2 - introduction to isotopes and what they are Flashcards

Question

direct counting method (method for determining decay constant of an unstable isotope)

Answer 1

Number of spontaneous decays on pure element can be counted. Several drawbacks to this method: abundance of unstable isotope may be very low and/or the decay constant is very low (e.g. 176Lu) so counting times may need to be long. Over this period, detector reliability becomes an issue. Also erroneous counts due to cosmic rays? For some elements the unstable isotope (e.g. 87Rb) emits low energy particles, which can be absorbed before they reach the detector – resulting in underestimate of the decay constant

Answer 2

(accumulation of daughter isotope in a sample of the parent element is directly proportional to the number of parent atoms that decay, so amount of daughter isotope can be measured. As with the direct counting method can take years but no counting is necessary and the sample can just be left in a sealed container for the desired time. at 0 time (start) the sample must contain no daughter atoms or the quantity must be accurately determined)

Answer 3

Decay constants for some elements are easier to determine by direct counting than others (e.g. U). If these elements are used to date a suite of rocks then the decay constants for other elements can be determined from those samples because the age t is known. The disadvantage is that it is best carried out on old rocks (often meteorites are used), which are usually associated with the greatest geological uncertainties (alteration, metamorphism and closure temperatures). (The concept of closure temperatures will be introduced next week). Nevertheless provides a useful check on laboratory measurements.

Answer 4

direct measurement, under extreme conditions frequencies and fading rates of gammer ray emissions from a supernovae over several billion years are predictable according to present day roles - no observable change different isotope systems give constant ages, highly fortuitous if decay rates aren't constant Okionatural nuclear reactor (1.7 Ga (long nuclear behavior) - constant as far as we can tell

Answer 5

The radioactive decay rates of nuclides used in radiometric dating have not been observed to vary since their rates were directly measurable, at least within limits of accuracy. This is despite experiments that attempt to change decay rates (Emery 1972). Extreme pressure can cause electron-capture decay rates to increase slightly (less than 0.2 percent), but the change is small enough that it has no detectable effect on dates.Supernovae are known to produce a large quantity of radioactive isotopes (Nomoto et al. 1997a, 1997b; Thielemann et al. 1998). These isotopes produce gamma rays with frequencies and fading rates that are predictable according to present decay rates. These predictions hold for supernova SN1987A, which is 169,000 light-years away (Knödlseder 2000). Therefore, radioactive decay rates were not significantly different 169,000 years ago. Present decay rates are likewise consistent with observations of the gamma rays and fading rates of supernova SN1991T, which is sixty million light-years away (Prantzos 1999), and with fading rate observations of supernovae billions of light-years away (Perlmutter et al. 1998).The Oklo reactor was the site of a natural nuclear reaction 1,800 million years ago. The fine structure constant affects neutron capture rates, which can be measured from the reactor's products. These measurements show no detectable change in the fine structure constant and neutron capture for almost two billion years (Fujii et al. 2000; Shlyakhter 1976).Radioactive decay at a rate fast enough to permit a young earth would have produced enough heat to melt the earth (Meert 2002).Different radioisotopes decay in different ways. It is unlikely that a variable rate would affect all the different mechanisms in the same way and to the same extent. Yet different radiometric dating techniques give consistent dates. Furthermore, radiometric dating techniques are consistent with other dating techniques, such as dendrochronology, ice core dating, and historical records (e.g., Renne et al. 1997).The half-lives of radioisotopes can be predicted from first principles through quantum mechanics. Any variation would have to come from changes to fundamental constants. According to the calculations that accurately predict half-lives, any change in fundamental constants would affect decay rates of different elements disproportionally, even when the elements decay by the same mechanism (Greenlees 2000; Krane 1987).

Answer 6

purely random process

Answer 7

one way street, can't convert back to parent

Answer 8

how close a series of measurements are to one another

Answer 9

how close a series of measurements are to the true/known/accepted value need standards to prove the accuracy of labs

Answer 10

- rate given by decay constant Lamda, or - by time it takes for half the parent isotope to decay to the daughter isotope: (half-life)

Answer 11

isochron equation