Walk-in measurement cabin for iodine-131 in the air. The prefilter and the TEDA cartridge are in the changer and the station counts them separately. The station subtracts radon with a constant of the decay scheme.
The Station Iodine measures only iodine-131. The ENVIRO Station Aerosol measures an aerosol spectrum and the alpha and beta activity.
Iodine occurs in 3 forms with different behaviour. A method for only one form measures only a part of the iodine.
| Form | Symbol | Behaviour |
|---|---|---|
| Elemental iodine | I₂ | Adsorbs physically on the charcoal. The retention increases when the bed temperature decreases. |
| Methyl iodide | CH₃I | Reacts chemically with the TEDA impregnation. The retention increases with the temperature. |
| Bound to aerosol | Particles | The prefilter catches this part. This part does not get to the bed. The station counts it separately. |
Cooling helps the elemental iodine. But cooling decreases the retention of methyl iodide and increases the humidity at the bed. Thus the bed operates at 50 to 100 °C, where the retention of methyl iodide is highest. The sample line stays above the dew point.
| Feature | Solution | Reason |
|---|---|---|
| Geometry | Diameter 50 to 60 mm, class CP-100 | fits all 2″ and 3″ detector faces |
| Retention | heated bed, 50 to 100 °C | practice in accordance with ISO 18417, higher retention for CH₃I |
| Counts | prefilter and cartridge separately | the 2 forms stay separate |
A cartridge with a diameter of 100 mm gets the necessary residence time directly. For this size, there is no detector and no laboratory geometry. Thus the station uses the market geometry of 50 to 60 mm.
The lead-214 line at 351.932 keV is 12.56 keV from the iodine line at 364.489 keV. A sodium iodide detector shows the 2 lines as one peak. Most of this peak comes from radon.
The station operates for 1 year in a measurement cabin with no visit. For this operation, a scintillator with no cooling is a better selection than germanium. Germanium is an option in the ENVIRO Station Aerosol.
Each iodine value shows the subtracted contribution in the window at 364 keV. The Environmental Solutions overview describes the physics, the resolution of the detectors and the compensation methods.
The station counts 2 of the 6 stages: the prefilter and the cartridge.
| Stage | Assembly | Design |
|---|---|---|
| 01 | Intake head | Omnidirectional, with louvres, heated against icing. |
| 02 | Sample line | Heated above the dew point, condensate separator at the lowest point. |
| 03 | Prefilter, counted | Glass-fibre disc. Holds the particle-bound iodine and the radon progeny. |
| 04 | Cartridge, counted | TEDA charcoal, 50 to 60 mm, heated bed. Service life 2 months. |
| 05 | Blower | Controlled, holds 6.0 m³/h during the full service life. |
| 06 | Gas meter | Volume referred to 0 °C and 1 013 hPa. |
The station counts the prefilter in its own geometry and the cartridge in its own geometry. Particle-bound iodine and gaseous iodine always stay separate.
The cartridge stays in the air flow for 2 months. The reported windows are from 1 hour to 1 week. The half-life is 8.02 days. Thus the collected activity saturates, and older iodine decays.
In accordance with ISO 11929 at 95 %, at 6.0 m³/h, with a CeBr₃ detector.
| Measurement point | 1 hour | 24 hours | 1 week |
|---|---|---|---|
| I-131 on the cartridge | 0.80 Bq/m³ | 40 mBq/m³ | 5.0 mBq/m³ |
| I-131 on the prefilter | 0.90 Bq/m³ | 45 mBq/m³ | 5.6 mBq/m³ |
Published values of comparable measuring instruments are 0.5 to less than 1 Bq/m³ in 1 hour and 44 mBq/m³ in 24 hours. The 1-hour limit of the station is 0.80 Bq/m³ and is in this range. A laboratory measurement with germanium for 1 week gets to approximately 3 mBq/m³.
A short cycle decreases the response time. A long integration time decreases the detection limit.
| Cycle | I-131, one window | Use |
|---|---|---|
| 2 minutes | 4 to 24 Bq/m³ · not reported | Status of changer and flow rate, early warning |
| 5 minutes | 2.8 to 10 Bq/m³ · not reported | Plume tracking during an event |
| 10 minutes | 2.0 to 4.8 Bq/m³ | Default value for the report |
| 1 hour | 0.80 Bq/m³ · design value | Point of comparison in the market |
| 24 hours | 40 mBq/m³ · design value | Monitoring limit |
Below 10 minutes, the radon reference is not stable. Short windows show the radon fluctuation and not the iodine. Thus the station gives iodine values only for windows of 10 minutes or longer.
The ranges for 2, 5 and 10 minutes come from the 1-hour value. The upper limit follows from 1/t, the lower limit from 1/√t. These ranges are an approximation for the counting window with a fully loaded cartridge. The relation with 1/t² and 1/t1.5 in the overview applies when the collection time is equal to the counting time. The Environmental Solutions overview gives the calculation of the detection limit with the emission probability of iodine-131.
One housing, one cabinet, one spare parts list. Operators of the 2 stations keep one set of wear parts and train the personnel one time.
The station monitors the door, the access and tamper attempts. It reports each event.
| Area | Solution | Report |
|---|---|---|
| Door | Steel, three-point lock, mechanical key as fallback | Status in the header of each screen view |
| Access | RFID card and PIN at the door, based on roles | each access in the audit log |
| Door contact | Monitored reed contact with tamper line | DOOR OPEN in the message stream |
| Tamper attempt | Tamper contacts on the cabinet front and the roof hatch | Alarm, remote, in less than 1 minute |
| Climate control | An open door stops the air conditioning | Note when the door stays open for longer than the set time |
The components are at the design stage. The selection of the components is not complete.
The same user interface as in the ENVIRO GO and in the Station Aerosol. The Station Iodine adds the changer module and shows the radon contribution next to each iodine value.
Spectrum view of the shared software, version with HPGe.
Each event with a time stamp.
Calendar grid, trends and reports.
Screenshots from the shared operating software. Operating language English, example values, no measurements.
One visit each year includes all tasks.
| Task | Interval | Duration |
|---|---|---|
| Replace the cartridge and the prefilter with the changer | automatic, each 2 months | none |
| Fill the changer, remove the used cartridges | each year | 15 minutes |
| Check source and energy calibration | automatic, each day | none |
| Flow rate calibration against a standard | each year | 1 hour |
| Efficiency calibration in cartridge geometry | each year | 2 hours |
The used cartridge goes to the laboratory with no change. It is not necessary to transfer, weigh or repack the sorbent. The laboratory counts in the same geometry as the station.
Standard detector CeBr₃. NaI(Tl) is an option.
| Parameter | Value |
|---|---|
| Flow rate | 6.0 m³/h nominal, controlled 3 to 12 m³/h |
| Prefilter | Glass-fibre disc for particle-bound iodine |
| Sorbent | TEDA charcoal 5 %, 50 to 60 mm, market geometry, class CP-100 |
| Retention | heated bed, 50 to 100 °C, practice in accordance with ISO 18417 |
| Detector | CeBr₃ 2″ × 2″, approximately 3.2 % at 662 keV |
| Shielding | 50 mm lead, on all sides, lining of copper and tin |
| Radon subtraction | Pb-214 at 295.2 keV, fixed ratio 1.933 |
| Cartridge changer | automatic, each 2 months, supply for 1 year on board |
| Measurement cabin | ISO container 10′, approximately 2.3 t |
| Power supply | 230 V, 50 Hz, approximately 0.6 kVA |
| Screen | 1920 × 1080, touch-sensitive, control areas from 20 mm |
| Interface | Ethernet, router as an option, N42.42, EURDEP, IRIX, Modbus TCP |
The detector values come from the data sheets of the component manufacturers. All other data are design values. The data go out of the measurement cabin through an Ethernet cable. A radio link is available only on request.
The measurement cabin comes fully wired, with a factory calibration certificate and a cartridge supply for 1 year.
The measurement cabin is built. The iodine measurement chain in the measurement cabin has no type test at this time.
Each performance value on this page is a design value or a value from experience. The detector values come from the data sheet of the manufacturer.
The design follows IEC 60761-1 and -4, IEC 61171, ISO 18417 and ISO 11929. We write “tested in accordance with” only after a satisfactory type test. Iodine monitors are in the scope of IEC 60761-4. Part 2 is about aerosols. For the CE marking in accordance with EN 61326 and EN 61010, an external test house does the tests.
The Station Iodine is for locations where only iodine is necessary. For aerosol and iodine together, we discuss with you which solution is applicable: 2 stations or one Station Aerosol with an iodine stage.
The detection limit of a combined solution is not yet specified.
