Radiant energy – Calibration or standardization methods
Patent
1984-12-27
1987-03-24
Fields, Carolyn E.
Radiant energy
Calibration or standardization methods
250328, 250362, G01T 1204
Patent
active
046527516
DESCRIPTION:
BRIEF SUMMARY
The invention comprises a method for calibrating liquid scintillation counters by using a number of reference solutions in which scintillation pulses, caused by photons liberated by the decay of isotopes, are registered by means of at least two photodetectors, cooperating so that only the pulses which are simultaneous in all the detectors within a certain interval, are counted and analyzed by means of pulse height analysers. Through the calibration, a number of calibration parameters are determined on basis of the pulse heights of the registered pulses and the counting efficiency determined for the reference samples. These parameters are specific for the liquid scintillation counter and for the scintillation solutions.
The aim of liquid scintillation counting (henceforth abbreviated LS counting) is to measure the amount of radioactive isotopes in a solution. This amount is directly proportional to the decay rate (the radioactivity). The method is used in different scientific research areas and can in principle be utilized to measure any radioactive isotope, but the largest group is definitely the .beta.-radiating isotopes and in this group there are especially two isotopes (of common use): .sup.3 H and .sup.14 C. Most commercial LS counters are built and the measuring programs are made for these two isotopes. Tritium (.sup.3 H) is the .beta.-radiating isotope which has the lowest radiation energy and is for that reason critical to measure, as often only less than a half of all decays can be detected.
When a charged particle (.beta.-electron) traverses a medium of aromatic molecules, it collides with the molecules of the medium and transfers part of its energy to the molecules in the form of excitation of their electrons. This excitation energy moves rapidly from one molecule to another until it is absorbed by a fluorescent molecule which emits a photon when it is de-excited. An electron having an initial energy of a few kilo electron Volts is able to excite and ionize a great number of solvent molecules before it is absorbed once and for all, but due to the low efficiency of the scintillation processes and the presence of many competitive processes only a few percent of the emitted energy is finally transformed to photons, while the rest is transformed to heat. Scintillation efficiency refers to a measure indicating how much of the energy having been absorbed in the form of excitation of the aromatic molecules, is finally transformed to energy in the form of photons.
As .beta.-radiating isotopes give rise to .beta.-electrons of different energies (.beta.-electron distribution), and as the number of photons arising from a decay is proportional to the energy of the electron, scintillations containing different numbers of photons are obtained. In commercial LS counters the photons are registered by two photomultiplier tubes in coincidence, and as the heights of the registered pulses are proportional to the number of photons which reaches the photomultiplier tubes, a distribution of pulses of different pulse heights is obtained, i.e. a so-called pulse height distribution or spectrum.
Quenching is a phenomenon whereby the overall number of photons per decay is reduced, resulting in lower probability that a decay will be detected. There are mainly four types of quenching which affect directly the scintillation solution and two which affect the measuring systems.
(i) absorption quenching, which implies that part of the energy of the .beta.-electron is absorbed by some inert material, such as tissues, filter paper or water droplets.
(ii) Dilution quenching, which implies that the .beta.-electron excites and ionizes non-aromatic molecules. This is the case, if the solution contains large amounts of non-aromatic solvents.
(iii) excitation quenching (chemical quenching), which implies that the scintillation efficiency decreases due to the fact that the excitation energy is absorbed by molecules which neither transport the energy further nor fluoresce.
(iv) colour quenching, which means that photons are absorbed by co
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patent: 3715584 (1973-02-01), Rosenstingl
patent: 3780289 (1973-12-01), Kulberg et al.
patent: 3899673 (1975-08-01), Packard
patent: 4085325 (1978-04-01), Atallah et al.
Kouru Heikki K. J.
Oikari Timo E. T.
Rundt Kenneth C. A.
Fields Carolyn E.
Wallac Oy
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