WO1991008497A1

WO1991008497A1 - Method for quench determination and compensation in scintillation counting utilizing pulse shape analysis and a scintillation counter

Info

Publication number: WO1991008497A1
Application number: PCT/FI1989/000223
Authority: WO
Inventors: Kenneth Rundt
Original assignee: Wallac Oy
Priority date: 1989-12-01
Filing date: 1989-12-01
Publication date: 1991-06-13

Abstract

A method and a scintillation counter in which quench determination and compensation can be automatically adjusted to fit the scintillation characteristics of each sample separately. This is accomplished by having means for measuring both the quench level of the sample and means for measuring the pulse length of the scintillation pulses from the sample, and means for using both parameters in quench determination and compensation.

Description

METHOD FOR QUENCH DETERMINATION AND COMPENSATION IN SCINTILLATION COUNTING UTILIZING PULSE SHAPE ANALYSIS AND A SCINTILLATION COUNTER

ABSTRACT

This invention describes a method and a scintillation counter in which quench determination and compensation can be automatically adjusted to fit the scintillation characteristics of each sample separately. This is accomplished by having means for measuring both the quench level of the sample and means for measuring the pulse length of the scintillation pulses from the sample, and means for using both parameters in quench determination and compensation.

FIELD OF THE INVENTION

This invention relates to quench correction in an apparatus for determining the amounts of a radioactive isotope in scintillation samples. In particular, this invention relates to method and means for using pulse length information in addition to any indications of quench level for quench correction.

BACKGROUND OF THE INVENTION

Scintillation counting of soft beta-emitters like tritium and carbon-14 is a very common analytical technique in life sciences. The aim of this technique is to accurately determine the activity of one or several radioactive isotopes dissolved in a special scintillation liquid held in a transparent vial. The scintillation counter can normally count several hundreds of vials (samples) in an automatic manner without attendance. The standard scintillation counter comprises a detector compartment for sequentially holding each sample at a time. Normally the detector comprises two photomultiplier (p ) tubes simultaneously converting photon pulses that are emitted from the sample into electrical pulses. The pm tubes normally work according to the coincidence technique. In this technique, the electrical pulses from both tubes are fed into an electronic circuit, called the coincidence analyzer, which passes pulses on to pulse height analyzers and sealers only if there is a pulse in both of the two photomultiplier tubes within a certain time period, called the coincidence resolving time. The function of the coincidence analyzer can shortly be described as follows: assume that a scintillation pulse causes an analog output pulse at the output of one of the two pm tubes. At a certain time, the analog output pulse exceeds a certain threshold and sets a logic signal that will prevail for a finite time period, equal to the coincidence resolving time. Normally, the coincidence resolving time is quite short, typically in the order of about 15 ns. If during the coincidence resolving time an analog output pulse from the other pm tube triggers the coincidence analyzer, the analyzer sets it output gate signal to indicate a coincidence pulse. This output gate signal causes the analog pulse height analyzer to accept the two analog pulses from the two pm tubes. Normally the two analog pulses are summed before further analysis by pulse height analyzers and sealers or a multichannel analyzer.

A radioactive disintegration is a fast phenomenon in itself, but the process, in which the disintegration energy is transformed into photons, may extend over a considerable time period, e.g. up to a few microseconds. The characteristics of this scintillation pulse, the intensity and its decay rate, depends on the scintillation medium. In most media, the decay consists of two parts: the prompt part, which is the major part, and the slow, or delayed part. The prompt part, which originates from the lower part. The prompt part, which originates from the lower excited singlet states immediately formed at the disintegration, is so short and instant that most of the photons can be observed during the first 20 ns after the disintegration. A typical scintillation pulse is shown in Figure 1. The delayed part, which is dependent on the formation of higher excited and ionized states, may extend over a considerable time period and photons in this part may not be noticed by the coincidence analyzer. This fact is of no concern when the total number of photons is high, as there in that case will be many photons in the prompt part and a high probability that both pm tubes will receive photons within the coincidence resolving time. But if only a few photons are emitted, the first photon has a high probability to occur within the prompt part, and the next may occur much later, or within the delayed part, after the coincidence resolving time. In this case the coincidence analyzer will not accept this pulse. Thus, if the coincidence resolving time is short in comparison to the decay rate of the delayed part, there is a certain chance that a disintegration resulting in only two or three detected photons will not cause a coincidence condition. This situation arises typically with low energy isotopes like tritium in certain scintillation media. In scintillation counting this may cause two undesirable effects: 1) the counting efficiency is unnecessarily reduced and 2) the pulse height spectrum is distorted as a result of a proportion of pulses with low pulse heights being missed.

In scintillation counting, quenching of the scintillation light is a very important factor to consider. Quenching in the samples means that the number of emitted photons is decreased. The counting efficiency, defined as the ratio between the detected pulse rate to the disintegration rate, is dependent of the degree of quenching. Usually the degree of quenching and the counting efficiency has to be determined for each sample separately. As the position of the scintillation spectrum on the pulse height scale also is effect for determining a value for a quench index, Q, indicating the quench level. By the use of an empirical calibration curve (quench curve) , the counting efficiency is computed from the determined quench index. The mean pulse height (MPH) is an often used measure for the position of the scintillation spectrum as this is relatively easy to compute. If the coincidence resolving time is short, then the shape of the spectrum, and also the mean pulse height will depend on the intensity and the decay rate of the delayed part. The higher the intensity and the slower the decay rate of the delayed part, the more pulses will be lost in the low amplitude region of the spectrum - causing the counting efficiency to. decrease and MPH to increase. Figure 2 portrays the general influence of the coincidence resolving time on the shape of a tritium spectrum. As a result of this effect, a quench curve based on standards prepared with a solvent having an insignificant delayed part can not be used for quench correction of samples prepared with a solvent having a significant delayed part. As an example, Fig. 3 shows four quench curves prepared with standards based on toluene, xylene, pseudocumene and di- isopropyl-naphthalene. These quench curves were measured on a normal scintillation counter with the coincidence resolving time equal to 15 ns.

Not only the solvent is important in this respect. Also the quenching agent has an effect on the decay rate of the delayed part. This effect is demonstrated in Fig. 4, which shows the quench curves for standards based on toluene, but with two different quenchers: carbon tetrachloride and acetone.

The two Figures 3 and 4 demonstrate a general problem in scintillation counting: the composition of the quench curve standards has to be exactly the same as for the samples. This is not always possible to accomplish. In most cases, only one quench curve is produced and used with all sorts of samples, causing systematical errors of more or less unknown magnitude in the computed radioactivity. One small improvement to the problem is achieved by using as a quench index some measure which reflects the endpoint of the spectrum, as this is not dependent on losses of low amplitude pulses, as can be seen in Fig. 2. The herein invention proposes a solution to this problem based on having means to measure the length of the scintillation pulses together with at least one quench index, and means to automatically perform quench correction by using these two values together.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a typical scintillation decay curve and two time intervals set so that a simple measure for the pulse length can be obtained.

Figure 2 shows the influence of the coincidence resolving time on the shape of a tritium spectrum.

Figure 3 shows four quench curves prepared with standards based on toluene, xylene, pseudocu ene and di- isopropyl-naphthalene.

Figure 4 shows the quench curves for standards based on toluene, but with two different quenchers: carbon tetrachloride and acetone.

Figure 5 shows the pulse length as a function of the quench level for the five solvents in Figure 3.

Figure 6 shows a block diagram of a general embodiment of a scintillation counter according to this invention.

DESCRIPTION OF THE INVENTION The objective of this invention is a scintillation counter in which the coincidence resolving time of the coincidence analyzer is so short that normally a large number of different quench curves would have to be used for quench correction of different scintillation systems. The herein invention proposes a number of embodiments which are much related to each other, and may even be combined into one system. Generally, all embodiments depend on that the pulse length (mpl) of the scintillation pulses produced by the sample is determined together with a quench index value Q. In all cases, mpl and Q can be determined either by using the pulses produced by the internal radioisotope dissolved in the sample or by an. external gamma-radiating source momentarily placed adjacent to the sample in the measuring compartment. The pulse length mpl may be used together with the determined quench index Q (internal or external) in different ways, which are described more in details in the following text.

Generally, although mpl can be determined in many different ways, it is not of importance for this invention how it is determined. Neither is it of importance that the value determined for mpl is actually and exactly equal to the mean value of the pulse length; it is only requested that mpl in a consistent way is proportional to the exact value. For example, mpl can be determined by registering the pulse shape for a number of pulses by using an analog-to-digital converter to convert each pulse into a digital form that can be stored as a histogram in a multichannel analyzer, where each channel corresponds to a small fraction of time. The centroid of this histogram may be used for mpl. By way of example only, a simpler procedure may be based on determining the area below the curve in two parts of the histogram, A and B in Fig. 1, and equating the pulse length to the ratio between these two areas.

A general embodiment of this invention is shown in the block diagram in Figure 6. In this figure, 1 is a sample to be measured placed in a measuring compartment, and 2 and 3 are photon detectors comprising preamplifiers for detecting the photons emitted by the sample 1. The detectors are connected to a coincidence analyzer 4. The outputs of the two detectors are also connected to a summing amplifier 5, which is connected to a pulse shape analyzing means 6, which measures the pulse length of the scintillation pulses and transfers this value to quench processing means 8. The analyzer 4 and the summing amplifier 5 are connected to a pulse height analyzer and sealer means 7, which analyzes and counts the pulses that are approved by the coincidence analyzer 4. The device 7 also computes a quench index. The computed quench index and the measured count rate is transferred from means 7 to quench processing means 8 for quench determination and compensation. The operations of processing means 8 is described more in detail in the next paragraph. The information produced by means 8 is transferred to processing means 9, for further data reduction and for output to an external device (not indicated in the diagram) .

EXAMPLES OF EMBODIMENTS

The main principle of quench processing means 8 is to take the pulse length and the quench index as its inputs and produce a value for a certain quench dependent parameter. The counting efficiency E is an example of one very important quench dependent parameter for which accurate quench determination is needed.

In a first embodiment of this invention, a number of quench curves are stored in and accessed by quench processing means 8. Each quench curve has been prepared with a set of calibration standards for which mpl also was determined together with the counting efficiency and the quench index Q. The mpl is not a constant but decreases with increasing quench level, as is shown in Figure 5. In this figure, mpl is equal to the ratio of count rates received with two different coincidence resolving times. As can be seen from this picture, the quench curve inheritance of an unknown sample can be determined by comparing the mpl of the sample with values computed from stored curves of mpl as a function of Q. The limitation with this solution is that it is difficult to handle all possible combinations of mpl and Q, or, there may be liquids for which the determined mpl does not correspond to any of the stored curves.

In a second embodiment of this invention, only one basic quench curve is actually used. Let Q denote the original quench index of the sample and Qc a corrected quench index of which the counting efficiency is a function, i.e. E = E(Qc) . The corrected quench index Qc can be computed by using an empirical relationship of the form

Qc = Q + DQ(mpl,Q) . This means using both the mpl value and the Q value to compute a displacement DQ which is then added to Q. For example, if the basic quench curve is produced with a fast liquid, for which mpl is small, then, for scintillation liquids having a large mpl value (slow liquids) the original quench index Q is too high and the displacement DQ is negative. This solution can handle all possible combinations of mpl and Q.

In a third embodiment of this invention, utilizing only one basic quench curve and capable of handling all possible combinations of mpl and Q, the original counting efficiency E determined from this quench curve is corrected by an amount DE, which is a function of both mpl and Q. If the basic quench curve is produced with a fast liquid, then, for slow scintillation liquids the counting efficiency E is too high and the displacement DE is negative.

In a fourth embodiment of this invention, the quench curve is not considered as a curve dependent on the quench index Qc or Q only, but as a surface dependent on both the Q and mpl. That is, instead of the function E = E(Q) , the function E = E (Q , mpl ) is used .

In a sixth embodiment of this invention, the quench curve is not considered as a curve dependent on the quench index Qc or Q only, but as a volume dependent on three parameters of which two are Q and mpl. The third parameter can for example be a second quench index, R, proportional to the amount of color in the sample (T. Oikari and K. Rundt, US Pat. No. 4,700,072) . Instead of the function E = E(Q), the function E = E(Q,R,mpl) is then used.

Generally, the invention described herein is not limited to correction of counting .efficiency. In multi-label applications it is necessary to use information retrieved form the shape of the spectra, and a calibration curve does not only store data on the behavior of counting efficiency, but also on the behavior of certain other parameters describing the behavior of the spectrum shape as quench level varies. Typical parameters of these kinds are the relative intensity of the spectrum in certain counting windows. Even the possibility to store complete spectra has been developed. In this case the spectrum shape is stored in the form of a certain number of functions describing the behavior of some parameters as quench level varies. The herein invention may be utilized also in this case in a manner corresponding to the above described embodiments, with the extension that these parameters are treated in an analogue manner to the counting efficiency.

Claims

1. Method for scintillation counting, in which a quench dependent parameter is to be determined, comprising by

- measuring sequentially and automatically a plurality of scintillation samples capable of emitting photons clustered together in scintillation pulses as a result of radioactive decays occurring either inside or outside of the sample,

- detecting said scintillation pulses emitted from each of said scintillation samples at least by one photodetector, - connecting a coincidence analyzer to said photodetectors, connecting one or more incremental sealers to said coincidence analyzer, determining the quench level of said sample, characterized by - determing the stored quench calibration data comprising the pulse length of the scintillation pulses,

- determining for each sample separately the pulse length of the scintillation pulses, and determining a value for said quench dependent parameter on the basis of said determined quench index and said determined pulse length.

2. Method according to claim 1, characterized by

- said stored quench calibration data being equal to a multiple of quench curves comprising the parameter to be determined as a function of the quench index reflecting the quench level of the sample and the pulse length as a function of said quench index,

- determining a value for said quench dependent parameter comprising means for selecting, for each sample separately, among said stored quench curves the one for which said determined pulse length is closest to the value computed from said stored pulse length functions.

3. Method according to claim 1, characterized by said stored quench calibration data being equal to at least one quench curve comprising the parameter to be determined as a function of a first quench index reflecting both the quench level of the sample and the pulse length, determining a value for said quench dependent parameter comprising means for determining a value for a second quench index reflecting only the quench level of the sample and combining said second quench index with said determined pulse length into said first quench index.

4. Method according to claim 1, characterized by said stored quench calibration data being equal to at least one quench curve comprising the parameter to be determined as a function of a quench index reflecting the quench level of the sample, determining a value for said quench dependent parameter comprising means for computing an approximate value for said quench dependent parameter by using said determined quench index, computing a displacement value for said quench dependent parameter by using said pulse length, and combining said approximate value and said displacement value into a final value for said quench dependent parameter.

5. Method according to claim 1, characterized by said stored quench calibration data being equal to a quench surface comprising the parameter to be determined as a function of both a quench index reflecting the quench level of the sample and the pulse length.

6. Method according to claim 1, characterized by said stored quench calibration data being equal to a quench volume comprising the parameter to be determined as a function of a first quench index reflecting the total quench level of the sample, a second quench index reflecting the color quench level of the sample, and the pulse length.

7. Method according to any of claims 1-6, characterized by that said parameter to be determined is the counting efficiency.

8. Method according to any of claims 1-6, characterized by determining the pulse length of the scintillation pulses for each sample separately comprising by, - measuring each scintillation pulse, a first intensity value in a first time interval and a second intensity value in a second time interval, - adding together a number of pulses recorded during said first measuring period, said first intensity value to yield a first total intensity and said second intensity values to yield a second total intensity computing the pulse length from the values of said first total intensity and said second total intensity.

9. Method according to any of claims 1-6, characterized by determining the pulse length of the scintillation pulses for each sample separately comprising by, determing a first and a second incremental sealer for counting pulses, measuring for each scintillation pulse, a first intensity value in a first time interval and a second intensity value in a second time interval, computing the ratio between said two intensities and comparing said ratio to a certain predetermined limit, incrementing said first sealer if said ratio exceeds said limit and incrementing said second sealer if said ratio does not exceed said limit, and - computing the pulse length from the number of pulses stored in said sealers.

10. Method according to any of claims 1-7, characterized by determining for each sample separately the pulse length comprises means for positioning a gamma-ray emitting source adjacent to said sample for causing compton electrons, and means for determining the pulse length of the scintillation pulses produced by said compton electrons.

11. Scintillation counter, in which a quench dependent parameter is to be determined, comprising - means for measuring sequentially and automatically a plurality of scintillation samples capable of emitting photons clustered together in scintillation pulses as a result of radioactive decays occurring either inside or outside of the sample, - at least one photodetector detecting said scintillation pulses emitted from each of said scintillation samples, a coincidence analyzer connected to said photodetectors, one or more incremental sealers connected to said coincidence analyzer, - means for determining the quench level of said sample characterized by comprising stored quench calibration data comprising the pulse length of the scintillation pulses, means for determining for each sample separately the pulse length of the scintillation pulses, and means for determining a value for said quench dependent parameter on the basis of said determined quench index and said determined pulse length.

12. Scintillation counter according to claim 11, characterized by said stored quench calibration data being equal to a multiple of quench curves comprising the parameter to be determined as a function of the quench index reflecting the quench level of the sample and the pulse length as a function of said quench index, said means for determining a value for said quench dependent parameter comprising means for selecting, for each sample separately, among said stored quench curves the one for which said determined pulse length is closest to the value computed from said stored pulse length functions.

13. Scintillation counter according to claim 11, characterized by

- said stored quench calibration data being equal to at least one quench curve comprising the parameter to be determined as a function of a first quench index reflecting both the quench level of the sample and the pulse length,

- said means for determining a value for said quench dependent parameter comprising means for determining a value for a second quench index reflecting only the quench level of the sample and combining said second quench index with said determined pulse length into said first quench index.

14. Scintillation counter according to claim 11, characterized by - said stored quench calibration data being equal to at least one quench curve comprising the parameter to be determined as a function of a quench index reflecting the quench level of the sample, said means for determining a value for said quench dependent parameter comprising means for computing an approximate value for said quench dependent parameter by using said determined quench index, computing a displacement value for said quench dependent parameter by using said pulse length, and combining said approximate value and said displacement value into a final value for said quench dependent parameter.

15. Scintillation counter according to claim 11, characterized by said stored quench calibration data being equal to a quench surface comprising the parameter to be determined as a function of both a quench index reflecting the quench level of the sample and the pulse length.

16. Scintillation counter according to claim 11, characterized by said- stored quench calibration data being equal to a quench volume comprising the parameter to be determined as a function of a first quench index reflecting the total quench level of the sample, a second quench i de.-: reflecting the color quench level of the sample, and the pulse length.

17. Scintillation counter according to any of claims 11-16, characterized by that said parameter to be determined is the counting efficiency.

18. Scintillation counter according to any of claims 11-16, characterized by that said means for determining for each sample separately the pulse length of the scintillation pulses comprises, means for measuring for each scintillation pulse, a first intensity value in a first time interval and a second intensity value in a second time interval, - means for adding together for a number of pulses recorded during said first measuring period, said first intensity value to yield a first total intensity and said second intensity values to yield a second total intensity - means for computing the pulse length from the values of said first total intensity and said second total intensity.

19. Scintillation counter according to any of claims 11-16, characterized by that said means for determining for each sample separately the pulse length of the scintillation pulses comprises,

- a first and a second incremental sealer for counting pulses, means for measuring for each scintillation pulse, a first intensity value in a first time interval and a second intensity value in a second time interval, means for computing he ratio between said two intensities and comparing said ratio to a certain predetermined limit,

- means for incrementing said first sealer if said ratio exceeds said limit and incrementing said second sealer if said ratio does not exceed said limit, and means for computing the pulse length from the number of pulses stored in said sealers.

20. Scintillation counter according to any of claims 11-17, characterized by that said means for determining for each sample separately the pulse length comprises means for positioning a gamma-ray emitting source adjacent to said sample for causing compton electrons, and means for determining the pulse length of the scintillation pulses produced by said compton electrons.