Page 583 - Instrumentation Reference Book 3E

References 565

For a typical system such as that of Libby so-called "fog-marking," reducing thereby the
(1985)., value of the cloth. In rubber manufacture for cable
coverings, automobile tires. etc., the rubber has to
t =48 h and T; = 5568yr so pass through rollers, where the static electricity so
Do = 6.7 cpm and = 5 cpm, generated causes the material to remain stuck to
DolJB M 3 the rollers instead of moving to the next processing
Hence stage.
All these static problems can be overcome, or
at least reduced by mounting a suitable radio-
active source close to the place where sfatic is
produced, so that ions of the appropriate sign
= 8034 x 4.038 are produced in quantities sufficient to neutralize
= 32442yr the charges built up by friction, etc.
A wide variety of sources are now available in
many shapes to allow them to be attached to the
Calibvation of the radiocarbon time scale A machines to give optimum ionization of the air at
number of corrections have to be applied to dates the critical locations. Long-strip sources of tri-
computed from the radiocarbon decay measure- tium, 9oSr190 Y, and 241Am are the most popular.
ments to arrive at a "true" age. First, a comparison The importance of preventing static discharges
of age by radiocarbon measurement with the age has been highlighted recently in two fields. The
of wood known historilzally initially showed good first is in the oil industry, where gas-filled oil
agreement in 1949. Wh.en the radiocarbon-dating tankers have exploded due to static discharges
system was improved by the use of CO? propor- in the empty tanks. The second is in the micro-
tional counters; in which the sample was intro- chip manufacturing industry, where static dis-
duced directly into the counter, closer inspection charges can destroy a complete integrated circuit.
using many more samples showed discrepancies
in the dates calculated by various methods. Some
of these discrepancies can be accounted for, such
as the effect on the atmosphere of all the burning 23.5 References
of wolod and coal since the nineteenth century-
Suess #called this the "fossil-fuel effect." Alternative Clayton, C. G. and J. F. Cameron. Radioisotope Instru-
methods of dating, such as dendrochronology nzents, Vol. 1, Pergamon Press, Oxford (1971) (Vol. 2
was not published). This has a very extensive biblio-
(the counting of tree rings), thermoluminescent graphy for further study.
dating, historical dating, etc., have demonstrated Gardner, R. P. and R. L. Ely, Jr, Radioisotope Measure-
that variations do occur in the curve relating radio- ment Applications in Engineering, Van Nostrand. New
carbon dating and other methods. Ottaway (1983) York (1967)
describes in more detail the problems and the pres- Hubbell, J. H., Photon Cross-sections, Attenuation Co-
ent state of the art. efficients, and Energy Absorption Co-efficients fioni
lOkeVtolOOGeV,NSRDSNBS29(1969)
Libby, W. F., Radiocarbon Dating, University of
23.4.3 Static elimination Chicago Press (1955)
Ottaway, B. S. (ed.); "Archaeology, dendrochronology
Although not strictly instrumentation, an inter- and the radiocarbon calibration curve." Edihburgh
esting application of radioactive sources is in the University Occasional Paper No. 9 (1983)
elimination of static electricity. In a number of Shumilovskii. N. N. and L. V. Mel'ttser, Radioactive
manufacturing processes static electricity is pro- Isotopes in Instrumentation and Control, Pergamon
duced, generally, by friction between, for exam- Press, Oxford (1964)
ple, a sheet of paper and the rollers used to pass it Sim, D. F., Summary of the Laiv Relating to Atoniic
throu,gh a printing process. This can cause tearing Energy and Radioactive Substances: UK Atomic
at the high speeds that are found in printing Energy Authority (yearly). Includes (a) Radioactive
Sz~bstances Act 1960, (b) Factories Act-The Ionizing
presses. In the weaving industry, when a loom is Radiations (Sealed Sources) Regulations 1969.
left standing overnight, the woven materials and (HMSO Stat. Inst. 1969, No. 808); (c) Factories Act-
the warp threads left in the loom remain charged The Ionisilzg Radiations Regulations, HMSO (1985)
for a iong period. It is found that dust particles Watt, D. E. and D. Ramsden, High Sensitivitj' Counting
become attracted to the cloth, producing the Techniques, Pergamon Press. Oxford (1964)