Cs-137 (Caesium-137) Radiation Safety Data
Published: Aug 21, 2023
Source: Ionactive Radiation Protection Resource
Cs-137 (Caesium-137)
Half life: 30 Years
Specific activity: 3.2×1012 Bq/g
Decay product: Ba-137 (non-radioactive) [See note on Ba-137m]
For most practicable radiation protection purposes it can be assumed that Cs-137 decays to Ba-137 (stable) and emits beta / gamma rays during the decay. In reality a small % of Cs-137 decays by direct beta emission to Ba-137, whereas most decays are by beta emission to Ba-137m. Ba-137m has a half life of 153 seconds and this then decays to stable Ba-137 by emission of a gamma ray. The classic 662 keV gamma emission, often used as a radiation instrument calibration energy peak, arises from Ba-137m and not Cs-137. However, Cs-137 is mostly quoted as an important beta / gamma emitter.
Significant emissions (keV) [Cs-137 (Caesium-137)]
In the data below, % refers to the probability of emission of a particular type of radiation at a specified energy
Gamma / x-ray: 662 keV (85 %), 36 keV (1 %), 32 keV (6 %) [662 keV direct from Ba-137m]
Beta (Emax): 1173 keV (5 %), 512 keV (95 %)
Electrons: 656 kev (1 %), 624 keV (8 %)
Alpha particles: n/a
Fairly high energy gamma emitter - the 662 keV energy peak is an important standard for radiation instrument calibration.
External exposure (in air) [Cs-137 (Caesium-137)]
The values below are specified as mSv/h for either 1 MBq of activity or an area source of 1 MBq/m2depending on the geometry. (HP 0.07) represents a skin dose rate and (HP 10) represents an 'at depth' tissue dose rate (> 10mm).
Point source (at 30cm)
Point source : 0.213 mSv/h (beta dose rate)
Point source: 0.00093 mSv/h (gamma dose rate), 0.93 micro Sv/h
The gamma dose rate data above can be derived directly from some recent Ionactive resource: Gamma dose rate (D=AE/6r2) - or is it? A widget to calculate the validity of this formula. Just calculate at 1m (for 1 MBq Cs-137) and then correct for 30cm by multiplying by 11.1111 (inverse square law modification).
Infinite place source (at 10 cm)
Beta (HP 0.07) : 0.09 mSv/h
Photon (HP 0.07) : 0.0039 mSv/h
Photons (HP 10) : 0.0037 mSv/h
Cs-137 is a fairly high energy gamma emitter [see Ba-137m comment]. In industry Cs-137 is often used in the GBq or TBq range. Looking at the Point Source gamma data given above, dose rates at 30 cm would then be 0.93 mSv/h (1 GBq) and 0.93 Sv/h (1 TBq). Lower activity in the kBq or low MBq range may be used for small radiation instrument functional check sources. All these values are for an unshielded point source.
External exposure (arising from personal contamination) [Cs-137 (Caesium-137)]
The values below are either for uniform contamination on the skin (for 1 kBq/cm2) or as a single droplet (1 kBq) and are specified in mSv/h. It is assumed that no PPE is being worn which would attenuate the radiation.
Uniform deposit on the skin: 1.57 mSv/h (beta / gamma dose rate)
0.05ml droplet on the skin: 0.708 mSv/h - (beta / gamma dose rate)
Shielding (external radiation) [Cs-137 (Caesium-137)]
In the data below beta shielding is specified as mm of material to provide 100% absorption of the beta particle (or electron). Gamma (and x-ray) shielding is specified in terms of mm of material relating to 1 TVT or 1 HVT. The TVT is the thickness of material (in mm) which will reduce the radiation intensity (dose rate) down to 1/10 of the pre-shielded dose rate. The HVT is the thickness of material (in mm) which will reduce the radiation intensity down to 1/2 of the pre-shielded dose rate
Beta radiation (for 100% absorption)
Glass: 2.1 mm
Plastic: 3.8 mm
Typical PPE (glove): negligible absorption (don't rely on gloves to shield for external radiation, they are a contamination control method).
Gamma / x-ray radiation (TVT and HVT)
Lead: 6.6mm (HVT) and 22mm (TVT)
Steel: 16 mm (HVT) and 53mm (TVT)
Note there is some variety in values across the internet - please use these values for planning and education. If you are planning to build a shield for CS-137 then please contact the Ionactive Radiation Protection Adviser (RPA).
Internal exposure [Cs-137 (Caesium-137)]
The data featured below is derived for employees who work with ionising radiation (and are therefore subject to dose limits specified by the Ionising Radiations Regulations 2017 in the UK, and similar regulation around the world). Whilst not directly applicable to public exposure (e.g. exposure resulting from environmental releases - either planned or accidental), the data will provide a good indication of likely exposures and is therefore adequate for general research, illustration and asking 'what if?' type questions. If you need professional advice, please consider consulting a Radiation Protection Adviser (RPA).
Internal radiation exposure generally means the intake of a radioactive substances by inhalation, ingestion or through cuts (or absorption) in the skin. For this resource assume cuts / absorption are similar to the ingestion values.
This data provides the dose delivered (committed effective dose equivalent) for an intake of [Cs-137 (Caesium-137)] by inhalation or ingestion to yield 1mSv effective dose. The dose value provided assumes that all the dose is delivered in the first year of intake, in reality this will vary greatly with radioisotope due to a combination of physical half-life, biological half-life and the biochemical behaviour of the particular radioactive substances in the body.
Target organ: Soft Tissue
Inhalation dose (1 mSv): 0.15MBq
Ingestion dose (1 mSv): 0.075 MBq
Workplace Monitoring and Dosimetry [Cs-137 (Caesium-137)]
General comment
Workplace monitoring means using techniques to detect ionising radiation sources in the working environment (e.g. on benchtops, floors, walls, drains and similar). The monitoring can be direct (e.g. using a radiation detector to gain real time measurements of dose rate and / or activity) or indirect (where a smear / wipe of an area is made and this is then offered up to a radiation monitoring probe or placed in scintillation fluid for liquid scintillation counting).
Dosimetry is used to measure personal exposure from a source of ionising radiation. Passive dosimetry (e.g. film badge, thermoluminescent dosimeter - TLD etc) is worn on the body (e.g. trunk / extremities / near eyes) and measures total integrated dose over time. Active dosimetry (e.g. electronic personal dosimeter - EPD, personal electronic dosimeter - PED etc) is worn on the trunk of the body and provides real time accumulated dose (and sometimes dose rate) - audible dose and dose rate alarms may also be available. Biological monitoring [bioassay] (e.g. urine sampling, faecal sampling etc) takes biological samples from the body where they are analysed in a sensitive detector to determine activity of radionuclides in the body (usually due to the intake of radioactive material by inhalation, ingestion or through absorption / cuts in the skin).
Workplace monitoring [Cs-137 (Caesium-137)]
- Sealed sources - dose rate monitoring using a variety of detectors (GM / ion chamber / scintillation)
- Unsealed Cs-137 - generally a GM based detector will be adequate for general contamination measurements. Wipe / smear monitoring / sample collection (followed by gamma counting) may be required for low level environmental contamination with Cs-137
Dosimetry [Cs-137 (Caesium-137)]
- Passive dosimetry (e.g TLD)
- Active dosimetry (e.g. electronic personal dosimeter - EPD)
- Internal dosimetry - whole body monitoring (using a whole body counter)
Occurrences and uses [Cs-137 (Caesium-137)]
Occurrences
- Not found in nature (artificially produced)
- Cs-137 is created in a nuclear reactor (i.e. A product of nuclear fission)
- Also produced during the detonation of a nuclear (atomic) bomb
Uses
- Calibration source - the 662 KeV gamma peak (technically from Ba-137m as noted earlier) is a gold standard for the calibration of radiation detection equipment. The activity of the source may be in the kBq / MBq range for an instrument functional test source, or in the GBq / TBq range for the calibration of high dose rate detectors.
- Process control - Cs-137 is often used in nuclear gauges (density, flow rate. level etc) for the detection / control / and feedback in manufacturing processes
- Multi Phase Flow Meters (MPFM) - Cs-137 is a popular choice of radioactive source for use in MPFM for the measurement of water / air / sand / petroleum in the oil industry
- Sterilisation - Cs-137 is often used in laboratory / research 'irradiators' (including blood irradiators)
- Flooded member detection (FMD) - Cs-137 sources are often used in combination with a ROV (remotely operated underwater vehicle) for the detection of water ingress into steel structures on oil platforms and similar