ENVIRO Station Iodine
One question, answered well: how much iodine-131 is in the air. Prefilter and TEDA cartridge in the changer, counted separately, radon subtracted using a constant of the decay scheme.
One measurement, done properly
This station is deliberately the simpler of the two. It measures neither an aerosol spectrum nor alpha or beta activity — it answers one question, and answers it thoroughly.
- Cabin
- ISO 10′ · approx. 2.3 t
- Flow
- 6.0 m³/h nominal
- Cartridge
- 2 months
- Supply on board
- one year
Three forms, one element
They do not behave remotely alike — and a method that captures only one of them does not measure iodine, but a part of it.
Physically bound
Adsorbs onto the charcoal. Retention improves the colder the bed is.
Chemically converted
Reacts with the TEDA impregnation. Retention improves the warmer it gets.
Bound to aerosol
Is caught on the prefilter and never reaches the bed. Counted on its own.
This is why heating is used, never cooling
Cooling helps elemental iodine, but harms methyl iodide and raises the humidity at the bed. We do the opposite: the bed runs at 50 to 100 °C, where retention for methyl iodide is best, and the sample line stays above the dew point.
| Feature | How it is solved | Why |
|---|---|---|
| Geometry | 50 to 60 mm diameter, class CP-100 | fits every 2″ and 3″ detector face |
| Retention | heated bed, 50 to 100 °C | practice to ISO 18417, better for CH₃I |
| Counting | prefilter and cartridge separately | the two forms stay separate |
An earlier design used an oversized cartridge 100 mm in diameter to reach the required residence time directly. We discarded it: for that size there is neither a detector nor a laboratory geometry. Compatibility beats the more elegant figure.
Twelve and a half kiloelectronvolts
Iodine-131 emits at 364.489 keV, lead-214 at 351.932 keV. The gap is 12.56 keV — and lead-214 is the strongest line in the whole natural background. On a sodium iodide detector both arrive as one peak, and that peak consists predominantly of radon.
Fig. 1 — Separation of the two lines at three resolutions, schematic. Each curve referred to its own maximum.
| Detector | FWHM at 364 keV | Gap divided by FWHM |
|---|---|---|
| NaI(Tl) | approx. 34 keV | 0.37 — one merged peak |
| CeBr₃ · our choice | approx. 16 keV | 0.79 — stable deconvolution |
| HPGe | approx. 1.05 keV | approx. 12 — fully separated |
Why not germanium straight away: because a station that is to run unattended in a cabin for three months is better served by a scintillator that never has to be cooled. Germanium is available as an option in the Station Aerosol.
How we subtract the radon
| Approach | How it works | Robustness |
|---|---|---|
| Trapezoid method | Two empirical side windows | weak at the edge of the evaluation window |
| Reference to bismuth-214 | A different nuclide as the reference | the equilibrium drifts |
| Lead-214 at 295.2 keV | The same nuclide, ratio 1.933 | invariable over time |
We take the third approach. Both lines come from lead-214, so their intensity ratio is fixed by the decay scheme. It does not drift with the weather, not with the radon concentration and not with the age of the air. How much of the 364 keV window was subtracted is stated next to every iodine value.
Six stages, two of them are counted
Intake head
Omnidirectional, with louvres, heated against icing.
Sample line
Heated above the dew point, condensate separator at the lowest point.
Prefilter — counted
Glass-fibre disc. Holds the particle-bound iodine and the radon progeny.
Cartridge — counted
TEDA charcoal, 50 to 60 mm, heated bed. Service life two months.
Blower
Controlled, holds 6.0 m³/h over the whole service life.
Gas meter
Volume referred to 0 °C and 1013 hPa.
Two geometries, two answers
The prefilter is counted in its own geometry, the cartridge in its own. Particle-bound and gaseous iodine are never thrown together.
Two months cost no sensitivity
The cartridge stays in the stream for two months; the reported windows range from one hour to one week. With a half-life of 8.02 days the collected activity saturates — old iodine decays away.
What the station can still detect
To ISO 11929 at 95 %, at 6.0 m³/h, with CeBr₃.
| Measuring point | 1 hour | 24 hours | One 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 |
The market standard is below 1 Bq/m³ in one hour; this limit is met with margin. Published values for comparison: Thermo FHT 1700 at 0.5 Bq/m³, MICASENSOR mIodine-101 below 1 Bq/m³ in one hour, GIHMM AMS02 at 44 mBq/m³ in 24 hours. A laboratory measurement with germanium over one week reaches approx. 3 mBq/m³.
The cycle buys time, integration buys sensitivity
| Cycle | I-131, one window | What it is good for |
|---|---|---|
| 2 minutes | 4 to 24 Bq/m³ | State of changer and flow, early warning |
| 5 minutes | 2.8 to 10 Bq/m³ | Plume tracking during an event |
| 10 minutes | 2.0 to 4.8 Bq/m³ | the default value for reporting |
| 1 hour | 0.80 Bq/m³ · design value | the point of comparison in the market |
| 24 hours | 40 mBq/m³ · design value | Monitoring limit |
Below ten minutes the radon reference becomes restless — short windows show the breathing of the radon, not the iodine. Iodine is therefore stated on windows of ten minutes and longer.
The emission probability p is 0.812 and is the easy part. The difficult one is B: in an iodine measurement the background is not the counting statistics of an empty detector, but what remains after the radon subtraction. That is why the compensation approach is set out at such length further up.
The same cabin as the aerosol station
One housing, one cabinet, one spare parts list. Anyone operating both stations keeps only one set of wear parts and trains their staff once.
- Cabin
- ISO 10′ · 2991 × 2438 × 2591 mm
- Mass
- approx. 2.3 t
- Controlled inside
- 5 to 35 °C
- Connected load
- approx. 0.6 kVA
Fig. 2 — Cabin with the door open. The same design as with the aerosol station.
Who gets in, and who hears about it
| Area | How it is solved | What is reported |
|---|---|---|
| Door | Steel, three-point locking, mechanical key as fallback | state in the header of every screen |
| Access | RFID card and PIN at the door, role-based | every entry in the audit log |
| Door contact | Monitored reed contact with tamper line | DOOR OPEN in the message stream |
| Tampering | Tamper contacts on the cabinet front and the roof hatch | alarm, remotely within one minute |
| Climate control | An open door suspends the air conditioning | note if open longer than set |
A station that is visited once a year is only as good as its door. The components are at design stage and not yet selected.
The same interface as in every ENVIRO instrument
On the screen: spectrum, fitted peak, radon share and limit. This station adds the changer module and shows the radon share next to every iodine value.
Fig. 3 to 5 — Screenshots from the software, taken on the ENVIRO GO, which runs the same software. Operating language English, values are examples.
One visit a year covers everything
| Task | Interval | Duration |
|---|---|---|
| Change cartridge and prefilter, by the changer | automatic, every 2 months | none |
| Refill the changer, take used cartridges away | annually | 15 minutes |
| Check source and energy calibration | automatic, daily | none |
| Flow calibration against a standard | annually | 1 hour |
| Efficiency calibration in cartridge geometry | annually | 2 hours |
The used cartridge goes to the laboratory as it is. Nothing has to be decanted, weighed or repacked, and the counting geometry in the laboratory is the same one the station itself used.
The data sheet on one page
controlled 3 to 12 m³/h
particle-bound iodine
market geometry, class CP-100
practice to ISO 18417
approx. 3.2 % at 662 keV
Lining of copper and tin
fixed ratio 1.933
supply for one year on board
approx. 2.3 t
approx. 0.6 kVA
control areas from 20 mm
N42.42, EURDEP, IRIX, Modbus TCP
Detector figures come from the data sheets of the component manufacturers. All other figures are design values. The data leave the cabin over an Ethernet cable; a radio link exists only if you ask for one.
What comes with it and what can be added
Included
- Cabin with cabinet, fully wired
- Intake head, mast and sample line
- Air conditioning and heating
- Changer for cartridge and prefilter
- Cartridge supply for one year on board
- Factory calibration certificate
- Efficiency calibration in the measuring geometry
- Interface description and test client
On request
- NaI(Tl) instead of CeBr₃, less expensive
- Silver zeolite instead of TEDA charcoal
- Second cartridge stage for species separation
- Weather station on the roof
- External uninterruptible power supply, recommended
- Mobile network router
- Stack probe to ISO 2889
- Spare cartridges and prefilters
The cabin exists; the iodine measuring chain inside it has not yet passed a type test. Every performance figure on this page is a design value or an experience value; detector figures come from the manufacturer's data sheet.
On the standards we write “design to follow” — IEC 60761-1 and -4, IEC 61171, ISO 18417 and ISO 11929. “Tested to” appears only after the type test. Iodine monitors fall under IEC 60761-4; part 2 covers aerosols — a distinction that several published data sheets get wrong. CE marking to EN 61326 and EN 61010 through an external test house.
Is iodine your only question?
If you need both, we will clarify in discussion whether two stations or an aerosol station extended by an iodine stage is the better route — which detection limit a combined solution reaches has not yet been finally determined. This station is for sites where iodine is the only question and the price is to reflect that.
