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Reducing Radiation Exposure and Time on Aircraft
When Performing Limited Radiographic Testing

by Stan Weatherly*

 

In an effort to reduce radiation exposure for faster X-ray testing, the Boeing Long Beach division pursued an alternative approach to conventional field radiography on the C-17 military transport aircraft.

Tests for short edge distance, improperly drilled holes and stack-ups often require carrying equipment up multiple flights of stairs. The logistics of this operation (positioning an X-ray tube, establishing a safe location for the control panel, draping power and cooling cables, and establishing a 0.5 µC/kg [2 mR] barrier) are quite burdensome. Performing these tasks is costly and time consuming. The clearing of the aircraft to establish a safe radiation perimeter within acceptable limits is also costly. Personnel must be evacuated from the aircraft and all adjacent areas before an exposure can be made. This usually leads to radiographic testing being performed during off hours, on the third shift or on weekends at a premium rate.

Our goal was to provide the beneficial results of X-ray testing while improving on all other aspects of the radiographic process, including the flexibility of a recording medium that could be manipulated into confined spaces.

Pertinent to the solution was the use of a portable, lightweight X-ray source using no power cables and producing minimal scatter radiation while penetrating a stack-up of aluminum skins, spars and ribs. The main goal, however, had to be reducing the primary and secondary radiation to a level where the X-rays could be taken while safely shrinking the barrier perimeter. This meant establishing a safe distance for non-radiographic personnel so they could continue working on the aircraft in the near vicinity of the radiographic testing being performed.

With this setup, the radiation barriers, total dose and
exposure times were significantly reduced.

A search of published materials and the Internet revealed several newer technologies using computed and digital radiography. However, the cost of switching from a conventional film to a digital imaging system did not fit our budget criteria (under $10,000). The application had to be portable while minimizing future costs. The search revealed that a portable, lightweight, battery powered radiation source might fit our application.

Several manufacturers produce portable X-ray sources that are powered by rechargeable 14.4 V batteries. These units do not look like conventional X-ray sources: only a single, thin cable controls the number of X-ray pulses applied to the exposure. There is no voltage, milliamperage or time to set - only the number of pulses. Each pulse is approximately 60 ns. The tubes usually weigh between 5.5 to 6.5 kg (12 to 15 lb) and are about the size of a lunch pail. They are advertised as having the capability to be used with either a conventional film or digital system.

After reviewing literature from several of the X-ray tube manufacturers, one of them was contacted. They agreed to send a demonstration X-ray unit for a trial. The unit they sent was a maximum potential 150 kV (peak) source with a 3 mm (0.12 in.) focal spot. The literature stated that its expected tube life was 100 000 pulses and that it produced a relatively low dose rate, comparable to a 1 mA, constant potential machine. The literature also cited its capability of penetrating up to 10 mm (0.4 in.) of steel with specific recording medium combinations.

Although these units are normally used with computed or digital radiography systems, we were only interested in using the system in a somewhat conventional mode, thereby eliminating the cost of purchasing a computed/digital recording medium.

Upon receiving the unit, we tried it with our fastest film, a very fine-grain, high-contrast ASTM Class 2 film suitable for light metals. The film we selected is advertised as being suitable for use with fluorescent or fluorometallic screens. We did not use fluorometallic screens to assist with the exposure. The initial exposures produced less than favorable results. We discovered that without the screens, we had very little density on the film.

We contacted both the X-ray tube and film manufacturers, who both recommend that we try the procedure again with rare earth or calcium tungstate screens. The X-ray tube manufacturer shipped a calcium tungstate screen to us to try. We placed the single calcium tungstate screen in a standard black vinyl cassette. The setup was as follows: source, stepwedge, vinyl cassette loaded with the screen facing tube-side and then the ASTM Class 2 film. Again the shots produced less than favorable results.

We then talked with some film and screen distributors about the acquisition and benefits of switching to a barium fluo-robromide (a rare earth) screen. These screens reportedly accelerate the exposure, therefore reducing the exposure time by one-half to one-third of the time required with the original calicum tungstate screen.

Table 1  Results of the rare earth screen setup at various distances and pulses

Pulses	Source to Film Distance	Aluminum Step Wedge	Density
15	635 mm (25 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	2.7 2.2 1.8
20	635 mm (25 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	3.1 2.6 2.2
30	635 mm (25 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	3.7 3.1 2.6
15	762 mm (30 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	2.1 1.6 1.3
20	762 mm (30 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	2.5 2.1 1.7
30	762 mm (30 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	3.1 2.6 2.2
15	889 mm (35 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	1.8 1.4 1.1
20	889 mm (35 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	2.0 1.6 1.3
30	889 mm (35 in.)	3.2 mm (0.125 in.) 6.4 mm (0.250 in.) 9.5 mm (0.375 in.)	2.5 2.0 1.7

 

One of the manufacturers generously sent us a blue-emitting rare earth screen to evaluate. This screen was evaluated with the same setup as above (source, stepwedge, vinyl cassette loaded with the rare earth screen and then the ASTM Class 2 film). Table 1 shows the results of this setup at various distances and pulses.

Based on the information obtained, we calculated that we could acquire approximately 2000 to 4000 exposures from the X-ray source depending on the type of screen used. At an approximate cost of $5,000 for the tube and the purchase of a minimal number of screens, this translates to a cost of approximately $2 per exposure to use this X-ray unit. The acquisition of the X-ray source used in conjunction with rare earth screens becomes very cost effective. With this setup, the radiation barriers, total dose and exposure times were significantly reduced. The radiation barriers were reduced by as much 75% from previously established barriers while maintaining an exceptionally low dose for total exposure time.

When compared to premium time for two technicians or lost time with builders off the aircraft, selection of this X-ray unit and the rare earth screen is a favorable solution. The purchase of the X-ray unit and rare earth screens was recommended.

 

* The Boeing Company, 2401 E. Wardlow Road, MC C054-0023, Long Beach, CA 90807; (562) 982-7073; fax (562) 593-9581; e-mail stanley.l.weatherly@boeing.com.

 

Copyright © 2007 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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