Who here knows much about RADIOMETRIC DATING?

by seattleniceguy 14 Replies latest jw friends

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  seattleniceguy

  I know a little about it, including the basic theory on how it works. But I'm interested in reading more. I figured some of the sciencephiles on the board would have some good resources on the topic....

  This is pertinent because the recent thread on Noah's flood has prompted discussions on whether alleged global events could have changed the rate of radioactive decay, a suggestion fundamentalist groups like the Witnesses like to make. From what I've read, there is no evidence that the rate of radioactive decay ever changes substantially, under any conditions.

  Anyway, I'd like to hear what knowledgable people have to say on the topic!

  SNG
• 

  ballistic

  I figured some of the sciencephiles on the board would have some good resources on the topic....

  what, are you calling us geeks or what?
• 

  upside/down

  AlanF... we're waiting.......
• 

  JamesThomas

  Went out with a lady from england who did a little disk-jockeying .... does that qualify?
• 

  Heatmiser

  edited: SEE NEXT POST
• 

  Heatmiser

  http://www.c14dating.com/

  http://www.101science.com/Carbon14.htm

  If you scroll down on the second link you will find a bunch of more links
• 

  Elsewhere

  discussions on whether alleged global events could have changed the rate of radioactive decay

  The answer to that is an unquestionable NO.

  Radioactive decay is the only 100% random event in nature and NOTHING can interfere with it, because... it is purely random.

  I know that some armchair scientists who's only education is based on watching the Discovery Channel will try to step up and say: "If it is random, then it cannot be predicted, therefore it can change."

  Anyone who says that is full of shit.

  Truly randomness does in fact have a mathematical pattern to it and that is why radioactive dating is so accurate. It is called Benford's Law - look it up and play with it... it is fun to play with using Excel because of how easy it is to create excel graphs based on it.

  The pattern is so reliable that accountants use it to detect "cooked books" at companies under investigation. (Accounting records should contain random numbers and therefore the pattern behind Benford?s Law, and accounting records that have been changed will break that pattern.)

  Now back to the pattern to random Radioactive decay... Every radioactive substance has a Half Life. What that means is that if a substance has a half life of 1 year then in one year 1/2 of it's mass will have disappeared in the form of radioactive particles "radiating" from it. Basically 10 pounds of a substance with a one year half life will weigh 5 pounds after one year... then 2.5 pounds after another year... then 1.25 pounds after another year... and so on until its mass reaches zero.

  Because of this pattern that cannot be interfered with, the age of any radioactive substance can be accurately measured.
• 

  Heatmiser

  The 14C Method

  There are three principal isotopes of carbon which occur naturally - C12, C13 (both stable) and C14 (unstable or radioactive). These isotopes are present in the following amounts C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010%. Thus, one carbon 14 atom exists in nature for every 1,000,000,000,000 C12 atoms in living material. The radiocarbon method is based on the rate of decay of the radioactive or unstable carbon isotope 14 (14C), which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14. The reaction is:

  14N + n => 14C + p

  (Where n is a neutron and p is a proton).
  The 14C formed is rapidly oxidised to 14CO2 and enters the earth's plant and animal lifeways through photosynthesis and the food chain. The rapidity of the dispersal of C14 into the atmosphere has been demonstrated by measurements of radioactive carbon produced from thermonuclear bomb testing. 14C also enters the Earth's oceans in an atmospheric exchange and as dissolved carbonate (the entire 14C inventory is termed the carbon exchange reservoir (Aitken, 1990)). Plants and animals which utilise carbon in biological foodchains take up 14C during their lifetimes. They exist in equilibrium with the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and non-radioactive carbon atoms stays approximately the same over time. As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no replenishment of radioactive carbon, only decay. There is a useful diagrammatic representation of this process given here

  Libby, Anderson and Arnold (1949) were the first to measure the rate of this decay. They found that after 5568 years, half the C14 in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on (see figure 1 below). The half-life ( t 1/2 ) is the name given to this value which Libby measured at 5568±30 years. This became known as the Libby half-life. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 - 60 000 years, then, the limit of the technique is reached (beyond this time, other radiometric techniques must be used for dating). By measuring the C14 concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the countrate or number of decay events per gram of Carbon. By comparing this with modern levels of activity (1890 wood corrected for decay to 1950 AD) and using the measured half-life it becomes possible to calculate a date for the death of the sample.

  As 14C decays it emits a weak beta particle (b ), or electron, which possesses an average energy of 160keV. The decay can be shown:

  14C => 14N + b

  Thus, the 14C decays back to 14N. There is a quantitative relationship between the decay of 14C and the production of a beta particle. The decay is constant but spontaneous. That is, the probability of decay for an atom of 14C in a discrete sample is constant, thereby requiring the application of statistical methods for the analysis of counting data.

  http://www.c14dating.com/int.html

  Modern standard

  The principal modern radiocarbon standard is N.I.S.T (National Institute of Standards and Technology; Gaithersburg, Maryland, USA) Oxalic Acid I (C 2 H 2 O 4 ). Oxalic acid I is N.I.S.T designation SRM 4990 B and is termed HOx1. This is the International Radiocarbon Dating Standard. Ninety-five percent of the activity of Oxalic Acid from the year 1950 is equal to the measured activity of the absolute radiocarbon standard which is 1890 wood. 1890 wood was chosen as the radiocarbon standard because it was growing prior to the fossil fuel effects of the industrial revolution. The activity of 1890 wood is corrected for radioactive decay to 1950. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'. 1950 was chosen for no particular reason other than to honour the publication of the first radiocarbon dates calculated in December 1949 (Taylor, 1987:97).

  The Oxalic acid standard was made from a crop of 1955 sugar beet. There were 1000 lbs made. The isotopic ratio of HOx I is -19.3 per mille with respect to (wrt) the PBD standard belemnite (Mann, 1983). The Oxalic acid standard which was developed is no longer commercially available. Another standard, Oxalic Acid II was prepared when stocks of HOx 1 began to dwindle. The Oxalic acid II standard (HOx 2; N.I.S.T designation SRM 4990 C) was made from a crop of 1977 French beet molasses. In the early 1980's, a group of 12 laboratories measured the ratios of the two standards. The ratio of the activity of Oxalic acid II to 1 is 1.2933±0.001 (the weighted mean) (Mann, 1983). The isotopic ratio of HOx II is -17.8 per mille. There are other secondary radiocarbon standards, the most common is ANU (Australian National University) sucrose. The ratio of the activity of sucrose with 0.95 Ox was first measured by Polach at 1.5007±0.0052 (Polach, 1976b:122). Later inter-laboratory measurements put the ratio at 1.5081 (Currie and Polach, 1980).

  According to Stuiver and Polach (1977), all laboratories should report their results either directly related to NBS Oxalic acid or indirectly using a sub-standard which is related to it.

  Background

  It is vital for a radiocarbon laboratory to know the contribution to routine sample activity of non-sample radioactivity. Obviously, this activity is additional and must be removed from calculations. In order to make allowances for background counts and to evaluate the limits of detection, materials which radiocarbon specialists can be fairly sure contain no activity are measured under identical counting conditions as normal samples. Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood. By measuring the activity of a background sample, the normal radioactivity present while a sample of unknown age is being measured can be accounted for and deducted.

  In an earlier section we mentioned that the limit of the technique is about 55-60 000 years. Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced. Amongst accelerator laboratories there has been mooted the theoretical possibility of extended range dating to 75 000 yr +, at present this seems difficult to attain because of the problems in accurately differentiating between ions that mimic the mass and charge characteristics of the C14 atom. Beukens (1994) for instance has stated that this means the limit of the range for his Isotrace laboratory is 60 000 yr which is very similar to the conventional range.

  Conventional radiocarbon ages (BP)

  A radiocarbon measurement, termed a conventional radiocarbon age (or CRA) is obtained using a set of parameters outlined by Stuiver and Polach (1977), in the journal Radiocarbon. A time-independent level of C14 activity for the past is assumed in the measurement of a CRA. The activity of this hypothetical level of C14 activity is equal to the activity of the absolute international radiocarbon standard.

  The Conventional Radiocarbon Age BP is calculated using the radiocarbon decay equation:

  t=-8033 ln(Asn/Aon)

  Where -8033 represents the mean lifetime of 14C (Stuiver and Polach, 1977). Aon is the activity in counts per minute of the modern standard, Asn is the equivalent cpm for the sample. 'ln' represents the natural logarithm.

  A CRA embraces the following recommended conventions:

  • a half-life of 5568 years;
  • the use of Oxalic acid I or II, or appropriate secondary radiocarbon standards (e.g. ANU sucrose) as the modern radiocarbon standard;
  • correction for sample isotopic fractionation (deltaC13) to a normalized or base value of -25.0 per mille relative to the ratio of C12/C13 in the carbonate standard VPDB (more on fractionation and deltaC13);
  • the use of 1950 AD as 0 BP, ie all C14 ages head back in time from 1950;
  • the assumption that all C14 reservoirs have remained constant through time.

  Three further terms are sometimes given with reported radiocarbon dates. d14C, D14C and deltaC13.

  All are expressed in per mille notation rather than per cent notation (%).
  d14C represents the per mille depletion in sample carbon 14 prior to isotopic fractionation correction and is measured by:

  d14C=((Asn/Aon) - 1)1000 per mille

  D14C represents the 'normalized' value of d14C. 'Normalized' means that the activity is scaled in relation to fractionation of the sample, or its deltaC13 value. All D14C values are normalized to the base value of -25.0 per mille with respect to the standard carbonate (VPDB). D14C is calculated using:

  D14C=d14C - 2(dC13 + 25)(1 + d14C/1000) per mille

  This value can then be used to calculate the CRA using the equation given above.

  Radiocarbon age=-8033 ln(1 + D14C/1000)

  http://www.c14dating.com/agecalc.html

  Hope this is a start for you.
• 

  AlanF

  Talk.origins, of course, is an excellent source of information on radiometric dating. A very readable, semi-technical book is:

  The Age of the Earth, G. Brent Dalrymple, Stanford University Press, 1991

  At the time of writing this book, Dalrymple was a Research Geologist at the U.S. Geological Survey, and had made major contributions to the science of geological dating. He later became a professor of geology at Oregon State University. The book treats everything but radiocarbon dating, since the scale of C14 dating is far too short to measure the age of the earth.

  Another excellent book (not on dating per se, but on the history of the critical years of the revolution in earth science that led to the theory of plate tectonics) is The Road to Jaramillo (William Glen, Stanford University Press, 1982). It taught me a great deal about radiometic dating and how solid and useful a science it is.

  Another excellent read is The Mediterranean Was A Desert (Kenneth Hsu, Princeton University Press, 1983), which shows how radiometric dating was used to date the time when the Mediterranean Sea dried up a number of times, roughly 3-5 million years ago, and how this dating technique proved that the Americas and Europe/Asia/Africa are moving apart at the Atlantic Mid-Ocean Ridge and have been doing so for about 150 million years.

  AlanF
• 

  Heatmiser

  Age reporting

  If the reservoir corrected conventional radiocarbon age calculated is within the past 200 years, it should by convention be termed 'Modern' (Stuiver and Polach, 1977:362). If a sample age falls after 1950, it is termed greater than Modern, or >Modern. Absolute percent modern (%M or pMC - 'percent modern carbon') is calculated using:
  

  %M=100 x Asn/Aabs
  

  or,

  Asn/Aon(1/8267(y-1950)) x 100 percent

  Where Aabs is the absolute international standard activity, 1/8267 is the lifetime based on the new half life (5730 yr), Y = the year of measurement of the appropriate standard. This is an expression of the ratio of the net modern activity against the residual normalised activity of the sample, expressed as a percentage and it represents the proportion of radiocarbon atoms in the sample compared to that present in the year 1950 AD. Thus, %Modern becomes a useful term in describing radiocarbon measurements for the past 45 years when, due to the influx of artificial radiocarbon into the atmosphere as a result of nuclear bomb testing the 'age' calculation becomes a 'future' calculation.

  If the sample approaches D14C = -1000 per mille within 2 standard deviations, it is considered to be indistinguishable from the laboratory background, ie, not able to be separated with confidence from the laboratory countrates which result from a sample which contains no radionuclide. In this instance, a minimum age is calculated. An example of a minimum age is >55, 000 yr or >50, 000 yr (Gupta and Polach, 1985).

  Should the activity of the sample be indistinguishable from the background activity at 1 standard deviation, it is released as background.

  Samples whose age falls between modern and background and are given finite ages.Standard errors released with each radiocarbon assay (see below) are usually rounded by convention (Stuiver and Polach, 1977). Again, not all laboratories subscibe to these conventions, some do not round up ages.

  Age (yr)	Radiocarbon date rounded to	Error (± value)
  0 - 1000	nearest 10	nearest 5 up
  1000 - 10000	nearest 10	nearest 10 up
  10000 - 25000	nearest 50	nearest 10 up
  >25000	nearest 100	nearest 50 up

  Standard error

  We mentioned previously that statistical analysis is necessary in radiocarbon dating because the decay of C14 although constant, is spontaneous. It is not possible to measure all of the radioactivity in a given sample, hence the need for some kind of statistical analysis of counted data. The distribution of counted C14 decay events will, over time, yield a pattern. The pattern is termed a "normal distribution curve". A normal or "Gaussian" distribution describes the symmetrical bell shaped cluster of events around the average or mean of the data. In a normal distribution, 2 out of 3, or 68% of the values or counts observed will fall within one standard deviation of the average of the data. At two standard deviations, 95% of the observed counts will fall within the range and at three standard deviations, 99% of the counts which comprise the normal distribution will fall within this region. Each radiocarbon date is released as a conventional radiocarbon age with 'standard error'. This is the '±' value and by convention is ± 1 sigma. The standard error is based principally upon counting statistics (but see below).
  

  According to Stuiver and Pearson (1992), the majority of laboratories report the measured counting statistics as a laboratory standard error. This results in considerable under-reporting (Scott, Long and Kra, 1990:253-393; Stuiver and Pearson, 1992:20) because a standard error based upon counting statistics alone does not include additional errors which may be incurred, for example, during sample preparation. Pearson (1979:21) has suggested:
  

  "Those involved in radiocarbon dating should be alert to the various possible sources of error and recognize that the precision quoted on a date may be quite unrealistic if the error sources have not been investigated in detail".

  Gordon Pearson. 1979. "Precise 14C Measurement by LS Counting". Radiocarbon 21(1):1-22.

  To present a date with a realistic standard error, Pearson (1980; 1983) and Pearson et al. (1986:929) have identified the factors which contributed significantly to errors in beta counting using Liquid Scintillation spectrometry. Ostensibly, this research was a precursor to high precision dating of Irish bog oak samples for the calibration of the radiocarbon timescale (Pearson, 1980, 1983; Pearson and Stuiver, 1993). He investigated each principal factor contributing to errors and considered their effect on overall laboratory precision and concluded that a standard error of ±25 radiocarbon years was possible in the Belfast laboratory. Standard errors quoted by the Belfast laboratory, then, are based upon this analysis.

  Many laboratories today calculate a laboratory error multiplier to account for all errors account for routine variation in reproducibility in radiocarbon dating. Stuiver (Stuiver and Pearson, 1993), for instance, has reported that the standard errors reported in the University of Washington laboratory results are based a lab multiplier. According to Stuiver and Pearson (1993), the error multiplier (or 'K') is a measure of the laboratory reproducibility, incorporating the errors resulting from the preparation of gas, its loading, memory effects and counting statistics. 'K' is defined as the actual standard error divided by the quoted standard error and is usually generated through repeat dating of a standard of known age or consensus age.

  In addition, inter-laboratory comparison exercises have been undertaken to evaluate laboratory variation and identify causes for it. We hope to include some information regarding these important programmes run principally by the International Atomic Energy Agency (IAEA) and Glasgow University Department of Statistics.

  Accuracy and Precision in Radiocarbon dating

  It is important to note the meaning of "accuracy" and "precision" in radiocarbon dating. Accuracy refers to the date being a 'true' estimate of the age of a sample within the range of the statistical limits or ± value of the date. Thus, for the sake of argument, if we were radiocarbon dating a sample of Harold 1's (d. 1066) remains, and obtained a date of 1040±40 AD, we would have dated the event of his death accurately. If however the date were 1000±15 AD, we would be inaccurate. In terms of precision, however, the former is imprecise in comparison to the latter. It can be seen that the date of 1000±15 AD while being highly precise is, in this instance, inaccurate.

  http://www.c14dating.com/agecalc.html