In early 2008, the reported loss of a Caseium-137 radiation source from a local hospital aroused the public's interest in radioactivity. This article shares with you more information about Caseium-137.

Caesium (chemical symbol Cs) is a soft silvery-gold metal. Naturally occurring Cs-133 is a stable isotope^[1], while other Cs isotopes are radioactive. Cs-137 is one of the by-products of nuclear fission process in nuclear bombs, nuclear weapon testing, and nuclear reactor operations. It emits gamma rays and because of its relatively long half-life of 30 years, Cs-137 causes long lasting effect to human body through intake of food or air contaminated by it or exposure to Cs deposited on the ground.

Cs-137 is a commonly-used gamma source. Radiation sources are used in the calibration of medical radiotherapy devices for treating cancer. They are also used to calibrate radiation detection instruments for monitoring the radiation received by patients and radiation personnel. Regular calibration of these devices and instruments is required to ensure their accuracy.

The Observatory uses Cs-137 for calibrating equipment used for environmental radiation monitoring. The radiation level of the Cs-137 calibration sources used by the Observatory is not high. They are either sealed or in epoxy matrix and their usage is under regulatory control. The Observatory personnel wear radiation badges to ensure the radiation they received is within safety limits.

The health effect of radiation depends on its intensity, exposure time and the kind of body cells being affected. In case the Cs-137 involved is a very strong radioactive source, it may acute radiation sickness e.g. nausea, fatigue, vomiting and loss of hair, or even death if the radiation dose arising from direct exposure to it reaches about 1 sievert^[2]. Handling of such strong sources requires professional or properly trained personnel. The Cs-137 calibration sources used by the Observatory are not strong sources. Intake of Cs-137 to the body results in the radioactive material being accumulated in the muscle tissue, and may increase the risk of cancer.

Radiation is present everywhere, a majority of which is naturally occurring. Protection from radiation can be done in three simple ways: distance, time and shielding. That is, stay as far away from a radiation source as possible, minimize the time spent with it, and use adequate shielding. In respect of shielding, lead is commonly used, e.g. lead shields and lead-lined clothing. If you visit a place involving the use of radiation, follow the recommended safety procedures and do not go into unauthorized areas.

Cs-137 is used to treat food, killing germs and microorganisms and thus lengthening the shelf-life. The process has the effect of suppressing sprouting and delaying ripening. The food after treatment by radiation contains no radioactivity or residual poisonous material. Cs-137 is also used in monitoring the weight, thickness and density of industrial products as well as spotting out cracks in welding and pipes.

Notes:
[1] Isotope - Atoms of an element having the same number of protons but different number of neutrons in the nuclei. Different isotopes have the same chemical properties, but somewhat different physical properties. Isotopes of an element can be classified as radioisotopes (those undergoing decay with the emission of characteristic radiation) or stable isotopes.
[2] Dose and Sievert - The effect of radiation on any material is determined by the "dose" of radiation that material receives. To put all ionising radiation on an equal basis with regard to the potential for causing harm, a quantity known as effective dose is introduced. The effective dose is a single number broadly representing the risk to health taking into consideration the susceptibility to harm of different tissues and weighted for the harmfulness of different types of radiation. The effective dose is frequently abbreviated to dose. The unit of dose is sievert (Sv). This is a very large unit and submultiples, in particular, the millisievert (mSv -- one thousandth of a sievert) are commonly used. The annual dose received by a member of the public in Hong Kong from natural background radiation is about 2 mSv. A radiation worker may legally receive up to 20 mSv per year. The effective dose received in a typical chest X-ray examination is about 0.05 mSv.