Fricke dosimeter

The Fricke dosimeter (also out of date Fricke ferrous sulfate detector ) is the best-known chemical dosimeter . It was developed in 1927 by Hugo Fricke and Sterne Morse . Its functional principle is based on the oxidation of iron (II) to iron (III) ions through the action of ionizing radiation .

composition

Fricke dosimeters consist of ampoules filled with an air-saturated iron (II) sulfate solution . A typical solution has the following composition:

• 0.001  mol / l ammonium iron (II) sulfate (Fe (NH ₄ ) ₂ (SO ₄ ) ₂ )
• 0.001 mol / l sodium chloride (NaCl)
• 0.4 mol / l sulfuric acid (H ₂ SO ₄ )

Mode of action

Radiolysis of water

When exposed to ionizing radiation, due to the dilution of the solution, primarily a radiolysis of water molecules initially takes place ; only at concentrations of over 0.1 mol / l could a dissolved substance noticeably undergo direct radiolysis. Radiolysis of water takes place in several steps, which are listed below.

${\displaystyle \mathrm {H_{2}O} \ {\xrightarrow[{\qquad }]{\gamma }}\ \mathrm {H_{2}O} ^{*}}$

${\displaystyle \mathrm {H_{2}O} \ {\xrightarrow[{\qquad }]{\gamma }}\ \mathrm {H_{2}O} ^{+}\ {+}\ \mathrm {e} ^{-}}$

${\displaystyle \mathrm {H_{2}O} ^{+}\ {+}\ \mathrm {H_{2}O} \ {\xrightarrow[{\qquad }]{}}\ {\cdot }\mathrm {OH} \ {+}\ \mathrm {H_{3}O} ^{+}}$

${\displaystyle \mathrm {H_{2}O} ^{*}\ {\xrightarrow[{\qquad }]{}}\ \mathrm {H} {\cdot }\ {+}\ {\cdot }\mathrm {OH} }$

${\displaystyle \mathrm {H_{2}O} ^{*}\ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}} \ {+}\ \mathrm {O} {\cdot }}$

${\displaystyle 2\;\mathrm {e_{aq}^{-}} \ {+}\ 2\;\mathrm {H_{2}O} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}} \ {+}\ 2\;\mathrm {OH} ^{-}}$

${\displaystyle \mathrm {e_{aq}^{-}} \ {+}\ {\cdot }\mathrm {OH} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {OH} ^{-}}$

${\displaystyle \mathrm {e_{aq}^{-}} \ {+}\ \mathrm {H_{3}O} ^{+}\ {\xrightarrow[{\qquad }]{}}\ \mathrm {H} {\cdot }\ {+}\ \mathrm {H_{2}O} }$

${\displaystyle \mathrm {e_{aq}^{-}} \ {+}\ \mathrm {H} {\cdot }\ {+}\ \mathrm {H_{2}O} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}} \ {+}\ \mathrm {OH} ^{-}}$

${\displaystyle 2\;\mathrm {H} {\cdot }\ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}} }$

${\displaystyle 2\;{\cdot }\mathrm {OH} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}O_{2}} }$

The presence of dissolved oxygen in the air-saturated solution also forms the hydroperoxy radical (∙ HO ₂ ):

${\displaystyle \mathrm {e_{aq}^{-}} \ {+}\ \mathrm {O_{2}} \ {\xrightarrow[{\qquad }]{}}\ {\cdot }\mathrm {O_{2}^{-}} }$

${\displaystyle \mathrm {H} {\cdot }\ {+}\ \mathrm {O_{2}} \ {\xrightarrow[{\qquad }]{}}\ {\cdot }\mathrm {HO_{2}} }$

${\displaystyle {\cdot }\mathrm {O_{2}^{-}} \ {+}\ \mathrm {H^{+}} \ \rightleftharpoons \ {\cdot }\mathrm {HO_{2}} }$

Oxidation of iron (II) to iron (III) ions

The iron (II) ions contained in the solution can be oxidized to iron (III) ions by hydroxyl radicals (∙ HO), hydroperoxy radicals (∙ HO ₂ ) or hydrogen peroxide (H ₂ O ₂ ):

${\displaystyle {\cdot }\mathrm {HO_{2}} \ {+}\ \mathrm {Fe^{2+}} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {Fe^{3+}} \ {+}\ \mathrm {HO_{2}^{-}} }$

${\displaystyle \mathrm {HO_{2}^{-}} \ {+}\ \mathrm {H^{+}} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {H_{2}O_{2}} }$

${\displaystyle \mathrm {H_{2}O_{2}} \ {+}\ \mathrm {Fe^{2+}} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {Fe^{3+}} \ {+}\ \mathrm {OH^{-}} \ {+}\ {\cdot }\mathrm {OH} }$

${\displaystyle {\cdot }\mathrm {OH} \ {+}\ \mathrm {Fe^{2+}} \ {\xrightarrow[{\qquad }]{}}\ \mathrm {Fe^{3+}} \ {+}\ \mathrm {OH^{-}} }$

The radiation-chemical yield G of iron (III) ions results from the equation

${\displaystyle G(\mathrm {Fe^{3+}} )=2\;G(\mathrm {H_{2}O_{2}} )+3\left[G(\mathrm {e_{aq}^{-}} )+G({\cdot }\mathrm {H} )+G({\cdot }\mathrm {HO_{2}} )\right]+G({\cdot }\mathrm {OH} )}$.

The value of the radiation chemical yield depends on the LET value of the radiation. For γ radiation it is about G (Fe ³⁺ ) = 0.155 (5) / eV ; which is about G (Fe ³⁺ ) = 1.6 mol / J . Thus the amount of substance n of iron (III) ions produced in a solution with mass m is proportional to the absorbed dose D :

${\displaystyle n(\mathrm {Fe^{3+}} )=m\cdot D\cdot G(\mathrm {Fe^{3+}} )}$

The same applies to the molar concentration c of the iron (III) ions generated in the Fricke dosimeter

${\displaystyle c(\mathrm {Fe^{3+}} )=\rho \cdot D\cdot G(\mathrm {Fe^{3+}} )}$.

Where ρ is the density of the solution. Due to the added sulfuric acid, it is approximately ρ  = 1.024 g / cm ³ .

The Fricke dosimeter can be used for dose rates of up to 2 · 10 ⁶  Gy / s and a dose in the range from 1 Gy to 500 Gy.

evaluation

The Fricke dosimeter is evaluated using a spectrophotometer . The generated molar concentration c of the iron (III) ions is determined by measuring the extinction E _λ at a wavelength λ of 304 nm. The difference to the extinction of a non-irradiated comparison solution results in Δ E _λ , whereby according to the Lambert-Beer law the following applies:

${\displaystyle {\operatorname {\Delta } }E_{\lambda }=\varepsilon \cdot d\cdot c(\mathrm {Fe^{3+}} )}$

It is d , the layer thickness of the used cuvette , z. B. d  = 10 mm. The molar extinction coefficient ε is 217.4 m ² / mol.

This gives D for the dose sought

${\displaystyle D={\frac {{\operatorname {\Delta } }E_{\lambda }}{\varepsilon \cdot d\cdot \rho \cdot G(\mathrm {Fe^{3+}} )}}}$.

Individual evidence

1. ↑ ^{a b} Lieselott Herforth , Hartwig Koch: Practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 98 .
2. ↑ ^{a b c d e f g} Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 184 .
3. ^ Hugo Fricke, Star Morse: The Chemical Action of Roentgen Rays on Dilute Ferrosulphate Solutions as a Measure of Dose . In: American Journal of Roentgenology and Radium Therapy . tape 18 , no. 5 , 1927, pp. 430-432 .
4. ↑ Lieselott Herforth , Hartwig Koch: practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 99-100 .
5. ^ ^{A b} Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 180 .
6. ^ ^{A b} Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 175-179 .
7. ^ ^{A b c} Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 366 .
8. ^ Gregory R. Choppin, Jan-Olov Liljenzin, Jan Rydberg: Radiochemistry and Nuclear Chemistry . 3. Edition. Butterworth-Heinemann, 2001, ISBN 978-0-7506-7463-8 , pp. 179-180 .
9. ^ Karl Heinrich Lieser: Introduction to nuclear chemistry . 3. Edition. VCH, Weinheim 1991, ISBN 3-527-28329-3 , pp. 367 .
10. ↑ ^{a b} Lieselott Herforth , Hartwig Koch: Practical course in radioactivity and radiochemistry . 3. Edition. Johann Ambrosius Barth, 1992, ISBN 3-335-00347-0 , p. 100 .