A METHOD FOR EVALUATING SAMPLE ACTIVITIES IN A MULTIDETECTOR LIQUID SCINTILLATION COUNTER

Abstract: The object of the invention is a method for the evaluation of sample activities in multidetector liquid scintillation counters. According to the invention all prepared quench curve standards are measured only in one reference detector in order to establish the relationship between the quench levels and the counting efficiencies. In addition, only two of said standards, preferably those with lowest and highest level of quench, are measured in every detector in order to establish the relationship of the quench levels between the reference detector and any other detector and in order to establish the relationship of the counting efficiencies between the reference detector and any other detector. The activities of unknown samples are evaluated by utilizing said predetermined relationships.

Description: A METHOD FOR EVALUATING SAMPLE ACTIVITIES IN A MULTIDETECTOR LIQUID SCINTILLATION COUNTER BACKGROUND OF THE INVENTION Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium, iodine-125, carbon-14, sulphur-35, calcium-40 and chromium-51. The range of the low energy beta particles in the sample is in generally few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with a scintillation medium by dissolving or suspending the sample within the liquid scintillation medium in a container so that the emitted beta particles can interact with the molecules of the liquid scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solution. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected by two, in coincidence operating, photomultiplier tubes producing electric pulses. The sum pulse height is proportional to the energy of the interacted beta particle. When measuring sample activities with the liquid scintillation counters, the basic problem is the reduction of the counting efficiency due to the quenching of the sample, which can be classified in two main types: the chemical quench and the color quench. The chemical quench is a phenomenon, where the solution formed by the sample and the scintillation medium contains some impurities. which reduce the efficiency of the counting system to detect the emitted beta particles by absorbing them. The color quench is a phenomenon, where the solution formed by the sample and the scintillation medium contains some impurities, which absorb produced scintillation photons. The consequence of this is also the reduction counting efficiency. It is known in the liquid scintillation counting that the reduction of the counting efficiency due to the quenching of the sample can be corrected by a means of a quench curve which describes the relationship between the counting efficiency and the amount of the quench of the sample. Normally liquid scintillation counters are provided with one detector and the quench curve is obtained by measuring a number of standards with identical activities but different quench levels in this detector. Difficulties will be encountered, when a liquid scintillation counter is provided with a plurality of detectors: all standards should be measured in each detector due to the variations in energy and efficiency responses of the detectors , which is a time consuming procedure. SUMMARY OF THE INVENTION The object of this invention is to produce a novel method, which differs from known level of techniques, for the evaluation of sample activities in multi-detector liquid scintillation counters. According to this method all quench curve standards are measured only in one detector and only two of the standards are measured in every detector. Thus a remarkable time saving is obtained compared to the known level of techniques. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram which illustrates a relationship between the counting efficiency and the SQP(I) in detector 1. Fig. 2 is a diagram which illustrates a relationship between the SQP(I) in detector 1 and the SQP(I) in detector j. Fig. 3 is a diagram which illustrates a relationship between the counting efficiency in detector j and the counting efficiency in detector 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS A number of standards with identical activities but different quench levels are prepared. All these standards are measured in the first detector of the multidetector liquid scintillation counter. This establishes the relationship illustrated in FIG. 1 between a quench related quantity called SQP(I) and efficiency. This relationship is stored in the memory of the apparatus. SQP(I) is a measure of the spectrum location (the centroid of the observed spectrum) and varies as the quench changes. It should be understood that -instead of SQP(I)- any other quench parameter can be applied. Two of said standards, preferably those with lowest and highest level of quench, are measured in every detector. The purpose of this is to establish the relationship illustrated in FIG. 2 of SQP(I) values between the first detector and any other detector and to establish the relationship illustrated in FIG. 3 of the efficiency values between the first detector and any other detector. These relationships are stored in the memory of the apparatus. The applicants have observed that these relationships are linear functions so that two points obtained with two standards are sufficient to define them. The actual sequence of events for evaluating activities of unknown samples is as follows: 1. The SQP(I) value and count rate of the sample are measured with any detector. 2. The SQP(I) value is converted as shown in Fig. 2 to correspond the SQP(I) value of the first detector using the relationship of SQP(I) values of the actual detector and the first detector. 3. The converted SQP(I) value is used as shown in Fig. 1 to read off the detector 1 efficiency from the quench curve made for samples counted with detector 1. 4. The efficiency value is converted as shown in Fig. 3 to correspond the actual detector efficiency using the relationship of the efficiency values of the first detector and the actual detector. 5. The measured count rate in the actual detector is divided by the actual detector efficiency to obtain the activity of the measured sample. The method according to the present invention is not confined to the above description alone, but it may show even considerable variation within the scope of the patent claims.

Claims: CLAIMS 1. Method for evaluating the activities of samples in multidetector liquid scintillation counter, in which method a number of standards with identical activities but different quench levels are measured for evaluating the variations in energy and efficiency responses of the detectors, characterized by that a number of standards with identical activities but different quench levels are measured only in one detector or in some detectors of said multidetector, - and at least two of said standards are measured in every other detector. 2. Method according to claim 1, characterized by that a number of standards are measured only in the first detector of said multidetector liquid scintillation counter, and two of said standards, preferably those with lowest and highest level of quench, are measured in every other detector. 3. Method according to claim 1 or 2, characterized by that due to the measurings the following relationships are established, due to the measurings of the number of standards in the first detector a relationship between the quench levels and the counting efficiencies is established, - due to the measurings of at least two of said standards in every other detector a relationship of the quench levels between said first detector and any other detector is established. and a relationship of the counting efficiencies between said first detector and any other detector is established. 4. Method according to claim 3, characterized by that in the memory of said multidetector liquid scintillation counter is stored said relationship between the quench levels and the counting efficiencies, said relationship of the quench levels between said first detector and any other detector, - said relationship of the counting efficiencies between said first detector and any other detector. 5. Method according to claim 4, characterized by that for evaluating activity of an unknown sample the following steps are taken, the quench level and the count rate of an unknown sample is measured by any detector of said multidetector liquid scintillation counter, said quench level to correspond the quench level of the reference detector is converted by using said stored relationship of the quench levels of the actual detector and said reference detector, said converted quench level is used to read off the corresponding counting efficiency of said reference detector by using said stored relationship of the counting efficiencies and the quench levels in said reference detector, said counting efficiency corresponding to said reference detector is converted to correspond to the counting efficiency of said actual detector by using said stored relationship of the counting efficiencies of said reference detector and said actual detector, said measured count rate in said actual detector is divided by said efficiency of the actual detector to obtain the activity of the said sample.

Inventors: KOURU HEIKKI, YRJOENEN TAPIO, KOURU, HEIKKI, YRJOENEN, TAPIO
Applicants: WALLAC OY
Application year: 1990
Main classification: G01T1
IPC codes: G01T1