A Brief History in Gamma Radiography Projectors
A short overview describing how Gamma Radiography has changed over the last 50 years.
Introduction: What is Gamma Radiography?
Gamma radiography uses radioisotopes to project gamma radiation through an object and onto radiographic film.
The structure and integrity of the object attenuates the radiation. If there is a flaw in the object more radiation will pass through and onto the film allowing the extent of the flaw to be assessed.This process is identical in theory to medical X-rays however the radioisotopes generate radiation at higher energies allowing more dense items to be assessed – such as steel.
How Gamma Radiography has developed:
Early gamma radiographic techniques involved the manual handling of the radioisotope. The isotope was removed from its storage container and placed in the desired position against the work piece using handling rods. This technique involved significant radiation dose to the radiographer and over time refinements were introduced to make the process safer.
The first refinement was a shutter designed to fit to the front of the shielded container so that the isotope did not have to be removed from the shield in order for an exposure to take place. This was a big step up in safety for the radiographer. However, the technique had limitations in how close the isotope could be placed in relation to the work piece.
The next enhancement was the introduction of the “Torch” container by such companies as Vitasonics, Bix and CS Products. The source had to be removed from its shield in order to be used, but some level of protection for the operator was achieved by the use of a depleted uranium shield behind the source although this did not offer protection from scatter radiation. DU offers a good level of shielding, however there is a strong movement to reduce the amount of DU used; further information can be found on the campaign against DU website (www.cadu.org.uk).
These gamma radiography techniques were inexpensive when compared with the use of X rays, and offered a solution for working in rugged environments where there may be no power supply, however they were considered to be less safe for the operator because of operator proximity to the isotope. The challenge was to develop a safe radiography system for gamma radiography.
The Next Step, Distance from the Isotope:
The next major progression for gamma radiography came with the introduction of remote controlled projection radiography systems during the 1970's. Due to the inherent design feature of the remote control, the operator was positioned at a safer distance from the exposure. As a result from the inverse square law, the distance from the isotope greatly reduced the dose of radiation received by the operator.
The operator sets up his equipment, retreats to the remote control unit and projects the isotope down the guide tube to the work piece for a set exposure period and then retracts it back into the shielded projector.
Since its introduction in the 1970s, projection radiography has become the most common gamma radiography practice. In a typical situation the radioisotope is transported to the work site stored in a shielded radiographic projector. The projector is linked to a remote control winding mechanism at its rear and guide tubes are attached at the front end to carry the radioisotope to the work piece. At the time, there were two main suppliers of projection radiography were Tech/Ops (USA) and Sauerwein (Germany).
To maintain safe working practices and protect personnel, a controlled area must be set up around the work piece to prevent access to any area where the dose rate exceeds a pre-set threshold (7.5uSv/hr in the UK). In situations where high activity sources are in use this can have a significant restrictive effect on the work which can be done while the radiography is in progress. As an example, if a 50Ci Ir192 source is in use the controlled area would need to be 179 metres around the work piece. This can be impractical and expensive if work around the site has to stop to allow the radiography to take place.
While the isotope is in the guide tube, it gives rise to high dose rates many metres from the equipment. Typically, in projection radiography situations, most of the dose will be received when the source is travelling to and from the work piece rather than during the exposure time. This is due to two reasons; firstly the source will reside in a collimator during the exposure cutting down the dose rate. Secondly, during the exposure the source is at its furthest point from the operator.
The introduction of Se75:
Selenium 75 is a less penetrating isotope than Ir192. Its use is limited to steel thicknesses up to 40mm but where it can be used, it allows for significantly smaller controlled working areas and an improved radiograph.
Specific systems were designed for use with Selenium 75 such as Gammamat SE, Saferad and BabySCAR. All these units however have particular restrictions.
Over the years, existing products have been adapted to facilitate progress towards safer working but the solutions have been piecemeal. Gilligan Engineering identified a need for a system which would allow for the use of Ir192, Se75 and Yb169 in either close proximity or projection mode.
The systems currently available may allow for a range of isotopes to be used but not permit both projection and close proximity working, or alternatively would allow close proximity or projection work, but not with the spectrum of isotopes required. To achieve flexibility to work in either mode and any isotope, several systems would have to be purchased and transported to site – all of which contain Depleted Uranium.
In line with environmental and safety considerations, we identified a demand for a projector which did not use depleted uranium as a shielding material.
The design brief was simple:
- Design a radiography system which does not use depleted uranium shielding, can operate as close proximity or projection, and with a range of isotopes including Ir192, Se75 and Yb169 and maximises safety for the operator.
The new design is constructed exclusively from Stainless Steel and Tungsten. There is no depleted uranium and the advantages of this are extensive:
- Health advantages with no unnecessary dose due to extra radioactive materials.
- No IAEA safeguards reporting.
- No risk of spread of contamination (common in older TechOps units).
- Flexibility to use the system under water and in wet conditions.
- No disposal costs at the end of the life of the SPS Viking – it remains an asset.
Using inert Tungsten as a radiation shield as opposed to radioactive Depleted Uranium the SPS Viking can achieve true close proximity. A Tungsten Collimator can be installed directly on the front of the SPS Viking allowing the isotope to be extended from the stored position to the working position without ever being unshielded. When the source is in the working position, being so directional, it can be shielded with ease after the radiation has passed through the work piece and film.
Use of the Viking in projection mode simply requires that a guide tube is attached. The nature of the shielding offered by the source holder ensures protection for the operator's fingers when attaching guide tubes, addressing an area of concern identified when analysing dose from projectors currently on the market.
New technologies are always going to develop and help gamma radiography as an industry to progress. Safety will always remain the main priority when designing radiography projectors whilst trying to reduce weight and bulkiness of each item. Take a look at our Viking system to find out more.