Module 7: Radiation Safety

7.1 Introduction

In recent years, extensive efforts have been made to reduce the risk of irradiation from radioactive sources at work by controlling the permitted levels of radioactivity. This was achieved by applying the three principles of radiation protection:

Justification

Any decision that alters the radiation exposure should do more good than harm.

Optimisation of Protection

The number of people exposed, and the magnitude of their individual doses should be kept as low as reasonably achievable (ALARA), taking into account economic and social factors.

Application of Dose limits

The total dose to any individual from regulated sources in planned exposure situations other than medical exposure of patients should not exceed the appropriate limits

Note that the optimisation principle requires actual operational dose limits for any work with radiation sources to be more restrictive than the maximum regulated dose limits.

7.2 Reduction of dose to personnel

Once radioactive materials are being considered as part of one’s research, an application for a radioisotope permit must be completed. During the process of obtaining the permit, radioisotope work procedures will be examined. Other considerations include the applicant’s training, previous work experience with radioactive materials, adequacy of workplace preparation and equipment, types of dosimeters used, protective equipment, etc.

No radioactive material can be ordered until the radioactive permit has been approved. Subsequently, only those isotopes listed on the permit can be ordered within the prescribed limits. However, new isotopes can be added and/or their limits changed, provided a written request by the permit holder is sent to the RPS for approval.

It is better to order radioactive materials as close as possible to the date of the experiment, from both experimental and ALARA perspectives. This will reduce the risks associated with long-term storage of labelled materials, as well as reducing the possibility of source leakage, external irradiation, etc.

7.3 Protection Against External Exposure

Protection against external exposure can be done by following three fundamental methods:

  1. Time
  2. Distance
  3. Shielding

7.3.1 Time

Because radiation is roughly emitted at a constant rate from its source, the radiation dose will be proportional to the amount of time spent in proximity to the source. Therefore, one should try to reduce the time spent working with radioactive materials as much as possible.

A good work practice is to run a mock experiment first, without any radioactive material, to get used to the procedures. Then, perform the first experiment with the smallest amount of radioactive material that will provide a readable result. After becoming familiar with the procedures and safe handling of these materials, the quantities can be increased if necessary.

Store the bulk of the radioactive material away from the work area or keep behind shielding. Only take the necessary amount of radioactive material for the experiment on the bench, returning the stock solution to the storage area. All radioactive waste should also be kept away from the work area or stored behind shielding.

Example:

a) Radiation is used in ferrokinetic studies of blood plasma. An injection of ferric chloride containing 18.5 MBq (0.5 mCi) of Fe-59 is to be administrated to a calf. The dose rate at the surface of the syringe is 0.40 mSv/minute. The syringe is handled for 1 1/2 minutes. Estimate the dose to the fingers.

b) If the procedure can be performed in half the time noted above, then the radiation dose received will be reduced. Estimate the new dose to the fingers.

Answers:

a) Dose = 0.40 mSv/min * 1.5 min = 0.60 mSv

b) Dose = 0.40 mSv/min * 0.75 min = 0.30 mSv

7.3.2 Distance

As described in Module 4, gamma and X-ray exposure decreases with distance from the source, according to the inverse square law (point source). As the potential for exposure decreases, so does the potential dose.

Example:

The gamma field from a particular Co-60 source is 0.10 mSv/hr at 0.5 meter.

What is the dose at 3 meters?

Answer:

According to the inverse square law:

Dose (3m) = Dose (0.5m) * 0.52 / 32= 0.10 * 0.25 / 9 = 0.0028 mSv/hr
An effective method of protection from gamma radiation is to maintain as great a distance as possible from the source. It is good practice to work with radioactive materials at arm’s length, minimising the radiation field to the trunk of the body. When handling sources of high activity, the use of long-handled tools is required. This will reduce exposure to the hands and fingers. For high activity sources such as greater than 50 MBq (1.35 mCi) of P-32 in the research laboratory, a whole body and ring TLD dosimeter are required.

Distance is also very effective in reducing dose received from alpha and beta radioisotopes. The resulting decrease in dose is even greater than in the case of gamma and X-ray sources due to the absorption of alphas and betas in air.

When maintaining a sufficient distance from beta and gamma radiation is not feasible, shielding becomes necessary.

7.3.3 Shielding

As explained previously, no shielding is required for alpha radiation unless beta and/or gamma emission is also associated with an alpha emission.

When working with beta radiation fields, X-rays also appear in the shielding material due to bremsstrahlung. It is recommended to use low atomic mass number (Z) materials such as plastics (e.g. Plexiglas) for shielding against beta radiation. If the radionuclide has gamma emissions associated with beta radiation (e.g., I-131), protection against gamma and X-rays is required.

Due to their weak interaction with matter, shielding against gamma and X-rays is the most difficult. High atomic mass number (Z) materials (lead, steel, or even high density concrete) are used. The thickness of the shielding material required depends on the intensity and energy of the X- and gamma radiation (for high energies, a thicker layer is necessary).

Interaction of gamma and X-rays with the shielding material usually produces secondary radiation. A build-up factor, specific to each type of material and radiation, estimates the resulting increases in the radiation field.

A simple approach to selecting shielding against gamma and X-ray is by using the HVT (Half Value Thickness) of a specific material. This is the thickness of material required to reduce X- and gamma radiation to half intensity. Another important value is the TVT (Tenth Value Thickness). It’s the thickness of material that reduces the intensity of the gamma filed at one tenth its initial value. The most commonly used material for gamma and X-ray shielding is lead. The following table lists lead HVT and TVT values for different radioisotopes used at U of T.

Lead HVT and TVT values (in cm) for different radionuclides

Tc- 99m I-131 Cs-137 Ir-192 Co-60
HVT(cm) 0.03 0.72 0.65 0.55 1.10
TVT (cm) 0.09 2.4 2.2 1.90 4.00

Example:

A Co-60 radiation source creates a gamma field of 20 Sv/hr. Find the lead thickness that will reduce the gamma filed to 2 mSv/hr

Answer:

TVT value for Co-60 lead is 4.0 cm.

To reduce the gamma field from 20 Sv/hr to 0.002 Sv/hr (four orders of magnitude), a layer of lead with thickness 4*TVT= 4*4.0 cm = 16.0 cm is necessary.

The best shielding against neutron radiation is a material with abundant Hydrogen content such as water, paraffin, wax, or concrete.

A large enough quantity of any material can shield from all types of radiation. Always remember to check the effectiveness of your shielding first. The shielding used for one experiment may not be appropriate for the next experiment, especially when larger quantities of radioactive material, or a different radioisotope are used.

7.4 Protection against internal radiation

As discussed previously, all types of radiation become more hazardous once inside the human body. The greatest internal hazard is alpha radiation, which is the least dangerous when outside the body.

Protection against internal radiation is the same as with other biological or chemical hazardous materials. Suitable handling precautions, good working procedures and protective equipment are necessary.

Eating or drinking, and even storage of food or drinks, are not permitted inside any designated radioactive laboratory. Lab coats and the use of disposable gloves are required when handling radioactive materials. Eyes or face protection (goggles or plastic face shield) is also recommended when using open sources of radioactive materials.

7.5 Radionuclides used at U of T

The Periodic Table links to some of the most commonly used radionuclides at U of T along with their half-life, disintegration mode/type, energy of emitted radiation, necessary protective equipment, etc.