Although digital radioscopy is useful for examining structures without damaging them, radiation can damage living tissue. Generally, harmful effects are associated with radioactive sources or levels of exposure much greater than used with the equipment in this research. Nonetheless, prior to using the radioscopy equipment, it is important to understand the potential hazards to people.
Genetic effects, which are passed on to offspring, and/or somatic effects, which are experienced by the individual who is exposed are potential health risks. Somatic effects can be immediate and acute (sickness or death) under extreme exposure or delayed (the development of cancer or a decrease in lifespan). Measurement units of radiation dosage are given in the table below. The severity of effects is a function of a number of factors, including:
- Cells can self repair and can keep pace with a certain amount of continuously administered radiation.
- Good physical condition may be a defense against radiation damage. Those in poor health are not as able to recover from radiation exposure.
- The type and energy of radiation controls the amount of damage. Beta particles and low energy x-rays will only damage the skin. High energy x- rays can penetrate deeper through the body and cause damage as they travel through tissue.
- The proportions of total dose and exposure time can have very different effects. It is safer to distribute a dose over time rather than through a single exposure. The concentrated one-time dose could be deadly, but spreading it over time can be better handled by the body. For example, “[i]f a person were to receive a dose of 500 Roentgens in one day, he would have less than a 50- percent chance of living. If this same dose were administered over a period of two months, the person would have a 90 percent chance of almost complete recovery.” (Field Radiography Employing Radiation Machines, Environmental Compliance & Health And Safety, Science Applications International Corporation (SAIC, no date, p. 5-6)
Site investigations using digital radioscopy require that a carefully organized safety plan be in place whether capturing images in the lab or in the field. This is especially important when individuals not associated with the investigation might be present, as with field work. The most important components of an adequate safety plan include:
- Operator training and radiation safety officer
- An individualized safety plan for the specific site
- Determination of safe distances
- Source key protocol
- Monitoring for radiation exposure
Operator Training and Radiation Safety Officer
The operator must be familiar with not only the operation of the x-ray source but also the regulations of the state where the work is being conducted to comply with radiation safety requirements. Fortunately, most regulations are very similar so the learning curve is not significant once an operator understands the basic issues.
Training may be available through the company selling equipment, through state certifying agencies and contractors. A list of state requirements and training programs is available through: www.hsrd.ornl.gov/nrc/ircertif.htm. The American Society of Nondestructive Testing (www.asnt.org/certification/irrsp/index.htm) also offers the Industrial Radiography and Radiation Safety Personnel training program.
At each job site, a radiation safety officer should be identified. This person should be trained in safe use of the equipment and should maintain the utilization and exposure records. He or she also serves as the contact for the state certifying agency. All operators of the equipment need to be trained to operate the equipment safely.
Safe Operation (Safe Distance, Signage, Source Key Protocol)
The safe operation of the equipment for a particular project should include:
- Defining the safe perimeter distance based on anticipated number of pulses and material being investigated
- Posting signage to warn the public in the immediate area
- Addressing line-of-site problems by using two people if the source and
imager are not visible from one location
It is important to maintain a safe perimeter on both sides of the equipment and the item being examined. This is done by marking off the area with ropes or tape, using radiation safety cards and using a second person when both sides of the radioscopy testing are not in view. The second person will assure that the area behind the testing is clear of people. This is particularly important when doing field work where the other side of the area being examined is another room in which people may not be aware of the work being done.
Since the energy of the source decreases with increasing distance (the inverse square law), a safe perimeter around shot site can be determined based on the anticipated number of pulses required to generate an acceptable image. It is important to maintain a safe perimeter for all individuals not associated with the investigation (using signage or ropes) and to determine a safe distance from which shots can be taken by the operator. Radiation signs should always be posted around the area of the investigation. One example of such a sign is included in the appendix.
Records of each firing of the x-ray source need to be maintained including the number of pulses used, the distance and angle of the firing, boundaries and the type of shielding (if any) used. Some certifying agencies may want to inspect these records periodically. The records track the amount of exposure the operators may receive. The radioscopy form that Anthony & Associates, Inc. uses records the date, site, source number, test location, number of pulses, imager orientation, file name, photo number, and specific measurements (a sample set- up form is in the appendix).
The primary control of the XR200® source (used with both radioscopy systems) is a removable key that prevents firing the source without the key being inserted. As part of any safety plan, the key should always be in the operator’s possession until the shot is completely set up. Only then should the immediate area be cleared, the key inserted and the shot taken, to avoid accidental discharge of the unit.
Monitoring for Radiation Exposure
Radiation exposure is obviously a concern when doing radioscopic examinations, and is the focus of the safety plan components identified above. Monitoring of radiation is important for both the operators and the site. Two types of monitoring devices were used during this investigation. The first were Pocket Ionization Chambers (PICs) from S. E. International, Inc., also known as pocket or pen dosimeters. They are used to determine exposure during individual x-ray operations. These 4.5-inch cylinders can clip on shirt pockets and are easy to use.
They read dosages up to 200 mREM and can be manually reset to zero for each x- ray session. They were also used to monitor for background radiation and any leakage near the source by positioning one immediately behind the source during the initial setup of each x-ray session. They are easily calibrated in the field and are typically read and recorded at the beginning and end of each session, at a minimum. Radiation dose limits are given in the table below.
Long-term exposure to radiation was monitored using the more accurate Thermoluminescent Dosimeter badges (TLD badges), obtained from Landauer, Inc. TLD badges can be worn daily to monitor exposure. They should be provided to operators for use within controlled areas. Three types of TLD badges measure different types of rays:
- “K”: For whole body exposures to x-rays, gamma rays, beta particles.
- “Z”: For whole body exposures to neutrons, x-rays, gamma rays, beta particles.
- “U”: For extremity exposures to x-rays, gamma rays, beta particles.
A new badge is provided for each individual every quarter and the old badge is returned, along with a control badge, for determination of cumulative dosage for that quarter, year-to-date, and lifetime exposure (all in mREM). For smaller organizations, this service is also available on a bi-annual basis.