The present invention relates generally to the field of liquid scintillation counting and in particular to an improved method and apparatus for indicating quench of a liquid scintillation solution. 
Liquid scintillation counting techniques are well known for measuring the activity of samples containing radionuclides.  Such a radioactive sample, typically a beta emitter, is dissolved or suspended in a liquid scintillation medium.  The liquid scintillation medium in turn comprises a solvent or solvents, typically methyl-benzene derivatives, and a solute or solutes present in a few percent by weight of the liquid scintillation medium.  It is theorized that most of the kinetic energy from the nuclear disintegrations of the radioactive sample is absorbed by the solvent and then transferred to the solute which emits photons as visible light flashes or scintillations.  The amount of light emitted from a scintillation is proportional to the energy of the corresponding nuclear disintegration. 
A liquid scintillation counter measures the relative intensities of scintillations occurring within a liquid scintillation solution.  As used herein, a liquid scintillation solution means a solution comprising the sample dissolved or suspended within the liquid scintillation medium.  Typically, scintillations occurring within the liquid scintillation solution are detected by a suitable photodetector which produces output pulses having pulse heights proportional to the number of photons by the corresponding scintillations.  The liquid scintillation counter counts the pulses in a plurality of pulse height channels or "windows" having upper and lower pulse height limits that together span a predetermined range of pulse heights.  The counts accumulated in the windows may be plotted with respect to corresponding pulse heights to provide a pulse height spectrum representing the energy spectrum of the nuclear radiation emitted by the radioactive sample. 
It is well known in the liquid scintillation counting art that materials present in the liquid scintillation solution can decrease the number of photons reaching the photodetector for a given nuclear disintegration.  For example, the production of photons in the solution may be decreased or emitted photons can be absorbed.  Such effects are commonly referred to as "quenching" and in each case result in the reduction in the number of photons detectable by the photodetector.  Because quenching decreases the number of photons applied to the photodetector, some scintillation events which would be detected in an unquenched sample are below the photodetector detection threshold in a quenched sample.  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 disintegration 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 disintegration by a given percentage, it will reduce all responses 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.  Many prior quench determination methods employ an external radiation source, such as a gamma source, which may be positioned so as to irradiate the liquid scintillation solution.  A pulse height spectrum of the solution in response to the external source is used to provide an indication of quench.  Several quench determination methods using an external source are discussed, for example, in U.S. Pat.  No.  4,075,480. 
When using an external source to determine quench, the solution pulse height spectrum in response to the external source radiation obscures or interferes with the pulse height spectrum produced by the sample in the solution.  Consequently, it is not possible to simultaneously count sample activity and perform a quench determination.  Typically, sample activity is counted during one time period and quench determination is performed during a different time period.  In a static liquid scintillation counting system, that is, one where a static or stationary sample is counted, the use of an external standard to determine quench lengthens the amount of time required to measure sample activity corrected for quench. 
External standard quench determination methods are even less desirable in flow liquid scintillation systems.  In such systems, a sample in a liquid scintillation medium is flowed through a flow detector.  Thus, the quench of the solution may change from the time that sample activity is measured to the time that a quench determination is performed, resulting in an inaccurate determination of quench.  Moreover, sample activity data is lost during the time that quench determination is being performed, a distinct disadvantage where the liquid scintillation solution, including the sample, is continuously flowing through the flow detector.