G-value (radiology)

The G value is a measure of the yield of a radiation chemical reaction.

definition

Originally, the G value was defined as the mean number N of the observed objects (e.g. molecules ), which are formed, consumed or changed by a transferred energy E of 100  eV :

${\displaystyle G={\frac {N}{E}}}$

G values ​​were expressed accordingly in the unit (100  eV ) ⁻¹ :

${\displaystyle [G]={\frac {1}{100\;\mathrm {eV} }}=(100\;\mathrm {eV} )^{-1}}$

The practical reason for this determination was that numerical values ​​below 10 were obtained for most reactions.

In the International System of Units (SI) , the radiation-chemical yield G is used instead. The size symbol G is the same, but the radiation-chemical yield is defined as the quotient of the considered (formed, used or changed) amount of substance n and the transferred energy E :

${\displaystyle G={\frac {n}{E}}}$

The unit symbol is accordingly mol / J :

${\displaystyle [G]={\frac {\mathrm {mol} }{\mathrm {J} }}}$

The conversion factor between numerical values ​​for the G value and for the radiation-chemical yield results from the Avogadro constant N _A

${\displaystyle N_{\mathrm {A} }={\frac {N}{n}}=6{,}022\;140\;76\cdot 10^{23}\;\mathrm {mol} ^{-1}}$

and the relationship

${\displaystyle 1\;\mathrm {eV} =1{,}602\;176\;634\cdot 10^{-19}\;\mathrm {J} }$

to

${\displaystyle (100\;\mathrm {eV} )^{-1}={\frac {1}{100\;\mathrm {eV} }}\approx 1{,}04\cdot 10^{-7}\;{\frac {\mathrm {mol} }{\mathrm {J} }}=0{,}104\;{\frac {\mathrm {\mu mol} }{\mathrm {J} }}}$.

Examples

During the radiolysis of pure liquid water by γ - or β ^- radiation, hydrogen (H ₂ ) is formed with a typical radiation-chemical yield of G (H ₂ ) = 0.047 µmol / J. This value shows, for example, that irradiating 1  l of water ( mass m  = 1  kg ) with a dose of D  = 1  Gy  = 1  J / kg results in an amount of hydrogen of n (H ₂ ) = 0.047  µmol :

${\displaystyle n(\mathrm {H_{2}} )=m\cdot D\cdot G(\mathrm {H_{2}} )=1\;\mathrm {kg} \cdot 1\;{\frac {\mathrm {J} }{\mathrm {kg} }}\cdot 0{,}047\;{\frac {\mathrm {\mu mol} }{\mathrm {J} }}=0{,}047\;\mathrm {\mu mol} }$

At the same time, water molecules are consumed by the radiolysis . A radiation-chemical yield can also be specified for this. A typical value for pure liquid water is G (−H ₂ O) = 0.43 µmol / J.

When the particularly radiation-sensitive plastic polytetrafluoroethylene (PTFE) is irradiated , the CC bonds of the linear molecular chains are preferably split , whereby the mean chain length is shorter and the strength of the plastic is reduced. The G value for such bond cleavages is 0.051 (100 eV) ⁻¹ . The corresponding radiation chemical yield is G  = 5.3 · 10 ⁻⁹  mol / J.

See also

• Quantum yield
• Fricke dosimeter

Individual evidence

1. ^ Arthur C. Wahl, Norman A. Bonner: Radioactivity Applied to Chemistry . John Wiley & Sons, New York 1951.
2. ↑ ^{a b c d} International Commission on Radiation Units and Measurements (ICRU): Fundamental Quantities and Units for Ionizing Radiation , ICRU Report 60, Bethesda, MD 1998.
3. ^ ^{A b c d} Gregory Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 .
4. ↑ ^{a b} L. Wojnárovits: Radiation Chemistry . In: Attila Vértes, Sándor Nagy, Zoltán Klencsár, Rezső G. Lovas, Frank Rösch (eds.): Handbook of Nuclear Chemistry . 2nd Edition. Volume 3. Springer, 2011, ISBN 978-1-4419-0719-6 , pp. 1281-1282 , doi : 10.1007 / 978-1-4419-0720-2 .
5. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 16, 2019 .  Value for the Avogadro constant. The value is exact, i.e. H. without uncertainty in the last specified places.
6. ↑ CODATA Recommended Values. National Institute of Standards and Technology, accessed July 16, 2019 .  Value for one electron volt. The value is exact, i.e. H. without uncertainty in the last specified places.
7. ^ KF Kircher, RE Bowman: Effects of Radiation on Materials and Components. Reinhold Corp., New York 1964.