WO1991010922A1

WO1991010922A1 - Method and a device for identifying radioisotopes in a liquid scintillation sample

Info

Publication number: WO1991010922A1
Application number: PCT/FI1990/000016
Authority: WO
Inventors: Kenneth Rundt; Heikki Kouru
Original assignee: Wallac Oy
Priority date: 1990-01-16
Filing date: 1990-01-16
Publication date: 1991-07-25

Abstract

This invention describes a method and a liquid scintillation counter capable of automatically determining for each sample separately both the number and type of radioisotopes present in the sample. This is accomplished by having means for measuring the quench level of the sample, means for producing model spectra of each feasible radioisotope at said quench level, and means for comparing said model spectra to the spectrum received from the sample.

Description

METHOD AND A DEVICE FOR IDENTIFYING RADIOISOTOPES IN A LIQUID SCINTILLATION SAMPLE

FIELD OF THE INVENTION

This invention relates to a method and an apparatus for identifying the radioisotopes present in a liquid scintillation samples. This invention also makes it possible to determine the number of radioisotopes in the sample.

BACKGROUND OF THE INVENTION

Liquid scintillation counting of beta-emitters like tritium, carbon-14 and sulfur-35 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 liquid scintillation counter can normally count several hundreds of vials (samples) in an automatic manner without attendance.

Beta-isotopes decay by emitting energy in the form of a fast electron and a neutrino. The energy liberated in the decay is always constant for a certain radioisotope, but is divided between the electron and the neutrino according to a distribution law. The neutrino can not be detected by using liquid scintillation counting but the electron will through collisional impact, transfer some of its energy to the liquid solvent molecules which are then ionized or excited to higher energy levels. Provided that the solvent molecules are predominantly of aromatic character and that certain fluorizing compounds are dissolved in the solution, part of the excitation energy deposited by the electron may cause an emission of photons which can be detected by a photosensitive device such as a photomultiplier. The intensity of the light pulse caused by a decay is proportional to the energy of the electron when ejected from the nucleus. The height of the electrical pulse measured at the output of the photomultiplier is again proportional to the number of photons in the light pulse. As each decay produces one distinct pulse, with a height proportional to the energy of the beta electron, a certain pulse height distribution or spectrum, can be recorded in a multichannel analyzer. The shape of this spectrum depends not only on the decay characteristics but also on the efficiency of the liquid to transform excitation energy into light and the efficiency of the detector to transform photons into detectable electrical pulses. The number of photons produced per unit of dissipated energy may vary from sample to sample due to the effect of quenching. In a more general definition, quenching comprises all mechanisms that reduce the size of the electrical signal produced by the photodetectors, but the most common forms of quenching, chemical quenching and color quenching, are due to impurities in the scintillation solution. Figure 1 shows spectra for five isotopes measured with nearly unquenched samples and Figure 2 shows spectra for the same isotopes measured with chemically quenched samples. In liquid scintillation counting it is usually essential to determine the degree of quenching for each sample separately, as. the counting efficiency also depends on the quench level. Before measuring samples with unknown activities, it is necessary to produce a counting efficiency calibration curve (or quench curve) for the counter by recording the counting efficiency and some quench index for a set of standard samples having known activities. The quench index is a measure of the quench level of the sample or standard. For a sample containing only one radioisotope the quench index can be determined from the position of the isotope spectrum. The sample channels ratio and the mean pulse height are commonly used for this purpose. In the case of a multi- labeled sample, or if the activity is very low, it becomes necessary to use an external gamma-radiating source, which may be momentarily placed adjacent to the vial for a short time period. The gamma rays cause formation of compton electrons in the liquid. These electrons "behave as electrons produced by beta-decay and also generate a spectrum of pulses which can be stored in the MCA. Hence, the effect of quench can be determined from this so called external standard spectrum. One measure of the degree of quenching is for example the mean pulse height of the spectrum, another similar measure is the endpoint of the spectrum.

It would be quite straightforward to identify the isotopes on basis of their spectra if they always looked the same, but due to the common effect of quenching, the spectra may alter both in shape and in location; see Figs 1 and 2. In a previous patent (Stanley J. De Filippis, US Pat. No 4,742,226) , one invention is disclosed wherein isotope identification is based on a relation between a quench index computed from the spectrum of the internal isotope (dissolevd in the liquid) and another quench index computed from the external standard. In particular, one embodiment is based on the mean pulse height of the internal isotope (SIS) and a measure of the end-point of the external standard

(tSIE) . For a number of common isotopes it has been found that linear relationships between SIS and tSIE exist. In the case of a single-labeled sample for which tSIE has. been determined, the SIS of the sample can be compared to values computed from the linear relationships for the alternative isotopes and an isotope assigned to the sample. One limitation of this procedure is that multi-labeled samples cannot be handled. Another limitation is that there may exist two isotopes the spectra of which have similar mean pulse heights, but still different shape. This situation is examplified in Figure 3.

The herein invention utilizes comparing the measured spectrum from the sample with spectra stored in a spectrum library. In order for this method to work for all kinds of samples, the spectrum library must be equipped with means for spectral interpolation. This invention is not concerned with how spectra are stored in and retrieved from the spectrum library, neither with how interpolation is performed. Generally, the spectrum library is herein only considered as a means, which takes at its input an external standard quench index and an isotope identifier and provides at its output a reference spectrum, which is normalized to e.g. 1 counts per minute (1 cpm) . The identifying system may request reference spectra for any number of isotopes from the library. If no data has been stored for a particular isotope, then no reference spectrum for that isotope is produced. After the requested reference spectra have been retrieved from the library, the next step is to try to fit each reference spectrum to the sample spectrum.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows spectra for the four isotopes ³H, ¹⁴C, ³³S and ⁴⁵ Ca recorded with nearly unquenched samples.

Figure 2 shows spectra for the same isotopes as in Figure 1 recorded with chemically quenched samples.

Figure 3 shows two spectra having clearly different shape but the same mean pulse height.

Figure 4 shows a block diagram of a general embodiment of a liquid scintillation counter according to this invention.

DESCRIPTION OF THE INVENTION

The objective of this invention is a liquid scintillation counter capable of identifying all the isotopes present in a sample, independently of the number of isotopes in the sample. Furtermore, it is another object of the invention to provide a liquid scintillation counter capable of determining for each sample separately the number of isotopes present in the sample and identifying all isotopes and setting the counting conditions accordingly.

A general embodiment of this invention is shown in the block diagram in Figure 4. 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. The analyzer 4 and the summing amplifier 5 are connected to a multichannel analyzer 6. The device 7 computes a quench index by using the spectrum stored in the multichannel analyzer 6. This quench index is taken to means 8, which produces a number of model spectra, which are transferred to spectrum processing means 9. The measured spectrum is also transfered to spectrum processing means 9. The operations of processing means 9 is described more in detail in the next paragraph.

EXAMPLES OF EMBODIMENTS

In a first embodiment of the herein invention, the sample is assumed to contain only one isotope, A. After the quen.ch level of the sample has been determined, the means 8 in Fig.

4 produces model spectra for the candidate isotopes, e.g. for the three isotopes A, B and C. Let si , j denote the intensity of channel i of the spectrum for isotope j (= A, B or C) . Let the model spectra be normalized, i.e. obey the following rule:

m

Σ Si , j = 1 i=l

where is the number of channels in the spectrum. Let the measured spectrum be denoted by yi . For all isotopes a merit value F may be computed, by using the equation

i=l

The isotope for which F is a maximum may then be assigned to the sample.

Another embodiment is based on using normal least squares fit. In this case the merit value F is computed by using the expression

m

F = ∑ (yi c Si )2 j=l

in which c is the total intensity of spectrum yi . The isotope for which F is a minimum may then be assigned to the sample.

If the sample is assumed to contain more than one isotope, only the procedure based on least squares may be used. Generally, in this procedure, each candidate isotope is assigned a separate multiplication factor cj , which originally is unknown and becomes determined in the least squares procedure. If the sample contains n isotopes, the expression to be minimized becomes

m n

i=l j=l wherein cj is the total intensity of isotope j. This function must be computed for all possible combinations of the n isotopes, and the sample assigned that combination for which F receives a minimum value.

Claims

1. A method for determining both the number N of radioactive isotopes present in a liquid scintillation sample and for identifying said N radioactive isotopes, comprising - measuring the photon emission from said sample with a detector, connected to a multi-channel analyzer storing a digital pulse height spectrum of said sample,

- determining a value for the quench level of said sample, characterized by, - producing model spectra of each of said N radioisotopes at said determined quench level, and

- comparing said model spectra to said sample spectrum stored in said multi-channel analyzer.

2. Method according to claim 1, characterized by that comparing said model spectra to said sample spectrum comprises determining for each model spectrum a factor by which said spectrum is to be multiplied in order to fit the sum of the multiplied spectra to the sample spectrum, each said factor being directly related to the amount of each isotope in the sample.

3. Method according to claim 1, characterized by that producing model spectra of each of said N radioisotopes at said determined quench level comprises

- storing experimental model spectra at certain quench levels, and

- interpolating between and extrapolating from said stored spectra, a new model spectrum at said determined quench level.

4. Method according to claim 1, characterized by that said model spectra of each of said N radioisotopes at said determined quench level comprises functions that describe for each radioistope the behavior of model spectra as quench level varies.

5. An apparatus for determining both the number N of radioactive isotopes present in a liquid scintillation sample and for identifying said N radioactive isotopes, comprising

- a detector for measuring the photon emission from said sample connected to a multi-channel analyzer for storing a digital pulse height spectrum of said sample,

- means for determining a value for the quench level of said sample, characterized by,

- means for producing model spectra of each of said N radioisotopes at said determined quench level, and - means for comparing said model spectra to said sample spectrum stored in said multi-channel analyzer.

6. Liquid scintillation counter according to claim 5, characterized by that said means for comparing said model spectra to said sample spectrum comprises means for determining for each model spectrum a factor by which said spectrum is to be multiplied in order to fit the sum of the multiplied spectra to the sample spectrum, each said factor being directly related to the amount of each isotope in the sample.

7. Liquid scintillation counter according to claim 5, characterized t that said means for producing model spectra of each of said N radioisotopes at said determined quench level comprises

- means for storing model spectra at certain quench levels, and

- means for interpolating between and extrapolating from said stored spectra, a new model spectrum at said determined quench level.

8. Liquid scintillation counter according to claim 5, characterized by that said means for producing model spectra of each of said N radioisotopes at said determined quench level comprises a function that describe for each radioistope the behavior of model spectra as quench level varies.