by using TDCR A and QP A for Ref A and TDCR B and QP B for Ref B. Then, the background count rates (Bkg A, Bkg B) are determined from the background reference curve 301 by finding the associated background count rates for the values of the background reference parameter.

[0065] FIG. 4 illustrates an example of a quench curve. The quench curve 401 represents a counting efficiency as a function of a quench parameter. The quench curve 401 was created by using a plurality of standard samples with known radioactivity and variable quench. For each standard sample, the quench parameter was measured, and the counting efficiency was calculated by dividing the observed radioactivity with the known radioactivity. The quench curve 401 was established by plotting all the datapoints to the counting efficiency versus the quench parameter graph and by fitting a curve to these datapoints. The quench curve 401 can be used to determine a counting efficiency for the samples A and B. The counting efficiency (Eff A, Eff B) for the samples A and B is determined from the quench curve 401, based on the quench parameters (QP A, QP B). The absolute activity in the samples A and B that eventually corresponds to the number of radionuclides can be determined by dividing the net sample count rate by the counting efficiency.

[0066] Only advantageous exemplary embodiments of the invention are described in the figures. It is clear to a person skilled in the art that the invention is not restricted only to the examples presented above, but the invention may vary within the limits of the claims presented hereafter. Some possible embodiments of the invention are described in the dependent claims, and they are not to be considered to restrict the scope of protection of the invention as such