Rafter Radiocarbon Laboratory

 

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Absolute Modern Standard

The ¹⁴C activity of the Modern Standard as it was in 1950. Since the oxalic acid standard used in ¹⁴C measurements is itself decaying, in order to represent the absolute ¹⁴C activity in a material, as distinct from the ratio of the activity to the standard, the decay of the standard must be taken into account. The modern standard activity is defined for 1950, so measurements made at a later time must correct the measured oxalic activity for decay since that year. For example, in the year 2000, the modern standard activity will have declined from 0.227 Becquerels/gram to 0.225 Becquerels/gram.

AMS

Abbreviation of Accelerator Mass Spectrometry, the technique by which a particle accelerator, usually a tandem, is configured as a mass spectrometer to separate the carbon isotopes in a sample, allowing milligram size samples to be dated.

AMU

Abbreviation of Atomic Mass Unit, a convenient way for representing the masses of atoms. The amu is defined by the mass of a neutral ¹²C atom, which weighs exactly 12 amu. On this scale the proton has a mass of 1.0073 amu and the neutron a mass of 1.0087 amu. 1 amu = 1.661 x 10^-27 kg.

Atomic Number

The number of protons in an atomic nucleus. Eg the hydrogen nucleus consists of a single proton, so hydrogen has atomic number 1, the carbon nucleus has 6 protons and carbon has atomic number 6. The atomic number defines each element: the carbon isotopes ¹²C, ¹³C and ¹⁴C all have atomic number 6.

B

Becquerel

SI unit of radioactivity, defined as one disintegration per second. Replaces the Curie.

Bomb Carbon

This expression refers to the significant quantity of ¹⁴C that was injected into the atmosphere by nuclear weapons testing between 1945 and the mid 1970s. The increased activity reached its peak around 1963; atmospheric ¹⁴C reached twice the normal level in the Northern Hemisphere and increased by about 60% in the Southern Hemisphere. It has since decreased to about 10% above "pre-bomb" levels, due mainly to the exchange of CO₂ between the atmosphere and the oceans.

BP

Abbreviation of Before Present. Radiocarbon ages are conventionally specified relative to the year 1950, defined as "present".

C

¹⁴C

Synonym for radiocarbon. The isotope of carbon that has a mass of 14 amu.

Calibration

The process of converting a conventional radiocarbon age to a "true" or calendar age. This is accomplished by comparing the measured CRA with a table of radiocarbon ages measured on samples of wood whose chronological ages are known exactly by tree-ring counting. Tables of tree-ring data going back 12,000 years are available (see Radiocarbon Calibration Issue, 1993), and work to refine the calibration data is continuing.

Conventional Radiocarbon Age

The age obtained from a radiocarbon measurement using conventions set out in a paper by Stuiver & Polach (1977). This is not the true chronological age of the sample, which is obtained by calibrating the conventional age. It is important to note that the CRA is calculated from a measurement of an isotopic ratio, ie radiocarbon dating does not measure time directly. Radiocarbon ages are inferred from the measurements subject to a number of assumptions and qualifications. See reservoir effect.

Cosmic Ray

The high energy atomic radiation that bombards the earth from space. Cosmic ray particles are predominantly high energy protons and ⁴He nuclei ( a -particles). Part of the cosmic ray flux originates at the sun, and is generated by solar flare events. The remainder comes from interstellar space. The mechanisms giving rise to the highest energy cosmic rays are not well understood. The impact of cosmic rays on the atmosphere gives rise to nuclear reactions, generating the cosmogenic nuclides such as ¹⁴C, ¹⁰Be and ²⁶Al.

Cosmogenic Nuclides

Rare isotopes that are generated mainly, or entirely, by the effects of cosmic rays on the earth's atmosphere or the first few metres of its surface. Examples are ¹⁴C, ¹⁰Be, ³⁶Cl, ²⁶Al, ³²Si.

CRA

Abbreviation of Conventional Radiocarbon Age

Curie

Obsolete unit of radioactivity, equal to 3.7x10¹⁰ disintegrations per second. Replaced by the Becquerel

D

d ¹³C

The carbon stable isotope ratio of a material relative to the PDB standard. Used to calculate the fractionation correction in radiocarbon dating.

d ¹⁴C

The ¹⁴C isotopic ratio of a material relative to the modern standard, without any correction for fractionation. As with d ¹³C it is expressed as parts per thousand greater than, or less than, the standard. Note that the ¹⁴C isotopic ratio means the ratio of ¹⁴C to total stable carbon, ie ¹²C + ¹³C.

D ¹⁴C

The ¹⁴C isotopic ratio of a material relative to the modern standard after correction for fractionation to d ¹³C = -25^o/_oo.

E

F

Fractionation

The change in isotopic ratio that can occur when a material undergoes a chemical reaction or certain types of physical processes. Although the different isotopes of an element are said to have identical chemical properties, the rate at which they take part in a chemical reaction depends on their mass. Generally, lighter isotopes will proceed more quickly than heavier isotopes. An example of fractionation occurs in the photosynthesis of atmospheric CO₂ to form wood cellulose. Atmospheric CO₂ has a d ¹³C of about -7^o/_oo whereas the d ¹³C of wood is about -25^o/_oo. The chain of chemical reactions leading to the formation of cellulose has resulted in the cellulose being depleted in ¹³C relative to the CO₂. Chemical reactions are not the only causes of fractionation. Evaporation, condensation, diffusion or the passage of vapour through a small aperture can also give rise to isotopic fractionation.

Correction for fractionation is a very important step in calculating a radiocarbon age. The processes that alter the stable isotope ratio will also affect the ¹⁴C, and it is necessary to distinguish between the change in ¹⁴C due to radioactive decay (which is the basis of radiocarbon dating) and that due to chemical fractionation effects. Fractionation will shift the ratio between ¹⁴C and ¹³C by about the same amount as it shifts the ratio between ¹³C and ¹²C, or alternatively, the shift in the ¹⁴C:¹²C ratio is about twice the shift in the ¹³C:¹²C ratio. By measuring the stable carbon isotope ratio by mass spectrometer it is possible to derive a correction to the ¹⁴C measurement that cancels out the fractionation effect, leaving only the radioactive decay component and enabling radiocarbon measurements on different materials to be compared on the same basis.

G

Geiger counter

A radiation detector first developed by Hans Geiger. It consists of a metal cylinder with a thin wire along its central axis, filled with with gas. A high voltage is connected between the wire (the anode) and the cylinder (the cathode). When radiation penetrates the cylinder it starts an electric discharge between the anode and the cathode which can be detected electronically as an electric pulse. The name Geiger refers not only to the design of the counter but also to the mode in which it is operated. The voltage between anode and cathode is set sufficiently high that the counter is extremely sensitive to radiation, but on the other hand the output of the counter gives no further information about the radiation except that it is present. See proportional counter.

H

Half life

The time taken for a given amount of a radioactive isotope to decay to half its original value. For each isotope the half life is constant and is not affected by changes in the environment. Because half lives do not change, radioactivity can be used as a "clock" to measure elapsed time, provided that the original quantity of the isotope present is known. Typical radioactive isotopes and their half lives, that are used to measure periods of time are ¹⁴C, half life 5730 years, ¹⁰Be, half life 1.5 million years and tritium (³H), half life 12.5 years.

Hard Water Effect

The "hard water effect" refers to the situation in which the radiocarbon age of fresh water (or organisms living in the water) in contact with limestone (calcium carbonate) can be increased by the dissolved carbonate in the water. As an ideal case, consider a freshwater lake in a limestone basin. Limestone is insoluble in pure water but dissolves in acid. As the lake absorbs CO₂ from the atmosphere the water becomes slightly acidic. If the CO₂ can be regarded as distributed uniformly through the water then in theory one atom of carbon will be released from the limestone for each molecule of absorbed CO₂. Since the limestone contains no ¹⁴C, the effect is to dilute the concentration of the ¹⁴C in the incoming CO₂ by half. The effect on the measured CRA of the carbon in the water is as if the carbon had decayed by one half life, so the dissolved carbon will give an age about 5,700 years older than the incoming CO₂. (See Fontes, 1992)

I

Isotope

Isotopes are atoms of an element having identical chemical properties but different masses. Thus, ¹²C, ¹³C and ¹⁴C are all isotopes of carbon having masses of 12, 13 and 14 amu, respectively. The atomic nuclei of these isotopes all contain 6 protons, which defines them as carbon, but they have 6, 7 and 8 neutrons, respectively, which gives them different masses. The extra neutron in ¹⁴C makes the nucleus unstable, which is why ¹⁴C is radioactive.

Isotopic Ratio

The relative quantity of the different isotopes of an element in a material. On earth, the relative number of ¹³C atoms to ¹²C atoms is about 0.0112, ie the isotopic ratio of ¹³C to ¹²C is 0.0112. Isotopic ratios are usually expressed relative to some standard material, as so many parts per thousand greater than or less than the corresponding ratio in the standard. Carbon isotopic ratios are given relative to the PDB limestone standard; eg wood has a ¹³C:¹²C ratio about 2.5% (=25 parts per thousand) smaller than the PDB standard. This is written as d ¹³C = -25^o/_oo (ie -25 per mille).

J

K

L

Libby

Willard Libby (1908 - 1980), American chemist who invented radiocarbon dating in 1946.

Libby half life

The half life of ¹⁴C obtained by Libby, 5568 years (Libby, 1955). This was the first precise determination of the ¹⁴C half life. Later measurements gave a more accurate value of 5730 ± 40 years (Godwin, 1962), but because the Libby value had become established in the radiocarbon literature, in 1962 it was decided to retain it in calculations of the Conventional Radiocarbon Age for published dates and apply a correction for actual chronological use. This correction is automatically applied in the calibration procedure.

Liquid Scintillation Counter

A type of radiation detector that exploits the property of some materials of emitting a pulse of light when radiation passes through them. Certain organic materials have this property, and it is used in ¹⁴C counting by converting the sample carbon to benzene in which some organic scintillator is dissolved. Beta particles emitted by decaying ¹⁴C atoms cause the solution to emit light pulses. These are detected by photomultiplier tubes placed close to the vial containing the liquid, which convert the light pulse to an electrical pulse amplified millions of times. Like the proportional counter, the liquid scintillation counter has the advantage that the magnitude of the output signal from the counter is proportional to the energy of the radiation detected, making it possible to discriminate against background radiation and monitor the performance of the counting electronics.

M

Mass Number

The total number of protons and neutrons in an atomic nucleus. ¹⁴C has mass number 14 because its nucleus contains 6 protons and 8 neutrons. ¹⁴N also has mass number 14 because its nucleus contains 7 protons and 7 neutrons.

Mass Spectrometer

An instrument to measure the isotopic ratio of an element in some material. It works by passing a beam of electrically charged atoms of the element through a magnetic field. The magnetic field causes the path of the atoms to follow a curve, with the lighter isotopes being deflected most and the heavier isotopes being deflected least. The instrument is designed so that the paths of the isotopes diverge sufficiently so that the intensity of the different beams can be measured separately.

Modern Standard

The reference standard used for radiocarbon measurements. It is defined as the radiocarbon activity measured in 1950 of a sample of wood growing in the Northern Hemisphere in 1890. The year 1890 was chosen to avoid the significant change in the concentration of ¹⁴C in the atmosphere that took place in the 20^th century due to the world wide increase in the consumption of fossil fuel. The reference activity corresponds to a ¹⁴C decay rate of 0.227 Becquerel per gram of carbon. Because of the presence of bomb carbon in the atmosphere since 1955, plant and animal material growng in the second half of the 20^th century contains much more radiocarbon than the modern standard.

For actual laboratory measurements, the standard used is taken from a batch of oxalic acid prepared by the US National Bureau of Standards (now NIST). The ¹⁴C activity of the oxalic acid (referred to as HOxI) multiplied by 0.95 is equal to the activity of the modern standard, defined above. In practice, then, radiocarbon measurements are compared with 0.95 x activity of the oxalic standard. To allow for fractionation of the oxalic acid during preparation for ¹⁴C counting, the measured activity is corrected to d ¹³C = -19^o/_oo.

The corresponding isotopic ratio in the modern standard, ¹⁴C/(¹²C + ¹³C) = 1.176 x 10^-12.

Since the original batch of oxalic acid is almost exhausted, a second batch has been prepared (HOxII), which has a ¹⁴C activity 1.2933 times HOxI, (Mann, 1983) and is now in use by many laboratories.

N

O

P

PDB

Abbreviation of PeeDee Belemnite, which is limestone used as the international reference standard for expressing carbon stable isotopic ratios. The carbon isotopic ratio of PDB is 0.0112372, and the carbon isotope ratios of other materials are expressed as parts per thousand (per mille) relative to this value.

Proportional Counter

A radiation detector similar to the Geiger counter, but operated in a mode where the magnitude of the output signal is proportional to the energy of the radiation triggering the counter. Proportional counters are used for ¹⁴C measurement by converting the sample material to methane or CO₂ which is placed in the counter body. As the ¹⁴C decays the beta particles that are emitted trigger the counter, enabling them to be counted. Because the output signal is proportional to the particle energy it is possible to discriminate against spurious events such as cosmic rays passing through the counter, background radiation from the surroundings or radioactive contaminants such as radon in the gas. This discrimination is not sufficient by itself, but is supplemented by heavy shielding and special measures to identify and reject cosmic ray events.

Q

R

Radiocarbon

The naturally occuring radioactive isotope of carbon, atomic number 6, mass number 14, symbol ¹⁴C. Radiocarbon has a half life of 5730 ± 40 years (Godwin, 1962) and is formed in the atmosphere as a result of cosmic ray collisions with air molecules. These collisions produce free neutrons that can be absorbed by the nuclei of ¹⁴N atoms in the atmosphere, converting them into ¹⁴C atoms. The ¹⁴C is quickly oxidised to carbon monoxide (CO) and then to carbon dioxide (CO₂). The CO₂ enters the biosphere either through photosynthesis in plants or by absorption in the oceans.

Radiometric Counting

A term sometimes used to describe the "traditional" method of measuring ¹⁴C, by measuring its radioactivity with radiation detectors. Three types of detector can be used for this: Geiger counters, proportional counters, and liquid scintillation counters. Of these three, Geiger counters are seldom used today.

Reservoir Effect

The shift in apparent radiocarbon age caused by the original carbon having an anomalously low or high ¹⁴C content. One of the fundamental assumptions underlying radiocarbon dating is that the material being dated was originally in equilibrium with the contemporary atmosphere, and drew its carbon (and hence, its ¹⁴C) directly from the atmospheric "reservoir". However, if the material was formed in an environment which had a different ¹⁴C content to the atmosphere then its radiocarbon age will be greater or less than that of contemporary wood, depending on whether its carbon reservoir contained less or more ¹⁴C than the atmosphere. The best known example of the reservoir effect is the shift in radiocarbon age of marine organisms. Marine shells from near coastal waters have radiocarbon ages about 350 years greater than contemporary land organisms because the sea has a lower ¹⁴C content than the atmosphere. See Hard Water Effect.

S

T

Tandem

A class of electrostatic linear particle accelerators in which negative ions are accelerated by a high positive potential, typically three to six million volts. Two or more electrons are then stripped from the ion, changing it to a positive ion. The same potential is then used to accelerate the positive ions through the second stage of the accelerator. Common types of tandem accelerator encountered in AMS work are the Van de Graaff and the Tandetron. They differ in the means used to generate the accelerating potential.

U

V

Varve

A regular band structure seen in some lake sediments. Varves are a seasonal effect caused by the regular annual variation in sediment load deposited in the lake by run-off from the surrounding land. Because varves mark annual changes they can act as absolute time markers, and material from varves can be used to calibrate the radiocarbon time scale.

W

X

Y

Z