Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Scintillation Methods
Scintillation Methods
Pages 4-44
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Modern scintillation counters followed closely the development of high-gain photomultiplier tubes. Combining various scintillating materials with a photomultiplier to count the scintillations has resulted in the most versatile detector available for nuclear research.
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1. Diagram of a scintillation counter, illustrating schematically the way in which light from the scintillator is coupled to a photomultiplier tube.
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Characteristics of anthracene and a number of other typical organic scintillators are shown in Table 1. It will be seen that although anthracene yields the largest light output of any organic scintillator, its fluorescence decay is the slowest listed; therefore, applications which demand very rapid pulse rise times may require a scintillator with faster response at a sacrifice in pulse height.
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The liquid scintillators have the advantage of easy fabrication in almost unlimited volumes. Plastic scintillators are mechanically rugged and can be readily machined.
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The amount of energy so transferred depends on the overlap between the emission spectrum of the solvent and the absorption spectrum of the primary solute. Many of the primary solutes which may be used with the common hydrocarbon solvents fluoresce at such a short wave length that a conventional photomultiplier tube cannot efficiently make use of the light; for this reason, the secondary solute is added to the solution.
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The details of liquid scintillator preparation, together with applications to various problems, will be found in several reviews. ' Except for the fact that they are solid solutions, the composition and fluorescence properties of plastic scintillators are similar to those of liquid scintillators.
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2. Mounting arrangements for electron detectors using organic scintillators.
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Hollow-crystal spectrometers have for several years been used to measure beta spectra, and they have consistently given improved performance over a flat scintillator, both as regards energy resolution and backscattering. ' A set of plastic scintillators machined and polished for use in hollowcrystal detectors is available commercially.
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. This is the basis of liquid scintillation counting, a method which has found widespread use in chemistry, particularly in tracer experiments using low-energy beta emitters such as C14, S35, Ca45, and H3.
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From page 13... ...
In addition to their use as counters, these organic scintillators are useful for determining electron and beta-ray energies. The consensus of the available experimental information indicates a linear pulse height-energy curve down to a low energy of -100 kev; below this energy the response is also nearly linear, but with a slightly different slope.17 The response of an organic scintillator to monoenergetic electrons is mainly a gaussian peak whose width varies inversely with the square root of the energy.
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From page 14... ...
. The typical response of a flat anthracene spectrometer to the electrons and beta rays from a Cs137 source is shown in Fig.
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The internal-conversion electron line at 624 kev and the continuum from the 523-kev beta group are shown. When the beta rays and electrons are stopped in an absorber, the background spectrum from the 662-kev gamma ray is obtained.
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The upper curve was corrected for the unique spectrum shape and the scintillation spectrometer resolution. The lower curve shows the improvement obtained when the correction for scintillator backscattering is included (Gardner and 3.
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Improved pulse height could probably be attained by using a photomultiplier tube with better response in the red, such as the low-noise, multialkali-cathode tubes. The decay time of the fluorescent light from CsI(Tl)
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detector is to be used as a spectrometer, it is important that any material surrounding the crystal be very thin. Otherwise, the gamma-ray spectrum will be distorted by Compton electrons and degraded gamma rays.
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5. Integral crystal mounting arrangement for a 3 x 3-inch Nal(Tl)
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Here, an assembly is often used consisting of a crystal enclosed with its reflector in a thin can and optically coupled to a transparent window. Various crystal assemblies may then be attached to the same photomultiplier tube as required.
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"well-type" crystal on a 2-inch photomultiplier tube (V.
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To prepare a sample may involve only the transfer of a few ml of solution to a small test tube. Because of the penetrating nature of gamma rays, self-absorption in such sources is small, and further treatement of the sample to reduce the mass is usually not necessary.
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and photomultiplier tubes with uniform high-efficiency photocathodes are available, it is possible to make a spectrometer which will not only measure energies of gamma rays to high precision but also yield their intensities. Much of the popularity of the Nal(Tl)
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Thus it is appropriate to discuss briefly the three processes (photoelectric effect, Compton effect, and pair production) by which gamma rays interact, in terms of their effects on the response of the scintillation spectrometer.
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* SMALL CRYSTAL (1V2 xlin.)
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The Compton electron e is stopped and yields a light pulse proportional to the electron's kinetic energy; on the other hand, if the crystal is small the scattered photon y2' may not be stopped, and its energy will then be lost. The figure shows that in a larger crystal, further Comptron processes may occur until the energy of the scattered photon is reduced to an energy so low that a photoelectric event finally transfers the remaining gamma energy to the crystal.
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10. A high-energy gamma ray forms a positive and negative electron pair which carry off as kinetic energy the original gamma-ray energy, minus the 1.02 Mev (two electron rest masses)
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11. Spectrum of 87.5-kev gamma rays and 22-kev X rays from a Cd109 source, illustrating the phenomenon of X-ray escape following detection of 87.5-kev gamma rays.
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COMPTON DISTRIBUTION^ -3 in.x 3in.:NaI(TI) 200 400 600 800 1000 PULSE HEIGHT 1200 with Fig.
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It may be of interest to note that the spectrum of the 1.38-Mev gamma ray shows no evidence of pair peaks; in practice, the effect of pair production is not detectable below about 1.5 Mev, even though the threshold falls at 1.02 Mev. Environmental Effects.
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AT 9.3 cm Na24 GAMMA SPECTRA 200 400 600 800 1000 1200 Fig.
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It is easy to show that, for Compton scattering at large angles, the energy of the scattered photon is nearly independent of the incident gamma-ray energy, and attains an almost constant value around 200 kev. Thus, large-angle scattering from shield walls, source holder, or other matter in the vicinity of the source (cf~., Fig.
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14. Cross-section of a typical scintillation spectrometer installation, showing the 3 x 3-inch Nal(Tl)
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The gamma-ray scintillation spectrometer has proved to be an important tool for the quantitative determination of gamma intensities. Since the true shape of the Compton electron distribution for a single gammaray energy is so obscured by the spurious effects which have just been described, the area of the full-energy peak is generally chosen as the basis for intensity measurements.
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10cm -BREMSSTRAHLUNG 0 200 400 600 800 PULSE HEIGHT 1000 1200 Fig.
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When using a system such as this, it is particularly important that no serious gain shifts occur with changes in counting rate. It may not be possible to measure directly the response functions for the component gamma rays of an unknown spectrum; in this case, it is necessary to synthesize the required function from a measurement of gamma-ray standards over the energy range of interest.
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Starting at the high-energy end of the spectrum, the ordinate scale of each appropriate standard response function is normalized at the full-energy peak and subtracted from the total spectrum in sequence.
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16. Decomposition of a composite gamma-ray spectrum into its components by successive subtraction of standard spectral shapes 38
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17. The analytical methods for generating response functions 23 27 have been used by West and Johnson, ' Strickfaden and Kloepper, and Heath in their computer programs, which closely simulate the manual "peel-off" technique.
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From page 41... ...
, which is simply the probability that a gamma ray denoted by y will produce a count once it strikes the crystal. The fraction of all counts in the spectrum which contribute to the full-energy peak is called the "peak-to-total" ratio, or "photofraction," and may be denoted by f (see Fig.
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From page 42... ...
It often happens that the gamma ray of interest is coinci dent with another gamma ray; in this instance the full-energy peak area will be decreased by coincident summing. This situation has been treated by Lazar and Klema, who derived the following equation for the emission rate: nep(y,)
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Coincident summing of gamma rays leads to another important experimental implication. The gamma rays which are lost to their respective full-energy peaks appear in a "sum peak," whose apparent energy is the total energy of both gamma 43
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An example of such a source is Mn5* , which emits only a single gamma ray at 0.838 Mev; consequently, the random sum peak will be found at about 1.7 Mev.
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