Patent Description:
Liquid scintillation counting systems are utilized to count radiation events in a sample being tested. Such systems often utilize a lead shield to block or inhibit background radiation, from the environment, which could cause unwanted noise events to be included when counting events in the sample. Typically, the lead shield is thickest at the top of the liquid scintillation counting system where cosmic ray flux is most intense. However, as more lead is added, the overall system becomes heavy and cumbersome. In addition to the lead shield, a guard subsystem may be utilized to detect and account for background radiation that was not inhibited by the shield. Radiation events coincidently detected by the sample counting system and the guard subsystem are classified as background radiation that should not be included in the count of events in the sample. However, due to the non-ideal performance of the guard subsystem, not all background noise events are coincidently detected, and thus unwanted background noise events may still be inadvertently included in the count of events in the sample.

The prior art document "<NPL> discloses liquid scintillation counting (LSC) as the most commonly used technique for measuring tritium.

The prior art document "<NPL> discloses the use of multichannel analyses of β-energy spectra with advantage to enhance resolution and precision of detection of <NUM>C or<NUM>H isotopes at naturally occurring concentrations (low-level counting).

The prior art document "<NPL> discloses the use of multichannel analyses of β-energy spectra with advantage to enhance resolution and precision of detection of <NUM>C or <NUM>H isotopes at naturally occurring concentrations (low-level counting).

The advantages of the embodiments herein may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which:.

A system and method for detecting radiation employs guard detector compensation based on one or more pre-determined guard efficiency values that may be determined utilizing a quenched or unquenched standard sample, to adjust sample event counts to compensate for a non-ideal guard which may not detect all background noise events. The system and method determines counts of events detected coincidently by a guard detector subsystem and a sample detector subsystem in one or more energy regions as well as counts of events that are detected by the sample detector subsystem and not coincidently detected by the guard detector subsystem for the respective energy regions. The system and method calculates correction values for the respective energy regions based on the counts of coincident and non-coincident events and the guard efficiency values associated with the respective energy regions. The system then applies the calculated correction values to sample counts for the respective energy regions, to produce corrected sample event counts.

Using the guard efficiency compensation system, the system improves the accuracy of the sample event counts by compensating for non-ideal guard performance. In addition, use of the guard detector compensation allows the system to operate efficiently in an environment in which the sample is less environmentally isolated during evaluation, and thus, more susceptible to unwanted background noise. For example, a liquid scintillation detection system using the guard detector compensation may be surrounded by relatively thin lead walls, without adversely affecting the counts of sample event of interest. Thus, the system size and weight may be significantly reduced.

<FIG> is a schematic block diagram of a liquid scintillation detection system <NUM> that may be advantageously used with the embodiments described herein. The liquid scintillation detection system <NUM> is configured to detect radioactive emissions (e.g., events) in a sample <NUM>. Sample detector subsystem <NUM> includes sample photomultiplier tubes (PMTs) <NUM> and sample counting chamber <NUM>. The sample <NUM> is placed in a vial <NUM> and then into the sample counting chamber <NUM> of the sample detector subsystem <NUM>, where a scintillating liquid is also added to the vial <NUM> to transform the radiation of the sample into light pulses detected by the sample PMTs <NUM>. The sample PMTs <NUM> operate simultaneously and the combined signal is analyzed to count the events in the sample <NUM>.

To reduce unwanted cosmic and environmental background radiations (e.g., BKG noise events) that effects the counting of events in the sample <NUM>, a lead shield <NUM> is utilized. In addition, a guard detector subsystem <NUM>, including guard PMTs <NUM> and a liquid scintillator <NUM>, is utilized to detect unwanted cosmic and environmental background radiations. Events that are coincidently detected at the sample PMTs <NUM> and the guard PMTs <NUM> are classified as BKG noise events that should not be included in the count of event in the sample <NUM>. However, the guard detector subsystem <NUM> is non-ideal and does not detect all BKG noise events, and thus unwanted BKG noise events that were not coincidently detected, may still be inadvertently included in the count of events in the sample <NUM>. As known by those skilled in the art, this may occur because when cosmic/gamma rays are missed by the guard detector and hit the sample, the rays can create Compton backscatter events as they travel through the sample medium.

In addition, a quenching agent, such as such as inorganic acids, organic acids, and extractive scintillators, may be added to the standard sample <NUM>. As known by those skilled in the art, quenched samples produce a different measured energy distribution of Compton backscatter than do unquenched samples. Advantageously, and by utilizing a quenched sample, light output from Compton backscatter in the standard sample <NUM> is quenched. Use of a quenched sample makes the guard look like it has lower cosmic/gamma ray detection efficiencies in lower energy regions, and higher efficiencies in higher energy regions, as compared to the use of an unquenched sample. This effect occurs because while quench in the standard sample can shift the measured energy of Compton backscatter events to lower energy regions, the energies of cosmic/gamma events as measured by the guard detector subsystem <NUM> remain approximately the same whether the standard sample is quenched or unquenched.

<FIG> is a schematic block diagram of an electronic architecture <NUM> for liquid scintillation counting that may be advantageously used with the embodiments described herein. The electronic architecture <NUM> includes multiple multi-channel analyzers (MCAs) <NUM>, one or more processors <NUM>, a memory <NUM>, one or more adapters <NUM>, an input device <NUM>, and a display <NUM> interconnected by a system bus <NUM>. Each MCA <NUM>, as known by those skilled in the art, provides sets of channels which represent different energy regions. For example, each energy region may be represented by a block of <NUM> channels of the MCA (e.g., energy region <NUM> is represented by channels <NUM> to <NUM> of the MCA, energy region <NUM> is represented by channels <NUM> to <NUM> of the MCA, etc.).

In an embodiment, the memory <NUM> includes memory locations that are addressable by the MCAs <NUM>, processor <NUM> and adapters <NUM> for storing software programs and/or processes and data structures associated with embodiments discussed herein. The processor <NUM> and adapters <NUM> may, in turn, include processing elements and/or logic circuitry configured to execute the software programs/processes, such as, calculate the values associated with the embodiments described herein. It will be apparent to those skilled in the art that other processing and memory means, including various computer readable media, may be used for storing and executing program instructions pertaining to the embodiments described herein. It is also expressly contemplated that the various software programs, processors and layers described herein may be embodied as modules configured to operate in accordance with the disclosure, e.g., according to the functionality of a software program, process or layer.

The input device <NUM> includes the mechanical, electrical and signaling circuitry needed to receive input commands (e.g., from a user) that in turn causes the other components (the MCA <NUM>, the processors <NUM>, the memory <NUM>, the adapters <NUM>, and the display <NUM>) to perform particular functions. For example, the input device may be a keyboard or a "touch screen. " Further, the display <NUM> includes the mechanical, electrical and signaling circuitry needed to display data and information to a user utilizing the guard compensation system <NUM>. For example, the display <NUM> may be a Liquid Crystal Display (LCD) screen.

The adapter <NUM> comprises the mechanical, electrical and signaling circuitry needed to connect the electronic architecture <NUM> to the liquid scintillation detection system <NUM> (<FIG>). For example, the adapters <NUM> may be an electronic subsystem comprised of comparators to detect event trigger signals from the PMTs, analog filters to process energy signals from the PMTs, digital state machines and timing elements, and analog-to-digital converters to convert energy information into a digital format which may be used by the MCA to store events.

<FIG> is a flowchart detailing the steps of a procedure 300A for calculating guard efficiencies associated with the guard detector subsystem <NUM>. The procedure starts at step <NUM> and continues to step <NUM>, where a standard sample (e.g., quenched or unquenched) having known properties is utilized as the sample <NUM>, placed in vial <NUM>, and placed into the sample counting chamber <NUM>.

A quenching agent may be added to the standard sample. The quench level of the standard sample may be chosen based on the quench level of the unknown sample (e.g., customer sample) whose count is to be corrected for one or more energy regions. For example, and based on the quench level of customer sample, a user may continuously add a quenching agent to the standard sample that is being utilized to calculate the guard efficiency values, until the quench level in the standard sample approximately matches the quench level in the customer sample. The quench level of the standard sample may be within a threshold amount of the quench level of the customer sample to determine that the quench level approximately match. Specifically, tSIE (transformed spectral index of the external standard) is a quench indicating parameter used in Tri-Carb LSAs. A standard sample with a tSIE of <NUM> would be an approximate match for an unknown sample with a tSIE from <NUM> to <NUM> (i.e., +/- <NUM> tSIE units). Approximate matching of the quench level of the standard sample used during the calculation of guard efficiency values to the quench level of the unknown sample provides significant improvements in BKG reduction and hence an improved figure of merit (efficiency<NUM>/background) can be observed.

More specifically, as known by those skilled in the art, adding a quenching agent to the standard sample prevents some light from getting to the detectors. Quench is a reduction in system efficiency as a result of energy loss in the liquid scintillation solution, i.e., the sample. Because of quench, the energy spectrum detected from radionuclide events appears to shift towards a lower energy. In addition, and due to this shift, at lower energy levels a sample with a quenching agent will have a higher count of Compton backscatter when compared to a sample that is unquenched, while at higher energy levels a sample with a quenching agent will have a lower count of Compton backscatter when compared to sample that is unquenched, as illustrated in <FIG>. As depicted in <FIG>, use of a quenched sample <NUM> makes the guard look like it has lower cosmic/gamma ray detection efficiencies in lower energy regions, and higher efficiencies in higher energy regions, as compared to the use of an unquenched sample <NUM>. It is noted that the values in <FIG> are simply for illustrative purposes and that other values may be associated with the energies values for the quenched and unquenched samples.

Further, as known by those skilled in the art, the standard sample exhibits low count per minute (CPM) and disintegrations per minute (dpm) values (i.e. a background standard). At step <NUM>, input is received on input device <NUM> to control the MCA <NUM> and select an energy region. For example, a user may enter the value of "<NUM>" on the input device <NUM> indicating that the user is selecting energy region <NUM> of the <NUM> available energy regions. In one or more alternative embodiments, a wide energy gamma emitter (e.g., 152Eu) may be utilized to externally irradiate liquid scintillation counting system <NUM> to greatly outnumber the effect of other types of BKG noise events which the guard detector subsystem <NUM> would not otherwise detect.

At step <NUM>, the count of coincident and non-coincident events are determined simultaneously. Specifically, the coincident events are the events that are coincidently detected at both the sample counting PMTs <NUM> and the guard PMTs <NUM> for the selected energy region. The non-coincident events are those events that are detected at the sample counting PMTs <NUM> but not detected at the guard PMTs <NUM> for the selected energy region. It is noted that the processor <NUM> determines the count of coincident and non-coincident events, or a separate counting electronic device determines the count of coincident events. At step <NUM>, and based on the count of coincident and non-coincident events, an efficiency of the guard detector subsystem <NUM> for the selected energy region is calculated. Specifically, the processor <NUM> performs the following calculation to calculate the efficiency of the guard detector subsystem <NUM> for the selected energy region (GER): <MAT> where SP12R is the count of coincident events, and SP11R is the count of non-coincident events count for the energy region R. It is noted that counts SP12R and SP11R are from events which were stored in MCAs where the energies of the events were determined by the sample counting PMTs. It is also noted that the efficiency of the guard subsystem is then determined for each of the other energy regions. At step <NUM> the procedure ends.

It is noted that the data obtained when calculating guard efficiencies is saved for later use, for example, in memory <NUM>, when unknown samples are counted and corrected using guard compensation from the electronic architecture <NUM>. Further it is noted that once the guard efficiencies for regions of interest have been calculated, any external gamma emitter may be removed and the system is ready to correct unknown samples using guard compensation.

<FIG> is a flowchart detailing the steps of a procedure <NUM> for calculating correction values associated with the guard detector subsystem <NUM> and its associated guard efficiencies. The procedure starts at step <NUM> and continues to step <NUM> where an unknown sample is utilized as the sample <NUM>, placed in vial <NUM>, and placed into the sample counting chamber <NUM>. It is noted that the unknown sample may have an associated quench level that, for example, approximately matches the quench level of the standard sample utilized to determine the guard efficiency values. That is, based on the calculated quench level of the unknown sample, guard efficiency values, calculated with a sample having a quench level that approximately matches the quench level of the unknown sample, may be utilized. At step <NUM>, the count of coincident and non-coincident events for the selected energy region are simultaneously determined. For example, it may be determined that the count of coincident events is <NUM> and the count of non-coincident events is <NUM>. At step <NUM>, a compensated guard count (e.g., the actual number of BKG noise events that should have been coincidently detected by the guard detector subsystem) is calculated. Specifically, the processor <NUM> performs the following calculation to calculate the compensated guard count for the selected energy region (CGCR): <MAT> where SP12R is equal to the counts from the unknown sample that were detected coincidentally by both the sample counting PMTs and guard PMTs. Thus, for the illustrative example, if the guard efficiency for the region of interest is <NUM>%, the compensated guard count for energy region <NUM> (CGC<NUM>) is <NUM> (<NUM>/. <NUM>), indicating that <NUM> events is the actual number of BKG noise events which should have been counted by both the sample PMTs <NUM> and guard PMTs <NUM>. Again, it is noted that the guard efficiency for the region may be calculated, in the manner described above, utilizing a standard sample having a quench level that approximately matches the quench level of the unknown sample.

At step <NUM>, the count of events missed by the guard detector subsystem <NUM> (that should have been coincidentally detected by the sample PMTs <NUM> and guard PMTs <NUM>) is calculated. Specifically, the processor <NUM> performs the following calculation to calculate the count of events missed (but were actually caused by background radiation) by the guard detector subsystem (GCMR): <MAT>.

Thus, for the illustrative example, the number of events missed by the guard detector subsystem <NUM> for energy region <NUM> is <NUM> (<NUM>-<NUM>).

At step <NUM>, the count of missed events (i.e., recovered guard count) is used to generate a background reduction factor (BRFR) that is applied to the normal sample PMT count data. Background reduction factors are used instead of subtracting the missed events from the sample count data so that generating negative CPM values is less likely, and spectral shape is maintained. The background reduction factors are typically limited in software from. <NUM> to <NUM> in order to prevent under or over correction of the sample (beta) spectrum counts. Specifically, the processor <NUM> performs the following calculation to calculate the background reduction factor for the particular region (BRFR): <MAT>.

Thus, for the illustrative example, the background reduction factor for energy region <NUM> (BRF<NUM>) is <NUM> (<NUM> - [<NUM>/<NUM>]). At step <NUM>, the background reduction factor (BRF R) is utilized to calculate a count of corrected non-coincident events (corrected SP11<NUM> ) indicating a number of BKG noise events that were incorrectly included in the count of events in the sample. Specifically, the processor <NUM> performs the following calculation to calculate a count of corrected non-coincident events (corrected SP11<NUM> ):
Corrected <MAT>.

Thus, in this example, the corrected SP11<NUM> is (<NUM>*. <NUM>), indicating that the actual number of non-coincident events is <NUM> for energy region <NUM>, to account for the non-ideal characteristics of the guard detector subsystem <NUM>. In practice, the background reduction factors for each energy region are applied directly to the counts in each channel of the specific energy regions in order to better maintain spectral shape.

It is noted that the calculated values may be displayed on display <NUM>. At step <NUM>, the calculated values (e.g., GER, CGCR, GCMR, and BRFR, and corrected SP11<NUM>) are stored. Specifically, the calculated values are stored in memory <NUM>. It is noted that input may be received on input device <NUM> (e.g., by a user) to alter the control of the MCA <NUM> to select one or more different energy regions to calculate the values associated with the guard detector subsystem <NUM> for such regions, in a similar manner as described above. Specifically, in an embodiment, the values may be calculated for <NUM> logarithmically spaced energy regions. At step <NUM>, the procedure ends.

It is noted that the strength of the guard compensation correction may be adjusted to account for different sample compositions and vial types. Specifically, the input device <NUM> may receive one or more input values associated with the sample compositions and vial type. The processor <NUM> may utilize the input values to add/subtract one or more standard deviations to the stored count data, which changes the measured (actual) guard efficiencies. It is noted that increasing the guard efficiencies decreases the amount of correction applied by the subsequent correction factor calculations, and decreasing the guard efficiencies increases the amount of correction applied by the subsequent correction factor calculations. When adjusting guard compensation correction strength, modifying the original stored data taken in manufacturing which is used to generate guard efficiencies is preferable to modifying the user's count data from the unknown sample. This is because the count time and therefore the statistics of the original stored data are well controlled in the manufacturing process. Mathematically, the guard efficiency strength equations are: <MAT> <MAT> <MAT> Where:.

It is noted that counts SP11R and SP12R are taken from the previously stored data generated when a background standard was counted while a wide energy gamma emitter (e.g., 152Eu) externally irradiated the liquid scintillation counting system <NUM>. It is also noted that the square root of the counts is equal to one standard deviation of the counts. Further, GERS values may then be used in place of GER in the flowchart of <FIG>.

<FIG> is a schematic block diagram of an alternative liquid scintillation detection system <NUM> that may be advantageously used with the embodiments described herein. The liquid scintillation detection system <NUM> is configured to detect radioactive emissions (e.g., events) in a sample <NUM>. A crystal scintillator guard <NUM> with a built in aperture for the sample vial is mounted in the counting chamber such that it is in intimate contact with the sample/guard PMTs <NUM>. The sample is placed in a vial <NUM> that is placed in sample counting chamber <NUM> and within the crystal scintillator guard <NUM>, where the radiation is detected as light by sample/guard PMTs <NUM>. Again, a quenching agent may be added to the sample to determine guard efficiency values associated with the crystal scintillator guard <NUM>. The quench level of the of the sample may be chosen based on the quench level of the standard sample whose count is to be corrected for one or more energy regions. For example, and based on the quench level of customer sample, a user may continuously add a quenching agent to the sample that is being utilized to calculate the guard efficiency values, until the quench level in the sample approximately matches the quench level in the customer sample. The crystal scintillator guard <NUM> is in close proximity with the sample/guard PMTs <NUM> for detecting cosmic and environmental background noise events as known by those skilled in the art.

The crystal scintillator guard <NUM> is non-ideal and does not detect all background noise events. Specifically, and as known by those skilled in the art, the background noise events are beta spectrum events in coincidence with external cosmic and gamma spectrum events, also known as the SP12 spectrum. However, in this particular embodiment, the gamma spectrum generated by the crystal scintillator guard <NUM> does not correlate well with the background count in the sample (beta) spectrum that was caused by external cosmic and gamma radiation. As such, the electronic architecture <NUM> may expressly transform the gamma spectrum into one that follows a Compton backscattering profile that approximates the SP12 spectrum as observed in the first liquid scintillation detector system <NUM> described previously. Specifically, the transform effectively changes the counts in each channel of the gamma MCA into a rectangle which starts at the energy channel and ends at <NUM> keV. The area of each rectangle is equal to the number of counts in the channel. The rectangles are then summed into the transformed spectrum. The number of counts in the transformed spectrum is equal to that of the original gamma spectrum, only the distribution has changed to better approximate that of a "beta in coincidence with gamma" spectrum. The transformed gamma spectrum may be used for the SP12 data on the liquid scintillation counting system having a crystal.

<FIG> is a flowchart detailing the steps of a procedure <NUM> for transforming the external cosmic and gamma spectrum associated with the liquid scintillation detection system <NUM>. The procedure starts at step <NUM> and continues to step <NUM> where the external cosmic and gamma spectrum is mathematically transformed into a spectrum that follows a Compton backscatter profile that approximates a SP12 spectrum. At step <NUM>, the procedure ends. This transformed spectrum is then used in place of the SP12 spectrum in flowcharts <NUM> and <NUM> and their associated methods and equations.

Using the system described herein, the accuracy of the sample event counts may be significantly improved without increased shielding or even with reduced shielding. In addition, the system allows for efficient operation in an environment in which the sample is less environmentally isolated during evaluation, and thus, more susceptible to unwanted background noise. For example, a liquid scintillation detection system may be built which is surrounded by relatively thin lead walls, without adversely affecting the counts of sample event of interest. Thus, the system size and weight may be significantly reduced. Further, and as known by those skilled in the art, performance of a liquid scintillation counting system may be measured based on its counting sensitivity, that is expressed as E<NUM>/B (where E is the counting efficiency and B is the background count rate). Tests have shown that utilizing the embodiments described herein, the sensitivity of the liquid scintillation counting system <NUM> may increase by <NUM>%.

Claim 1:
A radiation detection system (<NUM>) comprising:
a guard detector subsystem (<NUM>) comprising guard photomultiplier tubes (<NUM>) and a liquid scintillator (<NUM>), wherein the guard detector subsystem is configured to detect active guard events;
a sample detector subsystem (<NUM>) comprising sample photomultiplier tubes (<NUM>) and a sample counting chamber (<NUM>), the sample detector subsystem (<NUM>) configured to detect sample events in an unknown sample (<NUM>), wherein the sample photomultiplier tubes (<NUM>) are configured to detect light pulses generated by the transformation of radiation through a scintillating liquid added to the unknown sample (<NUM>);
one or more processors (<NUM>) adapted to:
count coincident events (SP12R) that are the detected sample events in the unknown sample and the detected guard events that are detected coincidently in a given energy region; and
for the given energy region:
calculate a compensation guard count (CGCR) for the unknown sample based on the count of coincident events (SP12R) and a predetermined guard efficiency value (GER) that is associated with the given energy region and determined based on utilization of a standard sample having a quench level that approximately matches the quench level of the unknown sample.