Answer to Question #11321 Submitted to "Ask the Experts"

I am studying biology and have some concerns regarding the use of a cesium-137/barium-137m (¹³⁷Cs/^137mBa) generator. In our studies we also have a course where we have to measure the gamma decay of the ^137mBa. There are some things which are not quite clear to me. It is said that working with a radionuclide generator is quite safe because the cesium is not washed out with the weak acid solution but is that completely true? What about the ^137mBa itself? Despite its short half-life, it is still emitting gamma rays, which are said to be the most penetrating type of radiation. In the experiments, we are relatively close to the barium solution, so are there any concerns or are the concentrations so low that there is no need to worry? Would those gamma rays be strong enough to penetrate through bone into the bone marrow, or through the skull into the brain?

I am not quite sure if these questions are appropriate, but please let me thank you in anticipation for your work in general. I hope the questions are not too trivial; it is just that I am not used to working with radioactivity and other people's panic about radioactivity makes me feel unsure about it.

Your questions are legitimate and you certainly are justified in asking them. I commend you for having the interest and taking the time to get your questions answered.

The bottom line is that the amount of radioactivity being used in the experimental work of concern is not sufficient to produce any detectable negative impact on individuals using the material. Even though the amount of radioactivity in this situation is not sufficient to produce any detectable negative impact on individuals using the material, for radiation protection purposes we make the assumption that any radiation exposure is potentially harmful (despite considerable evidence to the contrary). Therefore, it is appropriate to handle all radioactive materials in a fashion so as to minimize unnecessary external and internal exposure to such material. Abidance by this philosophy will stand one in good stead if future involvements lead to the use of larger amounts of radioactive materials.

The ¹³⁷Cs/^137mBa generators you describe have been in use as educational tools for many years. We should note up front that, so far as I am aware, all of the generator units that are used for educational purposes, as you describe, incorporate a small quantity of ¹³⁷Cs, typically no more than 3.7 × 10⁵ becquerels (Bq). This quantity of ¹³⁷Cs is often referred to as an exempt quantity, meaning that the person or facility that possesses such an amount need not hold a license from the pertinent regulating agency (in the United States such agencies are usually either the U.S. Nuclear Regulatory Commission or the radiation control program for the state in which the licensee resides). The implication of this exemption is that the quantity of activity is believed to be sufficiently small that it poses no significant health risk to users.

Regarding your specific concerns:

1. Small amounts of the ¹³⁷Cs may be washed out along with the ^137mBa, but you should not be concerned with potential exposure associated with usual use of the material. Typical estimates of cesium removal for these devices are on the order of 50 Bq of ¹³⁷Cs eluted per milliliter (ml) of effluent when the total ¹³⁷Cs content of the device is about 3.7 × 10⁵ Bq. For your reference, the allowed maximum annual ingestion intake of ¹³⁷Cs for occupationally exposed individuals is 3.7 × 10⁶ Bq.

2. It is true that ^137mBa emits the 662 kiloelectronvolt (keV) gamma rays commonly observed, but keep in mind that the amount of the radionuclide present is small—at most 95% of the ¹³⁷Cs activity, based on the decay scheme. This amount of ^137mBa taken into the body would have a negligible dose impact. With a 2.55-minute (min) physical half-life it would disappear within a short time, with more than 98% having decayed within 15 min. The external dose rate from the ¹³⁷Cs/^137mBa combined source, assuming 3.7 × 10⁶ Bq of cesium present and an equilibrium quantity of 3.5 × 10⁶ Bq of barium at a distance of 0.3 meters (m) from the source, would be about 3.4 × 10^-4  millisieverts hour^-1 (mSv h^-1), approximately three times a typical normal external radiation background dose rate. If you worked with the material at the 0.3-m distance for a nominal laboratory time of 3 h, the integral dose near the surface of your body would be about 1.0 × 10^-3 mSv, which is about 0.1% of the typical annual external radiation background dose from natural sources that an individual would receive in much of the United States. If we considered only the ^137mBa, once separated from the cesium, the dose would be much lower because the unsupported barium would decay away very quickly.

3. The gamma radiation from the ^137mBa is sufficiently penetrating that some which is incident on the body could reach any or all internal organs. However, this is also the case with the mix of gamma rays emitted from naturally occurring radioactive elements in the Earth's crust that impact us all on a daily basis. The accrued dose from the latter to any tissue in the body over time would greatly exceed the very small dose that might accrue from the limited use of the prepared sources in the laboratory.

I hope this is helpful. Best wishes in your continuing studies.

George Chabot, PhD