ABSTRACT
The interaction of energetic particles with matter has different kinds of effects on the biological system,materialsofindustrialimportance,environments,andsoforth.Thus,researchesusingvarious detectiontechniqueshavebeenperformedformanyyearsinordertoobtainaclearunderstanding oftheseeffectsonseveralionizingradiations.Thenumberofelectron-ionpairsformedinmatteruponthecompleteslowingdownofionizingradiationisanimportantquantitycharacterizing the ability of initiating a chemical reaction chain within a gas and/or of inducing certain radiation effectsinbiologicalsystems.Therefore,thestudyofexpectedvaluesofionpairshasdrawnthe attentionofmanyresearcherstowardŸeldssuchasradiationchemistry,charged-particlespectroscopy,anddosimetry(Inokuti,1975;SamsonandHaddad,1976;ICRU,1979;Combecher,1980;
6.1 Introduction .......................................................................................................................... 105 6.2Historical Background .......................................................................................................... 106 6.3Characteristics of W-Values in the Energy Region below 1 keV .......................................... 108 6.4Variation in Energy Dependence of Photon W-Values for Hydrocarbon Molecules ............ 111
6.4.1Introduction .............................................................................................................. 111 6.4.2 Measurements and Analysis ..................................................................................... 113 6.4.3Result of Ethylene ..................................................................................................... 114 6.4.4Simple Model for Explanation of Variation in W-Value ........................................... 115 6.4.5Methane and Propane ............................................................................................... 119
6.5Oscillatory Variation in Energy Dependence of Photon W-Values for Rare Gas Atoms and Atomic Shell Effects ...................................................................................................... 121 6.5.1Introduction .............................................................................................................. 121 6.5.2Measurements ........................................................................................................... 121 6.5.3Analysis .................................................................................................................... 122 6.5.4Result of Kr ...............................................................................................................124 6.5.5 Precise Model of the Atomic Shell Effect on Variation in the W-Value of Kr ......... 125 6.5.6Ar and Xe .................................................................................................................. 129
6.6Outlook ................................................................................................................................. 131 6.7Summary .............................................................................................................................. 132 Acknowledgments .......................................................................................................................... 132 References ...................................................................................................................................... 132
Inokuti et al., 1980; Waibel and Grosswendt, 1983; Grosswendt, 1984; Krajcar-Bronic et al. 1988; Kowari et al.1989;Kimuraetal.,1991;Panskyetal.,1996;Krajcar-Bronic,1998).Afundamental quantity,W-value,isdeŸnedastheaverageenergyexpendedbyradiationtoproduceanelectron-ion pairduringcompleteslowing-downinthematter.Avarietyofmeasurementshavebeenperformed onW-valuesofatomsandmoleculesfordifferentradiations(SamsonandHaddad,1976;ICRU, 1979; Combecher, 1980; Waibel and Grosswendt, 1983; Krajcar-Bronic et al., 1988; Pansky et al., 1996).TheoreticalapproacheshavepursuedclariŸcationofphysicalinsightintoW-valuesusing the Monte Carlo technique together with an analytical method using the Spencer-Fano equation (Inokuti et al., 1980; Grosswendt, 1984; Kowari et al., 1989; Kimura et al., 1991). These studies have shown that the W-value is insensitive to quality and energy for radiations above several keV (Inokuti, 1975; ICRU, 1979; Inokuti et al., 1980; Grosswendt, 1984; Kowari et al., 1989; Kimura et al., 1991; KrajcarBronic, 1998). A limited number of measurements were reported for W-values of some gases in energy regions below 1 keV (Samson and Haddad, 1976; Combecher, 1980; Waibel and Grosswendt, 1983; Krajcar-Bronic et al., 1988; Pansky et al., 1996). The W-value of electrons increases with decreasing electron energy. This behavior is explained as a phenomenon originating from the Ÿnding that the ratio of the probability of excitation to that of ionization in a single collision of an electron with a gas molecule increases as the electron energy decreases. Further, W-values of photons exhibit a variation in energy dependence in sub-keV regions where electrons ejected upon photoabsorption have low energies. This variation arises from the contributions of different electron orbitals responsible for the initial photoionization process, which critically depends on the photon energy.
