SINGLE SAMPLE QUENCH DETERMINATION

Abstract: A single sample quench determination method including generating a pulse height distribution spectrum for the sample in the presence of a standard source, measuring a pulse height value corresponding to a unique point on the spectrum, counting all pulses produced by a radionuclide within the sample, counting pulses produced by the radionuclide in the sample within a calibration window, optically varying the quench of the single sample and repeating such steps, correlating normalized pulse height values to the counts for all pulses produced by the radionuclide within the sample at varying degrees of quench to determine disintegrations per minute within the single sample, determining efficiency for the pulse counts measured within the calibration window, and relating such efficiencies to an indication of quench to thereby provide a quench relationship from a single sample.

Description: SING E SAMPLE QUENCH DETERMINATION Background of the Invention The present invention relates generally to liquid scintillation counting and, more particularly, to an im¬ proved method and apparatus for generating a quench rela¬ tionship, such as a quench curve, using a single sample. Liquid scintillation techniques are well known for measuring the activity of samples containing radio- nuclides. Such a radioactive sample, typically a beta emitter, is dissolved or suspended in a liquid scintilla¬ tion medium. Nuclear disintegrations occurring in the sample produce visible light flashes or scintillations in the scintillation medium with the amount of light emitted from a scintillation being proportional to the energy of the corresponding disintegration. A liquid scintillation counter measures the relative intensities of scintillations occurring in the liquid scintillation medium. Typically, scintillations occurring within the scintillating medium are detected by a suitable photodetector which produces output pulses having pulse heights proportional to the energy of the detected scin¬ tillations. The liquid scintillation counter counts the pulses in a plurality of pulse height channels or "win¬ dows" having upper and lower pulse height limits that together span a predetermined range of pulse heights. The counts accumulated in each window may then be plotted with respect to corresponding pulse heights to provide a pulse height spectrum representing the intensity spectrum of the nuclear radiation emitted by the radioactive sample. It is well known in the liquid scintillation art that materials present in the liquid scintillation medium can decrease the number of photons reaching the photodetector for a given nuclear disintegration in the liquid scintil¬ lation medium. For example, the production of photons in the scintillation medium may be decreased or emitted pho¬ tons may be absorbed. Such effects are commonly referred to as "quenching" and in each case result in the reduction of the number of photons which reach the photodetector. In particular, some scintillation events which would be detected in an unquenched sample will fall below the photodetector detection threshold in a quenched sample and thus go undetected. The result is that the number of counts per unit time detected by the photodetector for a quenched sample is decreased as compared with an otherwise identical unquenched sample. The scintillation count rate detected in a quenched sample as compared with the disin¬ tegration rate occurring within the sample is commonly referred to as "counting efficiency". Quenching acts equally on all events produced by the same type of excitation particle, for example, electron (beta), alpha, proton, and so on. Thus, if quenching is sufficient to reduce the measured response for one disin¬ tegration by a given percentage, it will reduce all res¬ ponses by the same percentage. In a liquid scintillation counter, quenching results in a shift of the pulse height spectrum detected by the counter to lower pulse height values, which is commonly referred to as "pulse height shift". Continuing efforts in the liquid scintillation art have been directed to measuring quench and relating quench to counting efficiency by means of a "quench curve" which plots counting efficiency with respect to the amount or degree of quench. The quench curve may then be used to correct the count rate of a sample for the effects of quench. For example, a sample may be counted to determine its pulse height spectrum and quench. Various methods for measuring quench are known in the prior art including, for example, the method and apparatus disclosed in U.S. Patent 4,075,480, which is assigned to the assignee of the pre¬ sent invention and which is incorporated herein by refer¬ ence. Using the measured quench, the quench curve pro¬ vides a counting efficiency for the sample which may be used to adjust the measured counts per unit time for a given portion of the pulse height spectrum to yield disin¬ tegrations per unit time. In the prior art, it is known to construct a quench curve by determining three parameters for each of a plur¬ ality of variously quenched samples. The three required parameters are disintegrations per minute occurring in the sample, the sample counts per minute detected by the liquid scintillation counter, and the degree of quench. Each sample is prepared by pipetting a known volume of calibration standard into a vial which also contains the liquid scintillation medium. The calibration standard may be obtained, for example, from a commercial supplier who identifies the distintegrations per minute (DPM) per unit volume present in the calibration standard. The sample DPM is determined by multiplying the volume of calibration standard present in the sample by the DPM per unit volume of the calibration standard. Quenching agents may then be introduced into the samples in varying amounts to provide a plurality of samples exhibiting varying degrees of quench. The samples are counted to determine counts per minute (CPM) and the quench of each sample is also meas¬ ured by, for example, using the method set forth in the above-referenced patent 4,075,480. The quench curve may then be constructed using the DPM calculated for each sample, the measured CPM for each sample, and the measured quench for each sample. Such a quench curve determination procedure has sev¬ eral inherent difficulties. For example, sample DPM accuracy is related to how accurately the calibration standard is pipetted into each of the samples and also the accuracy with which the supplier of the calibration stan¬ dard has determined the calibration standard DPM per unit volume. Thus, inaccuracies in pipetting the calibration, standard or in originally determining the activity of the calibration standard can adversely affect the accuracy of the resulting calibration curve. In an attempt to eliminate inaccuracies associated with pipetting the calibration standard into a plurality of samples, efforts have been made to construct a quench curve using a single sample into which a single aliquot of calibration standard is placed. The single sample is then successively quenched. Because it is necessary to deter¬ mine the sample DPM on the basis of the amount of calibra¬ tion standard pipetted into such sample, such a technique still suffers from pipetting inaccuracies and inaccuracies in the specification of the calibration standard acti¬ vity. Moreover, where chemical quenching is used to successively quench the sample, the sample must be repeat¬ edly opened, potentially allowing some of the calibration standard to evaporate from the quench sample or permitting some of the sample to be spilled. Furthermore, chemical quenching results in the destruction of the sample, making the quench determination destructive and thus not repeat- able. Another single sample quench determination method known to applicants is found in the Hat-Trick ® kit avail¬ able from LKB allac and disclosed in U.S. Pat. 3,763,371. The kit includes an automatic evaporation device which continuously adds quenching agent to a scin¬ tillation solution in a sample vial. To obtain the quench curve, the sample is counted repeatedly in a liquid scin¬ tillation counter as the amount of quenching increases. However, the scintillation counts detected by the liquid scintillation counter for each degree of quench must be averaged over some time interval during which quench with¬ in the sample is varying, introducing error into the de¬ termination of the quench curve. Moreover, the Hat-Trick kit method is destructive and thus the sample may not be retested. Also, the calibration standard must still be pipetted into the sample vial, subjecting the determina¬ tion of DPM to pipetting inaccuracies and any error in the specification of the calibration standard activity. Thus, there is a need for a quench determination method that is not subject to the pipetting inaccuracies or errors in calibration standard activity described above. Moreover, there is a need for a single sample quench determination method that is not subject to the difficulties in either repeated or continuous chemical quenching as described above and which does not destroy the sample so that quench determination for the sample may be repeated. Summary of the Invention The present invention resides in a method for deter¬ mining quench using a single sample and which is not sub¬ ject to pipetting inaccuracies or errors in calibration standard DPM. Moreover, the single sample need not be disturbed to repeatedly add quenching agent nor must the sample be counted while the degree of quench varies. Advantageously, quench determination in accordance with the present invention may be repeated with the same sam¬ ple, that is, the sample is not destroyed or altered dur¬ ing the determination of the quench curve. To the foregoing ends, a method in accordance with the present invention may include exposing a sample con¬ taining a radionuclide to radiation from a standard source, generating a pulse height distribution spectrum for the sample in response to the standard source, detect¬ ing a unique point on such spectrum and measuring the corresponding pulse height value of such point. All pulses produced by the radionuclide alone in the sample are counted as are all such pulses produced within a pre¬ determined pulse height window. The quench of the sample is. varied optically and the pulse height distribution spectrum determination, unique point pulse height deter¬ mination, and radionuclide counting steps are repeated. The pulse height values are normalized and correlated with corresponding pulse counts from the radionuclide alone to determine a pulse count corresponding to a DPM value for the sample. Counting efficiencies using such DPM value and CPM values produced within the predetermined pulse height windows are determined and the counting efficien¬ cies are related to an indication of quench to provide a quench relationship. Such a method may further include exposing a second sample containing a radionuclide to radiation from the standard source, determining CPM for the radionuclide within a predetermined counting window and quench of the second sample, determining the efficiency of the second sample using the previously determined quench relationship and determining a DPM value for the second sample by ad¬ justing the CPM value within the predetermined counting window according to the efficiency of the sample. Brief Description of the Drawings Fig. 1 is a block diagram of a liquid scintillation counting system for practicing the method of the present invention. Fig. 2 is a graphical plot of pulse height distribu¬ tion spectra of a liquid scintillation sample at three varying levels of quench. Fig. 3 is a graphical plot of log CPM versus normal¬ ized values (R) of pulse height values PH. Fig. 4 is a graphical plot of a quench curve deter¬ mined in accordance " with the present invention. Fig. 5 is a graphical plot of a pulse height spectrum for a radionuclide contained in an unknown sample wherein the DPM of such sample for various portions of the pulse height spectrum are determined in accordance with the present invention. Description of a Preferred Embodiment With reference now to Fig. 1, there is shown a liquid scintillation counting system which is adapted to perform the single sample quench relationship determination in accordance with the present invention. The scintillation counting system is arranged to measure radioactivity issu¬ ing from a sample generally indicated at 10 and comprising a vial disposed within a shielded counting chamber and containing a liquid scintillation medium and sample. A pair of photomultiplier tubes (PMTs) 12 and 14 is arrang¬ ed to detect and convert the scintillations of the sample 10 to corresponding output voltage pulses, each such pulse having an amplitude proportional to the photon intensity of the corresponding scintillation detected. The PMTs 12 and 14 produce a pair of coincident output pulses for each detected scintillation. Disposed between the sample 10 and the PMTs 12 and 14 is a sample optical quenching means 15. The quenching means 15 includes means for optically varying the apparent quench of the sample 10 as detected by the PMTs 12 and 14. In one embodiment of the present invention, the sam¬ ple optical quenching means 15 may include two liquid crystal light valves 15a and 15b disposed between the sample 10 and the PMTs 12 and 14, respectively. The light valves 15a and 15b variably attenuate the light from scin¬ tillations occuring within the sample 10 in response to an adjustable AC voltage from a controllable voltage source 15c of conventional design. In the embodiment disclosed herein, the adjustable voltage may be adjustable over a range of about 3 volts to 30 volts AC (r s), and may have a frequency in the range of about 30 Hz to 1000 Hz. The controllable voltage source 15c is in turn controlled by means of a control signal generated as is described here- inbelow. The liquid crystal light valves may be, for example, of a type CIN available from UCE, Inc., Norwalk, Connecticut. An example of a suitable commercially avail¬ able power supply usable as the controllable voltage source 15c is a Model 3310A available from Hewlett-Packard Company. Other types of optical quenching means may also be used. For example, the optical quenching means may take the form of a cylindrical optical absorber which can be controllably placed over the sample 10 by suitable mechan¬ ical means in response to the control signal, the cylind¬ rical optical absorber having differing neutral density absorbtivity bands along its length. In another form, the optical quenching means 15 may take the form of a filter wheel including a plurality of neutral density optical filters of varying transmission values. Two of such wheels may be placed between the sample 10 and the PMTs 12 and 14, the wheels being controllably rotatable together in response to the control signal so as to optically vary the apparent quench of the sample 10. ith continued reference to Fig. 1, the output of each of the PMTs 12 and 14 is coupled as an input to a combined pulse summation circuit and amplifier 16, the output of which is an analog pulse. By summing coincident output pulses from the pair of PMTs 12 and 14, and thereby detecting a larger combined pulse than either output pulse alone, the system signal-to-noise ratio is improved. Moreover, the system resolution is improved, that is, the uniformity of response between two equal-energy events is improved. The output of each PMT 12 and 14 is also coupled as an input to a coincidence circuit 18 which produces an output pulse upon receipt of essentially coincident input pulses. The outputs from the pulse summation and ampli¬ fier 16 and the coincidence circuit 18 are both applied to an analog gate 20 which passes the analog output from the pulse summation and amplifier 16 when the output pulse from the coincidence circuit 18 is also received by the gate 20. Thus, when a scintillation event within the sample 10 is detected by the PMTs 12 and 14, the coinci¬ dent pulses from such PMTs 12 and 14 are summed by the pulse summation and amplifier 16 and are applied to the gate 20. The coincident pulses from the PMTs 12 and 14 are also detected by the coincidence circuit 18 which applies a pulse to the gate 20. In the presence of the output pulse from the coincidence circuit 18, the analog output pulse from the pulse summation and amplifier 16 is passed by the gate 20. The output of the gate 20 is applied to an analog-to- digital (ADC) logarithmic pulse height converter 22 which provides a digital output logarithmically proportional to the height of the analog pulse applied thereto. The digi¬ tal output of the ADC pulse height converter 22 is applied to a microprocessor-based control unit 24. The control unit 24 is of conventional design and includes a micro¬ processor and related memory and input-output interface units, all well known in the art. The control unit 24 compares the value of the digital output from the ADC pulse height converter 24 to a plurality of predetermined values which define a plurality of energy ranges or win¬ dows together spanning a predetermined energy or pulse height range. According to the value represented by the digital output from the ADC pulse height converter 22, the control unit 24 determines which window the digital value falls within and accordingly increments one storage loca¬ tion within a pulse height distribution storage area 26. The pulse height distribution storage area 26 includes a plurality of storage locations corresponding to the win¬ dows established by the control unit 24. As the liquid scintillation counting process is performed, the values stored in the various storage locations within the storage area 26 together represent a pulse height distribution curve. The storage area 26 may comprise, for example, a portion of the memory accessible to and controlled by the microprocessor within the control unit 24, each storage location within such storage area 26 being cleared or reset prior to the start of a liquid scintillation count¬ ing procedure. The control unit 24 also directs the positioning of the sample 10 within a counting chamber by a conventional sample positioning means 28. The control unit 24 further actuates conventional external standard positioning means 30 for selectively positioning an external standard source, such as a cesium-137 gamma source, in an operative position for irradiating the sample 10 to generate a Comp- ton pulse height spectrum. The control unit 24 also pro¬ vides the control signal to the sample quenching means 15 and in the embodiment of Fig. 1 in particular to the con- trollable voltage source 15c. As described above, the degree of optical quenching of the sample 10 is varied in response to the control signal from the control unit 24. As will be recognized by those skilled in the art, the control signal may be either analog or digital depending upon the particular circuitry within the variable voltage source 15c, all of which may be accomplished using conven¬ tional means. The liquid scintillation counting system of Fig. 1 further includes a conventional display unit 32 such as a cathode ray tube (CRT) and a suitable input device such as a keyboard 34. The display unit 32 can display the count rate derived in a particular window or may display a curve graphically showing the pulse height distribution spect¬ rum. The display unit 32 also displays a pulse height value PH corresponding to an inflection point I on the Compton edge automatically located and determined as des¬ cribed hereinbelow and as also described in U.S. Pat. 4,075,480 referenced above. It will be recognized by those skilled in the art that the system of Fig. 1 is essentially a conventional multichannel liquid scintillation counting system or ana¬ lyzer modified in accordance with the teachings of the present invention. Such conventional multichannel ana¬ lyzers are well known in the art such as, for example, the 5800 and 9800 series liquid scintillation counters avail¬ able from Beckman Instruments, Inc. Turning now to a description of the operation of the apparatus of Fig. 1 and a preferred embodiment of the method of the present invention, such operation and method will first be briefly set forth. A more detailed descrip¬ tion of such operation and method is then made with refer¬ ence to Figs. 1-5. Briefly, the apparatus of Fig. 1 optically varies the quench of the single sample 10. At various quench levels, the system measures the CPM of the sample within a window wide enough to include all detected pulses produced by the sample, the CPM of the sample occurring within a predeter¬ mined window, sometimes referred to hereinafter as a "cal¬ ibration window", and the pulse height of a unique point on the pulse height spectrum when the sample is exposed to the standard source. The DPM of the single sample is then determined using the method of U.S. Patent 4,060,728 which is assigned to the assignee of the present application and which is incorporated by reference herein. For each CPM measurement within the calibration window, the efficiency is determined by dividing such CPM by DPM. A quench rela¬ tionship may then be generated by relating efficiency of each such CPM measurement to an indication of quench for each such measurement. As just described, a single sample in accordance with the method of the present invention uniquely yields a quench relationship or curve from a single sample without destroying the sample or relying upon DPM determined by pipetting a calibration standard into the sample. Furthermore, the DPM for an unknown sample may be determined using the quench relationship uniquely developed as just described. To do so, the CPM within the calibration window and quench are measured for the unknown sample. Having determined the quench, the efficiency for such sample is determined from the quench relationship. The DPM for the sample may then be found by dividing the CPM by the efficiency. Turning now to a more detailed description of the operation of the system of Fig. 1 and the method of the present invention, the control unit 24 begins by command¬ ing the sample positioning means 28 to position the sample 10 within the counting chamber adjacent the PMTs 12 and 14. The control unit 24 also adjusts the sample optical quenching means 15 for a first amount of quench which, in the embodiment disclosed herein, is a minimum amount of quench. In particular, the control unit 24 generates the control signal that is applied to the controllable voltage source 15c to thereby control the output of the voltage source 15c such that the valves 15a and 15b provide a minimum of optical attenuation. The control unit 24 com¬ mands the standard source positioning means 30 to position the standard source into an operative position adjacent the sample 10 for irradiating the sample 10. The system counts the pulses from the PMTs 12 and 14 in a plurality of counting windows to determine the counts developed for both the standard source and the sample. The control unit 24 commands the standard source positioning means 30 to remove the standard source from its operative position to an inoperative position where the standard source does not contribute to the scintillations occurring within the sample 10. The system again counts pulses from the PMTs 12 and 14 in a plurality of counting windows to determine the counts developed for the sample alone. The net counts developed in response to standard source are then deter¬ mined for the various counting windows by subtracting the counts obtained for the sample alone from the counts obtained for both the standard source and the sample. Such net counts are used to plot a standard source pulse height curve 50 as seen in Fig. 2. The control unit 24 determines a unique point on the pulse height curve which in the preferred embodiment is the inflection point I ] _ of the curve 50. A preferred method for locating such inflection point of the Compton edge, that is, where the second derivative of the edge is zero, is described in U.S. Patent 4,075,480. Briefly, the inflection point is located by counting in a plurality of windows along the Compton edge and by calculating the second derivative (that is, the slope of the slope of the edge) at spaced points along the edge by subtracting the counts of adjacent counting windows. When a group of two spaced derivatives is obtained, one being positive and the other being negative, the inflection point is thus deter¬ mined to be between the locations of the two deriva¬ tives. Thereafter, conventional interpolation techniques are employed to determine the exact location of the in¬ flection point ∑ι between the derivative values. When the inflection point is located, the corresponding pulse height value PH_ is determined by the control unit 24 and may be displayed for example, on the display unit 32. With PH ] _ determined and the standard source removed from its operative position to its inoperative position, the control unit 24 adjusts the lower and upper window limits corresponding to a selected storage location within the storage area 26 to establish a window W (Fig. 2) wide enough to include all the pulses issuing from the sample 10 alone. More particularly, the lower limit of the win¬ dow W is set at zero pulse height while the upper limit of the window W is set a sufficient distance outward on the pulse height axis so that the window W includes all of the pulses issuing from the sample alone. As seen in Fig. 2, the sample alone may have a pulse height spectrum 52 which is completely included within the window W. The count rate for the sample alone within window W is designated CPMj_. It will be understood that the control unit 24 may sum the contents of the storage locations within the stor¬ age area 26 which together span the window W rather than readjusting the upper and lower window limits for a selec¬ ted storage location as just described. In either case, the control unit 24 operates such that the system of Fig. 1 counts all of the pulses issuing from the sample alone, i.e., CPM^. The sample is also counted within a predetermined or calibration window to provide a value CPMCτ_. The calibra¬ tion window may be selected by the user and determines the window that will be represented by the efficiency curve developed as described below. If the calibration window is set equal to the window W, then CPM j _ = CPMC j . With PH ] _, CPM and CPMC-^ determined as just describ¬ ed, the control unit 24 varies the control signal applied to the sample optical quenching means 15 so as to vary the optical quench of the sample 10. In the embodiment dis¬ closed herein, the control signal is applied to the con¬ trollable voltage source 15c, causing the output of the voltage source 15c to vary and correspondingly increase the attenuation of the liquid crystal light valves 15a and 15b, thereby increasing the optical quench of the sample 10. The control unit 24 again positions the standard source first in an operative relationship and then in an inoperative relationship with the sample 10, determines a second unique point I2 on a second curve 54 (Fig. 2), and counts the sample 10 alone in the window W, all in a man¬ ner as previously described. With increased optical quench, a pulse height curve for the sample 10 above is as shov/n by curve 53 of Fig. 2. The system of Fig. 1 repeats the steps just described for varying degrees of optical quench, thereby generating a plurality of unique pulse height values PHι_-PH n and corresponding CPM and CPMC values, CP ^ through and in¬ cluding CPM n and CPMCi through and including CPMC n . One such value PH3 may correspond to an inflection point I3 on a third curve 56 as seen in Fig. 2. The sample 10 alone may have a pulse height curve 58 for the degree of quench represented by PH3. With the pulse height, CPM and CPMC values so deter¬ mined, the DPM for the sample 10 is determined in accord¬ ance with the teachings of U.S. Patent 4,060,728, refer¬ enced above. As taught therein, the pulse height values are normalized with respect to some selected common pulse height value to provide a plurality of normalized values R ] _ through and including R n for the corresponding pulse height values. After developing the normalized pulse height values R ] _ through and including R n , the values of R are correla¬ ted with corresponding pulse counts CPM^ through and in¬ cluding CPM n as by graphically plotting log CPM with res¬ pect to R as illustrated in Fig. 3. The intercept of the plot with the log CPM axis (that is, log CPMg for R = 0) establishes the logarithm of the sample DPM rate. Thus the antilog of such a value determines the sample disinte¬ gration rate or DPM. A quench relationship or quench curve in accordance with the present invention may now be determined. As is readily apparent, the quench relationship or curve is derived from a single optically quenched sample, providing advantages heretofore unattainable in the art. For each CPMC value measured as described above, a corresponding efficiency is determined by dividing CPMC by DPM. If efficiency is to be expressed in percentage, the result of the division is multiplied by 100. For each such efficiency value, a corresponding quench representation is also determined. In the embodi¬ ment disclosed herein, such quench representation is re¬ ferred to as an H number. The H number for each CPMC value and corresponding efficiency is found, in a system where pulse height is related to log energy, by subtract¬ ing the unique pulse height value corresponding to the CPMC value from a reference pulse height value, PH Q . The reference pulse height value PH 0 is preferably obtained using a sealed unquenched sample as is known in the art. It is to be noted, however, that any one of the pulse height values could be used as a reference. Thus for CPMC2, a corresponding efficiency is obtained by dividing CPMC2 by DPM and multiplying by 100. The corresponding quench indication is found by subtracting PH 2 from PH ] _ (Fig. 2). The resulting quench representation is H2. Similarly, efficiencies and quench indications are determined for the remaining values of CPMC determined above, that is, CPMC2 through and including CPMC n . Effi¬ ciency is then plotted with respect to quench indication (H number) to provide a quench relationship or curve 60- as seen in Fig. 4. Thus, the method in accordance with the present invention enables the determination of the quench curve 60 (Fig. 4) from a single sample using optical quen¬ ching. In a further aspect of the present invention, the quench curve 60 may be used to determine DPM for an un¬ known sample. As an example, the system of Fig. 1 may be operated as described above to determine a pulse height value along the Compton edge PH U and a CPMC value for the predetermined window previously used to determine effi¬ ciency, that is, the calibration window. For example, the unknown sample when counted alone may yield a pulse height curve 62 as shown in Fig. 5. With the sample exposed to the standard source, the Compton edge inflection point may correspond, for example, to a pulse height PH U of 700. With a standard unquenched pulse height PH U of 835, the H number is PH^ - PH U , or 835 - 700 = 135. Using the uni¬ quely derived quench curve 60 of Fig. 4, the H number of 135 corresponds to an efficiency of 40%. Thus, the CPMC for the calibration window may be corrected to yield DPM by dividing the CPMC for the calibration window by effi¬ ciency and multiplying the result by 100. For example, the CPMC within the calibration window for the unknown sample is 100,000. Consequently, DPM for the unknown sample is 250,000. While a preferred embodiment of the present invention has been described in detail herein, modifications and changes may be made without departing from the spirit of the present invention as defined by the appended claims.

Claims: What is claimed is: 1. A method of generating a quench relationship using a single sample comprising the steps of: (a) exposing the sample to radiation from a stan¬ dard source and generating a pulse height distribu¬ tion spectrum for the sample in response to the stan¬ dard source, (b) detecting a unique point on the spectrum and measuring the corresponding pulse height value, (c) counting pulses produced by a radionuclide in the sample in a window bounded on one side by a zero pulse height threshold detection limit and bounded on the other side by a limit the value of which at least includes all the pulses produced by the radionuclide in the sample, (d) counting pulses detected in a predetermined pulse height range, (e) optically varying the quench of the sample, (f) repeating steps (a) through (e), step (e) being repeated for at least all but the last repeti¬ tion of such steps to obtain a plurality of pulse height values and respective pulse counts, (g) developing normalized pulse height values, (h) correlating the normalized values with corre¬ sponding pulse counts obtained in step (c) to deter¬ mine the pulse count for a normalized value of zero. such pulse count corresponding to a disintegrations value for the sample, (i) determining counting efficiencies for at least some of the pulse counts obtained in step (d) by dividing such pulse counts each by the disintegra¬ tions value, and (j) relating the counting efficiencies to corres¬ ponding indications of quench to provide a quench relationship. 2. A method as in claim 1 wherein the method further includes: (k) exposing a second sample containing a radio s nuclide to radiation from the standard source and generating a pulse height distribution spectrum for the second sample in the presence of the standard source, (1) detecting an indication of quench for the second sample, (m) counting pulses produced by the radionuclide in the second sample in the predetermined counting window, (n) determining the efficiency of the second sample using the quench relationship and the indica¬ tion of quench for the second sample, and (o) determining a disintegrations value for the second sample by adjusting the pulse counts within the predetermined counting window according to the efficiency. 3. A method as in claim 1 wherein step (b) further includes providing an indication of quench related to the pulse height value measured in such step and wherein step (j) further includes relating the counting efficiencies to the indication of quench related to the pulse height val¬ ues at which such counting efficiencies occur to thereby provide a quench relationship. 4. A method as in claim 2 wherein step (1) of de¬ tecting an indication of quench for the second sample further includes detecting a unique point on the spectrum and measuring the corresponding pulse height value. 5. A method as in claim 4 wherein steps (b) and (1) further include exposing an unquenched sample to radiation from the standard source, generating a pulse height dis¬ tribution spectrum for the unquenched sample in the pres¬ ence of the standard source, detecting a unique point on such spectrum, measuring a corresponding pulse height value for such unique point, and relating such pulse height value to the pulse height values measured for the first and second named samples to provide the indication of quench. 6. A method of determining disintegrations per unit time occurring within an unknown sample using a quench relationship developed from a single sample, comprising the steps of: (a) exposing the single sample to radiation from a standard source and generating a pulse height dis¬ tribution spectrum in response to the standard source, (b) detecting a unique point on the spectrum and measuring the corresponding pulse height value, such corresponding pulse height value providing an indica¬ tion of quench, (c) counting pulses produced in the radionuclide in the single sample in a window bounded on one side by a zero pulse height threshold detection limit for the counting system and bounded on the other side by a limit the value of which at least includes all the pulses produced by the radionuclide in the sample, (d) counting pulses produced by the radionuclide in the single sample in a predetermined height range, (e) optically varying the quench of the single sample, (f) repeating steps (a) through (e), step (e) being repeated for at least all but the last repeti¬ tion of such steps to obtain a plurality of pulse height values and respective pulse counts, (g) developing normalized pulse height values, (h) correlating the normalized values with corre¬ sponding pulse counts obtained in step (c) to deter¬ mine the pulse count for a normalized value of zero, such pulse count corresponding to a disintegrations value for the single sample, (i) determining counting efficiencies for at least some of the pulse counts obtained in step (d) by dividing such pulse counts each by the disintegra¬ tions value, (j) relating the counting efficiencies to corres¬ ponding indications of quench, such indications of quench being related to the pulse height values at which such counting efficiencies occur to provide a ' quench relationship, (k) exposing the unknown sample to radiation from the standard source and generating a pulse height distribution spectrum for the unknown sample in the presence of the standard source, (1) detecting a unique point on the spectrum obtained in step (k) and measuring a corresponding pulse height value thereof, (m) counting pulses produced by the radionuclide in the unknown sample in the predetermined counting window, (n) determining the efficiency of the unknown sample in the predetermined counting window using the quench relationship and the indication of quench for the unknown sample, and (o) determining a disintegrations value for the second sample within the predetermined counting win¬ dow by adjusting the pulse counts for such window according to the efficiency. 7. A method as in claim 1 wherein step (a) includes the steps of counting pulses produced only by the radio¬ nuclide in the sample, exposing the sample to radiation from the standard source, counting pulses produced by both the radionuclide in the sample and the radiation from the standard source, determining net pulse counts produced in response only to the radiation from the standard source, and generating the pulse height spectrum using such net pulse counts.

Inventors: HORROCKS DONALD L, KOLB ALFRED J, HORROCKS, DONALD, L., KOLB, ALFRED, J.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1985
Main classification: G01T1
IPC codes: G01T1