290 Chapter 11
successfully used irradiation to sterilise archival and manuscript papers.
Decreases in mechanical strength of cellulose-based papers were observed after
gamma irradiation at a dose of 10 KGy. Nevertheless, the advantage of irradiat-
ing material after packaging has been recognised in the food industry.
Gamma rays are emitted from the radionuclides of cobalt 60 and caesium
- Caesium is a fission product obtained from spent nuclear fuel rods. This
must be extracted in processing plants before being used as a useful source
of radiation. Cobalt 60 is manufactured specifically for commercial use in
radiotherapy, radiography and food processing. It can be produced in nuclear
reactors in large quantities by neutron bombardment of inactive cobalt 59 for
periods usually in excess of 6 months. Almost all gamma ray facilities in the
world use cobalt 60 rather than caesium 137. The half-life of cobalt 60 is 5.3
years. The source must be changed periodically to maintain a given level of
radioactivity. Ionising radiation is only safe for use at energies below 5 MeV,
since radiation at higher energies may make materials such as wood radio-
active. Gamma radiation, however, does not result in any residual radioactiv-
ity in foods since cobalt 60 has an energy of only 1.17/1.33 MeV.
Research at the Mary RoseTrust has shown that gamma irradiation is a
superior alternative to low temperature and biocide treatments of polyethylene-
wrapped timbers and has been adopted by the Trust since 1998. Screening a
range of bacteria and fungi isolated from waterlogged archaeological wood
revealed that a dose of 15 KGy is required for the inactivation of most organ-
isms (see Table 2).
Gamma irradiation may also be desirable as a treatment for wood excav-
ated from polluted archaeological sites. Applying medical sterilisation stand-
ards of 25 KGy ensures complete inactivation of any human pathogens in
addition to wood decay organisms. A dose of 15–25 KGy had no adverse effects
on the chemical and physical properties of waterlogged archaeological wood.
However, at dose levels in excess of 100 KGy radiolytic damage was observed
resulting in increased hygroscopicity, warping, loss of surface texture, reduc-
tions in compressive and bending strength, and chemical alteration and degrad-
ation of wood cell wall components.
Table 2Radiosensitivity of wood decay organisms
(After Pointing, 1995)
Organism Data source Recommended lethal dose (KGy)
Aerobic bacteria Mary Rose timbers 2.4
Anaerobic bacteria National Collection of 2.5
Industrial Bacteria
Invertebrates (insects) Mary Rose timbers 3.2
Marine fungi Mary Rose timbers 12.0
Terrestrial fungi Mary Rose timbers 13.0