Skip to main content
Skip table of contents

Safety note on working with the rhoC5

Working principle of the rhoC5

The rhoC5 density meter measures the density of the soil by using a radioactive 22Na source. This isotope emits gamma radiation into the soil, and the intensity and spectral shape of the backscattered gamma-rays are used to derive the density of the soil. The relation between the backscattered gamma-rays and the soil density is determined by using Monte-Carlo simulations, soil composition models and calibration measurements.

The radioactive source in the rhoC5 has an activity below the exemption value defined by the European Atomic Energy Community1. It is considered safe to work with when using the recommended working procedures of operating the rhoC5.

Location of source in device

The radioactive source is located near the bottom of the titanium tube, just above the moisture sensor in the black nylon housing. This source is embedded in a circular disk, which is secured in a casing that is secured in the tube.


Source specifications



Half-life (T1/2)

2.6 years


Gamma – 1275 keV
Annihilation photons - 511 keV (180% abundance)
Beta – 546 keV (90% abundance)

Gamma Constant

1.34 mR/hr per mCi @ 1.0 meter
[3.62E-4 mSv/hr per MBq @ 1.0 meter]2

Specific activity at production

1.0 MBq (28.36 µCi)

General Precautions

The source present in the rhoC5 density meter is below the exemption value. However, the following precautions are advised:

  1. Maintain your occupational exposure to radiation As Low As Reasonably Achievable (ALARA). This can be achieved by maintaining a normal working distance between the source and the operator.

  2. Ensure all persons handling the rhoC5 are aware of the sensor's working principles and understand that there is a radioactive source located within the device.

  3. In the unlikely event that the sensor comes apart and the circular disk containing the radioactive source is removed from the sensor's housing, it is crucial that the source is not lost. The disk in which the source is embedded is safe to handle from the sides and should be placed in a container, preferably a heavy container made of material that shields gamma rays from the environment, such as metal. DO NOT PLACE THE RADIOACTIVE SOURCE CLOSE TO THE BODY, for example, in a pocket. Contact Medusa Radiometrics immediately.

  4. If there are any questions regarding radiation safety or the use of radioactive materials, please get in touch with Medusa Radiometrics.

Shelf life and Disposal

The 22Na source has a half-life of 2.6 years, meaning that about five years, almost two half-life periods have passed, and a quarter of the original activity is left. Since the measurement relies on scattered gamma-rays, the measurement time for acquiring a density reading increases when the source activity decreases. After five years, the 22Na source has to be replaced for the device to continue giving reliable readings.

After these five years, the rhoC5 sensor has to be shipped to the office of Medusa Radiometrics. Here the 22Na source is removed from the device and the 22Na source(s) will be disposed off in accordance with the legal regulations. If desired, the source can be replaced, and the sensor can be used for another five years.

1 Council Directive 2013/59/Euratom
2 Health Physics & Radiological Health Handbook, 3rd Ed. [Baltimore, MD; Williams & Wilkins, 1998], p. 6-9

JavaScript errors detected

Please note, these errors can depend on your browser setup.

If this problem persists, please contact our support.