Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Determination of the Disintegration Rate
Determination of the Disintegration Rate
Pages 114-139
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From page 114... ...
An end-window beta proportional counter of small dimensions will have a low background simply because its sensitive volume is small. If this small counter is placed inside a larger detector [Fig.
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From page 115... ...
Fast detectors suitable for precision alpha counting are scintillation counters, semiconductor detectors, and proportional counters.
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From page 116... ...
Some proportional counters also may require efficiency corrections because of the anode wire supports. The scintillation counter sketched in Fig.
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From page 117... ...
Introduction to Beta Counting Techniques. If an experimenter is so unfortunate as to be faced with the problem of determining the disintegration rate of a beta emitter, his 117
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From page 118... ...
The complications arise from the effects of scattering and absorption of electrons in matter, combined with the distribution in energy of the beta particles. It is usual to combine all experimental quantities which affect the counting rate into a counting yield e, which relates the observed counting rate N and the disintegration rate ND: e - £- • (21)
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From page 119... ...
for good sensitivity to low-energy beta particles. The mechanical dimensions of end-window Geiger and proportional counters are similar, as are the dimensions of their source holders; therefore, some of the published data for Geiger counters can still be used in modern counting applications.
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From page 120... ...
A well-designed 47T/3 proportional counter will have a geometrical efficiency in excess of 99.5% and a plateau whose slope is less than 0.1%; therefore, the accuracy with which the disintegration rate may be determined depends mainly on absorption in the source and in the mounting film. Absorption will in all likelihood remain the factor limiting the accuracy of 47T/3 counting.
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From page 121... ...
Similarly, the gamma-ray counting rate N is Ny = ND €y ' (23> Here, e is the counting yield of the gamma detector. The coincidence rate N is Nc " ND €0 €y ' (24)
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From page 122... ...
When counting nuclides with complex decay schemes by the 47T/3-y coincidence method, the corrections arising from the decay scheme usually turn out to be rather small. Since the beta-gamma coincidence technique is insensitive to counting losses from absorption in the source and its backing, it offers another way of calibrating the counting yield for thick sources in a 47T/3 counter.113'115 With quantitative information about the decay scheme of a particular nuclide, it should be possible to standardize sources by 47T/3-y coincidence counting to a few tenths of a per cent.
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From page 123... ...
SOURCE MOUNTING 1. Introduction By now it is probably obvious that the choice of a chemical separation procedure, the choice of a radiation detector, and the choice of source mount are not independent.
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From page 124... ...
118 A very useful spreading technique for preparation of uniform foils of heavy elements calls for mixing the nitrate of the desired element, dissolved in an organic solvent, with a dilute solution of Zapon lacquer. This mixture is painted on a metal plate, and, after drying, the plate is heated to destroy the organic residue and to convert the nitrate to the oxide.
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From page 125... ...
Frequently, it is convenient to transfer small amounts of precipitate to a source mount and evaporate the solvent. The precipitate may, for example, lie collected in the tip of a centrifuge tube at the last step of a chemical separation procedure.
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From page 126... ...
If there is a tendency for the cake of precipitate to break up, a dilute solution of organic binder such as collodion may be passed through the filter before the final drying. When the weight of final precipitate is needed to determine a chemical yield, a tare weight should be determined by using several filter paper disks identical to those employed for the unknown.
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From page 127... ...
A crucible, heated with electrical resistance wire or by electron bombardment, has similar collimating properties. For the preparation of thin sources, it is helpful to be able to swing the collector away during the initial heating of the sample.
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From page 128... ...
The experimenter may elect to introduce the gas into an ionization chamber, proportional counter, or Geiger counter as a component of the detector gas; or, he may choose to contain the gas in some way and mount it externally to the counter. The highly specialized and well-developed techniques for internal gas counting have been adequately described in the current literature, as the list of references given by Overman and Clark attests.
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From page 129... ...
. Larger aliquots of solution may be contained in centrifuge tubes of up to 50-ml capacity; these may be assayed in high-pressure, gammasensitive ionization chambers, such as were described in Section III.5.
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From page 130... ...
Bell, in K Siegbahn, ed., Beta- and Gamma-Ray Spectroscopy, Chapter 5, North Holland Publishing Company, Amsterdam,T955.
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From page 131... ...
R Burrus, "Unscrambling of Scintillation Spectra," in Neutron Physics Division Annual Progress Report for Period Encling September 1, 1960, Oak Ridge National Laboratory Report ORNL-3016, I960, p.
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From page 132... ...
L Heath, Calculated Efficiencies of Cylindrical Radiation Detectors^Phillips Petroleum Company,Atomic Energy Div.ReportIDO-16370, 1958 (O.T.S.
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From page 133... ...
H Elliott, Thick Radiation Detectors Made by Ion Drift, University of California Radiation Laboratory Report UCRL-9538, 1961.
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From page 134... ...
T Seaborg, ed., The Transuranium Elements, National Nuclear Energy Series,Div.IV, Vol.
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From page 135... ...
E Fairstein, in Instrumentation and Controls Division Annual Progress Report for Period Ending July 1, 1957, Oak Ridge National Laboratory Report ORNL-2480, 1957, pp.
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From page 136... ...
Siegbahn, ed., Beta- and Gamma-Ray Spectroscopy, Chapter XVIII, North Holland Publishing Company, Amsterdam, 1955.
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From page 137... ...
K Froman, eds., Miscellaneous Physical and Chemical Techniques of the Los Alamos Project, National Nuclear Energy Series, McGraw-Hill, New York, 1951, Chap.
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From page 138... ...
. USAEC Divilion of Technical Information Extension, Ook Ridge, T 138
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