**1. Introduction**

280 Gamma Radiation

Zhou, H.; Randers-Pehrson, G.; Waldren, C. & Hei, T. (2004). Radiation- induced bystander

1372.

effect and adaptive response in mammalian cells. *Advance Space Research*, 34:1368-

Wood as a natural organic material is susceptible to biodeterioration by insects, fungi, and bacteria. There is almost no wooden artefact or old wooden element, which is not infected and not damaged, at least partly, by wood-destroying organisms. As ancient wood and wooden artefacts are invaluable, their appropriate restoration is of particular importance. The first step in restoration is the detection and quantification of wood pests and decay and accordingly, under certain conditions, the disinfestation of ancient wood and old wooden artefacts could be needed.

Besides disinfestation prior to restoration, sterilization of wood is applied for testing the resistance of wood and wooden products against wood-destroying organisms. For both purposes, restoration and resistance testing, gamma radiation is considered as a suitable decontamination method.

### **2. Gamma radiation as a sterilisation method**

Gamma radiation, as a high energy, ionising electromagnetic radiation, easily penetrates through wooden objects. It is known to be very effective in the context of disinfestation of wooden artefacts (Unger *et al.*, 2001; Katušin-Ražem *et al.*, 2009; Fairand and Ražem, 2010) but also for wood sterilisation (Sharman and Smith, 1970; Shuler, 1971; Freitag and Morrell, 1998; Pratt *et al.*, 1999; Severiano *et al.*, 2010). In contrary to alpha and beta rays, which penetrate only very thin layers, gamma radiation fully penetrates wooden objects (Fengel and Wegener, 1989; Tišler and Medved, 1997). The energy-rich gamma rays modify molecular structures and lead to unexpected function of living cells or to their death.

Changes in Selected Properties of Wood Caused by Gamma Radiation 283

Klimentov and Bysotskaia (1979); Chawla (1985) and Šimkovic *et al.* (1991) reported that gamma irradiation as a pre-treatment for the "softening" of wood prior to chemical processing, can significantly reduce the use of chemicals. Oldham *et al.* (1990) used gamma radiographic technique for analysing wood destruction by fire. They determined the efficiency of fire retardants by measuring the difference in absorbed gamma radiation energy in wood protected by these retardants before and after burning the wood. Bogner (1993) and Bogner *et al.* (1997) used different surface activators in order to achieve greater adhesion of adhesives to wood. Among other used activators, gamma radiation at different doses was also used. The results show that after treatment of wood by gamma radiation in doses range of 0 and 100 kGy, glue improves adhesion to wood and thus increases the strength of glued bonds. The first investigations of the influence of gamma radiation on lignocellulose materials, in terms of increasing the solubility of insoluble high-polymerized sugars such as cellulose, were performed by Klimentov and Bysotskaia (1979) and

Sterilisation by gamma radiation is very easy, fast and effective, but at doses higher than disinfestation doses it changes the molecular structures not only in wood decaying organisms but also in wood cell walls. Although Severiano *et al.* (2010) reported no influence of gamma radiation on some wood physical, thermal and mechanical properties in the radiation dose range between 25 and 100 kGy, the majority of other studies reported

The random break-up of cellulose chains in gamma irradiated wood is a typical reaction (Seaman *et al.*, 1952; Kenaga and Cowling, 1959). Seifert (1964), Tabirih *et al.* (1977) and Cutter *et al.* (1980) found that the holocellulose portion of cell walls was degraded by gamma irradiation. After exposure of cypress wood (*Pseudotsuga mensziessi*) and tulipwood (*Liriodendron tulipifera*) to gamma rays, Lhoneaux *et al.* (1984) confirmed the occurrence of ultra structural changes in the cell walls of tested wood species. Following the above research results, Fengel and Wegener (1989) reported that gamma irradiation changes the anatomical and chemical structure of wood, but also the physical and mechanical properties. Seifert (1964) also found that small doses of gamma radiation lead to a destruction of hemicelluloses' pentose creating new compounds and new chemical bonds. Chawla (1985) reported that up to the dose of 500 kGy the increase of wood solubility occurs primarily because of hemicelluloses depolymerisation and destruction. The influence of gamma radiation was also investigated on compounds of cellulose-acetate and cellulose nitrate (Fadel and Kasim, 1977; Zamani *et al.*, 1981; Subrahmanyam *et al.*, 1998). They came to the conclusion that also modified cellulose chains broke up. Seifert (1964) came to the conclusion that, an average increase of 25 kGy of gamma radiation caused a loss of 1 % cellulose in the dose range of 0 – 1 MGy. This proportional loss in cellulose crystallinity with increasing radiation dose was confirmed by Zamani *et al.* (1981) in a narrower interval of doses from 0 to 0.5 MGy. Fedel and Kasim (1977) successfully used cellulose-acetate as an indicator of the amount of absorbed energy of gamma irradiated objects, which reaffirms the proportionality between the reduction of cellulose crystallinity and gamma radiation dose.

Klimentov *et al.* (1981).

**4. The influence of gamma radiation on wood** 

significant influence of gamma radiation on wood properties.

**4.1 Chemical properties of gamma irradiated wood** 

### **2.1 Doses of gamma radiation necessary for wood sterilisation**

According to Kunstadt (1998), insects do not withstand doses between 0.7 and 1.3 kGy, while elimination of fungi requires significantly higher doses. Unger *et al.* (2001) mentioned doses between 0.25 and 3 kGy to be adequate for extermination of wood-destroying insects, depending on species and developmental stage. Extermination of wood decay fungi in wood usually requires higher doses, ranging from 2 to 18 kGy depending on the fungus species. Mycelium of the fungus *Serpula lacrymans* can be killed off with 2 – 3 kGy, but it can be reduced to 0.5 kGy if the temperature rises to 50 °C. Unger *et al.* (2001) also stated that the bacteria elimination requires doses of 3 – 15 kGy. Lester *et al.* (2000) sterilised New Zealand soft woods including radiata pine (*Pinus radiata*) against Huhu beetle larvae (*Prionoplus reticularis*) and concluded that doses between 2.5 and 3.7 kGy are enough to control wood destroying insects and that moisture content of wood during irradiation has an important role in radiation dose determination. Magaudda *et al.* (2001) stated that the dose of 10 kGy is sufficient for practical disinfestations of Whatman paper-destroying insects. Freitag and Morrell (1998) reported on gamma radiation doses around 15 kGy to be adequate to combat pests in wood (predominantly insects). They have also stated that the doses for extermination of *Xylophagous* microorganisms in wood are much higher than for sterilisation of other materials. Csupor *et al.* (2000) irradiated wood decayed by *Xylophagous* fungi in the range from 2 to 1400 kGy, and they concluded that 12 kGy is sufficient for safe sterilisation of wood against fungi. The European Standard EN 113 (CEN, 1996) requires doses between 25 and 50 kGy for wood sterilisation in lab testing procedures.

The treatment time depends on the power of the irradiation source, and there is no significant difference if the wood was irradiated with a weaker source for a longer time or with a stronger source for a shorter time (Unger *et al.*, 2001; Hasan, 2006 mentioned Ražem, 2005). The important quantity is the absorbed dose (the amount of absorbed energy per mass unit) which irradiated substance receives (Fairand and Ražem, 2010). On the contrary Curling and Winandy (2008) reported that dose rate and total dose of gamma radiation differently affect both, bending strength and some chemical components in tested wood.
