Gauges Containing 241Am-Be Sources

Safety Data

Various OSU researchers have gauges manufactured by Troxler or CPN which use 241Am-Be neutron sources in the measurement of soil moisture content. The source consists of a compressed mixture of americium powder and beryllium powder, doubly encapsulated in welded stainless steel, with a neutron detector and associated electronics in a combination shield and housing. Data processing electronics may or may not be included. The device is hand portable and battery powered; thus it may be used in fields, forests, and other such difficult-access locations. Customary uses include determination of moisture content in soil to determine optimum irrigation; determination of soil density and compaction occurring by various treatments, etc.

Soil moisture gauges present only modest hazard so long as the radionuclide remains outside the body. 241Am decays via alpha particle emission with half life of about 432 years and average alpha energy of about 4 1/2 MeV. Also emitted are 60 keV photons (about 36% of decays) and 14 KeV photons (about 42%). Half-value layer for 60 keV photons is about 1 cm of A1, and <1 mm for Pb, Fe, Sn, etc. Theoretical dose rate is about 13 mrem/hr at a meter per curie; however self-absorption and capsule wall absorption result in measured dose rates of about 3 mrem/hr per curie at a meter. Thus, shielding is simple for the photons. Neutron emission from 1 Ci of 241Am-Be is about 2.7 x 106 n-s-1. For this neutron energy (about 5 1/2 MeV average) the rad/n-cm-2 is about 3.8 x 10-8 and Quality Factor is 7 (measured) or 10 (arbitrary). Thus, neutron dose rate is about 800-1200 mrem/hr at 5 cm per curie, or 2-3 mrem/hr at 1 meter per curie.

In contrast, the internal hazard is high. Annual Limit of Intake (ALI), per state regulations, is 0.006 mCi via inhalation (about 1/10,000,000 of the activity in one sealed source) and 0.8 mCi via ingestion. Maximum intake for soluble Be is also very small due to its extreme chemical toxicity. Thus, should source integrity be lost, the loose americium and beryllium pose a very great contamination hazard, and must be handled very carefully if internal contamination is to be avoided.

Cesium-137 decays via beta emission with a half-life of about thirty years. Emissions include 0.662 MeV gamma (95%) from the daughter barium nucleus plus beta spectra with Emax of 0.51 MeV (about 95%) and 1.81 MeV (about 5%). Note that the beta radiation will not penetrate the capsule wall. Gamma radiation level per Radiation Health Handbook is 3.3 rem/hr per mCi at 1 cm. Annual Limit of Intake (ALI), per state regulations, is 200 mCi via inhalation and 100 mCi via ingestion. Measured radiation levels at contact with a CPN device containing a 10 mCi source is 2-5 mrem/hr at contact, less than 0.5 mrem/hr at 3 feet. Hence, radiation levels from an intact device are modest. However, should the source become broken or removed it would constitute an appreciable safety hazard.

Adequate training and experience requires certification of completion of a safety and use training course presented by the manufacturer or similar authority. Use must be by such certified personnel, or under direct supervision of such personnel.

Compliance with safety provisions and Use Authorization provisions will ensure adequate procedures for use, storage, transport, dose control, contamination control, and emergency handling. Both Troxler and CPN gauges are specifically authorized by State license ORE-90005.